Microbial perchlorate reduction with elemental sulfur and other inorganic electron donors

ClO₄^- has recently been recognized as a widespread contaminant of surface and ground water. This research investigated chemolithotrophic perchlorate reduction by bacteria in soils and sludges utilizing inorganic electron-donating substrates such as hydrogen, elemental iron, and elemental sulfur. The bioassays were performed in anaerobic serum bottles with various inocula from anaerobic or aerobic environments. All the tested sludge inocula were capable of reducing perchlorate with H₂ as electron donor. Aerobic activated sludge was evaluated further and it supported perchlorate reduction with Fe⁰ and S⁰ additions under anaerobic conditions. Heat-killed sludge did not convert ClO₄^-, confirming the reactions were biologically catalyzed. ClO₄^- (3 mM) was almost completely removed by the first sampling time on d 8 with H₂ (≥0.37 mM d^-1), after 22 d with S⁰ (0.18 mM d^-1) and 84% removed after 37 d with Fe⁰ additions (0.085 mM d^-1). Perchlorate-reduction occurred at a much faster rate (1.12 mM d^-1), when using an enrichment culture developed from the activated sludge with S⁰ as an electron donor. The enrichment culture also utilized S^2- and S₂ O₃^{2 -} as electron-donating substrates to support ClO₄^- reduction. The mixed cultures also catalyzed the disproportionation of S⁰ to S^2- and SO₄^{2 -}. Evidence is presented demonstrating that S⁰ was directly utilized by microorganisms to support perchlorate-reduction. In all the experiments, ClO₄^- was stoichiometrically converted to chloride. The study demonstrates that microorganisms present in wastewater sludges can readily use a variety of inorganic compounds to support perchlorate reduction.