| Since the completion of the Three Gorges Dam,due to the influence of flood control,the water level in the reservoir area fluctuates periodically,forming a huge water level fluctuation zone on the bank.On the one hand,the material transport and energy transformation of the ecosystem are very active,on the other hand,the species composition and functional structure of the ecosystem are greatly limited,which makes the water level fluctuation zone gradually become a sensitive and fragile ecosystem.In recent years,the pollution of polycyclic aromatic hydrocarbons(PAHs)in different degrees has been found in many areas of the reservoir due to the frequent influence of shipping and human activities.Soil in the water level fluctuation zone is the main "sink" of PAHs,and the migration and transformation behavior of PAHs directly affects the water quality safety in the reservoir area.Plants in the water level fluctuation zone show a renewal process of growth-submersion decomposition-growth with the change of water level.Low molecular weight organic acids(LMWOAs)are released during plant growth and decomposition,which affect the migration and transformation of PAHs in soil.Therefore,based on the research theories of domestic and foreign scholars on LMWOAs,this paper takes the content of LMWOAs and PAHs in root system in the soil of the fluctuation zone as the research object.It studied the characteristics of LMWOAs in the root soil of plants in the fluctuating zone and their effects on the migration and transformation of soil PAHs in the fluctuating zone based on the dry-wet alternating characteristics of water level regulation in the Three Gorges Reservoir area.The main research contents and basic conclusions are as follows:(1)In this chapter,the soil vegetation at different altitudes in the water level fluctuation zone and the 16 PAHs monomers controlled by EPA are the research objects.When the water level dropped to 145 m in June 2020,the soil and vegetation in the water level fluctuation zone were completely exposed.The characteristics of vegetation and the distribution of soil PAHs at different altitudes in the water level fluctuation zone were monitored and analyzed.The results showed that the plants in the fluctuating zone were all herbs,and the annual herbs were absolutely dominant,and the Gramineae species were the most abundant,accounting for 78.78%of the total plants in the fluctuating zone of Xiangxi River;Cynodon dactylon is the most abundant and most widely distributed plant in the water level fluctuation zone,followed by Cyperus rotundus,Setaria viridis and Bidens pilosa,and the rest of the plants are distributed in a small amount at specific points and elevations;The total amount of PAHs in the soil in the water level fluctuation zone was between 110.48-982.99 ng/g,with an average value of 486.18ng/g,mainly in the middle and low rings,and Nap accounted for the highest proportion in the monomer,reaching 27.56%;The PAHs content of each sampling point was as follows:Y7>Y5>Y2>Y1>Y3>Y6>Y4.Among the four elevations,the PAHs content at the 175 m elevation was the highest,which was 539.24 ng/g,accounting for 27.73%.(2)In this chapter,Cynodon dactylon is taken as the research object,and the in-situ hanging bottle is used to simulate the flooding changes of plants in the fluctuation zone in the actual cycle.It analyzed the variation characteristics of LMWOAs of Cynodon dactylon under different flooding depths and different flooding times and their effects on the migration of PAHs monomers phenanthrene and pyrene at the soil-water interface.The results showed that the content of LMWOAs in soil: acetic acid > oxalic acid > citric acid > tartaric acid > malonic acid,LMWOAs showed significant differences in response to water level fluctuations,and the content of LMWOAs in soil also increased with the increase of flooding time;The concentrations of phenanthrene and pyrene in the soil increased with the increase of flooding time,and the variation trend of phenanthrene and pyrene in the soil was similar with the increase of flooding time and depth;The effects of LMWOAs on the migration of PAHs at the watersoil interface were analyzed by fugidity method.During the flooding process,the phenanthrene and pyrene showed a tendency to be released from the soil to the overlying water,and the soil in the water level fluctuation zone was the secondary release source of the phenanthrene and pyrene during the flooding process;Correlation analysis showed that under different flooding depths and times,the content of phenanthrene in the soil was significantly positively correlated with the content of tartaric acid(p< 0.01),and was significantly positively correlated with the content of oxalic acid(p< 0.05).The effects of phenanthrene and pyrene in soil are quite different.(3)In this chapter,the typical PAHs monomers phenanthrene and pyrene in the soil of the water level fluctuation zone are used as the research objects,and different dry-wet alternation frequencies and dry-wet alternation cycles are simulated by indoor water flooding experiments.It analyzed the morphological characteristics of soil LMWOAs,phenanthrene and pyrene,and studied the effect of soil LMWOAs on the morphological transformation and bioavailability of soil PAHs under different dry-wet alternation backgrounds.The results showed that there were significant differences in the composition of soil LMWOAs under the action of different drywetting cycles,and the time of flooding would promote the secretion of LMWOAs by plant roots;The contents of phenanthrene and pyrene were different under the action of different frequency of dry-wet alternation.The phenanthrene in the soil mainly existed in the organic bound state,and the pyrene mainly existed in the acid-soluble state and the organic bound state;Under the action of LMWOAs,the morphology of phenanthrene and pyrene in soil changed,the water-soluble and acid-soluble state of phenanthrene decreased with the increase of LMWOAs concentration,and the correlation between pyrene morphology and LMWOAs was low;Different types of LMWOAs had differences in the release of residual PAHs.The residual state of phenanthrene had a significant positive correlation with malonic acid and acetic acid(p< 0.05),a moderate positive correlation with citric acid,and a low negative correlation with oxalic acid.(4)Based on the above researches,this chapter takes tartaric acid,oxalic acid and malonic acid as the research objects to explore the adsorption-desorption effects of these three different LMWOAs on the typical PAHs monomer phenanthrene in the water level fluctuation zone.Then,the influence mechanism of LMWOAs on the migration and transformation of PAHs was analyzed.The results showed that the adsorption of phenanthrene in soil was significantly reduced in the presence of low molecular organic acid.Different types of LMWOAs had different effects on soil adsorption of phenanthrene,and the adsorption of phenanthrene was the lowest in the presence of oxalic acid.The second-order kinetic equation can better describe the kinetic process of soil adsorption of phenanthrene in the presence of LMWOAs;The presence of LMWOAs inhibited the adsorption of phenanthrene in soil.Soil adsorption of phenanthrene was mainly due to the combined action of inorganic minerals and soft carbon organic matter in the soil,and it was a linear adsorption dominated by distribution;In the adsorption of high concentration of phenanthrene,the analytical hysteresis of the three organic acids is: oxalic acid > tartaric acid > malonic acid.The addition of LMWOAs will promote the migration and transport of hydrophobic organic pollutants in the water environment. |
