Photocatalytic Degradation Of Gatifloxacin Antibiotics By Magnetic S-scheme BiOBr Heterojunction

Gatifloxacin（GAT）,as a drug for the cure of respiratory and urinary system infections,is often discharged into water bodies along with medical wastewater and sewage from pharmaceutical production enterprises,becoming a persistent organic pollutant in water bodies.It is difficult to remove by conventional water treatment processes.The photocatalytic technology can effectively degrade GAT in water.The special electric field structure of the stepped（S-scheme）heterojunction in the photocatalyst can effectively promote the separation of photogenerated hole electrons and improve the photocatalytic performance.Therefore,this study attempted to construct an S-scheme heterojunction for efficient photocatalytic degradation of GAT.In order to rapidly separate the S-scheme heterojunction from water after photocatalytic treatment of pollutants in water and recycle it,the photocatalyst is endowed with magnetism.In this study,a nanoscale magnetic S-scheme BiOBr/BiO（CH₃CH₂COO）_0.2Br_0.8 photocatalyst was prepared by chemical co-precipitation method for photocatalytic degradation of GAT.The preparation methods and conditions of photocatalyst composites were discussed and optimized,and the microscopic morphology,crystal phase,electron transport path and light absorption properties of the composites were characterized and analyzed,and the influencing factors,active species and degradation paths of the photocatalytic degradation of GAT were studied.The photocatalytic performance stability of its recycling was investigated.The reaction mechanism of photocatalytic degradation of GAT by magnetic BiOBr/BiO（CH₃CH₂COO）_0.2Br_0.8 was analyzed.In this thesis,different semiconductor materials for constructing S-scheme heterojunctions were screened,and the performance of different magnetic S-scheme heterojunctions for photocatalytic degradation of GAT was compared,and the composite materials was composed of Fe₃O₄,BiOBr and BiO（CH₃CH₂COO）_0.2Br_0.8.Photocatalytic degradation effect of GAT by magnetic BiOBr/BiO（CH₃CH₂COO）_0.2Br_0.8 was excellent.The preparation conditions of magnetic BiOBr/BiO（CH₃CH₂COO）_0.2Br_0.8 were optimized by orthogonal experiments,and it was found that the main factors of the photocatalytic performance of the magnetic BiOBr/BiO（CH₃CH₂COO）_0.2Br_0.8 composite were the mass ratio of Bi（NO₃）₃·5H₂O to Fe₃O₄,while the stirring reaction time,the mass ratio of urea to Bi（NO₃）₃·5H₂O,and the concentration of dilute nitric acid had little effect.The optimal preparation conditions were that the mass ratio of Bi（NO₃）₃·5H₂O to Fe₃O₄ was 2:1,the stirring reaction time was 12h,the mass ratio of urea to Bi（NO₃）₃·5H₂O was 15:1,and the concentration of nitric acid was 75mol/L.Under the optimal preparation conditions,the degradation rate of GAT reached 100%in 1h under the irradiation of 500W long arc xenon lamp at the magnetic BiOBr/BiO（CH₃CH₂COO）_0.2Br_0.8dosage of 1g/L and GAT concentration of 20mg/L.After the magnetic BiOBr/BiO（CH₃CH₂COO）_0.2Br_0.8 was recovered by magnetic separation,the photocatalytic degradation efficiency of GAT decreased,but after 4 cycle times,the photocatalytic degradation rate of GAT was still above80%in 3h.Field emission scanning electron microscopy（SEM）,energy X-ray spectroscopy（EDS）,X-ray diffraction（XRD）,field emission transmission electron microscopy（TEM）,X-ray photoelectron spectroscopy（XPS）,Fourier transform infrared spectroscopy（FT-IR）,ultraviolet-visible diffuse reflectance（UV-vis DRS）,photoluminescence spectroscopy（PL）,specific surface and pore size analysis（BET）,electrochemical impedance analysis（EIS）,Mott-Schottky analysis（MS）,Photocurrent analysis,electron paramagnetic resonance active material analysis（ESR）and magnetic properties measurement（VSM）various characterizations were carried out,a magnetic S-scheme heterojunction composed of Fe₃O₄,BiOBr and BiO（CH₃CH₂COO）_0.2Br_0.8 was successfully composited.The saturation magnetization value of magnetic BiOBr/BiO（CH₃CH₂COO）_0.2Br_0.8 was 29.13 emu/g.The UV-Vis diffuse reflection test showd that the absorption sideband of magnetic BiOBr/BiO（CH₃CH₂COO）_0.2Br_0.8 had a red-shifted compared with BiOBr and BiOBr/BiO（CH₃CH₂COO）_0.2Br_0.8,and its forbidden band width narrowed.The research on the photocatalytic degradation of GAT by magnetic BiOBr/BiO（CH₃CH₂COO）_0.2Br_0.8 showed that the photocatalytic degradation efficiency of GAT first increased with the increase of the photocatalyst dosage,but decreased after the dosage reached 1g/L,however,the degradation efficiency increased again above the dosage of 1.5g/L.The greater the initial concentration of GAT was the longer the time required for complete degradation of GAT.The degradation rate of GAT under alkaline conditions was faster than that of acidic conditions,and the stronger the acidity was,the slower the degradation rate of GAT was.When the p H value was at p H 7,the degradation rate of GAT was the fastest.Cl^-and SO₄^2-could promote the photocatalytic degradation of GAT by magnetic BiOBr/BiO（CH₃CH₂COO）_0.2Br_0.8 within 30min,while CO₃^2-had a slight inhibitory effect on the photocatalytic degradation of GAT.Magnetic BiOBr/BiO（CH₃CH₂COO）_0.2Br_0.8 had good adsorption property for GAT,and the adsorption process followed the pseudo-second-order kinetic model and the Freundlich adsorption isotherm.The photocatalytic degradation of GAT by magnetic BiOBr/BiO（CH₃CH₂COO）_0.2Br_0.8 conformed to the pseudo-first-order kinetic model.After the quenching experiment and the identification and analysis of the intermediate products,the active species in the reaction system were e^-,~1O₂,·OH,h⁺and O₂^·-,which could attack the benzene ring and side chain in GAT,and finally degraded GAT to CO₂ and H₂O.Compared to BiOBr,magnetic BiOBr/BiO（CH₃CH₂COO）_0.2Br_0.8 showed significantly enhanced photocatalytic degradation activity,and the study of degradation mechanism revealed that its electronic transfer path under illumination was S-scheme.The efficient degradation of GAT suggested that the S-scheme heterojunction could enhance the carrier separation ability and ultimately increased the photocatalytic activity.