Extrinsic defects-rich biochar for efficient peroxydisulfate activation: Electronic structure modulation and disparate nonradical mechanisms

Doping nitrogen atoms into carbon materials will produce abundant extrinsic defects, which can significantly improve the catalytic activity toward water remediation. However, there is still a lack of systematic research on which extrinsic defect is the real active site and how extrinsic defects enhance the catalytic activity of carbon materials. In this work, the sludge derived biochar with different configuration of extrinsic defects (pyrrolic-N, pyridinic-N, and graphitic-N.) were successfully synthesized by controlling nitrogen doping amount. The adsorption experiments showed that the biochar with rich graphitic-N significantly enhanced the adsorption ability of peroxydisulfate (PDS), which further led to the excellent catalytic activity toward norfloxacin (NOR) degradation, with 91 % removal efficiency within 90 min. The pseudo-first-order degradation rate constant (Kobs) was strongly positively correlated with the content of graphitic-N. Electron paramagnetic resonance (EPR) analysis and quenching experiments revealed that metastable reactive species (PDS*) were the dominated reactive species in NBC/PDS system. Besides, density functional theory (DFT) calculations demonstrated that the existence of graphitic-N most effectively improved the charge transfer among biochar, PDS and NOR compared to pyrrolic-N and pyridinic-N. Electrochemical characterization and Raman results further identified that graphitic-N heightened the direct electron transfer rate due to the formation of PDS*. The Fukui function predicted the reactive sites of NOR. All of these led to the high NOR removal efficiency. This study provided new insights for the design of non-metallic biochar catalysts with rich extrinsic defects for effectively activating PDS to degrade fluoroquinolone antibiotics.