Synergistic Mechanism Of Biochar-nano-TiO₂ Adsorption-photocatalytic Oxidation For Typical VOCs

Volatile organic compounds（VOCs）have become one of the major air pollutants in our country,and they will cause serious harm to the ecological environment and human health.VOCs pollution in our country is characterized by large total emissions,wide sources,and complex components.At the present stage,efficient and stable treatment methods are urgently needed.Based on the characteristics of the existing technology,this research combined biochar adsorption technology with TiO₂ photocatalytic technology to realize the integration of VOCs adsorption and photocatalytic degradation.To explore the two-way synergistic mechanism between biochar pore graded adsorption and nano-TiO₂ photocatalytic oxidation,this paper prepared biochar-nano-TiO₂cross-linked structure by the sol-gel method.Through adsorption-photocatalysis experiments,thermogravimetric desorption experiments and solid characterization analysis,the adsorption-photocatalytic properties of biochar-nano-TiO₂ cross-linked structure to typical VOCs（toluene）were explored,and the relationship between the synergistic mechanism of cross-linked structure and its physicochemical properties was investigated.In addition,in order to improve the efficiency of photocatalytic degradation of VOCs by the biochar-nano-TiO₂ cross-linked structure,the effects of the types and concentrations of doping elements on the photocatalytic properties of TiO₂were analyzed through simulation calculations,which provided theoretical guidance for the experimental work of TiO₂ doping modification.The results of adsorption-photocatalysis experiments and solid characterization show that the mesopore volume of biochar is linearly related to the amount of toluene adsorbed,and it is dominant in the physical adsorption process.A higher initial concentration of toluene can promote the photocatalytic rate,but a too high initial concentration will inhibit the reaction,and too high light intensity will cause the rate of the photocatalytic reaction to decrease.The removal ratio of biochar-nano-TiO₂cross-linked structure for high concentration of toluene is higher than the sum of the adsorption removal ratio of pure biochar and the photocatalytic degradation ratio of pure TiO₂,realizing the synergistic effect of adsorption and photocatalytic oxidation,and the synergy coefficient is 10.45.For the treatment of high concentration of VOCs,the synergistic effect of photocatalytic reaction after adsorption saturation is inhibited compared with the process of full illumination,and the synergy coefficient is reduced by 77.1%.The crystallite size of nano-TiO₂ is 6.22 nm,and the crystallite size of nano-TiO₂ cross-linked with biochar is 4.87 nm.The chemical bonding between biochar and nano-TiO₂ is achieved through Ti-C bond and N-Ti-O structure.The synergistic mechanism of biochar-nano-TiO₂ cross-linking structure includes:The adsorption-enrichment effect of biochar on VOCs promotes the photocatalytic oxidation reaction of nano-TiO₂.The cross-linking between biochar and TiO₂ improves the dispersibility of TiO₂ and reduces the size of single particles,which shortens the time for photogenerated carriers to migrate to the surface of nano-TiO₂ particles.The formation of chemical bonds between biochar and nano-TiO₂ enhances the interface contact effect and reduces the probability of photo-generated electron-hole pair recombination.The simulation results show that:C/N/Fe/Pt doped TiO₂ will form impurity levels,reduce the band gap,and promote the generation of photo-generated electron-hole pairs.Among them,C-doped TiO₂ has the largest light absorption coefficient at 300～550 nm,and the effect of Fe doping is best after 550 nm.The light absorption threshold of C-doped anatase TiO₂ is the largest.Judging by the band gap width,the optimal doping concentration of Pt is 8.33%,and the optimal doping concentration of C is 2.08%,but at the same time,the impurity level of C-doped TiO₂ is less,which is not conducive to the capture of photogenerated electrons.The increase of Pt doping concentration will lead to an increase in the red shift of the band edge,and the light absorption threshold of8.33%Pt doped TiO₂ is the largest,which is 884.4 nm.Under the excitation of visible light,the C-doped anatase TiO₂ with a concentration of 2.08%has the strongest photocatalytic activity.This study lays a theoretical foundation for elucidating the cross-linking mechanism and synergistic mechanism between biochar and nano-TiO₂,provides a theoretical reference for the experimental work on the doping modification of biochar-nano-TiO₂cross-linking structure,and provides theoretical support for the development of efficient VOCs removal technology.