Selenium oxyanion inhibition of hydrogenotrophic and acetoclastic methanogenesis

Inhibitory effects of selenite and selenate towards hydrogenotrophic and acetoclastic methanogenesis were evaluated in anaerobic toxicity assays. The 50% inhibitory concentration (IC50) of both selenium oxyanions was below 6.1 × 10−5 M in hydrogenotrophic assays, whereas acetoclastic methanogens were less inhibited: IC50 = 8.3 × 10−5 M and 5.5 × 10−4 M for selenite and selenate, respectively. Selenite completely inhibits methanogenesis from both substrates tested at concentrations ⩾10−3 M selenite, while only marginal methanogenic activities occur at equimolar concentrations of selenate. Selenite becomes irreversibly inhibitory upon a single exposure, whereas selenate inhibits methanogens upon repeated exposure. Consequently, methane recovery can be seriously hampered or even impossible during anaerobic treatment of highly selenium contaminated waste streams.