Surface Modulation and Chromium Complexation: All-in-One Solution for the Cr(VI) Sequestration with Bifunctional Molecules

The sulfidation of zero valent iron (ZVI) to an Fe@FeSx (S-ZVI) composite has been intensively explored in the ZVI field. Yet, further benefits from the FeSx coating layer are seldom realized, especially those effectively using its intrinsic physical and chemical properties for elaborate design. Here, we demonstrate that in a traditional Cr(VI) sequestration reaction, the FeSx layer displays a great utility in immobilizing molecules containing hydroxyl groups (-OH) and hence, attracting Cr(VI) complexes chelated with carboxyl organics (RCOOH). Such intermolecular attraction readily promotes the diffusion of the Cr(VI) complexes to the S-ZVI surface, affording a higher reaction rate for the Cr(VI) sequestration process. In addition, the above mechanism was used to guide a rational selection of molecules incorporating both hydroxyl and carboxyl functional groups with a proper ratio and thereby, a significantly improved reaction efficiency was achieved. Furthermore, the FeSx phase was revealed to be consumed in the reaction, acting as a supplementary reductant. This work is the first to unveil the relationship between molecules with specific functionalization and the FeSx phase, providing a general rule in choosing appropriate reaction media for Cr(VI) sequestration and related reactions.