Graphene oxide as electron shuttle for increased redox conversion of contaminants under methanogenic and sulfate-reducing conditions

•GO has the proper characteristics to promote the reduction of contaminants.•GO increased up to 10-fold the abiotic reduction of contaminants with sulfide.•GO increased up to 3.6-fold the microbial reduction of contaminants.•Biogenic sulfide plays a major role on the GO-mediated reduction of reactive red 2.•Effective electron shuttle under methanogenic and sulfate-reducing conditions.

Graphene oxide (GO) is reported for the first time as electron shuttle to increase the redox conversion of the azo compound, reactive red 2 (RR2, 0.5 mM), and the nitroaromatic, 3-chloronitrobenzene (3CNB, 0.5 mM). GO (5 mg L−1) increased 10-fold and 7.6-fold the reduction rate of RR2 and 3CNB, respectively, in abiotic incubations with sulfide (2.6 mM) as electron donor. GO also increased by 2-fold and 3.6-fold, the microbial reduction rate of RR2 by anaerobic sludge under methanogenic and sulfate-reducing conditions, respectively. Deep characterization of GO showed that it has a proper size distribution (predominantly between 450 and 700 nm) and redox potential (+50.8 mV) to promote the reduction of RR2 and 3CNB. Further analysis revealed that biogenic sulfide plays a major role on the GO-mediated reduction of RR2. GO is proposed as an electron shuttle to accelerate the redox conversion of recalcitrant pollutants, such as nitro-benzenes and azo dyes.