Fabrication Of Phosphorus-doped G-C₃N₄/Bismuth Oxy-acid Salt And Photocatalytic Degradation Of Tetracycline Hydrochloride

Graphite-phase carbon nitride（g-C₃N₄）and bismuth oxy-acid salt have good photocatalytic properties,and are often used to treat tetracycline hydrochloride（TC-HCl）wastewater.When used alone,both of them have the problem of low photocatalytic efficiency caused by narrow visible light absorption range and easy electron-hole recombination.Because they have very matched energy band structures,the combination of them to form g-C₃N₄/bismuth oxy-acid salt heterojunction photocatalyst can overcome the problems when they are used alone,but the photocatalytic performance needs to be further improved and the photocatalytic mechanism needs to be further studied.Therefore,the base photocatalyst P-doped g-C₃N₄（PCNS）was prepared by combining the strategy of element doping and morphology control,and then three kinds of multi-scale PCNS/bismuth oxy-acid salt photocatalysts were prepared by loading three different morphologies of bismuth oxy-acid salt on PCNS through the strategy of morphology and Z-scheme control.The performance,influencing factors,stability and ability to treat actual wastewater of photocatalytic degradation of TC-HCl were systematically investigated.Combining the calculation results of density functional theory（DFT）and the intermediate products detected by ultra-high resolution quadrupole electrostatic field orbital trap liquid mass spectrometer（HRAMLC-MS/MS）,the photocatalytic mechanism and degradation pathway of TC-HCl were proposed.The toxicity of degradation intermediates was evaluated,and finally the performance of three kinds of multi-scale photocatalysts was compared and evaluated.The main research results are as follows:（1）PCNS were prepared by combining morphology control and phosphorus（P）doping.The results of photocatalytic degradation of TC-HCl show that PCNS-0.5 has the highest degradation rate,which is 2.52 times that of bulk g-C₃N₄.·O₂^-and h⁺are the main active species in the degradation process.The good photocatalytic performance of PCNS is attributed to the good structural,optical and electronic properties produced by replacing carbon atoms in g-C₃N₄ with P atoms.TC-HCl was mainly degraded into seven kinds of intermediate products through de-CH₃ and oxidative ring opening,and the toxicity of most intermediate products was weakened.（2）A novel Z-scheme 0D/2D Bi₂Mo O₆ quantum dots/P-doped g-C₃N₄ nanosheets composite photocatalyst（BMO QDS/PCNS）was prepared by solvothermal method.The results showed that 50%BMO QDs/PCNS had the best degradation performance,and the degradation rate constant was 3.1 times and 2.1 times that of g-C₃N₄ nanosheets（CNS）and PCNS,respectively.Its excellent photocatalytic performance is attributed to the fact that P doping improves the visible light absorption range of CNS and improves its photoelectric properties.Second,the synergistic effect between BMO QDs and PCNS effectively inhibits charge recombination.The effects of humic acid（HA）,different anions and cations,p H value and water matrix on the degradation efficiency of TC-HCl were investigated.·O₂^-and h⁺are the main active species in the reaction system,and·OH also participates in the reaction.Combined with DFT calculation results and intermediate products detected by HRAMLC-MS/MS,it was concluded that TC-HCl was degraded mainly by ring opening,de-CH₃ and de-OH.The results of toxicity assessment showed that the toxicity of most intermediate products had been weakened.（3）Z-scheme 2D/2D P-doped g-C₃N₄/Bi₂WO₆ heterojunction（PCNS/BWO）was prepared by three strategies:P doping,morphology control and heterojunction.The results of degradation of TC-HCl by visible light show that the photocatalytic performance of 30%PCNS/BWO is the best,and the photocatalytic rate is 4.5 times that of CNS,2.2 times that of PCNS and 1.9 times that of BWO,respectively.The improvement of efficiency is attributed to the good structural and photoelectric properties produced by the synergistic effect among P doping,2D/2D structure and direct Z-scheme.The effects of HA,8 kinds of anions and cations,4 different p H values of raw water and 4 different water sources on degradation efficiency were discussed.Four cycle experiments show that 30%PCNS/BWO has good stability and repeatability.·O2^-and h⁺are the main active species in the photocatalytic reaction system.Based on DFT calculation and HRAMLC-MS/MS determination results,the degradation pathway of TC-HCl was deduced.TC-HCl was finally converted into CO₂ and H₂O through benzene ring opening,de-CH₃,deamidation,carbonylation and de-OH.The toxicity of most intermediate products after photocatalytic degradation was reduced.（4）A Z-scheme 3D/2D MT-Bi VO₄ microsphere/P-doped g-C₃N₄ nanoplate heterojunction composite photocatalyst with monoclinic scheelite（M-Bi VO₄）and tetragonal zircon（T-Bi VO₄）coexisting was synthesized by hydrothermal method.The results of photocatalytic degradation of TC-HCl show that the percentage of M-Bi VO₄to MT-Bi VO₄ has great influence on the photocatalytic activity of the composite catalyst.The excellent photocatalytic performance of 5%BVO/PCNS is attributed to the synergistic effect among P doping,3D/2D morphology,MT-Bi VO₄ homojunction and Z-scheme heterojunction constructed by BVO and PCNS.This synergistic effect not only improves the visible light absorption range of composite photocatalyst,but also accelerates the charge separation rate.h⁺and·O₂^-are the main active species in photocatalytic degradation.Based on the results of DFT calculation and HRAMLC-MS/MS determination,it was proposed that TC-HCl was degraded mainly by de-CH₃,de-NH₂ and oxidative ring opening.The toxicity of TC-HCl was reduced after photocatalysis.（5）The performances of three kinds of multi-scale p-doped g-C₃N₄/bismuth oxy-acid salt photocatalyst were comprehensively evaluated.The advantages and disadvantages of 0D Bi₂Mo O₆/2D PCNS、2D PCNS/2D Bi₂WO₆ and 3D Bi VO₄/2D PCNS were compared from the aspects of morphology,specific surface area,light absorption capacity,photocatalytic degradation of TC-HCl,HA impact resistance,anion and p H impact resistance,toxicity,stability and cost of active species and degradation intermediates.Considering comprehensively,2D PCNS/2D Bi₂WO₆ has the best photocatalytic performance,but its anti-interference ability is relatively poor.The photocatalytic performance of 3D Bi VO₄/2D PCNS is slightly poor,but it has strong anti-interference ability.The P-doped g-C₃N₄/bismuth oxy-acid salt heterojunction photocatalyst constructed in this paper overcomes the problems of low photocatalytic efficiency caused by narrow visible light absorption range and electron-hole recombination when g-C₃N₄ and bismuth oxy-acid salt are used alone,and greatly improves the photocatalytic performance.The research results can provide theoretical reference and technical support for the synergistic preparation of high-performance photocatalyst by various control strategies such as element doping,morphology control and Z-scheme heterojunction.