Design of Mg–Al layered double hydroxides for lead capture from polluted water: kinetics, isoelectric point, and pH effect

Physicochemical characterization and lead removal from synthetic polluted water were evaluated onto Mg–Al based layered double hydroxides (LDHs) as novel low-cost adsorbents whose facile pH co-precipitation synthesis approach is reported. The well elaborated X-ray characterization ascertained the obtaining of crystalline phases belonging to hydrotalcite-type materials. Anionic clay functions have also been verified by infrared analysis, consisting of OH- and CO₃²⁻ intercalation anions, as well as O- metal bonds. These functional groups which corresponding peaks were attenuated after calcination, are important aspects identifying LDH structures. Typical adsorption–desorption isotherms confirmed the formation of cluster mesopores within LDH texture, with a sufficiently high specific surface area of 91 m²/g, reduced after calcination. The morphological characterization revealed the presence of aggregates with irregular shapes and sizes, getting greater when calcined, suggesting a lamellae collapsing with temperature rise. A total mass loss of 53% of the non calcined Mg–Al-CO₃-NC was deduced from thermogravimetric analysis. The impact of several influential parameters on lead sorption onto Mg–Al-CO₃-NC and 500 °C-calcined Mg–Al-CO₃-C materials, such as adsorbent dose, contact time, pH, and isoelectric point was investigated. It revealed a rapid kinetic behavior with high adsorption amounts, clearly improved by the catalytic effect of calcination. Sorption data were in good agreement with the pseudo-second order mode rather than pseudo-first order model with maximum adsorption quantities of, 168.7 mg.g⁻¹ for Mg–Al-CO₃-NC and 197.9 mg.g⁻¹ for Mg–Al-CO₃-NC. Pb(II) adsorption mechanism was not limited only to the diffusion process, as evidenced by Weber–Morris approach with non-zero intercept. The aforementioned findings are of great utility, confirming the importance of the synthesized Mg–Al-CO₃ systems to serve as potential candidates for water environmental remediation.