Efficient removal of Cr(VI) using partially oxidized FeS under visible light

In this study, the removal of Cr(VI) using commercial FeS with the partially oxidized surface was investigated under visible light. The results demonstrated that the Cr(VI) removal using commercial FeS could be enhanced by 1.3 times in the presence of visible light. More importantly, the removed Cr(VI) was almost completely reduced to less toxic Cr(III), and most of the reduced Cr(III) was immobilized on the commercial FeS surface. In order to elucidate the plausible mechanism of the removal of Cr(VI) using partially oxidized FeS under visible-light irradiation, the role of electrons (e − ) and holes (h + ), the influence of dissolved oxygen, and the variation of the commercial FeS surface were carefully examined. The results revealed that the dissolved Fe(II)/S(-II), surface-bound Fe(II)/S(-II), and photogenerated electrons were directly responsible for the reduction of Cr(VI) to Cr(III). Additionally, the ·O 2 − derived from the dissolved oxygen activation by the photo-generated electrons also contributed to the reduction of Cr(VI). During the Cr(VI) removal process, due to the release of strong hydrophilic SO 4 2− ions from the partially oxidized FeS surface, the continuously exposed surface Fe(II) and S(-II) maintained the Cr(VI) reduction. Besides, the photo-generated electrons and ·O 2 − favored the regeneration of surface Fe(II), which helped sustain the reduction of Cr(VI) by the surface Fe(II). These findings clarify the transformation process of commercial FeS during the Cr(VI) removal under visible-light irradiation and help us to further understand the mechanism of Cr(VI) removal using partially oxidized FeS with a multipronged approach. This study also offers an alternative promising commercial material for Cr(VI)-bearing wastewater treatment.