Dye removal by adsorption using Fe₃O₄ and ε-Fe₂O₃-based kaolinite nanocomposites synthesized with an apricot kernels shell extract

In this study, Fe₃O₄/kaolinite and ε-Fe₂O₃/kaolinite nanocomposites were synthesized using apricot kernel extract as a green reductant. The prepared nanocomposites were characterized and evaluated for their adsorption efficiency in removing methylene blue (MB) from aqueous solutions. The synthesis was carried out at 80 °C using iron chloride precursors (FeCl₃·4 H₂O and FeCl₂·6 H₂O) and kaolinite. The nanocomposite physicochemical characterization (X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis and differential scanning calorimetry) confirmed the incorporation of crystalline Fe₃O₄ and ε-Fe₂O₃ structures in the kaolinite matrix (particle sizes from 30 to 70 nm). Adsorption experiments showed that 4 mg of Fe₃O₄/kaolinite and ε-Fe₂O₃/kaolinite removed 90.24% and 89.15% of MB (20 mg/L) in the first cycle, 67.95% and 77.60% in the second, and 59.53% and 57.29% in the third, respectively, within 1 h at room temperature. The adsorption experiments also showed that 4 mg of Fe₃O₄/kaolinite and ε-Fe₂O₃/kaolinite removed 29.95% and 10.59% of CoCl₂·6 H₂O (20 g/L), 27.96% and 19.42% of NiCl₂·6 H₂O (60 g/L), and 10.91% and 9.21% of Cu(CH₃COO)₂·H₂O (60 g/L), respectively, within one hour at room temperature. The adsorption isotherm data were best fitted by the Temkin model, with both nanocomposites exhibiting a maximum adsorption capacity (Qₘₐₓ) of 250 mg/g. MB adsorption could be modeled using a pseudo-second order kinetic model. The high correlation coefficients (R² = 0.991 for Fe₃O₄/kaolinite and 0.985 for ε-Fe₂O₃/kaolinite) suggested that chemisorption was the predominant mechanism. The intraparticle diffusion coefficients (24.857 mg/g·min¹/² for Fe₃O₄/kaolinite and 26.299 mg/g·min¹/² for ε-Fe₂O₃/kaolinite) indicated efficient internal diffusion. These results underscore the potential of Fe₃O₄- and ε-Fe₂O₃-based kaolinite nanocomposites as effective and green adsorbents for dye removal from wastewater.