A methodology relating changes in the isotherm parameters for sorption
of one solute by a heterogeneous sorbent to changes in the site energ
y distributions of that sorbent caused by prior irreversible sorption
(preloading) of other solutes is proposed. Approximate site energy dis
tributions underlying three isotherm models commonly used to describe
sorption of organic solutes from aqueous solutions are developed using
the theory of heterogeneous surf:aces. It is demonstrated that, regar
dless of the type of initial site energy distribution assumed, preload
ing by a non-desorbable solute results in a loss of surface heterogene
ity. The loss occurs preferentially across sites having the highest en
ergies, with the number of sites in the lowest energy ranges actually
increasing in some cases. Activated carbon is used to demonstrate the
methodology, but the approach is generally applicable to other heterog
eneous;adsorbents in both natural and engineered systems.