Chemically activated metakaolin as a novel and efficient adsorbent for the removal of Cd(II) ions from aqueous solutions

Unlike previous studies using untreated metakaolin, this work introduces a chemical activation route that significantly enhances surface charge, porosity, and Cd(II) affinity, providing a more efficient and sustainable pathway for Cd(II) removal. Brunauer–Emmett–Teller (BET), zeta potential, Fourier Transform Infrared (FTIR) spectra, Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), and Thermogravimetric analysis (TGA) were used to analyze the characteristics. The chemically activated metakaolin adsorption performance is attributed to its negative surface (−32 mv), enhanced surface area (258.674 m2/g), pore radius (5.2716 nm), pore volume (0.14 cm³/g), high density of reactive –OH and –Si–O− groups, and improved surface roughness. Different factors that affected the Cd(II) removal on metakaolin were investigated by batch adsorption experiments. The equilibrium values were 35 min contact time, 0.5 g adsorbent dose, pH 9, 25 °C temperature, and 200 rpm shaking speed. The kinetics of Cd(II) sorption were best fitted to the pseudo-second-order model, which yielded an R2 of 0.999 assuming that the rate-limiting step is chemisorption. The Sips model fitted the adsorption isotherms well, indicating that the sorption was heterogeneous and the sorption capacity was 10.7629 mg g−1. The thermodynamic evaluation shows that Cd(II) adsorption is endothermic and spontaneous, with ΔG° ≈ −15.6 kJ/mol, ΔH° ≈ 13 kJ/mol, and ΔS° ≈ 0.083 kJ/mol·K. An individual-stage batch adsorber was designed for the sorption of Cd(II) based on the optimum isotherm. The metakaolin exhibited excellent stability and retained more than 72% of its capacity after five regeneration cycles, demonstrating its potential for industrial-scale and environmentally sustainable remediation.