Preparation Of Carbon Nitride Based Photocatalysts Modified By Quantum Dots And Their Photocatalytic Degradation Of Antibiotics In Wastewater

Antibiotics are widely used in disease prevention and the treatment of bacterial infection.However,some antibiotics are discharged into the environment due to the incomplete metabolism of the organism,causing adverse influence and a serious threat to the ecosystem and human health.Traditional physical,chemical and biological methods for removing antibiotics in water have some problems such as low efficiency and secondary pollution,while photocatalytic technology is considered as an effective method for treating antibiotics in water due to its advantages such as low cost and environmental friendliness.Graphitic carbon nitride（g-C₃N₄）is a popular semiconductor material with suitable band gap and excellent chemical stability.However,its defects such as small specific surface area and slow charge separation and transfer limit its photocatalytic performance,so it is necessary to adopt some modification methods to improve its photocatalytic activity.In this work,boron nitride quantum dots（BNQDs）and bisphenol S（BPS）doped g-C₃N₄ nanosheets（BPS-CN）were combined to construct BNQDs/BPS-CN composite photocatalyst to overcome these defects and it was applied to the photodegradation of sulfamethazine（SMZ）under the visible light.Various characterization methods were applied to analyze the morphology structure,specific surface area,elemental composition,photochemical properties,and photocatalytic performances of BNQDs/BPS-CN composites.The main research contents and results were as follows:（1）New metal-free heterostructure photocatalysts named BNQDs/BPS-CN were synthesized for the first time.The photocatalytic activities of sulfamethazine under visible light were determined.The results showed that among all the prepared photocatalysts,BNQDs/BPS-CN-4 had the highest photocatalytic performance,and the degradation efficiency of SMZ reached 100%within 60 min.The first-order kinetic model further explored the photodegradation process,and the experimental data showed that the first-order rate constant of BNQDs/BPS-CN-4 was 0.0247 min^-1,which was 13.72 times higher than that of with g-C₃N₄.Meanwhile,after five cycles of BNQDS/BPS-CN,the removal rate of SMZ did not significantly decrease,which implieds the good photostability and reusability of BNQDs/BPS-CN composite.（2）The reasons for the improvement of the photocatalytic performance of BNQDs/BPS-CN and the photocatalytic degradation mechanism were explored:the doping of BPS shortens the band gap of carbon nitride,which is beneficial to enlarge the absorption range of visible light,and the introduction of electronegative BNQDs can improve the separation of photo-induced carriers.Meanwhile,the results of main active species trapping experiments and ESR spin-trap characterization demonstrate that the photogenerated holes and superoxide radicals play predominant roles in the photodegradation of SMZ.（3）The possible degradation pathways of SMZ were speculated by UPLC-MS/MS analysis of intermediates.The results show that it can be divided into three pathways,which are the hydroxylation process,the detachment of the sulfonyl group and the cleavage of the C-N bond,and the cleavage of the S-N bond,respectively.Finally,the main degradation products are bisphenol A,p-aminophenol,and other inorganics,which are easier to biodegrade than SMZ.This study provides an effective strategy to construct a new visible light driven photocatalyst using metal-free 2D materials and quantum dots,which provides an effective and green method for the removal of antibiotic pollutants in wastewater.