Influence of chlorinated solvents on the corrosion of iron in borate buffer and in simulated groundwater

Abstract

Contact with granular iron has emerged as a significant remediation technology for groundwater contaminated with chlorinated aliphatic hydrocarbons. The degradation of halogenated compounds by iron is a charge transfer process involving oxidation of iron and reduction of the organic compounds. To refine our understanding of the mechanism and kinetics of the charge transfer process, electrochemical and spectroscopic measurements were performed on iron electrodes in borate buffer and in simulated groundwater solutions of calcium carbonate and potassium bromide. These experiments, performed in the presence of two model compounds, a degradable compound (carbon tetrachloride) and a non-degradable compound (dichloromethane), indicated that carbon tetrachloride acts as an oxidizer of the iron surface while dichloromethane is nonreactive i.e., a non-oxidizer.

Based on electrochemical and spectral evidence, a new conceptual model of the reductive dehalogenation of chlorinated aliphatic hydrocarbons by iron is proposed. According to this model, the introduction of an oxidizer such as carbon tetrachloride will result in surface film formation. The composition and protective properties of these films were shown to be dependent on the corrosion behaviour of the iron prior to and after its exposure to the oxidizer, as well as on the ionic composition of the water.

In borate buffer, magnetite and hydrated magnetite were identified as the final products of the surface reactions, while carbonate-containing green rust complexes were detected in aqueous solutions of calcium carbonate. In these two solutions, the introduction of the oxidizer had no effect on the resistance of the iron/solution interface but resulted in an increase of the corrosion rate.

In aqueous solutions of potassium bromide, magnetite, green rusts, Fe(OH)3 and a-FeOOH were identified as the final products of the surface reactions while the resistance of the iron/solution interface increased and the corrosion rate decreased after exposure to carbon tetrachloride. The changes in resistance and corrosion rate could not have been anticipated based on measurements of the potential, since similar positive shifts of potential were observed in all the solutions after exposure to the oxidizer. These changes are, however, in agreement with the fact that, prior to the introduction of the oxidizer, the presence of a film was detected in the potassium bromide solutions; there was no evidence of such a film in the borate and calcium carbonate solutions.

With regard to the practical use of iron for the remediation of groundwater contaminated with chlorinated aliphatic hydrocarbons, the results suggest that surface films may, or may not, affect reaction rates. For example, magnetite and green rust compounds, being non-protective, will not prevent charge transfer at the iron/solution interface; charge transfer will however proceed at a slower rate than on a bare metal surface. In addition, in the case of green rust compounds, they may also play a role as reductants themselves. For Fe9OH)3 and a-FeOOH films, however, the situation is different. Being more protective, their presence could result in localized passivation of the iron, as seen in this study, and therefore be detrimental from a technological point-of-view.