External electricity-powered FeS/graphene/bacterial hybrid for Cr6+ removal from water lacking organic electron donors

Microbial reduction is an economical and environmentally benign route to deal with high-toxic Cr6+ widely present in aquatic environments, but faces major challenges such as cytotoxicity and insufficient organic electron donors. Herein, we propose a route for the removal of Cr6+ from water using a ternary FeS/reduced graphene oxide/Shewanella oneidensis MR-1 (FeS/rGO/MR-1) hybrid with external electricity supply at an electrode potential of −0.36 V. The biohybrid with excellent redox activity is in-situ biosynthesized by virtue of the versatile metabolic pathway and powerful extracellular electron transfer ability of MR-1 cells without the need of organic carbon sources. The biohybrid is directly used to remove Cr6+ under the condition of continuous electricity input, the removal rate is negatively correlated with the applied electrode potential. Compared with binary FeS/MR-1 and rGO/MR-1 counterpart, the FeS/rGO/MR-1 hybrid achieves an improved ability to remove Cr6+ at an initial concentration of 20 mg L−1 in three consecutive batches due to the synergistic effect. The FeS is the main active component for rapid chemical reduction of Cr6+ because of its highly reducible Fe2+ and S2-, and rGO acts as the bridge to accelerate electron transfer from the applied electrode to MR-1 cells. Then, the MR-1 cells use the obtained electrons for FeS regeneration, which in turn reduces Cr6+ again, resulting in the precipitation of less toxic Cr3+ [Cr(OH)3 and Cr2O3] in the biohybrid. This study provides a feasible route for removal of toxic Cr6+ from water lacking organic electron donor by in-situ synthesis of biohybrid with external electricity supply. [Display omitted] •A ternary FeS/rGO/S. oneidensis MR-1 hybrid was synthesized by electricity supply.•External electricity powered the biohybrid to improve Cr6+ removal.•Products of Cr6+ reduction were Cr(OH)3 and Cr2O3 deposited on the hybrid.