Selective production of singlet oxygen from zinc-etching hierarchically porous biochar for sulfamethoxazole degradation.

Porous carbons are appealing low-cost and metal-free catalysts in persulfate-based advanced oxidation processes. In this study, a family of porous biochar catalysts (ZnBC) with different porous structures and surface functionalities are synthesized using a chemical activation agent (ZnCl). The functional biochars are used to activate persulfate for sulfamethoxazole (SMX) degradation. ZnBC-3 with the highest content of ketonic group (CO, 1.25 at%) exhibits the best oxidation efficiency, attaining a rate constant (k) of 0.025 min. The correlation coefficient of the density of CO to k (R = 0.992) is much higher than the linearity of the organic adsorption capacity to k (R = 0.694), implying that CO is the intrinsic active site for persulfate activation. Moreover, the volume of mesopore (R = 0.987), and Zeta potential (R = 0.976) are also positive factors in PS adsorption and catalysis. In the mechanistic study, we identified that singlet oxygen is the primary reactive oxygen species. It can attack the -NH group aligned to the benzene ring to form dimer products which could be adsorbed on the biochar surface to reach complete removal of the SMX. The optimal pH range is 4-6 which will minimize the electrostatic repulsion between ZnBCs and the reactants. The SMX degradation in ZnBC/PS system was immune to inorganic anions but would compete with organic impurities in the real wastewater. Finally, the biochar catalysts are filled in hydrogel beads and packed in a flow-through packed-bed column. The continuous system yields a high removal efficiency of over 86% for 8 h without decline, this work provided a simple biochar-based persulfate catalyst for complete antibiotics removal in salty conditions.