Preparation Of MgO-biochar Via Co-pyrolysis Of Biomass And Mg-based Additives And Its Phosphorus Adsorption Performance

As an important product of biomass pyrolysis,biochar can be used as an adsorbent to recover phosphorus from wastewater,and then can be used as a raw material to prepare carbon-based fertilizer to improve soil fertility.Based on this,in this paper,a method of preparing magnesium-rich biochar by biomass magnesium-rich pyrolysis is proposed,so that micro-nano magnesium oxide particles are formed in the biochar generation process,and the phosphorus adsorption capacity is greatly improved.The main research conclusions are as follows:Firstly,TGA-FTIR,in-situ DRIFT and 2D-PCIS were used to analyze the pyrolysis characteristics and the evolution process of typical functional groups of the three components of biomass and magnesium citrate during the co-pyrolysis process.It was found that the glycosidic bond breaking and pyranoid ring opening reaction occurred at the beginning of the co-pyrolysis process of cellulose and magnesium citrate.In the co-pyrolysis process of xylan and magnesium citrate,the hydroxyl group in xylan takes place preferentially in dehydration reaction rather than ring-opening reaction.In the co-pyrolysis process of lignin and magnesium citrate,the bonding bonds between benzene rings of lignin were broken first,and the structures such as side chain aliphatic hydrocarbons and alcohol hydroxyl groups were gradually removed.After that,the aromatic ring structure reconstruction,poly-condensation and formation of thick ring structure began to occur.Secondly,the effects of pyrolysis temperature and mixing ratio on the magnesium-rich pyrolysis characteristics of lotus seedpod shell were studied in a fixed bed,and the evolution of organo-magnesium salts in the pyrolysis process were explored.The study found that with the increase of magnesium citrate,the formation of CO₂,phenols and aromatics products was promoted.While the temperature is below 450℃,magnesium-rich pyrolysis increases the yield of acetic acid,nitrogen compounds and dehydrated sugars.However,when the temperature is higher than 550℃,the Mg O and hydrogen radicals generated by the decomposition of magnesium citrate promote the generation of ketones and phenols,accompanied by the release of gas.Previous studies have shown that magnesium salt precursors have important effects on the structural properties of Mg O-biochar,thus the synergistic effects of four common magnesium salts（magnesium chloride,magnesium nitrate,magnesium acetate,and magnesium citrate）on biomass pyrolysis and biochar structure were studied.It was found that the addition of magnesium chloride and magnesium nitrate promoted the generation of ketones and nitrogenous components.The addition of magnesium acetate promoted the production of acetic acid and furan.In addition,magnesium chloride promoted the formation of magnesium oxide whisker,magnesium nitrate and magnesium acetate promoted the formation of flake magnesium oxide crystal,and magnesium citrate promoted the formation of nanometer scale and uniform distribution of spherical magnesium oxide particles.Then,the adsorption process of phosphate on Mg O-biochar obtained above was studied.It was found that the adsorption of phosphate firstly generated Mg OH⁺on the surface of biochar through Mg O protonation,and then recombined with phosphate ion through electrostatic attraction.Finally,Mg OH⁺reacts with the adsorbed phosphate ions to form Mg₃（PO₄）₂ and Mg HPO₄ precipitation.In addition,a large number of C-H/C-C functional groups of aliphatic chains on the surface of biochar are recombined with phosphoric acid through hydrogen bonds,which can greatly improve the adsorption capacity and adsorption rate of the rapid adsorption process.Finally,the influence of water environment on phosphorus Adsorption performance of Mg O-biochar was investigated,and Aspen Adsorption software was used to establish the mathematical model of phosphate adsorption,and economic analysis was conducted.The study found that the Mg O-biochar prepared by the co-pyrolysis of magnesium citrate and lotus seedpod has strong p H adaptability,faster adsorption speed,higher adsorption capacity,and the highest Mg O utilization efficiency.According to the fixed bed adsorption simulation,the penetration time of Mg O-biochar through the adsorption column can be up to 660 min.The cost-benefit method was used to evaluate the technology and economy of the project of converting lotus seedpod into magnesium-rich biochar.Based on sensitivity analysis,the market risk of the project mainly comes from the product price.