Active surface area determines the activity of biochar in Fenton-like oxidation processes

Biochar (BC) possesses diverse active sites (e.g., oxygen-containing groups OCGs, defects, and electronegative heteroatom) responsible for the catalytic reactions. As an ordinary indicator, specific surface area (SSA) can not accurately reflect the activity of biochar. Herein, we proposed active surface area (ASA), which referred to the surface containing active sites to characterize the activity of biochar in advanced oxidation processes (AOPs). We developed a simple and non-destructive etching method to realize the regulation of OCGs and SSA in silicon-abundant biochar. Intriguingly, peroxydisulfate (PDS) activation was improved whilst periodate (PI) activation was inhibited in HFBC-AOPs. Mechanistic study and theoretical calculations revealed that the electron-transfer between tetracycline (TC) and metastable high-potential complexes BC-PDS* dominated the TC oxidation in HFBC-PDS system. Meanwhile, the oxidation of TC was driven by the singlet oxygen (¹O₂) in PI system, where the surface -OH played a crucial role. The depletion of surface -OH well explained the inhibited TC oxidation in HFBC-PI system though the SSA was increased. Eventually, the established BC-AOPs performed excellent adaptability in complex scenarios and low eco-environmental risks through multi-level toxicity analysis. Overall, this study systematically scrutinized the critical role of ASA in BC-AOPs and enlightened the rational design of biochar for wastewater treatment.