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 VersionAbstract
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.
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 VersionAbstract
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 VersionAbstract
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 VersionAbstract
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 VersionAbstract
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 VersionAbstract
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 VersionAbstract
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 VersionAbstract
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&#x2212;">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&#x2212;">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 VersionAbstract
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.
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 VersionAbstract
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 VersionAbstract
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 VersionAbstract
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 VersionAbstract
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 VersionAbstract
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 VersionAbstract
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 VersionAbstract
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 VersionAbstract
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.
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 VersionAbstract
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 VersionAbstract
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.