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

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.