| In recent years,the frequent use of pesticides has caused widespread concerns for human health and the natural environment,and pesticides are the most important pollutants worldwide.Constructed wetlands are characterized by a variety of advantages,such as of low investment and operation costs,low management intensity,and the efficient removal of trace organic pollutants such as pesticides and antibiotics from wastewater.However,the mechanisms of the removal of pesticides via constructed wetland systems are still not entirely understood.Studies evaluating these mechanisms,along with increased measurements of typical pesticides in constructed wetlands,are therefore essential to provide a scientific basis for the efficient removal of pesticides from constructed wetlands.In this context,the objective of this thesis is the removal of commonly used pesticides from constructed wetlands,based on pollution surveys and the evaluation of pilot systems.The underlying mechanisms of pesticide removal on the basis of a process optimization scheme were analyzed;hereby,the major achievements of the study are as following:1)Overall,the potential impact of the pesticides detected in the rural stretch of the Guangzhou River on the ecosystem were evaluated.Specifically,19 commonly used pesticides were investigated in the water and sediment in different seasons.A total number of 11 pesticides were detected and their spatio-temporal distribution and ecological risks were assessed.Of these,five pesticides occurred in concentrations above 100 ng L-1,with dimethoate having the highest concentration(1,318 ng L-1)in surface water and quinalphos having the highest concentration(328 ng g-1 dry weight(dw))in sediment.The dominant pesticides were chlorpyrifos,acetochlor,and butachlor,with detection frequencies of 50-57% and 29-43% in water and sediment,respectively.In the rural stretch of the river in the agricultural area,Guangzhou,more pesticides in higher concentrations were detected compared to river stretches in industrial and residential areas,especially in the wet season.The ecological risks posed by nine pesticides,such as chlorpyrifos,acetochlor,butachlor,fenvalerate and so on,which necessitates adequate measures to reduce pesticide inputs into the river system.2)The use of iron(Fe)-impregnated biochar was assessed as a method to remove the pesticide chlorpyrifos,using a biochar/Fe Ox composite synthesized via chemical co-precipitation of Fe3+/Fe2+ onto Cyperus alternifolius biochar.The Fe-impregnated biochar exhibited a higher sorption capacity than pristine biochar,resulting in a more efficient removal of chlorpyrifos from the water.The soil was dosed with pristine or Fe-impregnated biochar at 0.1 or 1.0% w/w to evaluate chlorpyrifos uptake in Allium fistulosum L.(Welsh Onion).Based on the results,the average concentration of chlorpyrifos and its degradation product,3,5,6-trichloro-2-pyridinol(TCP),decreased in A.fistulosum L.with increasing levels of pristine biochar and Fe-biochar.However,Fe-biochar was more effective in reducing chlorpyrifos uptake by improving the sorption ability and increasing the plant root iron plaque,with the greatest persistence of chlorpyrifos residue at a dose of 1.0% of pristine biochar.The results of the microbial community analysis show that Fe-biochar had a positive impact on the efficiency of chlorpyrifos degradation in soils,possibly by altering the microbial community structure.3)Plant species carry out a series of important biological,chemical,and physical processes within a constructed wetland wastewater treatment system,significantly contributing to wastewater purification.One objective of this study was to investigate the removal of chlorpyrifos at realistic concentration levels(50 and 500 ?g L-1)in recirculating vertical flow constructed wetland(RVFCW)mesocosms planted with Cyperus alternifolius,Canna indica,Iris pseudacorus,Juncus effuses,and Typha orientalis.The results show that the removal of chlorpyrifos in different concentrations occurred at similar rates,and planted mesocosms had higher removal efficiencies than the unplanted controls.The first-order kinetics model fitted the removal of chlorpyrifos in all mesocosms,with half-lives of 10.66-15.43 h.The constructed wetland planted with C.indica had the lowest half-lives.The removal of chlorpyrifos by sorption to the bed substrate was high.A mass balance study revealed that sorption(64.6-86.4%)and biodegradation(8.1-33.7%)were the main removal processes in all mesocosms,while plant uptake was relatively low.Based on the results,wetlands planted with C.alternifolius,C.indica,and I.pseudacorus are promising systems to treat chlorpyrifos-contaminated water,as these plants can enhance chlorpyrifos removal through enhanced biodegradation in these systems.4)In this study,three models of integrated recirculating constructed wetlands(IRCWs),planted with and without C.alternifolius,were used to evaluate the removal of four pesticides(chlorpyrifos,endosulfan,fenvalerate,and diuron).Iron(Fe)-impregnated biochar of C.alternifolius was added as a primary substrate.All four pesticides could efficiently be removed in the three IRCWs.The highest removal rates were achieved when Fe-impregnated biochar was added to the IRCW(99%),followed by planted(64-99%)and plant-free IRCWs(45-99%).The removal of pesticides in the IRCWs followed a first-order kinetics,with half-lives of 1.5-11.6 h.A mass balance study revealed that sorption(32.2-98.6%)and biodegradation(1.3-52.8%)were the main removal processes in all IRCWs,which leads me to infer that the IRCW is a promising system to treat pesticide-contaminated water;in addition,the use of plants and Fe-impregnated biochar can further enhance pesticide removal.5)Carbon isotopic analysis was combined with 454 pyrosequence methods to investigate the in-situ biodegradation of chlorpyrifos during its transport in three models of integrated recirculating constructed wetlands(IRCWs).Based on the results,IRCWs containing plants and Fe-impregnated biochar promoted the degradation of chlorpyrifos and its metabolite 3,5,6-Trichloro-2-pyridinol(TCP).The carbon isotope ratios shifted to-31.24 ±0.58‰(IRCW1,without plants),-26.82 ±0.60‰(IRCW2,with plants),and-24.76 ±0.94 ‰(IRCW3,with plants and Fe-Biochar),respectively.The enrichment factor(?bulk,c)values were-0.69 ± 0.06‰(IRCW1),-0.91 ± 0.07‰(IRCW2),and-1.03 ± 0.09‰(IRCW3).Microbial community analysis further suggested that the enhanced chlorpyrifos degradation with plants and Fe-biochar was mediated by members of the genus Bacillus,which degrade chlorpyrifos.The results therefore suggest that plants and Fe-biochar can induce carbon isotope fractionation and have an impact on the chlorpyrifos degradation efficiency by influencing the development of the microbial community.In summary,the main mechanisms underlying the removal of pesticides from constructed wetlands are adsorption and biodegradation.In this study,typical wetland plants were used;however,after use in the wetlands,the plants can?t be disposed of,which represents an important issue in wetland management.This study indicates that the use of Fe-biochar directly and indirectly affects pesticide removal.In the future,the construction of wetlands for wastewater treatment should consider adsorption and biodegradation of the target substrates to further improve pesticide removal. |
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