Enhanced production of Fe(II) and Fe(0) by elemental sulfur modification on iron oxide to boost peroxymonosulfate activation for pollutants removal

Sulfidation treatment has been evidenced as an effective strategy in enhancing the catalytic ability of iron-based oxides. However, the proportion of highly reactive Fe(II) species remains to be low due to the use of weak reductive sulfur source. In this regard, this research found that the strongly reductive elemental sulfur treatment not only dramatically improved the content of Fe(II) in magnetic Fe3O4 (40.1–60.3 %), but also induced the generation of zero valent iron Fe(0) (16.95 %). Correspondingly, the SFe-2 exhibited the excellent performance in degrading biphenol A (BPA) with a removal rate of 98% within 30 min via peroxymonosulfate activation, far outperforming the other iron oxides-activating PMS system modified by Na2S, Na2SO3, and Na2SO4. This enhanced catalytic behavior was attributed to the generation of FeSO4 and FeS2 after sulfuration. The catalytic cycle of Fe(0) → Fe(II) ⇄ Fe(III) contributed to PMS activation, while FeS2 as a semiconductor enhanced the electron transfer, as a result quickening the reduction of Fe(III) to Fe(II). Both SO4−, OH, O2−, 1O2, and FeIVO2+ were involved. In addition, the validity of K2Cr2O7 in evaluating electron-transfer regime, the applicability of methyl phenyl sulfoxide (PMSO) in assessing the Fe(IV)-oxo complex with 1O2-engaged system, and the homogeneous/heterogeneous catalysis were both investigated. This research highlights the feasibility of improving Fe(II) proportion and catalytic capacity of iron-based oxides via simple sulfur-vapor modification strategy.