Visible-light-driven Activation Of Persulfate By N-CQDs/g-C₃N₄ Catalysts For Efficient Tetracycline Degradation

With the comprehensive advancement of industrialization and urbanization,the ensuing environmental problems have become one of the major challenges.A series of antibiotics,particularly tetracyclines,has attracted widespread attention because of their high toxicity,durability,refractory and bioaccumulation.Antibiotics like tetracycline are a class of refractory organic pollutants,they usually exhibit some characteristics,such as high stability and excellent solubility,so the traditional methods including adsorption filtration,membrane separation,biological filter or activated sludge treatment,cannot remove it completely.Compared with these traditional methods,the new advanced persulfate oxidation technology can remove refractory pollutants quickly.In addition,metal-free catalysts are used in this treatment process to activate persulfate under visible light and achieve efficient degradation of tetracycline wastewater.The degradation system possess the advantages of simple operation,high efficiency,energy-saving,low cost and no secondary pollution.The study was focused on the synthesis of efficient and environmentally-friendly metal-free catalysts to improve the activation performance of persulfate,and ultimately achieve the efficient degradation of tetracycline.Owing to the characteristics of low-cost,modest bandgap,and high chemical stability,g-C₃N₄ was regarded as a promising catalytic materials for the treatment of environmental problems.However,the rapid recombination of photogenerated carriers,low solar energy utilization rate,and insufficient specific surface area may further limite its catalytic performance.Furthermore,N-CQDs exhibited excellent properties such as charge transfer ability and up-conversion luminescence.Therefore,it is of great significance to find a g-C₃N₄-based catalytic system with excellent PS activation ability.In this experiment,bulk g-C₃N₄ catalysts were quickly prepared by calcination,and N-CQDs/g-C₃N₄ composite was synthesized through one-step hydrothermal method,and their physical and chemical characteristics were explored by X-ray diffraction,X-ray photoelectron spectroscopy,transmission electron microscopy,Brunauer-Emmett-Teller et al.The g-C₃N₄and N-CQDs/g-C₃N₄ composite was regarded as catalysts for activating persulfate and achieving oxidation degradation of tetracycline under visible light to verify their catalytic activity.The catalytic performance was expressed by the degradation rate of tetracycline.Then,different experimental variables in the reaction system were studied,such as catalyst dosages,persulfate concentrations,reaction pH,other inorganic anions.In order to further explore the degradation reaction mechanism,free radical trapping experiments and electron spin resonance analysis were used to determine the possible active radicals in the reaction.Finally,the chemical stability and application potential of the catalysts were demonstrated through repeated experiments and partial characterization.The results showed that the catalysts exhibited the highest degradation efficiency,and more than 91%removal rate was achieved under the optimum experimental conditions.In this reaction,persulfate absorbed the energy of ultraviolet light to break the peroxy bond,and the sulfate radicals（·SO₄^-）were generated,the N-CQDs/g-C₃N₄ catalyst activated the persulfate to produce sulfate radicals（·SO₄^-）and hydroxyl radicals（·OH）in the reaction,which can effectively eliminate tetracycline in wastewater.The optimum reaction rate(0.03203 min^-1)was almost 9.6 and 8.4 times higher than that of g-C₃N₄(0.00335 min^-1)and N-CQDs/g-C₃N₄composite(0.00383 min^-1)without the addition of persulfate.In addition,the N-CQDs/g-C₃N₄composite exhibited excellent chemical stability,and the oxidative degradation reaction can be effectively carried out in a wide pH range,which demonstrated that it can be widely applied in the treatment and disposal of organic wastewater under various conditions.