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.

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

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

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 CaCl(2) and Na(2)CO(3) at four different saturation states (Ω = [Ca(2+)][CO(3)(2-)]/K(spCaCO(3))) 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 CaCO(3) 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 (Ω(v) = 2.8 for vaterite) to 95% at the highest saturation state (Ω(v) = 4.5). Fluorescent tracer tests conducted before and after CaCO(3) precipitation indicate that pore spaces were occluded by CaCO(3) 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 CO(2) sequestration.

[1] Transverse dispersion across adjacent streamlines can control the amount of mixing and reaction between one or more contaminants and a limiting substrate along the fringes of groundwater plumes. Streamlines in groundwater converge and diverge in heterogeneous porous media, depending on the permeability distribution. When flow is focused in a high-permeability zone, the distance required for a solute to cross a given number of streamlines decreases, and the time allowed for mixing and reaction is reduced. Because the first effect outweighs the latter, the overall result is an enhancement of transverse mixing and reaction. Here we develop a conceptual model of heterogeneous two-dimensional structures facilitating flow focusing. We use the conceptual model to develop simple analytical expressions quantifying the extent to which mixing and reaction are enhanced when flow focusing occurs and compare these to results of numerical simulations. Significant enhancement of transverse mixing and reaction by flow focusing is observed; for the cases considered, flow focusing enhances the amount of reaction by a factor ranging from 1.8 to 11.9. The relatively simple analytical expressions demonstrate that the fraction of the domain height made up by highpermeability inclusions, the fraction of flow that passes through the inclusions, and the fringe bypassing of inclusions determine the amount of mixing and reaction enhancement for the permeability distributions considered. These results partially explain why field-scale dispersivities are larger than laboratory derived dispersivities, where homogeneous and isotropic sediments are typically used. Further work is needed to verify the theoretical results presented here with laboratory and field experiments and to expand the relatively simple analytical expressions to consider more heterogeneous three-dimensional permeability fields.Citation: Werth, C. J., O. A. Cirpka, and P. Grathwohl (2006), Enhanced mixing and reaction through flow focusing in heterogeneous porous media, Water Resour. Res., 42, W12414,

Catalytic nitrate reduction was evaluated for the purpose of drinking water treatment. Common anions present in natural waters and humic acid were evaluated for their effects on NO3(-) hydrogenation over a bimetallic supported catalyst (Pd-Cu/gamma-Al2O3). Groundwater samples, with and without powder activated carbon (PAC) pretreatment, were also evaluated. In the absence of inhibitors the NO3- reduction rate was 2.4 x 10(-01) L/min g cat. However, the addition of constituents (SO4(2-), SO3(2-), HS-, CI-, HCO3-, OH-, and humic acid) on the order of representative concentrations for drinking water decreased the NO3- reduction rate. Sulfite, sulfide, and elevated chloride decreased the NO3- reduction rate by over 2 orders of magnitude. Preferential adsorption of Cl- inhibited NO3- reduction to a greater extent than NO2- reduction. Partial regeneration of catalysts exposed to SO3(2-) was achieved by using a dilute hypochlorite solution, however Cu dissolution occurred. Dissolved constituents in the groundwater sample decreased the NO3- reduction rate to 3.7 x 10(-03) L/min g cat and increased ammonia production. Removal of dissolved organic matter from the groundwater using PAC increased the NO3- reduction rate to 5.06 x 10(-02) L/min g cat and decreased ammonia production. Elemental analyses of catalysts exposed to the natural groundwater suggest that mineral precipitation may also contribute to catalyst fouling.

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

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