Selective adsorption of dyes and other organic molecules to kaolinite and oxide surfaces

Adsorption of 23 organic molecules, including a number of polyaromatic dyes, from aqueous solution to kaolinite and amorphous alumina was measured at pH 9. For 22 of the 23 solutes, adsorption was greater to kaolinite than to alumina, and more than seven times as much was adsorbed to kaolinite for eight of the 23 adsorbates. Four dyes, 3,6-diaminoacridine, 9-aminoacridine, azure-A and safranin-O, were selected for further study, including an investigation of the effect of pH on adsorption, and measurement of adsorption to gibbsite (α-Al(OH)3) and silica at pH 9. Strong preferential adsorption to kaolinite was observed over the whole pH range, and at pH 9 the dyes also adsorbed much more to kaolinite than to gibbsite or silica. The basal spacing of kaolinite crystals, measured by X-ray diffraction, did not change when 3,6-diaminoacridine was adsorbed, indicating that the dye molecules were not intercalated between the crystalline layers. It is suggested that the negatively charged, flat silica faces of kaolinite crystals may serve as templates for pi-stacking association of the positively-charged polyaromatic molecules.