In-Depth Investigation of Role of -BCO₂ in the Degradation of Sulfamethazine by Metal-Free Biochar/Persulfate: The Mechanism of Occurrence of Nonradical Process

The remediation of organic wastewater through advanced oxidation processes (AOPs) based on metal-free biochar/persulfate systems has been extensively researched. In this work, boron-doped alkali lignin biochar (BKC_1:3) was utilized to activate peroxymonosulfate (PMS) for the removal of sulfamethazine (SMZ). The porous structure and substantial specific surface area of BKC_1:3 facilitated the adsorption and thus degradation of SMZ. The XPS characterization and density functional theory (DFT) calculations demonstrated that -BCO₂ was the main active site of BKC_1:3, which dominated the occurrence of nonradical pathways. Neither quenching experiments nor EPR characterization revealed the generation of free radical signals. Compared with KC, BKC_1:3 possessed more electron-rich regions. The narrow energy gap (Δ<i>E</i>_gap = 1.87 eV) of BKC (-BCO₂) promoted the electron transfer to the substable complex (BKC@PMS*) on SMZ, driving the electron transfer mechanism. In addition, the adsorption energy of BKC(-BCO₂)@PMS was lower (-0.75 eV → -5.12 eV), implying a more spontaneous adsorption process. The O-O (PMS) bond length in BKC(-BCO₂)@PMS increased significantly (1.412 Å → 1.481 Å), which led to the easier decomposition of PMS during adsorption and facilitated the generation of ¹O₂. More importantly, a combination of Gaussian and LC-MS techniques was hypothesized regarding the attack sites and degradation intermediates of the active species in this system. The synergistic T.E.S.T software and toxicity tests predicted low or even no toxicity of the intermediates. Overall, this study proposed a strategy for the preparation of metal-free biochar, aiming to inspire ideas for the treatment of organic-polluted wastewater through advanced oxidation processes (AOPs).