The influence of redox potential on the degradation of halogenated methanes

To determine the influence of redox potential on the reaction mechanism and to quantify kinetics of the dechlorination by digester sludge, the test compounds trichlorofluoromethane (CFCl₃), carbon tetrachloride (CCl₄), and chloroform (CHCl₃) were incubated in the presence of sludge and variable concentrations of reducing agent. Different sources of dehalogenation were examined, including live sludge and heat-killed sludge, and abiotic mechanisms were quantified in the absence of sludge. Batch incubations were done under redox conditions ranging from +534 to -348 mV. The highest rates for the dehalogenation of the three compounds were observed at -348 mV. The dechlorination rate of all the compounds by the heat-resistant catalysts was approximately twofold higher than the live treatments. It was proposed that the higher degradation rates by heat-killed sludge were due to the absence of physical barriers such as cell wall and cell membranes. There was no abiotic dechlorination of CFCl₃, whereas CCl₄ and CHCl₃ were both reduced in the absence of sludge catalyst by Ti (III) citrate at ≥2.5 mM. The degradation pathways of CFCl₃ and CHCl₃ appeared to be only partially reductive since the production of reduced metabolites was low in comparison with the total amount of original halogenated compounds degraded. For CFCl₃, the partial reductive degradation implied that different intra- and extra-cellular pathways were concurrent. The Gibbs free energy and the redox potential for the dehalogenation reactions utilizing Ti (III) citrate and acetate as electron donors are reported here for the first time.