Enhanced reduction of sulfate and chromium under sulfate-reducing condition by synergism between extracellular polymeric substances and graphene oxide

Microbial reduction of sulfate and metal were simultaneously enhanced in the presence of graphene oxide (GO)-like nanomaterials, however, the mechanism remained unclear. In this study, bio-reduction of Cr was compared between free-living bacterium BY7 and immobilized BY7 (BY-rGO) on reduced GO particles. The role of extracellular polymeric substances (EPS) and rGO material on reduction of sulfate and Cr was investigated. Cr(VI) was reduced to Cr(III) and elemental Cr by BY-rGO particles up to 51% and 28%, respectively. EPS produced by the bacterium BY7 mainly consisted of proteins, polysaccharides, nucleic acids and humic substances. Concentration of EPS was sharply increased (about 54%) with the addition of graphene oxide, while the composition of EPS components was strongly affected by the exposure to Cr. By removing surface EPS without breaking the cells, reduction activities of sulfate and chromium by both BY-rGO particles and free-living BY7 cells were decreased. In contrast, reduction of sulfate and Cr by the free-living BY7 cells was enhanced with external addition of extracted EPS. Based on electrochemical analysis, the reduction peak indicating enhanced electron transfer was lost after removing EPS. Moreover, the contribution of each EPS fractions on sulfate and Cr reduction followed an order of polysaccharides > proteins > humic substances. Therefore, microbial sulfate and Cr reduction processes in the presence of BY-rGO particles were enhanced by the increasing amounts of EPS, which likely mediated electron transfer during sulfate and Cr reduction, and relieved bacteria from metal toxicity. Nevertheless, the presence of rGO was crucially important for elemental Cr production under sulfate-reducing condition, which might contribute to lowering electric potential or reducing activation energy for Cr (III) reduction. This work provided direct evidences for enhancing sulfate and Cr reduction activities by supplement of EPS as an additive to increase treatment efficiency in environmental bioremediation.