Degradation Of Rac-and S-metolachlor In Constructed Wetlands

The entry of chiral pesticides into the environment is easy to destroy the ecology and endanger human health.Understanding the enantioselective degradation behavior of chiral pesticides and carrying out targeted treatment is of great practical significance for protecting the environment.In order to investigate the selective degradation of Rac-and S-metolachlor,reveal it`s stress to plants and explore the feasibility and rationality of using constructed wetlands to remove chiral pesticides,a method for the separation and detection of metolachlor enantiomers in actual samples was established.Through batch hydroponic experiment,the degradation law of wetland plants to metolachlor and the stress of metolachlor on wetland plants were investigated.The secondary vertical flow constructed wetlands was established to study the enantioselective degradation of metolachlor.The results are as follows:（1）An HPLC-CHIRALPAK AY-H column system was established to analyze the enantiomers of metolachlor.By optimizing the separation method,the enantiomeric resolution conditions were determined as:n-hexane/ethanol=96/4,velocity=0.6m L/min,column temperature=25°C,UV detection wavelength=230 nm.This method has good applicability and linear relationship（correlation coefficient r≥0.999）.The LOD,LOQ,precision,accuracy,and spiked recovery rate all meet the analysis requirements.The robustness and stability are good,and it is not easily affected by other factors.The chiral resolution ofαSS,αRS,αSR,andαRR enantiomers from mobile phase to stationary phase was separated by enthalpy.（2）The addition of metolachlor has no significant difference in the removal of COD from hydroponic system and constructed wetlands（p>0.05）,but it can promote the absorption and degradation of some nitrogen pollutants and TP to a certain extent.Metolachlor has a significant inhibitory effect on wetland plants,and its growth height,chlorophyll content,photosynthesis and light utilization efficiency are significantly reduced.Plants are able to produce a response to the stress of metolachlor,and achieve the elimination of active oxygen by adjusting the content of protein and antioxidant enzymes in plant tissues.In addition,the plants in the constructed wetlands have better resistance to stress.With the gradual stabilization of the constructed wetland system,the photosynthesis and antioxidant enzymes of the wetland plants are gradually restored,which is consistent with the control group.（3）In the hydroponic system,the degradation trend of metolachlor enantiomer accorded with the first-order kinetic equation,and the degradation rate of each enantiomer of the plant+metolachlor group was higher than that of the metolachlor group.In addition,the differential degradation behavior of metolachlor enantiomers in hydroponic systems is obvious.S-metolachlor with herbicidal activity is more easily degraded in hydroponic systems while plants can achieve enrichment of Rac-metolachlor in vivo.The removal of metolachlor in the constructed wetlands occurred mainly in the first-stage constructed wetland,which indicates that the aerobic environment is favorable for the degradation of metolachlor in the constructed wetlands.The degradation of metolachlor in the constructed wetlands also has stereoselectivity.The changes in EF（S-）and EF（Rac-）during the experiment are similar,and both significantly deviate from 0.5,the EF gradually decreased.The selective behavior is mainly manifested in the degradation of S-metolachlor and the accumulation of Rac-metolachlor.（4）The accumulation of metolachlor in the root system of constructed wetland plants was the most（2.37μg/kg）,followed by the stems and leaves,indicating that the absorption and migration of metolachlor in the plants occurred;The content of the first-stage constructed wetland was higher than that of the second-stage.The content of S-metolachlor in plant tissues and matrix was significantly lower than that of Rac-metolachlor,indicating that the constructed wetlands could intercept Rac-metolachlor through matrix adsorption and plant uptake.（5）The removal rate of S-metolachlor on constructed wetlands was significantly negatively related to the intercellular CO₂ concentration of plants while the differential degradation characteristics of metolachlor enantiomers in constructed wetlands were related to the DO and the removal rate of TP of constructed wetlands and negatively correlated with plant catalase（CAT）.