Structural characterization and ecological evaluation of natural clay mixtures for the removal of heavy metals (Cu(II), Co(II), and Zn(II)) from aqueous solutions: experimental study combined with RSM process optimization

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This study presents an eco-friendly strategy for removing Cu(II), Co(II), and Zn(II) ions from aqueous solutions using a natural clay mixture (KM-IQ), composed of kaolinite, montmorillonite, illite, chlorite, and quartz. Structural characterization of KM-IQ was performed using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM), confirming the crystalline phases, functional groups, and porous texture favorable for adsorption. Elemental composition and distribution are analyzed by X-ray fluorescence (XRF), energy-dispersive X-ray spectroscopy (EDS), and elemental mapping. Post-adsorption SEM-EDS mapping revealed significant retention of metal ions on the surface. Batch adsorption experiments showed that the kinetic data fit the pseudo-second-order model, while equilibrium data followed the Langmuir isotherm, indicating monolayer chemisorption. Maximum adsorption capacities are 131.66 mg/g for Zn(II), 130.91 mg/g for Co(II), and 98.15 mg/g for Cu(II). The adsorbent exhibited notable selectivity towards divalent metal ions, with preferential retention of Zn(II) and Co(II). Regeneration studies with HNO₃ demonstrated good reusability over successive cycles. Response Surface Methodology (RSM) is employed for process optimization, yielding high predictive accuracy (R² > 0.93). These findings confirm that KM-IQ is a structurally rich, selective, low-cost, and sustainable adsorbent for the efficient treatment of heavy-metal-contaminated water.