Construction Of Bismuth Based Composite Photocatalyst And Photocatalytic Degradation Of The Emerging Pollutants

Environmental pollution and energy crisis are two major problems faced by human sustainable development.Semiconductor photocatalytic technology can not only use solar energy to control environmental pollution,but also directly convert solar energy into clean energy such as hydrogen,low alcohol fuel,reducing environmental problems such as exhaust gas and dust pollution caused by the use of traditional fossil fuels,and has been praised as an ideal green technology for environmental pollution control and clean energy production.TiO₂ is the most widely used semiconductor photocatalytic material.However,the band gap of TiO₂ is large（3.0-3.2 eV）,it can only be excited by ultraviolet light.Therefore,research and development of new photocatalysts that can use visible light or sunlight as driving energy have important application value.In recent years,Bi-based photocatalysts have attracted much attention due to their unique lamellar crystal structure and excellent solar catalytic performance.Moreover,China is rich in bismuth resources,which is beneficial to the development of visible bismuth photocatalysts with high catalytic activity.However,the single bismuth-based photocatalyst has the disadvantages of limited visible light absorption range and low quantum efficiency.The combination of carbon-based materials or other semiconductor materials with bismuth-based photocatalysts with good visible light absorption can effectively broaden the light response range of photocatalyst and improve photocatalytic efficiency of photocatalysts,and is expected to solve the problems of low energy efficiency and quantum efficiency in the field of photocatalysis.In this paper,three composite bismuth based composite photocatalysts with high catalytic activity were prepared by low temperature hydrothermal method,respectively.The crystalline phase,morphology and microstructure,surface state and optical property were characterized by X-ray powder diffraction（XRD）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,high resolution transmission electron microscopy（HRTEM）,fourier transform infrared spectroscopy（FT-IR）,X-ray photoelectron spectroscopy（XPS）,UV-Vis diffuse reflection spectroscopy（UV-Vis DRS）and photoluminescence（PL）.Methyl orange（MO）and rhodamine B（RhB）were selected as evaluation object to systematically investigate the photocatalytic activity of the composite photocatalyst.A variety of emerging organic pollutants and Cr（VI）in water were selected as the target pollutants and the removal efficiencies of these pollutants by composite photocatalysts under simulated sunlight irradiation were studied.Based on the calculation of the energy band structure,the photocatalytic mechanism of composite photocatalyst for the degradation of compounds was proposed,and the degradation pathway of pollutants was deduced according to the detected intermediate degradation products.The main contents of this paper are as follows:（1）Multiwalled carbon nanotubes（MWCNTs）loaded MWCNTs/Bi₄VO₈Cl composite photocatalyst was prepared by a facile in-situ hydrothermal method.XRD,SEM,TEM,XPS and UV-Vis DRS technologies were used to investigate the crystal structure,morphology and microstructure,phase composition and optical property of the samples.The photocatalytic performance of the composite materials prepared under different conditions was evaluated by degradation of methyl orange under simulated sunlight irradiation.In addition,in order to further investigate the possibility of photosensitization,the broad applicability of the composite materials and their application in actual wastewater,the composite materials were applied to the simultaneous removal of Cr（VI）and bisphenol A in pure water and wastewater,and the mechanism of synergistic removal effect was analyzed.The results show that the bandgap and valence band potential of Bi₄VO₈Cl is 2.43 and 2.81 eV,respectively,indicating that it has strong oxidation capacity and can effectively response to visible light.The presence of moderate amount of CNTs in the composite can improve the photocatalytic activity.When the content of carbon nanotubes is 4wt.%,the activity of composite photocatalyst is 30%higher than that of single Bi₄VO₈Cl.When the composite photocatalyst is used in the coexistence of Cr（VI）and bisphenol A,it is found that the removal efficiency of the combined pollution is better than that of the single pollutant.Active substances trapping experiments show that the holes play the major role and·OH radicals play the miner role for the degradation of organic pollutants by MWCNTs/Bi₄VO₈Cl composite photocatalyst under light irradiation.The synergistic removal of Cr（VI）and bisphenol A is attributed to the simultaneous capture of electron and hole generated by photocatalysis by Cr（VI）and bisphenol A,by which the photocatalytic activity of the composite photocatalyst is greatly improved.（2）A novel BiVO₄/CH₃COO（BiO）heterojunction was successfully synthesized by a simple one-step mixed solvothermal method with the mixture of deionized water and glacial acetic acid used as solvent.The crystal structure,phase composition,morphology and microstructure,and optical properties of the as-synthesized samples were characterized by a variety of techniques.The photocatalytic properties of the composites prepared under different conditions were evaluated by the degradation of sulfamethoxazole,methyl orange and 4-aminoantipyrine.The mechanisms for the formation of composite photocatalysts and photocatalytic degradation of organic contaminants were studied in detail.In addition,BiVO₄/CH₃COO（BiO）photocatalyst was also used to degrade bisphenol A and ibuprofen.Through the comparative analysis of the removal effects of four target pollutants,the influence of the structure of organic pollutants on the degradation effect was revealed.The degradation pathway of organic pollutants was speculated by analyzing the degradation products of active substances and intermediates.Finally,the stability and reusability of the composite photocatalyst were evaluated,and the composite photocatalyst was attempted to remove pollutants from the actual wastewater.The results show that the pH value of the precursor solution and NH₄VO₃ dosage play significant roles on the morphologies and photocatalytic activities of as-synthesized composite photocatalysts.The photocatalytic performance achieves the best when the pH value of precursor and NH₄VO₃ dosage is 5.5 and 1.5 mmol,respectively.Compared with CH₃COO（BiO）and BiVO₄,photocatalytic activity of the composite photocatalyst is 2.5 and 14.8 times higher than that of individual CH₃COO（BiO）and BiVO₄,respectively.The mineralization rate can reach as high as 80-96%.The degradation pathway includes the cleavage of the side chain,heterocyclic ring opening as well as the hydroxylation of aromatic ring,and these intermediate products can be further attacked by hydroxyl radical,leading to eventual mineralization.In addition,the application of composite photocatalyst in practical wastewater has also achieved satisfactory results.After four consecutive cycles,heterojunction photocatalyst still maintains high photocatalytic activity.Therefore,the composite photocatalyst has potential application prospects in the purification of refractory pollutants in wastewater.（3）A novel iodine doped BiOIO₃/[Bi₆O₆（OH）₃]（NO₃）₃·1.5H₂O composite photocatalyst（I/BiOIO₃/BHN）was prepared by a low temperature hydrothermal method.The effects of different BiOI dosage,hydrothermal reaction temperature and reaction time on the photocatalytic property of the composite materials were evaluated by degradation of rhodamine B.The preparation conditions of the composite photocatalytic materials were optimized.The crystal phase,crystal morphology,surface oxidation state and optical properties of the composites were characterized and analyzed by using XRD,SEM,TEM,XPS,UV-Vis DRS and PL technologies.The mechanisms for the formation of composite photocatalyst and enhancement of photocatalytic activity were speculated.The main active species were identified by free radical capture experiments.In addition,the degradations of methyl orange and phenol by the composite photocatalyst under simulated sunlight were also investigated to further confirm the broad applicability of the composite photocatalyst.Finally,the stability and reusability of the material were evaluated through recycling experiment.The results show that when the dosage of BiOI is 0.4398 g,the reaction temperature is 180℃and the reaction time is 12 h,the photocatalytic activity of composite photocatalyst achieves the best.Compared with pure[Bi₆O₆（OH）₃]（NO₃）₃·1.5H₂O,BiOI and BiOIO₃,the photocatalytic activity of BB-2 composite is increased by 69.1,10.4 and 2.2 times,respectively.Rhodamine B can be completely removed in 40 min irradiation,and the removal efficiency of methyl orange and phenol can reached 94%and 84%after 100 min and 150 min of irradiation,respectively.The mechanism analysis shows that bismuth nitrate in precursor plays a key role in the formation of BiOIO₃.The synchronization implementation of the iodine doping and construction of the II heterostructures is the main reason for the enhancement of photocatalytic activity.The mechanism for the photocatalytic degradation of organic pollutants is hole oxidation.Especially,the composite photocatalyst can be easily separated and reused by simple free sedimentation,and good stability and photocatalytic activity can be maintained in the reuse process,therefore,it is predicted that the composite photocatalyst has good application prospects in the treatment of organic pollutants.