Mechanism for enhanced degradation of clofibric acid in aqueous by catalytic ozonation over MnO_x/SBA-15

Highlights: • Clofibric acid (CA) is efficiently mineralized by O₃/MnO_x/SBA-15. • Adsorption of CA and its intermediates on MnO_x/SBA-15 is proved unimportant. • Initiation of hydroxyl radicals (·OH) is enhanced in O₃/MnO_x/SBA-15. • Uniformly distributed MnO_x accounts for the high activity of MnO_x/SBA-15. • Degradation routes of CA in ozonation alone and catalytic ozonation are proposed. - Abstract: Comparative experiments were conducted to investigate the catalytic ability of MnO_x/SBA-15 for the ozonation of clofibric acid (CA) and its reaction mechanism. Compared with ozonation alone, the degradation of CA was barely enhanced, while the removal of TOC was significantly improved by catalytic ozonation (O₃/MnO_x/SBA-15). Adsorption of CA and its intermediates by MnO_x/SBA-15 was proved unimportant in O₃/MnO_x/SBA-15 due to the insignificant adsorption of CA and little TOC variation after ceasing ozone in stopped-flow experiment. The more remarkably inhibition effect of sodium bisulfite (NaHSO₃) on the removal of TOC in catalytic ozonation than in ozonation alone elucidated that MnO_x/SBA-15 facilitated the generation of hydroxyl radicals (·OH), which was further verified by electron spin-resonance spectroscopy (ESR). Highly dispersed MnO_x on SBA-15 were believed to be the main active component in MnO_x/SBA-15. Some intermediates were indentified and different degradation routes of CA were proposed in both ozonation alone and catalytic ozonation. The amounts of small molecular carboxylic acids (i.e., formic acid (FA), acetic acid (AA) and oxalic acid (OA)) generated in catalytic ozonation were lower than in ozonation alone, resulting from the generation of more ·OH