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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.