Effects of solution composition and pH on the reductive dechlorination of hexachloroethane by iron sulfide

Transition metal sulfide minerals are under investigation as potentially important abiotic reductants for chlorinated organic pollutants in anaerobic environments. This paper describes parametric rate studies done to evaluate the influence of environmental variables such as pH and ionic and organic solution composition on the reductive dechlorination of hexachloroethane (HCA) by FeS (poorly crystalline mackinawite). Results indicate that the reaction takes place at the mineral surface and is strongly PH-dependent. The influence of pH was explained by an acid/base equilibrium between two FeS surface species with different reactivities. Tetrachloroethylene was the principal reaction product, with pentachloroethane (PCA) as a minor intermediate and trichloroethylene, cis-1,2-dichloroethylene, and acetylene as minor products. Detection of PCA and the insensitivity of the reaction to numerous inorganic and organic solution species is consistent with an outer-sphere HCA dechlorination pathway involving two successive one-electron transfers. 2,2'-Bipyridine and 1,10-phenanthrolene significantly increased the rate of HCA dechlorination by FeS, which was explained by the participation of delocalized pi* molecular orbitals in the electron-transfer reaction. Cysteine and methionine were found to slow, but not stop, the reaction rate, and this was attributed to adsorption of thiol and sulfide functional groups to FeS surface iron atoms, causing an energetic or steric barrier to electron transfer. Rapid dechlorination rates and the insensitivity of the dechlorination reaction to numerous ionic and organic species suggest that FeS-mediated reductive dechlorination may be an important transformation pathway in natural systems.