Adsorption And Photocatalytic Degradation Of Gaseous Benzene By Cotton Stalk Derived Composites

At present,China’s environmental problems are becoming increasingly prominent.Benzene,as a common volatile organic compound（VOC）,is volatile and easy to harm human health and the overall ecosystem.Xinjiang has the largest cotton planting area in China,which produces a large amount of cotton straw every year.From this point of view,this thesis studies how to convert agricultural and forestry waste cotton stalk into high economic value-added products and apply it to the treatment of benzene.Firstly,this thesis takes cotton stalk as raw material and gaseous benzene as target pollutant.After carbonization of cotton stalk,modified biochar was prepared by KOH activation and used for benzene adsorption.Secondly,the liquid-phase product bio-oil produced during the carbonization of cotton stalk was separated and purified to obtain carbon dots（CDs）,and combined with semiconductor Cu Bi₂O₄/Zn O to construct photocatalyst for benzene degradation.Finally,biochar formed after carbonization of cotton stalk was compounded with semiconductor Cu Bi₂O₄/Bi OBr to construct photocatalyst for benzene degradation.The main results are as follows:（1）The effects of carbonization temperature,carbon alkali ratio,activation temperature and activation time on the preparation of activated carbon were investigated.Through the adsorption capacity,the optimal preparation process of adsorbent was determined as follows:carbonization temperature was 400℃,carbon alkali ratio was 1:5,activation temperature was 900℃and activation time was 2 h.the maximum adsorption capacity of the prepared adsorbent（CSAC）was 1388.7 mg/g and had good adsorption capacity for benzene.The morphology and structure of CSAC were analyzed by SEM,XPS,XRD,bet and FTIR.The specific surface area of CSAC was 2648.24 m²g^-1and contained chemical bonds such as sp²C,C=N,-C-O-C and C-N.The adsorption kinetics of CSAC accorded with the pseudo first-order kinetic equation,and the correlation coefficient R²was 0.9699.The adsorption isotherm can be fitted by Freundlich isotherm model,and the correlation coefficient R²is 0.991.The results show that the adsorption force is van der Waals force,and the adsorption of benzene is the combination of monolayer adsorption and multilayer adsorption.（2）The by-product bio-oil produced in the preparation of cotton stalk biochar at the optimal carbonization temperature was selected as the carbon precursor to prepare carbon dots（CDs）and compounded with Cu Bi₂O₄/Zn O（CBO/Zn O）to obtain CBO/Zn O/CDs composite photocatalyst.The morphology,structure and photoelectrochemical properties of the composite catalyst materials were analyzed by means of SEM,XRD,XPS,UV vis,i-t and EIS.Taking benzene as the target pollutant,the photocatalytic degradation properties of CBO,CBO/CDs,CBO/Zn O and CBO/Zn O/CDs were compared.Among them,CBO/Zn O/CDs has the best degradation effect.It can degrade 85%of benzene within 6hours.The first-order kinetic constant k=0.3244 h^-1.After 5 cycles,the degradation efficiency of CBO/Zn O/CDs can still reach 77%.ESR test showed that·OH and·O₂^-were the main active species involved in the photocatalytic degradation process.Z-type heterojunction can better explain the mechanism of photocatalytic enhancement.Heterojunction and CDs can enhance charge separation and light absorption.（3）Cotton stalk biochar（BC）prepared at the optimum carbonization temperature was selected and doped into Cu Bi₂O₄/biobr（CBO/B）.A composite photocatalyst was prepared.It was characterized by SEM,XRD,XPS,UV Vis,i-t and EIS.Taking benzene as the target pollutant,the photocatalytic degradation properties of BC,CBO,CBO/BC,CBO/B and CBO/B/BC were compared.Among them,CBO/B/BC has the best degradation effect,which can degrade 92%of benzene within 6 hours,and the first-order kinetic constant k=0.4016 h^-1.After 5 cycles,the degradation efficiency of CBO/B/BC can still reach 82%.In addition,the Z-type mechanism of CBO/B/BC photocatalyst was studied based on electronic structure and electron spin resonance（ESR）analysis.The effects of heterojunction and biochar on photodegradation were analyzed in detail.It is found that heterojunction can promote the effective separation of photogenerated electron hole pairs,and biochar can improve the light absorption and electron transport capacity of photocatalyst.