Key role of sulfur in sulfidated zerovalent iron during persulfate activation for the dynamic equilibrium of oxidative radicals including SO and OH.

Surface sulfidation has been widely investigated to effectively enhance the utilization and selectivity of iron electrons for enhanced pollutant reduction. However, there is relatively less knowledge on whether sulfidation facilitates the catalytic oxidation process and the mechanism of enhancement. Therefore, in this study, the role of surface sulfidation in modulating the oxidant decomposition pathway and reactive oxygen species generation was investigated with the sulfidated zerovalent iron (S-ZVI) activated persulfate (PS) system. The results revealed that sulfur on the surface of S-ZVI not only facilitates PS activation to generate more SO, but also acts as an essential in the dynamic equilibrium between SO and OH. Specifically, the S-ZVI surface sulfide first forms sulfur monomers during catalysis, which promotes electron transfer to accelerate Fe to Fe cycling, prompting the generation of more SO also generates SO. Then, SO is further reacted with OH to generate the [O-O-SO] intermediate of SO, which leads to a dynamic equilibrium of SO and OH, mitigating the further conversion of SO to OH. These findings unveiled the dynamic variation of sulfur on the surface of S-ZVI during PS activation, elevating new insights for the sulfate radical-based efficient degradation.