Study On Ammonium And Phosphate Adsorption Properties And Resource Recovery Of Modified Biochar In Aqueous Solution

Nitrogen and phosphorus are the two main nutrients that cause eutrophication of water bodies.Controlling the content of ammonium and phosphate in water is the key to controlling eutrophication of water bodies.Adsorption is a common method for removing ammonium and phosphate in water.However,pristine biochar（unmodified biochar）has a poor adsorption capacity on ammonium and phosphate.In this study,the discarded spruce sawdust was pyrolyzed at high temperature to generate biochar（Spruce Sawdust Biochar,SSB）,which was modified into MSSB2（the Second Modified Spruce Sawdust Biochar）and LDH/SSB（Layered Double Hydroxide/Spruce Sawdust Biochar）by acid-salt impregnation and co-precipitation,respectively.The factors influencing the ammonium adsorption of MSSB2,the adsorption mechanisms and the feasibility of resource recovery of N-laden MSSB2（MSSB2 adsorded ammonium）as fertilizer were studied.The adsorption characteristics and mechanisms of ammonium and phosphate by LDH/SSB were investigated,and the application potential of N-laden LDH/SSB（LDH/SSB adsorded ammonium）and P-laden LDH/SSB（LDH/SSB adsorded phosphate）as fertilizer was assessed.Compared with SSB（pristine biochar）,MSSB2 could adsorb more ammonium from aqueous solutions.The adsorption capacity of MSSB2 on ammonium first increased and then decreased with the increase of initial pH,and the optimal pH was 9.Under the condition that the concentration of co-existing cations and ammonium were both 40 mg/L,the order of influence of co-existing cations on ammonium adsorption was as follows:Mg²⁺>Ca²⁺>K⁺>Na⁺,where the influence of the presence of bivalent cations on ammonium adsorption is much greater than that of monovalent cations.At ammonium concentrations of20 mg/L and 40 mg/L,the pseudo-second order kinetic model was the best fitting model for adsorption data,and its R² were 0.9994 and 0.9999,respectively.The most suitable adsorption isotherm model is Freundlich model with R² of 0.998.Combined with adsorption behavior and a series of characterization results,it is found that ammonium adsorption is controlled by a variety of mechanisms,among which cationic exchange is the most important adsorption mechanism.Compared with the control group,the experimental group with N-laden MSSB2increased the germination rate by 20%,seedling height by 49.72%,and seedling biomass by82.06%,which shows that N-laden MSSB2 can be reused as N-Fertilizer.Compared with SSB,LDH/SSB has a higher adsorption capacity for ammonium and phosphate.When pH changed from 3 to 10,the ammonium adsorption capacity of LDH/SSB increased slightly at first and then decreased,the overall change range was small.The optimal pH range was 6-7.The adsorption amount of phosphate by LDH/SSB decreased with the increase of pH,and the optimal adsorption pH is 3.The co-existing cations(Na⁺,K⁺,Mg²⁺,and Ca²⁺)with the same concentration of ammonium（NH₄⁺-N）had little effect on the ammonium adsorption performance of LDH/SSB,and the order of influence is:Mg²⁺>Ca²⁺>Na⁺>K⁺,whereas phosphate ions(PO₄^3-)with the same concentration of NH₄⁺-N had no obvious effect on ammonium adsorption.Co-existing ions(SO₄^2-,NO₃^-,NO₂^-,Cl^-,and NH₄⁺-N)with the same concentration of phosphate(PO₄^3--P)had almost no effect on phosphate adsorption by LDH/SSB,indicating that LDH/SSB has good adsorption selectivity for phosphate.Both the optimal kinetic model and the optimal adsorption isotherm model of LDH/SSB for ammonium and phosphate were the pseudo-second order kinetic model and Freundlich model.The process of ammonium adsorption by LDH/SSB is mainly controlled by cation exchange.The adsorption of phosphate by LDH/SSB depends mainly on interlayer ion exchange and electrostatic attraction.LDH/SSB after adsorption of ammonium or phosphate could be used as fertilizer in agricultural production.Compared with the control group,the experimental group with N-laden LDH/SSB increased the germination rate by13.4%,seedling height by 24.92%and seedling biomass by 59.96%.The experimental group with P-laden LDH/SSB（LDH/SSB adsorbed phosphate）showed a 20%increase in germination rate,a 51.4%increase in seedling height,and an 84.15%increase in seedling biomass compared with the control group.Two modification methods improve the ammonium and phosphate adsorption performance of biochar,and the adsorbed modified biochar can realize the resource utilization as fertilizer.Therefore,the modified materials in this study are expected to be environmentally friendly adsorbents.