Preparation Of ZnO-Based Photocatalysts And Their Degradation Performance Against Sulfonamide Antibiotics In Water

Antibiotics are now found in freshwater environments（especially urban groundwater）worldwide and levels of antibiotics are increasing.In addition,such alarming levels of antibiotics severely affect the microbial community,which can lead to the development of antibiotic resistance groups and their spread in the environment.This could pose a significant risk to ecology and human health if appropriate treatment measures are not taken.Therefore,this paper aims to reduce and eliminate the harm of antibiotics in water bodies through photocatalytic degradation in order to achieve the goals of energy saving,reducing water pollution and protecting the environment with good environmental and social benefits.In this study,three photocatalysts,Zn O-ATP,Zn O-Ag-ATP and Zn O-Ag-Bi WO-ATP,were prepared using zinc oxide（Zn O）as the substrate material and loaded onto concave-planar clay（ATP）and doped with silver（Ag）and bismuth tungstate（Bi₂WO₆）to change the limitations of pure Zn O as a photocatalytic material.Secondly,the catalysts were characterised by scanning electron microscopy（SEM）to analyse the structural changes before and after modification;the presence of Ag and Bi₂WO₆ was verified by X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,X-ray energy spectroscopy（EDS）and Fourier infrared spectroscopy（FT-IR）;the catalysts were analysed by UV-vis diffuse reflectance（UV-vis DRS）characterisation The photocatalytic properties of the catalysts were inferred from their absorption of visible light by UV-vis DRS.The photocatalytic experiments were carried out to investigate the degradation effect and performance of the three photocatalysts on sulphonamide antibiotics,and to investigate the effects of different ratios of dopants,pollutant concentrations,p H,coexisting ions and humic acid on the photocatalytic activity,and to evaluate the stability of the photocatalysts by repeatability experiments.Free radical trapping experiments were used to verify the main active substances in the photocatalytic reaction,and the degradation products of sulfonamide antibiotics were analysed by LC-MS to explore the degradation mechanism and degradation process of the three photocatalysts.The main results were obtained as follows:（1）The successful preparation of Zn O-ATP photocatalyst was confirmed by SEM,EDS,XRD,XPS,FT-IR and UV-vis DRS characterisation.The experiments showed that 1.5mmol/g of Zn O-ATP exhibited excellent and stable photocatalytic performance.Kinetic experiments revealed that sulfapyridine（SPD）could be directly degraded under mercury lamp irradiation,but the degradation was incomplete;the quasi-primary reaction rate constants for the degradation of SPD by different catalysts were:0.0863 min^-1（stock solution）<0.1910 min^-1（ATP）<0.2545 min^-1（Zn O-ATP）,indicating that Zn O-ATP has excellent photocatalytic performance.The degradation efficiency of SPD by Zn O-ATP decreases with increasing pollutant concentration;the degradation of SPD by Zn O-ATP is more favourable under alkaline conditions.Ca²⁺,Fe³⁺,NO₃^-,SO₄^2-and HA all promoted the adsorption effect of Zn O-ATP on SPD under dark conditions;during photodegradation,the five co-existing substances inhibited the catalytic activity of Zn O-ATP to a certain extent.Zn O-ATP showed a relatively stable photocatalytic performance on SPD,which was still completely degraded within 30 min after five cycles.The SPD was degraded to smaller intermediate products by hydroxylation,desulphonation and splitting under the action of superoxide radicals（·O₂^-）.（2）The Zn O-Ag-ATP photocatalyst was successfully prepared by loading Ag nanoparticles onto the surface of Zn O-ATP composites as confirmed by various characterization methods.The photocatalytic performance of the 4%Zn O-Ag-ATP photocatalyst was the best and most stable,and the quasi-primary reaction rate constant of0.2594 min^-1 was better than that of the Zn O-ATP photocatalyst for the degradation of SPD.The degradation efficiency of Zn O-Ag-ATP for SPD decreased with the increase of pollutant concentration;it was more favourable under alkaline conditions The degradation of SPD by Zn O-Ag-ATP was more favourable under alkaline conditions.Ca²⁺,NO₃^-and SO₄^2-contributed to the adsorption and catalytic degradation of SPD by Zn O-Ag-ATP under dark conditions and at the beginning of the photodegradation process,and had little effect on the degradation efficiency of SPD as the reaction progressed;Fe³⁺and HA enhanced the degradation efficiency of SPD at the beginning of the photodegradation process,and inhibited the photocatalytic activity of the catalyst at the end of the reaction.Zn O-Ag-ATP showed a stable photocatalytic performance for SPD,with complete degradation within 30 min after 5cycles.·O₂^-was the main active substance and h⁺was the secondary active substance in the degradation of SPD by Zn O-Ag-ATP.（3）The generation and successful loading of Bi₂WO₆ onto the surface of Zn O-Ag-ATP composites was confirmed by various characterization methods,and Zn O-Ag-Bi WO-ATP photocatalysts were successfully prepared.The experiments showed that the 0.3Zn O-Ag-Bi WO-ATP photocatalyst had the best and most stable photocatalytic performance,and the quasi-primary reaction rate constants of 0.2708 min^-1 and 0.2856 min^-1 for the degradation of SPD and sulfamethoxazole（SMZ）by Zn O-Ag-Bi WO-ATP were better than those of other photocatalysts.The degradation efficiency of the composites for SPD decreased with increasing pollutant concentration,while that of SMZ increased with increasing concentration and then decreased when the concentration was too high;the degradation of SPD by Zn O-Ag-ATP was more favourable under alkaline conditions,and the photocatalytic degradation of SMZ by the catalyst was favoured at p H=5.Ca²⁺,Fe³⁺and NO₃^-contributed to the adsorption and catalytic degradation of SPD by Zn O-Ag-Bi WO-ATP,while SO₄^2-and HA inhibited the adsorption and catalytic degradation of SPD by the photocatalyst;the four co-existing ions and low concentrations of HA had little effect on the degradation efficiency of SMZ;the inhibitory effect of medium and high concentrations of HA gradually decreased with the prolongation of light exposure time.Zn O-Ag-Bi WO-ATP showed a stable photocatalytic performance for SPD and SMZ,which were completely degraded within 30min after 5 cycles.SMZ was degraded to other intermediate products through four pathways:photooxidation,hydroxylation,desulphonation and splitting under the action of·O₂^-and h⁺.