Preparation Of Modified Biochar And Its Adsorption Characteristics And Mechanism Of Phosphorus

Effect of modified biochar prepared by different metal chlorides（Fe Cl₃,Al Cl₃,La Cl₃,Mg Cl₂ and Ca Cl₂）and different biomass particle sizes on phosphorus removal had been developed to increase the utilization of wetland plants.In addition,this research used magnesium modified biochar（Mg-BC）pyrolysised from phragmites as adsorbent to study the effect of ammonia nitrogen in water on phosphorus removal.The mechanisms of phosphate adsorption by biochar analysised by FTIR,XRD,XPS and SEM.The results were as follows:（1）At an initial phosphate concentration of 20 mg/L,the phosphate adsorption equilibrium time by biochar and five kinds of modified biochar were sorted in order,La-BC<Mg-BC≈Ca-BC<Fe-BC<Al-BC<BC.At an initial phosphate concentration of1500 mg/L,the adsorption capacity of BC,Mg-BC,Al-BC,Fe-BC,Ca-BC and La-BC to phosphorus were 10.8,104.6,84.8,44.4,79.5 and 34.7 mg/g,respectively,the adsorption capacity of Mg-BC（104.6mg/g）was 9.69 times that of BC.The equilibrium adsorption capacity to phosphorus were sorted in order,Mg-BC>Al-BC>Ca-BC>Fe-BC>La-BC>BC.Therefore,the effect of phosphate adsorption by Mg-BC treated with Mg Cl₂ were better than that of Al-BC,Fe-BC,Ca-BC and La-BC modified with the same molar amount Al Cl₃,Fe Cl₃,Ca Cl₂ and La Cl₃.（2）The experimental result of Mg-BC fitted by pseudo first order and N＿th order kinetic models showed higher matching degree.Pseudo second order kinetic model had the best fitting effect on the kinetic test data of Ca-BC and La-BC.N＿th order kinetic and Elovich models could describe the adsorption kinetic of PO₄^3-by Al-BC and Fe-BC well.The Langmuir adsorption isotherm model could fit the phosphate adsorption isotherm datas of Mg-BC,Fe-BC and Ca-BC.The high correlation coefficients manifested that the phosphate adsorption on Al-BC and La-BC can confirmed to the Freundlich and Timkin models.There was no significant correlation between the maximum adsorption capacity of the five kinds of modified biochar and their specific surface area,total pore volume,average pore diameter and the solubility of phosphate compounds generated.（3）The phosphate adsorption equilibrium time and the equilibrium adsorption capacity by nine kinds of biochar were sorted in order,6～8 mm 0.12 Mg-BC>1～2 mm0.12 Mg-BC>0～0.5 mm 0.12 Mg-BC>1～2 mm 0.012 Mg-BC>6～8 mm 0.012Mg-BC>0-0.5 mm 0.012 Mg-BC>6～8 mm BC>1～2 mm BC>0～0.5 mm BC.（4）Pseudo first order and N＿th order kinetic models had the best fitting effect on the kinetic test data of 0.12 Mg-BC,the kinetic test data of 1～2 and 6～8 mm 0.012Mg-BC could fit N＿th order kinetic model.Langmuir and Langmuir-Freundlich isotherm model could fit the isotherm data of 0.12 Mg-BC.The phosphate adsorption isotherm on 0.012 Mg-BC could be well simulated by Freundlich isotherm model.Therefore,the adsorption effect of 6～8 mm 0.12 Mg-BC treated with 6～8 mm biomass particle size was better than other biochars.（5）The phosphous removal efficiency of the four treatment methods followed the order of Mg-BC>>BC+Mg²⁺≈Mg²⁺>BC.NH₄⁺in the solution promotes the phosphorus removal by Mg-BC.In addition,the larger of the ratio nitrogen to phosphorus and the higher the initial phosphorus concentration,the stronger the phosphorus removal capacity of Mg-BC is.The XRD analysis showed that in the three treatments with Mg-BC,BC+Mg²⁺and Mg²⁺,NH₄⁺reacted with Mg²⁺and PO₄^3-in the solution form Mg NH₄PO₄·6H₂O at N:P=5 or 10,promoting the removal of phosphorus.