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 (O-center dot(2)-). 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. (C) 2019 Elsevier B.V. All rights reserved.