Efficient peroxymonosulfate activation by magnesium-doped Co3O4 for thiacloprid degradation: regulation of Co2+/Co3+ ratios and degradation mechanism

Abstract

AS a low-cost and high-performance catalyst, spinel cobalt oxide (Co₃O₄) has two different catalytic active sites (tetrahedral Co²⁺ and octahedral Co³⁺) to drive the activation of peroxymonosulfate (PMS) through Co²⁺/Co³⁺ redox cycle. Tuning Co²⁺/Co³⁺ atomic ratio on the surface of Co₃O₄ for the construction of a synergy in the Co²⁺/Co³⁺ redox cycle might be an effective way to further boost PMS activation performance of Co₃O₄ catalyst. Herein, we suggested a metal-doping strategy to regulate Co²⁺/Co³⁺ atomic ratio of Co₃O₄ by partially substituting Co²⁺ with inert Mg²⁺ and formed a series of Mg doped Co₃O₄ (MCO) catalysts. Structural characterizations and experimental investigations demonstrated that Mg doping did not change Co₃O₄ host lattice and particle morphology, but could manipulate surface Co²⁺/Co³⁺ atomic ratio of Co₃O₄ for an improved PMS activation. The optimal MCO catalysts (MCO-0.2) with the suitable Co²⁺/Co³⁺ atomic ratios (1.13) exhibited the excellent thiacloprid (THIA) degradation performance through PMS activation, and the apparent degradation rate constant (0.2835 min⁻¹) was highly outperformed that of pure Co₃O₄ (0.09555 min⁻¹) and other similar cobalt-based catalysts. The optimal THIA degradation conditions might be: catalyst dose 100 mg L⁻¹, PMS concentration 0.8 mM, pH 7 and THIA concentration 20 mg L⁻¹. Quenching experiments and electron paramagnetic resonance (EPR) characterizations suggested SO₄˙⁻, HO˙ and ¹O₂ 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 SO₄˙⁻ and HO˙ were generated at lower steady-state concentrations than that of ¹O₂, 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.