Abstract:
A mix-culture of methanogenic and sulfate-reducing bacteria was capable of degrading trichloroethylene (TCE) to dichloroethylene (DCE). The culture was incubated under anaerobic conditions within a soil sample taken from Gilbert-Mosley site, Wichita, Kansas. To stimulate the growth of indigenous bacteria, a carbon and energy source was added in this microcosm study without the addition of nutrients. Each microcosm contained various amounts of groundwater and sediment. Benzoate was the primary carbon and energy source added to the various modified microcosms. Several other carbon and energy sources were also tested including: methanol, ethanol, acetate, propionate, butyrate, toluene and phenol. Except for toluene and phenol, the other substrates were able to enhance the degradation of TCE under sulfate-reducing conditions. The microcosm control to which no substrate was added showed no signs of TCE transformation. Addition of suitable carbon and energy sources did enhance the sulfate-reducing conditions and TCE transformation in this microcosm study.
No TCE transformation was observed under denitrifying conditions, although the
groundwater contained about 46 mg/L of nitrate. Nitrate reduction appeared earlier and the rate was faster than that of sulfate reduction. Addition of nitrate did enhance denitrifying conditions which are co-existent with sulfate reducing conditions.
Under sulfate reducing conditions, where the sulfate concentration was reduced to less than 150 mg/L, rapid degradation of TCE occurred within a time interval of 30 days. The transformation of TCE in some case followed a lag after sulfate reduction. The reduction mechanism of TCE transformation remained unknown. However, TCE transformation usually coincided with sulfate reduction. Some methane production was also observed at low sulfate concentrations. Under sulfate-reducing conditions, 1,2 c-DCE was the major end product and it persisted over 300 days without further degradation. It is consistent with the field observation. Some minor traces of ethylene were found, and accumulation of vinyl chloride was not observed.
