The Influence of Redox Potential on the Degradation of Halogenated Methanes

Abstract

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 > or =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 extracellular 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.

Biodegradability of chlorinated solvents and related chlorinated aliphatic compounds

“…Like corrinoids, another reduced porphyrinogen coenzyme, hematin, has been shown to catalyze the reduction of CFC-11 (Lovley & Woodward 1992) and CFC-113 (Lesage et al 1992). Heat-killed cells of anaerobic sludge supplied with the bulk reductant Ti(III)citrate also catalyzed the abiotic reduction of CFC-11 as was observed with the purified coenzymes (Olivas et al 2002). Cytochrome P450 monooxygenase is well known for carry out oxidative reactions; however, in the absence of elemental oxygen the reduced heme group can transfer its electrons to halogenated organics (Li & Wackett 1993).…”

“…Also methanogenic landfill leachate samples were shown to degrade CT (Kromann et al 1998). Both CT and CF are readily biotransformed by unadapted methanogenic sludge from various sources (Yu & Smith 1997;van Eekert et al 1998;Olivas et al 2002).…”

Spatial and temporal variations in pentachlorophenol dissipation at the aerobic–anaerobic interfaces of flooded paddy soils

“…SO 4 2À also limited the decrease in the redox potential in soil with increasing incubation time because of its oxidizing properties. The redox potential is an environmental parameter that affects oxidationereduction and reductive dechlorination of halogenated organic pollutants (Bagramyan et al, 2000;Olivas et al, 2002). The reduction of Fe(III) in the lower soil layers was also considered as an inhibitor for competition electron donors with PCP.…”

The use of redox potential to monitor biochemical HCBD dechlorination

“…Several studies have shown that low redox potentials are required for dechlorination (Pardue et al, 1988, Masscheleyn et al, 1991, Schumacher et al, 1997, Stuart et al, 1999, Arnold and Roberts, 2000, Olivas et al, 2002and Shimomura and Sanford, 2005. The highest rates of dechlorination were observed at lowest redox potentials tested (E Ag/AgCl = −348 mV) ( Olivas et al, 2002).…”

“…The highest rates of dechlorination were observed at lowest redox potentials tested (E Ag/AgCl = −348 mV) ( Olivas et al, 2002). Shulder (2006) noted that redox potential measurements can be an effective control parameter for maintaining an oxidizing, or reducing environment.…”