Study On The Efficiency And Mechanism Of Organic Degradation By Carbon-based Catalysts Via Persulfate Activation

Compared with the traditional advanced treatment process,the advanced oxidation technology（SR-AOPs）that generates sulfate radicals(SO₄^-·)by activating persulfate（PMS or PS）is characterized by its fast reaction rate,strong oxidation performance and wide application range of p H and other advantages,has broad application prospects in the efficient treatment of refractory organics.Carbon materials can effectively activate persulfate,but there are serious defects of low catalytic efficiency in general.However,a few new carbon materials with better catalytic performance have some problems,such as complex preparation process,high cost and unclear catalytic sites and mechanism of action.Therefore,in this paper,two types of carbon materials,biochar and graphene-TiO₂,were optimized and modified by heteroatom doping and surface modification to improve the efficiency of activating persulfate to degrade organic pollutants.Combining the characterization of the catalyst and the degradation efficiency of the target pollutants,the microstructure and catalytic performance were analyzed and verified.In addition,the influencing factors of the two activated persulfate reaction systems,the composition and relative contribution of the active species,the reaction mechanism and the practical application potential were systematically analyzed and compared.The main research content and conclusions of this study are as follows:（1）Developed a simple preparation method for synthesizing high-performance biochar materials from forest wastes.Using sawdust,dicyandiamide and ferric chloride as raw materials,the formation of carbon-based framework and heteroatom doping were achieved simultaneously through one-step pyrolysis.Two new types of biochar materials,nitrogen-doped biochar（N-biochar）and iron and nitrogen co-doped biochar（Fe/N-biochar）were successfully prepared.Characterization methods such as XPS,Raman,XRD,and TEM have confirmed the successful incorporation of elemental nitrogen and iron,ascertained the main occurrence forms of elemental nitrogen and iron.And the influence of the type and amount of nitrogen-containing organic compounds,the amount of ferric chloride and the pyrolysis temperature on the structure and catalytic performance of the two new biochar materials was systematically analyzed.Based on the system optimization of the preparation process,the final N-biochar and Fe/N-biochar exhibited excellent catalytic activity.The reaction rates of both activated PMS to degrade bisphenol A（a typical endocrine disruptor）were 6 times and 37 times higher than those of pure biochar,respectively,and were far better than those of metal materials such as Fe₃O₄ and Mn O₂.（2）A simple synthesis method for the preparation of novel graphene-based catalysts with high visible light photocatalytic activity was developed.Using tetrabutyl titanate,graphite oxide,urea and citric acid as raw materials,Ti³⁺doping in graphene-TiO₂ and surface modification of carbon quantum dots were realized simultaneously by hydrothermal anoxic calcination process.Thus,two new visible visible light photocatalysts TiO_2-x/rGO and CQDs-TiO_2-x/rGO were successfully prepared.The microstructures were characterized by XRD,XPS,DRS,TEM and PL.The effects of Ti³⁺self-doping and CQDs surface modification on graphene-TiO₂ photo absorption,carrier separation and visible light catalytic performance were revealed,and the synergistic effect of Ti³⁺self-doping and surface modification of CQDs was clarified.Due to the formation of Ti³⁺and C 2p doping energy levels and the unique photon conversion properties of CQDs,the maximum light absorption wavelength of graphene-TiO₂ has been broadened from 403 nm to 433 nm and 481 nm.In addition,graphene and CQDs can provide dual channels for the transmission of photogenerated electrons and accelerate the effective separation of photogenerated carriers.Therefore,TiO_2-x/rGO and CQDs-TiO_2-x/rGO can efficiently utilize visible light to activate persulfate to produce active substances,and their reaction rates of bisphenol A degradation are 6.5 times and 96.78 times that of unmodified graphene-TiO₂,respectively.（3）The catalytic action mechanism of the above two activated persulfate reaction systems was analyzed and compared from the aspects of the composition of active species,the relative contribution rate of each active material,and the analysis of catalytic active sites.The active substances in the new biochar activated PMS reaction system are mainly SO₄^·-,·OH and ¹O₂,while the new graphene-TiO₂ visible light activated PS system is dominated by SO₄^·-,·OH and valence band holes h⁺.Under different p H conditions,the relative contribution rates of the active substances in the two reaction systems to the degradation of bisphenol A are significantly different.Based on this,the internal reasons for the obvious differences in the resistance to the interference of p H and inorganic anions between the two activated persulfate systems were discussed.In addition,through the analysis of the correlation between the structure of the catalysts and the reaction rates,the changes in the microstructure of the catalysts before and after the reaction,and the quenching experiments,the main active sites and the strengthening mechanism of the two new types of carbon materials were analyzed.（4）Finally,the practical application potential of the two new types of carbon-based catalysts developed in this paper were analyzed and compared from the aspects of oxidative ability,antiinterference ability,stability and selectivity to organics of the raction systems.The results show that the comprehensive performance of Fe/N-biocahr activated PMS reaction system is significantly better than that of CQDs-TiO_2-x/rGO visible light activated PS system.Therefore,a small fixed-bed continuous flow reactor was further established with Fe/N-biochar to evaluate the advanced treatment effect of the new type of biochar activated persulfate advanced oxidation technology for practical printing and dyeing wastewater（secondary water）.It was found that the highest TOC removal efficiency of the device for actual printing and dyeing wastewater could reach 73.16%,and the TOC removal rate was still as high as 50%when the volume of the treated wastewater was close to 3 L（375 bed volumes）.When the treated water volumes were close to 10 L（1250 bed volumes）,the Fe/N-biochar fixed-bed was completely penetrated.However,the catalytic performance of the device that reached adsorption saturation could be well restored to more than 80%after being subjected to regeneration heat treatment,indicating that it has strong practical application potential.