Degradation of carbon tetrachloride by iron metal: Complexation effects on the oxide surface |
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Authors: | Timothy L. Johnson William Fish Yuri A. Gorby Paul G. Tratnyek |
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Affiliation: | aDepartment of Environmental Science and Engineering, Oregon Graduate Institute of Science and Technology, P.O. Box 91000, Portland, OR 97291-1000, USA;bEnvironmental Microbiology, Battelle Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, USA |
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Abstract: | Dehalogenation of chlorinated aliphatic contaminants at the surface of zero-valent iron metal (Fe0) is mediated by the thin film of iron (hydr)oxides found on Fe0 under environmental conditions. To evaluate the role this oxide film plays in the reduction of chlorinated methanes, carbon tetrachloride (CCl4) degradation by Fe0 was studied under the influence of various anions, ligands, and initial CCl4 concentrations ([P]o). Over the range of conditions examined in these batch experiments, the reaction kinetics could be characterized by surface-area-normalized rate constants that were pseudo-first order for CCl4 disappearance (kCCl4), and zero order for the appearance of dissolved Fe2+ (kFe2+). The rate of dechlorination exhibits saturation kinetics with respect to [P]o, suggesting that CCl4 is transformed at a limited number of reactive surface sites. Because oxidation of Fe0 by CCl4 is the major corrosion reaction in these systems, kFe2+ also approaches a limiting value at high CCl4 concentrations. The adsorption of borate strongly inhibited reduction of CCl4, but a concomitant addition of chloride partially offset this effect by destabilizing the film. Redox active ligands (catechol and ascorbate), and those that are not redox active (EDTA and acetate), all decreased kCCl4 (and kFe2+). Thus, it appears that the relatively strong complexation of these ligands at the oxide–electrolyte interface blocks the sites where weak interactions with the metal oxide lead to dehalogenation of chlorinated aliphatic compounds. |
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Keywords: | Iron oxide Passive film Groundwater remediation Dechlorination Corrosion |
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