Repulsion between surface complexes?

[Charlie]

I have been trying to use PHREEQC to model sorption of Pb at high sorbate concentrations (onto Mn oxides).  However, the results so far predict a plateaux of maximum sorption using ~75% of the total number of surface sites available, rather than reaching saturation of 100% of total sites.  I am wondering if this is possibly to do with electrostatic repulsion between the surface complexes on the surface (not repulsion between the charged surface and the EDL) , but am not sure if this is explicitly considered by the PHREEQC code.  Does anybody have suggestions or can point out something I might have missed?  Thanks.

[dlparkhurst]

The theory has a relation between the charge density on the surface and the potential at the surface. In turn, the potential is used in a factor that is something like an activity coefficient in that it is a multiplier in the mass-action expression, but accounts for the work necessary to move a charged particle from the bulk solution to the surface. The charge density is the charge per m^2 (Coulomb/m^2) so as more charge develops on the surface, the charge density goes up and the work necessary to move an ion to the surface increases.