Hydrothermally prepared mesoporous silica gel (prepared under acidic condition) was calcined at different temperatures to control concentration of surface hydroxyl groups. These materials were functionalized with 3-aminopropyltrimethoxysilane (APTMS) and used for the adsorption of Cu2+ ions. Adsorption isotherm and adsorption capacity of the amine functionalized materials for Cu2+ were followed by inductively coupled plasma optical emission spectrometry (ICP-OES). The adsorption capacity for functionalized silica gel was correlated with population density of hydroxyl groups and hence amine content in the materials, which was determined by CHN analysis. Nitrogen adsorption/desorption measurements at liquid N-2 temperature, Fourier transform infrared spectrometry (FTIR), and Si-29 MAS NMR spectra were used to follow the changes in the textural parameters and surface properties of the mesoporous silica gel samples. This study indicates that thermal treatment of silica gel determines the population density of surface hydroxyl groups and hence the NH2 population density after functionalization of the gels, which in turn determines the NH2/Cu2+ ratio at saturation of Cu2+ on the functionalized silica gel.