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

A possible overall sulfite activation mechanism on Mn 2 O 3 @Mn 5 O 8 catalyst for organic contaminant degradation. [Display omitted] • Mn 2 O 3 @Mn 5 O 8 shows better catalytic performance than other S(IV) activators. • S(IV) is activated by Mn 2 O 3 @Mn 5 O 8 , generating SO 3 •−, SO 5 •−, SO 4 •−, and •OH radicals. • Experiments show phenol is mainly removed by •OH, and partially by SO 4 •− and SO 5 •−. • At an initial solution pH of 3.0–7.0, stable degradation of phenol is achieved. • Low toxicity and high S(IV) activation ability of Mn 2 O 3 @Mn 5 O 8 affords a green method. 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 Mn 2 O 3 @Mn 5 O 8 that activates S(IV) more efficiently than other Mn and transition-metal oxides. Mn 2 O 3 @Mn 5 O 8 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 Mn 2 O 3 @Mn 5 O 8 comprises well-combined cubic Mn 2 O 3 and monoclinic Mn 5 O 8 crystalline structures, whereas electron paramagnetic resonance spectroscopy and scavenging tests showed that SO 5 •−, SO 4 •−, and •OH radicals are generated during S(IV) activation, with SO 3 •− 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. Mn 2 O 3 @Mn 5 O 8 /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. [ABSTRACT FROM AUTHOR]