Reduction of ZnO nanoparticles toxicity to methanogenic wastewater treatment by the presence of Fe3O4 nanoparticles: Focusing on nanomaterial interaction and direct interspecies electron transfer

Abstract

The extensive production and wide use of ZnO and Fe3O4 nanoparticles (NPs) potentially leads to their synchronous release into the environment, and the exposure of ZnO NPs can inhibit the methanogenic process. This study investigated the effect of Fe3O4 NPs on the toxicity of ZnO NPs to methanogenesis of wastewater and the related mechanism. It was observed that the presence of 50 mg/L Fe3O4 NPs entirely mitigated the inhibition caused by 100 mg/L ZnO NPs, which had been decreased by 39.0% compared with the control group. This was because that ZnO NPs induced the accumulation of short-chain fatty acid, deterioration of extracellular polymeric substances, low activities of acetate kinase and coenzyme F420, low levels of intracellular NADH/NAD+ and ATP, and even the death of methanogenic Archaea. However, all these suppressed activities were recovered when co-presence of Fe3O4 NPs. Moreover, Fe3O4 NPs not only suppressed the levels of hydrogenase and c-type cytochrome but also enriched the direct interspecies electron transfer (DIET)-linking partners, suggesting a lower interspecies hydrogen transfer and higher DIET activities occurred. The investigations of NPs interactions demonstrated that the agglomeration of NPs, the decrease of Zn2+ and release of Fe2+ were primarily responsible for the reduced toxicity of ZnO NPs.