Reduction of Hexavalent Chromium with Elemental Sulfur by Sulfur-oxidizing Bacteria.

The reduction of hexavalent chromium (Cr6+) by sulfur-oxidizing bacteria was investigated using elemental sulfur as the substrate for bacteria in 9K medium (without FeSO4).The test bacteria were collected from a neutralization plant at the abandoned Matsuo mine and cultured in iron-free 9K medium with elemental sulfur.The bacteria oxidized elemental sulfur to sulfate, and main strains of them must be Thiobacillus ferrooxidans and Thiobacillus thiooxidans.The results obtained are summarized as follows.1) Cr6+ was reduced when the bacteria were cultured on elemental sulfur.It could be due to sulfurous acid formed as a intermediate during the oxidation of elemental sulfur to sulfate by the bacteria.2)Cr6+ inhibited the sulfur-oxidizing ability of the bacteria, but the oxidation rate of elemental sulfur increased remarkably after the reduction of Cr6+.3)The rate of Cr6+ reduction increased with the inoculum size and the concentration of elemental sulfur, and the optimum initial pH was 2.0.