Performance And Mechanism Of Peroxydisulfate Activation By Nitrogen-doped Biochar For Tetracycline Removal From Water

As the most widely used antibiotics in our country,tetracycline antibiotics have been frequently detected in water effluents,which has aroused emerging concerns due to their seriously hazardous to the environment.The advanced oxidation process based on persulfate（SR-AOPs）is one of the effective strategies for removing tetracycline pollutants from water.Biochar,which is environment-friendly and has abundant pore structures,holds great promise for versatile applications in heterogeneous SR-AOPs.Biochar activates persulfate to degrade tetracycline,including radical and non-radical pathways,wherein the non-radical pathway is relatively mild and resistant to anions and background natural organic matter（NOMs）.Studies have found that the catalytic activity of most pristine biochar is unsatisfactory.In this regard,nitrogen doping has been found to effectively promote the catalytic activation of persulfate by biochar.However,the structures that play a significant role in enhancing persulfate activation by nitrogen-doped biochar and its mechanism remain ambiguous,which limits the directional design of nitrogen-doped biochar.In this study,tetracycline hydrochloride（TCH）was adopted as the target pollutant,corn stalk powder as raw material,and melamine as nitrogen precursor to prepare a series of pristine biochar（BC）and nitrogen-doped biochar（N-BC）at different pyrolysis temperatures to explore the effect of biochar structure on its peroxydisulfate（PDS）activation performance.In addition,electronic paramagnetic resonance（EPR）,quenching experiments,electrochemical analysis techniques,and high-performance liquid chromatography-mass spectrometry（LC-MS）were used to analyze the mechanism of PDS activation by BC or N-BC,providing a theoretical basis for the directional design of nitrogen-doped biochar.The main results of this study are as follows:（1）Nitrogen doping significantly changed the physical and chemical properties of biochar.N-BC prepared at 900℃（N-BC900）showed the highest specific surface area（1357 m²/g）,graphitization degree,and the relative amount of carbonyl groups(A_C=O/A_C-O=1.07).The defects level of N-BC900 was also increased（I_D/I_G=1.14）.Notably,BC showed a higher content of persistent free radicals（PFRs）,in which BC700 had the highest content of PFRs.（2）Nitrogen doping successfully improved the catalytic performance of BC.Under the conditions that the N-BC900 dosage was 0.10 g/L,the PDS dosage was 1.00m M,and the initial TCH concentration was 20.00 mg/L,the TCH removal efficiency of N-BC900/PDS system reached 98.77%within 40 min.The TCH degradation followed the first-order kinetics model with the reaction rate constant(k_obs)of 0.0989min^-1,which is 34 times higher than the optimal BC700/PDS system for the pristine biochar.（3）The excellent catalytic performance of N-BC900 is due to its huge specific surface area and high graphitization degree,which facilitates TCH adsorption,and thus improving the TCH degradation effect.Meanwhile,the defects,C=O groups,pyridine N and graphite N as active sites play an important role in TCH degradation.While PFRs on the BC surface act as the dominant active site to achieve TCH degradation.（4）The degradation of TCH by the N-BC/PDS system is mainly through the non-radical mechanism.Among,O₂·^-/¹O₂ act as the dominant reactive oxygen species.There is also an electron transfer pathway from TCH to PDS mediated by N-BC.In contrast,TCH degradation by BC/PDS system is a radical mechanism dominated by·OH/SO₄·^-.The above analysis indicated that nitrogen doping induced the transformation of biochar-activated PDS from a free radical mechanism to a non-free radical mechanism.（5）Owing to the non-radical dominant mechanism of the N-BC/PDS system,it exhibits excellent degradation performance at the initial p H of 3-7 and can still maintain over 90%TCH removal efficiency in the presence of different anions（Cl^-,NO₃^-,HCO₃^-）or different water substrates（tap water,natural water）,suggesting that the system has a strong anti-interference ability to NOMs in water.