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 (h ydr)oxides found on Fe-0 under environmental conditions. To evaluate t he role this oxide film plays in the reduction of chlorinated methanes , carbon tetrachloride (CCl4) degradation by Fe-0 was studied under th e influence of various anions, ligands, and initial CCl4 concentration s ([P](0)). Over the range of conditions examined in these batch exper iments, the reaction kinetics could be characterized by surface-area-n ormalized rate constants that were pseudo-first order for CCl4 disappe arance (k(CCl4)), and zero order for the appearance of dissolved Fe2(k(Fe2+)). The rate of dechlorination exhibits saturation kinetics wit h respect to [P](0), suggesting that CCl4 is transformed at a limited number of reactive surface sites. Because oxidation of Fe-0 by CCl4 is the major corrosion reaction in these systems, k(Fe2+) also approache s a limiting value at high CCl4 concentrations. The adsorption of bera te strongly inhibited reduction of CCl4, but a concomitant addition of chloride partially offset this effect by destabilizing the film. Redo x active ligands (catechol and ascorbate), and those that are not redo x 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 interac tions with the metal oxide lead to dehalogenation of chlorinated aliph atic compounds. (C) 1998 Elsevier Science B.V.