Peroxymonosulfate Activation by Rice-Husk-Derived Biochar (RBC) for the Degradation of Sulfamethoxazole: The Key Role of Hydroxyl Groups

In this work, rice-husk-derived biochar (RBC) was synthesized by using simple one-step pyrolysis strategies and served as catalysts to activate peroxymonosulfate (PMS) for degrading sulfamethoxazole (SMX). When the annealing temperature (T) = 800 °C, RBC800 exhibits the typical hardwood structure with several micropores and mesoporous. Furthermore, RBC800 obtains more defect sites than RBC600, RBC700, and RBC900. In the RBC800/PMS system, the removal rate of the SMX reached 92.0% under optimal conditions. The kinetic reaction rate constant (kobs) of the RBC800/PMS system was 0.009 min−1, which was about 1.50, 1.28, and 4.50 times that of the RBC600/PMS (kobs = 0.006 min−1), RBC700/PMS (kobs = 0.007 min−1), and RBC900/PMS (kobs = 0.002 min−1) systems, respectively. In the RBC800/PMS system, sulfate radical (SO4•−) is the main active species. Compared with other active sites, the hydroxyl group (C-OH) on the surface of RBC800 interacts more strongly with PMS, which is more likely to promote the stretching of the O-O bond of the PMS, thus breaking into the activated state and significantly reducing the activation energy required for reaction. The degradation intermediates of SMX were speculated, and the toxicity analysis was conducted. Generally, this work reveals in depth the interaction between reactive sites of biochar-based catalysts and PMS at the molecular level.