Biogenic Fe₃O₄@eggshell Nanocomposite: Synthesis, Physicochemical Properties, and Cr(VI) Removal in Aqueous Media

A magnetite-eggshell nanocomposite (Fe₃O₄@eggshell NC) was synthesized by green coprecipitation and evaluated for Cr(VI) removal from water. Passiflora ligularis peel extract, evaluated using a 2³ factorial design, served as the biogenic medium, while eggshell waste acted as the support. Characterization included X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning and transmission electron microscopy (SEM and TEM), Brunauer-Emmett-Teller (BET) analysis, zeta potential measurements, thermogravimetric analysis (TGA), vibrating sample magnetometry (VSM), and Mössbauer spectroscopy. The NC preserved Fe₃O₄ and CaCO₃ crystalline phases, contained nanoparticles averaging 18 nm, and exhibited oxygenated surface functionalities, a BET surface area of 134.93 m²/g, amphoteric behavior, and a predominantly superparamagnetic response suitable for magnetic recovery. Under conditions of pH 4, 180 rpm, 60 min, 0.15 g NC, and 50 mg/L Cr(VI), removal reached 75.98%. Kinetic data followed the pseudo-first-order model, whereas equilibrium data were well described by the Sips isotherm, with an estimated capacity of 50.81 mg/g. At the investigated initial concentration of 50 mg/L, adsorption was exothermic and favored at temperatures up to 308 K, and the material retained moderate reusability during the first three alkaline-regeneration cycles. Overall, Fe₃O₄@eggshell NC is a waste-derived, magnetically recoverable adsorbent with favorable Cr(VI) uptake under moderately acidic conditions.