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Publications

#indicates work by my graduate student or post-doc

2021

Zhou, L., R.E. Alcalde, J. Deng, B. Zuniga, R.A. Sanford, B.W. Fouke, C.J. Werth, Impact of antibiotic concentration gradients on nitrate reduction and antibiotic resistance in a microfluidic gradient chamber, Science of the Total Environment, In Press, 2021.

#Esfahani, S.G., A.J. Valocchi, C.J. Werth, Using MODFLOW and RT3D to Simulate Diffusion and Reaction without Discretizing Low Permeability Zones, 239, 103777, 2021.

#Alcalde, R., C. Dundas, Y. Dong, R. Sanford, B. Keitz, B. Fouke, C.J. Werth, The Role of Chemotaxis and Efflux Pumps on Nitrate Reduction in the Toxic Regions of a Ciprofloxacin Concentration Gradient, ISME, In Press, 2021.

Akono, A.-T. C. Werth, Z. Shi, K. Jessen, T. Tsotsis, Advanced geomechanical model to predict the impact of CO2-induced microstructural alterations on the cohesive-frictional behavior of Mt. Simon sandstone, Minerals, In Press, 2021.

Engelmann, C., K.S. Lari, L. Schmidt, C.J. Werth, M. Walther, Towards predicting DNAPL source zone formation to improve plume assessment: using robust laboratory and numerical experiments to evaluate the relevance of retention curve characteristics, J. Hazardous Materials, 407, 124741, 2021.

Yan, C., S. Kakuturu, A. Hesterberg Butzlaff, D. Cwiertny, S. Mubeen, C.J. Werth, Scalable Reactor Design for Electrocatalytic Nitrite Reduction with Minimal Mass Transfer Limitations, ACS ES&T Engineering, 1(2), 204-215, 2021.

#Werth, C.J., C. Yan, J. Troutman, Factors impeding replacement of ion exchange with (electro)catalytic treatment for nitrate removal from drinking water, ACS ES&T Engineering, 1(1), 6-20, 2021.

2020

Fouke, B.W., A.S. Bhattacharjee, G.A. Fried, M. Sivaguru, R.A. Sanford, L. Zhou, R.E. Alcalde, K. Wunch, A. Stephenson, J.A. Ferrar, A.G. Hernandez, C. Wright, C.J. Fields, L.G. Todorov, K.W. Fouke, E.M. Fried, C.W. Werth, Sulfate Reducing Bacteria Streamers and Iron Sulfides Rapidly Occlude Porosity and Increase Hydraulic Resistance in Proppant-Filled Shale Fractures, AAPG Bulletin, In Press, 2020.

#Harbert, W., A.. Goodmad, R. Spaulding, I. Haljasmaa, D. Crandall, S. Sanguinito, B. Kutchko, M. Tkach, S. Fuchs, C.J. Werth, T. Tsotsis, L. Dalton, K. Jessen, Z. Shi, S. Frailey, CO2 induced changes in Mount Simon sandstone: Understanding links to post CO2 injection monitoring, seismicity, and reservoir integrity, International J. of Greenhouse Gas Control, 100, 103109, 2020.

#Li, H., C. Yan, H. Guo, K. Shin, S. Humphrey, C. Werth, G. Henkelman, CuxIr1-x nanoalloy catalysts achieve near 100% selectivity for aqueous nitrite reduction to NH3, ACS Catalysis, 10, 14, 7915–7921, 2020.

#Troutman, J., H. Li, A. Haddix, B. Kienzle, G. Henkelman, S. Humphrey, C. Werth, PdAg alloy nanocatalysts: Toward economically viable nitrite reduction in drinking water, ACS Catalysis, 10, 14, 7979–7989, 2020.

Zeng, T., K.T. Kim, C.J. Werth, L.E. Katz, K.K. Mohanty, Surfactant Adsorption on Shale Samples: Experiments and an Additive Model, Energy and Fuels, ef-2019-04016z (10.1021/acs.energyfuels.9b04016), 2020.

Dávila, G., L. Dalton, D.M.Crandall, C. Garing, C.J.Werth, J.L.Druhan, Reactive alteration of a Mt. Simon Sandstone due to CO2-rich brine displacement, Geochimica et Cosmochimica Acta, Volume 271, 227-247, 2020.

2019

Dávila, G., L. Dalton, D.M.Crandall, C. Garing, C.J.Werth, J.L.Druhan, Reactive alteration of a Mt. Simon Sandstone due to CO2-rich brine displacement, Geochimica et Cosmochimica Acta, Volume 271, 227-247, 2020.

Engelmann, C., L. Schmidt, C.J. Werth, M. Walther, Quantification of uncertainties from image processing and analysis in laboratory-scale DNAPL release studies evaluated by reflective optical imaging, Water, In Press, 2019.

Fuchs, S., D.N. Espinoza, A.T. Akono, C.J. Werth, Geochemical and geomechanical alteration of siliciclastic reservoir rock by supercritical CO2-saturated brine formed during geological carbon sequestration, International Journal of Greenhouse Gas Control, 88, 251-260, 2019.

Dong, Y., Sanford, R.A., Inskeep, W.P., Srivastava, S., Bulone, V., Fields, C., Yau, P.M., Sivaguru, M., Ahren, D., Fouke, K.W., Weber, J., Werth, C.J., Cann, I. K., Keating, M. K., Khetani, R., Hernandez, H.G., Wright, C., Band, M., Imai, B.S., Fried, G.A., and Fouke, B. W., 2019. Physiology, Metabolism and Fossilization of Hot-Spring Filamentous Microbial Mats. Astrobiology, 19(12), 1-17, 2019.

Deng, J., L. Zhou, R.A. Sanford, L.A. Shechtman, Y. Dong, R.E. Alcalde, M. Sivaguru, G.A. Fried, C.J. Werth, B.W. Fouke, Adaptive evolution of Escherichia coli to ciprofloxacin in controlled stress environments: Contrasting patterns of resistance in spatially varying versus uniformly, Environmental Science & Technology, 53(14), 7996-8005, 2019.

Akono, A.-T., J.L. Druhan, G. Dávila, T. Tsotsis, K. Jessen, S. Fuchs, D. Crandall, Z. Shi, L. Dalton, M.K. Tkach, P. Kabir, A.L. Goodman, S. Frailey, C.J. Werth, A Review of Geo-Chemical-Mechanical Impacts in Geological Carbon Storage Reservoirs, Greenhouse Gases: Science and Technology, 9(3), 474-504, 2019.

Berns, E.C., R.A. Sanford, A.J. Valocchi, T.J. Strathmann, C.E. Schaefer, C.J. Werth, Contributions of Biotic and Abiotic Pathways to Anaerobic Trichloroethene Transformation in Low Permeability Source Zones, J. Contam. Journal of Contaminant Hydrology, 224, 103480, 2019.

Michelson, K., R. Alcalde, R.A. Sanford, A.J. Valocchi, C.J. Werth, Diffusion-based recycling of flavins allows Shewanella oneidensis MR-1 to yield energy from metal reduction across physical separations, Environmental Science & Technology., 53(7), 3480-3487, 2019.

Alcalde, R.E., K. Michelson, L. Zhou, E.V. Schmitz, J. Deng, R.A. Sanford, B.W. Fouke, C.J. Werth, Shewanella oneidensis MR-1 Motility Allows for Nitrate Reduction in the Toxic Region of a Ciprofloxacin Concentration Gradient in a Diffusion-Controlled Microfluidic Reactor, Environmental Science & Technology, 53(5), 2778-2787, 2019.

2018

Schaeffer, C.E. P. Ho, E. Berns, C.J. Werth, Mechanisms for abiotic dechlorination of TCE by ferrous minerals under oxic and anoxic conditions in natural sediments, Environmental Science & Technology, 52(23), 13747-13755, 2018.

A. J. Valocchi, Bolster, D., and Werth, C. J., “Mixing-limited reactions in porous media,” Transport in Porous Media, pp. 1-26, 2018. Publisher’s Version

Abstract
Mixing-driven reactions in porous media are ubiquitous and span natural and engineered environments, yet predicting where and how quickly reactions occur is immensely challenging due to the complex and nonuniform nature of porous media flows. In particular, in many instances, there is an enormous range of spatial and temporal scales over which reactants can mix. This paper aims to review factors that affect mixing-limited reactions in porous media, and approaches used to predict such processes across scales. We focus primarily on the challenges of mixing-driven reactions in porous media at pore scales to provide a concise, but comprehensive picture. We balance our discussion between state-of-the-art experiments, theory and numerical methods, introducing the reader to factors that affect mixing, focusing on the bracketing cases of transverse and longitudinal mixing. We introduce the governing equations for mixing-limited reactions and then summarize several upscaling methods that aim to account for complex pore-scale flow fields. We conclude with perspectives on where the field is going, along with other insights gleaned from this review.

 

A. – T. Akono, Kabir, P., Shi, Z., Fuchs, S. J., Tsotsis, T. T., Jessen, K., and Werth, C. J., “Modeling CO2-induced alterations in Mt. Simon sandstone via nanomechanics,” Rock Mechanics and Rock Engineering, pp. 1-23, 2018. Publisher’s Version

Abstract
The objective of this work is to formulate a novel and physics-based nanomechanics framework to connect geochemical reactions in host rock to the resulting morphological changes at the microscopic lengthscale and to the resulting geomechanical changes at the macroscopic lengthscale. The key idea is to monitor the fraction of minerals based on their mechanical signature. We illustrate this procedure on the Mt. Simon sandstone from the Illinois Basin. To this end, various acidic fluid systems were applied to Mt. Simon sandstone specimens. The chemistry, morphology, microstructure, and mechanical characteristics were investigated across multiple lengthscales. Grid indentation was carried out with a total of 6900 individual indentation tests performed on 24 specimens. A good agreement was observed between the composition computed using statistical nanoindentation and measurements employing independent methods such as scanning electron microscopy, electron-dispersive X-ray spectroscopy, X-ray diffraction analyses, mercury intrusion porosimetry, flow perporometry, and helium pycnometry. An increase in porosity and a decrease in K-feldspar content were observed following the incubation in CO2-saturated brine, suggesting dissolution reactions involving feldspar. Thus, a rigorous methodology is presented to connect geochemical reactions and related compositional changes at the nano- and microscopic scales to alterations of the constitutive behavior at the macroscopic level.

 

C. E. Schaefer, Ho, P., Berns, E., and Werth, C. J., “Mechanisms for abiotic dechlorination of TCE by ferrous minerals under oxic and anoxic conditions in natural sediments,” Environmental Science & Technology, 2018. Publisher’s Version

Abstract
Bench-scale experiments were performed on natural sediments to assess abiotic dechlorination of trichloroethene (TCE) under both aerobic and anaerobic conditions. In the absence of oxygen (<26 µM), TCE dechlorination proceeded via a reductive pathway generating acetylene and/or ethene. Reductive dechlorination rate constants up to 3.1 x 10-5 d-1 were measured, after scaling to in situ solid water ratios. In the presence of oxygen greater than 120 µM, TCE dechlorination proceeded via an oxidative pathway generating formic/glyoxylic and glycolic/acetic acids, and oxidative dechlorination rate constants (again scaled to in situ conditions) up to 7.4 x 10-3 d-1 were measured. These rates correspond to half-lives of 60 and 0.25 years for abiotic TCE dechlorination under anaerobic and aerobic conditions, respectively, indicating the potentially large impact of aerobic TCE oxidation in the field. For both reductive and oxidative TCE dechlorination pathways, measured first-order rate constants increased with increasing ferrous iron content, suggesting the role of iron oxidation. Hydroxyl radical formation was measured and increased with increasing oxygen and ferrous iron content. Rate constants associated with TCE oxidation products increased with increasing hydroxyl radical generation rates, and are zero in the presence of a hydroxyl radical scavenger, suggesting that oxidative TCE dechlorination is a hydroxyl radical driven process.

 

S. U. Akki and Werth, C. J., “Critical review: DNA aptasensors, are they ready for monitoring organic pollutants in natural and treated water sources?,” Environmental Science and Technology, vol. 52, no. 16, pp. 8989-9007, 2018. Publisher’s Version

Abstract
There is a growing need to monitor anthropogenic organic contaminants detected in water sources. DNA aptamers are synthetic single-stranded oligonucleotides, selected to bind to target contaminants with favorable selectivity and sensitivity. These aptamers can be functionalized and are used with a variety of sensing platforms to develop sensors, or aptasensors. In this critical review, we (1) identify the state-of-the-art in DNA aptamer selection, (2) evaluate target and aptamer properties that make for sensitive and selective binding and sensing, (3) determine strengths and weaknesses of alternative sensing platforms, and (4) assess the potential for aptasensors to quantify environmentally relevant concentrations of organic contaminants in water. Among a suite of target and aptamer properties, binding affinity is either directly (e.g., organic carbon partition coefficient) or inversely (e.g., polar surface area) correlated to properties that indicate greater target hydrophobicity results in the strongest binding aptamers, and binding affinity is correlated to aptasensor limits of detection. Electrochemical-based aptasensors show the greatest sensitivity, which is similar to ELISA-based methods. Only a handful of aptasensors can detect organic pollutants at environmentally relevant concentrations, and interference from structurally similar analogs commonly present in natural waters is a yet-to-be overcome challenge. These findings lead to recommendations to improve aptasensor performance.

 

W. Lin, Bergquist, A. M., Mohanty, K. K., and Werth, C. J., “Environmental impacts of replacing slickwater with low/no-water fracturing fluids for shale gas recovery,” ACS Sustainable Chemistry & Engineering, vol. 6, no. 6, pp. 7515-7524, 2018. Publisher’s Version

Abstract
The environmental impacts of a typical hydraulic fracturing operation for shale gas recovery were evaluated using life cycle assessment, with energy demands for well drilling and fracturing determined from GHGfrack model. Dominant environmental impacts stem from well construction, which are >63% in all categories (e.g., global warming and eutrophication), and mainly due to diesel fuel combustion and steel production. The relative impacts related to water use (i.e., fracturing fluid components, water/wastewater transportation, and wastewater disposal) are relatively small, ranging from 5 to 22% of total impacts in all categories; freshwater consumption for fracturing is also a small fraction of available water resources for the shale play considered. The impacts of replacing slickwater with CO2 or CH4-foam fracturing fluid (≤10 vol % water) were evaluated; total impacts decrease <12%, and relative impacts related to water use decrease to 2–9% of total impacts. Hence, switching to a foam-based fracturing fluid can substantially decrease water-related impacts (>60%) but has only marginal effects on total environmental impacts. Changes in lateral well length, produced to fresh-water ratios, fracturing fluid composition, and LCA control volume do not change these findings. More benefits could potentially be realized by considering water versus foam-related impacts of ecological health and energy production.

 

M. Sivaguru, Saw, J. J., Jr., J. W. C., Lieske, J. C., Krambeck, A. E., Romero, M. F., Chia, N., Schwaderer, A. L., Alcalde, R. E., Bruce, W. J., Wildman, D. E., Fried, G. A., Werth, C. J., Reeder, R. J., Sanford, R. A., and Fouke, B. W., “Geobiology reveals how human kidney stones dissolve in vivo,” Scientific Reports, vol. 8, pp. 13731, 2018. Publisher’s Version

Abstract
More than 10% of the global human population is now afflicted with kidney stones, which are commonly associated with other significant health problems including diabetes, hypertension and obesity. Nearly 70% of these stones are primarily composed of calcium oxalate, a mineral previously assumed to be effectively insoluble within the kidney. This has limited currently available treatment options to painful passage and/or invasive surgical procedures. We analyze kidney stone thin sections with a combination of optical techniques, which include bright field, polarization, confocal and super-resolution nanometer-scale auto-fluorescence microscopy. Here we demonstrate using interdisciplinary geology and biology (geobiology) approaches that calcium oxalate stones undergo multiple events of dissolution as they crystallize and grow within the kidney. These observations open a fundamentally new paradigm for clinical approaches that include in vivo stone dissolution and identify high-frequency layering of organic matter and minerals as a template for biomineralization in natural and engineered settings.

 

2017

C. E. Schaefer, Ho, P., Gurr, C., Berns, E., and Werth, C. J., “Abiotic dechlorination of chlorinated ethenes in natural clayey soils: impacts of mineralogy and temperature,” Journal of Contaminant Hydrology, 2017. Publisher’s Version

Abstract
Laboratory batch experiments were performed to assess the impacts of temperature and mineralogy on the abiotic dechlorination of tetrachloroethene (PCE) or trichloroethene (TCE) due to the presence of ferrous minerals in natural aquifer clayey soils under anaerobic conditions. A combination of x-ray diffraction (XRD), magnetic susceptibility, and ferrous mineral content were used to characterize each of the 3 natural soils tested in this study, and dechlorination at temperatures ranging from 20 to 55 °C were examined. Results showed that abiotic dechlorination occurred in all 3 soils examined, yielding reduced gas abiotic dechlorination products acetylene, butane, ethene, and/or propane. Bulk first-order dechlorination rate constants (kbulk), scaled to the soil:water ratio expected for in situ conditions, ranged from 2.0 × 10−5 day−1 at 20 °C, to 32 × 10−5 day−1 at 55 °C in the soil with the greatest ferrous mineral content. For the generation of acetylene and ethene from PCE, the reaction was well described by Arrhenius kinetics, with an activation energy of 91 kJ/mol. For the generation of coupling products butane and propane, the Arrhenius equation did not provide a satisfactory description of the data, likely owing to the complex reaction mechanisms associated with these products and/or diffusional mass transfer processes associated with the ferrous minerals likely responsible for these coupling reactions. Although the data set was too limited to determine a definitive correlation, the two soils with elevated ferrous mineral contents had elevated abiotic dechlorination rate constants, while the one soil with a low ferrous mineral content had a relatively low abiotic dechlorination rate constant. Overall, results suggest intrinsic abiotic dechlorination rates may be an important long-term natural attenuation component in site conceptual models for clays that have the appropriate iron mineralogy.

 

A. M. Bergquist, Bertoch, M., Gildert, G., Strathmann, T. J., and Werth, C. J., “Catalytic denitrification in a trickle bed reactor: ion exchange waste brine treatment,” Journal-American Water Works Association, vol. 109, no. 5, pp. E129-E143, 2017. Publisher’s Version

Abstract
Catalytic reduction of nitrate in ion exchange (IX) waste brine for reuse is a promising option for reducing IX costs and environmental impacts. A recycling trickle bed reactor (TBR) was designed and optimized using 0.5 percent byweight (wt%) palladium–0.05 wt% indium catalysts supported on US mesh size 12 × 14 or 12 × 30 activated carbon particles. Various liquid superficial velocities (Ur) and hydrogen gas superficial velocities (Ug-H2) were evaluated to assess performance in different flow regimes; catalyst activity increased with Ug-H2 at all Ur for both catalysts and was greatest for the 12 × 30 catalyst at thelowest Ur (8.9 m/h). The 12 × 30 catalyst demonstrated up to 100% higher catalytic activity and 280% higher mass transfer rate compared with the 12 × 14 catalyst. Optimal TBR performance was achieved with both catalysts in thetrickle flow regime. The results indicate that the TBR is a promising step forward, and continued improvements are possible to overcome remaining mass transfer limitations.

 

M. Bertoch, Bergquist, A. M., Gildert, G., Strathmann, T. J., and Werth, C. J., “Catalytic nitrate removal in a trickle bed reactor: direct drinking water treatment,” Journal-American Water Works Association, vol. 109, no. 5, pp. E144, 2017. Publisher’s Version

Abstract
Palladium (Pd)-based catalysts hold promise as an alternative water treatment technology for nitrate (NO3–), but practical application requires a flow-through reactor that efficiently delivers hydrogen (H2) from gas to water. A trickle bed reactor (TBR) packed with a 0.1 percent by weight (wt%) Pd–0.01 wt% In/γ-Al2O3 (indium and porous aluminum oxide) catalyst was evaluated to address this challenge. Catalytic activity generally increased with H2 superficial velocity (0.65–29.6 m/h) and liquid (deionized water) superficial velocities from 14.8 to 26.6 m/h before decreasing at 38.5 m/h. This decrease corresponded to a change in flow regime and suggests that optimal TBR performance occurs at the transition from pulse to bubble flow. An optimal TBR activity of 19.5 ± 1.3 mg NO3–/min-g Pd was obtained; this is only ~18% of the batch reactor activity as a result of H2 mass transfer limitations, but three to 15 times greater than activities obtained with previous flow-through reactors. Catalyst deactivation occurred in the TBR after 41 days of operation, motivating the need for improved fouling mitigation strategies.

 

J. Botto, Fuchs, S. J., Fouke, B. W., Clarens, A. F., Freiburg, J. T., Berger, P. M., and Werth, C. J., “Effects of mineral surface properties on supercritical CO2 wettability in a siliciclastic reservoir,” Energy & Fuels, vol. 31, no. 5, pp. 5275-5285, 2017. Publisher’s Version

Abstract
Wettability is a key reservoir characteristic influencing geological carbon sequestration (GCS) processes, such as CO2 transport and storage capacity. Wettability is often determined on a limited number of reservoir samples by measuring the contact angle at the CO2/brine/mineral interface, but the ability to predict this value remains a challenge. In this work, minerals comprising a natural reservoir sample were identified, and the influence of their surface roughness and mineralogy on the contact angle was quantified to evaluate predictive models and controlling mechanisms. The natural sample was obtained from the Mount Simon formation, a representative siliciclastic reservoir that is the site of a United States Department of Energy CO2 injection project. A thin section of the Mount Simon sandstone was examined with compound light microscopy and environmental scanning electron microscopy (ESEM) coupled with energy-dispersive X-ray spectroscopy (EDS). Quartz and feldspar were identified as dominant minerals and were coated with various reddish black precipitates consistent with illite clay and iron oxide hematite. Contact angle (θ) measurements were conducted for the four representative minerals and the Mount Simon sample over a range of pressures (2–25 MPa) at 40 °C. At supercritical conditions, all samples are strongly water-wet, with contact angles between 27° and 45°. Several predictive models for contact angle were evaluated for the mineral and Mount Simon samples, including the Wenzel and Cassie–Baxter models, plus newly proposed modifications of these that account for the fraction of different minerals comprising the reservoir sample surface, the surface roughness, and the extent that roughness pits are filled with brine. Modeling results suggest that the fraction of mineral surfaces containing roughness pits filled with brine is the most important reservoir characteristic that controls wettability in the Mount Simon sandstone, followed by surface mineralogy. To our knowledge, this is one of the few studies to investigate the effects of individual minerals on the wettability of a natural reservoir sample.

 

X. Chen, Huo, X., Liu, J., Wang, Y., Werth, C. J., and Strathmann, T. J., “Exploring beyond palladium: catalytic reduction of aqueous oxyanion pollutants with alternative platinum group metals and new mechanistic implications,” Chemical Engineering Journal, vol. 313, pp. 745-752, 2017. Publisher’s Version

Abstract
For over two decades, Pd has been the primary hydrogenation metal studied for reductive catalytic water treatment applications. Herein, we report that alternative platinum group metals (Rh, Ru, Pt and Ir) can exhibit substantially higher activity, wider substrate selectivity and variable pH dependence in comparison to Pd. Cross comparison of multiple metals and oxyanion substrates provides new mechanistic insights into the heterogeneous reactions. Activity differences and pH effects mainly originate from the chemical nature of individual metals. Considering the advantages in performance and cost, results support renewed investigation of alternative hydrogenation metals to advance catalytic technologies for water purification and other environmental applications.

 

J. Tudek, Crandall, D., Fuchs, S. J., Werth, C. J., Valocchi, A. J., Chen, Y., and Goodman, A., “In situ contact angle measurements of liquid CO2, brine, and Mount Simon sandstone core using micro X-ray CT imaging, sessile drop, and lattice Boltzmann modeling,” Journal of Petroleum Science and Engineering, vol. 155, pp. 3-10, 2017. Publisher’s Version

Abstract
Three techniques to measure and understand the contact angle, θ, of a CO2/brine/rock system relevant to geologic carbon storage were performed with Mount Simon sandstone. Traditional sessile drop measurements of CO2/brine on the sample were conducted and a water-wet system was observed, as is expected. A novel series of measurements inside of a Mount Simon core, using a micro X-ray computed tomography imaging system with the ability to scan samples at elevated pressures, was used to examine the θ of residual bubbles of CO2. Within the sandstone core the matrix appeared to be neutrally wetting, with an average θ around 90°. A large standard deviation of θ (20.8°) within the core was also observed. To resolve this discrepancy between experimental measurements, a series of Lattice Boltzmann model simulations were performed with differing intrinsic θ values. The model results with a θ=80° were shown to match the core measurements closely, in both magnitude and variation. The small volume and complex geometry of the pore spaces that CO2 was trapped in is the most likely explanation of this discrepancy between measured values, though further work is warranted.

 

J. Liu, Su, X., Han, M., Wu, D., Gray, D. L., Shapley, J. R., Werth, C. J., and Strathmann, T. J., “Ligand design for isomer-selective oxorhenium(V) complex synthesis,” Inorganic Chemistry, vol. 56, no. 3, pp. 1757-1769, 2017. Publisher’s Version

Abstract
Recently, N,N–trans Re(O)(LN–O)2X (LN–O = monoanionic N–O chelates; X = Cl or Br prior to being replaced by solvents or alkoxides) complexes have been found to be superior to the corresponding N,N–cis isomers in the catalytic reduction of perchlorate via oxygen atom transfer. However, reported methods for Re(O)(LN–O)2X synthesis often yield only the N,N–cis complex or a mixture of trans and cis isomers. This study reports a geometry-inspired ligand design rationale that selectively yields N,N–trans Re(O)(LN–O)2Cl complexes. Analysis of the crystal structures revealed that the dihedral angles (DAs) between the two LN–O ligands of N,N–cis Re(O)(LN–O)2Cl complexes are less than 90°, whereas the DAs in most N,N–trans complexes are greater than 90°. Variably sized alkyl groups (−Me, −CH2Ph, and −CH2Cy) were then introduced to the 2-(2′-hydroxyphenyl)-2-oxazoline (Hhoz) ligand to increase steric hindrance in the N,N–cis structure, and it was found that substituents as small as −Me completely eliminate the formation of N,N–cisisomers. The generality of the relationship between N,N–trans/cis isomerism and DAs is further established from a literature survey of 56 crystal structures of Re(O)(LN–O)2X, Re(O)(LO–N–N–O)X, and Tc(O)(LN–O)2X congeners. Density functional theory calculations support the general strategy of introducing ligand steric hindrance to favor synthesis of N,N–trans Re(O)(LN–O)2X and Tc(O)(LN–O)2X complexes. This study demonstrates the promise of applying rational ligand design for isomeric control of metal complex structures, providing a path forward for innovations in a number of catalytic, environmental, and biomedical applications.

 

K. Michelson, Sanford, R. A., Valocchi, A. J., and Werth, C. J., “Nanowires of Geobacter sulfurreducens require redox cofactors to reduce metals in pore spaces too small for cell passage,” Environmental Science & Technology, 2017. Publisher’s Version

Abstract
Members of the Geobacteraceae family are ubiquitous metal reducers that utilize conductive ‘nanowires’ to reduce Mn(IV) and Fe(III) oxides in anaerobic sediments. However, it is not currently known if and to what extent the Mn(IV) and Fe(III) oxides in soil grains and low permeability sediments that are sequestered in pore spaces too small for cell passage can be reduced by long-range extracellular electron transport via Geobacter nanowires, and what mechanisms control this reduction. We developed a microfluidic reactor that physically separates Geobacter sulfurreducens from the Mn(IV) mineral birnessite by a 1.4 μm thick wall containing <200 nm pores. Using optical microscopy and Raman spectroscopy, we show that birnessite can be reduced up to 15 μm away from cell bodies, similar to the reported length of Geobacter nanowires. Reduction across the nanoporous wall required reducing conditions, provided by Escherichia coli, and an exogenous supply of riboflavin. Our results discount electron shuttling by dissolved flavins, and instead support their role as bound redox cofactors in electron transport from nanowires to metal oxides. We also show that upon addition of a soluble electron shuttle (i.e., AQDS), reduction extends beyond the reported nanowire length up to 40 μm into a layer of birnessite.

 

S. Seraj, Kunal, P., Li, H., Henkelman, G., Humphrey, S. M., and Werth, C. J., “PdAu alloy nanoparticle catalysts: effective candidates for nitrite reduction in water,” ACS Catalysis, vol. 7, no. 5, pp. 3268-3276, 2017. Publisher’s Version

Abstract
Well-defined palladium–gold nanoparticles (PdAuNPs) with randomly alloyed structures and broadly tunable compositions were studied in catalytic nitrite (NO2–) reduction. The catalysts were synthesized using a microwave-assisted polyol coreduction method. PdxAu100–xNPs with systematically varied compositions (x = 18–83) were supported on amorphous silica (SiO2) and studied as model catalysts for aqueous NO2– reduction in a batch reactor, using H2 as the electron donor. The reactions followed pseudo-first-order kinetics for ≥80% NO2– conversion. The PdxAu100–xNP-SiO2 catalysts showed a volcano-like correlation between NO2– reduction activity and x; the highest activity was observed for Pd53Au47, with an associated first-order rate constant of 5.12 L min–1 gmetal–1. Alloy NPs with greater proportions of Au were found to reduce the loss in catalytic activity due to sulfide fouling. Density functional theory calculations indicate that this is because Au weakens sulfur binding at PdAuNP surfaces due to atomic ensemble, electronic, and strain effects and thus reduces sulfur poisoning. The environmental relevance of the most active supported catalyst was evaluated by subjecting it to five cycles of catalytic NO2– reduction. The catalytic activity decreased over multiple cycles, but analysis of the postreaction PdxAu100–xNP-SiO2 materials using complementary techniques indicated that there were no significant structural changes. Most importantly, we show that PdxAu100–xNP-SiO2 alloys are significantly more active NO2– reduction catalysts in comparison to pure Pd catalysts.

 

R. Singh, Sivaguru, M., Fried, G. A., Fouke, B. W., Sanford, R. A., Carrera, M., and Werth, C. J., “Real rock-microfluidic flow cell: a test bed for real-time in situ analysis of flow, transport, and reaction in a subsurface reactive transport environment,” Journal of Contaminant Hydrology, vol. 204, pp. 28-39, 2017. Publisher’s Version

Abstract
Physical, chemical, and biological interactions between groundwater and sedimentary rock directly control the fundamental subsurface properties such as porosity, permeability, and flow. This is true for a variety of subsurface scenarios, ranging from shallow groundwater aquifers to deeply buried hydrocarbon reservoirs. Microfluidic flow cells are now commonly being used to study these processes at the pore scale in simplified pore structures meant to mimic subsurface reservoirs. However, these micromodels are typically fabricated from glass, silicon, or polydimethylsiloxane (PDMS), and are therefore incapable of replicating the geochemical reactivity and complex three-dimensional pore networks present in subsurface lithologies. To address these limitations, we developed a new microfluidic experimental test bed, herein called the Real Rock-Microfluidic Flow Cell (RR-MFC). A porous 500 μm-thick real rock sample of the Clair Group sandstone from a subsurface hydrocarbon reservoir of the North Sea was prepared and mounted inside a PDMS microfluidic channel, creating a dynamic flow-through experimental platform for real-time tracking of subsurface reactive transport. Transmitted and reflected microscopy, cathodoluminescence microscopy, Raman spectroscopy, and confocal laser microscopy techniques were used to (1) determine the mineralogy, geochemistry, and pore networks within the sandstone inserted in the RR-MFC, (2) analyze non-reactive tracer breakthrough in two- and (depth-limited) three-dimensions, and (3) characterize multiphase flow. The RR-MFC is the first microfluidic experimental platform that allows direct visualization of flow and transport in the pore space of a real subsurface reservoir rock sample, and holds potential to advance our understandings of reactive transport and other subsurface processes relevant to pollutant transport and cleanup in groundwater, as well as energy recovery.

 

2016

J. Liu, Wu, D., Su, X., Han, M., Kimura, S. Y., Gray, D. L., Shapley, J. R., Abu-Omar, M. M., Werth, C. J., and Strathmann, T. J., “Configuration control in the synthesis of homo- and heteroleptic bis(oxazo/thiazolinylphenolato) chelate ligand complexes of oxorhenium(V): isomer effect on ancillary ligand exchange dynamics and implications for perchlorate reduction catalysis,” Inorganic Chemistry, vol. 55, no. 5, pp. 2597-2611, 2016. Publisher’s Version

Abstract
This study develops synthetic strategies for N,N-trans and N,N-cis Re(O)(LO–N)2Cl complexes and investigates the effects of the coordination spheres and ligand structures on ancillary ligand exchange dynamics and catalytic perchlorate reduction activities of the corresponding [Re(O)(LO–N)2]+ cations. The 2-(2′-hydroxyphenyl)-2-oxazoline (Hhoz) and 2-(2′-hydroxyphenyl)-2-thiazoline (Hhtz) ligands are used to prepare homoleptic N,N-trans and N,N-cis isomers of both Re(O)(hoz)2Cl and Re(O)(htz)2Cl and one heteroleptic N,N-trans Re(O)(hoz)(htz)Cl. Selection of hoz/htzligands determines the preferred isomeric coordination sphere, and the use of substituted pyridine bases with varying degrees of steric hindrance during complex synthesis controls the rate of isomer interconversion. The five corresponding [Re(O)(LO–N)2]+ cations exhibit a wide range of solvent exchange rates (1.4 to 24,000 s–1 at 25 °C) and different LO–N movement patterns, as influenced by the coordination sphere of Re (trans/cis), the noncoordinating heteroatom on LO–N ligands (O/S), and the combination of the two LO–N ligands (homoleptic/heteroleptic). Ligand exchange dynamics also correlate with the activity of catalytic reduction of aqueous ClO4– by H2 when the Re(O)(LO–N)2Cl complexes are immobilized onto Pd/C. Findings from this study provide novel synthetic strategies and mechanistic insights for innovations in catalytic, environmental, and biomedical research.

 

A. M. Bergquist, Choe, J. K., Strathmann, T. J., and Werth, C. J., “Evaluation of a hybrid ion exchange-catalyst treatment technology for nitrate removal from drinking water,” Water Research, vol. 96, pp. 177-187, 2016. Publisher’s Version

Abstract
Ion exchange (IX) is the most common approach to treating nitrate-contaminated drinking water sources, but the cost of salt to make regeneration brine, as well as the cost and environmental burden of waste brine disposal, are major disadvantages. A hybrid ion exchange-catalyst treatment system, in which waste brine is catalytically treated for reuse, shows promise for reducing costs and environmental burdens of the conventional IX system. An IX model with separate treatment and regeneration cycles was developed, and ion selectivity coefficients for each cycle were separately calibrated by fitting experimental data. Of note, selectivity coefficients for the regeneration cycle required fitting the second treatment cycle after incomplete resin regeneration. The calibrated and validated model was used to simulate many cycles of treatment and regeneration using the hybrid system. Simulated waste brines and a real brine obtained from a California utility were also evaluated for catalytic nitrate treatment in a packed-bed, flow-through column with 0.5 wt%Pd–0.05 wt%In/activated carbon support (PdIn/AC). Consistent nitrate removal and no apparent catalyst deactivation were observed over 23 d (synthetic brine) and 45 d (real waste brine) of continuous-flow treatment. Ion exchange and catalyst results were used to evaluate treatment of 1 billion gallons of nitrate-contaminated source water at a 0.5 MGD water treatment plant. Switching from a conventional IX system with a two bed volume regeneration to a hybrid system with the same regeneration length and sequencing batch catalytic reactor treatment would save 76% in salt cost. The results suggest the hybrid system has the potential to address the disadvantages of a conventional IX treatment systems.

 

J. Liu, Han, M., Wu, D., Chen, X., Choe, J. K., Werth, C. J., and Strathmann, T. J., “A new bioinspired perchlorate reduction catalyst with significantly enhanced stability via rational tuning of rhenium coordination chemistry and heterogeneous reaction pathway,” Environmental Science & Technology, vol. 50, no. 11, pp. 5874-5881, 2016. Publisher’s Version

Abstract
Rapid reduction of aqueous ClO4– to Cl– by H2 has been realized by a heterogeneous Re(hoz)2–Pd/C catalyst integrating Re(O)(hoz)2Cl complex (hoz = oxazolinyl-phenolato bidentate ligand) and Pd nanoparticles on carbon support, but ClOx– intermediates formed during reactions with concentrated ClO4– promote irreversible Re complex decomposition and catalyst deactivation. The original catalyst design mimics the microbial ClO4– reductase, which integrates Mo(MGD)2 complex (MGD = molybdopterin guanine dinucleotide) for oxygen atom transfer (OAT). Perchlorate-reducing microorganisms employ a separate enzyme, chlorite dismutase, to prevent accumulation of the destructive ClO2– intermediate. The structural intricacy of MGD ligand and the two-enzyme mechanism for microbial ClO4– reduction inspired us to improve catalyst stability by rationally tuning Re ligand structure and adding a ClOx– scavenger. Two new Re complexes, Re(O)(htz)2Cl and Re(O)(hoz)(htz)Cl (htz = thiazolinyl-phenolato bidentate ligand), significantly mitigate Re complex decomposition by slightly lowering the OAT activity when immobilized in Pd/C. Further stability enhancement is then obtained by switching the nanoparticles from Pd to Rh, which exhibits high reactivity with ClOx– intermediates and thus prevents their deactivating reaction with the Re complex. Compared to Re(hoz)2–Pd/C, the new Re(hoz)(htz)–Rh/C catalyst exhibits similar ClO4– reduction activity but superior stability, evidenced by a decrease of Re leaching from 37% to 0.25% and stability of surface Re speciation following the treatment of a concentrated “challenge” solution containing 1000 ppm of ClO4–. This work demonstrates the pivotal roles of coordination chemistry control and tuning of individual catalyst components for achieving both high activity and stability in environmental catalyst applications.

 

M. Oostrom, Mehmani, Y., Romero-Gomez, P., Tang, Y., Liu, H., Yoon, H., Kang, Q., Joekar-Niasar, V., Balhoff, M. T., Dewers, T., Tartakovsky, G. D., Leist, E. A., Hess, N. J., Perkins, W. A., Rakowski, C. L., Richmond, M. C., Serkowski, J. A., Werth, C. J., Valocchi, A. J., Wietsma, T. W., and Zhang, C., “Pore-scale and continuum simulations of solute transport micromodel benchmark experiments,” Computational Geosciences, vol. 20, no. 4, pp. 857-879, 2016. Publisher’s Version

Abstract
Four sets of nonreactive solute transport experiments were conducted with micromodels. Each set consisted of three experiments with one variable, i.e., flow velocity, grain diameter, pore-aspect ratio, and flow-focusing heterogeneity. The data sets were offered to pore-scale modeling groups to test their numerical simulators. Each set consisted of two learning experiments, for which all results were made available, and one challenge experiment, for which only the experimental description and base input parameters were provided. The experimental results showed a nonlinear dependence of the transverse dispersion coefficient on the Peclet number, a negligible effect of the pore-aspect ratio on transverse mixing, and considerably enhanced mixing due to flow focusing. Five pore-scale models and one continuum-scale model were used to simulate the experiments. Of the pore-scale models, two used a pore-network (PN) method, two others are based on a lattice Boltzmann (LB) approach, and one used a computational fluid dynamics (CFD) technique. The learning experiments were used by the PN models to modify the standard perfect mixing approach in pore bodies into approaches to simulate the observed incomplete mixing. The LB and CFD models used the learning experiments to appropriately discretize the spatial grid representations. For the continuum modeling, the required dispersivity input values were estimated based on published nonlinear relations between transverse dispersion coefficients and Peclet number. Comparisons between experimental and numerical results for the four challenge experiments show that all pore-scale models were all able to satisfactorily simulate the experiments. The continuum model underestimated the required dispersivity values, resulting in reduced dispersion. The PN models were able to complete the simulations in a few minutes, whereas the direct models, which account for the micromodel geometry and underlying flow and transport physics, needed up to several days on supercomputers to resolve the more complex problems.

 

J. Lee, Yoon, H., Kitanidis, P. K., Werth, C. J., and Valocchi, A. J., “Scalable subsurface inverse modeling of huge data sets with an application to tracer concentration breakthrough data from magnetic resonance imaging,” Water Resources Research, vol. 52, no. 7, pp. 5213-5231, 2016. Publisher’s Version

Abstract
Characterizing subsurface properties is crucial for reliable and cost-effective groundwater supply management and contaminant remediation. With recent advances in sensor technology, large volumes of hydrogeophysical and geochemical data can be obtained to achieve high-resolution images of subsurface properties. However, characterization with such a large amount of information requires prohibitive computational costs associated with “big data” processing and numerous large-scale numerical simulations. To tackle such difficulties, the principal component geostatistical approach (PCGA) has been proposed as a “Jacobian-free” inversion method that requires much smaller forward simulation runs for each iteration than the number of unknown parameters and measurements needed in the traditional inversion methods. PCGA can be conveniently linked to any multiphysics simulation software with independent parallel executions. In this paper, we extend PCGA to handle a large number of measurements (e.g., 106 or more) by constructing a fast preconditioner whose computational cost scales linearly with the data size. For illustration, we characterize the heterogeneous hydraulic conductivity (K) distribution in a laboratory-scale 3-D sand box using about 6 million transient tracer concentration measurements obtained using magnetic resonance imaging. Since each individual observation has little information on the K distribution, the data were compressed by the zeroth temporal moment of breakthrough curves, which is equivalent to the mean travel time under the experimental setting. Only about 2000 forward simulations in total were required to obtain the best estimate with corresponding estimation uncertainty, and the estimated K field captured key patterns of the original packing design, showing the efficiency and effectiveness of the proposed method.

 

2015

J. Liu, Chen, X., Wang, Y., Strathmann, T. J., and Werth, C. J., “Mechanism and mitigation of the decomposition of an oxorhenium complex-based heterogeneous catalyst for perchlorate reduction in water,” Environmental Science & Technology, vol. 49, no. 21, pp. 12932-12940, 2015. Publisher’s Version

Abstract
A biomimetic heterogeneous catalyst combining palladium nanoparticles and an organic ligand-coordinated oxorhenium complex on activated carbon, Re(hoz)2–Pd/C, was previously developed and shown to reduce aqueous perchlorate (ClO4–) with H2 at a rate ∼100 times faster than the first generation ReOx–Pd/C catalyst prepared from perrhenate (ReO4–). However, the immobilized Re(hoz)2 complex was shown to partially decompose and leach into water as ReO4–, leading to an irreversible loss of catalytic activity. In this work, the stability of the immobilized Re(hoz)2 complex is shown to depend on kinetic competition between three processes: (1) ReV(hoz)2 oxidation by ClO4– and its reduction intermediates ClOx–, (2) ReVII(hoz)2 reduction by Pd-activated hydrogen, and (3) hydrolytic ReVII(hoz)2 decomposition. When ReV(hoz)2 oxidation is faster than ReVII(hoz)2 reduction, the ReVII(hoz)2 concentration builds up and leads to hydrolytic decomposition to ReO4– and free hoz ligand. Rapid ReV(hoz)2 oxidation is mainly promoted by highly reactive ClOx– formed from the reduction of ClO4–. To mitigate Re(hoz)2 decomposition and preserve catalytic activity, ruthenium (Ru) and rhodium (Rh) were evaluated as alternative H2 activators to Pd. Rh showed superior activity for reducing the ClO3– intermediate to Cl–, thereby preventing ClOx– buildup and lowering Re complex decomposition in the Re(hoz)2–Rh/C catalyst. In contrast, Ru showed the lowest ClO3– reduction activity and resulted in the most Re(hoz)2 decomposition among the Re(hoz)2–M/C catalysts. This work highlights the importance of using mechanistic insights from kinetic and spectroscopic tests to rationally design water treatment catalysts for enhanced performance and stability.

 

R. Singh, Yoon, H., Sanford, R. A., Katz, L., Fouke, B. W., and Werth, C. J., “Metabolism-induced CaCO3 biomineralization during reactive transport in a micromodel: implications for porosity alteration,” Environmental Science & Technology, vol. 49, no. 20, pp. 12094-12104, 2015. Publisher’s Version

Abstract
The ability of Pseudomonas stutzeri strain DCP-Ps1 to drive CaCO3 biomineralization has been investigated in a microfluidic flowcell (i.e., micromodel) that simulates subsurface porous media. Results indicate that CaCO3 precipitation occurs during NO3– reduction with a maximum saturation index (SIcalcite) of ∼1.56, but not when NO3– was removed, inactive biomass remained, and pH and alkalinity were adjusted to SIcalcite ∼ 1.56. CaCO3 precipitation was promoted by metabolically active cultures of strain DCP-Ps1, which at similar values of SIcalcite, have a more negative surface charge than inactive strain DCP-Ps1. A two-stage NO3– reduction (NO3– → NO2– → N2) pore-scale reactive transport model was used to evaluate denitrification kinetics, which was observed in the micromodel as upper (NO3– reduction) and lower (NO2– reduction) horizontal zones of biomass growth with CaCO3 precipitation exclusively in the lower zone. Model results are consistent with two biomass growth regions and indicate that precipitation occurred in the lower zone because the largest increase in pH and alkalinity is associated with NO2– reduction. CaCO3 precipitates typically occupied the entire vertical depth of pores and impacted porosity, permeability, and flow. This study provides a framework for incorporating microbial activity in biogeochemistry models, which often base biomineralization only on SI (caused by biotic or abiotic reactions) and, thereby, underpredict the extent of this complex process. These results have wide-ranging implications for understanding reactive transport in relevance to groundwater remediation, CO2 sequestration, and enhanced oil recovery.

 

Z. Gao, Zhang, Y., Li, D., Werth, C. J., Zhang, Y., and Zhou, X., “Highly active Pd–In/mesoporous alumina catalyst for nitrate reduction,” Journal of Hazardous Materials, vol. 286, pp. 425-431, 2015. Publisher’s Version

Abstract
The catalytic reduction of nitrate is a promising technology for groundwater purification because it transforms nitrate into nitrogen and water. Recent studies have mainly focused on new catalysts with higher activities for the reduction of nitrate. Consequently, metal nanoparticles supported on mesoporous metal oxides have become a major research direction. However, the complex surface chemistry and porous structures of mesoporous metal oxides lead to a non-uniform distribution of metal nanoparticles, thereby resulting in a low catalytic efficiency. In this paper, a method for synthesizing the sustainable nitrate reduction catalyst Pd–In/Al2O3 with a dimensional structure is introduced. The TEM results indicated that Pd and In nanoparticles could efficiently disperse into the mesopores of the alumina. At room temperature in CO2-buffered water and under continuous H2 as the electron donor, the synthesized material (4.9 wt% Pd) was the most active at a Pd–In ratio of 4, with a first-order rate constant (kobs = 0.241 L min−1 gcata−1) that was 1.3× higher than that of conventional Pd–In/Al2O3 (5 wt% Pd; 0.19 L min−1 gcata−1). The Pd–In/mesoporous alumina is a promising catalyst for improving the catalytic reduction of nitrate.

 

Y. Tang, Valocchi, A. J., and Werth, C. J., “A hybrid pore‐scale and continuum‐scale model for solute diffusion, reaction, and biofilm development in porous media,” Water Resources Research, vol. 51, no. 3, pp. 1846-1859, 2015. Publisher’s Version

Abstract
It is a challenge to upscale solute transport in porous media for multispecies bio-kinetic reactions because of incomplete mixing within the elementary volume and because biofilm growth can change porosity and affect pore-scale flow and diffusion. To address this challenge, we present a hybrid model that couples pore-scale subdomains to continuum-scale subdomains. While the pore-scale subdomains involving significant biofilm growth and reaction are simulated using pore-scale equations, the other subdomains are simulated using continuum-scale equations to save computational time. The pore-scale and continuum-scale subdomains are coupled using a mortar method to ensure continuity of solute concentration and flux at the interfaces. We present results for a simplified two-dimensional system, neglect advection, and use dual Monod kinetics for solute utilization and biofilm growth. The results based on the hybrid model are consistent with the results based on a pore-scale model for three test cases that cover a wide range of Damköhler (Da = reaction rate/diffusion rate) numbers for both homogeneous (spatially periodic) and heterogeneous pore structures. We compare results from the hybrid method with an upscaled continuum model and show that the latter is valid only for cases of small Damköhler numbers, consistent with other results reported in the literature.

 

Y. Tang, Werth, C. J., Sanford, R. A., Singh, R., Michelson, K., Nobu, M., Liu, W. – T., and Valocchi, A. J., “Immobilization of selenite via two parallel pathways during in situ bioremediation,” Environmental Science & Technology, vol. 49, no. 7, pp. 4543-4550, 2015. Publisher’s Version

Abstract
It is widely understood that selenite can be biologically reduced to elemental selenium. Limited studies have shown that selenite can also be immobilized through abiotic precipitation with sulfide, a product of biological sulfate reduction. We demonstrate that both pathways significantly contribute to selenite immobilization in a microfluidic flow cell having a transverse mixing zone between propionate and selenite that mimics the reaction zone along the margins of a selenite plume undergoing bioremediation in the presence of background sulfate. The experiment showed that red particles of amorphous elemental selenium precipitate on the selenite-rich side of the mixing zone, while long crystals of selenium sulfides precipitate on the propionate-rich side of the mixing zone. We developed a continuum-scale reactive transport model that includes both pathways. The simulated results are consistent with the experimental results, and indicate that spatial segregation of the two selenium precipitates is due to the segregation of the more thermodynamic favorable selenite reduction and the less thermodynamically favorable sulfate reduction. The improved understanding of selenite immobilization and the improved model can help to better design in situ bioremediation processes for groundwater contaminated by selenite or other contaminants (e.g., uranium(IV)) that can be immobilized via similar pathways.

 

A. Laleian, Valocchi, A. J., and Werth, C. J., “An incompressible, depth-averaged lattice Boltzmann method for liquid flow in microfluidic devices with variable aperture,” Computation, vol. 3, no. 4, pp. 600-615, 2015. Publisher’s Version

Abstract
Two-dimensional (2D) pore-scale models have successfully simulated microfluidic experiments of aqueous-phase flow with mixing-controlled reactions in devices with small aperture. A standard 2D model is not generally appropriate when the presence of mineral precipitate or biomass creates complex and irregular three-dimensional (3D) pore geometries. We modify the 2D lattice Boltzmann method (LBM) to incorporate viscous drag from the top and bottom microfluidic device (micromodel) surfaces, typically excluded in a 2D model. Viscous drag from these surfaces can be approximated by uniformly scaling a steady-state 2D velocity field at low Reynolds number. We demonstrate increased accuracy by approximating the viscous drag with an analytically-derived body force which assumes a local parabolic velocity profile across the micromodel depth. Accuracy of the generated 2D velocity field and simulation permeability have not been evaluated in geometries with variable aperture. We obtain permeabilities within approximately 10% error and accurate streamlines from the proposed 2D method relative to results obtained from 3D simulations. In addition, the proposed method requires a CPU run time approximately 40 times less than a standard 3D method, representing a significant computational benefit for permeability calculations.

 

J. K. Choe, Bergquist, A. M., Jeong, S., Guest, J. S., Werth, C. J., and Strathmann, T. J., “Performance and life cycle environmental benefits of recycling spent ion exchange brines by catalytic treatment of nitrate,” Water Research, vol. 80, pp. 267-280, 2015. Publisher’s Version

Abstract
Salt used to make brines for regeneration of ion exchange (IX) resins is the dominant economic and environmental liability of IX treatment systems for nitrate-contaminated drinking water sources. To reduce salt usage, the applicability and environmental benefits of using a catalytic reduction technology to treat nitrate in spent IX brines and enable their reuse for IX resin regeneration were evaluated. Hybrid IX/catalyst systems were designed and life cycle assessment of process consumables are used to set performance targets for the catalyst reactor. Nitrate reduction was measured in a typical spent brine (i.e., 5000 mg/L NO3−”>NO3− and 70,000 mg/L NaCl) using bimetallic Pd–In hydrogenation catalysts with variable Pd (0.2–2.5 wt%) and In (0.0125–0.25 wt%) loadings on pelletized activated carbon support (Pd–In/C). The highest activity of 50 mgNO3−”>NO3−/(min − gPd) was obtained with a 0.5 wt%Pd–0.1 wt%In/C catalyst. Catalyst longevity was demonstrated by observing no decrease in catalyst activity over more than 60 days in a packed-bed reactor. Based on catalyst activity measured in batch and packed-bed reactors, environmental impacts of hybrid IX/catalyst systems were evaluated for both sequencing-batch and continuous-flow packed-bed reactor designs and environmental impacts of the sequencing-batch hybrid system were found to be 38–81% of those of conventional IX. Major environmental impact contributors other than salt consumption include Pd metal, hydrogen (electron donor), and carbon dioxide (pH buffer). Sensitivity of environmental impacts of the sequencing-batch hybrid reactor system to sulfate and bicarbonate anions indicate the hybrid system is more sustainable than conventional IX when influent water contains <80 mg/L sulfate (at any bicarbonate level up to 100 mg/L) or <20 mg/L bicarbonate (at any sulfate level up to 100 mg/L) assuming 15 brine reuse cycles. The study showed that hybrid IX/catalyst reactor systems have potential to reduce resource consumption and improve environmental impacts associated with treating nitrate-contaminated water sources.

 

S. U. Akki, Werth, C. J., and Silverman, S. K., “Selective aptamers for detection of estradiol and ethynylestradiol in natural waters,” Environmental Science & Technology, vol. 49, no. 16, pp. 9905-9913, 2015. Publisher’s Version

Abstract
We used in vitro selection to identify new DNA aptamers for two endocrine-disrupting compounds often found in treated and natural waters, 17β-estradiol (E2) and 17α-ethynylestradiol (EE). We used equilibrium filtration to determine aptamer sensitivity/selectivity and dimethyl sulfate (DMS) probing to explore aptamer binding sites. The new E2 aptamers are at least 74-fold more sensitive for E2 than is a previously reported DNA aptamer, with dissociation constants (Kd values) of 0.6 μM. Similarly, the EE aptamers are highly sensitive for EE, with Kd of 0.5–1.0 μM. Selectivity values indicate that the E2 aptamers bind E2 and a structural analogue, estrone (E1), equally well and are up to 74-fold selective over EE. One EE aptamer is 53-fold more selective for EE over E2 or E1, but the other binds EE, E2, and E1 with similar affinity. The new aptamers do not lose sensitivity or selectivity in natural water from a local lake, despite the presence of natural organic matter (∼4 mg/L TOC). DMS probing suggests that E2 binding occurs in relatively flexible single-stranded DNA regions, an important finding for rational redesign of aptamers and their incorporation into sensing platforms. This is the first report of aptamers with strong selectivity for E2 and E1 over EE, or with strong selectivity for EE over E2 and E1. Such selectivity is important for achieving the goal of creating practically useful DNA-based sensors that can distinguish structurally similar estrogenic compounds in natural waters.

 

2014

T. J. Strathmann, Werth, C. J., and Shapley, J. R., “Chapter 22 – Heterogeneous catalytic reduction for water purification: nanoscale effects on catalytic activity, selectivity, and sustainability,” in Nanotechnology Applications for Clean Water, 2nd ed., Oxford: William Andrew, 2014, pp. 339-349. Publisher’s Version

Abstract
Reductive catalysis is a promising water treatment technology that employs heterogeneous metal catalysts (e.g., Pd nanoparticles on a support) to convert dihydrogen to adsorbed atomic hydrogen in order to promote reactions with functional groups in various contaminants. Reductive catalysis has several potential advantages, including high selectivity for a given target, fast rates under mild conditions, and low production of harmful by-products. The technology has been applied mostly for remediation of groundwater contaminated with halogenated hydrocarbons and for treatment of nitrate, but recent studies have expanded the range of target contaminants to include perchlorate and N-nitrosamines. Palladium-based catalysts hold tremendous promise for their ability to selectively destroy several drinking water contaminants, and some compounds that exhibit slow reaction kinetics with Pd alone are rapidly degraded when a second, promoter metal is added to the catalyst. However, there is a lack of information about the long-term sustainability of these catalytic treatment processes, which is a major consideration in their possible adoption for remediation applications. Recent research has focused on the nanoscale characterization of these heterogeneous catalysts in order to develop an improved understanding of their mechanisms of deactivation and the pathways for regeneration. Two examples of studies from the authors’ laboratories, involving (i) hydrodehalogenation of iodinated X-ray contrast media with Ni or Pd catalysts and (ii) selective reduction of nitrate with a regenerable Pd-In/alumina catalyst, are discussed in this chapter.

 

A. Kokkinaki, Werth, C. J., and Sleep, B. E., “Comparison of upscaled models for multistage mass discharge from DNAPL source zones,” Water Resources Research, vol. 50, pp. 3187-3205, 2014. Publisher’s Version

Abstract
Analytical upscaled models that can describe the depletion of dense nonaqueous phase liquids (DNAPLs) and the associated mass discharge are a practical alternative to computationally demanding and data‐intensive multiphase numerical simulators. A major shortcoming of most existing upscaled models is that they cannot reproduce the nonmonotonic, multistage effluent concentrations often observed in experiments and numerical simulations. Upscaled models that can produce multistage concentrations either require calibration, which increases the cost of applying them in the field, or use dual‐domain conceptual models that may not apply for spatially complex source zones. In this study, a new upscaled model is presented that can describe the nonmonotonic, multistage average concentrations emanating from complex DNAPL source zones. This is achieved by explicitly considering the temporal evolution of three source zone parameters, namely source zone projected area, the average of local‐scale DNAPL saturations, and the average of local‐scale aqueous relative permeability, without using empirical parameters. The model is evaluated for two real and twelve hypothetical centimeter‐scale complex source zones. The proposed model captures the temporal variations in concentrations better than an empirical model and a dual‐domain ganglia‐to‐pool ratio model. The results provide evidence that effluent concentrations downgradient of DNAPL source zones are controlled by the evolution of the aforementioned macroscopic parameters. This knowledge can be useful for the interpretation of field observations of effluent concentrations downstream of DNAPL source zones, and for the development of predictive upscaled models. Advances in DNAPL characterization techniques are needed to quantify these macroscopic parameters that can be used to guide DNAPL remediation efforts.

 

Y. Wang, Liu, J., Wang, P., Werth, C. J., and Strathmann, T. J., “Palladium nanoparticles encapsulated in core–shell silica: a structured hydrogenation catalyst with enhanced activity for reduction of oxyanion water pollutants,” ACS Catalysis, vol. 4, no. 10, pp. 3551–3559, 2014. Publisher’s Version

Abstract
Noble metal nanoparticles have been applied to mediate catalytic removal of toxic oxyanions and halogenated hydrocarbons in contaminated water using H2 as a clean and sustainable reductant. However, activity loss by nanoparticle aggregation and difficulty of nanoparticle recovery are two major challenges to widespread technology adoption. Herein, we report the synthesis of a core–shell-structured catalyst with encapsulated Pd nanoparticles and its enhanced catalytic activity in reduction of bromate (BrO3–), a regulated carcinogenic oxyanion produced during drinking water disinfection process, using 1 atm H2 at room temperature. The catalyst material consists of a nonporous silica core decorated with preformed octahedral Pd nanoparticles that were further encapsulated within an ordered mesoporous silica shell (i.e., SiO2@Pd@mSiO2). Well-defined mesopores (2.3 nm) provide a physical barrier to prevent Pd nanoparticle (∼6 nm) movement, aggregation, and detachment from the support into water. Compared to freely suspended Pd nanoparticles and SiO2@Pd, encapsulation in the mesoporous silica shell significantly enhanced Pd catalytic activity (by a factor of 10) under circumneutral pH conditions that are most relevant to water purification applications. Mechanistic investigation of material surface properties combined with Langmuir–Hinshelwood modeling of kinetic data suggest that mesoporous silica shell enhances activity by promoting BrO3– adsorption near the Pd active sites. The dual function of the mesoporous shell, enhancing Pd catalyst activity and preventing aggregation of active nanoparticles, suggests a promising general strategy of using metal nanoparticle catalysts for water purification and related aqueous-phase applications.

 

H. Yoon, Leibeling, S., Zhang, C., Mueller, R. H., Werth, C. J., and Zilles, J., “Adaptation of Delftia acidovorans for degradation of 2,4-dichlorophenoxyacetate in a microfluidic porous medium,” Biodegradation, vol. 25, no. 4, pp. 595–604, 2014. Publisher’s Version

Abstract
Delftia acidovorans MC1071 can productively degrade R-2-(2,4-dichlorophenoxy)propionate (R-2,4-DP) but not 2,4-dichlorophenoxyacetate (2,4-D) herbicides. This work demonstrates adaptation of MC1071 to degrade 2,4-D in a model two-dimensional porous medium (referred to here as a micromodel). Adaptation for 2,4-D degradation in the 2 cm-long micromodel occurred within 35 days of exposure to 2,4-D, as documented by substrate removal. The amount of 2,4-D degradation in the adapted cultures in two replicate micromodels (~10 and 20 % over 142 days) was higher than a theoretical maximum (4 %) predicted using published numerical simulation methods, assuming instantaneous biodegradation and a transverse dispersion coefficient obtained for the same pore structure without biomass present. This suggests that the presence of biomass enhances substrate mixing. Additional evidence for adaptation was provided by operation without R-2,4-DP, where degradation of 2,4-D slowly decreased over 20 days, but was restored almost immediately when R-2,4-DP was again provided. Compared to suspended growth systems, the micromodel system retained the ability to degrade 2,4-D longer in the absence of R-2,4-DP, suggesting slower responses and greater resilience to fluctuations in substrates might be expected in the soil environment than in a chemostat.

 

M. R. Saat, Werth, C. J., Schaeffer, D., Yoon, H., and Barkan, C. P. L., “Environmental risk analysis of hazardous material rail transportation,” Journal of Hazardous Materials, vol. 264, no. 15, pp. 560–569, 2014. Publisher’s Version

Abstract
An important aspect of railroad environmental risk management involves tank car transportation of hazardous materials. This paper describes a quantitative, environmental risk analysis of rail transportation of a group of light, non-aqueous-phase liquid (LNAPL) chemicals commonly transported by rail in North America. The Hazardous Materials Transportation Environmental Consequence Model (HMTECM) was used in conjunction with a geographic information system (GIS) analysis of environmental characteristics to develop probabilistic estimates of exposure to different spill scenarios along the North American rail network. The risk analysis incorporated the estimated clean-up cost developed using the HMTECM, route-specific probability distributions of soil type and depth to groundwater, annual traffic volume, railcar accident rate, and tank car safety features, to estimate the nationwide annual risk of transporting each product. The annual risk per car-mile (car-km) and per ton-mile (ton-km) was also calculated to enable comparison between chemicals and to provide information on the risk cost associated with shipments of these products. The analysis and the methodology provide a quantitative approach that will enable more effective management of the environmental risk of transporting hazardous materials.

 

V. Boyd, Yoon, H., Zhang, C., Oostrom, M., Hess, N., Fouke, B. W., Valocchi, A. J., and Werth, C. J., “

Influence of Mg2+ on CaCO3 precipitation during subsurface reactive transport in a homogeneous silicon-etched pore network

,” Geochimica et Cosmochimica Acta, vol. 135, pp. 321-335, 2014. Publisher’s Version

Abstract
Calcium carbonate (CaCO3) geochemical reactions exert a fundamental control on the evolution of porosity and permeability in shallow-to-deep subsurface siliciclastic and limestone rock reservoirs. As a result, these carbonate water–rock interactions play a critically important role in research on groundwater remediation, geological carbon sequestration, and hydrocarbon exploration. A study was undertaken to determine the effects of Mg2+ concentration on CaCO3 crystal morphology, precipitation rate, and porosity occlusion under flow and mixing conditions similar to those in subsurface aquifers. This was accomplished by promoting CaCO3 precipitation through the mixing of two solutions flowing parallel to each other in a microfluidic pore structure, containing uniform concentrations of dissolved Ca2+ and carbonate (CO32−), and systematic variations in the concentration of Mg2+. Raman spectroscopy indicates that all three polymorphs of CaCO3 (calcite, aragonite, and vaterite) were present under all experimental conditions. Coordinated brightfield imaging results show the morphology of calcite with increasing Mg2+ progressed from blocky and dogtooth approximately 10–80 μm in size, to anhedral spheroidal approximately 5–30 μm in size. The morphology of aragonite with increasing Mg2+ progressed from shrubs and fuzzy dumbells to spheroidal, and the size increased from approximately 5–60 μm to 20–200 μm. Recrystallization was observed in all experiments, but more so at low Mg2+, in which many small microcrystals dissolved and re-precipitated as one or a few larger calcite crystals. Analysis of brightfield images indicates calcite is the most abundant polymorph under all conditions. However, the area of pore space with aragonite increased from <5% when no Mg2+ was present to >20% at the highest Mg2+ concentration. The initial apparent precipitation rate of mineral polymorphs with no Mg2+ present was 2.5 times greater than when 40 mM Mg2+ was added, and large (20–200 μm) aragonite crystals formed primarily near to and below the center mixing zone with increasing Mg2+ concentration. Pore-scale modeling results are consistent with experiments, and indicate that all three polymorphs are thermodynamically favorable, with calcite and aragonite being the most favorable and having similar saturation ratios (SR > 100). The influence of Mg2+ on mineral precipitation rates is consistent with previous studies showing that calcite precipitation rates decrease with increasing Mg2+ concentrations. The precipitation of aragonite below the center-mixing zone is not predicted by thermodynamic SRs, but is consistent with the literature and our modeling results showing aragonite precipitation is kinetically more favorable in regions with higher Mg2+/Ca2+ ratios. Hence, both thermodynamic and kinetic constraints affect precipitation rates, the distribution of mineral polymorphs, and the corresponding extent of porosity occlusion. A tracer study demonstrated that mineral precipitation along the center-mixing zone under all experimental conditions led to substantial pore blockage. Imaging results suggest that with increasing Mg2+ concentration, slower crystal growth rates will increase the time period before pore blockage occurs, and the transition to more spherical and larger aragonite crystals below the center mixing line will increase pore occlusion and decrease mixing. Hence, understanding how Mg2+ affects calcium carbonate precipitation is very important for predicting mixing and reactive transport in subsurface reservoirs.

 

H. Liu, Valocchi, A. J., Werth, C. J., Kang, Q., and Oostrom, M., “

Pore-scale simulation of liquid CO2 displacement of water using a two-phase lattice Boltzmann model

,” Advances in Water Resources, vol. 73, pp. 144-158, 2014. Publisher’s Version

Abstract
A lattice Boltzmann color-fluid model, which was recently proposed by Liu et al. (2012) based on a concept of continuum surface force, is improved to simulate immiscible two-phase flows in porous media. The new improvements allow the model to account for different kinematic viscosities of both fluids and to model fluid–solid interactions. The capability and accuracy of this model is first validated by two benchmark tests: a layered two-phase flow with a variable viscosity ratio, and a dynamic capillary intrusion. This model is then used to simulate liquid CO2 (LCO2) displacing water in a dual-permeability pore network. The extent and behavior of LCO2 preferential flow (i.e., fingering) is found to depend on the capillary number (Ca), and three different displacement patterns observed in previous micromodel experiments are reproduced. The predicted variation of LCO2 saturation with Ca, as well as variation of specific interfacial length with LCO2 saturation, are both in reasonable agreement with the experimental observations. To understand the effect of heterogeneity on pore-scale displacement, we also simulate LCO2 displacing water in a randomly heterogeneous pore network, which has the same size and porosity as the simulated dual-permeability pore network. In comparison to the dual-permeability case, the transition from capillary fingering to viscous fingering occurs at a higher Ca, and LCO2 saturation is higher at low Ca but lower at high Ca. In either pore network, the LCO2–water specific interfacial length is found to obey a power-law dependence on LCO2 saturation.

 

J. K. Choe, Boyanov, M., Liu, J., Kemner, K., Werth, C. J., and Strathmann, T. J., “X-ray spectroscopic characterization of immobilized rhenium species in hydrated rhenium-palladium bimetallic catalysts used for perchlorate water treatment,” Journal of Physical Chemistry C, vol. 118, no. 22, pp. 11666-11676, 2014. Publisher’s Version

Abstract
Carbon-supported rhenium–palladium catalysts (Re–Pd/C) effectively transform aqueous perchlorate, a widespread drinking water pollutant, via chemical reduction using hydrogen as an electron donor at ambient temperature and pressure. Previous work demonstrated that catalyst activity and stability are heavily dependent on solution composition and Re content in the catalyst. This study relates these parameters to changes in the speciation and molecular structure of Re immobilized on the catalyst. Using X-ray spectroscopy techniques, we show that Re is immobilized as ReVII under oxic solution conditions, but transforms to a mixture of reduced, O-coordinated Re species under reducing solution conditions induced by H2 sparging. Under oxic solution conditions, extended X-ray absorption fine structure (EXAFS) analysis showed that the immobilized ReVII species is indistinguishable from the dissolved tetrahedral perrhenate (ReO4–) anion, suggesting outer-sphere adsorption to the catalyst surface. Under reducing solution conditions, two Re species were identified. At low Re loading (≤1 wt %), monomeric ReI species form in direct contact with Pd nanoclusters. With increased Re loading, speciation gradually shifts to oxidic ReV clusters. The identified Re structures support a revised mechanism for catalytic reduction of ClO4– involving oxygen atom transfer reactions between odd-valence oxorhenium species and the oxyanion (Re oxidation steps) and atomic hydrogen species (Re reduction steps) formed by Pd-catalyzed dissociation of H2.

 

2013

M. F. Fanizza, Yoon, H., Zhang, C., Oostrom, M., Wietsma, T. W., Hess, N. J., Bowden, M. E., Strathmann, T. J., Finneran, K. T., and Werth, C. J., “Pore scale evaluation of uranyl phosphate precipitation in a model groundwater system,” Water Resour. Res., {DOI}:, vol. 10., 2013.

J. Liu, Choe, J. K., Sasnow, Z., Shapley, J. R., Werth, C. J., and Strathmann, T. J., “Application of a Re-Pd bimetallic catalyst for treatment of perchlorate in waste ion-exchange regenerant brine,” Water Research, vol. 47, no. 1, pp. 91–101, 2013. Publisher’s Version

Abstract
Concentrated sodium chloride (NaCl) brines are often used to regenerate ion-exchange (IX) resins applied to treat drinking water sources contaminated with perchlorate (ClO4−), generating large volumes of contaminated waste brine. Chemical and biological processes for ClO4− reduction are often inhibited severely by high salt levels, making it difficult to recycle waste brines. Recent work demonstrated that novel rhenium–palladium bimetallic catalysts on activated carbon support (Re–Pd/C) can efficiently reduce ClO4− to chloride (Cl−) under acidic conditions, and here the applicability of the process for treating waste IX brines was examined. Experiments conducted in synthetic NaCl-only brine (6–12 wt%) showed higher Re–Pd/C catalyst activity than in comparable freshwater solutions, but the rate constant for ClO4− reduction measured in a real IX waste brine was found to be 65 times lower than in the synthetic NaCl brine. Through a series of experiments, co-contamination of the IX waste brine by excess NO3− (which the catalyst reduces principally to NH4+) was found to be the primary cause for deactivation of the Re–Pd/C catalyst, most likely by altering the immobilized Re component. Pre-treatment of NO3− using a different bimetallic catalyst (In–Pd/Al2O3) improved selectivity for N2 over NH4+ and enabled facile ClO4− reduction by the Re–Pd/C catalyst. Thus, sequential catalytic treatment may be a promising strategy for enabling reuse of waste IX brine containing NO3− and ClO4−.

 

J. K. Choe, Mehnert, M. H., Guest, J. S., Strathmann, T. J., and Werth, C. J., “Comparative assessment of the environmental sustainability of existing and emerging perchlorate treatment technologies for drinking water,” Environmental Science & Technology, vol. 47, no. 9, pp. 4644–4652, 2013. Publisher’s Version

Abstract
Environmental impacts of conventional and emerging perchlorate drinking water treatment technologies were assessed using life cycle assessment (LCA). Comparison of two ion exchange (IX) technologies (i.e., nonselective IX with periodic regeneration using brines and perchlorate-selective IX without regeneration) at an existing plant shows that brine is the dominant contributor for nonselective IX, which shows higher impact than perchlorate-selective IX. Resource consumption during the operational phase comprises >80% of the total impacts. Having identified consumables as the driving force behind environmental impacts, the relative environmental sustainability of IX, biological treatment, and catalytic reduction technologies are compared more generally using consumable inputs. The analysis indicates that the environmental impacts of heterotrophic biological treatment are 2–5 times more sensitive to influent conditions (i.e., nitrate/oxygen concentration) and are 3–14 times higher compared to IX. However, autotrophic biological treatment is most environmentally beneficial among all. Catalytic treatment using carbon-supported Re–Pd has a higher (ca. 4600 times) impact than others, but is within 0.9–30 times the impact of IX with a newly developed ligand-complexed Re–Pd catalyst formulation. This suggests catalytic reduction can be competitive with increased activity. Our assessment shows that while IX is an environmentally competitive, emerging technologies also show great promise from an environmental sustainability perspective.

 

A. Kokkinaki, O’Carroll, D. M., Werth, C. J., and Sleep, B. E., “Coupled simulation of DNAPL infiltration and dissolution in three-dimensional heterogeneous domains: Process model validation,” Water Resources Research, vol. 49, no. 10, pp. 7023-7036, 2013. Publisher’s Version

Abstract
A three-dimensional multiphase numerical model was used to simulate the infiltration and dissolution of a dense nonaqueous phase liquid (DNAPL) release in two experimental flow cells containing different heterogeneous and well-characterized permeability fields. DNAPL infiltration was modeled using Brooks-Corey-Burdine hysteretic constitutive relationships. DNAPL dissolution was simulated using a rate-limited mass transfer expression with a velocity-dependent mass transfer coefficient and a thermodynamically based calculation of DNAPL-water interfacial area. The model did not require calibration of any parameters. The model predictions were compared to experimental measurements of high-resolution DNAPL saturations and effluent concentrations. The predicted concentrations were in close agreement with measurements for both domains, indicating that important processes were effectively captured by the model. DNAPL saturations greatly influenced mass transfer rates through their effect on relative permeability and velocity. Areas with low DNAPL saturation were associated with low interfacial areas, which resulted in reduced mass transfer rates and nonequilibrium dissolution. This was captured by the thermodynamic interfacial area model, while a geometric model overestimated the interfacial areas and the overall mass transfer. This study presents the first validation of the thermodynamic dissolution model in three dimensions and for high aqueous phase velocities; such conditions are typical for remediation operations, especially in heterogeneous aquifers. The demonstrated ability to predict DNAPL dissolution, only requiring prior characterization of soil properties and DNAPL release conditions, represents a significant improvement compared to empirical dissolution models and provides an opportunity to delineate the relationship between source zone architecture and the remediation potential for complex DNAPL source zones.

 

R. Zhang, Shuai, D., Guy, K. A., Shapley, J. R., Strathmann, T. J., and Werth, C. J., “Elucidation of nitrate reduction mechanisms on a Pd-In bimetallic catalyst using isotope labeled nitrogen species,” ChemCatChem, vol. 5, no. 1, pp. 313–321, 2013. Publisher’s Version

Abstract
To understand nitrate reduction pathway and to improve selectivity towards dinitrogen (N2) over toxic ammonia species (NH4+, NH3), aqueous reduction experiments with an Al2O3‐supported Pd‐In bimetallic catalyst were conducted by using isotope‐labeled nitrite (15NO2−). Nitrite is the first reduction intermediate of nitrate. Experiments were performed using nitrite alone and in combination with unlabeled proposed reduction intermediates (N2O, NO), and using only N2O and NO alone, each as a starting reactant. Use of 15N‐labeled species eliminates interference from ambient N2 when assessing mass balances and product distributions. Simultaneous catalytic reduction of 15NO2− and 14N2O shows no isotope mixing in the final N2 product, demonstrating that N2O does not react with other NO2− reduction intermediates; N2O reduction alone yielded only N2. In contrast, simultaneous catalytic reduction of 15NO2− and 14NO yielded mixed‐labeled 15/14N2 (MW: 29), whereas reduction of 15NO alone yields a mixture N2 and NH4+, the ratio of which varies with initial 15NO concentration. These findings, along with those from a new kinetic model we propose, indicate that highly reactive adsorbed NO (NO*), or other unspecified adsorbed N species (Nads), is a key intermediate involved in determining final product selectivity.

 

A. Kokkinaki, O’Carroll, D. M., Werth, C. J., and Sleep, B. E., “An evaluation of Sherwood–Gilland models for NAPL dissolution and their relationship to soil properties,” Journal of Contaminant Hydrology, vol. 155, pp. 87–98, 2013. Publisher’s Version

Abstract
Predicting the longevity of non-aqueous phase liquid (NAPL) source zones has proven to be a difficult modeling problem that has yet to be resolved. Research efforts towards understanding NAPL depletion have focused on developing empirical models that relate lumped mass transfer rates to velocities and organic saturations. These empirical models are often unable to predict NAPL dissolution for systems different from those used to calibrate them, indicating that system-specific factors important for dissolution are not considered. This introduces the need for a calibration step before these models can be reliably used to predict NAPL dissolution for systems of arbitrary characteristics.
In this paper, five published Sherwood–Gilland models are evaluated using experimental observations from the dissolution of two laboratory-scale complex three-dimensional NAPL source zones. It is shown that the relative behavior of the five models depends on the system and source zone characteristics. Through a theoretical analysis, comparing Sherwood–Gilland type models to a process-based, thermodynamic dissolution model, it is shown that the coefficients of the Sherwood–Gilland models can be related to measurable soil properties. The derived dissolution model with soil-dependent coefficients predicts concentrations identical to those predicted by the thermodynamic dissolution model for cases with negligible hysteresis. This correspondence breaks down when hysteresis has a significant impact on interfacial areas. In such cases, the derived dissolution model will slightly underestimate dissolved concentrations at later times, but is more likely to capture system-specific dissolution rates than Sherwood–Gilland models.

 

Y. – S. Jun, Giammar, D. E., and Werth, C. J., “Impacts of geochemical reactions on geologic carbon sequestration,” Environmental Science & Technology, vol. 47, no. 1, pp. 3–8, 2013. Publisher’s Version

Abstract
In the face of increasing energy demands, geologic CO2 sequestration (GCS) is a promising option to mitigate the adverse effects of climate change. To ensure the environmental sustainability of this option, we must understand the rates and mechanisms of key geochemical reactions and their impacts on GCS performance, the multiphase reactive transport of CO2, and the management of environmental risks. Strong interdisciplinary collaborations are required to minimize environmental impacts and optimize the performance of GCS operations.

 

Y. Tang, Valocchi, A. J., Werth, C. J., and Liu, H., “An improved pore-scale biofilm model and comparison with a microfluidic flow cell experiment,” Water Resources Research, vol. 49, no. 12, pp. 8370-8382, 2013. Publisher’s Version

Abstract
This work presents a pore-scale biofilm model that solves the flow field using the lattice Boltzmann method, the concentration field of chemical species using the finite difference method, and biofilm development using the cellular automaton method. We adapt the model from a previous work and expand it by implementing biofilm shrinkage in the cellular automaton method. The new pore-scale biofilm model is then evaluated against a previously published pore-scale biofilm experiment, in which two microfluidic flow cells, one with a homogeneous pore network and the other with an aggregate pore network, were tested for aerobic degradation of a herbicide. The simulated biofilm distribution and morphology, biomass accumulation, and contaminant removal are generally consistent with the experimental data. Biofilm detachment in this model occurs when the local shear stress is above a critical value. We use the critical value from our previously published modeling study and find it works well in this case, even though we now have a different pore network and a different microbial species. We also use the model to show that the interaction between flow and biofilm growth is important to predict contaminant removal. The computational time of the new model is reduced 90% compared to our prior work due to implementation of biofilm shrinkage in the cellular automaton method. To the best of our knowledge, this is the first time that biofilm shrinkage has been incorporated into a pore-scale model for simulation of pollutant biodegradation in porous media.

 

H. Liu, Valocchi, A. J., Kang, Q., and Werth, C. J., “

Pore-scale simulations of gas displacing liquid in a homogeneous pore network using the lattice Boltzmann method

,” Transport in Porous Media, vol. 99, no. 3, pp. 555-580, 2013. Publisher’s Version

Abstract
A lattice Boltzmann high-density-ratio model, which uses diffuse interface theory to describe the interfacial dynamics and was proposed originally by Lee and Liu (J Comput Phys 229:8045–8063, 2010), is extended to simulate immiscible multiphase flows in porous media. A wetting boundary treatment is proposed for concave and convex corners. The capability and accuracy of this model is first validated by simulations of equilibrium contact angle, injection of a non-wetting gas into two parallel capillary tubes, and dynamic capillary intrusion. The model is then used to simulate gas displacement of liquid in a homogenous two-dimensional pore network consisting of uniformly spaced square obstructions. The influence of capillary number (Ca), viscosity ratio (M), surface wettability, and Bond number (Bo) is studied systematically. In the drainage displacement, we have identified three different regimes, namely stable displacement, capillary fingering, and viscous fingering, all of which are strongly dependent upon the capillary number, viscosity ratio, and Bond number. Gas saturation generally increases with an increase in capillary number at breakthrough, whereas a slight decrease occurs when Ca is increased from   8.66×10−4  to   4.33×10−3 , which is associated with the viscous instability at high Ca. Increasing the viscosity ratio can enhance stability during displacement, leading to an increase in gas saturation. In the two-dimensional phase diagram, our results show that the viscous fingering regime occupies a zone markedly different from those obtained in previous numerical and experimental studies. When the surface wettability is taken into account, the residual liquid blob decreases in size with the affinity of the displacing gas to the solid surface. Increasing Bo can increase the gas saturation, and stable displacement is observed for   Bo>1  because the applied gravity has a stabilizing influence on the drainage process.

 

D. Shuai, McCalman, D. C., Choe, J. K., Shapley, J. R., Schneider, W. F., and Werth, C. J., “Structure sensitivity study of waterborne contaminant hydrogenation using shape- and size-controlled Pd nanoparticles,” ACS Catalysis, vol. 3, no. 3, pp. 453–463, 2013. Publisher’s Version

Abstract
Catalytic reduction with Pd has emerged as a promising technology to remove a suite of contaminants from drinking water, such as oxyanions, disinfection byproducts, and halogenated pollutants, but low activity is a major challenge for application. To address this challenge, we synthesized a set of shape- and size-controlled Pd nanoparticles and evaluated the activity of three probe contaminants (i.e., nitrite, N-nitrosodimethylamine (NDMA), and diatrizoate) as a function of facet type (e.g., (100), (110), (111)), ratios of low- to high-coordination sites, and ratios of surface sites to total Pd (i.e., dispersion). Reduction results for an initial contaminant concentration of 100 μM show that initial turnover frequency (TOF0) for nitrite increases 4.7-fold with increasing percent of (100) surface Pd sites (from 0% to 95.3%), whereas the TOF0 for NDMA and for diatrizoate increases 4.5- and 3.6-fold, respectively, with an increasing percent of terrace surface Pd sites (from 79.8% to 95.3%). Results for an initial nitrite concentration of 2 mM show that TOF0 is the same for all shape- and size-controlled Pd nanoparticles. Trends for TOF0 were supported by results showing that all catalysts but one were stable in shape and size up to 12 days; for the exception, iodide liberation in diatrizoate reduction appeared to be responsible for a shape change of 4 nm octahedral Pd nanoparticles. Density functional theory (DFT) simulations for the free energy change of hydrogen (H2), nitrite, and nitric oxide (NO) adsorption and a two-site model based on the Langmuir–Hinshelwood mechanism suggest that competition of adsorbates for different Pd sites can explain the TOF0 results. Our study shows for the first time that catalytic reduction activity for waterborne contaminant removal varies with the Pd shape and size, and it suggests that Pd catalysts can be tailored for optimal performance to treat a variety of contaminants for drinking water.

 

2012

B. P. Chaplin, Reinhard, M., Schneider, W. F., Schüth, C., Shapley, J. R., Strathmann, T. J., and Werth, C. J., “Critical review of Pd-based catalytic treatment of priority contaminants in water,” Environmental Science & Technology, vol. 46, no. 7, pp. 3655-3670, 2012. Publisher’s Version

Abstract
Catalytic reduction of water contaminants using palladium (Pd)-based catalysts and hydrogen gas as a reductant has been extensively studied at the bench-scale, but due to technical challenges it has only been limitedly applied at the field-scale. To motivate research that can overcome these technical challenges, this review critically analyzes the published research in the area of Pd-based catalytic reduction of priority drinking water contaminants (i.e., halogenated organics, oxyanions, and nitrosamines), and identifies key research areas that should be addressed. Specifically, the review summarizes the state of knowledge related to (1) proposed reaction pathways for important classes of contaminants, (2) rates of contaminant reduction with different catalyst formulations, (3) long-term sustainability of catalyst activity with respect to natural water foulants and regeneration strategies, and (4) technology applications. Critical barriers hindering implementation of the technology are related to catalyst activity (for some contaminants), stability, fouling, and regeneration. New developments overcoming these limitations will be needed for more extensive field-scale application of this technology.

 

B. P. Chaplin, Reinhard, M., Schneider, W. F., Schüth, C., Shapley, J. R., Strathmann, T. J., and Werth, C. J., “Response to comment on “critical review of Pd-based catalytic treatment of priority contaminants in water”,” Environmental Science & Technology, vol. 46, no. 20, pp. 11469-11470, 2012. Publisher’s Version

Abstract
We appreciate the opportunity provided by Dr. Kopinke’s comment(1) to address the calculation error in our manuscript(2) regarding the external mass-transfer coefficient, and to discuss the possibility of internal mass-transfer limitations during catalytic treatment of water contaminants. Other errors in our manuscript pointed out by Dr. Kopinke are corrected here.

 

D. C. McCalman, Kelley, K. H., Werth, C. J., Shapley, J. R., and Schneider, W. F., “Aqueous N2O reduction with H2 over Pd-based catalyst: mechanistic insights from experiment and simulation,” Topics in Catalysis, vol. 55, no. 5-6, pp. 300–312, 2012. Publisher’s Version

Abstract
Nitrous Oxide (N2O), an ozone depleting greenhouse gas, is an observed intermediate in aqueous nitrate/nitrite reduction mediated by both natural microbial and synthetic laboratory catalysts. Because of our interest in catalytic nitrate/nitrite remediation, we have endeavored to develop a detailed concordant experimental/theoretical picture of N2O reduction with H2 over a Pd catalyst in an aqueous environment. We use batch experiments in H2 excess and limiting conditions to examine the reduction kinetics. We use density functional theory (DFT) to model the elementary steps in N2O reduction on model Pd(100), Pd(110), Pd(111) and Pd(211) facets and including the influence of adsorbed O, H, and of H2O. Both experiments and theory agree that hydrogen is necessary for removal of adsorbed oxygen from the catalyst surface. The dissociation of N2O to N2(g) and O(ads) is facile and in the absence of H proceeds until the catalyst is O-covered. Water itself is proposed to facilitate the hydrogenation of surface O by transferring absorbed hydrogen to Pd-absorbed O and OH. We measure an apparent activation energy of 41.4 kJ/mol (0.43 eV) for N2O reduction in the presence of excess H2, a value that is within 0.1 eV of the barriers determined theoretically.

 

D. Shuai, Choe, J. K., Shapley, J. R., and Werth, C. J., “Enhanced activity and selectivity of carbon nanofiber supported Pd catalysts for nitrite reduction,” Environmental Science & Technology, vol. 46, no. 5, pp. 2847–2855, 2012. Publisher’s Version

Abstract
Pd-based catalyst treatment represents an emerging technology that shows promise to remove nitrate and nitrite from drinking water. In this work we use vapor-grown carbon nanofiber (CNF) supports in order to explore the effects of Pd nanoparticle size and interior versus exterior loading on nitrite reduction activity and selectivity (i.e., dinitrogen over ammonia production). Results show that nitrite reduction activity increases by 3.1-fold and selectivity decreases by 8.0-fold, with decreasing Pd nanoparticle size from 1.4 to 9.6 nm. Both activity and selectivity are not significantly influenced by Pd interior versus exterior CNF loading. Consequently, turnover frequencies (TOFs) among all CNF catalysts are similar, suggesting nitrite reduction is not sensitive to Pd location on CNFs nor Pd structure. CNF-based catalysts compare favorably to conventional Pd catalysts (i.e., Pd on activated carbon or alumina) with respect to nitrite reduction activity and selectivity, and they maintain activity over multiple reduction cycles. Hence, our results suggest new insights that an optimum Pd nanoparticle size on CNFs balances faster kinetics with lower ammonia production, that catalysts can be tailored at the nanoscale to improve catalytic performance for nitrite, and that CNFs hold promise as highly effective catalyst supports in drinking water treatment.

 

Y. Guo, Li, W., Yan, J., Moosa, B., Amad, M., Werth, C. J., and Khashab, N. M., “Fullerene-catalyzed reduction of azo derivatives in water under UV irradiation,” Chemistry- An Asian Journal, vol. 7, no. 12, pp. 2842–2847, 2012. Publisher’s Version

Abstract
Metal‐free fullerene (C60) was found to be an effective catalyst for the reduction of azo groups in basic aqueous solution under UV irradiation in the presence of NaBH4. Use of NaBH4 by itself is not sufficient to reduce the azo dyes without the assistance of a metal catalyst such as Pd and Ag. Experimental and theoretical results suggest that C60 catalyzes this reaction by using its vacant orbital to accept the electron in the bonding orbital of azo dyes, which leads to the activation of the NN bond. UV irradiation increases the ability of C60 to interact with electron‐donor moieties in azo dyes.

 

A. Marruffo, Yoon, H., Schaeffer, D. J., Barkan, C. P. L., Saat, M. R., and Werth, C. J., “

NAPL source zone depletion model and its application to railroad-tank-car spills

,” Ground Water, vol. 50, no. 4, pp. 627-632, 2012.

H. Yoon, Valocchi, A. J., Werth, C. J., and Dewers, T., “Pore-scale simulation of mixing-induced calcium carbonate precipitation and dissolution in a microfluidic pore network,” Water Resources Research, vol. 48, no. 2, pp. W02524, 2012. Publisher’s Version

Abstract
We develop a 2‐D pore scale model of coupled fluid flow, reactive transport, and calcium carbonate (CaCO3) precipitation and dissolution. The model is used to simulate transient experimental results of CaCO3 precipitation and dissolution under supersaturated conditions in a microfluidic pore network (i.e., micromodel) in order to improve understanding of coupled reactive transport systems perturbed by geological CO2 injection. In the micromodel, precipitation is induced by transverse mixing along the centerline in pore bodies. The reactive transport model includes the impact of pH upon carbonate speciation and a CaCO3 reaction rate constant, the effect of changing reactive surface area upon the reaction, and the impact of pore blockage from CaCO3 precipitation on diffusion and flow. Overall, the pore scale model qualitatively captured the precipitate morphology, precipitation rate, and maximum precipitation area using parameter values from the literature. In particular, we found that proper estimation of the effective diffusion coefficient (Deff) and the reactive surface area is necessary to adequately simulate precipitation and dissolution rates. In order to match the initial phase of fast precipitation, it was necessary to consider the top and bottom of the micromodel as additional reactive surfaces. In order to match a later phase when dissolution occurred, it was necessary to increase the dissolution rate compared to the precipitation rate, but the simulated precipitate area was still higher than the experimental results after ∼30 min, highlighting the need for future study. The model presented here allows us to simulate and mechanistically evaluate precipitation and dissolution of CaCO3 observed in a micromodel pore network. This study leads to improved understanding of the fundamental physicochemical processes of CaCO3precipitation and dissolution under far‐from‐equilibrium conditions.

 

2011

D. Shuai, Wang, C., Genc, A., and Werth, C. J., “A new geometric method based on two-dimensional transmission electron microscopy for analysis of interior versus exterior Pd loading on hollow carbon nanofibers,” Journal of Physical Chemistry Letters, vol. 2, no. 9, pp. 1082–1087, 2011. Publisher’s Version

Abstract
Hollow carbon nanofibers (CNFs) are being explored as catalyst supports because of their unique properties. Internal versus external loading of metal nanoparticles impacts catalytic performance; we developed a fast and accurate geometric analysis method based on two-dimensional transmission electron microscopy (2D TEM) images to estimate Pd internal versus external loading percentages. Three different Pd-loaded CNF catalysts were prepared using methods reported in the literature to yield different amounts of Pd inside loading. Results indicate the percentage of inside-loaded Pd increases as expected in the three samples (from 22.7 ± 17.8%, to 47.2 ± 22.8%, to 71.4 ± 19.7%, based on Pd nanoparticle number). We compared percent inside loading values for one segment of a Pd-loaded CNF using our method and three-dimensional scanning transmission electron microscopy (3D STEM), and observed adequate agreement (27.8% vs 32.7%). Our geometric analysis method is proposed as a more straightforward and fast way to evaluate metal nanoparticles on tubular supports.

 

2010

C. Zhang, Kang, Q., Wang, X., Zilles, J., Muller, R. H., and Werth, C. J., “Effects of pore-scale heterogeneity and transverse mixing on bacterial growth in porous media,” Environmental Science & Technology, vol. 44, no. 8, pp. 3085–3092, 2010. Publisher’s Version

Abstract
Microbial degradation of contaminants in the subsurface requires the availability of nutrients; this is impacted by porous media heterogeneity and the degree of transverse mixing. Two types of microfluidic pore structures etched into silicon wafers (i.e., micromodels), (i) a homogeneous distribution of cylindrical posts and (ii) aggregates of large and small cylindrical posts, were used to evaluate the impact of heterogeneity on growth of a pure culture (Delftia acidovorans) that degrades (R)-2-(2,4-dichlorophenoxy)propionate (R-2,4-DP). Following inoculation, dissolved O2 and R-2,4-DP were introduced as two parallel streams that mixed transverse to the direction of flow. In the homogeneous micromodel, biomass growth was uniform in pore bodies along the center mixing line, while in the aggregate micromodel, preferential growth occurred between aggregates and slower less dense growth occurred throughout aggregates along the center mixing line. The homogeneous micromodel had more rapid growth overall (2 times) and more R-2,4-DP degradation (9.5%) than the aggregate pore structure (5.7%). Simulation results from a pore-scale reactive transport model indicate mass transfer limitations within aggregates along the center mixing line decreased overall reaction; hence, slower biomass growth rates relative to the homogeneous micromodel are expected. Results from this study contribute to a better understanding of the coupling between mass transfer, reaction rates, and biomass growth in complex porous media and suggest successful implementation and analysis of bioremediation systems requires knowledge of subsurface heterogeneity.

 

D. Shuai, Chaplin, B. P., Shapley, J. R., Menendez, N. P., McCalman, D. C., Schneider, W. F., and Werth, C. J., “Enhancement of oxyanion and diatrizoate reduction kinetics using selected azo dyes on Pd-based catalysts,” Environmental Science & Technology, vol. 44, no. 5, pp. 1773–1779, 2010. Publisher’s Version

Abstract
Azo dyes are widespread pollutants and potential cocontaminants for nitrate; we evaluated their effect on catalytic reduction of a suite of oxyanions, diatrizoate, and N-nitrosodimethylamine (NDMA). The azo dye methyl orange significantly enhanced (less than or equal to a factor of 5.24) the catalytic reduction kinetics of nitrate, nitrite, bromate, perchlorate, chlorate, and diatrizoate with several different Pd-based catalysts; NDMA reduction was not enhanced. Nitrate was selected as a probe contaminant, and a variety of azo dyes (methyl orange, methyl red, fast yellow AB, metanil yellow, acid orange 7, congo red, eriochrome black T, acid red 27, acid yellow 11, and acid yellow 17) were evaluated for their ability to enhance reduction. Hydrogenation energies of azo dyes were calculated using density functional theory and a volcano relationship between hydrogenation energies and reduction rate enhancement was observed. A kinetic model based on Brønsted−Evans−Polanyi (BEP) theory matched the volcano relationship and suggests sorbed azo dyes enhance reduction kinetics through hydrogen atom shuttling between reduced azo dyes (i.e., hydrazo dyes) and oxyanions or diatrizoate. This is the first research that has identified this synergetic effect, and it has implications for designing more efficient catalysts and reducing Pd costs in water treatment systems.

 

Y. Yang, Metre, V. P., Mahler, B., Wilson, J., Ligouis, B., Razzaque, M., Schaeffer, D., and Werth, C. J., “Influence of coal-tar sealcoat and other carbonaceous materials on polycyclic aromatic hydrocarbon loading in an urban watershed,” Environmental Science & Technology, vol. 44, no. 4, pp. 1217–1223, 2010. Publisher’s Version

Abstract
Carbonaceous material (CM) particles are the principal vectors transporting polycyclic aromatic hydrocarbons (PAHs) into urban waters via runoff; however, characteristics of CM particles in urban watersheds and their relative contributions to PAH contamination remain unclear. Our objectives were to identify the sources and distribution of CM particles in an urban watershed and to determine the types of CMs that were the dominant sources of PAHs in the lake and stream sediments. Samples of soils, parking lot and street dust, and streambed and lake sediment were collected from the Lake Como watershed in Fort Worth, Texas. Characteristics of CM particles determined by organic petrography and a significant correlation between PAH concentrations and organic carbon in coal tar, asphalt, and soot indicate that these three CM particle types are the major sources and carriers of PAHs in the watershed. Estimates of the distribution of PAHs in CM particles indicate that coal-tar pitch, used in some pavement sealcoats, is a dominant source of PAHs in the watershed, and contributes as much as 99% of the PAHs in sealed parking lot dust, 92% in unsealed parking lot dust, 88% in commercial area soil, 71% in streambed sediment, and 84% in surficial lake sediment.

 

J. K. Choe, Shapley, J. R., Strathmann, T. J., and Werth, C. J., “Influence of rhenium speciation on the stability and activity of Re/Pd bimetal catalysts used for perchlorate reduction,” Environmental Science & Technology , vol. 44, no. 12, pp. 4716–4721, 2010. Publisher’s Version

Abstract
Recent work demonstrates reduction of aqueous perchlorate by hydrogen at ambient temperatures and pressures using a novel rhenium−palladium bimetal catalyst immobilized on activated carbon (Re/Pd-AC). This study examines the influence of Re speciation on catalyst activity and stability. Rates of perchlorate reduction are linearly dependent on Re content from 0−6 wt %, but no further increases are observed at higher Re contents. Surface-immobilized Re shows varying stability and speciation both in oxic versus H2-reducing environments and as a function of Re content. In oxic solutions, Re immobilization is dictated by sorption of the Re(VII) precursor, perrhenate (ReO4−), to activated carbon via electrostatic interactions. Under H2-reducing conditions, Re immobilization is significantly improved and leaching is minimized by ReO4− reduction to more reduced species on the catalyst surface. X-ray photoelectron spectroscopy shows two different Re binding energy states under H2-reducing conditions that correspond most closely to Re(V)/Re(IV) and Re(I) reference standards, respectively. The distribution of the two redox states varies with Re content, with the latter predominating at lower Re contents where catalyst activity is more strongly dependent on Re content. Results demonstrate that both lower Re contents and the maintenance of H2-reducing conditions are key elements in stabilizing the active Re surface species that are needed for sustained catalytic perchlorate treatment.

 

C. Zhang, Dehoff, K., Hess, N., Oostrom, M., Wietsma, T. W., Valocchi, A. J., Fouke, B. W., and Werth, C. J., “Pore-scale study of transverse mixing induced CaCO3 precipitation and permeability reduction in a model subsurface sedimentary system,” Environmental Science & Technology, vol. 44, no. 20, pp. 7833–7838, 2010. Publisher’s Version

Abstract
A microfluidic pore structure etched into a silicon wafer was used as a two-dimensional model subsurface sedimentary system (i.e., micromodel) to study mineral precipitation and permeability reduction relevant to groundwater remediation and geological carbon sequestration. Solutions containing CaCl2 and Na2CO3 at four different saturation states (Ω = [Ca2+][CO32−]/KspCaCO3) were introduced through two separate inlets, and they mixed by diffusion transverse to the main flow direction along the center of the micromodel resulting in CaCO3 precipitation. Precipitation rates increased and the total amount of precipitates decreased with increasing saturation state, and only vaterite and calcite crystals were formed (no aragonite). The relative amount of vaterite increased from 80% at the lowest saturation state (Ωv = 2.8 for vaterite) to 95% at the highest saturation state (Ωv = 4.5). Fluorescent tracer tests conducted before and after CaCO3 precipitation indicate that pore spaces were occluded by CaCO3 precipitates along the transverse mixing zone, thus substantially reducing porosity and permeability, and potentially limiting transformation from vaterite to the more stable calcite. The results suggest that mineral precipitation along plume margins can decrease both reactant mixing during groundwater remediation, and injection and storage efficiency during CO2 sequestration.

 

Y. Yang, Mahler, B. J., VanMetre, P. C., Ligouis, B., and Werth, C. J., “Potential contributions of asphalt and coal tar to black carbon quantification in urban dust, soils, and sediments,” Geochimica et Cosmochimica Acta, vol. 74, no. 23, pp. 6830–6840, 2010. Publisher’s Version

Abstract
Measurements of black carbon (BC) using either chemical or thermal oxidation methods are generally thought to indicate the amount of char and/or soot present in a sample. In urban environments, however, asphalt and coal-tar particles worn from pavement are ubiquitous and, because of their pyrogenic origin, could contribute to measurements of BC. Here we explored the effect of the presence of asphalt and coal-tar particles on the quantification of BC in a range of urban environmental sample types, and evaluated biases in the different methods used for quantifying BC. Samples evaluated were pavement dust, residential and commercial area soils, lake sediments from a small urban watershed, and reference materials of asphalt and coal tar. Total BC was quantified using chemical treatment through acid dichromate (Cr2O7) oxidation and chemo-thermal oxidation at 375 °C (CTO-375). BC species, including soot and char/charcoal, asphalt, and coal tar, were quantified with organic petrographic analysis. Comparison of results by the two oxidation methods and organic petrography indicates that both coal tar and asphalt contribute to BC quantified by Cr2O7 oxidation, and that coal tar contributes to BC quantified by CTO-375. These results are supported by treatment of asphalt and coal-tar reference samples with Cr2O7 oxidation and CTO-375. The reference asphalt is resistant to Cr2O7 oxidation but not to CTO-375, and the reference coal tar is resistant to both Cr2O7 oxidation and CTO-375. These results indicate that coal tar and/or asphalt can contribute to BC measurements in samples from urban areas using Cr2O7 oxidation or CTO-375, and caution is advised when interpreting BC measurements made with these methods.

 

C. J. Werth, Zhang, C., Brusseau, M., Oostrom, M., and Baumann, T., “A review of non-invasive imaging methods and applications in contaminant hydrogeology research,” Journal of Contaminant Hydrology, vol. 113, no. 1-4, pp. 1–24, 2010. Publisher’s Version

Abstract
Contaminant hydrogeological processes occurring in porous media are typically not amenable to direct observation. As a result, indirect measurements (e.g., contaminant breakthrough at a fixed location) are often used to infer processes occurring at different scales, locations, or times. To overcome this limitation, non-invasive imaging methods are increasingly being used in contaminant hydrogeology research. Four of the most common methods, and the subjects of this review, are optical imaging using UV or visible light, dual-energy gamma radiation, X-ray microtomography, and magnetic resonance imaging (MRI). Non-invasive imaging techniques have provided valuable insights into a variety of complex systems and processes, including porous media characterization, multiphase fluid distribution, fluid flow, solute transport and mixing, colloidal transport and deposition, and reactions. In this paper we review the theory underlying these methods, applications of these methods to contaminant hydrogeology research, and methods’ advantages and disadvantages. As expected, there is no perfect method or tool for non-invasive imaging. However, optical methods generally present the least expensive and easiest options for imaging fluid distribution, solute and fluid flow, colloid transport, and reactions in artificial two-dimensional (2D) porous media. Gamma radiation methods present the best opportunity for characterization of fluid distributions in 2D at the Darcy scale. X-ray methods present the highest resolution and flexibility for three-dimensional (3D) natural porous media characterization, and 3D characterization of fluid distributions in natural porous media. And MRI presents the best option for 3D characterization of fluid distribution, fluid flow, colloid transport, and reaction in artificial porous media. Obvious deficiencies ripe for method development are the ability to image transient processes such as fluid flow and colloid transport in natural porous media in three dimensions, the ability to image many reactions of environmental interest in artificial and natural porous media, and the ability to image selected processes over a range of scales in artificial and natural porous media.

 

T. W. Willingham, Zhang, C., Werth, C. J., Valocchi, A. J., Oostrom, M., and Wietsma, T. W., “Using dispersivity values to quantify the effects of pore-scale flow focusing on enhanced reaction along a transverse mixing zone,” Advances in Water Resources, vol. 33, no. 4, pp. 525–535, 2010. Publisher’s Version

Abstract
A key challenge for predictive modeling of transverse mixing and reaction of solutes in groundwater is to determine values of transverse dispersivity  in heterogeneous flow fields that accurately describe mixing and reaction at the pore scale. We evaluated the effects of flow focusing in high permeability zones on mixing enhancement using experimental micromodel flow cells and pore-scale lattice-Boltzmann-finite-volume model (LB-FVM) simulations. Micromodel results were directly compared to LB-FVM simulations using two different pore structures, and excellent agreement was obtained. Six different flow focusing pore structures were then systematically tested using LB-FVM, and both analytical solutions and a two-dimensional (2D) continuum-scale model were used to fit  values to pore-scale results. Pore-scale results indicate that the overall rate of mixing-limited reaction increased by up to 40% when flow focusing occurred, and it was greater in pore structures with longer flow focusing regions and greater porosity contrast. For each pore structure, values from analytical solutions of transverse concentration profiles or total product at a given longitudinal location showed good agreement for nonreactive and reactive solutes, and values determined in flow focusing zones were always smaller than those downgradient after the flow focusing zone. Transverse dispersivity values from the 2D continuum model were between values within and downgradient from the flow focusing zone determined from analytical solutions. Also, total product and transverse concentration profiles along the entire pore structure from the 2D continuum model matched pore scale results. These results indicate that accurate quantification of pore-scale flow focusing with transverse dispersion coefficients is possible only when the entire flow and concentration fields are considered.

 

2009

G. Gopalakrishnan, Burken, J., and Werth, C. J., “Lignin and lipid impact on sorption and diffusion of trichloroethylene in tree branches for determining contaminant fate during plant sampling and phytoremediation,” Environ. Sci. Technol., vol. 43, 2009.

G. Gopalakrishnan, Werth, C. J., and Negri, M. C., “Mass recovery methods for trichloroethylene in plant tissue,” Environ. Toxic. & Chem., {DOI}:, vol. 10., 2009.

H. Yoon, Oostrom, M., Wietsma, T. W., Werth, C. J., and Valocchi, A. J., “Numerical and experimental investigation of DNAPL removal mechanisms in a layered porous medium by means of soil vapor extraction,” J. Contam. Hydrol., pp. 1–13, 2009.

S. R. Nellis, Yoon, H., Werth, C. J., Oostrom, M., and Valocchi, A. J., “Surface and interfacial properties of representative nonaqueous phase liquid mixtures released to the subsurface at the U,” S. {DOE} {Hanford} site in {Washington} state, {Vadose} {Zone} {J}., vol. 8, pp. 343–351, 2009.

K. A. Guy, Xu, H., Yang, J. C., Werth, C. J., and Shapley, J. R., “Catalytic nitrate and nitrite reduction with Pd−Cu/PVP colloids in water: composition, structure, and reactivity correlations,” Journal of Physical Chemistry, vol. 113, no. 19, pp. 8177–8185, 2009. Publisher’s Version

Abstract
A set of bimetallic Pd−Cu/PVP (PVP = poly(N-vinylpyrrolidone)) colloids, with copper proportions ranging from 0 to 50 atom %, has been examined as catalysts in a batch reactor with flowing hydrogen for the reduction of aqueous nitrate and/or nitrite. The encapsulated Pd−Cu nanoparticles were characterized by powder XRD, TEM, EDX, and IR of adsorbed CO. A significant decrease in average particle diameter and changes in the Pd−Cu crystallinity occurred above ca. 30% copper content, and this transition corresponded with a significant increase in observed nitrate reduction rates. The strong dependence on composition suggests that specific Cun ensembles on the surface of the Pd−Cu nanoparticles are needed for effective nitrate-to-nitrite conversion. In contrast, nitrite reduction rates were only minimally enhanced by the presence of copper. Increasing pH had little effect on the nitrate reduction rates, but it strongly inhibited the rate of nitrite reduction. The requisite protonation of a palladium−nitrite surface intermediate is proposed.

 

H. Yoon, Oostrom, M., and Werth, C. J., “Estimation of interfacial tension between organic liquid mixtures and water,” Environmental Science & Technology, vol. 43, no. 20, pp. 7754–7761, 2009. Publisher’s Version

Abstract
Knowledge of IFT values for chemical mixtures helps guide the design and analysis of various processes, including NAPL remediation with surfactants or alcohol flushing, enhanced oil recovery, and chemical separation technologies, yet available literature values are sparse. A comprehensive comparison of thermodynamic and empirical models for estimating interfacial tension (IFT) of organic chemical mixtures with water is conducted, mainly focusing on chlorinated organic compounds for 14 ternary, three quaternary, and one quinary systems. Emphasis is placed on novel results for systems with three and four organic chemical compounds, and for systems with composite organic compounds like lard oil and mineral oil. Seven models are evaluated: the ideal and nonideal monolayer models (MLID and MLNID), the ideal and nonideal mutual solubility models (MSID and MSNID), an empirical model for ternary systems (EM), a linear mixing model based on mole fractions (LMMM), and a newly developed linear mixing model based on volume fractions of organic mixtures (LMMV) for higher order systems. The two ideal models (MLID and MSID) fit ternary systems of chlorinated organic compounds without surface active compounds relatively well. However, both ideal models did not perform well for the mixtures containing a surface active compound. However, for these systems, both the MLNID and MSNID models matched the IFT data well. It is shown that the MLNID model with a surface coverage value (0.00341 mmol/m2) obtained in this study can practically be used for chlorinated organic compounds. The LMMM results in poorer estimates of the IFT as the difference in IFT values of individual organic compounds in a mixture increases. The EM, with two fitting parameters, provided accurate results for all 14 ternary systems including composite organic compounds. The new LMMV method for quaternary and higher component systems was successfully tested. This study shows that the LMMV may be able to be used for higher component systems and it can be easily incorporated into compositional multiphase flow models using only parameters from ternary systems.

 

B. P. Chaplin, Shapley, J. R., and Werth, C. J., “Oxidative regeneration of sulfide fouled catalysts for water treatment,” Catalysis Letters, vol. 132, no. 1-2, pp. 174-181, 2009. Publisher’s Version

Abstract
This study tested the stability, activity, and selectivity of an alumina-supported Pd–In bimetallic catalyst during repetitive sulfide fouling and oxidative regeneration conditions. Nitrate reduction with hydrogen was used as the probe reaction in a continuous-flow packed-bed reactor to assess changes in the catalyst structure as a result of the fouling and regeneration processes. Partial regeneration of a severely sulfide-fouled Pd–In catalyst was achieved with a NaOCl/NaHCO3 solution. However, the regenerated catalyst had a reduced activity for NO3 − reduction and increased selectivity towards NH3. Analysis of the catalyst bed after regeneration experiments using XPS, ICP-MS, and BET surface area revealed that bulk structural transformations of the Pd–In bimetallic catalyst occurred, as a result of preferential Pd dissolution near the column influent. The dissolved Pd showed limited mobility in the column, and was re-deposited on the catalyst, resulting in Pd enrichment on the catalyst surface and redistribution of Pd towards the end of the column. These changes along with residual sulfur content on the catalyst surface were likely responsible for the increased selectivity towards NH3. These results indicate the importance of limiting the exposure of reduced sulfur species to Pd-based catalysts, especially when treating contaminants like NO3 −, where product selectivity is a priority.

 

H. Yoon, Werth, C. J., Barkan, C. P. L., Schaeffer, D. J., and Anand, P., “

An environmental screening model to assess the consequences to soil and groundwater from railroad-tank-car spills of light non-aqueous phase liquids

,” Journal of Hazardous Materials, vol. 165, no. 1-3, pp. 332-344, 2009. Publisher’s Version

Abstract
North American railroads transport a wide variety of chemicals, chemical mixtures and solutions in railroad tank cars. In the event of an accident, these materials may be spilled and impact the environment. Among the chemicals commonly transported are a number of light non-aqueous phase liquids (LNAPLs). If these are spilled they can contaminate soil and groundwater and result in costly cleanups. Railroads need a means of objectively assessing the relative risk to the environment due to spills of these different materials. Environmental models are often used to determine the extent of contamination, and the associated environmental risks. For LNAPL spills, these models must account for NAPL infiltration and redistribution, NAPL dissolution and volatilization, and remediation systems such as pump and treat. This study presents the development and application of an environmental screening model to assess NAPL infiltration and redistribution in soils and groundwater, and to assess groundwater cleanup time using a pumping system. Model simulations use parameters and conditions representing LNAPL releases from railroad tank cars. To take into account unique features of railroad-tank-car spill sites, the hydrocarbon spill screening model (HSSM), which assumes a circular surface spill area and a circular NAPL lens, was modified to account for a rectangular spill area and corresponding lens shape at the groundwater table, as well as the effects of excavation and NAPL evaporation to the atmosphere. The modified HSSM was first used to simulate NAPL infiltration and redistribution. A NAPL dissolution and groundwater transport module, and a pumping system module were then implemented and used to simulate the effects of chemical properties, excavation, and free NAPL removal on NAPL redistribution and cleanup time. The amount of NAPL that reached the groundwater table was greater in coarse sand with high permeability than in fine sand or silt with lower permeabilities. Excavation can reduce the amount of NAPL that reaches the groundwater more effectively in lower permeability soils. The effect of chemical properties including vapor pressure and the ratio of density to viscosity become more important in fine sand and silt soil due to slow NAPL movement in the vadose zone. As expected, a pumping system was effective for high solubility chemicals, but it was not effective for low solubility chemicals due to rate-limited mass transfer by transverse dispersion and flow bypassing. Free NAPL removal can improve the removal efficiency for moderately low solubility chemicals like benzene, but cleanup times even after free NAPL removal can be prolonged for very low solubility chemicals like cyclohexane and styrene.

 

B. P. Chaplin, Shapley, J. R., and Werth, C. J., “The selectivity and sustainability of a Pd–In/γ-Al2O3catalyst in a packed-bed reactor: the effect of solution composition,” Catalysis Letters, vol. 130, no. 1-2, pp. 56–62, 2009. Publisher’s Version

Abstract
This study tested the selectivity and sustainability of an alumina-supported Pd–In bimetallic catalyst for nitrate reduction with H2 in a continuous-flow packed-bed reactor in the presence of: (i) dissolved oxygen (DO), an alternative electron acceptor to nitrate, (ii) variable NO3 −:H2 influent loadings, and (iii) the presence of a known foulant, sulfide. The sustainability of the catalyst was promising, as the catalyst was found to be stable under all conditions tested with respect to metal leaching. The presence of DO at concentrations typical of treatment conditions will increase H2 demand for NO3 − reduction, but has no negative impact on the selectivity of the catalyst. Under optimal conditions, i.e., a pH of 5.0 and a high NO3 −:H2 influent loading, low NH3 selectivity (5%) was achieved for extended periods (36 days), resulting in sustained levels of NH3 that approached the European legal limit. The biggest challenge to the sustainability of the catalyst was the addition of sulfide, that initially increased NH3 selectivity and ultimately resulted in complete deactivation of the catalyst. Further work is required to identify regeneration methods to restore sulfide-fouled catalyst activity and selectivity; however, the most effective use would be to remove sulfide prior to catalytic treatment.

 

2008

H. Yoon, Zhang, C., Werth, C. J., Valocchi, A. J., and Webb, A. G., “Three dimensional characterization of water flow in heterogeneous porous media using magnetic resonance imaging,” Water Resour. Res., vol. 44, 2008.

C. Zhang, Yoon, H., Werth, C. J., Valocchi, A. J., Basu, N. B., and Jawitz, J. W., “Evaluation of simplified mass transfer models to simulate the impacts of source zone architecture on nonaqueous phase liquid dissolution in heterogeneous porous media,” Journal of Contaminant Hydrology, vol. 102, no. 1-2, pp. 49–60, 2008. Publisher’s Version

Abstract
Nonaqueous phase liquid (NAPL) dissolution was studied in three-dimensional (3D) heterogeneous experimental aquifers (25.5 cm × 9 cm × 8.5 cm) with two different longitudinal correlation lengths (2.1 cm and 1.1 cm) and initial spill volumes (22.5 ml and 10.5 ml). Spatial and temporal distributions of NAPL during dissolution were measured using magnetic resonance imaging (MRI). At high NAPL spill volume, average effluent concentrations initially increased during dissolution, as NAPL pools transitioned to NAPL ganglia, and then decreased as the total NAPL–water interfacial area decreased over time. Experimental results were used to test six dissolution models: (i and ii) a one-dimensional (1D) model using either specific NAPL–water interfacial area values estimated from MR images at each time step (i.e., 1D quasi-steady state model), or an empirical mass transfer (Sh′) correlation (i.e., 1D transient model), (iii and iv) a multiple analytical source superposition technique (MASST) using either the NAPL distribution determined from MR images at each time step (i.e., MASST steady state model), or the NAPL distribution determined from mass balance calculations (i.e., MASST transient model), (v) an equilibrium streamtube model, and (vi) a 3D grid-scale pool dissolution model (PDM) with a dispersive mass flux term. The 1D quasi-steady state model and 3D PDM captured effluent concentration values most closely, including some concentration fluctuations due to changes in the extent of flow reduction. The 1D transient, MASST steady state and transient, and streamtube models all showed a monotonic decrease in effluent concentration values over time, and the streamtube model was the most computationally efficient. Changes during dissolution of the effective NAPL–water interfacial area estimated from imaging data are similar to changes in effluent concentration values. The 1D steady state model incorporates estimates of the effective NAPL–water interfacial area directly at each time point; the 3D PDM does so indirectly through mass balance and a relative permeability function, which causes reduced water flow through high saturation NAPL regions. Hence, when model accuracy is required, the results indicate that a surrogate of this effective interfacial area is required. Approaches to include this surrogate in the MASST and streamtube models are recommended.

 

T. M. Willingham, Werth, C. J., and Valocchi, A. J., “Evaluation of the effects of porous media structure on mixing-controlled reactions using pore-scale modeling and micromodel experiments,” Environmental Science & Technoloty, vol. 42, no. 9, pp. 3185–3193, 2008. Publisher’s Version

Abstract
The objectives of this work were to determine if a pore-scale model could accurately capture the physical and chemical processes that control transverse mixing and reaction in microfluidic pore structures (i.e., micromodels), and to directly evaluate the effects of porous media geometry on a transverse mixing-limited chemical reaction. We directly compare pore-scale numerical simulations using a lattice-Boltzmann finite volume model (LB-FVM) with micromodel experiments using identical pore structures and flow rates, and we examine the effects of grain size, grain orientation, and intraparticle porosity upon the extent of a fast bimolecular reaction. For both the micromodel experiments and LB-FVM simulations, two reactive substrates are introduced into a network of pores via two separate and parallel fluid streams. The substrates mix within the porous media transverse to flow and undergo instantaneous reaction. Results indicate that (i) the LB-FVM simulations accurately captured the physical and chemical process in the micromodel experiments, (ii) grain size alone is not sufficient to quantify mixing at the pore scale, (iii) interfacial contact area between reactive species plumes is a controlling factor for mixing and extent of chemical reaction, (iv) at steady state, mixing and chemical reaction can occur within aggregates due to interconnected intra-aggregate porosity, (v) grain orientation significantly affects mixing and extent of reaction, and (vi) flow focusing enhances transverse mixing by bringing stream lines which were initially distal into close proximity thereby enhancing transverse concentration gradients. This study suggests that subcontinuum effects can play an important role in the overall extent of mixing and reaction in groundwater, and hence may need to be considered when evaluating reactive transport.

 

H. Yoon, Werth, C. J., Valocchi, A. J., and Oostron, M., “Impact of nonaqueous phase liquid (NAPL) source zone architecture on mass removal mechanisms in strongly layered heterogeneous porous media during soil vapor extraction,” Journal of Contaminant Hydrology, vol. 100, no. 1-2, pp. 58–71, 2008. Publisher’s Version

Abstract
An existing multiphase flow simulator was modified in order to determine the effects of four mechanisms on NAPL mass removal in a strongly layered heterogeneous vadose zone during soil vapor extraction (SVE): a) NAPL flow, b) diffusion and dispersion from low permeability zones, c) slow desorption from sediment grains, and d) rate-limited dissolution of trapped NAPL. The impacts of water and NAPL saturation distribution, NAPL-type (i.e., free, residual, or trapped) distribution, and spatial heterogeneity of the permeability field on these mechanisms were evaluated. Two different initial source zone architectures (one with and one without trapped NAPL) were considered and these architectures were used to evaluate seven different SVE scenarios. For all runs, slow diffusion from low permeability zones that gas flow bypassed was a dominant factor for diminished SVE effectiveness at later times. This effect was more significant at high water saturation due to the decrease of gas-phase relative permeability. Transverse dispersion contributed to fast NAPL mass removal from the low permeability layer in both source zone architectures, but longitudinal dispersion did not affect overall mass removal time. Both slow desorption from sediment grains and rate-limited mass transfer from trapped NAPL only marginally affected removal times. However, mass transfer from trapped NAPL did affect mass removal at later time, as well as the NAPL distribution. NAPL flow from low to high permeability zones contributed to faster mass removal from the low permeability layer, and this effect increased when water infiltration was eliminated. These simulations indicate that if trapped NAPL exists in heterogeneous porous media, mass transfer can be improved by delivering gas directly to zones with trapped NAPL and by lowering the water content, which increases the gas relative permeability and changes trapped NAPL to free NAPL.

 

C. Zhang, Werth, C. J., and Webb, A. G., “Investigation of surfactant-enhanced mass removal and flux reduction in 3D correlated permeability fields using magnetic resonance imaging,” Journal of Contaminant Hydrology, vol. 100, no. 3-4, pp. 116–126, 2008. Publisher’s Version

Abstract
Magnetic resonance imaging (MRI) was used to visualize the NAPL source zone architecture before and after surfactant-enhanced NAPL dissolution in three-dimensional (3D) heterogeneously packed flowcells characterized by different longitudinal correlation lengths: 2.1 cm (aquifer 1) and 1.1 cm (aquifer 2). Surfactant flowpaths were determined by imaging the breakthrough of a paramagnetic tracer (MnCl2) analyzed by the method of moments. In both experimental aquifers, preferential flow occurred in high permeability materials with low NAPL saturations, and NAPL was preferentially removed from the top of the aquifers with low saturation. Alternate flushing with water and two surfactant pulses (5–6 pore volumes each) resulted in ∼ 63% of NAPL mass removal from both aquifers. However, overall reduction in mass flux (Mass Flux 1) exiting the flowcell was lower in aquifer 2 (68%) than in aquifer 1 (81%), and local effluent concentrations were found to increase by as high as 120 times at local sampling ports from aquifer 2 after surfactant flushing. 3D MRI images of NAPL revealed that NAPL migrated downward and created additional NAPL source zones in previously uncontaminated areas at the bottom of the aquifers. The additional NAPL source zones were created in the direction transverse to flow in aquifer 2, which explains the higher mass flux relative to aquifer 1. Analysis using a total trapping number indicates that mobilization of NAPL trapped in the two coarsest sand fractions is possible when saturation is below 0.5 and 0.4, respectively. Results from this study highlight the potential impacts of porous media heterogeneity and NAPL source zone architecture on advanced in-situ flushing technologies.

 

S. Jeong, Wander, M. M., Kleineidam, S., Grathwohl, P., Ligouis, B., and Werth, C. J., “The role of condensed carbonaceous materials on the sorption of hydrophobic organic contaminants in subsurface sediments,” Environmental Science & Technology, vol. 42, no. 5, pp. 1458-1464, 2008. Publisher’s Version

Abstract
The identification and characterization of carbonaceous materials (CMs) that control hydrophobic organic chemical (HOC) sorption is essential to predict the fate and transport of HOCs in soils and sediments. The objectives of this paper are to determine the types of CMs that control HOC sorption in the oxidized and reduced zones of a glacially deposited groundwater sediment in central Illinois, with a special emphasis on the roles of kerogen and black carbon. After collection, the sediments were treated to obtain fractions of the sediment samples enriched in different types of CMs (e.g., humic acid, kerogen, black carbon), and selected fractions were subject to quantitative petrographic analysis. The original sediments and their enrichment fractions were evaluated for their ability to sorb trichloroethene (TCE), a common groundwater pollutant. Isotherm results and mass fractions of CM enrichments were used to calculate sorption contributions of different CMs. The results indicate that CMs in the heavy fractions dominate sorption because of their greater mass. Black carbon mass fractions of total CMs in the reduced sediments were calculated and used to estimate the sorption contribution of these materials. Results indicate that in the reduced sediments, black carbon may sequester as much as 32% of the sorbed TCE mass, but that kerogen and humin are the dominant sorption environments. Organic carbon normalized sorption coefficients (KOC) were compared to literature values. Values for the central Illinois sediments are relatively large and in the range of values determined for materials high in kerogen and humin. This work demonstrates the advantage of using both sequential chemical treatment and petrographic analysis to analyze the sorption contributions of different CMs in natural soils and sediments, and the importance of sorption to natural geopolymers in groundwater sediments not impacted by anthropogenic sources of black carbon.

 

2007

C. Zhang, Werth, C. J., and Webb, A. G., “Characterization of NAPL source zone architecture and dissolution kinetics in heterogeneous porous media using magnetic resonance imaging,” Environ. Sci. Technol., vol. 41, pp. 3672–3678, 2007.

H. Yoon, Valocchi, A. J., and Werth, C. J., “Effect of soil moisture dynamics on dense nonaqueous phase liquid (DNAPL) spill zone architecture in heterogeneous porous media,” Journal of Contaminant Hydrology, vol. 90, no. 3-4, pp. 159–183, 2007. Publisher’s Version

Abstract
The amount, location, and form of NAPL in contaminated vadose zones are controlled by the spatial distribution of water saturation and soil permeability, the NAPL spill scenario, water infiltration events, and vapor transport. To evaluate the effects of these processes, we used the three-phase flow simulator STOMP, which includes a new permeability–liquid saturation–capillary pressure (k–S–P) constitutive model. This new constitutive model considers three NAPL forms: free, residual, and trapped. A 2-D vertical cross-section with five stratigraphic layers was assumed, and simulations were performed for seven cases. The conceptual model of the soil heterogeneity was based upon the stratigraphy at the Hanford carbon tetrachloride (CT) spill site. Some cases considered co-disposal of NAPL with large volumes of wastewater, as also occurred at the Hanford CT site. In these cases, the form and location of NAPL were most strongly influenced by high water discharge rates and NAPL evaporation to the atmosphere. In order to investigate the impact of heterogeneity, the hydraulic conductivity within the lower permeability layer was modeled as a realization of a random field having three different classes. For six extreme cases of 100 realizations, the CT mass that reached the water table varied by a factor of two, and was primarily controlled by the degree of lateral connectivity of the low conductivity class within the lowest permeability layer. The grid size at the top boundary had a dramatic impact on NAPL diffusive flux just after the spill event when the NAPL was present near the ground surface. NAPL evaporation with a fine grid spacing at the top boundary decreased CT mass that reached the water table by 74%, compared to the case with a coarse grid spacing, while barometric pumping had a marginal effect for the case of a continuous NAPL spill scenario considered in this work. For low water infiltration rate scenarios, the distribution of water content prior to a NAPL spill event decreased CT mass that reached the water table by 98% and had a significant impact on the formation of trapped NAPL. For all cases simulated, use of the new constitutive model that allows the formation of residual NAPL increased the amount of NAPL retained in the vadose zone. Density-driven advective gas flow from the ground surface controlled vapor migration in strongly anisotropic layers, causing NAPL mass flux to the lower layer to be reduced. These simulations indicate that consideration of the formation of residual and trapped NAPLs and dynamic boundary conditions (e.g., areas, rates, and periods of different NAPL and water discharge and fluctuations of atmospheric pressure) in the context of full three-phase flow are needed, especially for NAPL spill events at the ground surface. In addition, NAPL evaporation, density-driven gas advection, and NAPL vertical movement enhanced by water flow must be considered in order to predict NAPL distribution and migration in the vadose zone.

 

C. Knutson, Valocchi, A. J., and Werth, C. J., “

Comparison of continuum and pore-scale models of nutrient biodegradation under transverse mixing conditions

,” Advances in Water Resources, vol. 30, no. 6-7, pp. 1421-1431, 2007. Publisher’s Version

Abstract
Recent studies indicate that during in situ bioremediation of contaminated groundwater, degradation occurs primarily along transverse mixing zones. Classical reactive-transport models overpredict the amount of degradation because solute spreading and mixing are not distinguished. Efforts to correct this have focused on modifying both dispersion and reaction terms, but no consensus on the best approach has emerged. In this work, a pore-scale model was used to simulate degradation along a transverse mixing zone between two required nutrients, and a continuum model with fitted parameters was used to match degradation rates from the pore-scale model. The pore-scale model solves for the flow field, concentration field, and biomass development within pore spaces of porous medium. For the continuum model, the flow field and biomass distributions are assumed to be homogeneous, and the fitting parameters are the transverse dispersion coefficient (DT) and maximum substrate utilization rate (kS,c). Results from the pore-scale model show that degradation rates near the system inlet are limited by the reaction rate, while degradation rates downgradient are limited by transverse mixing. For the continuum model, the value of DT may be adjusted so that the degradation rate with distance matches that from the pore-scale model in the mixing-limited region. However, adjusting the value of kS only improves the fit to pore-scale results within the reaction-limited region. Comparison with field and laboratory experiments suggests that the length of the reaction rate-limited region is small compared to the length scale over which degradation occurs. This indicates that along transverse mixing zones in the field, values of kS are unimportant and only the value of DT must be accurately fit.

 

G. Gopalakrishnan, Negri, C. M., Minsker, B. S., and Werth, C. J., “

Monitoring subsurface contamination using tree branches

,” Ground Water Monitoring Remediation, vol. 27, no. 1, pp. 65-74, 2007. Publisher’s Version

Abstract
This paper proposes a method of assessing the distribution of chlorinated solvents in soil and ground water using tree branches. Sampling branches is a potentially more cost‐effective and easier method than sampling tree cores, with less risk of damage to the tree. This approach was tested at Argonne National Laboratory, where phytoremediation is being used to remove tetrachloroethene (PCE), trichloroethene (TCE), and carbon tetrachloride (CCl4) from soil and ground water. The phytoremediation system consists of shallow‐rooted willows planted in an area with contaminated soil and deep‐rooted poplars planted in an area with clean soil and contaminated ground water. Branch samples were collected from 126 willows and 120 poplars. Contaminant concentrations from 31 soil borings and six monitoring wells were compared to those from branches of adjacent trees. Regression equations with correlation coefficients of at least 0.89 were obtained, which were found to be chemical specific. Kriged profiles of TCE concentration based on soil and willow branch data were developed and showed good agreement. Profiles based on ground water data could not be developed due to lack of sufficient monitoring wells for a meaningful statistical analysis. An analytical model was used to simulate TCE concentrations in tree branches from soil concentrations; the diffusion coefficient for TCE in the tree was used as the fitting parameter and the best‐fit value was two orders of magnitude greater than literature values. This work indicates that tree branch sampling is a useful approach to assess contaminant distribution and potentially to determine where to locate monitoring wells or perform detailed soil analysis. Further research is necessary prior to using this method as a quantitative monitoring tool for soil and ground water.

 

R. C. Acharya, Valocchi, A. J., Werth, C. J., and Willingham, T. W., “Pore-scale simulation of dispersion and reaction along a transverse mixing zone in two-dimensional porous media,” Water Resources Research, vol. 43, no. 10, pp. W10435, 2007. Publisher’s Version

Abstract
Several studies have demonstrated that the success of natural and engineered in situ remediation of groundwater pollutants relies on the transverse mixing of reactive chemicals or nutrients along plume margins. Efforts to predict reactions in groundwater generally rely on dispersion coefficients obtained from nonreactive tracer experiments to determine the amount of mixing, but these coefficients may be affected by spreading, which does not contribute to reaction. Mixing is controlled only by molecular diffusion in pore spaces, and the length scale of transverse mixing zones can be small, often on the order of millimeters to centimeters. We use 2D pore‐scale simulation to investigate whether classical transverse dispersion coefficients can be applied to model mixing‐controlled reactive transport in three different porous media geometries: periodic, random, and macroscopically trending. The lattice‐Boltzmann method is used to solve the steady flow field; a finite volume code is used to solve for reactive transport. Nonreactive dispersion coefficients are determined from the transverse spreading of a conservative tracer. Reactive dispersion coefficients are determined by fitting a continuum model which calculates the total product formation as a function of distance to the results from our pore scale simulation. Nonreactive and reactive dispersion coefficients from these simulations are compared. Results indicate that, regardless of the geometrical properties of the media, product formation can be predicted using transverse dispersion coefficients determined from a conservative tracer, provided dispersion coefficients are determined beyond some critical distance downgradient where the plume has spread over a sufficiently large transverse distance compared to the mean grain diameter. This result contrasts with other studies where reactant mixing was controlled by longitudinal hydrodynamic dispersion; in those studies longitudinal dispersion coefficients determined from nonreactive tracer experiments over‐estimated the extent of reaction and product formation. Additional work is called for in order to confirm that these findings hold for a wider variety of grain sizes and geometries.

 

B. P. Chaplin, Shapley, J., and Werth, C. J., “Regeneration of sulfur fouled bimetallic Pd-based catalysts,” Environmental Science & Technology, vol. 41, no. 15, pp. 5491-5497, 2007. Publisher’s Version

Abstract
Pd-based catalysts provide efficient and selective reduction of several drinking water contaminants, but their long-term application requires effective treatments for catalyst regeneration following fouling by constituents in natural waters. This study tested alumina-supported Pd−Cu and Pd−In bimetallic catalysts for nitrate reduction with H2 after sulfide fouling and oxidative regeneration procedures. Both catalysts were severely deactivated after treatment with μM levels of sulfide. Regeneration was attempted with dissolved oxygen, hydrogen peroxide, sodium hypochlorite, and heated air. Only sodium hypochlorite and heated air were effective regenerants, specifically restoring nitrate reduction rates for a Pd−In/γ-Al2O3 catalyst from 20% to between 39 and 60% of original levels. Results from ICP−MS revealed that sodium hypochlorite caused dissolution of Cu from the Pd−Cu catalyst but that the Pd−In catalyst was chemically stable over a range of sulfide fouling and oxidative regenerative conditions. Analysis by XPS indicated that PdS and In2S3 complexes form during sulfide fouling, where sulfur is present as S2-, and that regeneration with sodium hypochlorite converts a portion of the S2- to S6+, with a corresponding increase in reduction rates. These results indicate that Pd−In catalysts show exceptional promise for being robust under fouling and regeneration conditions that may occur when treating natural waters.

 

2006

R. A. Brennan, Sanford, R. A., and Werth, C. J., “Biodegradation of tetrachloroethene by chitin fermentation products in a continuous flow column system,” Journal of Environmental Engineering, vol. 132, no. 6, pp. 664-673, 2006. Publisher’s Version

Abstract
The ability of chitin fermentation products to promote tetrachloroethene (PCE) reduction was evaluated in a continuous-flow column system to identify how different electron donors affect reductive dechlorination. Natural chitin fermentation products were initially used to support PCE reduction. Acetate (3.5mM) was the dominant fermentation product, followed by propionate (0.1mM), butyrate (0.1mM), and hydrogen (100nM). After chlorinated ethene concentration profiles reached pseudo steady state, the ability of individual fermentation products (acetate, acetate+propionate, propionate, or formate) to support PCE reduction was evaluated. None of the fermentation products tested stimulated dechlorination as well as the suite generated from chitin (kPCE=6.9day−1); however, acetate-stimulated PCE dechlorination the best (kPCE=5.3day−1), followed by formate (kPCE=2.4day−1), acetate+propionate (kPCE=1.8day−1), and propionate (kPCE=1.2day−1). Similar trends were observed for the PCE daughter products trichloroethene and dichloroethene. Free energies of individual fatty acid reactions were calculated and shown to be useful predictors of dechlorination performance, except for the case of acetate+propionate. Hence, acetate is the dominant fatty acid controlling dechlorination in the chitin-enhanced system, propionate appears to have an inhibitory effect when present with acetate alone, and other unidentified nutrients produced during chitin fermentation likely contribute to dechlorination activity as well.

 

R. A. Brennan, Sanford, R. A., and Werth, C. J., “Chitin and corncobs as electron donor sources for the reductive dechlorination of tetrachloroethene,” Water Research, vol. 40, no. 11, pp. 2125–2134, 2006. Publisher’s Version

Abstract
Chitin, corncobs, and a mixture of chitin and corncobs were tested as potential electron donor sources for stimulating the reductive dechlorination of tetrachloroethene (PCE). Semi-batch, sand-packed columns were used to evaluate the donors with aerobic and anaerobic groundwaters containing varying degrees of alkalinity. In all experiments, acetate and butyrate were the dominant fatty acids produced, although propionate, valerate, formate, and succinate were also detected. From a multivariable regression analysis on the data, the presence of chitin, limestone, and dechlorinating culture inoculum were determined to be the most positive predictors of dechlorination activity. Chitin fermentation products supported the degradation of PCE to trichloroethene (TCE), cis-1,2-dichloroethene (DCE), and vinyl chloride (VC), even in columns containing PCE DNAPL, whereas dechlorination activity was not observed in any of the columns containing corncobs alone. The longevity and efficiency of chitin as an electron donor source demonstrates its potential usefulness for passive, in situ field applications.

 

B. P. Chaplin, Roundy, E., Guy, K. A., Shapley, J. R., and Werth, C. J., “Effects of natural water ions and humic acid on nitrate reduction using an alumina supported Pd-Cu catalyst,” Environmental Science & Technology, vol. 40, no. 9, pp. 3075-3081, 2006. Publisher’s Version

Abstract
Catalytic nitrate reduction was evaluated for the purpose of drinking water treatment. Common anions present in natural waters and humic acid were evaluated for their effects on NO3- hydrogenation over a bimetallic supported catalyst (Pd−Cu/γ-Al2O3). Groundwater samples, with and without powder activated carbon (PAC) pretreatment, were also evaluated. In the absence of inhibitors the NO3- reduction rate was 2.4 × 10-01 L/min g cat. However, the addition of constituents (SO42-, SO32-, HS-, Cl-, HCO3-, OH-, and humic acid) on the order of representative concentrations for drinking water decreased the NO3- reduction rate. Sulfite, sulfide, and elevated chloride decreased the NO3- reduction rate by over 2 orders of magnitude. Preferential adsorption of Cl- inhibited NO3- reduction to a greater extent than NO2- reduction. Partial regeneration of catalysts exposed to SO32- was achieved by using a dilute hypochlorite solution, however Cu dissolution occurred. Dissolved constituents in the groundwater sample decreased the NO3- reduction rate to 3.7 × 10-03 L/min g cat and increased ammonia production. Removal of dissolved organic matter from the groundwater using PAC increased the NO3- reduction rate to 5.06 × 10-02 L/min g cat and decreased ammonia production. Elemental analyses of catalysts exposed to the natural groundwater suggest that mineral precipitation may also contribute to catalyst fouling.

 

C. J. Werth, Cirpka, O., and Grathwohl, P., “Enhanced mixing and reaction through flow focusing in heterogeneous porous media,” Water Resources Research, vol. 42, pp. W12414, 2006. Publisher’s Version

Abstract
Transverse dispersion across adjacent streamlines can control the amount of mixing and reaction between one or more contaminants and a limiting substrate along the fringes of groundwater plumes. Streamlines in groundwater converge and diverge in heterogeneous porous media, depending on the permeability distribution. When flow is focused in a high‐permeability zone, the distance required for a solute to cross a given number of streamlines decreases, and the time allowed for mixing and reaction is reduced. Because the first effect outweighs the latter, the overall result is an enhancement of transverse mixing and reaction. Here we develop a conceptual model of heterogeneous two‐dimensional structures facilitating flow focusing. We use the conceptual model to develop simple analytical expressions quantifying the extent to which mixing and reaction are enhanced when flow focusing occurs and compare these to results of numerical simulations. Significant enhancement of transverse mixing and reaction by flow focusing is observed; for the cases considered, flow focusing enhances the amount of reaction by a factor ranging from 1.8 to 11.9. The relatively simple analytical expressions demonstrate that the fraction of the domain height made up by high‐permeability inclusions, the fraction of flow that passes through the inclusions, and the fringe bypassing of inclusions determine the amount of mixing and reaction enhancement for the permeability distributions considered. These results partially explain why field‐scale dispersivities are larger than laboratory derived dispersivities, where homogeneous and isotropic sediments are typically used. Further work is needed to verify the theoretical results presented here with laboratory and field experiments and to expand the relatively simple analytical expressions to consider more heterogeneous three‐dimensional permeability fields.

 

2005

S. Jeong and Werth, C. J., “Evaluation of methods to obtain geosorbent fractions enriched in carbonaceous materials that affect hydrophobic organic chemical sorption,” Environmental Science & Technology, vol. 39, no. 9, pp. 3279–3288, 2005. Publisher’s Version

Abstract
To better understand sorption, separation methods are needed to enrich soils and sediments in one or more types of carbonaceous materials (CM), especially in fine grain materials where physical separation is not possible. We evaluated a series of chemical and thermal treatment methods by applying them to four different CMs prepared in our laboratory: a humic acid (HA), a char, a soot, and a heat-treated soot (HN-soot). Before and after each treatment step, CM properties were evaluated including aqueous phase sorption with trichloroethene (TCE). Results indicate that treatment with hydrofluoric (HF) and hydrochloric acid (HCl) to remove silicate minerals, and with trifluoroacetic acid (TFA) to remove easily hydrolyzable organic matter, has relatively little effect on the humic acid mass (<19% change) and TCE sorption to this material. Subsequent treatment with NaOH to extract fulvic and humic acids results in almost complete removal of the humic acid mass (>92%) and has little to no effect on the masses of the char and two soots (<8% change) and TCE sorption to these materials. Treatment with acid dichromate to remove kerogen and humin also has little effect on masses of the char and soots (<16% change), but TCE sorption to these materials is significantly altered (by >10× in some cases), and there is strong evidence of surface oxidation based on X-ray photoelectron and diffuse reflectance Fourier transform infrared spectroscopy results. The last step, thermal treatment, which targets char removal, also destroys >96% of the soots pretreated with acid dichromate. However, when thermal treatment is applied to the original soots, <32% of these materials are destroyed. Thermal oxidation also affects sorption to one of the soots (by ~2× at low concentration), and surface oxidation is evident. These results suggest that treatment with HCl, HCl/HF, TFA, and NaOH can be applied to soils and sediments to obtain CM enrichment fractions for sorption evaluation, but that acid dichromate and heat treatment may not be appropriate for these purposes.

 

C. E. Knutson, Werth, C. J., and Valocchi, A. J., “Pore-scale simulation of biomass growth along the transverse mixing zone of a model two-dimensional porous medium,” Water Resources Research, vol. 41, no. 7, 2005. Publisher’s Version

Abstract
The success of in situ bioremediation projects depends on the mixing of contaminants and nutrients in the presence of microbes. In this work, a pore-scale model is developed to simulate biomass growth that is controlled by the mixing of an electron donor and acceptor. A homogeneous packing of cylinders representing solid grains is used as the model two-dimensional porous medium. The system is initially seeded with microbes in computational cells located at grain-water interfaces. The solutes enter the system completely unmixed; each solute is input over one half of the inlet boundary. Solute mixing is controlled by molecular diffusion transverse to the flow direction, and solutes are biotransformed according to dual Monod kinetics only where biomass is present. Simulation of biomass growth requires calculation of the water flow field as well as transport and reaction of solutes. The lattice Boltzmann method is used to obtain the flow field. Transport and reaction of the solutes is modeled by a finite volume discretization of the advection-diffusion-reaction equation. Biomass is allowed to grow and spread by means of a cellular automata algorithm. Model parameters are systematically varied to understand their effects on biomass development. Base case parameter values are obtained from batch experiments reported in the literature and are modified to achieve agreement between simulation results and previously reported micromodel experimental results. The most significant mechanisms that control biomass development are shear strength of new biomass and solute degradation rates. The biomass growth model achieves good qualitative agreement with experimental results.

 

T. Baumann and Werth, C. J., “Visualization of colloid transport through heterogeneous porous media using magnetic resonance imaging,” Colloids and Surfaces A, vol. 265, no. 1-3, pp. 2–10, 2005. Publisher’s Version

Abstract
The effects of heterogeneous grain packing on colloid transport were evaluated in flow-through columns using magnetic resonance imaging (MRI). Two columns were packed, each with a core of fine-grained silica gel surrounded by a shell of coarse-grained silica gel. In column 1, 600–850 μm silica gel was surrounded by 850–1000 μm silica gel. In column 2, 250–600 μm silica gel was surrounded by 850–1000 μm silica gel. Both columns were continuously purged with water and colloids were introduced as pulses.MRI images of column 1 showed that colloid transport in the core and shell was not distinguishable. However, colloid transport was slightly faster along the bottom of the column. T1-weighted images showed that small variations in the packing density of silica gel caused this effect. MRI images of column 2 showed that colloid transport in the core was much slower than colloid transport in the shell. Colloid exchange between the shell and the core was also observed.Colloid transport velocities and collision efficiencies were calculated from the images. In agreement with the visualization, velocities for column 1 increased from the top to bottom of the column and velocities for column 2 were greater in the shell than in the core. Collision efficiencies were calculated, but trends were not apparent because of the difficulty of applying filtration theory to heterogeneous media. Velocities from images were compared to those from conventional experiments where colloid concentrations were measured at the column effluent. While often comparable, results from the latter mask many of the complexities that control the overall rate of colloid transport. Since these complexities can give rise to very different transport behavior, it is critical to understand their effects in real systems. Hence, MRI is a technique that has the power to elucidate many of the small-scale processes that affect the behavior of colloids in the field.

 

2004

Y. Chu, Werth, C. J., Valocchi, A. J., Yoon, H., and Webb, A. G., “Magnetic resonance imaging of nonaqueous phase liquid during soil vapor extraction in heterogeneous porous media,” J. Contam. Hydrol., vol. 73, pp. 15–37, 2004.

T. Willingham, Werth, C. J., Valocchi, A. J., Krapac, I., Stark, T., and Daniel, D., “Evaluation of multi-dimensional transport through a field-scale compacted soil liner,” Journal of Geotechnical and Geoenvironmental Engineering, vol. 130, no. 9, pp. 887–895, 2004. Publisher’s Version

Abstract
A field-scale compacted soil liner was constructed at the University of Illinois at Urbana-Champaign by the U.S. Environmental Protection Agency (USEPA) and Illinois State Geological Survey in 1988 to investigate chemical transport rates through low permeability compacted clay liners (CCLs). Four tracers (bromide and three benzoic acid tracers) were each added to one of four large ring infiltrometers (LRIs) while tritium was added to the pond water (excluding the infiltrometers). Results from the long-term transport of Br− from the localized source zone of LRI are presented in this paper. Core samples were taken radially outward from the center of the Br− LRI and concentration depth profiles were obtained. Transport properties were evaluated using an axially symmetric transport model. Results indicate that (1) transport was diffusion controlled; (2) transport due to advection was negligible and well within the regulatory limits of ksat⩽1×10−7cm/s; (3) diffusion rates in the horizontal and vertical directions were the same; and (4) small positioning errors due to compression during soil sampling did not affect the best fit advection and diffusion values. The best-fit diffusion coefficient for bromide was equal to the molecular diffusion coefficient multiplied by a tortuosity factor of 0.27, which is within 8% of the tortuosity factor (0.25) found in a related study where tritium transport through the same liner was evaluated. This suggests that the governing mechanisms for the transport of tritium and bromide through the CCL were similar. These results are significant because they address transport through a composite liner from a localized source zone which occurs when defects or punctures in the geomembrane of a composite system are present.

 

J. Li and Werth, C. J., “Slow desorption mechanisms of volatile organic chemical mixtures in soil and sediment micropores,” Environmental Science & Technology, vol. 38, no. 2, pp. 440–448, 2004. Publisher’s Version

Abstract
Desorption profiles of trichloroethylene (TCE), tetrachloro- ethylene (PCE), and a TCE−PCE mixture were measured for three natural solids and four zeolites. Initial sorbed mass (Mi) in slow desorbing sites of natural solids and in micropores of zeolites were obtained from desorption profiles. In natural solids, Mi increases with recalcitrant organic matter content. In zeolites, Mi generally increases with decreasing micropore width and increasing micropore hydrophobicity, but the effect of hydrophobicity is stronger. In both natural solids and zeolites, competition between TCE and PCE causes Mi for each sorbate in the mixture to be less than or similar to that for each sorbate alone. Zeolite results indicate that micropore width affects this competition more than micropore hydrophobicity for the solids examined. Desorption in all solids was simulated with the radial diffusion model, either alone or coupled with the advection−dispersion equation when necessary; diffusion rate constants (D/R2) were obtained. In natural solids, mean values of D/R2 increase with decreasing recalcitrant organic matter content. In zeolites, values of D/R2 generally increase with increasing micropore width, while they are a weak function of hydrophobicity. In both natural solids and zeolites, competition between TCE and PCE causes D/R2 for each sorbate in the mixture to generally be larger than that for each sorbate alone. Zeolite results indicate that the effects of competition on D/R2 generally decrease with decreasing micropore width for the solids examined; a trend with micropore hydrophobicity is not apparent. For the three natural solids and four zeolites examined in this study, the similar effects of competition between TCE and PCE on values of Mi and D/R2 and the overlapping range of D/R2 values support the hypothesis that diffusion through hydrophobic micropores affects and may control slow mass transfer processes in the recalcitrant organic matter of natural solids. These results contribute to the fundamental understanding of slow mass transfer processes in natural solids, and they indicate that characterization of micropore width and polarity may be necessary to predict organic chemical transport and fate.

 

T. Baumann and Werth, C. J., “Visualisation and modelling of polystyrol colloid transport in a silicon micromodel,” Vadose Zone Journal, vol. 3, no. 2, pp. 434-443, 2004. Publisher’s Version

Abstract
A new experimental approach and complementary model analysis are presented for studying colloid transport and fate in porous media. The experimental approach combines high precision etching to create a controlled pore network in a silicon wafer (i.e., micromodel), with epifluorescent microscopy. Two different sizes of latex colloids were used; each was stained with a fluorescent dye. During an experiment, water with colloids was purged through a micromodel at different flow rates. Flow paths and particle velocities were determined and compared with flow paths calculated using a two-dimensional (2D) lattice Boltzmann (LB) model. For 50% of the colloids evaluated, agreement between measured and calculated flow paths and velocities were excellent. For 20%, flow paths agreed, but calculated velocities were less. This is attributed to the parabolic velocity profile across the micromodel depth, which was not accounted for in the 2D flow model. For 12%, flow paths also agreed, but calculated velocities were less. These colloids were close to grain surfaces, where model errors increase. Also, particle–surface interactions were not accounted for in the model; this may have contributed to the discrepancy. For the remaining 18% of colloids evaluated, neither flow paths nor velocities agreed. The majority of colloids in this last case were observed after breakthrough, when concentrations were high. The discrepancies may be due to particle–particle interactions that were not accounted for in the model. Filtration efficiencies for all colloid sizes at different flow rates were calculated from filtration theory. Attachment rates were obtained from successive images during an experiment. With these, attachment efficiencies were calculated, and these agreed with literature values. The study demonstrates that excellent agreement between experimental and model results for colloid transport at the pore scale can be obtained. The results also demonstrate that when experimental and model results do not agree, mechanistic inferences and system limitations can be evaluated.

 

A. J. Valocchi and Werth, C. J., “Web-based interactive simulation of groundwater pollutant fate and transport,” Computer Applications in Engineering Education, vol. 12, no. 2, pp. 75–83, 2004. Publisher’s Version

Abstract
A series of interactive web simulation models were developed to help students understand the coupled physical, chemical, and microbiological processes that affect the transport and fate of pollutants in groundwater. Conventional models that simulate coupled processes are often not effective learning tools because they are too complex, they suffer from cumbersome interfaces, and/or they are difficult to install and run. The web models are fully interactive Java applets that run locally through a web browser. They have graphical user interfaces, straightforward input and output fields, and rapid response times. These features enhance learning because students can rapidly visualize the impact of changes to parameter values and boundary and initial conditions, and explore the effect of different reaction processes. Presently, six different web models have been developed to explore coupled processes such as advection, longitudinal and transverse dispersion, linear or rate limited sorption, and first order decay. A web model was also developed to study the flow patterns caused by multiple pumping wells in two‐dimensional steady flow. Several examples of how the models can be used to teach students about coupled processes are discussed. Last, an assessment of the effectiveness of the models to enhance student learning is presented.

 

2003

H. Yoon, Valocchi, A. J., and Werth, C. J., “The influence of water content on soil vapor extraction,” Vadose Zone J., vol. 2, pp. 368–381, 2003.

C. Chomsurin and Werth, C. J., “Analysis of pore-scale nonaqueous phase liquid dissolution in etched silicon pore networks,” Water Resources Research, vol. 39, no. 9, pp. 1265, 2003. Publisher’s Version

Abstract
Predicting the dissolution rate of nonaqueous phase liquids (NAPLs) in groundwater is difficult, as the effects of variable pore and NAPL blob geometry are poorly understood. To elucidate these effects, fluorescence microscopy and digital image analysis were used to quantify the size and location of variably distributed NAPL blobs during dissolution in homogeneous and heterogeneous pore networks etched into silicon wafers. Results show that the dissolution rate constant (expressed as the Sherwood number, Sh) is relatively constant regardless of pore and NAPL blob geometry when the average mass transfer length scale remains constant during dissolution. Results also show that Sh increases with Peclet (Pe) between 2 and 26 and then levels off. The limiting value of Sh reached depends on the average diffusion length scale; this length scale was directly calculated and found to vary depending on the pore and NAPL blob geometry. For example, the average diffusion length scale decreases (and Sh increases) as the pore throat width to grain diameter increases. Last, results show that the volumetric NAPL content (θn) is linearly related to the specific NAPL-water interfacial area (ait) over much of the dissolution process. However, this relationship depends on the pore and blob size distribution. For example, when multipore blobs control dissolution, the relationship between these parameters will change as smaller blobs dominate dissolution at low θn. These results are important because existing mass transfer correlations do not account for limiting values of Sh that can be obtained at high Pe for the effect of blob or pore geometry on the average diffusion length scale (and therefore on Sh) or for the effect of pore geometry and transient blob size distribution on the relationship between ait and θn.

 

I. Nambi, Werth, C. J., Sanford, R. A., and Valocchi, A. J., “Pore-scale analysis of anaerobic halorespiring bacterial growth along the transverse mixing zone of an etched silicon pore network,” Environmental Science & Technology, vol. 37, no. 24, pp. 5617–5624, 2003. Publisher’s Version

Abstract
The anaerobic halorespiring microorganism, Sulfurospirillum multivorans, was observed in the pore structure of an etched silicon wafer to determine how flow hydrodynamics and mass transfer limitations along a transverse mixing zone affect biomass growth. Tetrachloroethene (PCE, an electron acceptor, 0.2 mM) and lactate (an electron donor, 2 mM) were introduced as two separate and parallel streams that mixed along a reaction line in the pore structure. The first visible biomass occupied a single line of pores in the direction of flow, a few pore bodies from the micromodel centerline. This growth was initially present as small aggregates; over time, these grew and fused to form finger-like structures with one end attached to downgradient ends of the silicon posts and the other end extending into pore bodies in the direction of flow. Biomass did not grow in pore throats as expected, presumably because shear forces were not favorable. Over the next few weeks, the line of growth migrated upward into the PCE zone and extended over a width of up to five pore spaces. When the PCE concentration was increased to 0.5 mM, the microbial biomass increased and growth migrated down toward the lactate side of the micromodel. A new analytical model was developed and used to demonstrate that transverse hydrodynamic dispersion likely caused the biomass to move in the direction observed when the PCE concentration was changed. The model was unable, however, to explain why growth migrated upward when the PCE concentration was initially constant. We postulate that this occurred because PCE, not lactate, sorbed to biofilm components and that biomass on the lactate side of the micromodel was limited in PCE. A fluorescent tracer experiment showed that biomass growth changed the water flow paths, creating a higher velocity zone in the PCE half of the micromodel. These results contribute to our understanding of biofilm growth and will help in the development of new models to describe this complex process.

 

2002

C. J. Werth and Hansen, K. M., “Modeling the effects of concentration history on the slow desorption of trichloroethene from a soil,” J. Contam. Hydrol., vol. 54, pp. 307–327, 2002.

C. Toupiol, Willingham, T., Valocchi, A. J., Werth, C. J., Krapac, I. G., Stark, T. D., and Daniel, D. E., “Long-term tritium transport through a field-scale compacted soil liner,” Journal of Geotechnical and Geoenvironmental Engineering, vol. 128, no. 8, pp. 640–650, 2002. Publisher’s Version

Abstract
A 13-year study of tritium transport through a field-scale earthen liner was conducted by the Illinois State Geological Survey to determine the long-term performance of compacted soil liners in limiting chemical transport. Two field-sampling procedures (pressure-vacuum lysimeter and core sampling) were used to determine the vertical tritium concentration profiles at different times and locations within the liner. Profiles determined by the two methods were similar and consistent. Analyses of the concentration profiles showed that the tritium concentration was relatively uniformly distributed horizontally at each sampling depth within the liner and thus there was no apparent preferential transport. A simple one-dimensional analytical solution to the advective–dispersive solute transport equation was used to model tritium transport through the liner. Modeling results showed that diffusion was the dominant contaminant transport mechanism. The measured tritium concentration profiles were accurately modeled with an effective diffusion coefficient of 6×10−4mm2/s, which is in the middle of the range of values reported in the literature.

 

C. Zhang, Werth, C. J., and Webb, A. G., “A magnetic resonance imaging study of dense nonaqueous phase liquid dissolution from angular porous media,” Environmental Science & Technology, vol. 36, no. 15, pp. 3310–3317, 2002. Publisher’s Version

Abstract
Magnetic resonance imaging (MRI) was used to determine the effects of pore-scale heterogeneity on the dissolution of a nonaqueous phase liquid (NAPL) in water-saturated flow-through columns (1.2 cm in diameter) packed with either ∼500 or ∼1000 micron diameter angular silica gel (referred to as SG500 and SG1000, respectively). Columns were contaminated with 1,3,5-trifluorobenzene at residual saturation and then purged with water at a constant Darcy velocity of 1.83 m/day. Three-dimensional 19F images were acquired every 2−5 h at an imaging resolution of 59 × 234 × 234 μm3. Imaging results show that the specific NAPL surface area (at) is linearly related to the NAPL volumetric fraction (θn) and that the constant of proportionality between these parameters is determined by the blob size and geometry distribution. Overall (expressed as the modified Sherwood number, Sh’) and intrinsic (expressed as the apparent Sherwood number, Shapt) mass transfer rate coefficients were calculated. Values of Sh’ and Shapt for SG500 were approximately three times less than those for SG1000. For both solids, Sh’ first increased or stayed the same and then decreased with decreasing θn, while Shapt generally increased with decreasing θn. These results suggest that during dissolution new flow paths were created (i.e., bypass zones were eliminated) as NAPL dissolved, decreasing the fraction of NAPL−water interfaces adjacent to pores filled with stagnant water and the average diffusion length scale. Since at for SG500 was dominated by less spherical multipore blobs (as opposed to more spherical singlets for SG1000), these results also suggest that the extent of flow bypassing (and the average diffusion length scale) increases in systems with more irregular blobs. These results are important because Sh’ correlations and a “sphere” dissolution model do not account for transient changes in the fraction of NAPL surface area that contributes to dissolution or for the effect of initial blob size and geometry distribution on this fraction.

 

J. Li and Werth, C. J., “Modeling sorption isotherms of volatile organic chemical mixtures in model and natural solids,” Environmental Toxicology and Chemistry, vol. 21, no. 7, pp. 1377–1383, 2002. Publisher’s Version

Abstract
Parameters from single‐component isotherm models were used in multicomponent isotherm models to predict the aqueous phase sorption of trichloroethylene (TCE) in the presence of tetrachloroethylene (PCE) in four zeolites, Tenax, and three natural solids. The Langmuir, the Polanyi‐Dubinin, and the Freundlich or the Langmuir‐Freundlich isotherm models were used to simulate single‐component sorption in zeolites. The Langmuir two‐site, the Polanyi‐Dubinin two‐site, and the Freundlich or the Langmuir‐Freundlich isotherm models were used to simulate single‐component sorption in Tenax and natural solids. Two‐site models have been used previously to model sorption in soils and sediments, and they combine an adsorption component (e.g., Langmuir) with a linear partitioning component. By using parameters from the different single‐component isotherm models, the multicomponent Langmuir, the ideal adsorbed solution theory, and the Polanyi theory were each used to predict multicomponent sorption. In general, the ability to predict TCE sorption in the presence of PCE depended more on the choice of the single‐component model than the multicomponent model, and better results were obtained when the Freundlich or the Langmuir‐Freundlich isotherm was used for single‐component sorption. This suggests that the more mechanistically based Langmuir and Polanyi‐type models may not adequately describe the distribution of adsorption sites in some model and natural solids. The Freundlich or the Langmuir‐Freundlich model, although empirical, has greater flexibility in characterizing sorbent heterogeneity and results in better multicomponent model predictions. However, this last statement is tenuous, because more solids must be tested against various model combinations.

 

2001

J. Li and Werth, C. J., “Evaluating competitive sorption mechanisms of volatile organic compounds in soils and sediments using polymers and zeolites,” Environmental Science & Technology, vol. 35, no. 3, pp. 568–574, 2001. Publisher’s Version

Abstract
The competitive sorption of trichloroethene (TCE) and tetrachloroethene (PCE) was investigated in three natural solids, two polymers, and four zeolites. Competition was observed in natural solids with high contents of recalcitrant organic carbon, in the glassy polymer, and in zeolites with strongly and moderately hydrophobic micropores of large (7.5 × 10 Å) and small pore widths (∼5.4 Å), respectively. Isotherm results and recalcitrant OC% values for natural solids indicate that the extent of competition between TCE and PCE is related to the amount of hard organic carbon. Gas adsorption results and the variability in C/H values suggest that natural organic matter contains micropores with varying width and polarity. Isotherm results for zeolites indicate that competition between TCE and PCE increases with increasing hydrophobicity and decreasing micropore width. We suggest that competition between volatile organic contaminants in the subsurface is controlled by competition for hydrophobic micropores in hard organic matter and that smaller more hydrophobic micropores result in stronger competition.

 

B. K. Grens and Werth, C. J., “

Durability of wood-based versus coal-based granular activated carbon

,” Journal American Water Works Association, vol. 93, no. 4, pp. 175–181, 2001.

S. M. Vera, Werth, C. J., and Sanford, R. A., “

Evaluation of different polymeric organic materials for creating conditions that favor reductive processes in groundwater

,” Bioremediation Journal, vol. 53, no. 3, pp. 169-181, 2001.

C. E. Knutson, Werth, C. J., and Valocchi, A. J., “Pore-scale modeling of dissolution from variably distributed nonaqueous phase liquid blobs,” Water Resources Research, vol. 37, no. 12, pp. 2951–2963, 2001. Publisher’s Version

Abstract
Contamination of groundwater by nonaqueous phase liquids (NAPLs) is widely recognized as a serious environmental problem. Predicting the dissolution, fate, and transport of these organic chemicals in the subsurface is challenging because geological heterogeneity exists at numerous scales. To better understand heterogeneity at the pore scale, we use the lattice Boltzmann (LB) method to simulate water flow and solute transport from distributed NAPL blobs in a two‐dimensional porous media. The LB method approximates the momentum and mass transport equations at the pore scale, easily incorporating complex boundary conditions of the porous media. The effects of NAPL blob configuration and Peclet number (Pe) on steady state mass transfer are studied at 7% and 15% NAPL saturation. We find that the solute flux out of the simulated system decreases substantially as the transverse length over which NAPL blobs are distributed decreases; for example, the solute flux is reduced by a factor of 2 by confining the NAPL blobs to only half of the transverse length. Values of Sherwood numbers determined from our simulations are slightly less than values determined from previously published mass transfer correlations. Our results indicate that pore‐scale NAPL configuration significantly affects mass transfer and that correlations should be modified to account for it. We find that the dimensionless mass transfer coefficient increases with Pe for the values used in our simulations, where the rate of increase decreases with increasing Pe. We observe that much of the variability in computed mass transfer coefficients is accounted for by differences in the NAPL‐water interfacial area at high Pe. However, at lower Pe, variability remains due to NAPL configuration.

 

2000

C. E. Schaefer, Schuth, C., Werth, C. J., and Reinhard, M., “Binary desorption isotherms of TCE and PCE from silica gel and natural solids,” Environmental Science & Technology, vol. 34, no. 20, pp. 4341–4347, 2000. Publisher’s Version

Abstract
Binary solute desorption isotherms of trichloroethylene (TCE) and tetrachloroethylene (PCE) at 100% relative humidity from silica gel and two well-characterized natural solids were investigated. Results indicated that the ideal adsorbed solution theory (IAST) was able to describe desorption isotherms for the silica gel. For the natural solids, IAST was not able to describe desorption isotherms for the full concentration range examined. Failure of IAST was greatest for the most heterogeneous sorbent, even when considering multiple sorption domains. In addition, IAST predictions worsened as nonlinear uptake mechanisms began to dominate. Several possible explanations for the failure of the IAST are given, including the possibility that complex interactions between the sorbing solutes and the sorbent may exist, causing deviations from ideal sorption behavior.

 

H. J. Castilla, Werth, C. J., and McMillan, S. A., “Structural evaluation of slow desorbing sites in model and natural solids using temperature stepped desorption profiles. 2. column results,” Environmental Science & Technology, vol. 34, no. 14, pp. 2966–2972, 2000. Publisher’s Version

Abstract
Results from temperature stepped desorption (TSD) experiments are presented and compared with simulations from the TSD model presented in the first of this two-paper series. TSD columns were filled with a sand, a sediment, a soil, or a silica gel, all at 100% relative humidity. Next, TSD columns were equilibrated with trichloroethene (TCE), initially purged at 30 °C, and then heated to 60 °C after 100, 1000, or 10 000 min of slow desorption. One γ distribution of diffusion rate constants at 30 °C and one γ distribution of diffusion rate constants at 60 °C were used to simulate column results at all three heating times for a single solid. At each heating time, diffusion rate constants of the γ distributions at 30 °C and 60 °C were used to calculated an effective activation energy, Eact,eff. Values of Eact,eff for all solids were between 47 and 94 kJ/mol, on the order of activation energy values found for diffusion in microporous solids. Between 100 and 10 000 min heating times, the value of Eact,eff increased by a factor of 1.7 for the sand and by a factor of ∼1.1 for the sediment and the soil. This suggests that diffusion occurs from micropores with a wider distribution of widths in the sand than in the other solids and that with decreasing mass remaining diffusion occurs from successively smaller width micropores. For the sediment, values of Eact,eff and 〈D/lm2〉 were lower than those in the other solids. For a given sorbate, larger width micropores are associated with smaller values of Eact,eff and larger values of D. Hence, it is likely that micropores in the sediment are both wider and longer (i.e. larger value of lm2) than those in the other solids. These results suggest that micropore geometry varies between natural solids, and it is an important parameter that must be quantified to predict rates of slow desorption.

 

C. J. Werth, McMillan, S. A., and Castilla, H. J., “Structural evaluation of slow desorbing sites in model and natural solids using temperature stepped desorption profiles. 1. model development,” Environmental Science & Technology, vol. 34, no. 14, pp. 2959–2965, 2000. Publisher’s Version

Abstract
In the first of this two-paper series, a new model is presented that simulates the effects of a temperature perturbation on the rate of slow desorption as a function of mass remaining. The model assumes slow desorption is controlled by one-dimensional diffusion from a single or many hydrophobic micropores and that the micropores of a geosorbent are defined by a γ distribution of diffusion rate constants. Simulation results indicate that during slow desorption the relative increase in flux upon heating increases with decreasing micropore width. Simulation results also indicate that the relative increase in flux upon heating increases with desorption time when diffusion occurs from successively smaller width micropores with decreasing mass remaining. In paper 2, the model is tested and used to examine micropore geometry in natural and model solids by simulating results from temperature stepped desorption (TSD) experiments.

 

1999

C. J. Werth and Reinhard, M., “Sterically-hindered counter-diffusion of isotopically-labeled trichloroethylene in silica gel and geosorbent micropores,” Column results, {Environ}. {Sci}. {Technol}., vol. 33, pp. 730–736, 1999.

K. J. Hollenbeck, Harvey, C. F., Haggerty, R., and Werth, C. J., “Estimation of continuous mass-transfer rate distributions,” J. Contam. Hydrol., vol. 37, pp. 367–388, 1999.

S. A. McMillan and Werth, C. J., “Sterically-hindered counter-diffusion of trichloroethylene isotopes in silica gel and geosorbent micropores,” Model Development, Environ. Sci. Technol., vol. 33, pp. 2178–2185, 1999.

1997

C. J. Werth, Cunningham, J., Roberts, P. V., and Reinhard, M., “Effects of grain-scale mass transfer on the transport of volatile organics through sediments: 2. column results,” Water Resources Research, vol. 33, no. 12, pp. 2727–2740, 1997. Publisher’s Version

Abstract
Trichloroethylene (TCE) elution profiles for purged and unswept columns are presented and simulated with the Distributed Dual Diffusion Model (DDDM) presented in the first of this two‐paper series. Elution profiles were measured at 15, 22, 30, and/or 60°C for a silica gel, a Livermore sand fraction, a Livermore clay and silt fraction, a Santa Clara sediment, and/or a Norwood soil, all at 100% relative humidity. Advection and dispersion control TCE transport through the vapor phase of purged columns. Diffusion controls TCE transport through the vapor phase of unswept columns. For both purged and unswept columns a fast and a slow desorbing fraction of TCE were observed. The DDDM effectively simulated both of these fractions. For the fast fraction the DDDM predicted desorption with no fitting parameters. For the slow fraction the DDDM was not predictive but it simulated desorption using either a single (for silica gel) or a gamma distribution (for soil and sediments) of micropore diffusion rate constant(s) and a micropore capacity factor. Micropore capacity factors obtained by fitting the DDDM to purged column results were used to predict the onset of slow desorption in unswept columns of the same solid.

 

J. Cunningham, Werth, C. J., Reinhard, M., and Roberts, P. V., “Effects of grain-scale mass transfer on the transport of volatile organics through sediments: 1. model development,” Water Resources Research, vol. 33, no. 12, pp. 2713–2726, 1997. Publisher’s Version

Abstract
In the first paper of this two‐paper series, we present a new model that attributes nonequilibrium sorption of moderately hydrophobia, volatile organic compounds to intragranular diffusion. The model differs from those of previous researchers in that for the first time, it combines the following elements: (1) We account for two distinct intragranular rate‐limiting diffusion processes, occurring in series and at widely different timescales; (2) we describe the slower of the two processes with a gamma distribution of diffusion rates; and (3) we use the disparity of timescales of the two processes to approximate a boundary condition for the distributed diffusion equation, allowing it to be solved analytically. The slower diffusion process is attributed to activated diffusion through very small pores, called micropores. In paper 2 [Werth et al., this issue] we evaluate the capabilities of the model and use it to interpret experimental results.

 

C. J. Werth and Reinhard, M., “Effects of temperature on trichloroethylene desorption from silica gel and natural sediments. 1. isotherms,” Environmental Science & Technology, vol. 31, no. 3, pp. 689–696, 1997. Publisher’s Version

Abstract
Aqueous phase isotherms were calculated from vapor phase desorption isotherms measured at 15, 30, and 60 °C for trichloroethylene on a silica gel, an aquifer sediment, a soil, a sand fraction, and a clay and silt fraction, all at 100% relative humidity. Isosteric heats of adsorption (Qst(q)) were calculated as a function of the sorbed concentration, q, and examined with respect to the following mechanisms: adsorption on water wet mineral surfaces, sorption in amorphous organic matter (AOM), and adsorption in hydrophobic micropores. Silica gel, sand fraction, and clay and silt fraction 60 °C isotherms are characterized by a Freundlich region and a region at very low concentrations where isotherm points deviate from log-log linear behavior. The latter is designated the non-Freundlich region. For the silica gel, values of Qst(q) (9.5−45 kJ/mol) in both regions are consistent with adsorption in hydrophobic micropores. For the natural solids, values of Qst(q) in the Freundlich regions are less than or equal to zero and are consistent with sorption on water wet mineral surfaces and in AOM. In the non-Freundlich regions, diverging different temperature isotherms with decreasing q and a Qst(q) value of 34 kJ/mol for the clay and silt fraction suggest that adsorption is occurring in hydrophobic micropores. The General Adsorption Isotherm is used to capture this adsorption heterogeneity.

 

C. J. Werth and Reinhard, M., “Effects of temperature on trichloroethylene desorption from silica gel and natural sediments. 2. kinetics,” Environmental Science & Technology, vol. 31, no. 3, pp. 697–703, 1997. Publisher’s Version

Abstract
Isothermal desorption rates were measured at 15, 30, and 60 °C for trichloroethylene (TCE) on a silica gel, an aquifer sediment, a soil, a sand fraction, and a clay and silt fraction, all at 100% relative humidity. Temperature-stepped desorption (TSD) rates were measured for these solids in columns prepared and equilibrated at 30 °C, but heated instantaneously to 60 °C after ∼1000 min of slow desorption. Fast and slow elution rates are observed for all solids. Modeling results for the fast eluting fraction of TCE show that fast desorption is controlled by diffusion through aqueous filled mesopores. Rates predicted from diffusion and surface-barrier models are compared to slow isothermal and TSD rates. Diffusion model fits are superior to surface-barrier model fits in all cases. Slow diffusion coefficients and a high activation energy calculated from silica gel data (∼34 kJ/mol) indicate that slow desorption is controlled by activated diffusion in micropores. Initial amounts of slow desorbing TCE do not affect these rates and are found to obey Polanyi’s equation. The mass adsorbed in non-Freundlich isotherm regions, where micropores are hypothesized to control adsorption, is 10 times greater than the mass adsorbed at the onset of slow desorption, suggesting that these pores are undulating in nature. TSD column results are consistent with a mechanism where slow diffusion rates are controlled by sorptive forces at hydrophobic micropore constrictions.