Synthesis of silica gel supported salicylaldehyde modified PAMAM dendrimers for the effective removal of Hg(II) from aqueous solution

Highlights: • Silica gel supported salicylaldehyde modified PAMAM dendrimers were synthesized. • SiO₂-G0-SA∼SiO₂-G2.0-SA were promising adsorbents for the removal of Hg(II). • The pseudo-second-order model described adequately the adsorption kinetics data. • The film diffusion mechanism dominated the adsorption processes of Hg(II). • Langmuir model provide best correlation of the experimental data. - Abstract: A series of silica gel supported salicylaldehyde modified PAMAM dendrimers (SiO₂-G0-SA∼SiO₂-G2.0-SA) were synthesized and their structures were characterized by FTIR, XRD, SEM, TGA, and porous structure analysis. The feasibility of these adsorbents for the removal of Hg(II) from aqueous solution was first described and the adsorption mechanism was proposed. The adsorption was found to depend on solution pH, the generation number of salicylaldehyde modified PAMAM dendrimers, contact time, temperature, and initial concentration. Results showed that the optimal pH was about 6 and the adsorption capacity increased with the increasing of generation number. Density functional theory (DFT) method was used to investigate the coordination geometries and the chelating mechanism. Adsorption kinetics was found to follow the pseudo-second-order model with film diffusion process as rate controlling step. Adsorption isotherms revealed that adsorption capacities increased with the increasing of temperature. Langmuir, Freundlich and Dubinin-Radushkevich (D-R) isotherm models were employed to analyze the equilibrium data. The adsorption can be well described by Langmuir isotherm model and took place by chemical mechanism. The thermodynamics properties indicated the adsorption processes were spontaneous and endothermic nature. The maximum adsorption capacity of SiO₂-G0-SA, SiO₂-G1.0-SA, and SiO₂-G2.0-SA were 0.91, 1.52, and 1.81 mmol g⁻¹, respectively. The considerable higher adsorption capacity compared with other adsorbents indicates SiO₂-G0-SA∼SiO₂-G2.0-SA are favorable and useful for the uptake of Hg (II), and can be potentially used as promising adsorbents for the effective removal of Hg(II) from aqueous solution