Reduction of chromium(VI) by bacterially produced hydrogen sulphide in a marine environment

Chromium in both the Cr(III) and Cr(VI) state enters Otago Harbour in effluent from a tannery. The Cr(III) is precipitated together with organic matter from the effluent into the sediment. Chromium(VI) which might be expected from thermodynamic considerations to remain in the overlying water does not. Instead it is converted to the Cr(III) state by the hydrogen sulphide diffusing into the overlying water produced by sulphate-reducing bacteria.