Mn2O3@Mn5O8 as an efficient catalyst for the degradation of organic contaminants in aqueous media through sulfite activation

Abstract

Less-toxic, cost-effective, stable, and highly efficient catalysts for sodium sulfite (S(IV)) activation are required to degrade organic pollutants from wastewater. Herein, we report the facile thermal synthesis of Mn2O3@Mn5O8 that activates S(IV) more efficiently than other Mn and transition-metal oxides. Mn2O3@Mn5O8 exhibits good performance and long-term stability for eliminating various contaminants from aqueous media, including phenol, bisphenol A, nitrobenzene, 2,4-dichlorophenol, and acetaminophen. Its high performance is attributed to its multivalency, unique architecture, surface hydroxyl groups (–OH), and high surface area. X-ray diffractometry and high-resolution transmission electron microscopy revealed that Mn2O3@Mn5O8 comprises well-combined cubic Mn2O3 and monoclinic Mn5O8 crystalline structures, whereas electron paramagnetic resonance spectroscopy and scavenging tests showed that SO5•−, SO4•−, and •OH radicals are generated during S(IV) activation, with SO3•− as a precursor. The mixed-valence state provides effective and favorable electron transfer via Mn redox cycling (Mn(II) ↔ Mn(III) ↔ Mn(IV)), improving the S(IV) activation performance and catalytic activity. Mn2O3@Mn5O8/S(IV) system shows stable performance in the 3.0–7.0 pH range. Density functional theory calculations confirmed the higher catalytic activity as indicated by high –OH adsorption energy and significant inter-charge transformation. This study provides new insights and strategies for the activation of S(IV) using less-toxic metal oxides as catalysts and broadens the scope of heterogeneous Mn-based catalysts and S(IV) chemistry in real-world applications, particularly for the treatment of wastewater.