Biosorption is one of the useful phenomena that can be used for removal of heavy metals in wastewater. To date, many researchers have used Langmuir or Freundlich isotherms to quantify sorption capacity; however, these isotherms lack physical meaning for the adsorption mechanism, and parameters in isotherm equations must be obtained by experiment whenever environmental conditions change. We used a surface complexation model that considered adsorption phenomena as chemical reactions in solution. Using titration, we determined a surface active site and equilibrium constants for binding parameters. This model can predict the pH effect on adsorption of Pb and could be applied to explain multi-ion and other competent chemicals such as EDTA.