Hexavalent chromium removal from aqueous systems using magnetic graphene oxide NiFe₂O₄ nanocomposites

Hexavalent chromium (Cr(VI)) contamination poses persistent risks to aquatic ecosystems and human health, requiring the development of adsorbents that combine rapid uptake, high capacity, and easy magnetic recovery. The aim of this study was to synthesize a novel graphene oxide/nickel ferrite nanocomposite (GO/NiFe₂O₄) utilizing a hydrothermal route to integrate the surface functionality of GO with ferrimagnetic properties of NiFe₂O₄ for efficient Cr(VI) removal. The hybrid material was characterized by SEM-EDS, XRD, FT-IR, Raman spectroscopy, and vibrating sample magnetometry, confirming ferrimagnetic behavior at room temperature and retention of GO's oxygenated functional groups that serve as high-affinity binding sites. Batch adsorption experiments (40-300 mg/L) in an acidic medium revealed rapid kinetics and short equilibrium times. Nonlinear modeling identified the Elovich model as the best fit, indicating heterogeneous surface energies and multistage mass transfer. Equilibrium data (20-40 °C) fitted the Sips isotherm, confirming heterogeneous multilayer adsorption. Thermodynamic parameters (ΔG⁰ < 0; ΔH⁰ < 0) indicated a spontaneous and exothermic process. Regeneration tests using HCl demonstrated efficient desorption and reusability. The proposed mechanism involves electrostatic attraction of Cr(VI) oxyanions, interfacial reduction to Cr(III), coordination to ferrite and oxygenated sites, and post-reduction cation-π stabilization acting synergistically. These results demonstrate the novelty of this study-the first systematic integration of Elovich and Sips modeling to correlate kinetic and equilibrium behaviors in a GO/NiFe₂O₄ system-providing mechanistic insight and confirming the material's high performance and reusability for scalable Cr(VI) remediation.