Fate and toxicity of pentachlorophenol and nitrophenols in anaerobic acidogenic culture
Description
Organic pollutants in anaerobic acidogenic systems have not been studied extensively despite the potential of acidogenic treatment to be an efficient and economical pretreatment process. In this study, fate and toxicity of four priority pollutant phenols (pentachlorophenol, PCP; 2-nitrophenol, 2-NP; 4-nitrophenol, 4-NP; and 2,4-dinitrophenol, 2,4-DNP) were investigated in an acidogenic, glucose-degrading anaerobic enrichment system Chemostat cultures (1.5-day SRT) were acclimated to 21, 42, and 59 muM (6, 12, and 17 mg/l) PCP for up to 261 days. No biodegradation of PCP was observed even in acclimated cultures. Extraction of PCP in serum bottles showed sorption to biomass was the dominant mechanism for PCP removal. PCP concentrations of 21--121 muM (6--35 mg/l) partially inhibited glucose degradation. In contrast to the control, the test chemostats with PCP showed formation of lactate and ethanol with lower acetate concentrations. Hydrogen-utilizing methanogens were inhibited by 21--121 muM (6--35 mg/l) PCP. Hence, contrary to expectations, the usefulness of anaerobic acidogenic cultures for degradation of PCP could not be substantiated In contrast to PCP dechlorination, nitrophenol transformation occurred rapidly in serum bottles with unacclimated acidogenic culture, mainly to the corresponding aminophenols. Abiotic nitrophenol removal was negligible. The ortho-nitro group was much more rapidly reduced than the para-nitro group, and removal of 2-NP and 2,4-DNP was faster than removal of 4-NP by almost an order of magnitude. With initial nitrophenol concentrations of 328--414 muM, first-order transformation rate constants were 0.061 hr-1 and 0.079 hr-1, respectively, for 2-NP and 2,4-DNP, versus 0.0034 hr-1 for 4-NP. Competitive inhibition coefficients for glucose uptake were comparable for 4-NP and 2,4-DNP and much lower than those for 2-NP. Literature values for methanogens and sulfate-reducers show that acidogens are more resistant to nitrophenol toxicity. The yield constant Y was significantly lower than in control serum bottles at high concentrations of all three nitrophenols tested, and the decay constant b was negligible at high concentrations of 2-NP and 2,4-DNP. Overall, the rapid removal of nitrophenols makes acidogenic systems a suitable pretreatment step for subsequent aerobic systems, while further research is necessary on factors stimulating PCP degradation by non-methanogenic anaerobes