70130 100200 1040 315 1040 1035

*From Grim.2 and Weaver and Pollard.1

The BEC and the species of exchange cations may be determined in ihe laboratory by leaching the clay with excess of a suitable salt, such as ammonium acetate, which displaces both the adsorbed cations and those in the interstitial water. Then, another sample is leached with distilled water, which displaces only the ions in the interstitial water. Both filtrates are analyzed for the common exchange cations: the difference between the ionic content of the acetate and water leachates gives the meq of each species adsorbed on the clay, and the total meq of all species of cations gives the BF.(" A field test for the approximate determination of the BEC (but not the species of cations) based on the adsorption of methylene blue is given in Chapter 3.

Clays with a single species of exchange cation may be prepared by leaching with an appropriate salt and washing to remove excess ions. Alternatively, they may be prepared by passing a dilute clay suspension through an exchange resin, such as Dowex 50", which has been saturated with the desired cation Since clays are analogous to large multivalent anions, it is customary to call mono-ionic clays by the name of the adsorbed cation; thus we speak of sodium montmorillonite, calcium montmorillonite, etc.

Anion exchange capacities are much less than base exchange capacities about 10 20 meq/100 g for minerals in the smectite group. With some clay minerals, anion exchange capacities are difficult to determine because of the small amounts involved.

Clay Swelling Mechanisms

All classes of clay minerals adsorb water, but smectites take up much larger volumes than do other classes, because of their expanding lattice. For this reason, most of the studies on clay swelling have been made with smectites, particularly with montmorillonite.

Two swelling mechanisms are recognised: crystalline and osmotic. Crystalline swelling (sometimes called surface hydration), results from the adsorption of mono-molecular layers of water on the basal crystal surfaces— on both the external, and, in the case of expanding lattice clays, the inter-layer surfaces (see Fig. 4-6). The first layer of water is held on the surface by hydrogen bonding to the hexagonal network of oxygen atoms,22 as shown in Fig. 4 11. Consequently, the water molecules are also in hexagonal coordination, as shown in Figure 4-12. The next layer is simarly co-ordinated and bonded to the first, and so on with succeeding layers. The strength of the bonds decreases with distance from the surface, but structured water is believed to persist to distances of 75—100 A from an external surface.: 1

The structured nature of the water gives it quasi-crystalline properties Thus, water within 10 A of the surface has a specific volume about 3"0 less than that of free water.24 (Compared with the specific volume of ice, which is

Figure 4-11. Combined water layers between layers of partially dehydrated ver-miculite. (From Hendricks and Jefferson.22 Courtesy of American Minerologist.)

8% greater than that of free water.) The structured water also has a viscosity greater than that of free water.

The exchangeable cations influence the crystalline water in two ways. First, many of the cations are themselves hydrated i.e., they have shells of water molecules (exceptions are NH4 + , K + , and Na + ). Second, they bond to the crystal surface in competition with the water molecules, and thus tend to disrupt the water structure. Exceptions are Na+ and Li + , which are lightly bonded and tend to diffuse away.

When dry montmorillonite is exposed to water vapor, water condenses between the layers, and the lattice expands. Figure 4-13 shows the relationships between the water vapor pressure, the amount of water adsorbed, and the increase in c-spacing.25 It is evident that the energy of adsorption of the first layer is extremely high, but that it decreases rapidly with succeeding layers. The relation between the vapor pressure and the potential swelling pressure is given in Equation 8-8.

Figure 4-12. Combination of water and vermiculite layers by binding through hydrogen. The oxygen atoms represented by large dotted circles are 2.73 A below the plane of the water molecules. (From Hendricks and Jefferson.22 Courtesy of American Minerologist.)

Norrish20 used an X-ray diffraction technique to measure the c-spacing of flakes of monoionic montmorillonites while they were immersed in saturated solutions of a salt of the cation on the clay. Then, the spacing was observed in progressively more dilute solutions and, finally, in pure water.

In all cases, the spacing at first increased in discrete steps with decrease in concentration, each step corresponding to the adsorption of a mono-layer of water molecules. Table 4-4 shows the maximum spacing observed with most monoionic clays. The values obtained indicate that no more than four layers of water were adsorbed.

With monoionic sodium montmorillonite, however, a jump in spacing from I1) to 40 angstroms was observed at a concentration of 0.3N, and the X-ray patterns changed from sharp to diffuse. At still lower concentrations, the spacings increased linearly with the reciprocal of the square root of the concentration as shown in Figure 4-14. The patterns became more diffuse as the spacing increased, so that spacings above the maximum of 130 A shown in Figure 4 -14 may have occurred, but could not be detected. Similar behavior was observed with lithium chloride and hydrogen chloride, except that the stepwise expansion persisted until the spacing was 22.5 A, which occurred at a concentration of 0.66 N. However, the diffuse spacings observed in dilute hydrogen chloride solutions collapsed on aging, probably

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