Ultrafast and high-capacity Cr(VI) removal by a nanorod-like chitosan-iron composite

The persistent presence of hexavalent chromium (Cr(VI)) in aquatic environments necessitates the development of efficient, reusable, and sustainable adsorbents. In this study, a novel chitosan (CS)-based nanocomposite, CS@Fe-X@restruct (X = 1-5, representing the mass of FeCl₂·4H₂O in grams used in synthesis), was prepared via sodium borohydride (NaBH₄)- assisted surface restructuring of Fe-loaded CS for Cr(VI) removal, which created abundant edge sites with rough surface, significantly enhance reactivity and accessibility of active sites. A series of batch adsorption experiments, combined with structural and surface analyses, were conducted to evaluate performance and elucidate the removal mechanism. Among CS@Fe-X@restruct, CS@Fe-4@restruct exhibited rapid and high-capacity Cr(VI) adsorption, with 92.4 % removal achieved in 10 min and a maximum uptake of 151.5 mg/g. Kinetic, isothermal, and thermodynamic studies revealed a spontaneous, endothermic process involving both chemisorption and physisorption, with a temperature-dependent transition from monolayer to multilayer adsorption behavior. Mechanistically, Cr(VI) removal was driven by a synergistic interplay of Fe(II)-mediated reduction, surface complexation, co-precipitation, and electrostatic attraction. In addition, CS@Fe-4@restruct exhibited excellent stability and reusability, and demonstrated resilience against common anions such as Cl^- and NO₃^-. This work presents a rational strategy for engineering high-performance bio-based adsorbents, offering promising prospects for heavy metal remediation in water treatment.