Chromate Adsorption by Kaolinite

J. M. Zachara, C. E. Cowan, R. L. Schmidt and C. C. Ainsworth
Battelle, Pacific Northwest Laboratories P.O. Box 999, Richland, Washington 99352

Abstract: Chromate (CrO42-) adsorption was investigated on kaolinite (0.2–2 µm) saturated with NaClO4 over a range of pH. Adsorption increased with decreasing pH because of protonation of chromate and/or variable charge sites on kaolinite. Chemical pretreatment to remove noncrystalline and crystalline oxide contaminants affected the magnitude of CrO42- adsorption, but not the pH range over which CrO42- adsorbed. Chromate adsorption at different sorbate and sorbent concentrations increased below the pHzpc for the kaolinite edge, suggesting the formation of weak surface complexes. If CrO42- and SO42- were present at equal concentration (5.0 x 10−7 M), the two solutes sorbed independently, suggesting binding to separate sites. The presence of excess SO42- (5.0 x 10−4 M), however, unexplainably enhanced CrO42- adsorption. The adsorption of both chromate and sulfate can be described in terms of a site-binding model of the kaolinite edge, in which the edge is viewed as composite layers of Al and Si oxide. Surface complexation constants for CrO42- on kaolinite were similar to those for alumina, pointing to the importance of Al-OH edge sites in chromate adsorption.

Key Words: Adsorption • Chromate • Kaolinite • Sulfate • Surface complex

Clays and Clay Minerals; August 1988 v. 36; no. 4; p. 317-326; DOI: 10.1346/CCMN.1988.0360405
© 1988, The Clay Minerals Society
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