Reduction of polychlorinated ethanes and carbon tetrachloride by structural Fe(II) in smectites

Ferrous iron associated with clay minerals can be important for the reductive transformation of organic contaminants in anoxic soils and groundwaters. We investigated the reactivity of structural Fe(II) in ferruginous smectite for the reduction of a series of polychlorinated alkanes (hexa-, penta-, 1,1,1,2-and 1,1,2,2-tetrachloroethane, and carbon tetrachloride (CCl4)) in laboratory batch reactors. Evaluation of reaction kinetics, product distribution, and C-isotope fractionation suggest that polychlorinated ethanes containing three alpha-Cl atoms reacted via reductive beta-elimination to the corresponding ethenes while CCl4-reduction leads predominantly to the formation of chloroform. Reduction kinetics followed a typical biphasic behavior characteristic of the presence of two types of Fe(II) species exhibiting different reactivity in the octahedral sheet of smectites, and reaction rate constants were pH-independent. Dehydrochlorination reactions of chloroethanes containing at least one beta-H atom were found to compete with or even dominate over the reduction reaction with increasing suspension pH. Reference experiments in homogeneous solution and with non-reduced smectite performed in the pH range of 5.5-8.5 suggest that the HCl-elimination is not catalyzed at mineral surfaces. From the observed slow transformation of chloroalkanes, we hypothesize that structural Fe(II) in smectites will be important mainly as a reductant in the subsurface once iron(hydr)oxides have been reductively dissolved.