Efficient peroxymonosulfate activation by magnesium-doped CoO for thiacloprid degradation: regulation of Co/Co ratios and degradation mechanism.

AS a low-cost and high-performance catalyst, spinel cobalt oxide (CoO) has two different catalytic active sites (tetrahedral Co and octahedral Co) to drive the activation of peroxymonosulfate (PMS) through Co/Co redox cycle. Tuning Co/Co atomic ratio on the surface of CoO for the construction of a synergy in the Co/Co redox cycle might be an effective way to further boost PMS activation performance of CoO catalyst. Herein, we suggested a metal-doping strategy to regulate Co/Co atomic ratio of CoO by partially substituting Co with inert Mg and formed a series of Mg doped CoO (MCO) catalysts. Structural characterizations and experimental investigations demonstrated that Mg doping did not change CoO host lattice and particle morphology, but could manipulate surface Co/Co atomic ratio of CoO for an improved PMS activation. The optimal MCO catalysts (MCO-0.2) with the suitable Co/Co atomic ratios (1.13) exhibited the excellent thiacloprid (THIA) degradation performance through PMS activation, and the apparent degradation rate constant (0.2835 min) was highly outperformed that of pure CoO (0.09555 min) and other similar cobalt-based catalysts. The optimal THIA degradation conditions might be: catalyst dose 100 mg L, PMS concentration 0.8 mM, pH 7 and THIA concentration 20 mg L. Quenching experiments and electron paramagnetic resonance (EPR) characterizations suggested SO˙, HO˙ and O were all involved in THIA degradation during the MCO-0.2/PMS process. Furthermore, the steady-state concentrations of these reactive species and their relative contributions to THIA degradation were also calculated by combining a kinetic model and a series of probe compound-based experiments. The results indicated that SO˙ and HO˙ were generated at lower steady-state concentrations than that of O, but they dominated THIA abatement during the MCO-0.2/PMS process. This study presented new insights into the construction of efficient PMS activator and a mechanistic understanding for PMS-mediated reaction.