Bioreduction Mechanisms of High - Concentration Hexavalent Chromium Using Sulfur Salts Byphotosynthetic Bacteria

Sulfur is an essential element widely required by living organisms, thus sulfur starvation caused by competition for sulfate transporters between chromate and sulfate would affect Cr(VI) reduction in bacteria. In this study, we investigated the effects of six different sulfur salts on Cr(VI) bioreduction. Thiosulfate, sulfite, and metabisulfite markedly increased the concentration of reduced Cr(VI) while maintaining stable Cr(VI) reduction efficiency in Rhodobacter sphaeroides SC01. High Cr(VI) reduction rates were achieved under optimum conditions of an initial OD600 of 0.33, 2.0 g L-1 sulfur salts, and 96 h of treatment. The highest reduction efficiency for 500 mg L-1 Cr(VI) of 91.30% was attained under these conditions in SC01 combined with metabisulfite, which markedly increased the activity of glutathione peroxidase and glutathione content while decreasing ROS accumulation and cell death compared with other sulfur salts. Furthermore, SEM, FTIR, and XRD indicated that Cr(VI) and Cr(III) were efficiently absorbed on cell surfaces of SC01. Three sulfur salts activated sulfonate and disulfide groups on cell surfaces, resulting in the production of Cr2P3S9 and CrPS4. Overall, the results of this research revealed a highly efficient and reliable strategy for Cr(VI) removal by photosynthetic bacteria combined with sulfur salts in high-concentration Cr(VI)-contaminated wastewater.