Effect Of Microplastics On Decontamination Efficiency And Plant Stress Of Constructed Wetland

Microplastic is an emerging pollutant that has been widely recognized.It can enter the sewage treatment system along with the discharge of domestic sewage,and has a negative impact on the removal process and removal effect of pollutants.The impact of microplastics on the performance of constructed wetlands（CWs）has not been reported previously.In this work,the physiological toxicity of microplastics to Canna indica was compared under different experimental conditions using three types of microplastics（polyamide microparticles（PA-MP_S）,polyethylene microparticles（PE-MPs）,polystyrene microparticles（PS-MPs））and four particle sizes（5μm,50μm,100μm,500μm）by hydroponic experiments.The effects of PS-MPs on the removal efficiency of conventional pollutants in CWs were analyzed.In addition,the stress effects of PS-MPs on three typical wetland plants were evaluated by detecting photosynthesis and stress resistance indexes.The results of hydroponic experiment showed that the order of inhibition and physiological toxicity of three kinds of microplastics on Canna was PS-MPs>PA-MPs>PE-MPs.The order of inhibition of photosynthesis and physiological toxicity of PS MPs with four particle sizes was PS MPs 500μm>PS-MPs 100μm>PS-MPs 50μm>PS-MPs 5μm.Taking 500μm PS-MPs as the research object,the effect of PS-MPs on the decontamination efficiency of constructed wetland was investigated.The results showed that the exposure level of PS-MPs had a significant difference in the effect on the decontamination efficiency of Canna CWs.With the increase of the concentration of PS-MPs,the removal efficiency of conventional pollutants in Canna CWs was more inhibited.The effect of 1mg/L PS-MPs on the decontamination efficiency of constructed wetland was not significant.PS-MPs10mg/L had little effect on nitrogen and COD removal,but great effect on phosphorus removal,and removal rate of TP was 18.44%lower than that of control group.When PS-MPs concentration was 20mg/L and 50mg/L,it had minor effect on nitrogen removal,but had noticeable effect on phosphorus and COD removal.The removal rate of TP was 31.47%and35.35%lower than control group respectively,and the removal rate of COD was 11.88%and19.80%lower than control group respectively.When PS-MPs exposure level was 20mg/L,the nitrogen removal efficiency of Canna indica CWs,Iris pseudacorus CWs,Thalia dealbata CWs and non-vegetated CWs was not inhibited.Compared with non-vegetated CWs,the removal of TP and COD in three vegetated wetlands was less inhibited.There was no significant difference found in the inhibition of COD removal among the three vegetated wetlands,while the inhibition of phosphorus removal in Iris pseudacorus CWs was less than that in Canna indica CWs and Thalia dealbata CWs.The physiological characteristics of wetland plants showed that the chlorophyll content,net photosynthetic rate,intercellular CO₂ concentration,transpiration rate and stomatal conductance of Canna leaves were all affected by PS-MPs exposure.With the increase of PS-MPs concentration,the effect on photosynthesis was more significant.When PS-MPs was exposed at 20mg/L,the order of photosynthetic inhibition of the three wetland plants was as follows:Iris pseudacorus>Thalia dealbata>Canna indica.The stress resistance indexes of Canna leaves did not change significantly when exposed to PS-MPs at 1mg/L and 10mg/L.When PS-MPs was exposed at 20mg/L and 50mg/L,the results showed that the activities of peroxidase（POD）and catalase（CAT）in Canna leaves had no significant changes,while the activities of superoxide dismutase（SOD）,the contents of hydrogen peroxide（H₂O₂）,malondialdehyde（MDA）,total phenolic content and anthocyanin content increased significantly.Compared with the control group,SOD activity increased by 21.34%and 31.81%,respectively;H₂O₂ content increased by 18.41%and 45.80%,respectively;MDA content increased by 19.37%and 28.77%,respectively;total phenol content increased by 8.04%and14.79%,respectively;and anthocyanin content increased by 12.59%and 15.44%,respectively.When PS-MPs was exposed at 20mg/L,the order of stress resistance of the three wetland plants was as follows:Canna indica>Thalia dealbata>Iris pseudacorus.The correlation analysis between plant physiological characteristics and the decontamination efficiency of wetlands showed that when exposure levels of PS-MPs were different,the activities of SOD and POD,the contents of total phenols and anthocyanins in Canna leaves were significantly negatively correlated with the removal rates of COD,NH₄⁺-N,TN and TP.The chlorophyll content was significantly positively correlated with the removal rates of COD,NH₄⁺-N,TN and TP.MDA content and H₂O₂content only had significant negative correlation with the removal rate COD.There was no significant correlation between CAT activity and the removal efficiency of all pollutants.When PS-MPs exposure level was20mg/L,the changes of MDA and H₂O₂contents in the leaves of three wetland plants were significantly positively correlated with the inhibition degree of TP removal.The research results provide a basic reference for evaluation of the effects of microplastics exposure on the decontamination efficiency of CWs,especially the stress effects of microplastics on wetland plants.