Sodium-cation-exchanged clays

Addition of a salt can transform the shale by cation exchange to a less sensitive form of clay, or reduce the osmotic swelling effect by reducing the water activity in the mud below that which occurs in the shale. These effects depend on the salt concentration and the nature of the cation. Salts containing sodium, potassium, calcium, magnesium, and ammonium ions ate used to varying degrees. 

Calcium sources, such as gypsum and lime, promote cation exchange from sodium clay to a less-sweUing calcium clay. Calcium concentrations ate normally low (<1000 mg/L) and osmotic swelling is only reduced if other salts are present. Calcium chloride has been used infrequently for this purpose but systems are available that allow high calcium chloride levels to be carried in the mud system (98). 

A theoretical model for the adsorption of metals on to clay particles (<0.5 pm) of sodium montmorillonite, has been proposed, and experimental data on the adsorption of nickel and zinc have been discussed in terms of fitting the model and comparison with the Gouy-Chapman theory [10]. In clays, two processes occur. The first is a pH-independent process involving cation exchange in the interlayers and electrostatic interactions. The second is a pH-dependent process involving the formation of surface complexes. The data generally fitted the clay model and were seen as an extension to the Gouy-Chapman model from the surface reactivity to the interior of the hydrated clay particle. 

Most cation exchange occurs in estuaries and the coastal ocean due to the large difference in cation concentrations between river and seawater. As riverborne clay minerals enter seawater, exchangeable potassium and calcium are displaced by sodium and magnesium because the Na /K and Mg /Ca ratios are higher in seawater than in river water. Trace metals are similarly displaced. 

The clay minerals carried by rivers into the ocean represent a net annual addition of 5.2 X 10 mEq of cation exchange capacity. Most of these exchange sites are occupied by calcivun. Within a few weeks to months following introduction into seawater, sodium, potassium, and magnesium displace most of the calcium. As shown in Table 21.7, this uptake removes a significant fraction of the river input of sodium, magnesium, and potassium. 

Kennedy VC, Brown TC (1965) Experiments with a sodium ion electrode as a mean to studying cation exchange rate. Clays Clay Minerals 13 351-352 Khachikian C, Harmon TC (2000) Nonaqueous phase liquid dissolution in porous media Current state of knowledge and research needs. Trans Porous Media 38 3-28 Kookana RS, Aylmore LAG (1993) Retention and release of diquat and paraquat herbicides in soils. Austral J Soil Res 31 97-109... 

The peculiar layer structure of these clays gives them cation exchange and intercalation properties that can be very useful. Molecules, such as water, and polar organic molecules, such as glycol, can easily intercalate between the layers and cause the clay to swell. Water enters the interlayer region as integral numbers of complete layers. Calcium montmorillonite usually has two layers of water molecules but the sodium form can have one, two, or three water layers this causes the interlayer spacing to increase stepwise from about 960 pm in the dehydrated clay to 1250, 1550, and 1900 pm as each successive layer of water forms. 

Clays (aluminosilicates) in which the sodium cations have been partially or fully exchanged by surfactants. 

Figure 11.13 Adsorption isotherms for a series of alkyl ammonium compounds on sodium montmoril-lonite (adapted from Cowan and White. 1958). The horizontal dashed line indicates the cation exchange capacity of the clay.

Sodium bentonite with a cation exchange capacity (CEC) of 75 meq/100 g of clay, supplied by Commercial Minerals Ltd., Australia, was used as starting clay material, to prepare samples for SCD and surfactant treatments. Besides, sodium montmorillonite (Kunipia G), from Kunimine Industrial Company, Japan, was used as the starting clay for samples of pore opening modification. CEC of this clay is 100 meq/100 g of clay. 

Kennedy, V. C., and Brown, T. C. (1965). Experiments with a sodium-ion electrode as a means of studying cation exchange rates. Clays Clay Miner. 13, 351-352. 

I was lucky to receive some jars of sodium-substituted Eucatex vermiculite and n-butylammonium-substituted Eucatex vermiculite. These synthetic systems had been obtained by cation exchange on the raw minerals. Such cation exchange plays a major role in clay chemistry, and the process is described in detail in standard books on clay colloids, like that of van Olphen [2], Some years later, I was able to obtain the following chemical formula for the dry sodium Eucatex vermiculite [3]... 

The use of modifiers occasionally improves the extraction process. Water as extractant can be modified with organic solvents such as methanol, acetone or acetonitrile in low proportions (< 5%) in order to decrease its dielectric constant — and hence its polarity — without the need for a drastic temperature increase [37]. Also, an acid or base can be used to alter the pH in those cases where it significantly influences the extraetion yield [29,46]. On the other hand, surfactants facilitate the extraction of non-polar compounds by formation of micelles [47]. Modifiers are less frequently used with extractants other than water. One example is the addition of sodium acetate to methanol to extract organotins (OTs) the additive increases the efficiency in two ways, namely (a) acetate ion by complexing OTs and (b) sodium ion through cation exchange of OTs sorbed to the clay fraction of sediments [21]. 

Ion exchangers can preferentially absorb some types of ions relative to other ions therefore, the ratio between different counterions on an ion exchanger is usually not the same as the ratio between those ions in solution. For example, consider a clay ion exchanger that is selective for calcium. In a groundwater regime that initially contains sodium as the only cation, the clay... 

---