The Mechanism Of Simultaneous Removal Of Chlorpyrifos And Cadmium From Farmland Drainage Based On Iron-carbon Enhanced Constructed Wetland

In recent years,pesticides,heavy metals,nitrogen and phosphorus in farmland have enterd the natural water through surface runoff and farmland drainage under the way of rainfall.This has caused the serious threaten to water ecological environment and the safety of human drinking water.How to solve farmland drainage pollution economically and effectively has become an urgent problem.Constructed wetland（CW）have been successfully applied to the treatment of farmland drainage polluted water due to low operating cost,good purification effect and high removal efficiency.Traditional CW showed high purification ability to nitrogen and phosphorus removal,while showed the low adsorption capacity for pesticides and heavy metals removal.In this study,iron-carbon micro-electrolysis（ICME）enhanced constructed wetland（ICME-CW）to investigate the effects of ICME-CW on chlorpyrifos（CP）and cadmium（Cd）removal performance.In addition,the interaction between CP degradation and Cd removal processes was analyzed,and the responses of microbial communities and related functional genes were evaluated.In addition,the mechanism of CP and Cd removal by ICME-CW was elucidated.The main conclusions were shown as follows:（1）The optional preparation method of ZB-ICME substrate was determined,and the changes of morphology,structure and functional groups of ZB-ICME substrate were analyzed.ZB-ICME substrate was prepared from five raw materials:reduced iron powder,straw biochar,zeolite powder,soluble starch and ammonium bicarbonate.The optimal ratio of raw materials was 2:1:5:1:1 in volume ratio.ZB-ICME substrate was used to strength the CW system（ZB-ICME-CW）and microelectrolysis environment was formed spontaneously in the system,thereby the iron-oxidation corrosion was occurred in ZB-ICME-CW system.The characterization results showed that iron corrosion particles was appeared on the surface of ZB-ICME substrate after the experiment,and the corrosion extent of ZB-ICME substrate under the coexistence of CP and Cd was higher than CW.After the experiment,different iron oxidation states（Fe OOH,Fe O,Fe₂O₃ and Fe₃O₄）were detected in the ZB-ICME substrate,which proved the micro-electrolysis reaction.In addition,the detection of Cd（OH）₂ and Cd_xFe_（1-x）（OH）₂ co-precipitation indicated that the Cd removal was mainly through the precipitation process.（2）The purification effect of CW on CP and Cd pollutants was clarified.The ZB-ICME-CW system was used to synchronically remove CP and Cd from farmland drainage,and the removal rates can reach 99.6%and 98.6%,respectively,which was higher than the treatment effect of zeolite based constructed wetland（ZB-CW）（CP=92.9%;Cd=63.6%）.The addition of CP and Cd could promote the removal of N and P in the ZB-ICME-CW system,which increased from 53.1%and 63.9%to 99.1%and96.6%,respectively.Among them,CW could use CP as an additional carbon source to promote the denitrification process.The denitrifying phosphorus collecting microorganisms may be produced under high concentration of CP（100μg/L）,which could promote denitrifying dephosphorization.In addition,iron oxidation reaction could occur in the ZB-ICME micro-electrolysis process,and iron oxide could combine with phosphate in water to achieve the purpose of removal.（3）The microbial community diversity and characteristics in the seed sludge and ZB-ICME-CW system were analyzed,and the related functional genes were sequenced and analyzed.After exposure to CP and Cd,the abundance of CP-resistant bacteria such as Firmicutes,Clostridia,Acetobacterium and Cd-resistant bacteria such as Bacteroidetes in the ZB-ICME-CW system was increased.The abundance of related functional genes（czc A and opd）was also increased,which promoted the removal efficient of CP and Cd by ZB-ICME-CW system.In addition,ICME environment also promoted the growth of denitrifying bacteria such as Proteobacteria,Patescibacteria,Bacteroidetes,Pseudomonas and Paracoccus,which improved the nitrogen removal ability of ZB-ICME-CW system.（4）The removal mechanism of CP and Cd in ZB-ICME-CW system and the interaction between CP and Cd removal was illustrated.In this study,the removal mechanism of CP and Cd by ZB-ICME substrate were mainly summarized as follows:the degradation of CP was promoted by electron transfer during cathode hydrogen production,and the removal of Cd was immobilized by iron corrosion during anodic oxidation.At the same time,the iron oxidation reaction of ZB-ICME substrate would increase the consumption of dissolved oxygen in the process of CP degradation,and the reduced hydrogen production reaction of cathode would consume hydrogen ions in the solution,hence the p H of the solution was increased,thus promoting the generation of Cd and Cd/Fe co-precipitation to achieve Cd removal.The ZB-ICME substrate could enhance the removal effect of CP and Cd pollutants in the CW system,and increase the richness and diversity of microbial community in the system,thus could effectively improve the removal of pesticides and heavy metals in the system.This study provided theoretical support for CW purification of farmland drainage containing pesticides and heavy metals.