Chromate reduction at low sulphate concentration in hydrogen-fed bioreactors

Aimed at developing a bioremediation process to treat Cr(VI)-bearing water at low sulphate concentration in order to reduce excess sulphide production, the highly toxic, mutagenic, and soluble Cr(VI) was reduced to the less toxic and insoluble Cr(III) in 2-litre fixed-bed reactors inoculated with the sulphate-reducing bacterium (SRB) Desulfomicrobium norvegicum, capable of performing direct enzymatic Cr(VI) reduction. H2 was used as the electron source. The fixed-films were developed on three different supports: a PVC cross-flow material, a pozzolana, and a ceramic granulate. The phased experiments began with a progressive increase of the Cr(VI) concentration in the feed to the column reactors, followed by a progressive decrease of the sulphate concentration. Inhibition by Cr(VI) was less pronounced with pozzolana than with the other supports; when the pozzolana column was fed with a medium containing 100 mg l(-1) Cr(VI) and only 250 mg l(-1) sulphate, the lowest residence time that could be applied for complete Cr(VI) reduction was 16 h. The molar ratio between the sulphate and Cr(VI) reduction rates was decreased down to 1.5, suggesting that indirect reaction with HS was not the sole mechanism of Cr(VI) reduction.