Degradation of carbon tetrachloride by iron metal: Complexation effects on the oxide surface

Dehalogenation of chlorinated aliphatic contaminants at the surface of zero-valent iron metal (Fe 0) is mediated by the thin film of iron (hydr)oxides found on Fe 0 under environmental conditions. To evaluate the role this oxide film plays in the reduction of chlorinated methanes, carbon tetrachloride (CCl 4) degradation by Fe 0 was studied under the influence of various anions, ligands, and initial CCl 4 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 CCl 4 disappearance ( k CCl 4 ), and zero order for the appearance of dissolved Fe 2+ ( k Fe 2+ ). The rate of dechlorination exhibits saturation kinetics with respect to [ P] o, suggesting that CCl 4 is transformed at a limited number of reactive surface sites. Because oxidation of Fe 0 by CCl 4 is the major corrosion reaction in these systems, k Fe 2+ also approaches a limiting value at high CCl 4 concentrations. The adsorption of borate strongly inhibited reduction of CCl 4, 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 k CCl 4 (and k Fe 2+ ). 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.