Steric and energetic studies on the retention of Cd (II) and Zn (II) metal ions into magnesium rich zeolite-A synthesized from natural talc: Experimental and theoretical studies

Abstract

Magnesium-rich zeolite-A (Mg.ZA) was successfully synthesized from natural talc minerals as a novel form of magnesium silicate zeolitic structure with a surface area of 187 m²/g and mesoporous nature (1.5 to 10 nm). Different characterization techniques, including X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR), energy dispersive X-ray spectroscopy (EDX), scanning electron microscope (SEM), and Brunner−Emmet−Teller (BET), confirmed the formation of Mg.ZA. The adsorption properties of Mg.ZA were evaluated for effective retention of toxic Cd (II) and Zn (II) metallic ions from the water supplies. The Mg.ZA structure displays significant capacities for the two ions, with saturation capacities (278 mg/g (Cd (II)) and 221.2 mg/g (Zn (II)) higher than several investigated adsorbents. Based on the steric analysis of the applied advanced isotherm models, the structure of Mg.ZA was enriched in 150 and 106.9 mg/g effective uptake sites during the retention of Cd (II) and Zn (II), respectively. Up to three ions of Cd (II) and Zn (II) can occupy each of these sites, contributing to their adsorption in a vertical orientation through multi-interaction or multi-ionic mechanisms. The energetic assessment, either based on Gaussian energy (< 7 kJ/mol) or adsorption energy (< 6 kJ/mol), suggested the predominant impact of physical mechanisms (hydrogen bonds and electrostatic attraction), in addition to the impact of the zeolitic ion exchange process (0.6 to 25 kJ/mol). Furthermore, the thermodynamic functions declare the retention of these ions into the framework of Mg.ZA by exothermic and spontaneous reactions.