From the model, the surface site density for different rocks and the binding constant for different cations are found and they are in good agreement with those reported in literature. The result also shows that (1) the surface site density of Bentheim sandstone mostly composed of silica is the largest of five rock samples; (2) the binding constant is almost the same for a given cation but it increases in the order KMe(Na+ ) SiO− + H+, for deprotonation of silanol groups and >SiOH0 + Me+ >SiOMe0 + H+, (2) for cation adsorption on silica surfaces ( Me+ refer to monovalent cations in the electrolytes such as K+ or Na+). It should be noted that further protonation of the silanol surfaces is expected only under extremely acidic conditions (pH Si2O0) is .
