Ion chromatography analyses revealed that Ca2+ is the major
desorbed cation relative to the others (Fig. 3). Moreover, stoichiometric
relation between the total cations desorbed and CIP
adsorbed yielded a slope of ∼1 with a negligibly small intercept,
confirming cation exchange as the dominant mechanism for CIP
adsorption. The equilibrium solution pH decreased from 6.5 to 5 as
the initial CIP concentrations increased from 500 to 4000 mg/L. In
the acidic aqueous solution, the CIP molecules are expected to be
in the cationic and zwitterionic forms (Fig. 1). The positive charge
of the amine group might be responsible for the exchange between
CIP molecules and cations in the interlayer sites of montmorillonite.
The stoichiometric correlation between CIP adsorption and cation
desorption provides a direct evidence of cation exchange being the
dominant adsorption mechanism on montmorillonite. It is noteworthy
to mention that such an analysis has not received much
attention that it deserved for studies of adsorption processes on
swelling clays in the past