Study On Phosphorus Fractions And Adsorption/Release Characteristics Of Sediments In Shuangji River,Xinmi City

Xinmi Shuangji River is a tailwater type river,mainly composed of tailwater from the sewage treatment plant,coal mine well water and tailwater from paper wastewater treatment plant.The water quality category of the outbound water in 2017 was surface water nationalⅤstandard.After a series of practical measures since 2018,the water quality has been effectively improved,and the water quality of the outbound water in2019 was the surface water nationalⅣstandard.However,the Total Phosphorus concentration in the water body was significantly higher in summer,with an average value of 0.25 mg/L,which exceeds the surface water national III standard.In order to further improve water quality and reach the target of surface water national III standard in the whole basin of Xinmi Shuangji River,it is necessary to study the characteristics of sediment phosphorus distribution,adsorption/release characteristics,discuss the risk of phosphorus release,and provide management directions for the surface water environment management personnel.Through the study,it was found that:（1）The Comprehensive Pollution Index method’s evaluation results show that the overall water quality of the Shuangji River basin in Xinmi City was in a moderate pollution status,with the most polluted upstream section being in the heavy pollution class.It was also found that the distribution of water quality conditions in the river was influenced by the type of overlying water and was similar to the distribution of the scope and intensity of human activities.Besides,phosphorus concentrations in the overlying water bodies showed differences in spatial and temporal distribution,with high phosphorus concentrations in summer and low phosphorus concentrations in winter in the temporal distribution.In terms of spatial distribution,the phosphorus concentration in the upstream section,which was heavily influenced by human activities,was much higher than that in the middle and downstream sections.（2）The phosphorus content in the Shuangji River basin’s surface sediment was determined by sequential extraction methods.The total phosphorus content ranged from 354 to 1626 mg/kg,with inorganic phosphorus being the main component.For the heavily polluted river sections,such as the upper reaches,the organic phosphorus（OP）content was relatively high.The Fe-P content accounted for the most significant proportion of inorganic phosphorus.For the overlying water,which was cleaner water such as coal mine well water,the sediment’s Ca-P content was higher,which was related to the Cinnamon soil type in Xinmi and was mainly influenced by geographical conditions.Correlation analysis showed that the sediment’s phosphorus content was influenced by elements such as organic matter,Fe,Al and Ca,and physicochemical properties such as particle size.（3）The Degree of Phosphorus Saturation（DPS）and Phosphorus Sorption Index（PSI）of sediments were determined experimentally.It was found that the active iron content in the sediments of the Shuangji River Basin in Xinmi City was the main element influencing phosphorus adsorption in sediments and played a significant role in phosphorus adsorption and fixation.The phosphorus release risk index（ERI）was found to be high or high risk in the sections of the river and tributaries that are heavily influenced by human activities.（4）To investigate the influence of environmental factors on the release of phosphorus from sediments,it was found that the higher the temperature,the more favourable the release of phosphorus,and the higher temperature promoted the release of organic phosphorus and Fe-P indirectly by promoting microbial activity and changing the redox potential of the sediment.Under alkaline conditions,phosphorus release was increased through ion exchange.The release of phosphorus from anaerobic conditions was relatively easy compared to aerobic conditions,mainly by reducing the redox potential of the sediment,reducing Fe³⁺to Fe²⁺in the sediment and promoting the release of phosphorus bound to Fe（OH）₃.Disturbance accelerates sediment-water exchange mainly through physical processes,causing sediment resuspension and increasing the diffusion of Labile-P into water.（5）The experimental results of sorption kinetics and sorption thermodynamics showed that phosphorus sorption processes were predominantly chemical and readily spontaneous.A modified Langmuir model can better describe the sorption thermodynamics for sediments that have already sorbed significant amounts of phosphorus.The equilibrium phosphorus concentration（EPC₀）was calculated for each river section by linear fitting of sorption isotherms at low concentrations.It found that EPC₀ showed spatial and temporal variability,being higher in summer and autumn,closely related to human activities.