Mechanism And Application Of Using Conductive Materials To Reduce ZnO NPs Toxicity In Wastewater Anaerobic Digestion

ZnO nanoparticles?ZnO NPs?have been widely used as environmental protection,bioengineering,chemical engineering and medicine due to their unique excellent magnetic,electrochemical and optical properties.The wide application and mass production will inevitably lead to the discharge into sewage water plants through the conventional emissions or some accidents.The exposure of ZnO NPs has been reported to induce a negative impact on the nitrogen and phosphorus removal process of the wastewater treatment plant,resulting in unqualified effluent.ZnO NPs could also inhibit the methane production from excess sludge anaerobic fermentation.This study used conductive materials to reduce the inhibitory effect of ZnO NPs on wastewater anaerobic digestion for methane production,and granular anaerobic sludge was used as the inoculum.Fe₃O₄ nanoparticles?Fe₃O₄ NPs?,the representative of non-carbon-based conductive materials,were selected to be mitigator for reducing ZnO NPs toxicity.The effect of Fe₃O₄NPs on the methane synthesis pathway were fistly studied,it was found that both acetoclastic pathway and mixed pathway were affected by Fe₃O₄ NPs.Their methane productions were observed to be increased initially and decreased afterwards.The optimum Fe₃O₄ NPs dosages were both 300 mg/g-TS.The suitable Fe₃O₄ NPs addition could promote the hydrogenotrophic methanogenesis and the appropriate dosages were 300-600 mg/g-TS.Subsequently,the disinhibition effect of Fe₃O₄ NPs on wastewater methanogenic process caused by ZnO NPs was studied.It was found that compared with the inhibition caused by ZnO NPs,the methane productions were respectively increased by 101.0%and 84.5%in the presence of 50 and 100 mg/g-TS Fe₃O₄ NPs.It indicated that Fe₃O₄ NPs significantly reduced the toxicity of ZnO NPs on wastewater methanogenic process.Obviously,50 mg/g-TS Fe₃O₄NPs was the optimal concentration for reducing ZnO NPs toxicity,which completely eliminated the inhibitory effect of ZnO NPs on methane production.Further mechanistic studies indicated that Fe₃O₄ NPs could solve the problem of organic acid accumulation caused by ZnO NPs,promote acidification and methanation of fermentation.In addition,Fe₃O₄ NPs might repair EPS and microbial structure damaged by ZnO NPs stress.Granular activated carbon?GAC?,the representative of carbon-based conductive materials,were selected to be mitigator for reducing ZnO NPs toxicity.The results showed compared with the inhibition caused by ZnO NPs,the methane productions were respectively increased by 15.6%,50.0%and 95.2%in the presence of 10 g/L,20 g/L and 30 g/L GAC.It indicated that GAC significantly reduced the toxicity of ZnO NPs on wastewater methanogenic process.Obviously,30 g/L-TS GAC was the optimal concentration for reducing ZnO NPs toxicity,which completely eliminated the inhibitory effect of ZnO NPs on methane production.Further mechanistic studies have shown that GAC can solve the problem of organic acid accumulation caused by ZnO NPs,promote acidification and methanation of fermentation.In addition,GAC might repair EPS and microbial structure damaged by ZnO NPs stress.GAC can also adsorb some ZnO NPs,reduce the effective contact between ZnO NPs and microorganisms,and reduce the damage of ZnO NPs to microorganisms.Finally,the toxicity reduction application of Fe₃O₄ NPs and GAC to ZnO NPs in anaerobic fermentation of actual wastewater?food industry wastewater?in UASB reactor was studied.The results showed that compared with the inhibition of ZnO NPs exposure on methane production and COD degradation rate,the addition of Fe₃O₄ NPs and GAC could increase the daily methane production by 80.0%and 81.4%,respectively,and the COD removal rate increased by 80.2%and 80.5%,achieved the purpose of reducing the toxicity of ZnO NPs during fermentation of food industry wastewater by Fe₃O₄ NPs and GAC in UASB process.