New insights of FeS–Cr interaction in sequestration remediation: Model, mechanism, and DFT calculation

Chromium (Cr), especially hexavalent Cr, is a commonly found heavy metal in both water and soil. In this work, we synthesized FeS nanoparticles (NPs) stabilized per carboxymethyl cellulose (CMC) for reductive immobilization of Cr(VI) in both soil/water matrix. A CMC-to-FeS molar ratio of 0.0010 was determined to ensure effective immobilization (96.3%) and high soil mobility. The overall removal kinetic was rapid and the removal capacity reached up to 327.9 mg/g. Reduction of Cr(VI) to Cr(III) (accounting for 91.7%) was the major immobilization mechanism, rather than adsorption (only 4.5%). Fe(II) was the predominant reducing agents, but S species in FeS also served as electron donors for Cr(VI). Moreover, characterization results and theoretical calculations indicated that the Sulfur sites on the FeS surface played a crucial role in adsorbing CrO42− molecules and facilitating the rapid conversion of Cr(VI) to Cr(III) via the electron pathway Fe(II)–S=O–Cr(VI). The removal isotherm curve exhibited a “S” shape at low Ce concentrations and could be adequately fitted by a modified dual mode model (R2 = 0.995). The efficient immobilization of Cr(VI) in water could be achieved in pH range of 5–9. Humic at high concentration can strongly complex with Cr(VI) oxyanions leading to an inhibited removal efficiency. Both soil batch tests and column tests demonstrated the high mobility and reactivity of CMC-FeS with suppressed Cr release. Our study gave new insights on the role of S in FeS–Cr(VI) interactions and theoretical support for nanoparticle injection to immobilize Cr in soil matrix.