Plant Action Mechanism Of Degradation Of Amide Pesticides In Constructed Wetlands

In order to enhance the degradation of amide pesticides in constructed wetlands（CWs）,the mechanism of plant action in CWs was studied.The pretreatment methods of liquid and solid samples and the analysis methods of Metolachlor by liquid chromatography were established.The degradation kinetics of pesticides on plants was studied through batch experiments,and the effects of pesticides on chlorophyll fluorescence,growth characteristics and enzyme activities of plants were discussed.The effects of plant cultivation on the degradation of conventional pollutants and metolachlor in CWs,their spatial distribution and degradation pathways were studied through simulated experiments of CWs.The results are as follows:（1）For the analysis of metolachlor in liquid and solid samples（plant and soil）,centrifugal-filtration-high performance liquid chromatography（CE-FI-HPLC）and solid phase extraction-high performance liquid chromatography（SPE-HPLC）were established.The chromatographic analysis conditions of high concentration Metolachlor（200μg/L-20 mg/L）were as follows:the mobile phase was acetonitrile:water=60:40,1 mL/min flow rate,column temperature 30℃,injection volume 20μL;the low concentration of Metolachlor（2-200 ug/L）was acetonitrile:water=70:30,0.4 mL/min flow rate,injection volume 100μL.（2）The presence of plants can improve the degradation of pesticides in the reaction system.The removal rate of metolachlor in the plant+pesticide group is 23.4%higher than that in the pesticide group,and the degradation of amide pesticides by plants accords with the first-order kinetic equation C/C₀=1.1434 e^-0.0545t.The presence of pesticides has a stress on plants.The chlorophyll fluorescence parameters F_o,F_m,F_v/F_m,Y,ETR,qP and qN of plants decreased by 79.6%,93.2%,96.8%,85.4%,93.1%,96.8%and 95.8%respectively compared with the initial values.In addition,the stress degree of pesticides on plants exceeded the clearance rate of antioxidant enzymes.The superoxide dismutase（SOD）and glutathione reductase（GR）in plants+pesticides group increased first and then decreased,while the peroxidase（POD）and peroxide（CAT）decreased first and then increased,while the content of malondialdehyde（MDA）increased significantly.（3）The presence of plants strengthened the pesticide degradation ability of CWs.The average pesticide removal rate of CWs planting plants（54.4±20.5%）was higher than that of non-plant CWs（46.0±21.8%）（P>0.05）.The decrease of temperature will weaken the pesticide degradation ability of CWs.The average removal rates of pesticides in planted and non-planted CWs decreased by 31.63%and 28.36%respectively with the decrease of temperature.In addition,along the horizontal distance,the concentration of metolachlor in the two CWs decreased gradually;however,the two CWs had different vertical removal laws of metolachlor.The removal of pesticides in planted wetlands mainly concentrated in the upper and middle layer of plant root growth,while the removal of pesticides in blank wetlands gradually expanded with the depth of filling.Quantitative analysis of pesticide degradation in CWs shows that plants can not only absorb and remove pesticides directly（9.56μg/m²）,but also improve the adsorption and accumulation of pesticides by wetland substrates（1.67μg/m² vs 0.76μg/m²）.（4）According to redundancy analysis（RDA）,the removal of metolachlor in wetland has a good correlation with the internal environmental factors of the wetland,the concentration of each pollutant in the influent,the amount of each pollutant removed,and the plant growth characteristics.Environmental factors such asρ（DO）,T,pH were positively correlated with pesticide removal rate and their correlation decreased in turn.The concentrations of NH₄⁺-N,TN,pesticides,and TP in influent were positively correlated with pesticide removal rate and the correlation increased in turn.In addition,the pesticide removal rate was positively correlated with the removal rate of TP,TN,NH₄⁺-N and plant height,and the correlation decreased in turn.