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](0)). 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(CCl4)), and zero order for the appearance of dissolved Fe(2+) (k(Fe2+)). The rate of dechlorination exhibits saturation kinetics with respect to [P](0), 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(Fe2+) 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(CCl4) (and k(Fe2+)). 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. (C) 1998 Elsevier Science B.V.