Adsorption of CIP on montmorillonite was well fitted with
the Langmuir sorption equation with a regression coefficient (r2)
of 0.9997 (Fig. 2). In contrast, fitting with the Freundlich equation
resulted in a low r2 of 0.8744. The adsorption capacity was
330 mg/g, corresponding to 1.0 mmol/g. With an initial concentration
of 3000 mg/L, the equilibrium concentration of CIP was
only 80 mg/L, suggesting higher than 97% of CIP removal from
the solution. Using the specific surface area (SSA) of 65.2m2/g of
the montmorillonite [28], at the maximum adsorption capacity
of 1.0 mmol/g, the adsorption area of each CIP molecule would
be 11Å2, much less than 17Å2, the reported area of the cationic
amine [19]. The ratio between the external and internal surface
areas of the montmorillonite is about 1:10 [30]. Each CIP molecule
could have an area of 110Å2 available for adsorption if both external
and internal surface areas are considered available. Interlayer
adsorption of CIP by montmorillonite is sustained by this estimation.
The close agreement between the maximum CIP adsorption
capacity and the CEC of the mineral suggests that it is the charge
density, rather than the SSA, that is the limiting factor controlling
the amount of CIP adsorbed on montmorillonite, a strong indication
of adsorption dominated by a cation-exchange mechanism