Impacts Of Ag And CeO₂ Nanoparticles On Nitrogen Removal Performance And Microbial Community Of MBBR

Ag nanoparticles?AgNPs?and cerium oxide nanoparticles?CeO₂ NPs?are widely used in many fields due to their excellent properties.The production,transportation,use and discharge of AgNPs and CeO₂ NPs have inevitably caused their release into water environment such as sewage treatment system by various environment media,which generated potential adverse effects on its performance.The effects of 10 mg/L AgNPs,CeO₂ NPs,AgNPs and CeO₂ NPs mixture on biological nitrogen removal?short-term and long-term?and microbial community?long-term?in a moving bed biofilm reactor?MBBR?were investigated,after building efficient nitrogen removal system.The main conclusions were as follows:?1?The TN removal efficiencies of MBBRs were affected after short-term?12 h?exposure to AgNPs and CeO₂ NPs,by inhibiting the activity of relevant ammonia monooxygenase?AMO?and nitrate/nitrite reductase?NR?.The TN removal efficiencies reduced from 79.00%?control?to 73.13%,76.82%and 76.18%,respectively,when exposed to 10 mg/L AgNPs,CeO₂ NPs,Ag and CeO₂ NPs mixture for short-term.After long-term?100 d?exposure to Ag and CeO₂ NPs,the activity of AMO recovered,the TN removal efficiencies of MBBRs decreased due to the inhibition of NR activity and the reduce of biomass.The TN removal efficiencies were 75.49%,72.10%,69.98%,respectively,after long-term exposure to AgNPs,CeO₂ NPs,Ag and CeO₂ NPs mixture,which were significant different from that of the control?80.07%?.?2?After exposure to AgNPs and CeO₂ NPs,there were a large amount of aggregated NPs adhering onto the surface of the biofilm,which damaged the cell membrane integrity after short time.With the extension of exposure time,effects of AgNPs and CeO₂ NPs on cell membrane integrity were decreased.?3?The production and characteristic of extracellular polymeric substances?EPS?of MBBRs were changed by AgNPs and CeO₂ NPs.For short-term exposure,the loosely bound extracellular polymeric substances?LB-EPS?contents in all MBBRs were incerased,and tightly bound extracellular polymeric substances?TB-EPS?only significantly increased when faced to AgNPs and CeO₂ NPs mixture.For long-term exposure,the secretion of EPS increased when exposed to AgNPs,TB-EPS decreased significantly when attached with CeO₂ NPs,AgNPs and Ce02 NPs mixture.Protein?PRO?and polysaccharide?PS?in EPS played an important role in resisting the toxicity of AgNPs and CeO₂ NPs.?4?After long-term exposure to NPs,the order of MBBR microbial diversity was:the addition of Ce02 NPs>the addition of AgNPs>control>the addition of AgNPs and CeO₂ NPs mixture.The results showed that 10 mg/L AgNPs or CeO₂ NPs increased the biological community diversity to resist the toxicity of NPs.However,the shock of 10 mg/L AgNPs and CeO₂ NPs mixture was too large for microbes to endure,thus the biological community diversity was reduced after long-term exposure.The results of microbial community diversity suggested that the communities in systems with AgNPs and CeO₂ NPs were very different from the control.Microbial communities showed differences at the phylum,class,order and genus levels.There was a shift in the abundance of the dominant bacteria.The increase of nitrifying bacteria?Nitrosomonas and Nitrospira?and the significant decrease of denitrifying bacteria?Dechloromonas and Thauera?after exposure to NPs lead to the lower nitrogen removal efficiency.Besides,the bacteria related to heavy metal resistance?Zoogloea?increased when exposed to AgNPs,but decreased when exposed to CeO₂ NPs,AgNPs and CeO₂ NPs mixture.?5?In general,the CeO₂ NPs was more toxic to biofilms in MBBR than AgNPs,and there was the biggest effect on nitrogen removal when exposed to AgNPs and CeO₂ NPs mixture...