Constructing amorphous-crystalline FeS via coordination regulation of ethylenediamine for efficient Cr(VI) removal from groundwater.

The low reactivity of crystalline ferrous sulfide (FeS) limits its application for hexavalent chromium (Cr(VI)) remediation in groundwater. Herein, we synthesize low-crystallinity FeS via coordination regulation using ethylenediamine (EDA), N,N'-dimethylethylenediamine (DMEDA), ethylenediaminetetraacetic acid (EDTA), and citric acid (CA). Among these, EDA-FeS exhibits the highest Cr(VI) removal efficiency (26.55 times that of unmodified FeS) with an optimal EDA-to-Fe(II) molar ratio of 2:1 and a maximum removal capacity of 159.04 mg/g. The material demonstrates robust environmental applicability, maintaining excellent performance across a wide pH range and in the presence of coexisting ions and humic acid. Integrated characterization and density functional theory (DFT) calculations suggest that EDA overcomes steric hindrance, competes with HS- for coordination sites on Fe(II), and occupies the active growth sites on the FeS crystal surface through strong chemical adsorption via d-p orbital hybridization, which directs the formation of an amorphous and crystalline combined architecture and yields a loose, porous morphology with a specific surface area of 50 m2/g, which is 72 times that of unmodified FeS, facilitating Cr(VI) penetration to internal active sites. Subsequently, the adsorbed Cr(VI) is reduced and precipitated through synergistic electron donation from highly reactive Fe(II) and S(II)/S(I) species. This coordination regulation strategy offers mechanistic insights for designing high-performance mineral materials for groundwater remediation.