Chunhua Xu

The crystallinity of the iron sulfide (FeS_x) 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 FeS_x crystallinity, limiting interfacial electron transfer. Therefore, developing regulatory strategies to promote the ordered growth of FeS_x 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 FeS_x. 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 FeS_x crystallization and provides a generalizable strategy for designing high-performance ZVI-based materials.

The environmental remediation performance of zero-valent iron (ZVI) fundamentally relies on physicochemical processes at its solid-liquid interface. Although phosphorus modification is widely employed to regulate the interfacial behavior of ZVI, the evolution of iron-phosphorus interfacial species and the impact on the electronic structure and reactivity remain insufficiently understood. This work systematically investigated the evolution of the ZVI interfacial structure and behavior by controlling the phosphate supply. Phosphorus species are predominantly monodispersed on the ZVI surface via inner-sphere coordination under a moderate phosphate supply, forming an amorphous structure characterized by Fe-O-P. Dynamic reconstruction of the Fe-O-P microstructure upshifted the d-band center of ZVI, thereby enhancing electron transport and boosting the apparent rate constant for Cr(VI) removal by up to 158-fold. The interfacial reaction gradually shifted from surface-restricted coordination to structural reconstruction involving near-surface iron species as the phosphate supply increased. Consequently, the Fe-O-P coordination units undergo spatial expansion and structural rearrangement within the interfacial region, ultimately forming crystalline Fe₃(PO₄)₂, which severely suppresses electron migration and reactivity. This work reveals a systematic volcano-type structure-activity relationship in which microstructurally evolving phosphate species regulate the electronic structure of ZVI, providing a critical theoretical foundation for the rational design of next-generation iron-based materials.