Effects of nitrobenzene and zinc on acetate utilizing methanogens

Determination of anaerobic degradation rates and toxic effects of nitrobenzene (NB) on acetate utilizing methanogens was the first objective of this research. Serum bottles were used for anaerobic toxicity assays with an acetate enrichment culture of methanogens. Ten mg/l of nitrobenzene did not inhibit total gas production in the acetate enrichment methanogenic culture. Twenty and thirty mg/l of nitrobenzene caused reversible inhibition of methanogenesis. Batch kinetic experiments showed 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 about 1 mg/l. The second objective was to determine the effect of zinc on 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 and 20 mg/l of nitrobenzene. With 10 and 20 mg/l of added zinc, the k value 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 toxicity of Zn to methanogenesis. Abiotic control tests with autoclaved culture showed that adsorption alone could remove 60–70% of spiked nitrobenzene in 36 days. However, the samples extracted from solids in the methanogenic test systems showed that nitrobenzene was below the detection limit of 0.1 mg/l, indicating biodegradation of nitrobenzene in these systems. Traces of benzene were seen as an intermediate in the liquid samples. Headspace analysis showed that nitrobenzene and benzene were below detection limits.