| The changes of water and sediment environment caused by the construction of large reservoirs in the Lancang River Basin have been concerned by the downstream countries and even the international community.Due to damming and impoundment of rivers,the flow velocity of reservoirs decreases,the hydraulic retention time increases and the nutrient accumulation continues,which leads to the decline of water ecological environment quality.Phosphorus is the main limiting factor of water eutrophication,which is constantly transferred and transformed between overlying water and sediment.The“sink”and“source”effect of phosphorus released by sediment adsorption has an important influence on the eutrophication of reservoir.In this paper,the sediment of Xiaowan Reservoir was taken as the research object.Firstly,the physical and chemical properties of the sediment and the occurrence form of phosphorus in the sediment were analyzed to understand the differences between the sediments of each sampling point,and then the characteristics of phosphorus adsorption and release in the sediment were studied.The single-factor control variable method was used to simulate the adsorption and release of phosphorus by sediments under different initial p H values,disturbance intensities and reaction temperatures.Combined with the mathematical model,the adsorption and release characteristics of phosphorus by sediments in Xiaowan Reservoir were quantitively analyzed.The main results are concluded as follows:（1）The sediment particles at upper Xiaowan dam and Yubapingtan（middle part of the reservoir）are mainly silt,while the sediment particles at the Wayao wharf（tail of reservoir）are mainly sand.The composition of clay in the sediments of Yubapingtan is the highest,accounting for 29.41%,and the composition of clay minerals such as Muscovite,calcite and chlorophyllinite is the highest,accounting for about 49%.（2）The contents of organic matter,total nitrogen and total phosphorus in the sediments at upper dam were the highest,and the contents of all the sediments in the Yubapingtan were the lowest.The main component of total phosphorus（TP）in Xiaowan reservoir is inorganic phosphorus（IP）,and the main component of IP is calcium bounded phosphorus（Ca-P）.The organic phosphorus（OP）content in all sediments had little difference.The IP content in the sediments at upper dam accounted for the highest proportion of TP（86.58%）,and the Ca-P content in the sediments also accounted for the highest proportion of IP（74.43%）.The proportion of active phosphorus in the sediments at upper dam,Yubapingtan and Wayao wharf was 36.78%,64.55%and 56.74%,respectively,indicating that the risk of releasing phosphorus in the sediments of Yubapingtan was the highest,while the risk of releasing phosphorus in the sediments at upper dam was low.However,the active phosphorus in the sediments at upper dam was as high as412.68 mg·kg-1,and the potential of releasing phosphorus into the water was the highest.（3）The results of acid-base experiments on phosphorus adsorption and release from sediments showed that Ca-P was easily dissolved in large quantities under acidic conditions,while iron/aluminum bounded phosphorus（Fe/Al-P）was easily dissolved in large quantities under alkaline conditions.With the increase of initial p H value of overlying water solution from 2 to 12,the phosphorus release of sediments showed a symmetrical“U”trend,indicating that the higher the acidity of water was,the greater the phosphorus release was.Similarly,the stronger the alkalinity of water was,the greater the phosphorus release was,and the phosphorus release of sediments was the smallest in the neutral range.When the p H of overlying water phosphorus solution was in the range of 4～11,the adsorption amount was basically positive,and there was no significant difference in the value.Under extremely acidic（p H=2,3）and extremely alkaline（p H=12）conditions,the adsorption capacity was negative.After the reaction,the p H values of the overlying solutions varied between 5 and8,indicating that the sediments had a strong buffer capacity for p H values.（4）The experimental results of phosphorus adsorption and release on sediments under disturbance intensity showed that the greater the disturbance intensity was,the greater the phosphorus adsorption amount was.At the end of the adsorption reaction,the phosphorus adsorption capacity of sediments at upper dam,Yubapingtan and Wayao wharf under 200 r/min disturbance intensity was1.59,1.68 and 1.58 times of that under 0 r/min,respectively.The greater the disturbance intensity,the greater the sediment adsorption rate of phosphorus,shorten the adsorption time to reach equilibrium.The higher the disturbance intensity,the higher the rate of phosphorus adsorption by sediments,and the shorter the time of adsorption to equilibrium.With the increase of disturbance intensity from 0 r/min to 200 r/min,the release of unit sediment at upper dam,Yubapingtan and Wayao wharf increases by 0.75,0.22 and 0.30 times,respectively.Under the same water disturbance,more phosphorus will be released from the sediments in front of Xiaowan dam.（5）The experiment of phosphorus adsorption and release by reaction temperature on sediments showed that when the temperature of overburden water increased,the phosphorus adsorption capacity of sediments increased.When the reaction temperature was 31℃,the maximum adsorption capacity at upper dam,Yubapingtan and Wayao wharf were 1.75,1.25 and 1.29 times of that of 8℃,respectively.The increase of temperature increases the rate of phosphorus adsorption and release from sediments.The initial adsorption rates at 10℃,17℃,24℃and 31℃are 0.016,0.027,0.070 and 0.067 mg.（g.h）-1,respectively,and the time to complete the process of phosphorus release from sediments at 31℃is half shorter than that at 10℃.The effect of temperature on phosphorus adsorption/release from sediments is asymmetric,and the effect of temperature on the process of phosphorus adsorption is greater than that of phosphorus release from sediments.（6）The experimental data fitting results of phosphorus adsorption and release kinetics of sediments in Xiaowan reservoir area showed that the whole process of adsorption and release can be divided into rapid stage,slow stage and equilibrium stage.The pseudo-second-order kinetic model is suitable to describe the adsorption and release process of phosphorus in sediments of Xiaowan reservoir area.The pseudo-first-order kinetic model can well simulate the adsorption and release process without disturbance.The results of isothermal adsorption experiments showed that the larger the disturbance intensity and the higher the reaction temperature,the greater the isothermal adsorption of phosphorus on sediments.When describing and characterizing the isothermal adsorption process of phosphorus in sediments of Xiaowan Reservoir,the modified Langmuir model should be selected.The adsorption results of adsorption thermodynamics showed thatΔG~0<0,ΔH~0>0 andΔS~0>0were the adsorption reactions of phosphorus by sediments in the reservoir area,which were spontaneous endothermic reactions mainly driven by entropy.This study revealed the main physical and chemical characteristics of the sediments of Xiaowan Reservoir,as well as the characteristics of phosphorus adsorption and release by the sediments in the reservoir area.The dynamic model and isothermal adsorption model suitable for simulating the process of phosphorus adsorption and release by the sediments of Xiaowan Reservoir were selected.The results of this study provide a theoretical basis for the study of the properties of reservoir sediments and the mechanism of nutrient adsorption and release,as well as the improvement of ecological environment and sustainable development of the reservoir area,and have a very important practical guiding significance for the prevention and control of eutrophication in the reservoir. |
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