Groundwater contaminated by hazardous chlorinated compounds, especially chlorinated ethenes, continues to be a significant environmental problem in industrialized nations. The conventional treatment methods of activated carbon adsorption and air-stripping successfully remove these compounds by way of transferring them from the water phase into the solid or gas phase. Catalysis is a promising approach to remove chlorinated compounds completely from the environment, by converting them into safer, non-chlorinated compounds. Palladium-based materials have been shown to be very effective as hydrodechlorination catalysts for the removal of chlorinated ethenes and other related compounds. However, relatively low catalytic activity and a propensity for deactivation are significant issues that prevent their widespread use in groundwater remediation. Palladiumon-gold bimetallic nanoparticles, in contrast, were recently discovered to exhibit superior catalyst activity and improved deactivation resistance. This new type of material is a significant next-step in the development of a viable hydrodechlorination catalysis technology.

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The aqueous-phase hydrodechlorination (HDC) of trichloroethene (TCE) is an important chemical reaction for water pollution control, for which unsupported palladium-ongold and palladium nanoparticles (Pd/Au and Pd NPs) definitively show the beneficial effects of gold on palladium catalysis. The observed batch reactor kinetics can be erroneously oversimplified when concentration and mass transfer effects are neglected. A comprehensive treatment of NP catalysis is presented here using Pd-based NPs as the catalytic colloid and TCE HDC as the model reaction. Mass transfer effects were quantified for three specific compositions (Pd/Au NPs with 30% and 60% Pd surface coverages, and pure Pd NPs) by analyzing the observed reaction rates as functions of stirring rate and initial catalyst charge. The largest effect on observed reaction rates came from gas-liquid mass transfer. The TCE HDC reaction was modeled as a Langmuir-Hinshelwood mechanism involving competitive chemisorption of dihydrogen and TCE for all three NP compositions. Differences in adsorption affinities of the reactant molecules for the Pd/Au and Pd surfaces are suggested as responsible for the observed difference in TCE reaction order at high TCE concentrations; that is, first-order for Pd/Au NPs and non-first-order for Pd NPs.

Although potassium permanganate (KMnO4) flushing is commonly used to destroy chlorinated solvents in groundwater, many of the problems associated with this treatment scheme have not been examined in detail. We conducted a KMnO4 flushing experiment in a large sand-filled flow tank (L x W x D = 180 cm x 60 cm x 90 cm) to remove TCE emplaced as a DNAPL in a source zone. The study was specifically designed to investigate cleanup progress and problems of pore plugging associated with the dynamics of the solid-phase reaction front (i.e., MnO2) using chemical and optical monitoring techniques. Ambient flow through the source zone formed a plume of dissolved TCE across the flow tank. The volume and concentration of TCE plume diminished with time because of the in situ oxidation of the DNAPL source. The migration velocity of the MnO2 reaction front decreased with time, suggesting that the kinetics of the DNAPL oxidation process became diffusion-controlled because of the pore plugging. A mass balance calculation indicated that only approximately 18% of the total applied KMnO4 (MnO4- = 1250 mg/ L) participated in the oxidation reaction to destroy approximately 41% of emplaced TCE. Evidently, the efficiency of KMnO4 flushing scheme diminished with time due to pore plugging by MnO2 and likely CO2, particularly in the TCE source zone. In addition, the excess KMnO4 used for flushing may cause secondary aquifer contamination. One needs to be concerned about the efficacy of KMnO4 flushing in the field applications. Development of a new approach that can provide both contaminant destruction and plugging/ MnO4- control is required.