Enhancement of anaerobic carbon tetrachloride biotransformation in methanogenic sludge with redox active vitamins

Carbon tetrachloride (CT) is an important groundwater pollutant which is only subject to biotransformation in the absence of oxygen. The anaerobic biotransformation of CT is influenced by electron shuttling compounds. The purpose of this study was to evaluate the impact of redox active vitamins on CT (100 μM) metabolism in a methanogenic sludge consortium (0.5 g VSS1 ^-1) supplied with volatile fatty acids as electron donor (0.2 g COD1^-1). The redox active vitamins, tested at concentrations ranging from 0.5 to 20 μM, were riboflavin (RF) and two forms of vitamin B12, cyanocobalamin (CNB12) and hydroxycobalamin (HOB12), and these were compared with a redox mediating quinone, anthraquinone-2,6-disulfonate (AQDS). Substoichiometric concentrations of RF, CNB12, HOB12 at molar ratios of vitamin:CT as low as 0.005 significantly increased rates of CT-bioconversion. These are the lowest molar ratios of vitamin B12 reported having an impact on dechlorination. Additionally, this study constitutes the first report of RF having a role in reductive dechlorination. At molar ratios of 0.1 vitamin:CT, RF, CNB12, HOB12 increased the first order rate constant of CT bioconversion by 4.0-, 13.3-and 13.6-fold, respectively. The redox active vitamins also enhanced the rates of abiotic CT conversion in heat killed sludge treatments, but the rates were approximately 4- to 5-fold lower than the corresponding vitamin enhanced rates of biological CT conversion. The addition of CNB12 or HOB12 to the live methanogenic sludge consortium increased the yield of inorganic chloride (Cl^-) from CT-converted. Chloroform was a transient intermediate in CNB12 or HOB12 supplemented cultures. In contrast, the addition of RF increased the yield of chloroform from CT-converted. Taken as a whole the results clearly demonstrate that very low concentrations of redox active vitamins could potentially play an important role in accelerating the anaerobic the bioremediation of CT as well as influencing the proportions of biotransformation products formed.