The Poisson-Boltzmann equation was applied to simulation of published data on adsorption of bivalent metal ions on silica. The surface charge of silica was approximated with the basic Stern model (without coordinate binding of sodium ions). It was found that the adsorption of alkaline-earth metal ions is closely consistent with the Poisson-Boltzmann equation: i.e., it may be completely explained by electrostatic attraction in the diffuse layer. Adsorption in the diffuse layer was also found important for some heavy metals, such as cadmium. (C) 2010 Elsevier Inc. All rights reserved.