The adsorption of acids and bases onto the surface of silicon dioxide (SiO2) particles was systematically studied as a function of several variables, including activation conditions, contact time, specific surface area, particle size, concentration and temperature. The physical properties of SiO2 particles were investigated, where characterizations were carried out by FT-IR spectroscopy, and morphology was examined by scanning electron microscopy (SEM). The SEM of samples showed good dispersion and uniform SiO2 particles with an average diameter of about 1-1.5 mu m. The adsorption results revealed that SiO2 surfaces possessed effective interactions with acids and bases, and greatest adsorption capacity was achieved with NaOH, where the best fit isotherm model was the Freundlich adsorption model. The adsorption properties of raw SiO2 particles were further improved by ultrasonication. Langmuir monolayer adsorption capacity of NaOH adsorbate at 25 degrees C on sonicated SiO2 (182.6 mg/g) was found to be greater than that of the unsonicated SiO2 (154.3 mg/g). The spontaneity of the adsorption process was established by decreases in Delta G(ads)(0), which varied from -10.5 to -13.6 kJ mol(-1), in the temperature range 283-338 K. (C) 2009 Elsevier B.V. All rights reserved.