The anodic potential shaped a cryptic sulfur cycling with forming thiosulfate in a microbial fuel cell treating hydraulic fracturing flowback water

Abstract

The flowback water (FW) from shale gas exploitation can be effectively treated by bioelectrochemical technology, but sulfide overproduction remains to be addressed. Herein, sulfate-reducing bacteria (SRB) meditated microbial fuel cells (MFCs) with anodic potential control were used. COD removal gradually increased to 67.4 ± 5.1% in electrode-potential-control (EPC) MFCs and 78.9 ± 2.4% in the MFC with open circuit (OC-MFC). However, in EPC MFCs sulfate removal stabilized at much lower levels (no more than 19.9 ± 1.9%) along with much lower sulfide concentrations, but in OC-MFC it increased and finally stabilized at 59.9 ± 0.1%. Partial sulfur reuse in EPC MFCs was indicated by the current production. Notably, thiosulfate was specially detected under low potentials and effectively oxidized in EPC MFCs, especially under -0.1 V vs. SHE, which probably related to the sulfur reuse. Metagenomics analysis showed that the anode with -0.1 and -0.2 V likely shunted electrons from cytochromes that used for reducing DsrC-S0 trisulfide and thus contributed to producing thiosulfate and decreasing sulfide production. Meanwhile, the anode with -0.1 V specially accumulated sulfur-oxidizing system (Sox) genes regarding thiosulfate and sulfite oxidation to sulfate, which concurred to the effective thiosulfate oxidation and also indicated the possible direct sulfite oxidation to sulfate during the sulfur cycling. But the anode of -0.2 V highly accumulated genes for thiosulfate and sulfite reduction. Both anodes also distinctly accumulated genes regarding thiosulfate oxidation to tetrathionate and sulfide oxidation to sulfur or polysulfide. Further, sulfur-oxidizing bacteria were specially enriched in EPC MFCs and likely contributed to thiosulfate and sulfite oxidation. Thus, we suggested that the higher electrode potential (e.g. -0.1 V) can shape a cryptic sulfur cycling, in which sulfate was first reduced to sulfite, and then reoxidized to sulfate by forming thiosulfate as an important intermediate or by direct sulfite oxidation. The results provide new sights on the bioelectrochemical treatment of wastewater containing complex organics and sulfate.