Competitive Effects on the Dechlorination of Chlorinated Aliphatic Hydrocarbons by Zero-valent Iron

Dechlorination of chlorinated aliphatic hydrocarbons (CAHs) by zero-valent iron (Fe 0 ) was found to be influenced by the competitive effects exerted by other groundwater contaminants. Laboratory column study of the competitive effects on CAH dechlorination by Fe 0 indicated that the presence of 1,1,1-trichloroethane (1,1,1-TCA) in the trichloroethylene (TCE)-contaminated groundwater could decrease the normalized dechlorination rate constant (k SA ) of TCE from 3.04 x 10 -2 to 2.74 x 10 -2 mL m -2 hr -1 . In a similar fashion, introduction of chloroform (TCM) into the synthetic groundwater containing TCE and 1,1,1-TCA led to a 40 to 54% drop in TCE and 1,1,1-TCA k SA , thus indicating competition among TCE, 1,1,1-TCA and TCM during dechlorination reactions induced by Fe 0 . Activation energy ranging from 34.3 to 53.7 kJ/mol for the simultaneous dechlorination of TCE, 1,1,1-TCA and TCM by Fe 0 showed that the process of the electron transfer from Fe 0 to the CAHs is the dominant step limiting the rate of the dechlorination reactions so that the electron released from Fe 0 is most likely in competition with TCE, 1,1,1-TCA and TCM during the dechlorination reactions. In addition to CAHs, abiotic reduction of hexavalent chromium [Cr(VI)] by Fe 0 also exerted effects on TCE dechlorination leading to a 31% drop in TCE k SA after the addition of Cr(VI) into the TCE-contaminated groundwater. Groundwater geochemical factors such as alkalinity and contaminant concentration could potentially influence competition among TCE, 1,1,1-TCA, TCM and Cr(VI) during the abiotic reduction of chemical substances by Fe 0 .