Photodegradation Of Pollutants In Swine Wastewater By Persulfate Activated With Modified Bismuth-based Materials

With the continuous development of scale-intensive animal husbandry in southern Xinjiang,the discharge of swine wastewater has been increasing.The efficient removal of high-concentration organic pollutants,nitrogen and phosphorus,and antibiotics in swine wastewater is currently a hot research topic.Due to its clean,green,and efficient characteristics,photocatalysis in advanced oxidation technology has received widespread attention,and it mainly focuses on finding new photocatalytic materials and improving photocatalytic efficiency.Sodium bismuthate（Na Bi O₃）has strong visible light absorption ability,but the fast recombination rate of photo-generated electron-hole pairs severely affects its degradation efficiency.Therefore,it is crucial to improve its photocatalytic activity.In this study,Fe-Na Bi O₃ and Co Fe₂O₄/Na Bi O₃were prepared by ultrasound-assisted and solvothermal methods using Na Bi O₃ as the raw material.Coupled with peroxydisulfate（PDS）and persulfate（PMS）,the Vis（light irradiation conditions）+Fe-Na Bi O₃+PDS and Vis+Co Fe₂O₄/Na Bi O₃+PMS systems were constructed.Degradation experiments of tetracycline hydrochloride（TCH）and organic pollutants in actual wastewater were conducted to investigate the photocatalytic degradation performance and mechanism of the two systems towards the target pollutants.The main research contents and conclusions are as follows:（1）Fe³⁺loaded bismuth-based photocatalytic material Fe-Na Bi O₃ and Co Fe₂O₄/Na Bi O₃ composite catalytic material were synthesized respectively by a simple ultrasonic stirring method and solvothermal method,and the morphology,physicochemical properties,etc.of the catalyst were analyzed by scanning electron microscopy（SEM）,specific surface area（BET）,X-ray photoelectron spectroscopy（XPS）,X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FT-IR）,ultraviolet-visible diffuse reflectance spectroscopy（UV-Vis）,and fluorescence spectroscopy（PL）.The results showed that the above materials could all broaden the absorption range of visible light,reduce the bandgap width,and effectively suppress the compounding efficiency of photogenerated electron-hole pairs after the composite.Among them,the composite of Co Fe₂O₄and Na Bi O₃ could effectively improve the aggregation phenomenon of Co Fe₂O₄,enhance the photocatalytic activity and magnetism of the catalyst,and solve the problem of the difficult recovery of Fe-Na Bi O₃.（2）The synthesized Fe-Na Bi O₃ and Co Fe₂O₄/Na Bi O₃ catalysts were coupled with persulfate（PDS,PMS）to construct Vis+Fe-Na Bi O₃+PDS（System I）and Vis+Co Fe₂O₄/Na Bi O₃+PMS systems（SystemⅡ）,which were applied to the degradation of tetracycline hydrochloride（TCH）.The results showed that the System I exhibited high degradation efficiency towards TCH compared to Vis+Fe-Na Bi O₃ and Vis+PDS systems.Under the optimal reaction conditions of Fe-Na Bi O₃ concentration of 1 g/L,PDS concentration of 100 mg/L,initial TCH concentration of 10 mg/L,natural p H（6-8）,and with no obvious inhibition by organic compounds,the degradation rate could reach 98.8%.The cycling stability test showed that after 5 cycles,the degradation rate of TCH decreased only from 96.1%to 85.4%,and the characteristic peak of Fe-Na Bi O₃ showed no significant change,indicating its good stability.The free radical capture test showed that the contribution of active species during the TCH degradation process was in the order of superoxide radicals（·O₂^-）>holes（h⁺）>sulfate radicals(SO₄^-·)>hydroxyl radicals（·OH）.SystemⅡ,the optimal reaction conditions of the system under this test condition were:Co Fe₂O₄/Na Bi O₃ concentration of 1 g/L,PMS concentration of 100 mg/L,initial concentration of TCH of 10 mg/L,and natural p H（6-8）,and the degradation rate of TCH under this condition was as high as 94%;however,the degradation rate of TCH was significantly inhibited after the addition of organic matter（FA）;for the magnetic recovery of Co Fe₂O₄/Na Bi O₃ could reach 90.3%.After five cycles of stability test,the degradation rate of the reaction system for TCH decreased from 90.5%to 73.5%,and the characteristic peaks of the catalyst before and after the cycle did not change significantly,which proved its good stability.The free radical capture test showed that the contribution of active species in the reaction system was in the order of hydroxyl radicals（·OH）>sulfate radicals(SO₄^-·)>holes（h⁺）>superoxide radicals（·O₂^-）.（3）Based on the above materials,Fe-Na Bi O₃ and Co Fe₂O₄/Na Bi O₃ composite materials showed good photocatalytic activity in the oxidation degradation process of TCH.Actual wastewater containing organic pollutants（COD,DOC）was selected to further investigate the degradation effect of the two composites.The two reaction systems were tested under different experimental conditions（catalyst concentration,peroxydisulfate concentration,initial pollutant concentration）to investigate their effects on chemical oxygen demand（COD）and soluble organic matter（DOC）in actual wastewater.It was found that the Vis+Fe-Na Bi O₃+PDS system had the optimal degradation rates for COD and DOC at 53.2%and 38.6%,respectively,while the Vis+Co Fe₂O₄/Na Bi O₃+PMS system had the optimal degradation rates for these two pollutants at51.2%and 35.7%,respectively.Three cycles of stability tests showed that,except for a decrease of about 4%in the degradation rate of COD for the Vis+Fe-Na Bi O₃+PDS system,the degradation rates of both DOC and COD for the two systems decreased by about 10%.Three-dimensional fluorescence spectra（3D-EEM）analysis showed that the soluble organic matter degraded by the two systems was organic matter metabolites and humic acid.EPR testing showed that both reaction systems could produce·OH,SO₄^-·,h⁺and·O₂^-.