EFFECTS OF NITROBENZENE AND ZINC ON ACETATE UTILIZING METHANOGENS

Determination of anaerobic degradation rates and toxic effects of nitr obenzene (NB) on acetate utilizing methanogens was the first objective of this research. Serum bottles were used for anaerobic toxicity assa ys with an acetate enrichment culture of methanogens. Ten mg/l of nitr obenzene did not inhibit total gas production in the acetate enrichmen t methanogenic culture. Twenty and thirty mg/l of nitrobenzene caused reversible inhibition of methanogenesis. Batch kinetic experiments sho wed that 20 mg/l of nitrobenzene was degraded with a first-order rate constant, k, of 0.37 d(-1). Acetate was not degraded during the first 7 days when the measured nitrobenzene concentration was higher than ab out 1 mg/l. The second objective was to determine the effect of zinc o n nitrobenzene degradation in methanogenic systems. Ten mg/l of spiked zinc caused a reduction of gas production in the systems with 10 mg/l of nitrobenzene; 20 mg/l of zinc led to failures of systems with 10 a nd 20 mg/l of nitrobenzene. With 10 and 20 mg/l of added zinc, the k v alue for nitrobenzene degradation decreased to 0.18 d(-1) and 0.14 d(- 1), respectively. With 20 mg/l of Zn, acetate was not degraded at all even after nitrobenzene concentration reached 0.1 mg/l, indicating tox icity of Zn to methanogenesis. Abiotic control tests with autoclaved c ulture showed that adsorption alone could remove 60-70% of spiked nitr obenzene in 36 days. However, the samples extracted from solids in the methanogenic test systems showed that nitrobenzene was below the dete ction limit of 0.1 mg/l, indicating biodegradation of nitrobenzene in these systems. Traces of benzene were seen as an intermediate in the l iquid samples. Headspace analysis showed that nitrobenzene and benzene were below detection limits. Copyright (C) 1996 Elsevier Science Ltd