Adsorption And Desorption Characteristics Of Phosphorus In Sediments From The Three Gorges Reservoir Area

Since the operation,the hydrological situation in the Three Gorges reservoir has changed greatly.Especially,parts of branches inclined to eutrophication because of low velocity of flow.In addition,due to the interception of the Three Gorges Dam,sediment from the upper reaches of the Yangtze River deposition in the bottom of the bottom of the reservoir area.The "source" and "sink" roles of the sediment on the pollutants make the eutrophication problem of the reservoir area more complicated.Phosphorus is the main limiting factor of eutrophication,and has a strong affinity with sediment.Sediment adsorption and desorption process of phosphorus is an important process of phosphorus transport and transport in water.In this article,sediment samples were washed with different HC1 concentration(0～12 mol/L),and the washing effects on the native absorptive substandes,equilibrium adsorptive phosphorus as well as the micro morphology and surface characters of washed sediment were studied.Based on this,the best HCl concentration for the washing pretreatment in the experiment was chose.Then the effects of disturbance intensity,reaction temperature,sediment concentration and initial pH on the adsorption and desorption characteristics of sediment phosphorus were studied by single factor control experiment.Even for the same sediment sample,the equilibrium adsorptive phosphorus varied several times when washed with different of HCl concentration.Most of the native adsorbed substances were released by 0.5 mol/L HCl,and the concentration of HCl higher than 1 mol/L may have impact on the sediment structural framework,even destroyed the sediment structural framework.So the 0.5 mol/L HCl washing pretreatment can be used for the analysis of other substances because of the similar impacts on the sediment erosion and the washing efficiency.Adsorption and desorption of phosphorus by sediment showed that:(1)The adsorption process of phosphorus by sediment could be divided into two stages:a high-speed physical adsorption and a slow-speed physicochemical adsorption stage.A pseudo second-order reaction kinetic equation fitted above the dynamic process.And the desorption of phosphorus from polluted sediment was also divided into the fast and the slow stages,in line with pseudo-second order kinetic model.(2)The Langmuir equation showed better fitting results for the curves adsorption isotherms.With the increase of the disturbance intensity,the increase of the reaction temperature and the decrease of the sediment concentration,the maximum adsorption capacity(Qm)in the adsorption equation increased.The △G°<0,△H°>0 and △S°>0 showed that phosphate adsorption onto sediment was an endothermic process.(3)The greater the disturbance intensity,the greater the content of adsorption.When the adsorption reaches equilibrium,the adsorption capacity of phosphorus at 250 r/min perturbation intensity is 1.39 times than 0 r/min.Under the medium and intensity disturbance,the desorption of phosphorus from polluted sediment increases with the increase of disturbance intensity,but when the disturbance intensity is small(such as 100 r/min),the desorption capacity was similar with the 0 r/min,indicating that the adsorption of phosphorus on the sediment has a certain stability.(4)Temperature is a crucial factor affecting the P adsorption in sediment.When the temperature increased from 8 ℃ to 35 ℃,the maximum adsorption capacity of sediment increased from 0.142 mg/g to 0.207 mg/g,increased 1.54 times.The increase of temperature also promoted the desorption of phosphorus.When the temperature rose from 8 ℃ to 35 ℃,the maximum desorption capacity increased 1.54 times.(5)With the increase of sediment concentration,more phosphorus transferred from the solution to the sediment particles,but the adsorption capacity of phosphorus per unit sediment decreased,which showed a solid concentration effect.When the sediment concentration is higher,the desorption amount of sediment per unit mass is less,but the amount of desorption increases,so that the phosphorus content of water phase increases significantly.(6)The initial pH value of solution had important influence on the adsorption process that sediment adsorption of phosphorus showed a "U"type curve,acid and alkaline conditions can promote the sediment adsorption of phosphorus.The amount of desorption was the least in the neutral solution,and was most significant in alkaline solution.When pH=12,the desorption capacity reached 0.284 mg/g,which was 2.9 times of the desorption capacity at pH = 7.