Charles J. Werth

Critical review of Pd-based catalytic treatment of priority contaminants in 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.

Effects of Temperature on Trichloroethylene Desorption from Silica Gel and Natural Sediments. 2. Kinetics

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.

Effects of grain-scale mass transfer on the transport of volatile organics through sediments 2. Column results

Trichloroethylene (TCE) elution profiles for purged and unswept columns are presented and simulated with the Distributed Dual Diffusion Model (DDDM) presented in the first of this two-paper series. Elution profiles were measured at 15, 22, 30, and/or 60°C for a silica gel, a Livermore sand fraction, a Livermore clay and silt fraction, a Santa Clara sediment, and/or a Norwood soil, all at 100% relative humidity. Advection and dispersion control TCE transport through the vapor phase of purged columns. Diffusion controls TCE transport through the vapor phase of unswept columns. For both purged and unswept columns a fast and a slow desorbing fraction of TCE were observed. The DDDM effectively simulated both of these fractions. For the fast fraction the DDDM predicted desorption with no fitting parameters. For the slow fraction the DDDM was not predictive but it simulated desorption using either a single (for silica gel) or a gamma distribution (for soil and sediments) of micropore diffusion rate constant(s) and a micropore capacity factor. Micropore capacity factors obtained by fitting the DDDM to purged column results were used to predict the onset of slow desorption in unswept columns of the same solid.

Pore-scale study of transverse mixing induced CaCO3 precipitation and permeability reduction in a model subsurface sedimentary system

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.

Pore-scale modeling of dissolution from variably distributed nonaqueous phase liquid blobs

Contamination of groundwater by nonaqueous phase liquids (NAPLs) is widely recognized as a serious environmental problem. Predicting the dissolution, fate, and transport of these organic chemicals in the subsurface is challenging because geological heterogeneity exists at numerous scales. To better understand heterogeneity at the pore scale, we use the lattice Boltzmann (LB) method to simulate water flow and solute transport from distributed NAPL blobs in a two-dimensional porous media. The LB method approximates the momentum and mass transport equations at the pore scale, easily incorporating complex boundary conditions of the porous media. The effects of NAPL blob configuration and Peclet number (Pe) on steady state mass transfer are studied at 7% and 15% NAPL saturation. We find that the solute flux out of the simulated system decreases substantially as the transverse length over which NAPL blobs are distributed decreases; for example, the solute flux is reduced by a factor of 2 by confining the NAPL blobs to only half of the transverse length. Values of Sherwood numbers determined from our simulations are slightly less than values determined from previously published mass transfer correlations. Our results indicate that pore-scale NAPL configuration significantly affects mass transfer and that correlations should be modified to account for it. We find that the dimensionless mass transfer coefficient increases with Pe for the values used in our simulations, where the rate of increase decreases with increasing Pe. We observe that much of the variability in computed mass transfer coefficients is accounted for by differences in the NAPL-water interfacial area at high Pe. However, at lower Pe, variability remains due to NAPL configuration.

Pore-Scale Simulations of Gas Displacing Liquid in a Homogeneous Pore Network Using the Lattice Boltzmann Method

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 (

Enhanced activity and selectivity of carbon nanofiber supported Pd catalysts for nitrite reduction

M

Comparative assessment of the environmental sustainability of existing and emerging perchlorate treatment technologies for drinking water.

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