Study On The Micro-mechanism For Enhancing The Activity Of Titanium-based Catalyst And Its Degradation Performance Of Diclofenac

In recent years,pharmaceuticals and personal care products?PPCPs?have been continuously detected in water environment.Although most of the PPCPs at trace levels,they may cause environmental risks due to their environmental persistence,bioaccumulation and biological toxicity.Hence the development of technology research for the effective treatment of PPCPs is extremely urgent.In this study,non-metallic element S was used to dope in TiO₂,and the obtained S-TiO₂ was combined with graphene oxide?GO?and porous graphite phase carbonitride?p-g-C₃N₄?to prepare the binary composites GO/S-TiO₂ and p-g-C₃N₄/S-TiO₂,respectively.The micro-mechanisms of modified TiO₂ for improving its visible light photocatalytic activity in atomic level were studied by the density functional theory?DFT?calculations.One typical PPCPs,diclofenac?DCF?,was employed as the target pollutant in this work.The properties of the prepared materials and the mechanisms of their catalytic degradation of DCF under visible light were studied.p-g-C₃N₄/S-TiO₂ was chosen for visible light catalytic degradation of DCF and other refractory organic pollutants in actual wastewater.The main research contents and conclusions are as follows.S-doped TiO₂?S-TiO₂?was prepared by sol-gel method using thiourea and tetrabutyl titanate as precursors at low calcination temperature?250°C?.The surface morphologies,crystal structures,elemental compositions and optical properties of S-TiO₂ were analyzed by a serial of characterization analysis methods,such as SEM,XRD,XPS,UV-vis DRS,VB-XPS,etc.The micro-mechanism in atomic level of S doping for enhancing the visible light photocatalytic activity of TiO₂ was investigated by DFT calculation.The results showed that the doped S atoms replaced O atoms in the TiO₂ structure and introduced S2p impurity level,which reduced its band gap and improved the absorption capacity of visible light.The effective mass of photogenerated electrons reduced from 1.35 m₀ to1.16 m₀,which indicated that the migration rate of electrons was enhanced.S doping effectively improved the visible light catalytic activity of the material by extending the absorption range of the light and promoting the separation of photogenerated electron-hole pairs.The rate constant(k_obs)of S-TiO₂ during DCF degradation was to be around three times higher than that of the pure TiO₂ under visiable light.Under the dominate effect of photogenerated holes?h⁺?,a series of intermediate products was generated and gradually oxidized throguh addition,substitution,carboxylation,dehydrogenation reaction.In order to further promote the visible light catalytic performance,the prepared S-TiO₂ was modified to improve the adsorption performance and enhance the photo-generated electron transport.GO and S-TiO₂ were combined to form a binary composite material GO/S-TiO₂.The degradation effect and reaction mechanism of DCF using GO/S-TiO₂ under visible light were studied.The results showed that the composition with GO showed no obvious influence on the crystal structure of S-TiO₂.After S-TiO₂ was combined with GO,the specific surface area of GO/S-TiO₂ increased from 278.58 m²/g to 346.05 m²/g,the band gap decreased from 2.90 eV to 2.81 eV,and the photocurrent density also increased.The visible light photocatalytic degradation rate of DCF by GO/S-TiO₂ was closed to 1.3 times higher than that of S-TiO₂,and GO/S-TiO₂showed a good reusability.The DFT calculation results showed that the introduction of GO greatly reduced the effective mass of electrons?from 1.16 m₀ to 0.88 m₀?,which enhanced the migration rate of electrons and effectively improved the photocatalytic activity of the material.To further improve the visible light catalytic activity,S-TiO₂ was composited with the porous graphite phase carbon nitride?p-g-C₃N₄?which had a narrow band gap.The characterizations of this binary heterojunction structure p-g-C₃N₄/S-TiO₂ were conducted.The efficiency and mechanism of the visible light degradation of DCF were studied.Besides,the electronic structures and optical properties of p-g-C₃N₄/S-TiO₂ were investigated via DFT calculation from the microscopic point of view.The results indicated that p-g-C₃N₄/S-TiO₂ showed a good visible light response with the band gap of2.41 eV,the k_obs of DCF degradation was 0.0143 min^-1,closed to 1.7 times higher than that of S-TiO₂.Furthermore,photogenerated holes?h⁺?played a major role in photocatalytic degradation of DCF under visible light irradiation,and·O₂^-also participated in the reaction.The DFT calculation results showed that g-C₃N₄ was stimulated by visible light radiation,the photogenerated electrons were transferred to the conduction band of S-TiO₂ through the heterojunction,which reduced the electron-hole recombination.The photogenerated holes existing in the valence band of g-C₃N₄ and the photogenerated electrons existing in the conduction band of S-TiO₂ can both act on the degradation of DCF.Thus,although the electron effective mass of g-C₃N₄/S-TiO₂ reached3.18 m₀,much higher than that of S-TiO₂?1.16 m₀?,it still exhibited a strong photoactivity.In order to explore the effect and application feasibility of p-g-C₃N₄/S-TiO₂ binary heterojunction material in the treatment of PPCPs and other practical refractory organic wastewater,the influences of inorganic anions on the photocatalytic degradation of DCF by p-g-C₃N₄/S-TiO₂ were investigated in the simulated wastewater.The treatment effect of photocatalytic degradation by p-g-C₃N₄/S-TiO₂ in actual wastewater was studied by adding DCF in hospital wastewater and reverse osmosis concentrated water obtained from municipal sewage treatment.The effects of visible light catalytic degradation on the refractory organic pollutants in hospital wastewater,reverse osmosis concentrated water obtained from municipal sewage treatment and landfill leachate concentrate by p-g-C₃N₄/S-TiO₂ were evaluated.The results demonstrated that inorganic anions,such as Cl^-and SO₄^2-,shown little influence on the visible light catalytic activity of p-g-C₃N₄/S-TiO₂,and the degradation efficiencies were 42.84%and 36.93%for hospital wastewater and reverse osmosis concentrated water obtained from municipal sewage treatment respectively.The COD removal rate could reach 35.0%and 38.4%,respectively.Moreover,the refractory organic substances such as fulvic acid and humic acid in wastewater were oxidized into aromatic protein substances with good biodegradability,and the biological toxicity was also reduced to some extent.However,p-g-C₃N₄/S-TiO₂shown a poor effect on the photocatalytic degradation of pollutants in the landfill leachate concentrate.