Efficiency And Mechanism Of Removing Tetracycline By Peroxymonosulfate Activated With Cobalt And Nitrogen Co-doped Biochar In Aqueous

Water is an indispensable resource for human survival and plays a major role in all aspects of human production and life.However,at this stage,China’s water shortage is very serious,and one of the many factors that cause the problem of water shortage is water environmental pollution.Advanced persulfate-based oxidation has proven to be an efficient technique for removing various contaminants from water.Although metal-doped modified biochar-activated persulfate has many advantages,single-metal-doped biochar catalysts have poor cyclability during the reaction process,and their catalytic activity is easily limited by environmental factors due to the instability of metal ions and easy leaching.In order to design a system with high catalytic performance and stability,this paper uses lignin as the precursor of biochar to synthesize a series of cobalt-nitrogen co-doped biochar（Co_xN_yBC）by introducing different proportions of cobalt acetate and dicyandiamide,and Cox Ny BC and PMS form a heterogeneous system to remove tetracycline,and the degradation effect,influencing factors,circularity,degradation mechanism and mineralization of the system are analyzed,and the contribution of each component in the material to the catalytic degradation reaction is clarified.The main conclusions are as follows:（1）The cobalt-nitrogen co-doped biochar has abundant pleated and spherical structures,which effectively suppress the clustering of Co on the surface when the nitrogen doping increases.The specific surface area and defect structure of the material increased with the increase of nitrogen doping.The total nitrogen content and pyridine N content of the material also increased with the increase of nitrogen doping.Although the total Co content was not regular,the Co²⁺/Co³⁺ratio was correlated with the nitrogen doping amount.The same trend was observed for the doping amounts of 8 mmol and 16 mmol.（2）The adsorption of TC by the cobalt-nitrogen co-doped biochar materials was in accordance with the proposed secondary kinetics with R2 range from 0.997 to0.999,and the isothermal adsorption pattern was more in accordance with the Langmuir model with fitted values ranging from 0.900 to 0.997.The maximum adsorption（Qm）of the Co_xN_yC catalysts was fitted using the Langmuir isotherm model,and the maximum adsorption of the nine materials was in the range of 17.51mg/g to 50 mg/g.The pore structure,specific surface area and the degree of defects may be the factors affecting the adsorption of the materials by preliminary analysis with the characterization data.（3）The Co_xN_yC/PMS system all showed high removal effect on TC,and the TC was completely removed within 25 min.The material Co₂₄N₂₅C had the best catalytic ability,and the removal rate of TC reached 100%within 10 min with kobs reaching0.6685 min^-1.By investigating the catalytic ability of the Co₂₄N₂₅C/PMS system under different parameters.The catalyst dosage was positively correlated with the degradation efficiency of TC,and the final removal rate of Co₂₄N₂5C could still reach96.21%at the concentration of 20.00 mg/L of TC.Co₂₄N₂5C could still maintain its good catalytic performance in a wide pH range（pH=5-9）,and the common anions in water had less influence on the catalytic performance of the Co₂₄N₂₅C/PMS system The effect of common anions in water on the catalytic performance of Co₂₄N₂₅C/PMS system is small.（4）The correlation and multi-component regression analysis of the degradation rate k_obsof TC with the characterization parameters showed that the degree of defects,pyridine N content and Co²⁺/Co³⁺ratio of the cobalt-nitrogen co-doped biochar material can be considered as the decisive factors for the degradation of TC by activated PMS,with the highest contribution of defects,followed by pyridine N and Co²⁺/Co³⁺.The results of Co₂₄N₂₅C characterization before and after the reaction can further prove that pyridine N,defective structure and Co²⁺/Co³⁺are the main active sites of the reaction.（5）Comparing the reusability of Co₂₄BC and Co₂₄N₂₅C materials,it was found that Co₂₄N₂₅C has better recyclability,and the structure of XPS elements before and after the reaction of Co₂₄N₂₅C material did not change significantly,which proved that the introduction of nitrogen can effectively disperse and protect the highly reactive cobalt material,and make Co₂₄N₂₅C have good stability and recyclability.（6）EPR analysis,burst experiments and electrochemical tests were used to demonstrate the existence of both free radical and non-free radical pathways in the Co₂₄N₂₅C/PMS system,generating·OH、SO₄·^-、¹O₂and O₂·^-to degrade TC,as well as the presence of the system Co₂₄N₂₅C as an electron transfer mediator mediates the direct electron transfer pathway between TC and PMS.O₂·^-and ¹O₂play a dominant role in the reaction in the Co₂₄N₂₅C/PMS/TC system,and LC-MS analysis was used to identify the intermediates in the catalytic degradation process to speculate the possible degradation of TC pathway.