Solid base Mg-doped ZnO for heterogeneous catalytic ozonation of isoniazid: Performance and mechanism

Magnesium-doped ZnO (denoted as x-MgZnO where x represented the molar ratio of Mg to the sum of Mg and Zn) powders synthesized by the traditional thermal decomposition were used as catalysts for ozonation of isoniazid (20 mg/L) at the initial pH of 7.2. Magnesium substituted zinc in wurtzite structure and the Zn-O-Mg bond was formed in Mg-doped ZnO on the basis of the results of X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses. The removal efficiencies of isoniazid were enhanced in Mg-doped ZnO catalytic ozonation processes (57.7% by 0.05-MgZnO and 76.3% by 0.10-MgZnO in 9 min), compared with ozonation alone (50.5%) and ZnO catalytic ozonation (49.5%). The removal efficiencies of total organic carbon (TOC) were also improved in Mg-doped ZnO catalytic ozonation processes. When the initial pH of 7.2 was lower than the pH_PZC (point of zero charge) of Mg-doped ZnO, surface hydroxyl groups of the catalysts were protonated and the solution pH gradually increased during Mg-doped ZnO catalytic ozonation. The increase in the solution pH value mainly induced ozone decomposition into superoxide radical (O₂^-). Furthermore, protonated surface hydroxyl groups (S-OH₂⁺) on Mg-doped ZnO also contributed a little to ozone decomposition. The 0.10-MgZnO powder showed high stability after continuous use in the process. Additionally, we proposed a possible degradation pathway for the oxidation of isoniazid in Mg-doped ZnO catalytic ozonation on the basis of intermediates detected. This work provides an insight into the mechanism for basic sites of solid base in heterogeneous catalytic ozonation.