Powdered carboxymethyl chitosan-engineered nZVI nanocomposites for rapid and ultrahigh-capacity Cr(VI) removal

The aggregation and surface passivation of nanoscale zero-valent iron (nZVI) significantly limit its practical efficiency in aqueous heavy metal remediation. Herein, we report the synthesis of carboxymethyl chitosan-engineered nZVI (ChMC-nZVI) nanocomposites in a free powder form via an in situ borohydride reduction method to enhance particle stability, surface accessibility, and Cr(VI) removal performance. Structural and interfacial characterization studies (XRD, TEM, SEM-EDS, and FTIR) confirm the formation of a core-shell Fe⁰ structure uniformly coated with ChMC through carboxylate-mediated coordination, which suppresses agglomeration and mitigates surface oxidation. The optimized nanocomposite (15 wt% ChMC) achieved complete removal of 50 mg L^-1 Cr(VI) at pH 3 within 180 min using a low adsorbent dosage (0.16 g L^-1). A maximum experimental adsorption capacity of 439.55 mg g^-1 was obtained at higher initial concentration, while Langmuir modeling yielded a maximum monolayer capacity of 424.46 mg g^-1, indicating homogeneous surface adsorption. Kinetic analysis revealed rapid uptake, with ∼40% removal within the first 5 min, and excellent agreement with the pseudo-second-order model (R² > 0.99), suggesting a chemisorption-dominated redox mechanism. The enhanced performance is attributed to the synergistic effects of steric stabilization, improved mass transfer in powder form, and facilitated electron transfer between Fe⁰/Fe²⁺ and Cr(VI). This study demonstrates that polymer-engineered nZVI nanocomposites offer a highly efficient and structurally robust platform for advanced water remediation applications.