Study On Microwave Carbonization Of Bagasse With Metal Oxides And Its Adsorption Mechanism On Cr(Ⅵ) And As(Ⅴ)

Heavy metal（HM）pollution is one of the most hazardous and persistent water pollutions,As one of the main techniques for removing heavy metals in water,the adsorption method has the advantages of simple processing,fast speed,low cost,and low residual concentration of pollutants.The key to successfully commercializing the technology is to develop new adsorptive materials from renewable sources at low cost with excellent performance.Bagasse is characterized by high yield,simple collection,relatively stable composition,and homogeneous properties,it can be used as raw material for preparing new heavy metal adsorption materials.The purpose of this work is to improve the carbonization rate of sugarcane bagasse to reduce the preparation cost,maximize the heavy metal adsorption capacity in water bodies,and rapidly achieve solid-liquid separation.Based on thermodynamics,kinetics and molecular orbital theory,etc.,and supplemented with modern analytical techniques,the adsorption performance of the new composite adsorbent materials on the simulated wastewater containing chromium and arsenic and the corresponding adsorption mechanisms were systematically investigated.The main research results were shown as follows:1.When metal oxides were selected as the wave absorption additives for microwave carbonization of sugarcane bagasse,different metal oxides exhibited different microwave heating temperature rise properties.The heating characteristics of metal oxides during microwave heating were not only related to the phase transformation of metal oxides during microwave heating,but also related to the crystal structure,electronic structure,and optical properties of metal oxides calculated by first principles.Among these microstructure properties,the smaller the band gap,the larger the peak value of the optical absorption coefficient and dielectric constant in the microwave energy range,and the faster the heating rate of metal oxides in the microwave field.Therefore,based on the results of these correlation characteristics,the band gap,optical absorption coefficient,dielectric constant,and other characteristic microstructure property parameters of metal oxides can be directly calculated by first principles to theoretically predict and directly select the efficient absorbing additives for microwave composite carbonization of bagasse.The crystal structure,electronic structure,and optical properties of metal oxides had different correlations with their microwave heating rate.The correlation coefficient between the band gap size and the heating rate of metal oxides was the highest.The correlation coefficient between the optical absorption coefficient,dielectric constant,and the heating rate of metal oxide,which characterizes the optical properties,was the second.The total energy,bond length,bond population,and other parameters of crystal structure have the lowest correlation coefficient with the heating rate of metal oxides.The smaller the band gap,the greater the peak value of the light absorption coefficient and dielectric constant in the microwave energy range,and the faster the heating rate of the metal oxide in the microwave field.2.Based on the frontier orbit theory,the minimum energy difference between the HOMO and LUMO orbitals of adsorbent and adsorbent was calculated.It is proved theoretically that iron oxide is the most suitable modifier for the adsorption of heavy metal ions chromium（VI）and arsenic（V）.Combined with the microwave heating characteristics and magnetic characteristics of iron oxide,a novel high efficiency iron oxide/bagasse biochar composite was prepared.The LUMO orbital of iron oxide was closest to the HOMO of HCr O₄^-,Cr₂O₇^2-,H₂As O₄^-,and HAs O₄^2-.The absolute energy difference between the LUMO value of iron oxide and the HOMO of HCr O4^-,Cr₂O₇^2-,H₂As O₄^-,and HAs O₄^2-was the smallest,and much smaller than that of other metal oxides tested.The minimum energy difference of HOMO and LUMO orbitals of other metal oxides with solution ionic compounds of chromium（VI）and arsenic（V）was in the following order from smallest to largest:Ti O₂,Cu O,Ni O,Zn O,Cr₂O₃,which indicated that iron oxide is most likely to interact with HCr O4^-,Cr₂O₇^2-,H₂As O₄^-,and HAs O₄^2-.From this point of view,iron oxide is the most suitable bagasse modifier.3.Bagasse biomass carbon/nano iron oxide composite had good adsorption performance for Cr（VI）in water.The adsorption mechanism involved surface diffusion,electrostatic attraction,REDOX,and complexation.The adsorption mechanism of Cr（VI）on Fe₂O₃/Fe₃O₄/WBC composites follows:various forms of Cr（VI）ions entered into the pores of Fe₂O₃/Fe₃O₄/WBC composites through surface diffusion and electrostatic attraction to complete the physical adsorption process;the Fe-O group on the surface of the composite material underwent a redox reaction with Cr（VI）,which converted Cr（VI）into Cr（III）,coupled with the OH on the surface of the composite material and deposited in the form of a complex state on the material surface.4.Bagasse-based biomass carbon/nano iron oxide composites had good adsorption performance for As（V）in water,and the adsorption mechanism involved surface diffusion,electrostatic attraction,complexation reaction,hydrolytic,flocculation,etc.The adsorption mechanism of As（V）on Fe₂O₃/Fe₃O₄/WBC composites was as follows:various forms of As（V）ions entered into the pores of Fe₂O₃/Fe₃O₄/WBC composite material through electrostatic attraction to complete the physical adsorption process;under acidic and neutral conditions,iron oxide mainly existed in the form of Fe（OH）₂⁺and Fe（OH）²⁺,and the anion of arsenic was adsorbed to the positively charged iron oxide surface due to electrostatic attraction,resulting in a complexation reaction（electrons were transferred from the oxygen atom in the As-O to the iron atom（d orbitals））,and the Fe（OH）₃ generated by the hydrolysis of iron ions in the solution can undergo flocculation,so that part of the arsenic can be removed.The results have good theoretical guidance and practical value for developing microwave heated carbonization of biomass materials and the development and application of biomass adsorption materials.