Rational Modification Of Monoclinic Bismuth Vanadate And Insight Into Their Performance And Mechanism In Catalytic Degradation Of Antibiotic Under Visible Light

The frequent detection of antibiotics in the environment and the accelerated formation of bacterial resistance have made the treatment of antibiotic-contaminated water a focus and hotspot of research.Photocatalysis relying on reactive species generated within the system can achieve oxidative degradation and even mineralization of antibiotics,which is a green,efficient and easy-to-operate technology for the elimination of aqueous refractory organic pollutants.Monoclinic bismuth vanadate（BiVO₄）,as a representative visible light responsive photocatalyst,has attracted intensive attention in the field of water decontamination owing to its moderate band gap,good stability,non-toxic accessibility,and so on.However,the narrow visible light response region,the rapid recombination of photogenerated electron-hole pairs,low mass transfer efficiency and small specific surface area are still the key factors limiting the improvement of quantum efficiency of BiVO₄.In this regard,the reasonable modification of BiVO₄ to improve its photocatalytic activity is essential.In this dissertation,a series of modified BiVO₄ photocatalysts with optimized chemical structure,morphological features and electronic structure were developed based on multifarious strategies such as constructing heterostructure,defect regulation,morphological design and surface modification,and applied in the catalytic removal of typical antibiotics in water under visible light,with the emphasis on elucidating the relationship between photocatalytic activity and multi-component interfacial interactions,surface defect states,energy band structure and microscopic morphology.Based on the experimental results and their discussion and analysis,the intrinsic mechanisms of the upgraded photocatalytic activity were elucidated in depth,which would furnish theoretical support for further relevant studies.The specific research work and creative achievements of this dissertation include the following four aspects.To surmount the shortcomings of narrow light-response range and rapid carrier recombination of BiVO₄,BiVO₄/Ag₃PO₄/PANI heterojunction was developed in Part one based on the interfacial modification of heterostructures,and applied to the degradation removal of ciprofloxacin（CIP）from water under visible light irradiation.The results showed that the Z-scheme heterojunction between BiVO₄ and Ag₃PO₄promoted the transfer and separation of photogenerated carriers,and PANI could transfer the VB-holes of Ag₃PO₄ to its HOMO orbitals to prevent the self-oxidation of Ag₃PO₄,as well as broaden the light response range of BiVO₄.As a result,the synergistic modification of Ag₃PO₄ and PANI significantly improved the photocatalytic performance of BiVO₄,in which BiVO₄/Ag₃PO₄/PANI-6%exhibited the highest photocatalytic activity with CIP degradation efficiency of 86.21%and a reaction rate constant of 0.0102 L mg^-1 min^-1 under visible light for 60 min,which was 10.9,5.7 and1.8 times that of BiVO₄,BiVO₄/PANI and BiVO₄/Ag₃PO₄,respectively.Quenching experiments and ESR detection showed that O₂^·-and h⁺were the dominant active species for CIP degradation,where O₂^·-originated from the reduction of surface-adsorbed oxygen molecules by CB-electrons of BiVO₄.（Chapter 2）Part two simplified the catalytic system and photocatalyst preparation process in Part one by defect modification,and upgraded the utilization of photogenerated carriers by regulating the concentration and distribution of oxygen vacancies（OVs）for the simultaneous removal of heavy metal and antibiotic,and provided insight into the mechanism of OVs-mediated synchronous photocatalytic redox.The results showed that the oxygen defect level could reduce the energy required for excited electrons to transition from the valence band（VB）to the conduction band（CB）,and accelerate the separation and transfer of photogenerated carriers.Meanwhile,modulating the concentration and distribution of OVs could tailor the CB and VB positions of the resulting samples,with the optimal BVO possessing the most positive potential at the maximum of VB and the minimum of CB among all series.As a result,BVO exhibited the highest photocatalytic activity under the coexistence of TC-HCl and Cr（Ⅵ）,with TC-HCl degradation efficiency of 81.22%and Cr（Ⅵ）reduction efficiency of 83.42%under visible light for 90 min,where Cr（Ⅵ）reduction rate(1.799 h^-1)increased by 71times compared to individual Cr（Ⅵ）system.In the Cr（Ⅵ）/TC-HCl co-existence environment,the dominant reactive species for TC degradation were singlet oxygen（¹O₂）and h⁺,while it was photogenerated electrons（e^-）and ¹O₂ for Cr（Ⅵ）reduction,among the generated ¹O₂ originated from the oxidation of O₂^·-.（Chapter 3）In Part three,two-dimensional（2D）BiVO₄ nanosheets（NS）containing OVs were prepared on the basis of Part two supplemented with morphological design,which could remove chloride ions（Cl^-）from strongly acidic wastewater of metal smelting by-products via in-situ topological transformation reaction at room temperature,and the product was ultrathin Bi OCl NS with OVs.Bi OCl-3 exhibited the highest catalytic activity towards CIP removal under visible light,and the CIP degradation efficiency was 93.58%,the apparent quantum efficiency was 0.411%,and the apparent rate constant was 0.04756 min^-1.The excellent activity of Bi OCl-3 was attributed to the synergy of OVs and ultrathin 2D structures,the OVs-mediated defect state acted as an electron capture center to capture the excited VB-electrons to inhibit the recombination of photogenerated electron-hole pairs,while the ultrathin 2D nanosheet structure shortened the carrier diffusion distance from the bulk to the surface to improve the mass transfer efficiency,allowing more available carriers to participate in the surface molecular oxygen activation reaction.NBT tests showed that the transformation efficiency of NBT in Bi OCl-3/Vis system was much higher than others,with the highest transformation efficiency of 64.59%,and the O₂^·-was quantified as 3.320×10^-10 M.（Chapter 4）In the fourth part,on the basis of the above three parts（Chapters 2 to 4）,a coupled system of photocatalysis and persulfate activation was constructed based on surface modification and applied to the catalytic degradation of CIP.The results showed that BiVO₄ prepared with CTAB as intercalator and carbon source could introduce carbon-containing functional groups on its surface upon post calcination,among which carbonyl group（C=O）acted as Lewis basic site to induce PMS hydrolysis under acidic conditions for ¹O₂ generation.Aside from PMS hydrolysis,the oxidation of lattice oxygen(O_L^2-)was corroborated to induce O₂^·-generation as well,which involved the formation of lattice oxygen reversible redox(O_L^-/O_L^2-).Meanwhile,PMS could be reduced by photogenerated electrons to generate SO₄^·-and·OH.Based on the above activation pathway,BVO-2/PMS/Vis system delivered eminent catalytic activity and stability towards CIP degradation,and the apparent rate constants(k_obs=0.4264 min^-1)were 118.4,99.2,67.7 and 10.4 times higher than those of Vis,PMS/Vis,BVO-2/PMS and BVO-2/Vis systems,and exhibited high anti-interference to ambient background water matrix.Moreover,the toxicity tests using the live cell density of E.coli showed that the BVO-2/PMS/Vis system not only achieved rapid oxidative degradation of CIP,but also reduced the biotoxicity of the CIP stock solution.（Chapter 5）This dissertation revealed the mechanisms of catalytic degradation of typical antibiotics by modified BiVO₄ under visible light irradiation and explored the biological toxicity of the degradation products,which would provide methods and fundamentals for upgrading the photocatalytic performance of BiVO₄ and provide new insights and reference for broadening the application of BiVO₄-based photocatalysis technology in water decontamination.