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.
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.
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., “
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.
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.
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.
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.
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.
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.
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 NN 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., “
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.
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.
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.
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.
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.
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.
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.
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.
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.