Impacts Mechanism Of Titanium Oxidation Nanoparticles On Functionalities Of Constructed Wetlands

As an ecological wastewater treatment technology,constructed wetlands?CWs?have been widely used in the treatment and reuse of sewage treatment plants.However,with the rapid development of nanotechnology,engineering nanomaterials?ENMs?have been widely used in industrial and commercial products.The effluent of urban wastewater treatment plants?WWTPs?not only contain the conventional pollutants such as COD,ammonia nitrogen,phosphate,etc.,but also there is metal and metal oxide nanoparticles,such as Ag NPs,TiO₂ NPs and CeO₂ NPs,etc.But the mechanism of the effects of influence containing nanoparticles on CWs is not yet clear,this pollutants brought potential ecological and health risks on CWs.Nano titanium dioxide?TiO₂ NPs?is one of the most widely used engineering nanomaterials.In recent years,with the widespread use of TiO₂ NPs,it is inevitable that it will enter into the water treatment system with effluent.Aiming at these problems,in this study,we regarded the synthetic wastewater containing TiO₂ NPs as the inlet of wetland reactors.In addition,we detected the routine index of wastewater,microbial physiological activity and wetland plant physiological parameter.And it was designed to understand the effect of TiO₂ NPs on treatment efficiency of CWs,to verify the effects of TiO₂ NPs to on the microbial population and the inhibition of key enzyme activity under long-and short-term exposure and to identify the response of photosynthesis and root activity of wetland plants under the long-and short-term exposure of TiO₂ NPs.The simulated wetland wastewater treatment system was exposed to TiO₂ NPs with different concentrations?0mg/L,1mg/L,and 50 mg/L?.The effects of TiO₂ NPs on water treatment efficiency of wetland system during the short-?5 d?and long-?60 d?term were observed respectively.The short-term exposure test results showed that the exposure of low concentration and high concentration of TiO₂ NPs did not significantly affect the water treatment efficiency.However,the long-term exposure test results showed that the pollutant removal efficiency of constructed wetlands was significantly suppressed.Moreover,the degree of inhibition is nonlinear with the concentration of TiO₂ NPs.When the concentration scale of TiO₂ NPs was 1mg/L,the removal rate of COD,TN and NH₄⁺-N decreased to 85.21%?38.55%?28.24%of the control,and the removal rate of COD,TN and NH₄⁺-N decreased to 78.33%?51.45%?47.02%of the control respectively,while the concentration of TiO₂ NPs was 50mg/L.The removal rate of TP is not significant in the whole experimental period.SEM observation showed that a large number of nanoparticles gathered on the surface of biofilms.The results of metagenomics analysis showed that the relative abundance of Nitrospira and Nitrosomonas related to denitrification decreased at the level of portal and genera.In the control,the proportion of two species of bacteria in the Ti-0 mg/L group was 5.17%and 2.55%,respectively,while in the experimental group,the Ti-50 mg/L group was 1.38%and 0.96%respectively.The proportion of Candidatus Accumulibacter in microbial community decreased significantly.The proportion of PAOs abundance accounted for 1.08%in the control,the experimental group was 0.56%,felled by nearly half.The key enzyme activity in the Ti-1 mg/L and Ti-50mg/L group compared with the control group decreased by 6.05%and 13.22%respectively after the end of the short-term exposure experiment.The urease activity was reduced by 10.90%and 17.65%after the end of the short-term exposure experiment,and 30.22%and 56.21%after the end of the long-term exposure experiment.The activity of AMO was not significantly inhibited after the end of the short-term exposure experiment,but reduced27.42%and 35.17%after the long-term exposure experiment.There was no significant difference in the activity of phosphatase activity at the end of the short-term exposure experiment,and reduced 26.85%and 38.18%respectively after the long-term exposure experiment.The release of lactate dehydrogenase?LDH?is an indicative enzyme,and its release indicates the persecution of TiO₂ NPs to microbes after long-term accumulation,which indicates that TiO₂ NPs have seriously damaged the membrane structure of microorganism,which leads to the change of microorganism morphology and even death.In addition,the normal physiological activity of plants on CWs will be also affected by the exposure of TiO₂ NPs.In this study,plants were not negatively affected under short-term exposure to TiO₂ NPs,but during the long-term exposure,all the physiological activities of plants were promoted.At the end of short-term exposure experiment,root activity showed the promotion at 1 mg/L?higher than 3.85%of the control?,and inhibition of 50 mg/L was lower than that of control group?less than 5.78%of the control?.There was no significant difference in the chlorophy ll between the experimental group and the control group during the short-term experiment.However,after the long-term experiment,the Ti-1 mg/L and Ti-50 mg/L were higher than 9.86%and 13.41%of the control respectively,indicating that the long-term exposure of TiO₂NPs to the wetland plants was positive.The results showed that the net photosynthetic rate of the Ti-1 mg/L and the Ti-50 mg/L group was 15.71%and 21.31%higher than that of the control after the long-term.In addition,the concentration of CO₂ was 12.99%and 16.88%,respectively;the stomatal conductance was 23.53%and 36.32%of the control,and the transpiration rate was 11.70%and 30.63%respectively.At the same time,the ability of plants to remove nutrients will also be enhanced.After the long-term exposure of TiO₂ NPs,on the one hand,it could inhibit microbial activity and promote the physiological activity of wetland plants on the other hand,because plants contribute less to the nitrogen removal from the CWs than the microbes.Therefore,the results of the two aspects showed that the long-term exposure of TiO₂NPs inhibits the efficiency of wastewater treatment in the CWs,and the inhibitory effect of low concentration is stronger than that of high concentration.