Study On Decontamination Performance Of Constructed Wetland Ecosystem Under The Stress Of Graphene Oxide

With the large-scale production and application of graphene oxide（GO）products,GO will inevitably be released into the ecological environment during its life cycle.The adverse effects of GO exposure on the decontamination performance,microbial community structure,matrix key enzyme activity and plant physiological characteristics of sewage biological treatment system have been confirmed.Constructed wetlands,as an economical and efficient ecological treatment technology for sewage,not only need to treat conventional pollutants（organic matter,nitrogen,phosphorus,etc.）,but also need to have the potential to treat artificial pollutants constantly emerging in wastewater,such as nano pollutants（GO,nano silver,nano titanium dioxide,etc.）.It is reported that the environmental background concentration of graphene-based nanomaterials has been reached as high as ug/L.However,the potential impact of GO exposure on the constructed wetland sewage ecological treatment system is still unknown.Therefore,this article selected Iris pseudacorus tidal flow constructed wetland as the research object,and carried out a 120-day experimental study to explore the effect of GO exposure on the decontamination performance of the constructed wetland,analyze the removal potential of the wetland system to GO.The main results are as follows:The study investigated the effect of two concentrations（0.5mg/L and 5mg/L）GO exposure on the decontamination performance of tidal flow constructed wetlands and the removal effect of wetlands to GO.The results showed that the effect of GO exposure on pollutant removal was different.There was no significant change in COD removal under GO exposure.Compared with the control group,the removal efficiency of NH₄⁺-N,TN and TP in GO test groups were significantly lower（p<0.05）:the removal efficiency of NH₄⁺-N,TN and TP in the 0.5mg/L GO test group was 92.20%,91.36%and 98.99%of that in the control group;the removal efficiency of NH₄⁺-N,TN,and TP in the 5mg/L GO test group was 92.82%,86.94%and 99.22%of that in the control group.In addition,in the initial stage of GO dosing（0-30 days）,the removal of NH₄⁺-N,TN,TP in the wetland was significantly inhibited（p<0.05）,especially in the high concentration 5mg/L GO test group.As for the nitrogen conversion process in wetlands,the conversion of NO₂^--N and NO₃^--N in wetlands was affected by GO exposure,and the mass concentration of nitrogen in each group of wetland effluent was mainly NO₃^--N.For phosphorus removal,the removal efficiency of dissolved orthophosphate（SOP）was also inhibited by GO exposure,and the main form of phosphorus in each wetland effluent was SOP.On the other hand,wetlands have high potential for removal of GO:high concentration of 5mg/L GO continuously exposed for 120 days,and the removal efficiency of GO by tidal flow constructed wetlands was more than 90%.The study investigated the effect of two concentrations（0.5mg/L and 5mg/L）GO on the activity of matrix enzymes of wetlands.The results showed that matrix enzyme activity is one of the decisive factors for improving the decontamination performance of wetlands.After 120days of GO exposure,the activities of matrix dehydrogenase,β-glucosidase and arylsulfatase were not significantly affected（p>0.05）.The urease activity was not affected by the 0.5 mg/L GO exposure,but significantly increased at the early stage of 5 mg/L GO exposure（p<0.05）,and the stimulating effect persisted during the 120-day GO exposure,with an average activity being 260.27%of that of the control group（p<0.05）.Similarly,phosphatase activity also increased significantly under GO exposure（p<0.05）,especially under 5 mg/L GO exposure,with the average activity being 350.78%of that of the control group.On the other hand,the key enzymes in the process of nitrification and denitrification:ammonia monooxygenase and nitrate reductase,their activities were inhibited by GO exposure,especially ammonia monooxygenase（p<0.05）.Under the exposure of 0.5mg/L and 5mg/L GO,ammonia monooxygenase activity was significantly lower than that of the control group on days 30-60th and 0-30th,respectively（p<0.05）,which were 47.89%and 51.36%of that of the control group,respectively,and consistent with the removal effect of NH₄⁺-N in water quality.The study investigated the effect of two concentrations（0.5mg/L and 5mg/L）GO exposure on the antioxidant system and photosynthesis of wetland plant-Iris pseudacorus.The results showed that after exposure to GO,Iris pseudacorus produced different degrees of oxidative stress response,which induced the expression of antioxidant enzyme system in plants.Among them,the activities of superoxide dismutase（SOD）and peroxidase（POD）increased significantly（p<0.05）,while the activity of catalase（CAT）decreased.In the antioxidant enzyme system,SOD acts as the first line of defense,and its activity was 105.22%and 112.44%of that of the control group when exposed to 0.5mg/L and 5mg/L GO,respectively.It can be seen that the SOD activity of the high concentration 5mg/L GO test group was more significantly induced.At the same time,POD and CAT cooperated in the removal of H₂O₂ in plants,and POD was the main role.In addition,the content of malondialdehyde（MDA）in the leaves of the Iris pseudacorus in the GO test groups were significantly higher than that in the control group（p<0.05）.Excess reactive oxygen species（ROS）damaged the cell membrane of plants,increased the degree of membrane lipid peroxidation,and decreased the self-defense ability of Iris pseudacorus.The decrease of chlorophyll content in the leaves of Iris pseudacorus indicated that the photosynthesis of the plant was affected.The chlorophyll in the leaves of Iris pseudacorus was mainly chlorophyll a,accounting for about 70%to 75%,and its inhibition degree was slightly higher than that of chlorophyll b.