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The Role of Edge Surfaces in Flocculation of 2:1 Clay Minerals

Inst. of Soils and Water, The Volcani Center, ARO, P.O. Box 6, Bet Dagan, Israel

D. L. Sparks

Dep. of Plant and Soil Sciences, Univ. of Delaware, Newark, DE 19717-1303

* Corresponding author (vwrmen*volcani.agri.gov.il).

ABSTRACT

The effect of pH on the flocculation, settling rate, and gel volume of pyrophyllite in dilute suspension was studied in solutions of various NaNO₃ concentrations to evaluate the role of the edge surfaces in particle-particle interaction. The critical flocculation concentration (CFC) increased with increasing pH, being 1.6 x 10^–4, 4 x 10^–4, 1.5 x 10^–3, 9.5 x 10^–3, 1.6 x 10^–2, and 0.1 mol_c L^–1 at pH 3.2, 4.4, 5.3, 6.0, 7.4, 8.5, and 10.2, respectively. The CFC increased significantly in the presence of small amounts of sodium hexametaphosphate in the suspension. The changes in CFC from low values in the absence of the polymer to higher values in the presence of it support the hypothesis that the edge surfaces are involved in particle-particle interactions. The settling rate of the clay particles at pH 8.5 increased as the electrolyte concentration was decreased. The effect of electrolyte concentration on settling rate was explained by the presence of an electrical field associated with the edge surfaces. The gel volume, after reaching a stable volume, increased with the increasing of electrolyte concentration. This increase in water content, indicating an open structure of clay platelets, also supports the hypothesis that edge surfaces are associated with clay flocculation. The electrical field associated with the edge surface also has an effect on the gel volume.

Joint contribution from the ARO, The Volcani Center, Bet Dagan, Israel, and the Univ. of Delaware. Supported by a grant from U.S. Borax Corp.

Received for publication September 1, 1993.