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

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 SO32- 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 S2O32- and SO32- 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.