Preparation Of CDs/TiO₂ Composite Catalyst And Photocatalytic Degradation Of Bisphenol A Under Visible Light Irradiation

Bisphenol A is a synthetic raw material,widely used in the production of epoxy resin,polycarbonate and so on.Toxicological studies have confirmed that bisphenol A,as an endocrine disruptor,has immunotoxicity,neurotoxicity,carcinogenicity and other toxic effects,and its ecological and environmental hazards have attracted much attention.At present,the main removal methods of BPA include biological method,adsorption method,photocatalytic oxidation method and so on.Among them,photocatalytic oxidation is considered as one of the most promising treatment technologies because of its simple operation,stable operation,mild reaction conditions and strong degradation ability.Titanium dioxide（TiO₂）has been widely used in the field of photocatalysis due to its low production cost,high-efficiency and non-toxicity,but pure TiO₂ has a low utilization rate of visible light.Therefore,the preparation of TiO₂ photocatalytic materials with high performance and corresponding visible light is the focus of current research.In this study,CDs was firstly prepared by microwave method,the optimal preparation conditions were obtained by single factor experiment:the ratio of citric acid and urea was 1:2,microwave power was 650 W,and microwave time was 5 min.Then,TiO₂ was prepared by hydrothermal method.the optimum preparation conditions were obtained by single factor experiment:glycerol content was 5 m L,hydrothermal temperature was 180℃,hydrothermal time was 24 h,calcination temperature was 450℃.Finally,CDs/TiO₂ was prepared by calcination method,the optimal preparation conditions were obtained through single factor experiment:CDs doping amount is 6%,calcination temperature is 300℃,calcination time is 3 h.CDs,TiO₂ and CDs/TiO₂ were characterized by SEM,TEM,XRD,BET,FT-IR and UV-Vis DRS.SEM and TEM confirmed that CDs was supported on the surface of TiO₂.XRD confirmed that both TiO₂and CDs/TiO₂ were anatase phase,and CDs doping did not affect the crystal structure of TiO₂.BET confirmed that TiO₂ had a large specific surface area（90.981 m²/g）,and the specific surface area of CDs/TiO₂ sample was 83.914 m²/g.UV-Vis DRS confirmed the red shift of CDs/TiO₂ samples,and the band gap was about 2.93 e V,which was smaller than that of TiO₂（3.07 e V）.FT-IR confirmed that the doping of CDs results in some new characteristic peaks of TiO₂ that CDs/TiO₂ produced C=O and C-H groups,and the new chemical bonds generated and the strength of chemical bonds affected the activity of the whole photocatalytic system.The photocatalytic reaction of CDs/TiO₂ degradation of BPA under visible light was studied,and the effect of reaction conditions on photocatalytic efficiency was investigated.The results are as follows:the dosage of CDs/TiO₂ is 2 g/L,the initial pH of the solution is 7,and the initial concentration of BPA is 5 mg/L,the degradation efficiency of BPA can reach 99.9%for 300 min,which is much higher than the photocatalytic efficiency of BPA degradation by TiO₂.With the photocatalytic reaction,the pH of the system decreased slightly,and the mineralization rate increased.The mineralization rate of BPA could reach 70.1%.It was confirmed that·O₂^-,h⁺,and·OH free radicals are responsible for the degradation of BPA by reactive group capture experiment.CDs/TiO₂ photocatalyst is stable and reusable,and the degradation efficiency of BPA can still reach 91.8%after5 cycles.The photocatalytic mechanism of CDs/TiO₂ degradation of BPA under visible light was discussed.The main intermediates are 2-isopropyl phenol,hydroquinone and 4-hydroxyacetophenone,which were further oxidized and decomposed into intermediates such as phenol and p-benzoquinone through chromatographic analysis.The acute toxicity and bioaccumulation factors of BPA decreased after CDs/TiO₂ photocatalytic degradation.The reaction mechanism is as follows:the doping of CDs reduces the recombination of electrons and holes,reduces the band gap width of TiO₂,and makes the material surface have more active sites,thus enhancing the photocatalytic activity of CDs/TiO₂ composites.Under visible light,the electrons and holes of CDs/TiO₂ photocatalyst are excited,and·O₂^-,h⁺,·OH and other free radicals are generated to attack BPA,as intermediate products such as phenol are finally oxidized to generate CO₂ and H₂O.The kinetic equation of CDs/TiO₂ photocatalytic degradation of BPA in visible light was studied,and the apparent reaction rate constant was calculated.The experimental results confirmed that the photocatalytic reaction of CDs/TiO₂ degradation of BPA under visible light conforms to the first-order kinetic equation.