Study On The Occurrence Characteristics And Migration And Transformation Of Phosphorus In The Sediments Of Nanyi Lake

The protection and restoration of the hydrological and ecological environment in the Yangtze River basin plays a significant role in the development of modern ecological and environmental civilization.However,there are many lakes in the Yangtze River basin,and lake eutrophication is one of the important issues facing us today.It is of great significance to carry out lake environmental pollution control and governance.In order to study the eutrophication status of typical shallow lakes in the lower reaches of the Yangtze River and the migration and transformation characteristics of endogenous phosphorus,based on the environmental background of"Yangtze River Protection",the research object is Nanyi Lake in Xuancheng City,the largest lake in southern Anhui,a freshwater lake flowing out of the southern bank of the lower reaches of the Yangtze River in China.Based on the key research and development plan of Anhui Province,"Analysis of phosphorus pollution sources and study on endogenous phosphorus release characteristics of typical small and medium-sized lakes in the Anhui River Basin"（202004i07020006）,Through the combination of field investigation and indoor simulation experiment,the occurrence forms of phosphorus in sediment were systematically analyzed by using the method of graded extraction of inorganic phosphorus in sediment,the distribution characteristics of phosphorus forms in sediment and water body were studied,and the relationship between phosphorus forms in sediment and overlying water body and interstitial water was analyzed to reveal the main source of phosphorus in Nanyi Lake.Through the indoor simulated sediment phosphorus adsorption equilibrium experiment,the migration and transformation of P at the water-sediment interface under the influence of different environmental factors were explored.At the same time,the fuzzy comprehensive evaluation method is used to evaluate the change and development trend of water quality indicators of Nanyi Lake from 2015 to 2020.It is intended to provide data support for further studying the distribution characteristics of phosphorus occurrence and migration and transformation in its sediments,and improving the relevant data,systematic management and risk prevention and control of lakes in the middle and lower reaches of the Yangtze River.The main research conclusions are as follows:（1）The analysis of the concentration of water quality factors(DO,COD_Mn,NH₃-N,TP,TN)in Nanyi Lake from 2015 to 2020 showed that the content of NH₃-N was significantly reduced,the mean value of TN,TP and COD_Mn did not change significantly,and the overall concentration of TN and TP increased first and then decreased.The change trend of DO concentration was first decreased and then increased,and its mean value was slightly increased.Except for DO,the water quality index data of Shizikou are significantly higher than the monitoring surface of the lake center.DO,COD_Mn,NH₃-N are basically in the I-III standard value,while TN and TP are in the III-IV standard limit value fluctuation.There are even seasonal concentrations exceeding the standard.With the passage of time,the water quality of the monitoring area in the center of the lake showed an overall trend of improvement.In space,the overall water quality of the west lake is higher than that of the east lake.（2）The content of TP in sediment is 463.3～1016.6 mg/kg,with an average of654.7 mg/kg.The spatial distribution shows a trend of gradual decrease from the center to the four sides.The spatial distribution of the six forms of P is significantly different.The order of their relative content proportion is:calcium-bound phosphorus（Ca-P）>iron-bound phosphorus（Fe-P）>aluminum-bound phosphorus（Al-P）>reducing agent soluble phosphorus（RS-P）>residual phosphorus（Res-P）>weakly bound phosphorus（L-P）.The relative proportion of the total amount of L-P,Al-P,Fe-P and RS-P with high bioavailability in the phosphorus occurrence forms of sediment can reach 60%.According to the correlation analysis between the phosphorus forms in sediment and the phosphorus concentration in interstitial water and overlying water,most of the phosphorus forms are significantly correlated,indicating that they have important effects on each other during their migration and transformation.The variation range of EPC₀ in the sediments from the main entrance and exit lakes and estuaries is0.009～0.014 mg/L,all of which show that EPC₀<SRP,which may indicate that the main estuarine sediments are still the"sink"of exogenous phosphorus input at present.（3）The phosphorus adsorption equilibrium experiment of the sediment showed that the adsorption rate of P on the sediment decreased with the increase of time,and basically reached the equilibrium within 0～10h,showing a fast adsorption stage.After10h,the adsorption rate is less than 3 mg/（kg·h）,which is a slow adsorption stage.At low initial P concentration,the capacity of Nanyi Lake sediment to adsorb P in water body gradually increases with the increase of initial concentration.Temperature rise does not promote the migration of P from overlying water to sediment,but promotes the release of P from sediment,indicating that the adsorption of P by sediment is an exothermic process.PSO model and Langmuir model are the best fit for the kinetics and isothermal adsorption process of phosphorus adsorption by sediment.When the concentration of P in the overlying water is lower than 1mg/L,the disturbance intensity has no obvious effect on the release of P from the sediment.On the contrary,increasing the disturbance intensity will promote the adsorption of low concentration of P in the sediment.When the concentration of P in the overlying water is higher than 1mg/L,increasing the disturbance intensity will promote the release of P in the sediment.Adding FA/HA to the overlying water can improve the phosphorus adsorption capacity of the sediment.