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

• Published in: Journal of contaminant hydrology Vol. 29; no. 4; pp. 379 - 398
• Main Authors: Johnson, Timothy L., Fish, William, Gorby, Yuri A., Tratnyek, Paul G.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01-03-1998; Elsevier Science
• Subjects: Corrosion; Dechlorination; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Exact sciences and technology; groundwater contamination; Groundwater remediation; Hydrogeology; Hydrology. Hydrogeology; Iron oxide; Passive film; Pollution, environment geology; water pollution; water purification; Groundwater remediation; Dechlorination; Iron oxide; Passive film; Corrosion; chlorides; ligands; models; corrosion; oxides; iron oxides; ferrous iron; ground water; oxidation; aqueous solutions; concentration; degradation; electrolytes; pollution; halides; saturation; carbon; methane; iron; kinetics; anions
• ISSN: 0169-7722; 1873-6009
• DOI: 10.1016/S0169-7722(97)00063-6

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.