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

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 x 10", 1.5 x 10-, 9.5 x 10-, 1.6 x ID", and 0.1 moU L-' 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. M PLATELETS in aqUCOUS SUSpCHsion may flocculate in three possible modes of particle association (Van Olphen, 1977): (i) association between siloxane planes of two parallel platelets (FF), (ii) association between edge surfaces of neighboring particles (EE), and (iii) association between an edge surface and a siloxane planar surface (EF). In the presence of electrolyte (e.g., NaCl) at low concentration, the double layers on the Na-montmorillonite particles are well developed and osmotic repulsion prevents particle association. Thus, a stable suspension of individual platelets is obtained (Van Olphen, 1977). As the concentration of NaCl in the suspension increases, double layers at both the planar and the edge surfaces are compressed, and at the CFC, both EF and EE association can occur. As the electrolyte concentration increases further, the FF mode of particle association occurs, and "oriented aggregates" are formed. Although the van der Waals attraction energy between two plates is independent of the charge density of the plate, the repulsion energy depends on it along with the composition and concentration of the electrolyte in suspension (Derjaguin and Landau, 1941; Verwey and Overbeek, 1948; Pashley, 1981). Thus, the CFC of 2:1 clay minerals should decrease with the surface electrical potential (\|/0). The CFC values for Namontmorillonite range from 7 to 24 mmolc L" NaCl (Arora and Coleman, 1979; Goldberg and Foster, 1990; R. Keren, Inst. of Soils and Water, The Volcani Center, ARO, P.O. Box 6, Bet Dagan, Israel; and D.L. Sparks, Dep. of Plant and Soil Sciences, Univ. of Delaware, Newark, DE 19717-1303. 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 1 Sept. 1993. *Corresponding author (vwrmen*volcani.agri.gov.il). Published in Soil Sci. Soc. Am. J. 59:430-435 (1995). Keren et al., 1988; Oster et al., 1980), whereas for illite, the CFC has ranged from 55 mmolc Lr NaCl (Oster et al., 1980) to 90 mmolc L" NaCl (Goldberg and Foster, 1990), depending on pH. Recent investigations have related the CFC of montmorillonite and illite to suspension pH (Goldberg and Foster, 1990; Hesterberg and Page, 1990; Keren et al., 1988). For example, at pH 9.8 (a value that is well above the PZC of the edge surfaces), the CFC of Na-montmorillonite is 44 mmolc Lr NaCl (Keren et al., 1988). At such a high pH, however, an EE or EF association is not likely to occur. Moreover, at 44 mmolc L" NaCl, the repulsion energy due to the double layer at distances >0.25 nm from the clay surface exceeds the attraction energy, despite the fact that at this NaCl concentration and pH, the clay flocculates. Keren et al. (1988) suggested that FF association between two platelets takes place at surface regions that have lower specific charge density than the average value of the entire montmorillonite surface (0.117 C m-). As mentioned above, the CFC of 2:1 clay minerals decreases as the specific charge density decreases. Thus, clay that has an electrostatic charge close to zero may flocculate regardless of the electrolyte concentration in suspension, unless the edge surfaces are involved in this system. This hypothesis is tested in this study. Pyrophyllite, in contrast to montmorillonite, shows little deviation from the ideal formula. The dioctahedral structure of this clay mineral consists of essentially neutral tetrahedral-octahedral-tetrahedral layers and is the prototype of the structure of montmorillonite and illite groups. The platelets are held together by van der Waals forces, which are comparatively weak with respect to the primary valence forces that hold the atoms in the unit layers together. Consequently, cleavage parallel to the unit layers is relatively easy, and the mineral occurs in the form of flakes (Van Olphen, 1977). Despite the lack of a charged planar layer, the edge surface properties are similar to those of the rest of the phyllosilicates. The lack of electrical field associated with the planar surfaces may allow the evaluation of the edge surface reactivity in clay particle interaction. The purposes of this study were to determine the influence of pH and Na-hexametaphosphate on CFC and the effect of electrolyte concentration on the gel water content. MATERIALS AND METHODS