Peroxymonosulfate (PMS) activation by mackinawite for the degradation of organic pollutants: Underappreciated role of dissolved sulfur derivatives

Abstract

The internal Fe²⁺/Fe³⁺ cycle is important for peroxymonosulfate (PMS) activation by iron-based materials to produce the reactive oxidative species (ROS) for the breakdown of organic contaminants. Previous studies have focused on the contribution of heterogeneous sulfur species to the Fe²⁺/Fe³⁺ cycle such as lattice S(-II) and surface SO₃^2- of iron sulfides. In this study, we found that the dissolved S(-II) from mackinawite (FeS) had a substantial contribution to the Fe²⁺/Fe³⁺ cycle. Furthermore, the oxidation intermediates of the dissolved S(-II) such as S₂O₃^2- and SO₃^2- ions could convert Fe³⁺ to Fe²⁺ in solution. The elimination of target organic pollutant bisphenol A (BPA) derived from PMS activation triggered by the dissolved Fe²⁺ might be enhanced by the equivalent dissolved S(-II) in the FeS/PMS system. These results revealed that previous studies underestimated the significance of PMS activation by dissolved Fe²⁺ of iron sulfides to organic pollutant degradation. Moreover, SO₄^•- and •OH were more likely to be the main ROS for BPA degradation in the FeS/PMS system compared with FeO²⁺. Considering that the metal sulfides have been widely used to activate PMS, H₂O₂ and peroxydisulfate, this study offers a new perspective on the function of sulfur in these advanced oxidation processes.

Keywords: Bisphenol A; Dissolved S(-II); Fe(2+)/Fe(3+) cycle; Mackinawite; Peroxymonosulfate.