A comprehensive study of Cd(II) ions removal utilizing highsurface- area binary Mg–Si hybrid oxide adsorbent

Presented work concerns the application of synthetic oxide adsorbent in the removal of cadmium ions from its model, water solutions. In this study, a novel magnesium–silicon (Mg–Si) binary oxide adsorbent was prepared by a modified co-precipitation method, utilizing sodium silicate and magnesium sulphate solutions as precursors of silica and magnesium oxide, respectively. The material was thoroughly characterized in order to evaluate chemical composition (AAS, EDS and gravimetric method), crystalline structure (XRD), morphology (SEM), particle size distribution (DLS), characteristic functional groups (FTIR) and porous structure parameters (BET and BJH models). It was proved that the adsorbent is amorphous, with a micrometric-sized, irregular-shaped particles and relatively large surface area of 540 m²/g. Batch adsorption experiments were conducted to investigate the adsorption of Cd(II) ions on the prepared adsorbent, including evaluation of adsorption kinetics, the intraparticle diffusion model, the effect of pH, contact time, mass of the adsorbent, temperature and the effect of competitive Cl^- and NO₃^- anions. During the study, it was confirmed that the adsorption of Cd(II) ions reached equilibrium within 30 min, which was found to fit well with a pseudo-secondorder kinetic model type 1 (r² = 0.998–0.999). The Mg–Si adsorbent exhibited high adsorption capacity for Cd(II) ions at pH above 7, and the maximum quantity of cadmium( II) ions adsorbed in optimal time was achieved for the highest metal ion concentrations: 18.22 (Cl^-) and 15.46 (NO₃^-) mg/g. The competitive anions present in the model cadmium salt solutions hindered adsorption in the sequence Cl^->NO₃^-.

Keywords
Mg–Si binary oxide adsorbent; Porous structure; Cd(II) ions; Adsorption kinetics