Study On Simultaneous Removal Of Nitrogen And Phosphorus By Modified Low-grade Sepiolite

In recent years,China's eutrophication wastewater has seriously threatened the water quality of the Bohai Sea,Xiangjiang River and other rivers and lakes,directly affecting the healthy living and social and economic development of residents.The high content of ammonia nitrogen and phosphorus is the cause of eutrophication wastewater.The traditional chemical and biological methods have high cost,easy to produce secondary pollution and long treatment time.Therefore,it is of great significance to develop green and efficient nitrogen and phosphorus adsorbents.In this paper,calcium modified sepiolite?Ca-Sep?and sodium lanthanum modified sepiolite?Na/La-Sep?were prepared by modification of low grade sepiolite with calcium hydroxide,lanthanum oxide and sodium chloride as active substances.The effect of simultaneous nitrogen and phosphorus removal was greatly improved.The main research contents of the thesis are as follows:?1?Inorganic acid-calcium modification,calcination-calcium modification and different concentraton and different concentration of calcium hydroxide were used to treat raw sepiolite,and the dephosphorization rate of different modified sepiolite was investigated.The results showed that the pretreatment of hydrochloric acid,sulfuric acid and nitric acid was not conducive to improving the dephosphorization rate of Ca-Sep,and above 600?was beneficial to the remolval of phosphate by Ca-Sep.The optimal concentration of Ca?OH?₂ solution is 0.85M,and the saturated adsorption capacity of phosphorus can reach 81.2mg P/g,which is better than of most adsorbents.?2?The effects of solution pH,impurity anion and cation type,ionic strength and surface zero charge on the dephosphorization rate of Ca-Sep were investigated.At the same time,the isotherm adsorption model and kinetic model of dephosphorization were established.Calcium carbonate and hydroxyl groups were found to be active sites for dephosphorization by surface zero point charge analysis.The coexisting metal cations(Fe²⁺,Fe³⁺,Mn²⁺,Cu²⁺,Ni⁺)are all beneficial to the removal of phosphate.While the Coexisting anions are disadvantageous to the removal of phosphate.The interference degree of coexisting anion from small to large is:F^-<NO₃^-<SO₄^2-=Cl^-<CO₃^2-<HCO₃^-.The effect of ionic strength on the ability of Ca-Sep to remove phosphate was studied.The mechanism of Ca-Sep phosphorus removal was obtained as the inner layer complex.The quasi-secondary kinetic model is more in line with the Ca-Sep adsorption phosphate process.By isothermal adsorption studies,Freundlich is more compatible with Ca-Sep adsorption phosphate.Analysis by XRD,SEM-EDS and XPS showed that chemical precipitation and complexation were the main ways of phosphate removal.?3?Study on the concentration of sodium chloride,calcination temperature and three different kinds of inorganic acids to remove ammonia nitrogen from sepiolite.The results show that the optimal sodium chloride concentration of 0.5M,5wt%sulfuric acid is the best condition,but the calcination temperature has almost no effect on nitrogen removal.The optimum conditions for Na/La-Sep modification are:LaCl₃concentration of 0.9 wt%,pH modification condition of 10 and temperature calcination of 200°C.?4?The effects of solution pH,surface zero charge,ionic strength and cation exchange characteristics on Na/La-Sep nitrogen and phosphorus removal were investigated.At the same time,an isothermal adsorption model and a kinetic model of nitrogen and phosphorus removal were established.Neutral solution is beneficial to the removal of ammonia nitrogen,and the removal efficiency of Na/La-Sep on phosphate decreases with the pH of the initial solution.The maximum saturated adsorption capacities of Na/La-Sep for ammonia nitrogen and phosphate were 2.01and 8.15 mg/g,respectively.Cl^-,SO₄^2-and NO₃^-hardly affect the phosphate adsorption capacity of Na/La-Sep,CO₃^2-forms a significant competition for phosphate adsorption,and Mg²⁺and Ca²⁺hardly affect the adsorption of ammonia nitrogen by Na/La-Sep,but Fe³⁺And K⁺have great influence on the adsorption of ammonia nitrogen.Kinetic studies show that the adsorption of ammonia nitrogen is more in line with the quasi-first-order kinetic curve,while the adsorption of phosphate is more in line with the quasi-second-order kinetic curve.Adsorption isotherm studies show that the Langmuir curve is more consistent with Na/La-Sep adsorption of phosphate and ammonia nitrogen.SEM-EDS,XRD and FTIR results show that phosphate is mainly removed by chemical precipitation and internal layer complexes,and ammonia nitrogen is mainly removed by ion exchange.