Degradation Mechanism Of Tetracycline In Wastewater By Photocatalytic G-C₃N₄/BiOBr/NaHSO₃ Oxidation System

Tetracycline wastewater has high toxicity,complex components,poor biodegradability.It was difficult to be oxidated by conventional technology.In this study,Vis/g-C₃N₄/BiOBr/NaHSO₃ system was constructed to degrade tetracycline.（1）The material structure and photoelectric properties of g-C₃N₄/BiOBr were studied.The results showed that the material had a good loose structure,which provided active sites for the reaction.To further explore the type of photocatalyst and its electron transfer mechanism,it was proved by various characterization means that the synthesis of g-C₃N₄/BiOBr material effectively reduced the electron-hole recombination rate and promoted the transmission efficiency of photogenerated electrons between semiconductor components,improving the photocatalytic effect.The optimum ratio of g-C₃N₄/BiOBr was 10%（CNB10）.In addition,NaHSO₃ was added into CNB10/Vis system to improve the tetracycline degradation rate of the system.（2）The factors affecting the degradation of tetracycline in CNB10/NaHSO₃/Vis system were studied.The results showed that the optimum reaction conditions were CNB10 dosage:0.1g/L,NaHSO₃ dosage:2.0mM,light intensity:300W,initial pH of solution:6.0.The common anions SO42-,Cl-,NO₃-and natural organics（EDTA,HA）in water inhibited the degradation of tetracycline.The order of influence factors on the degradation were as follows:photocatalyst dosage>light intensity>sulfite dosage>initial pH value.（3）The photocatalytic reaction mechanism,free radical transformation path,and quantitative change of tetracycline degradation in CNB10/NaHSO₃/Vis system were revealed through quenching and ESR tests.The introduction of sulfite provided a variety of free radicals such as SO₃·-,SO5·-,SO4·-,·OH for the system.In the reaction process,sulfite was activated to produce SO₃·-which was further oxidized to SO5·under the action of O2,and then the SO5·-produced SO4·-via the disproportionation,and part of SO4-· could be converted to SO₃·-after reacting with HSO₃-.As a result,the cyclic conversion of sulfur oxygen free radicals was realized,which accelerated the degradation reaction.At the same time,due to the reaction of electron gain and loss in sulfite activation,the electron-hole recombination effect in photocatalyst was weakened,and the photocatalytic effect of photocatalyst was improved.According to the principle of Z-type heterojunction photocatalysis and the law of free radical action,the degradation mechanism of CNB10/NaHSO₃/Vis system was described.15 intermediates were found by LC-MS,and two possible degradation routes were inferred.The toxicological test showed that the tetracycline solution after treatment become low toxic,which was convenient for the follow-up biochemical reaction.After CNB10 was recycled for 4 times,the degradation rate could be maintained at 87.24%,proving that CNB10 had good recyclability.CNB10/NaHSO₃/Vis system could also degrade oxytetracycline,ofloxacin and norfloxacin.The degradation rate of oxytetracycline with a similar structure could reach more than 70%,and the degradation rate of ofloxacin and norfloxacin with different structures can also reach more than 40%,which proved the universality of the system.（4）The possibility of introducing photosensitized dye RhB into CNB10/NaHSO₃/Vis system to treat waste was studied,and its degradation process and mechanism were speculated.At the same time,the optimal dosage ratio of Rhodamine B and sulfite was explored.When the initial concentration of tetracycline was 10mg/L,C（sulfite）:C（Rhodamine B）=1:5,and the final degradation rate was 97.3%,the mineralization rate reached the highest.The reason could be explained that rhodamine B was excited to form a high-energy state under visible light which participated in the activation of sulfite,increasing the type and quantity of free radicals in the system.Furthermore,it cooperated with photocatalytic degradation of tetracycline and accelerated the degradation of tetracycline.It not only provided a new strategy and new idea for waste control by waste but also provided a reference for the treatment of antibiotic wastewater.