pH-Independent and pH-Dependent Surface Charges on Kaolinite

M. D. A. Bolland1, A. M. Posner and J. P. Quirk2
Department of Soil Science and Plant Nutrition, Institute of Agriculture University of Western Australia, Nedlands, Western Australia 6009
Deceased August 1980.
1 Present address: Western Australian Department of Agriculture, Esperance, Western Australia 6450.
2 Present address: Waite Agricultural Research Institute, University of Adelaide, Glen Osmond, South Australia 5064.

Abstract: Model cation-exchange curves are presented for an idealized kaolinite surface where the charge on the surface (1) has its origin in cation substitution in the structure, and hence, is pH independent; and (2) is produced by protonation/deprotonation reactions of oxide-like sites, and hence, depends on the acid and base strengths of the surface sites, as welt as the ionic strength. Two pH-independent situations are considered: one where the exchanging ions have no selectivity for the surface and are all in the diffuse layer; and one where selectivity exists for one ion and where that ion is partly in a Langmuir-Stern layer and partly in the diffuse layer. If one of the exchanging ions is a proton, the shape of the curves and their position on the pH scale depend on ionic strength and ionic selectivity.

The model curves are compared with data for actual kaolinites. Under most conditions exchangeable Al is released from the structure, and the shape of the charging curves becomes similar to that of an oxide-like surface. However, if titration is carried out rapidly, or account is taken of the presence of Al, the proton binding curves are similar in shape to those expected for sites resulting from cation substitution in the structure of kaolinite, either near the surface or at the edge of the crystal.

Key Words: Cation substitution • Ion selectivity • Kaolinite • pH • Protonation/deprotonation • Surface charge

Clays and Clay Minerals; December 1980 v. 28; no. 6; p. 412-418; DOI: 10.1346/CCMN.1980.0280602
© 1980, The Clay Minerals Society
Clay Minerals Society (www.clays.org)