Surfactant-free porous nano-Mn3O4 as a recyclable Fenton-like reagent that can rapidly scavenge phenolics without H2O2

Abstract

Current methods that utilize Fenton-like reagents to remove persistent organic pollutants suffer from several limitations, i.e., the reagents may release metal ions into the sludge and cause secondary pollution, the reagents are easily inactivated after use, and the use of additives (e.g., 2KHSO₅·KHSO₄·K₂SO₄, Na₂S₂O₈ and H₂O₂) can be costly but cannot be avoided. In this work, through a one-pot reaction, we prepared surfactant-free porous nano-Mn₃O₄ as an efficient Fenton-like reagent. Without the addition of external H₂O₂, it could remove 86.8% and 96.9% of the phenol from an aqueous solution within 1 and 5 min, respectively, and thus outperformed many other reported reagents. The underlying mechanism for the removal of organic pollutants by this Mn₃O₄ reagent was examined. It was found that the low-valence Mn in Mn₃O₄ reacted with the dissolved O₂ to generate H₂O₂, and the H₂O₂ in turn created hydroxyl radicals (HO˙) to degrade organic pollutants. The Mn₃O₄ was also annealed to generate porous nano-Mn₂O₃, and a comparison of the performance of Mn₃O₄ and Mn₂O₃ in removing organic pollutants revealed that low-valence Mn in the oxides played a critical role in scavenging the pollutants. Note that this novel nano-Mn₃O₄ could remove pollutants without the addition of external H₂O₂, and it could be easily reactivated after use through reduction by NaBH₄ and thus be recycled. The Mn₃O₄ can also be successfully applied to complex polluted water containing a mixture of phenolics. These findings pave the way for developing other efficient and environmentally friendly Fenton-like reagents for removing organic pollutants in wastewater.