In-Depth Investigation\nof Role of -BCO<sub>2</sub> in the Degradation of Sulfamethazine by\nMetal-Free Biochar/Persulfate:\nThe Mechanism of Occurrence of Nonradical Process

The remediation of organic wastewater through advanced\noxidation\nprocesses (AOPs) based on metal-free biochar/persulfate systems has\nbeen extensively researched. In this work, boron-doped alkali lignin\nbiochar (BKC<sub>1:3</sub>) was utilized to activate peroxymonosulfate\n(PMS) for the removal of sulfamethazine (SMZ). The porous structure\nand substantial specific surface area of BKC<sub>1:3</sub> facilitated\nthe adsorption and thus degradation of SMZ. The XPS characterization\nand density functional theory (DFT) calculations demonstrated that\n-BCO<sub>2</sub> was the main active site of BKC<sub>1:3</sub>, which\ndominated the occurrence of nonradical pathways. Neither quenching\nexperiments nor EPR characterization revealed the generation of free\nradical signals. Compared with KC, BKC<sub>1:3</sub> possessed more\nelectron-rich regions. The narrow energy gap (Δ<i>E</i><sub>gap</sub> = 1.87 eV) of BKC (-BCO<sub>2</sub>) promoted the\nelectron transfer to the substable complex (BKC@PMS*) on SMZ, driving\nthe electron transfer mechanism. In addition, the adsorption energy\nof BKC(-BCO<sub>2</sub>)@PMS was lower (−0.75 eV → −5.12\neV), implying a more spontaneous adsorption process. The O–O\n(PMS) bond length in BKC(-BCO<sub>2</sub>)@PMS increased significantly\n(1.412 Å → 1.481 Å), which led to the easier decomposition\nof PMS during adsorption and facilitated the generation of <sup>1</sup>O<sub>2</sub>. More importantly, a combination of Gaussian and LC-MS\ntechniques was hypothesized regarding the attack sites and degradation\nintermediates of the active species in this system. The synergistic\nT.E.S.T software and toxicity tests predicted low or even no toxicity\nof the intermediates. Overall, this study proposed a strategy for\nthe preparation of metal-free biochar, aiming to inspire ideas for\nthe treatment of organic-polluted wastewater through advanced oxidation\nprocesses (AOPs).