Construction Of Bismuth-vanadate-based Catalytic System And Its Performance And Mechanism For The Degradation Of Organic Pollutants In Water Under Visible Light

Environmental pollution and energy shortage have become two critical issues for the sustainable development of human beings.As an environment-friendly technology,semiconductor-based photocatalysis not only can convert solar energy into chemical energy for the efficient removal of toxic pollutants,but also directly realize the convertion from solar energy into fuels or other storable energies.The focus of semiconductor-based photocatalysis is to develop the photocatalyst with wide light response,satisfactory efficiency and high stability,thus visible-light-response photocatalysts become the research hotspots.In recent years,monoclinic bismuth vanadate?m-BiVO₄?has been recognized as one typical photocatalyst for organic pollutants degradation under visible light irradiation,which is attributable to its advantages such as suitable band structure,earth-abundant elemental composition,decent chemical and thermal stability,non-toxic,relatively narrow band gap.Nevertheless,the low photogenerated electron-hole pairs transfer and separation efficiency,rapid recombination rate of photoinduced charge carriers and relatively poor visible light absorption limit its photocatalytic performance,thus it is very necessary to modify BiVO₄ or construct novel BiVO₄-based visible-light-driven photocatalytic systems to boost its photoactivity.Consequently,in this dissertation,BiVO ₄ was chosen as the basic photocatalytic material.A series of BiVO₄-based heterojunctions photocatalysts had been fabricated by water bath,solvethermal,chemical precipitate and photoreduction method.Furthermore,new persulfate activation system by combining BiVO₄ with visible light was constructed to realize the high effective removal of organic pollutants.The as-prepared BiVO₄ composites were characterized by various techniques to determine their compositions,morphologies,charge carriers separation and photoelectric properties etc.Simultaneously,major active species in the reactions,pollutant degradation mechanism,possible effects of operational parameters and the stability were also explored.The details are summarized briefly as follows:?1?p-Ag₂O/n-BiVO₄ p-n heterojunction photocatalyst was successfully fabricated by a facile pH-mediated chemical precipation method and it could realize high efficiency removal towards dyes in water.Among all the prepared photocatalysts,the composite with an initial Ag₂O/BiVO₄ mass ratio of 1:10 exhibited the highest photocatalytic activity,where single rhodamine B?RhB?or methyl blue?MB?could be degraded within 15 min or 48 min,respectively.The corresponding reaction rate was49.0/2.5 and 72.0/2.2 times as high as those of pure BiVO₄ and Ag₂O.For RhB+MB mixture,desirable degradation was also obtained by Ag₂O/BiVO₄?1:10?.The effect of initial dye concentration,initial pH,calcination temperature on the photodegradation of single dye as well as the inhibition mechanism for mixed dyes were inveatigated in detail.A slight amount of metallic Ag produced from Ag₂O during reaction process could enhance visible light absorption and promote photo-induced carriers generation,resulting in photocatalytic activity improvement.Dye degradation and mineralization depended on both holes?h⁺?and superoxide radical?�O₂^-?.The cyclic experiments also demonstrated the reusability and stability of composite photocatalysts.?2?A novel graphene-bridged Ag/Ag₃PO₄/BiVO₄?040?Z-scheme heterojunction was prepared via a facile in-situ deposition method followed by photo-reduction.Various characterization methods were employed to determine its phase composition,morphology structure and surface chemical state.The Ag/Ag₃PO₄/BiVO₄/RGO nanocomposites could rapidly degrade antibiotic tetracycline?TC?and the removal efficiency reached to 94.96%.The photocatalytic activity also depended on catalyst dosage,initial TC concentration,reaction pH,light irradiation condition and supporting electrolytes.The large surface area,enhanced visible light absorption and efficient photo-generated electron-hole pairs separation were the major reasons for the improved photoactivity.The introduction of metallic Ag in the composite could facilitate a Z-scheme hterojunction,the presence of graphene could effectively increase the surface area for holding more pollutants and reaction interspace.The synergistic effect between Ag/Ag₃PO₄ nanoparticles,graphene and BiVO₄ could simultaneously realize high efficiency of TC removal and solve the problem of Ag₃PO₄ photocorrosion to certain degree.The radical trapping experiments and electron spin response?ESR?measurements revealed that the photo-induced superoxide radiacal?�O₂^-?and holes?h⁺?were the predominant active species in catalytic system.The Ag/Ag₃PO₄/BiVO₄/RGO composite also processed desiable photocatalytic performance on the degradation of TC from different water sources,further to demonstrate its practical application potentials.?3?A ternary heterojunction photocatalyst,g-C₃N₄ nanosheets and Ag nanoparticles co-modified BiVO₄ was synthezied through a wet-impregnation and photodeposition procedure.The ternary composite possessed the distinctly enhanced photocatalytic activity towards organic polluatnts degradation under solar light,visible light even near infrared light irradiation.The coexisting Fe³⁺would be beneficial to the photoactivity enhancement because it could absorp light to produce active species as�OH under weakly acidic solution and capture photoinduced electrons and holes.The presence of NO₃^-and Ca²⁺played a negative role in the photodegradation process,which could be ascribed to the increased electrostatic repulsion and the formation of metal complexes.The Ag/g-C₃N₄/BiVO₄ composite photocatalyst showed saticifatroy stability and resyclability under both simulated light and visible light.Various characterization methods such as XRD,SEM,FTIR,XPS etc were used and the results demonstrated that Ag nanoparticles and g-C₃N₄ nanosheets were introduced to contruct ternary composite photocatalyst with BiVO₄.On one hand,g-C₃N₄ and BiVO₄ could form traditional heterojuntion;on the other hand,the introduction of metallic Ag could contribute to a Z-scheme charge transfer and separation mechanism,which could ensure their high redox performance of electrons and holes.The active species�O₂^-,h⁺and�OH were generated in TC photodegradation pocess and confirmed by active species trapping experiments and ESR tests.Compared with single BiVO ₄ and binary g-C₃N₄/BiVO₄ II heterojunction,Ag/g-C₃N₄/BiVO₄ Z-scheme heterojunction exhibited wider light absorption,more efficient charge carriers separation and longer carrier lifetime,further to boost their possibilities in catalytic reaction and photocatalytic activity.?4?Palladium?Pd?nanoparticles were uniformly photo-deposited onto BiVO₄nanosheets to effectively activate persulfate?PS?under visible-light irradiation and employed to efficiently degrade pharmaceutical residues amiloride?AML?in aqueous solution.Experimental results demonstrated that the removal efficiency of AML in Pd/BiVO₄/PS/vis system reached to 96.43%within 30 min and the degradation rate was 5.22,2.92,2.48 times of BiVO₄/vis,Pd/BiVO₄/vis and BiVO₄/PS/vis system,respectively.The effects of PS dosage,AML concentration,HA concentration,coexisting anions and cations and water sources on the degradation performance of Pd/BiVO₄/PS/vis system were deeply investigated.The suitable PS addition was in favor of high catalytic performance achievement.The HA at lower concentration?<1mg/L?would promote AML degradation while the inhibition led by high concentration HA should be released by related pretreatment.It was necessary to remove the coexisting anions especially CO₃^2-and HCO₃^-in wastewater due to their capture properties of SO₄�^-and�OH.The Pd/BiVO₄/PS/vis system could maintain good performance under different AML concentration and water sources.ESR test and active species quenching results confirmed that PS could be effectively activated with the help of visible light and Pd/BiVO₄ photocatalyst.The enhanced amouts of�OH and SO₄�^-radicals,efficient charge separation and enhanced visible light absorption attributable to SPR effect of Pd nanoparticles could be concluded as the reasons of photoactivity enhancement.Furthermore,based on LC-MS results,the main degradation byproducts in the Pd/BiVO₄/PS/vis system were identified and the possible AML decomposition pathways were proposed.