Comparative study on adsorption performance of Pb(II) using fly ash-based mesoporous silica through sol-gel method with different aging temperature

Abstract

The preparation of mesoporous silica materials from fly ash for the adsorption of heavy metals represents a significant approach to resource utilization. In this study, the sol-gel process was employed to synthesize amino-functionalized mesoporous silica with varying pore structure properties (denoted as MS-NH₂-T, where T = 20 °C, 40 °C, and 90 °C) by controlling the aging temperature. The effects of adsorption conditions on the adsorption efficiency of Pb(II) were investigated, and the differences in the adsorption behavior of amino-functionalized mesoporous silica with distinct pore structures were compared and analyzed using typical adsorption isotherms, adsorption kinetics, and rate-limiting adsorption kinetics models. The results indicated variations in specific surface area (625 m² g⁻¹, 640 m² g⁻¹, and 345 m² g⁻¹), average pore size (3 nm, 7 nm, and 15 nm), and total pore volume (0.5 cm³ g⁻¹, 1.0 cm³ g⁻¹, and 1.3 cm³ g⁻¹) among the mesoporous silica synthesized at different aging temperatures. Following amino modification, Pb(II) can be adsorbed through mechanisms such as pore adsorption, electrostatic attraction, and complexation. The adsorption capacities of the amino-functionalized mesoporous silica with varying pore structures were found to be 330 mg g⁻¹, 303 mg g⁻¹, and 204 mg g⁻¹, respectively. Furthermore, the adsorption rate constant of the MS-NH₂-90 material, which features a larger pore size and volume, was determined to be 0.0290 and 0.0008, with a time to reach adsorption equilibrium of 60 min, outperforming the MS-NH₂-20 and MS-NH₂-40 materials under identical conditions (initial concentration of 300 mg L⁻¹). Overall, mesoporous silica with a larger specific surface area demonstrates a higher adsorption capacity, while materials with increased pore size and pore volume exhibit faster adsorption rates. The findings of this study will assist in the selection of the most appropriate amino-functionalized mesoporous silica materials for the adsorption of Pb(II), based on whether the emphasis is on adsorption rate or capacity.