Adsorption of paraquat ions on clay minerals. Electrophoresis of clay particles

Abstract

The adsorption of paraquat (1,1′-dimethyl-4,4′-dipyridinium chloride) on montmorillonite, illite, and kaolinite was investigated. The organic cation was strongly adsorbed up to the cation exchange capacity (CEC) of montmorillonite and illite. On kaolinite, adsorption took place only up to 57% of the CEC. From the low concentration parts of the adsorption isotherms it was concluded that paraquat has a p reference for the interlayer spacings over the exterior surface of the clay particles. Paraquat adsorption is reversible with respect to variation of the indifferent electrolyte concentration. The effect of electrolyte on the ζ-potential of the different clay particles is small. The effect of pH on the ζ-potentials is small for illite and montmorillonite, but large for kaolinite, corresponding with a high fraction of variable charge. The overall iso-electrical point of kaolinite is around pH=2.5. It appears that paraquat adsorption has a minor effect on the ζ-potential until approximately 80% coverage is obtained. This typical effect is explained by the difference in affinity of paraquat for the interlayer spacings and the exterior surface. No charge reversal takes place. The behavior of kaolinite differs in some respects from montmorillonite and illite due to the large fraction of variable charge and its high charge density.