Electrokinetic study of the adsorption of ethyl violet and crystal violet by montmorillonite clay particles

Electrophoretic mobilities of mixtures of a montmorillonite clay (in different exchangeable metal cation forms) and increasing amounts of two cationic dyes (ethyl violet and crystal violet) were measured. Electrophoretic mobilities were found to vary between -60 x 10(-5) and +40 x 10(-5) cm(2) s(-1) V-1. For both the dyes, the degree of saturation at which the isoelectric point (IEP) occurs, decreases with increasing valency of the metal cations. An effort was made to connect the IEP, maximum flocculation, and dye adsorption parameters. An important adsorption parameter is the transition saturation (TS), the saturation beyond which adsorption by organophilic attractions occurs in addition to adsorption by electrical and (or) pi-interactions. It was found that maximum flocculation occurs before the IEP for all the exchangeable cations tested, but the IEP is reached at similar saturations to the TS for most of the cations. These results have been interpreted in terms of different types of adsorption phenomena and particle associations.