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