The competitive adsorption to kaolinite between Cd(II) and four polyaromatic dyes (9-aminoacridine, 3,6-diaminoacridine, azure A and safranin O) was studied in 5 mM KNO3 at 25°C. Under these conditions, Cd adsorbs to the silica face of kaolinite between about pH 4 and 6.5, but at higher pH, adsorbed Cd is progressively relocated to the crystal edges. In the presence of dye, less Cd adsorbed to kaolinite below pH 7. If sufficient dye was added to saturate the kaolinite surface, Cd adsorption was totally suppressed up to ~pH 6. At higher pH, Cd followed the characteristic pattern for edge adsorption. In separate experiments 9-aminoacridine and azure A displaced pre-adsorbed Cd from kaolinite. The displacement curves were initially linear, with one Cd ion being displaced for every 13 dye molecules adsorbed at pH 5.5, and one Cd ion for every 35 dye molecules at pH 7.5. The interpretation of these results is that the dyes bind to kaolinite much more strongly than Cd(II) does, but only to the silica face.

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