Sequestration of Cr (III) and Cd (II) ions from aqueous matrices using an Fe3O4@CNF nanocomposite sorbent: sorption insights and mechanisms

In this study, novel magnetic sorbent, Fe3O4@CNF, was developed for the magnetic solid-phase extraction (MSPE) of Cr3+ and Cd2+ from aqueous and real wastewater samples. The adsorbent was produced using a chemical precipitation process and functionalized with cellulose nanofibers (CNFs) derived from Sargassum wrightii. Comprehensive physicochemical evaluation, which included FTIR, XRD, FESEM–EDS, BET analysis, zeta potential, and vibrating sample magnetometry (VSM), confirmed the material’s successful manufacturing and structural integrity. The saturation magnetization (Ms) values of Fe3O4 NPs and Fe3O4@CNF were 0.519 and 0.234 emu/g, indicating effective magnetic responsiveness. The effects of adsorption time, pH, temperature, starting concentrations, and interfering ions on metal uptake were thoroughly investigated. Results from batch adsorption tests indicate qmax capacities of 19.55 mg/g for Cr³+ and 45.32 mg/g for Cd2+, with Cd2+ over 90% efficiency. Both metals exhibited adsorption behavior that fitted the Redlich–Peterson and the Freundlich isotherms (R2 = 0.96) and a pseudo-second-order kinetic model (R2 = 0.99), confirming monolayer chemisorption. Using MSPE procedure, the limits of detection (LODs) for Cr³+ and Cd2+ were 0.641 and 0.071 mg/L, while the limits of quantification (LOQs) were 2.14 and 0.237 mg/L, respectively. Inter-day and intra-day precision demonstrated Cr³+ %RSD of 3.34 and Cd2+ of 1.92. Removal efficiency in wastewater samples was 69.9% for Cr³+ and 67.41% for Cd2+ (starting concentration: 0.70 mg/L for Cr3+, 0.80 mg/L for Cd2+). The sorbent exhibited considerable reusability, retaining 22.8% (Cd2+) and 19.3% (Cr³+) efficiency after four cycles. The method’s AGREE (0.63) and ComplexGAPI (81) scores showed its environmental compatibility and cost-effectiveness.