Mechanism Of Oxygen/nitrogen Surface Group Construction Of Biochar And Its Effective Adsorption Of VOCs/CO₂/NH₃ In Biochar

The rapid development of our economy has brought a series of environmental pollution problems.Among them,volatile organic compounds?VOCs?are bioaccumulative and carcinogenic,teratogenic and mutagenic,which seriously endanger human health and cause many environmental problems.Affected by Chinese energy structure,the control of coal-burning VOCs has become the focus of the prevention and control of organic pollutants.Due to the characteristics of low VOCs concentration,large total volume,and complex components in complex coal-fired flue gas?CO2/H2O/O2?,effective control methods are still lacking at home and abroad.The biochar adsorption method is regarded as the most potential VOCs removal technology because of its advantages of wide source of raw materials,strong controllability of physical and chemical characteristics and high adaptability of target adsorbate.In order to analyze the adsorption mechanism of porous biochar to VOCs,and to elucidate the structure-effect relationship of biochar adsorbing VOCs,this paper used corn stalk as the biomass raw material,through the gas physical activation method and chemical agent activation method to construct the hierarchical pore-functionalized biochar structure system.The adsorption characteristics of biochar to VOCs were explored by various methods such as liquid phase adsorption test,molecular dynamics simulation and quantum chemical simulation,and the specific mechanism of hierarchical pore-functionalized structure adsorbing VOCs molecules was analyzed.The solid characterization results of biochar indicate that: The chemical activation of FeCl₃-ZnCl₂ makes two typical pore structures on the surface of biochar: the surface pores and crack-like pore channels,and the activated biochar had a high mesopore ratio and successfully constructed a hierarchical pore structure.In the competition for the surface active sites of biochar,NH3 is more dominant than H2 O.The chemical activation of KOH makes the biochar present a "honeycomb" morphology,which plays an important role in the formation of micropores.The KOH activation process is conducive to the construction of graphite lamellar structure,but it is still difficult to dissect the lamellar graphite structure.The activation process of FeCl₃-ZnCl₂ is more conducive to the conversion of biochar edge carbon structure to graphite,and the edge single layer graphite structure can be observed on biochar.Chemical activation can build more oxygen/nitrogen functional groups on the pore surface of biochar.The gasification effect of H2 O on biochar increases the number of structural defects in biochar,resulting in the preferential consumption of small aromatic ring structures or carbon matrix defects with smaller sizes.Liquid Zn Cl2 will penetrate into the carbon matrix as a dehydrogenation agent to promote the dehydrogenation and dissolution of cellulose,thereby increasing the proportion of small molecular aromatic rings in the process of biochar aromatization.The results of adsorption experiments and molecular calculation simulations show that: The biochar activated by FeCl₃-ZnCl₂ has the best phenol adsorption effect.The well-developed pore structure,especially the mesoporous structure and the pyridine nitrogen on the surface of biochar have a very good promotion effect on the adsorption of phenol.The micropore filling mechanism caused by the superposition of pore wall force fields is the main adsorption mechanism of small molecular adsorbates such as formaldehyde,in which the primary adsorption caused by the enhanced adsorption potential dominates.The adsorption process of macromolecular VOCs with benzene ring tends to form an adsorption film on the mesopore wall.None of the VOCs involved in this study can form a stable lateral adsorption configuration.The adsorbent molecules with aromatic ring structure are more stable than the adsorption configuration of formaldehyde,carbon dioxide and ammonia.The adsorption energy of VOCs molecules and biochar containing N functional groups is larger than that of other configurations,and the adsorption configuration is more stable.The adsorption stability of carbon dioxide,ammonia and other small molecules with different characteristics of biochar base is not much different,but when adsorbed on the side of biochar molecules doped with N/O functional groups,the adsorption energy is tripled.Doping N/O functional groups on biochar can produce different degrees of adsorption promotion for the above 6 molecules.This study lays the foundation for clarifying the specific mechanism of VOCs adsorption by hierarchical-functionalized biochar,and provides theoretical support for the practical application of biochar efficient removal of VOCs from coal combustion.