Chitosan-diatom hybrid beads embedding zero-valent iron nanoparticles as a bio-based adsorbent for Cr(VI) removal:...

Hexavalent chromium [Cr(VI)] is a highly toxic and persistent water pollutant, generating a vital need for efficient and sustainable removal strategies. To address this challenge, this study aims to develop a novel bio-based hybrid composite with multiple active sites to effectively remove Cr(VI) from aqueous solutions. Herein, hybrid chitosan (CS) and separated diatom frustule (SDF) beads functionalized with nanoscale zero-valent iron (nZVI) were developed as an eco-friendly adsorbent for Cr(VI) removal in aqueous media (pH 2.0, 25-55 °C). The resulting nZVI@CS/SDF biocomposite was systematically characterized by XRD, TGA, DSC, FTIR, SEM-EDX, TEM, and zeta potential analyses to elucidate its structural, thermal, and surface properties. By combining the high surface area of SDF, the abundant functional groups of CS, and the strong reactivity of nZVI, the as-synthesized nZVI@CS/SDF can achieve a high adsorption efficiency. This interaction resulted in enhancing the stability of the tested composite and reducing secondary pollution risks commonly associated with free nanoparticles. Batch adsorption studies using classical equilibrium models indicated that the process followed the Langmuir isotherm, with the maximum adsorption capacity increasing from 163.32 to 200.44 mg/g as the temperature was elevated from 25 °C to 55 °C. Advanced statistical monolayer adsorption modeling provided molecular-level insights, including the number of Cr(VI) ions accommodated per functional group, the density of nZVI@CS/SDF active sites, and the corresponding adsorption energies. The results revealed a multi-docking mechanism, where both Cr(VI) and its reduced form Cr(III) interacted with the functionalized composite surface. Particularly, the active site density increased substantially with temperature (159.42-447.53 mg/g), indicating the generation of new binding sites and an enhancement in adsorption capacity Adsorption energies ranging from 24.44 kJ/mol (25 °C) to 25.78 kJ/mol (55 °C) indicated that electrostatic attraction, surface complexation, and pore-filling were the predominant mechanisms governing Cr(VI) uptake. Thermodynamic analysis demonstrated that the Cr(VI) binding was spontaneous and endothermic, while regeneration tests confirmed the stability and reusability of the composite. Overall, because of its low cost, high adsorption capacity, remarkable regeneration ability, and high stability, the synthesized nZVI@CS/SDF composite offers a promising and environmentally friendly approach for industrial wastewater purification, especially for the elimination of harmful heavy metal ions.