Relationship Between Sulphate Starvation and Chromate Reduction in a H2-fed Fixed-film Bioreactor

Abstract

While developing a low-sulphate system combining indirect chromate-reduction by biologically-produced hydrogen sulphide and direct biological chromate-reduction to treat chromate-bearing waters, the aim of the present work was to evaluate the influence of sulphate and H2 starvation on chromate reduction. Chromate-reduction was performed under continuous-feed conditions in a fixed-film column bioreactor originally inoculated with a bacterial consortium containing Desulfomicrobium norvegicum, and fed with H2. With 500 mg l−1 of sulphate in the feed solution, total chromate-reduction was observed in the effluent whereas sulphate-reduction was strongly decreased, as also confirmed by measurements of isotopic ratios for sulphur. In the absence of sulphate, a chromate-reduction activity was still observed but was lower than in the presence of sulphate, and chromate-reduction was H2-dependent. Molecular biology techniques revealed the composition of the bacterial population in the effluent. D. norvegicum together with other micro-organisms of the Bacteria domain were detected. They include members related to the genera Acinetobacter, Acetobacterium and Rhodocyclus. Even when sulphate-reduction was strongly decreased, the presence of sulphate enhances the efficiency of the H2-dependent chromate-reduction. A H2- and CO2-consuming bacterial population may be used in a globally autotrophic process to reduce chromate at low sulphate concentration, thus avoiding excess sulphide production.