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Abstract: A one dimensional, multiple reaction pathway model of the dechlorination reactions of trichloroethylene (TCE) and tetrachloroethylene (PCE) as these species pass through a zero valent iron permeable reactive barrier (PRB) was produced. Three different types of rate equations were tested; first order, surface controlled with interspecies competition, and surface controlled with inter and intra species competition. The first order rate equations predicted the most accurate results when compared to actual data from permeable reactive barriers. Sensitivity analysis shows that the most important variable in determining TCE concentration in the barrier is the first order rate constant for the degradation of TCE. The velocity of the water through the barrier is the second most important variable determining TCE concentration. For PCE the concentration in the barrier is most sensitive to the velocity of the water and to the first order degradation rate constant for the PCE to dichloroacetylene reaction. Overall, zero valent iron barriers are more effective for the treatment of TCE than PCE.
Permeable reactive barriers (PRB) containing zero-valent iron (ZVI) have been used for the in situ remediation of groundwater contaminated with trichloroethylene (TCE) for almost two decades; however, ZVI is susceptible to passivation over time, which greatly reduces its treatment efficiency. Several recent studies have suggested that electrically-induced reduction (EIR), the application of low-level direct current, may restore the reactivity of passivated ZVI in situ. In this study, a continuous-flow column reactor was fabricated to treat TCE-contaminated groundwater and assess the effects of low-voltage direct current (0-12 V) on abiotic TCE reduction. In experiments with partially passivated ZVI, application of current increased the rate and the extent of abiotic TCE reduction; both were correlated with voltage. Based on calculated reaction rate coefficients, TCE reduction in passivated ZVI is a first-order reaction. While several mechanisms contribute to the abiotic reduction of TCE in passivated ZVI, they are difficult to isolate experimentally.
Evaluates the effects of external cathodic current on the trichloroethylene (TCE) degradation rate by zero-value iron (ZVI). Sets up a bench scale test and a reactor where electrolytic reduction can take place.
Trichloroethylene (TCE) is a volatile, chlorinated aliphatic organic compound. It has been used ubiquitously as an ingredient in industrial cleaning agents and as a degreasing agent, which has resulted in widespread contamination of groundwater. Since TCE is a highly oxidized compound, reduction reactions are considered a promising way to treat it. Permeable reactive barriers (PRB) containing highly-reducing reactive media [e.g., zero-valent iron (Fe° or ZVI)] have been successfully used in situ to remediate TCE in groundwater. There have been more than 200 ZVI PRBs installed worldwide since the mid-1990s. Despite their promise, ZVI PRBs are susceptible to passivation over time, largely due to oxidation of the Fe° by dissolved oxygen or nitrate. Several recent studies have suggested that electrically-induced reduction (EIR) is a promising approach to restore reductive activity in passivated ZVI PRBs. The overall objectives of this study were to simulate in the laboratory a ZVI PRB to remediate TCE-contaminated groundwater and systematically evaluate the effect of applying direct current. Results suggest that: (1) any enhancements in the rate or extent of TCE removal in `fresh' ZVI that may be the result of application of direct current are indistinguishable from those due to TCE reduction by ZVI alone; (2) upon application of direct current (6V and 12V) to partially passivated ZVI, a significant improvement in TCE reduction was observed; (3) the longevity of ZVI de-passivated by EIR was not determined, but preliminary results suggest the observed effect may be short-lived and be due to direct reduction of TCE by electrons; and (4) experiments in a partially passivated ZVI-sand column suggest that observed enhancements in TCE removal are correlated with voltage, with higher current densities resulting in faster rates of TCE reduction.