Flocculation Value and Gel Structure of Sodium/Calcium Montmorillonite and Illite Suspensions

The flocculation value, the minimum salt concentration which causes flocculation within 24 hours, was determined for mixtures of Naand Ca-montmorillonite (API-25) and Fithian illite (API-35). The equivalent fraction of exchangeable Na, £Na, varied from 0 to 1. We prepared Na/Ca clay suspensions by adding the appropriate amounts of homoionic dry clays to a series of Na/Ca chloride solutions (0.2 to 80 molc nr; 0.2 to 80 meq liter') with appropriate sodium adsorption ratio and shaking the mixture for 1 hour. The flocculation value of Camontmorillonite (0.25 mol,, nr) increased rapidly with small amounts of exchangeable Na: the flocculation value for an JENa of 0, 0.2, 0.4, and 1 was 0.25, 6, 9, and 12 molc m-", respectively. The flocculation values for Caand Na-illite were 0.25 and 55 molc nr, respectively, and increased linearly with £Na. Van der Waals attraction forces are mainly responsible for flocculation of Ca-montmorillonite and illite systems. Edgeto-face attraction is dominant in flocculating Na-montmorillonite. The large effect of small amounts of exchangeable Na in montmorillonite suspensions results from demising of exchangeable ions whereby Na* is the predominant cation on the external surfaces when CHa < 0.20. Additional Index Words: Wyoming montmorillonite, Fithian illite, van der Waals forces, double-layer theory, (NaPOaJu, electrophoretic mobility, swelling pressure, sodium adsorption ratio, clay gel water content. Oster, J. D., I. Shainberg, and J. D. Wood. 1980. Flocculation value and gel structure of sodium/calcium montmorillonite and illite suspensions. Soil Sci. Soc. Am. J. 44:955-959. 1 Contribution from U. S. Salinity Laboratory, AR-SEA, USDA, 4500 Glenwood Drive, Riverside, CA 92501. Received 26 Nov. 1979. Approved 18 June 1980. "Soil Scientists. Dr. Shainberg's permanent address is Institute of Soils and Water, ARO, The Volcani Center, Bet-Dagan, Israel. R PUBLICATIONS report unusually large effects of low exchangeable Na fractions, ENa. on the hydraulic conductivity (Frenkel et al., 1978; Pupisky and Shainberg, 1979) and on infiltration rate (Oster and Schroer, 1979) of soils leached with dilute solutions where the salt concentration was < 18 molc m3~ (18 meq liter"). Clays moved within the soil pores when £Na ^0.15 and hydraulic conductivity decreased drastically within the soil layers where clay particles lodged. Infiltration rates increased, however, with irrigation water concentration at an E^& as low as 0.02. These results suggested that a small amount of adsorbed Na markedly increase the dispersivity and mobility of the clay fraction in dilute solutions. Little information exists on clay dispersivity, or its counterpart flocculation, of mixed Na/Ca clay systems (Na/Ca indicates both Na and Ca are present in the exchanger phase). The minimum NaCl concentration that causes flocculation, referred to here as flocculation value, of Na montmorillonite is 12 to 16 molc m~; the flocculation value for Ca montmorillonite in a CaCl2 solution is 0.2 molc m~ (van Olphen, 1977). The flocculation values of Naand Ca-illite suspensions are 40 and 0.25 molc m~, respectively (ElSwaify, 1976; Greene et al., 1978, respectively). Sodium montmorillonite in dilute suspensions exists as single platelets (van Olphen, 1977). Calcium montmorillonite exists in packets (tactoids or quasicrystals), each consisting of several (4-9) clay platelets with a film of water 0.45 nm (4.5A) on each internal surface 3 Concentrations such as cation exchange capacity, exchangeable cations, and solute concentrations are expressed in moles of (either negative or positive) charge, with the symbol mole. Published September, 1980