Post-synthesis surface-modified silicas as adsorbents for heavy metal ion contaminants Cd(II), Cu(II), Cr(III), and Sr(II) in aqueous solutions.

To analyze the influence of silica surface modification and confined space effects on specific interactions of divalent and trivalent metal cations with surface functionalities, three different high surface area silicas with different pore size distributions were modified with the following organosilanes: 3-aminopropyltriethoxysilane, N-(2-aminoethyl)-3-aminopropyltriethoxysilane, 3-(trimethoxysilylpropyl)diethylenetriamine, N-(triethoxysilylpropyl)ethylenediaminetriacetic acid (EDTrA), and 3-(2,4-dinitrophenylamino)propyltriethoxysilane. The silicas were characterized by N(2) adsorption and reflectance FTIR spectroscopy before and after surface modification. N(2) adsorption and pore size distributions showed an increase in the pore width for all EDTrA-modified silicas, opposite to what occurred with the other organosilanes. Adsorption isotherms of Cd(II), Cr(III), Cu(II), and Sr(II) obtained from aqueous solutions were compared and analyzed by silica type, organosilane functional group, and metal adsorbed. Reflectance FTIR spectroscopy was used to probe the acetate functionality in EDTrA as a function of adsorbed metal content. A band shift to higher energy for Cr(III) on the wide pore silica studied indicated that the interaction with the acetate groups can be probed in this manner. In general, the wider pore distribution silica provided larger adsorption maxima, whereas the narrower pore distribution silica provided more favorable ΔG because of stronger binding of the cations. Cr(III) and Cu(II) exhibited larger adsorption maxima compared to Cd(II) and Sr(II), with the grafted organosilanes studied since the first cations have a greater charge/radius ratio than the second ones that provide a greater binding energy.