| In addition to a small amount of the soil lead?Pb?from the natural sources,most of Pb pollution is caused by human activities,mainly coming from vehicle exhaust,manufacturing and using of Pb products,sewage irrigation of farmland,pesticides,fertilizers application etc.Studies have shown that Pb will be discharged to the environment up to 5 million tons every year,and most of them go into the water and soil.Pb can be accumulated in the body,and it will damage the internal organs of the organism and interfere with the biochemical and physiological activities.Three Gorges Reservoir?TGR?is the largest regulating reservoir in China.Studies have shown that from 2008 to 2013,water quality of TGR is stable.However,total phosphorus,total nitrogen and Pb are the main pollutants of TGR.The content of Pb is far beyond the World Health Organization recommended content,and also,it exceeds the Chinese drinking water standard and the US Environmental Protection Agency drinking water standard.Heavy metals and other pollutants in the river can be transferred to the soil through adsorption,precipitation and other forms in the water-level-fluctuating zone?WLFZ?.Soil heavy metals and other pollutants,through being dissolved,exchanged and diffused,go into the water and become a potential pollution source.So the WLFZ is the source and the sink of pollutants.Vegetations will be grown vigorously in the WLFZ of Three Gorges Reservoir during the nonflooded period,including Cynodon dactylon,Echinochloa crusgalli,Setaria viridis,Xanthium sibiricum and Alternanthera Philoxeroides.In the process of the plant growth,the roots will excrete organic acids,amino acids,carbohydrates,phenolic acids and other low molecular weight organic matter.LWMOAs are the main components of plant root exudates.LWMOAs will participate in the material cycle and exchange in Three Gorges Reservoir,which will influence the total amount of heavy metals and its speciation in soil/sediment system.Then it will influence the whole Three Gorges Reservoir ecological system.Therefore,in order to provide the reference data for assessing the effects of plant root exudates on the activation of heavy metals in soil and its ecological risk of TGR,this paper qualitatively and quantitatively analyzes the LMWOAs released from Cynodon dactylon and Echinochloa crusgalli roots,which are the dominant species in WLFZ.Furthermore,single and mixed species of LMWOAs are also used to investigate the desorption kinetics of Pb in the vegetation covered soil of TGR.Through alternating drying and flooding in simulation experiments,this paper analyzes the changes of total Pb content and Pb speciation in soil with LMWOAs excreted by dominant plant roots.The results show that:1.The variety and content of LMWOAs secreted from Cynodon dactylon roots were larger and higher than those of Echinochloa crusgalli.Citric acid,malonic acid,acetic acid and malic acid were LMWOAs released from the two types of plants.The contents of acetic acid were the highest among other LMWOAs,which are 0.765 and 0.261 mmol·kg-1in Cynodon dactylon and Echinochloa crusgalli roots.Citric acid,malonic acid and acetic acid were the same LMWOAs which were detected from their rhizosphere soils.2.In the citric acid,malonic acid and acetic acid desorption test,the desorption captivities of Pb was inhibited when the concentration of LMWOAs were less than 1 mmol·L-1.The desorption ability displays a decreasing order like malonic acid> citric acid> acetic acid.However,the three LMWOAs promoted the desorption of Pb in soil when their concentrations were more than 1 mmol·L-1 respectively.The desorption ability displayed a decreasing order like citric acid>malonic acid> acetic acid.3.The desorption of Pb by LMWOAs in the soil was dominated by chemical processes,with heterogeneous diffusion and intraparticle diffusion,which was a complex dynamic process.The fitting effect of desorption kinetic equation displayed a decreasing order like pseudo-secondorder kineti > double constant equation> Elovich equation> parabolic equation.4.Through alternating drying and flooding in simulation experiments,the Pb content was decreased in the soil surface.The percentage of Pb content in the surface soil after the second flooding was higher than the first alternation of drying and flooding.After two cycles of drying and flooding,the Pb content of each treatment group displayed a decreasing order like mixed acid treatment> citric acid treatment and acetic acid treatment> malonic acid treatment > control treatment.The results indicated that LWMOAs could promote the adsorption of Pb when the Pb content was high in the WLFZ soil.For the underlying soil,the Pb content increased gradually,and the Pb content of each treatment group displayed a decreasing order like mixed acid treatment> citric acid treatment and acetic acid treatment> malonic acid treatment> control treatment.There was no significant difference in the content of Pb in the overlying water of the two flooding.Nevertheless,the Pb content of the first flooding was higher than the second flooding in the overlying water.5.A small amount of LWMOAs could promote the Pb speciation from low stable state to high stable state in the surface of the soil.Fe/Mn oxides fraction,organic fraction and residual fraction Pb were increased in the underlying soil.Comparison of the two drying and flooding cycles,the content of exchangeable fraction,Fe/Mn oxides fraction,organic fraction and residual fraction Pb after the second drying and flooding cycle were higher than the first time after the end of the drying and flooding cycle,and the content of carbonate fraction Pb was decreased in the soil surface.The content of exchangeable fraction,carbonate fraction and residual fraction Pb changed little during the two alternation of drying and flooding cycles in the underlying soil,but the content of Fe/Mn oxides fraction was increased and the content of organic fraction was decreased after the second drying and flooding cycle in the underlying soil.Pb of the surface soil was very high risk,while Pb of the underlying soil was high risk,and all of them had greater environmental ecological risk to TGR. |
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