Adsorption-desorption Characteristics Of Nitrogen And Phosphorus In Water By Biochar Prepared By Co-pyrolysis Of Sludge And Walnut Shell

The problem of water eutrophication treatment needs to be solved.Which the removal of excessive N/P in eutrophic water is of great significance for the prevention and control of water eutrophication.The biochar adsorption method has the advantages of low cost and wide sources of materials in various treatment methods.Meanwhile,the increasing sludge production needs safe,effective,resource-based and harmless treatment.High temperature pyrolysis of sludge to prepare biochar is an environmentally friendly sludge treatment method.Therefore,our paper aims to use sludge-based biochar to deal with water N/P pollution.In this study,the biochar was prepared by co-pyrolysis of sludge and walnut shell.The adsorption-desorption of ammonium and phosphate in water was studied.The physical and chemical properties of sludge-based biochar were analyzed by using Brunauer-Emmett-Teller analysis,X-ray diffraction,Scanning Electron Microscope and Fourier transform infrared technology.When the ratio of sludge to walnut shell is3:1,the obtained biochar MBC3-1 has a higher adsorption capacity for NH₄⁺in neutral or weak alkaline water.The adsorption of PO₄^3-by SBC in a wide p H range was studied.The adsorption of NH₄⁺and PO₄^3-by MBC3-1 and SBC are controlled by the intraparticle diffusion and pseudo-second-order model.Meanwhile,under different p H value,temperature,time and other conditions,0.1mol/L HCl and Na OH,two different desorption agents respectively to explore the desorption characteristics of the two materials on the adsorbed sludge-based biochar.The desorption results was analyzed by the thermodynamic performance.The highest desorption of NH₄⁺and PO₄^3-reached 0.242mg/g and 2.1mg/g at p H=2,T=25℃and p H=2,T=35℃,respectively.The low p H and temperature are beneficial to the desorption of biochar.With the increase of time,the desorption of PO₄^3-with negative charge decreases gradually due to the existence of desorption lag and the influence of acid environment on chemical reaction,while the desorption of NH₄⁺with positive charge increases gradually.At the micro level,density functional theory（DFT）was used to explore the effects of oxygen-containing functional groups on biochar and the presence of metals on the adsorption of H₂PO₄^-.Different kinds and amounts of oxygen-containing functional groups（hydroxyl,carboxyl and carbonyl）were added to the biochar.Mg/Ca was used to modify the original carbon structure to adsorb H₂PO₄^-in aqueous solution.The electrostatic potential,frontier molecular orbital and exponential reaction,structure analysis,intramolecular atom（AIM）analysis and electronic density of states are analyzed.The strong oxidation and electrostatic attraction of functional groups are the main functions of the adsorption of H₂PO₄^-on carbon materials,and the presence of metal plays an important role in the adsorption of H₂PO₄^-on carbon structure.The adsorption effect of metal modified carbon structure on H₂PO₄^-is stronger than that of unmodified carbon structure.The adsorption effect of Ca modified carbon structure is better than that of Mg modified carbon structure.Through the AIM,the bonding characteristics after adsorption were obtained,which includes electrostatic interaction and covalent bond.Through the comprehensive analysis of biochar adsorption-desorption by macro experimental analysis and micro molecular simulation,a relatively complete mechanism of biochar adsorption of N/P in water is obtained,which provides a reference for the comprehensive treatment of eutrophic water by sludge-based biochar in the future.