Enhanced Nitrobenzene Anaerobic Biotransformation By Reduced Graphene Oxide (RGO)

Currently, nitrobenzene as a kind of widely used chemical raw materials is heavily discharged. Due to the great environmental harm, it has been already listed as one of the priority pollutants. The treatment methods mainly include physical, chemical, biological and the combined processes, among which biological method has the significant advantages and the sequential anaerobic-aerobic biological process (A/O) is suggested as one of the most effective technologies for nitrobenzene treatments. However, anaerobic reduction of nitrobenzene was generally a rate-limiting step in the conventional anaerobic process, resulting in low biodegradation rates of nitrobenzene. Although it has been found that some redox mediators such as anthraquinone-2,6-disulfonate (AQDS) can increase the anaerobic biotransformation rates of nitroaromatics by accelerating the electron transfer from electron donors to terminal electron acceptors, continuous dosing implies continuous expense of AQDS as well as continuous discharge of this kind of biologically recalcitrant compound, resulting in secondary pollution.Based on these, graphene-anaerobic sludge composite as a novel biocatalyst was proposed for the enhancement of nitrobenzene biotransformation.Batch test results showed that reduced graphene oxide/anaerobic sludge (RGO/AS) composite presented good settling performance when graphene oxide (GO) was added into an AS system for24h cultivation. The presence of RGO resulted in enhanced nitrobenzene biotransformation by AS, and the highest removal efficiency of nitrobenzene was observed with initial GO (3～5μm) vs sludge ratio of0.075(w/w). Dehydrogenase activity in the RGO/AS system increased approximately2-folds over that in the AS system and redox active species appeared in supernatant from the RGO/AS system. The ratio of acetate to propionate increased in. the RGO/AS system when glucose as electron donor, and nitrobenzene biotransformation by both AS and RGO/AS systems was independent of methanogenesis, but dependent on acetogenesis. Moreover, bound and free extracellular polymeric substances (EPS) from RGO/AS composite were involved in direct biotransformation of nitrobenzene, and bound EPS might interact with secreted redox active species to accelerate nitrobenzene biotransformation by extracellular electron transfer. RGO had little effects on microbial community composition from AS, the analysis of PCR-DGGE showed that the dominant bacteria were Flavobacterium, Acidaminococcus, Mucilaginibacter, Riemerella sp. Q27and Bacillus sp. in the two systems.Continuous treatment of nitrobenzene simulation wastewater by RGO/AS composite results showed that the existence of RGO can obviously shorten the start-up time of UASB reactor, and at higher concentration of nitrobenzene and lower hydraulic retention time, RGO/AS system compared to AS system showed better removal capacity and stability. When the influent was in high salt conditions, RGO/AS system still had a higher salt tolerance, showing steady nitrobenzene removal capacity.In summary, the existence of RGO can significantly accelerate biotransformation of nitrobenzene by AS, and RGO/AS composite has better adaptability to external environmental conditions.