of the study is an extension of an adsorption model (1-4) Adsorption of the divalent organic cations paraquat (PQ) and for the case of divalent organic cations. diquat (DQ) to montmorillonite was studied experimentally and sim-Three main elements characterize the adsorption model ulated by a model which combines the Gouy-Chapman solution and (1, 2): specific cation binding in a closed system. The model allows for coulombic and noncoulombic interactions as previously considered (i) Specific and nonspecific binding; the total amount of for monovalent organic cations. The coulombic interactions were cations adsorbed is assumed to consist of (a) cations bound expressed in terms of complexes formed between a divalent cation to the surface and (b) cations residing in the double-layer and one or two singly charged surface sites. The noncoulombic interregion. actions also included mixed complexes in which a monovalent or-(ii) The electrostatic Gouy-Chapman equations are ganic cation could bind to a complex composed of a divalent organic solved for a solid-liquid system containing several cations cation and one or two surface sites, and vice versa, with asymmetric binding coefficients. PQ adsorbed up to the cation exchange capacity of various valencies interacting with particles whose surfaces (CEC) of the clay, whereas adsorbed amounts of DQ exceeded the are charged and partially neutralized by cation binding.
CEC by 18%. The binding coefficients determined for PQ and DQ
(iii) The concentration of surface sites is explicitly taken exceed those found for Mg or Ca by several orders of magnitude but into account since it affects the concentration of nonadsorbed are below those previously found for several monovalent organic cations remaining in solution. The concentration of free catcations, such as acriflavin (AF) and crystal violet (CV). Unlike the ions in solution may be significantly reduced as a result of unmodified, or enhanced, adsorption of these monovalent organic cation adsorption to the particles.
cations with an increase in ionic strength, the adsorbed amounts of PQ and DQ decreased with increasing CsCl concentrations. This
The adsorption of organic cations can involve interactions trend necessitated ignoring noncoulombic interactions when only a between the organic ligands, which can result in adsorption divalent organic cation interacted with the clay. The adsorption in excess of the cation exchange capacity (CEC) of the clay model could simulate and predict the adsorption of PQ and DQ when (3-6). Indeed, electrophoretic measurements demonstrated added alone or in competition. The model explained qualitatively the charge reversal of montmorillonite particles due to adsorplarger adsorbed amounts of AF in competition with PQ or DQ but tion of monovalent organic cations in excess of the CEC overestimated the amounts of adsorbed AF. We propose that steric restrictions imposed by low basal spacing in montmorillonite inter-(3). The adsorption of divalent organic cations can involve acting with PQ or DQ reduce the amounts of adsorbed AF. This more types of complexes than in the case of monovalent proposal is supported by X-ray diffraction measurements which yield organic cations. basal spacing values of 1.3 nm when DQ and CV are added at
We demonstrate how the effect of variations in ionic amounts in excess of the CEC, whereas with CV alone the respective strength can be utilized in the determination of the types of values were larger than 1.8 nm.