G. Rytwo et al., A MODEL FOR ADSORPTION OF DIVALENT ORGANIC CATIONS TO MONTMORILLONITE, Journal of colloid and interface science, 181(2), 1996, pp. 551-560
Adsorption of the divalent organic cations paraquat (PQ) and diquat (D
Q) to montmorillonite was studied experimentally and simulated by a mo
del which combines the Gouy-Chapman solution and specific cation bindi
ng 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 forme
d between a divalent cation and one or two singly charged surface site
s, The noncoulombic interactions also included mixed complexes in whic
h a monovalent organic cation could bind to a complex composed of a di
valent organic cation and one or two surface sites, and vice versa, wi
th asymmetric binding coefficients, PQ adsorbed up to the cation excha
nge capacity (CEC) of the clay, whereas adsorbed amounts of DQ exceede
d the CEC by 18%, The binding coefficients determined for PQ and DQ ex
ceed those found for Mg or Ca by several orders of magnitude but are b
elow those previously found for several monovalent organic cations, su
ch as acriflavin (AF) and crystal violet (CV), Unlike the unmodified,
or enhanced, adsorption of these monovalent organic cations with an in
crease 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 interact
ed 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 competiti
on with PQ or DQ but overestimated the amounts of adsorbed AF, We prop
ose that steric restrictions imposed by low basal spacing in montmoril
lonite interacting with PQ or DQ reduce the amounts of adsorbed AF, Th
is 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 l
arger than 1.8 nm. (C) 1996 Academic Press, Inc.