Dominant pathways of FeSx formation and evolution during sulfidation of zero-valent iron by different sulfur agents

Sulfidation of zero-valent iron (SZVI) is an effective method to improve the properties of ZVI for decontamination. Among various sulfidation methods, sulfur agent and sulfidation parameters play decisive roles in the sulfidation reactions. A correct understanding of the reactions to form FeSx is helpful to regulate the sulfidation. In this study, Na2S, Na2S2O3 and Na2S2O4 were used as sulfur agents to reveal the dominant pathways of FeSx formation. Results demonstrate that FeSx tends to form via the reaction of S2- and Fe2+ (S2- pathway) when Na2S and Na2S2O3 are used as sulfur agents, whereas it tends to occur via the reaction of S0 and Fe0 (S0 pathway) when Na2S2O4 is used. During the sulfidation, FeSx gradually grows and covers the surface, and then the FeSx layer flaks off locally. The sulfur agents can also affect the FeSx composition, that is, Na2S and Na2S2O3 tend to form FeS, while Na2S2O4 tends to form FeS2. Such FeSx composition can further affect the performance of SZVI to Cr(VI) removal due to their different conductivity. The study of the influence on SZVI properties with different sulfur agents is helpful to select sulfur agents for targeting design of SZVI.