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 e ight of the 23 adsorbates. Four dyes, 3,6-diaminoacridine, 9-aminoacridine, azure-A and safranin-O, were selected for further study, including an inve stigation of the effect of pH on adsorption, and measurement of adsorption to gibbsite (alpha -Al(OH)(3)) and silica at pH 9. Strong preferential adso rption to kaolinite was observed over the whole pH range, and at pH 9 the d yes also adsorbed much more to kaolinite than to gibbsite or silica. The ba sal spacing of kaolinite crystals, measured by X-ray diffraction, did not c hange when 3,6-diaminoacridine was adsorbed, indicating that the dye molecu les were not intercalated between the crystalline layers. It is suggested t hat the negatively charged, flat silica faces of kaolinite crystals may ser ve as templates for pi-stacking association of the positively-charged polya romatic molecules. (C) 2001 Elsevier Science B.V. All rights reserved.