Rapid Preparation Of Biochar By Microwave-Molten Salt Method And Its Adsorption Of Pb(Ⅱ) In Water

With the rapid development of industry and agriculture and the acceleration of urbanization,a large amount of waste water is discharged directly without proper treatment,causing water pollution and seriously threatening the survival of human beings and other kinds of living things.Therefore,the development of efficient,economical,convenient and environmentally friendly heavy metal removal technology is of great significance for solving the problem of polluted water,ensuring the ecological security of the basin and ensuring the health of people’s drinking water.Biochar is a carbon-rich solid produced by pyrolysis of biomass under oxygen-limited conditions.It has the characteristics of large specific surface area,porous structure,abundant surface functional groups（carboxyl,hydroxyl and phenolic functional groups,etc.）,high mineral content and stable structure,and is considered as an ideal adsorbent for water.However,due to the lack of abundant pore structure,low preparation efficiency,unreasonable distribution of functional groups on the surface,and lack of secondary recovery methods,the adsorption performance of protobiochar in water is not ideal.Therefore,improving activation and surface interface modification methods of biochar has become the key to improve the adsorption performance of biochar.In this paper,a new method for rapid preparation of microwave-molten salt coupled biochar was proposed by using molten salt as sealing,activation and microwave absorber.The influence of biochar under different conditions on heavy metal pollutants was investigated,and the mechanism of improving the performance of biochar materials was revealed.The possible influencing factors in the practical application of modified biochar were preliminarily defined,which provided the foundation and technical support for the application of biochar in the field of environment.The main research contents of this paper are as follows:Using ZnCl₂ as molten salt,rapid carbonization of biomass was realized in 2-4 min under the action of microwave by virtue of its swelling,catalytic dehydration and pore-forming effects on biomass raw materials at high temperature.The effects of different mass ratios of biomass straw and Zn Cl₂,microwave power and time on the performance of biochar were investigated.The results showed that when the mass ratio of straw to Zn Cl₂ was 1:17 absolute mass of 2 grams, microwave power was 1000 W and time was 3 min,the prepared samples had the best performance,and the specific surface area reached 1270 m²·g^-1.The maximum adsorption capacity of Pb（II）is143.5 mg·g^-1,and the equilibrium of the adsorption process can be reached within 60 min.In order to further increase the alkaline functional groups on the surface of biochar and enhance its adsorption performance,Na OH was used to soak and pretreat the straw,and then the biomass raw materials were evenly mixed with Zn Cl₂ at the mass ratio of 1:17 and then microwave heated to obtain biochar（A-MBC）.Infrared spectroscopy（FTIR）and X-ray photoelectron spectroscopy（XPS）analysis showed that Na OH pretreatment increased the contents of carboxyl and methoxyl groups on the surface of biochar,and the maximum adsorption capacity of A-MBC reached 218.28mg·g^-1 through the optimization of functional groups.The adsorption process of Pb（II）on A-MBC is consistent with Redlich-Prterson isothermal adsorption model,indicating that the adsorption behavior of Pb（II）on A-MBC is monomolecular adsorption on homogeneous surface.On the basis of the above experiments,in order to improve the difficult recovery of biochar after adsorption and avoid the problem of secondary pollution,metal salts（Fe Cl₃）impregnation treatment of biomass raw materials was carried out in advance,and metal ions were evenly diffused to the surface of the carbon materials formed by the medium of molten salt at high temperature.Based on the skin effect of nano-phase metal elements formed in situ during carbonization in microwave field,the functional phase complexes with transition metal oxides were constructed in situ on the surface of biochar.The results showed that when the mass ratio of iron ion to biomass was 1:9,the microwave power was 1000 W and the time was 3 min,the biochar（F-Z-MBC）with magnetic Zn Fe₂O₄ modified by Zn O nanoparticles was prepared in one step.X-ray diffraction（XRD）and infrared spectroscopy（FTIR）analysis showed that Zn O and Zn Fe₂O₄ were successfully loaded on biochar.Due to the activation of hydroxyl groups on the surface of biochar by Zn O,the maximum adsorption capacity of Pb（II）on the prepared F-Z-MBC sample reached 225.80 mg·g^-1,and the adsorption process was in accordance with the Langmuir isothermal adsorption model,indicating that the adsorption of Pb（II）in water by F-Z-MBC was mainly monolayer adsorption.