Wang Jiankang, Farrell James
Department of Chemical and Environmental Engineering, University of Arizona, Tucson, Arizona 85721, USA.
Environ Sci Technol. 2003 Sep 1;37(17):3891-6. doi: 10.1021/es0264605.
Metallic iron filings are commonly employed as reducing agents in permeable barriers used for remediating groundwater contaminated by chlorinated solvents. Reactions of trichloroethylene (TCE) and tetrachloroethylene (PCE) with zerovalent iron were investigated to determine the role of atomic hydrogen in their reductive dechlorination. Experiments simultaneously measuring dechlorination and iron corrosion rates were performed to determine the fractions of the total current going toward dechlorination and hydrogen evolution. Corrosion rates were determined using Tafel analysis, and dechlorination rates were determined from rates of byproduct generation. Electrochemical impedance spectroscopy (EIS) was used to determine the number of reactions that controlled the observed rates of chlorocarbon disappearance, as well as the role of atomic hydrogen in TCE and PCE reduction. Comparison of iron corrosion rates with those for TCE reaction showed that TCE reduction occurred almost exclusively via atomic hydrogen at low pH values and via atomic hydrogen and direct electron transfer at neutral pH values. In contrast, reduction of PCE occurred primarily via direct electron transfer at both low and neutral pH values. At low pH values and micromolar concentrations, TCE reaction rates were faster than those for PCE due to more rapid reduction of TCE by atomic hydrogen. At neutral pH values and millimolar concentrations, PCE reaction rates were faster than those for TCE. This shift in relative reaction rates was attributed to a decreasing contribution of the atomic hydrogen reaction mechanism with increasing halocarbon concentrations and pH values. The EIS data showed that all the rate limitations for TCE and PCE dechlorination occurred during the transfer of the first two electrons. Results from this study show that differences in relative reaction rates of TCE and PCE with iron are dependent on the significance of the reduction pathway involving atomic hydrogen.
金属铁屑通常用作渗透屏障中的还原剂，该渗透屏障用于修复被氯代溶剂污染的地下水。研究了三氯乙烯（TCE）和四氯乙烯（PCE）与零价铁的反应，以确定原子氢在其还原脱氯中的作用。进行了同时测量脱氯和铁腐蚀速率的实验，以确定总电流中用于脱氯和析氢的比例。使用塔菲尔分析确定腐蚀速率，根据副产物生成速率确定脱氯速率。采用电化学阻抗谱（EIS）来确定控制观察到的氯代烃消失速率的反应数量，以及原子氢在TCE和PCE还原中的作用。铁腐蚀速率与TCE反应速率的比较表明，在低pH值下，TCE的还原几乎完全通过原子氢进行，而在中性pH值下则通过原子氢和直接电子转移进行。相比之下，PCE的还原在低pH值和中性pH值下主要通过直接电子转移进行。在低pH值和微摩尔浓度下，由于原子氢对TCE的还原更快，TCE的反应速率比PCE快。在中性pH值和毫摩尔浓度下，PCE的反应速率比TCE快。相对反应速率的这种变化归因于随着卤代烃浓度和pH值的增加，原子氢反应机制的贡献逐渐减小。EIS数据表明，TCE和PCE脱氯的所有速率限制都发生在前两个电子的转移过程中。本研究结果表明，TCE和PCE与铁的相对反应速率差异取决于涉及原子氢的还原途径的重要性。
