Impacts Of Typical Metal Oxide Nanoparticles On The Anaerobic Methanogenic System With Anaerobic Granular Sludge

Over the past decade,due to the unique physicochemical properties,since large numbers of nanoparticles(NPs)have been manufactured and applied in our daily life.However,this is cause for concern,as some toxicology researches have pointed out the potential risks of NPs to human health and ecosystems.Wastewater treatment process are considered as the last barriers to the release of NPs from domestic and industrial sources to aquatic or terrestrial environments,and sorption by activated sludge is one of the primary physical removal mechanisms of NPs,resulting in accumulations of these NPs in biosolids and thus the possible negative impacts on microbial communities.The objective of the study presented the impact factors and mechanism of adsorption between TiO2 NPs and anaerobic granular sludge(AGS),as well as the short-term sudden exposure and the long-term impacts of three metal oxide NPs(TiO2,CuO and ZnO)on the metabolism of microbial community and transformation or biophysicochemical interactions occurring between metal oxide NPs and granules.Specifically,this study focused on the dynamic responses of AGS to different NPs exposure.The results of this study have important implications for limiting the emission of harmful metal oxide NPs,and operating of bioreactors treating NPs-containing wastewater.This study could also provide fundamental knowledge for assessing the fate in the environment and potential risks to humans of metal oxide NPs.Firstly,this study investigated the effect of various factors on the adsorption of TiO2 NPs onto AGS by batch tests,including particle size of AGS,p H,biomass concentration,temperature,TiO2 NPs initial concentration as well as ionic strength(IS).Among all the above factors,p H and IS were found to be quite significant on the adsorption effect.The entire process of adsoption between TiO2 NPs and AGS can not be explained completely by the DLVO theory.After determined the optimal experimental parameters,the adsorption kinetic and isotherm study,as well as the thermodynamic analysis were investigated find proper explanation for the adsorption mechanism of TiO2 NPs on AGS.Results showed that mono-layer chemisorption processes could be main adsorption mechanism,while the multi-interfaces adsoption was also involved in the adsorption process.The interaction process between TiO2 NPs and AGS can be divided into four steps: the existence of effective adsorption space,the migration of NPs to the surface of AGS,the formation of outer-sphere complex through hydrogen bonds between > TiOH and carboxyl or hydroxyl groups,and the formation of inner-sphere complex further.Short-term sudden exposure experiments are conducted to investigate the potential risk of typical metal oxide(TiO2,CuO and ZnO)NPs and their released ions to microbial communities,and the respective roles of EPS under different NPs exposure.It can be concluded that only CuO NPs and higher dosages of ZnO NPs(>50 mg/L)induced negative effect on VFAs production during acidification step.The exposure of metal oxide NPs coud slow hydrogen-producing rate and prolong the lag time during acetogensis.Various species of microflora appeared in the presence of the metal oxide NPs during fermentative hydrogen production,which may explain the different hydrogen yield among experimental groups.Acetoclastic methanogens was more susceptible to NPs sudden exposure than hydrogenotrophic methanogens.Comparing with the methanation process,the lower sensitivity of the whole methanogensis to metal oxide NPs sudden exposure was the joint effect of the multi-layered structure of AGS,the phenomenon that NPs were mainly located superficially in the AGS,and the syntrophic interaction between functionally and phylogenetically distinct microbial groups.The short-term inhibition effect of TiO2 NPs to AGS might be attributed to the physical restraints which might creat a physical barrier to substrate and gas,and metabolite transport between the acidogens cells and bulk solution.The toxic effect of ZnO NPs on AGS was mainly due to the release of Zn2+,which may cause the inhibitory effects on the key enzyme activity,while the most possible toxicity mechanism of CuO NPs might have been the direct physical interaction between the microorganism cell in AGS and CuO NPs,which severely compromised the integrity of the cytoplasmic membrane.The limited protective capability of EPS could be attributed to instability or sensitivity of the EPS protein under ZnO or CuO NPs exposure.The addition of TiO2 NPs could increase the relative abundance of archaea to 62.08% and the Methanosaeta comprised a larger portion(61.46%)of archaea.However,the relative abundance of Methanosarcina and Methanobacteriales were increased to 41.34% and 32.57%after sudden exposure of 200 mg/L ZnO and CuO NPs,respectively.The cumulative impacts of three metal oxide(TiO2,CuO and ZnO)NPs on the performance of anaerobic methanogenic system were evaluated over an extended period(90 d),and the fate of the NPs during anaerobic treatment was assessed.Likewise,the microbial communities and abundance in each reactor with different NPs were investigated using 454 pyrosequencing technology.The existence of ?delay phase? was found in this study,that is,remarkabe influence of metal oxide NPs on AGS have to make dosage accumulated to the certain degree.Long-term negative influence of TiO2 NPs on the operation of anaerobic reactor may be the outcome of substrate shortage of methanogens caused by disturbance of acidogens metabolism,while the cumulative production of ROS or metal ions under long exposure to metal oxide NPs should explain the high toxicity of ZnO or CuO NPs.The results also indicate that most of NPs were largely retained inside the reactor,which led to a much higher NPs concentration in biomass than the effluent.A higher number of members in the microbial community was triggered to respond to the long-term exposure of TiO2 NPs.Although the bacterial richness at 97% sequence similarity of the AGS sample from ZnO NPs reactorwas higher than that from the control,ZnO NPs could lower the richness in archaea.The CuO NPs cumulative effects on AGS could reduce the richness in both bacteria and archaea.Different metal oxide NPs can drive the succession of different bacterial and methanogenic populations in a microbial community,which may lead to the significant differences in the microbial community structure among the active members under different NPs exposure and the control in taxonomic classifications.