Metachromasy in clay-mineral systems: adsorption of cationic dyes crystal violet and ethyl violet by kaolinite from aqueous and organic solutions

Abstract

Adsorption of two cationic dyes, crystal violet (CV) and ethyl violet (EV), by kaolinite has been investigated by normal and comparative electronic spectroscopy. It was found that adsorption of both dyes from dilute aqueous solutions resulted in metachromasy. Adsorption from organic solvents (isopropyl alcohol or glycerol) led to metachromasy with CV but not with EV. Adsorption from mixed water–organic solvents was dependent on both the cationic dye used and the composition of the solvent. Quantitative determinations showed that both dyes were adsorbed to a greater extent from aqueous solutions than from isopropyl alcohol or aqueous isopropyl alcohol solutions, with more CV than EV being adsorbed.