Study On The Preparation And Adsorption Properties For Phosphorus And Fluorine Of Lanthanum-Modified Biochar

In the world,freshwater resources were threatened by anion pollution in water bodies,such as eutrophication of natural surface water bodies and excessive fluoride ions in underground drinking water bodies.So far,Adsorption method was considered to be suitable for removing phosphorus（P）and fluorine（F）pollutants in water because of its simplicity,efficiency,safety and low effluent concentration.In this work,lanthanum-loaded biochar was prepared by a simple one-step pyrolysis method using pretreated agricultural waste-peanut shells and LaCl₃·7H₂O as the lanthanum source.In order to obtain the best material for removing phosphorus and fluorine from lanthanum-loaded biochar,the mass ratio of peanut shells and LaCl₃?7H₂O,the pyrolysis temperature and the holding time were optimized.In the process of phosphorus removal and fluorine removal,the effects of solution pH,reaction time,adsorbent dosage,initial concentration and reaction temperature on the adsorption performance of lanthanum-loaded biochar were investigated,respectively.Meanwhile,the practical application potential of the adsorbent material was tested through the ion coexistence interference experiment and the actual water sample spike experiment.In addition,combined with kinetic model,thermodynamic model and SEM,XRD,FTIR results,the mechanism of phosphorus removal and fluorine removal by lanthanum-loaded biochar was explored,respectively.When the mass ratio of peanut shell to LaCl₃?7H₂O was 5:3,and the pyrolysis temperature and holding time were 600°C,1 h and 500°C,1 h,the obtained lanthanum-loaded biochars La-BC-600 and La-BC-500 showed the best phosphorus removal performance and fluorine removal performance,respectively.The SEM,XRD and FTIR results showed that lanthanum modification was beneficial to increase roughness of the carbon-based surface,and pyrolysis temperature maked lanthanum crystal types different.Among them,the lanthanum species of La-BC-600 were La OCl and La（OH）₃,while the lanthanum species of La-BC-500 mainly included La OCl,La（OH）₃and La（OH）₂Cl,and a small amount of La₂（CO₃）₃and La PO₄.The zero charge point(pH_pzc)of La-BC-600 was 6.2,and the phosphorus removal process was mainly affected by factors other than temperature,including solution pH,reaction time,La-BC-600 dosage and initial phosphate concentration.The adsorption process of La-BC-600 obeyed the Pseudo-second-order model and the Langmuir isotherm.Meanwhile,thermodynamic parametersΔH⁰<0,ΔS⁰>0,ΔG⁰<0,indicating that the reaction was a spontaneous exothermic process of entropy increase.When the reaction temperature was 45℃,the maximum theoretical adsorption capacity of La-BC-600 was33.98 mg/g.The adsorption process of La-BC-600 was greatly affected by SO₄^2-,CO₃^2-and HCO₃^-.And,the phosphorus removal efficiency of La-BC-600 in the actual surface water environment application was improved by increasing the dosage of adsorbent,so that the effluent concentration was lower than 0.02 mg/L.Phosphate was removed by reaction with La（OH）₃in La-BC-600 and converted to La PO₄by electrostatic interaction and chemical precipitation.More interestingly,a large number of stable La PO₄nanofibers were formed on the La-BC-600 surface via the reaction between the dissolved phosphate anions and La（OH）₃loaded on the adsorbent.Their intertwining facilitated the formation of the floc,which was conducive to the solid–liquid separation.In addition,P-loaded La-BC-600 was not eluted by0.1 mol/L Na OH,suggesting that the La-BC-600 was not only a P-capping agent but also a P-immobilizing agent.The zero charge point(pH_pzc)of La-BC-500 was 6.4.When the pH of the solution was5～8,the fluorine removal efficiency remained stable.Therefore,in the first 15 min of the reaction,the fluorine removal efficiency of La-BC-500 has exceeded 50%of the equilibrium time.The increasing rate of fluorine removal efficiency of La-BC-500 was positively correlated with the dosage of adsorbent and reaction temperature,and negatively correlated with the initial fluoride ion concentration.The adsorption process of La-BC-500 conformed to the Pseudo-second-order model and Langmuir isotherm,indicating that the process was a homogeneous monolayer chemical adsorption.In addition,the thermodynamic parameters?H⁰>0,?S⁰>0,?G⁰<0,indicating that the reaction was a spontaneous endothermic process with entropy increase.When the reaction temperature was 35℃,the maximum theoretical adsorption capacity of La-BC-500 was 33.17 mg/g.XRD results showed that the lanthanum-containing species on the La-BC-500 included La（OH）₃,La OCl,La（OH）₂Cl and a small amount of La₂（CO₃）₃and La PO₄.Fluoride was removed mainly by reaction with La（OH）₂Cl and converted to La F₃by electrostatic interaction and chemical precipitation.The adsorption and defluorination process of La-BC-500 was most affected by CO₃^2-and HCO₃^-.In the actual groundwater environment application potential test,the fluoride removal efficiency was improved by acidification pretreatment,so that the effluent concentration was lower than 1.0 mg/L,which met the Chinese domestic drinking water standard（GB5749-2006）.