Sustained-Release Reducing Microenvironment Tailors Highly Crystalline FeSx Shells on Zero-Valent Iron for Enhanced Electron Transfer.

The crystallinity of the iron sulfide (FeSx) layer critically determines the interfacial electron transfer and reactivity of sulfidated zero-valent iron (SZVI). However, in heterogeneous synthesis systems, the native oxide layer on ZVI and inefficient sulfur utilization lead to poor FeSx crystallinity, limiting interfacial electron transfer. Therefore, developing regulatory strategies to promote the ordered growth of FeSx is crucial for enhancing the reactivity and environmental adaptability of the SZVI. Herein, we develop a surface-engineered strategy in which a sustained-release agent, L-ascorbic acid 2-phosphate sesquimagnesium salt hydrate (AAPS), in situ generates ascorbic acid (AA) to construct a reductive microenvironment. This microenvironment suppresses H2S generation (91.2% sulfur utilization) and removes surface passivation layers, synergistically guiding the formation of high crystallinity FeSx. AA engineering lowers the work function and enhances electron transfer, endowing AAPS-modified SZVI (S-LZVI) with a Cr(VI) removal rate constant 2.44-fold higher than that of SZVI, along with superior stability under high salinity, wide pH ranges, and continuous-flow conditions. Furthermore, the efficacy of AAPS is closely related to the sulfur precursor, with the greatest enhancement in systems suffering from sulfide loss and passivation. This study highlights the critical role of interfacial microenvironment regulation in controlling FeSx crystallization and provides a generalizable strategy for designing high-performance ZVI-based materials.