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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.
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.
Abstract: Trichloroethylene (TCE) is one of the most common groundwater pollutants in the United States and is a suspected carcinogen. The United States Environmental Protection Agency (EPA) estimated that between 9% and 34% of the drinking water sources in the United States may contain TCE, and have set a maximum contaminant level of 5 [mu]g/L for drinking water. Traditional treatment technologies such as granular activated carbon and air stripping have only had marginal success at removing TCE from contaminated sites. Chemical oxidation processes have provided a promising alternative to traditional treatment methods. The objective of this research was to examine the conditions under which zero valent iron (Fe0) activates persulfate anions to produce sulfate free radicals, a powerful oxidant used for destroying organic contaminants in water. With batch experiments, it was found that persulfate activated by zero valent iron removed TCE more effectively than persulfate oxidation activated by ferrous iron. This laboratory study also investigated the influence of pH (from 2 to 10) on TCE removal. TCE was prepared in purified water and a fixed persulfate/TCE molar ratio was employed in all tests. The results indicated that this reaction occurred over a wide range of pH values. The production and destruction of daughter products cis 1,2 dichloroethylene and vinyl chloride were observed. The effect of persulfate dose on this reaction was also studied. Results showed that a molar ratio of 10/1/1 (persulfate/ZVI/TCE) yielded over 95 percent TCE destruction. Increasing the persulfate dose resulted in greater TCE destruction as well as destruction of the daughter products. Kinetic experiments at a molar ratio of 10/1/1 (persulfate/ZVI/TCE) show that approximately 90 percent of the TCE was destroyed in less than 15 minutes.
Groundwater contamination with chlorinated solvents, such as trichloroethylene or TCE, is a major environmental challenge. The development of innovative, efficient, and sustainable remediation technologies is needed. In this study, iron electrolysis is assessed as a potential technology for the remediation of TCE contaminated groundwater. A three-stage experimental program is conducted in this study: (i) the evaluation of chemical changes in the electrolyte due to iron electrolysis in batch reactors, as well as the investigation of TCE degradation rates; (ii) the optimization of electrochemical operating variables using a multivariable statistical approach; and (iii) the assessment of a proposed electrochemical system under flow conditions for the remediation of groundwater contaminated with TCE. The first phase of this study focuses on the temporal chemical changes in the electrolyte due to iron electrolysis and TCE degradation kinetics with various electrode materials. Unlike an inert anode, an iron anode releases Fe (II) into the system and generates a highly reducing electrolyte condition (lower oxidation-reduction potential). This reducing electrolyte condition facilitates the reductive dechlorination of TCE. The TCE dechlorination rate of various anode materials is investigated. The iron anode coupled with a copper foam cathode provides the best TCE dechlorination performance. In the second stage, the significance of changes in operating variables on final TCE elimination efficiency (FEE) and specific energy consumption (SEC) is investigated using an iron anode-copper cathode couple. Under the same total charge conditions, changes in applied current impact FEE the most. For SEC, the ionic conductivity of the electrolyte is the most influential parameter. In the final stage, a three-electrode (the sequence of an iron anode, a copper foam cathode, and an MMO anode) electrochemical system is implemented for the remediation of TCE in groundwater under flow conditions. Higher TCE removal efficiencies are reached at a lower flow rate, supporting the conclusion that a longer residence time of the electrolyte improves TCE removal efficiency. Conversely, the treating capacity of TCE is higher for a higher flow rate.
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.
Performs column studies to determine the effects of pH and EDTA (ethylenediaminetetraacetic acid) concentration on the degradation of TCE (trichloroethene) by ZVI (zero-valent iron). The aim of the study is to provide additional kinetic data for more efficient methods of using ZVI for the remediation of TCE-contaminated groundwaters.