Looking deeper: Decoding the core structure of a micron-sized S-ZVI particle

Abstract

• The diffusion of sulfur to the core of the micron ZVI was confirmed for the first time. • The inner core composition of the micron ZVI was confirmed and quantified. • XRPD and XANES analysis pointed out that about 12.5% iron was transformed to FeS x . Although it has been widely accepted that the sulfidation of zero-valent iron (ZVI) represents a typical diffusionless process, which mainly happens on the surface of the ZVI particle and results in a FeS x @Fe core-shell structure, such an assumption has never been experimentally proved due to the difficulties in probing the interior domains of the large particles, especially those in micro size. In this work, we demonstrate that the sulfidation process not only takes place on the surface but also synchronizes throughout the entire body of the micron-sized ZVI, which was confirmed by the close inspection of the cross-section of a finely sliced ZVI particle. Coupled with the X-ray powder diffraction and X-ray absorption near-edge structure analysis, it is revealed that the whole S-ZVI particle consists of a mixed lattice of the dominant metallic iron (87.5%) and minor iron sulfides (12.5%), according to the established averaged oxidation state for iron. This work is the first to unambiguously reveal the compositional information of the micron-sized S-ZVI particles, especially the inner components that are typically difficult to reach. It clarified a long-time misunderstanding that the sulfidation process on ZVI is initiated from the surface and continuously undergoes a diffusion pathway, reaching to the center of the particle.