Impact Mechanism Of Cerium Oxidation Nanoparticles On Wastewater Treatment In Constructed Wetlands

For the past decade,with the rapid development of nanotechnology,a large number of nanoparticles?NPs?enter the environment during the process of production,use and disposal,which poses a threat to the ecosystem and even human health.Previous studies have shown that NPs may enter the sewage treatment system in the process of translocation,leading to the decrease of the treatment efficiency of activated sludge process.As an ecological sewage treatment technology,constructed wetlands are widely used in domestic sewage and industrial wastewater treatment.However,the impact of NPs on the constructed wetland system is still unclear.Hence,in this study,the typical NPs in sewage—cerium dioxide nanoparticles?CeO₂ NPs?were regarded as the research object to investigate the influence of CeO₂ NPs on the wastewater treatment efficiency of the subsurface flow constructed wetland system,and the influence mechanism was emphatically explored from the aspects of wetland microorganisms and plants.The main research results are as follows:?1?The effects of CeO₂ NPs with different concentrations?0 mg/L,1 mg/L and 50mg/L?on wastewater treatment efficiency of subsurface flow constructed wetland system were investigated in short-term?5 days?and long-term?60 days?exposure experiment.Short-term exposure experiments showed that CeO₂ NPs had no significant effect on the removals of COD and TP,whereas significantly inhibited the removals of TN and NH₄⁺-N.Under the exposure of CeO₂ NPs at 1 and 50 mg/L,the removal efficiencies were reduced by 28.66%,33.73%and 23.22%,21.76%,respectively.Long-term exposure experiments showed that CeO₂ NPs still had no significant effect on the removal of TP,whereas slightly inhibited COD removal and significantly inhibited the removals of TN and NH₄⁺-N.Under the exposure of CeO₂ NPs at 1 and 50mg/L,the removal efficiencies were reduced by 5.24%,60.74%and 67.38%and 6.75%,51.17%and 59.07%.?2?According to scanning electron microscopy?SEM?observation and lactate dehydrogenase?LDH?activity determination,it was found that a large amount of CeO₂NPs was adsorbed on the surface of the biofilm,which roughened the surface of the biofilm and damaged the integrity of the cell membrane.Metagenome analyses revealed that the exposure of CeO₂ NPs significantly changed the bacteria abundance of functional microorganisms at the phylum and genus levels.Enzyme activitiy determinations indicated that microbial enzyme?dehydrogenase,urease,ammonia monooxygenase and phosphatase?activities were affected in different degrees.Short-term exposure of 1 and 50 mg/L CeO₂ NPs significantly inhibited urease and phosphatase activities,which were reduced by 13.6%?16.6%and 20.7%?28.7%,respectively.Under the long-term exposure of 1 and 50 mg/L CeO₂ NPs,four key enzyme activites were inhibited sifnificantly,which were reduced by 16.2%?23.8%?55.6%?52.9%and 23.5%?40.1%?71.0%?70.5%,respectively.Hence,wastewater treatment efficiency of wetland system was consequently affected.?3?According to the detection of plant biomass,it was found that low concentration of CeO₂ NPs slightly promoted the growth of plants,while high concentration of CeO₂NPs significantly inhibited the growth of plants.By measuring the contents of photosynthetic pigments and photosynthetic parameters,it was found that the short-term exposure of CeO₂ NPs had no obvious influence on the synthesis of photosynthetic pigments.The net photosynthetic rate,stomatal conductance and transpiration rate were enhanced by 53%,168%,119%,and 36%,63%,47%under exposure of 1 and 50 mg/L CeO₂ NPs,respectively.Under long-term exposure,the chlorophyll a and chlorophyll b,total chlorophyll and carotenoid contents increased by 12.4%,42.3%,19.6%and 23.7%under exposure of 1 mg/L CeO₂ NPs,respectively,50 mg/L CeO₂ NPs slightly reduced the photosynthetic pigment contents.Net photosynthetic rate,transpiration rate,stomatal conductance increased by 16%,26%and 35%under exposure of 1 mg/L of CeO₂ NPs,respectively,and net photosynthetic rate and intercellular carbon dioxide concentration decreased and increased by 45%and 27%under exposure of 50 mg/L CeO₂ NPs,respectively.By means of the determination of the relevant indicators of the antioxidant defense system of wetland plants,it was found that under short-term exposure of 1 and 50 mg/L of CeO₂ NPs,malondialdehyde?MDA?content increased by27%and 57%,and superoxide dismutase?SOD?,peroxidase?POD?activities were reduced and enhanced by 38%,56%and 54%,219%,respectively,indicating that CeO₂NPs induced oxidative stress on wetland plants.Under long-term exposure,no obvious changes were found in MDA content,and SOD and POD enzyme activities were reduced and enhanced by 34%,40%,and 21%,160%under exposure of 1 and 50 mg/L of CeO₂ NPs,respectively,and activity of catalase?CAT?increased by 21%under exposure of 1 mg/L of CeO₂ NPs.This indicated that oxidative stress of wetland plants was relieved under the role of antioxidant enzymes,such as SOD,POD and CAT.In conclusion,under exposure of CeO₂ NPs,the activity of wetland microorganisms was significantly inhibited,and low concentration and high concentration of CeO₂ NPs respectively promoted and inhibited the growth and physiological activity of plants.However,wetland plants contributed less to pollutant removal than microorganisms in constructed wetlands.Thus,exposure of CeO₂ NPs inhibits wastewater treatment efficiency of the wetland system.