A Model for Adsorption of Divalent Organic Cations to Montmorillonite

Abstract

Adsorption of the divalent organic cations paraquat (PQ) and diquat (DQ) to montmorillonite was studied experimentally and simulated by a model which combines the Gouy–Chapman solution and specific cation binding in a closed system. The model allows for coulombic and noncoulombic interactions as previously considered for monovalent organic cations. The coulombic interactions were expressed in terms of complexes formed between a divalent cation and one or two singly charged surface sites. The noncoulombic interactions also included mixed complexes in which a monovalent organic cation could bind to a complex composed of a divalent organic cation and one or two surface sites, and vice versa, with asymmetric binding coefficients. PQ adsorbed up to the cation exchange capacity (CEC) of the clay, whereas adsorbed amounts of DQ exceeded the CEC by 18%. The binding coefficients determined for PQ and DQ exceed those found for Mg or Ca by several orders of magnitude but are below those previously found for several monovalent organic cations, such as acriflavin (AF) and crystal violet (CV). Unlike the unmodified, or enhanced, adsorption of these monovalent organic cations with an increase in ionic strength, the adsorbed amounts of PQ and DQ decreased with increasing CsCl concentrations. This trend necessitated ignoring noncoulombic interactions when only a divalent organic cation interacted with the clay. The adsorption model could simulate and predict the adsorption of PQ and DQ when added alone or in competition. The model explained qualitatively the larger adsorbed amounts of AF in competition with PQ or DQ but overestimated the amounts of adsorbed AF. We propose that steric restrictions imposed by low basal spacing in montmorillonite interacting with PQ or DQ reduce the amounts of adsorbed AF. This proposal is supported by X-ray diffraction measurements which yield basal spacing values of 1.3 nm when DQ and CV are added at amounts in excess of the CEC, whereas with CV alone the respective values were larger than 1.8 nm.