Encapsulating carboxymethyl cellulose stabilized nanoscale ferrous sulfide with layered magnesium hydroxide shell for controlled reactivity release and long-term sequestration of Cr(Ⅵ)

Nanoparticulate iron sulfide (nano-FeS) is a highly effective and environmentally-friendly reducing agent for decontamination. However, challenges such as aggregation, poor mobility, and oxidative aging limit its application of in situ groundwater and soil remediation. Herein, we present a novel approach to enhance the feasibility and effectiveness of nano-FeS for in situ remediation by encapsulating carboxymethyl cellulose stabilized FeS nanoparticles with a layered magnesium hydroxide shell (referred to as CMC-FeS@Mg(OH)2). The novel CMC-FeS@Mg(OH)2 nanoparticles, with a suitable coating ratio (10 %–50 %), demonstrate considerably increased colloidal stability and enhanced mobility in porous media. This is mainly due to steric hindrance, and decreased attachment, blocking, and straining effects. The Mg(OH)2 shell obviously protects the reactivity of FeS nanoparticles and enhances their resistance to oxidation, ensuring their longevity and long-term decontamination effectiveness during practical field applications. The preserved reactivity of FeS core can be progressively released and fully recovered by modulating the Mg(OH)2 coating ration and the extent of dissolution. After the complete dissolution of the Mg(OH)2 shell, CMC-FeS@50 % Mg(OH)2 exhibits enhanced efficiency in Cr(VI) sequestration, increasing from 58 % to 92 % compared to bare FeS. The removal of Cr(VI) by CMC-FeS@Mg(OH)2 is a synergistic process, and the main mechanisms includes adsorption, reduction, ion exchange, and subsequent precipitation. These results demonstrate a promising and effective method to improve the mobility and longevity of FeS and enable the controlled reactivity release for in situ remediation of subsurface contamination.