Microbial reduction of Ferrihydrite in the presence of reduced Graphene oxide materials: Alteration of Fe(III) reduction rate, biomineralization product and settling behavior

Although graphene-based materials have been shown to physicochemically interact with biotic or abiotic particles after entering into the environment, little is known about their impacts on biotransformation of inorganic colloids. Accordingly, the effects of different reduced graphene oxide (rGO) materials on ferrihydrite reduction were investigated by using Shewanella oneidensis MR-1 and lactate as Fe(III) reducer and electron donor, respectively. rGO at 0.5 mg L-1 stimulated the transformation of ferrihydrite to magnetite, possibly by improving electron transfer. In contrast, higher concentrations of rGO (10-100 mg L-1) hindered the exposure, accessibility and therefore reduction of ferrihydrite, resulting in the formation of a mixture of goethite and magnetite. The adsorption of soil humic acid by coexisting ferrihydrite and rGO resulted in further inhibition of Fe(III) reduction. On the other hand, the enhanced Fe(III) bioreduction by the less adsorbable aquatic humic acid or simpler flavin molecules was further improved by 0.5 mg L-1 rGO but inhibited by 50 mg L-1 rGO. Additionally, the settling stability of the rGO-mineral aggregates was altered during bioreduction, which could influence the transport and reactivity of iron mineral and the fate of associated rGO. This study improves our understanding of the impact and behavior of rGO in the environment.