Study On Removal Characteristics Of Typical Pesticidesin Stormwater By Enhanced Bioretention System

There are a lot of refractory organics in urban runoff due to plant aintenance and pesticide spraying,which causes water pollution and poses a threat to water ecology.Bioretention systems are a widely used stormwater management measure and one of the preferred technologies for the construction of sponge cities in China.Bioretention systems can not only reduce the total volume and peak flow of runoff,but also purify runoff pollutants,but they are less effective and unstable in removing pesticides from runoff stormwater.In this study,the performance of biochar-enhanced bioretention systems（BC-EBRS）,electrochemical-enhanced bioretention systems（E-EBRS）and coupled biochar-electrochemical-enhanced bioretention systems（BC-E-EBRS）for the removal of typical pesticides（atrazine,ATZ）and conventional pollutants was investigated by constructing conventional（BRS）and enhanced bioretention systems（EBRS）.The effects of the conditioning parameters（biochar addition,biochar addition location,current density,electrolysis time）on pollutant removal in the enhanced bioretention system and the effects of environmental factors such as initial ATZ concentration,runoff volume and rainfall intensity on the removal performance of different enhanced systems were investigated.Through characterisation of biochar,determination of ATZ content in soil,identification of degradation intermediates and resolution of microbial community structure,the removal mechanism of ATZ in different systems was investigated to further elucidate the mechanism of action of enhanced bioretention systems on the removal of difficult to degrade pollutants.The main findings of this study are as follows.（1）The effects of biochar addition,biochar addition location,current density and electrolysis time on pollutant removal were investigated.the removal capacity of BC-EBRS for ATZ was positively correlated with the amount of biochar added（r=0.983,p<0.05）,in which the addition of 6%biochar could remove 72.64%of ATZ.The addition of biochar to the submerged layer was more beneficial to the removal of contaminants than the planted layer.Increasing the current density was beneficial to the removal of ATZ,NO₃^--N and TN by the E-EBRS,but had no effect on the removal of TP,but reduced the removal of NH₄⁺-N.The removal of COD showed an increasing trend with the increase of current density,and the optimal current density should be2.01 m A/cm².Increasing the electrolysis time is beneficial to the removal of ATZ and COD,but has no effect on the removal of TP,while the removal of NH₄⁺-N,NO₃^--N and TN decreases with the increase of electrolysis time.Therefore,the electrolysis time should be controlled for 4 h.（2）The effects of initial ATZ concentration,runoff volume and rainfall intensity on the removal performance of the different enhanced systems were investigated;the initial ATZ concentration significantly affected the ATZ removal performance of the BRS and EBRS systems,with the removal rate decreasing with increasing initial ATZ concentration.The concentrations of ATZ,NO₃^--N,COD and TN in the effluent increased first and then decreased,and even NO₃^--N leaching occurred with the increase of runoff.However,runoff had no significant effect on the removal of NH₄⁺-N and TP.The removal rates of ATZ,NH₄⁺-N and TN all decreased with the increase of rainfall intensity,while the removal rates of NO₃^--N and COD fluctuated,while rainfall intensity had no significant effect on the removal of TP.（3）The removal mechanism of ATZ in the enhanced bioretention system was analysed based on the microbial community structure,biochar characterisation and determination of ATZ degradation intermediates.The results showed that E-EBRS could achieve 74.60%ATZ removal by electrochemical oxidation and enrichment of electroactive bacteria（Desulfovibrio,Clostridium,Bacillus,etc.）through electrode formation,which was 22.17%higher than that of BRS.This system can enhance the removal of ATZ by coupling electrochemical and solid-state electron transfer with in situ Fe modification of biochar.The coupling of biochar and electrochemistry at the same time can promote the enrichment of dominant ATZ degrading bacteria such as Paenarthrobacter,Arthrobacter and Bacillus,resulting in a higher ATZ degradation capacity.ATZ is mainly removed by soil adsorption,biochar adsorption,electrochemical oxidation and microbial degradation in the enhanced bioretention system.