Natural attenuation of organic contaminants in groundwater : biodegradation of high phenol concentrations under sulphate-reducing conditions and anaerobic oxidation of vinyl chloride.

A plume of phenolic compounds (phenols, cresols, xylenols) with a total organic compound concentration of 24,800mg/L (in the core of the plume), including 12,500mg/L phenol, was found at a site (four-ashes) that has been used in the production of chemicals since 1950. Phenol biodegradation studies of microcosms with four-ashes inoculum demonstrated that phenol degraded readily, with concurrent SO42--reduction, at initial concentrations of ≤235 mg/L, at 770 mg/L phenol and also at ~ 900 mg/L phenol. Oxidation of phenol at such high concentrations, under sulphidogenic conditions, has not been reported in bacteria from sediment or groundwater systems. Previous studies have shown phenol to be inhibitory or toxic at concentrations between 200 mg/L and 600 mg/L, to sulphate-reducing bacteria and also to bacteria utilising alternative electron acceptors under anaerobic conditions. Phenol biodegradation curves obtained from the Monod model correlate well with the experimental data, as do predicted biomass concentrations at the conclusion of the experiment. Values for kmax (maximum phenol utilisation rate) are between 4.90 x 10-8/s and 3.74 x 10-6/s. Half-saturation constants Kp (phenol) and Kso4 (sulphate) were determined to be 2.0 x 10-4 mol/L and 3.7 x 10-4, respectively. These values are of the same order of magnitude but higher than those reported in the literature. Half-life calculations suggest that concentrations of up to 575 mg/L phenol may be remediated within 6 years, if environmental conditions were suitable. VC concentrations were accurately determined in microcosms investigating VC oxidation under SO42-reducing conditions, using SPME/direct headspace sampling coupled with GC-MS. Initial experimentation suggested that VC oxidation may have occurred, however this could not be confirmed in subsequent experiments. Cometabolic VC degradation did not occur under S042--reducing conditions. The methodology presented in these experiments is suitable for long-term VC microcosm experiments, and forms a sound basis for future studies of VC degradation.