Kinetics of the Transformation of Halogenated Aliphatic Compounds by Iron Sulfide

The transformation of nine halogenated aliphatic compounds by 10 g/L (0.5 m 2 /L) FeS at pH 8.3 was studied in batch experiments. These compounds were as follows: pentachloroethane (PCA), 1,1,2,2- and 1,1,1,2-tetrachloroethanes (1122-TeCA and 1112-TeCA), 1,1,1- and 1,1,2-trichloroethanes (111-TCA and 112-TCA), 1,1- and 1,2-dichloroethanes (11-DCA and 12-DCA), carbon tetrachloride (CT), and tribromomethane (TBM). 11-DCA, 12-DCA, and 112-TCA showed no appreciable transformation by FeS over approximately 120 days, but the other compounds were transformed with half-lives of hours to days. PCA and 1122-TeCA underwent dehydrohalogenation faster than FeS-mediated reductive dehalogenation reactions under the conditions of these experiments. The remaining compounds for which significant transformation was observed underwent FeS-mediated reactions more rapidly than hydrolysis or dehydrohalogenation. For 1112-TeCA, the dihaloelimination product (1,1-dichloroethylene) was the only reaction product detected. For 111-TCA, CT, and TBM, hydrogenolysis products were the only products detected, although their mass recoveries were considerably less than 100%. Two simple log-linear correlations between rate constants and either one-electron reduction potentials or homolytic bond dissociation enthalpies were developed, with coefficients of determination ( R 2 values) of 0.48 and 0.82, respectively. These findings are consistent with a rate-limiting step involving homolytic bond dissociation. However, neither correlation accurately described the reactivity of all the compounds that were studied, suggesting distinctions among the mechanisms for reductive dehalogenation of these compounds by FeS or the influence of additional molecular or thermodynamic parameters on rate constants.