Electrolytes, smectites

Our model for the adsorption of water on silicates was developed for a system with few if any interlayer cations. However, it strongly resembles the model proposed by Mamy (12.) for smectites with monovalent interlayer cations. The presence of divalent interlayer cations, as shown by studies of smectites and vermiculites, should result in a strong structuring of their primary hydration sphere and probably the next nearest neighbor water molecules as well. If the concentration of the divalent cations is low, then the water in interlayer space between the divalent cations will correspond to the present model. On the other hand, if the concentration of divalent cations approaches the number of ditrigonal sites, this model will not be applicable. Such a situation would only be found in concentrated electrolyte solutions. 

Of particular interest for chemical transport into a predominantly smectite medium is the shrink-swell property of the clay material. The swelling properties of smectites are explained by two concepts. The first one, developed by Sposito (1973), shows that smectite swelling is caused by the hydration and mobility of the cations, which in turn balance the negative charge of the layer silicates. The second concept, presented by Low (1981), emphasizes the direct interaction of water molecules with the silicate surface. Both viewpoints fit the common observation that smectite swells in a high-hydration environment and at low electrolyte concentrations and shrinks when water is lost and salt is added to the bulk solution. 

TABLE 3 Effect of 1 1 Electrolyte Concentration on the DDL Inner Potentials for Smectites... 

Whitworth T. M. and Fritz S. J. (1994) Electrolyte-induced solute permeability effects in compacted smectite membranes. Appl. Geochem. 9, 533-546. 

The pH-dependence of radionuclide cation adsorption partly reflects competition with H ions for adsorption sites. Competition for adsorption sites by major electrolyte ions also limits radionuclide adsorption in saline waters (cf. Mahoney and Langmuir 1991). Also, competition for sites with Ca can significantly reduce Ra adsorption by quartz and kaolinite (Riese 1982), as well as NpOJ adsorption by smectite clays (Kozai et al. 1995). 

PZC/IEP of Smectite from Clay Minerals Society Repository Electrolyte T Method Instrument pH, Reference... 

Figure 3.6. Calculated isotherms for divalent cation (B ) adsorption on a smectite initially saturated with monovalent cations at three electrolyte concentrations in solution.

Figure 3.7. Measured isotherms at low electrolyte concentrations for Ca adsorption on K -, and -saturated smectites.

Figure 3.13. Variation of the selectivity coefficient for Cd /Na exchange on smectite as a function of the adsorption level of Cd " and the electrolyte...

If we reconsider K —Na exchange on smectite, nearly ideal two-ion exchange behavior is seen when the process is conducted in 0,01 MNaCl that is, the selectivity coefficient Ks is fairly constant. On the other hand, if the electrolyte concentration is very low, Na cations, and to a lesser extent cations, are exchanged from the clay by protons (more precisely, HjP ions) generated from carbonic acid dissociation. As a result, the apparent value of Ks, if based on the incorrect assumption that only... 

Significant replacement of monovalent metal cations on layer silicate clay surfaces by protons can occur if the electrolyte concentration is very low. The long-term result, beyond hydrolytic exchange, is acidic decomposition of the clay structure in part, and release of structural AP or Mg + to solution. These multivalent cations may then readsorb onto exchange sites, influencing the rheological properties of clays in very dilute salts. Some of the anomalous behavior of Na -smectites suspended in... 

See, e.g., D.M.C. MacEwan and M. J. Wilson, Interlayer and intercalation compounds of clay minerals, in G. W. Brindley and G. Brown, op cit. That 1 1 electrolytes cannot be dissolved completely in adsorbed water on smectites and illitic micas has been shown by A. M. Posner and J. P. Quirk, The adsorption of water from concentrated electrolyte solutions by montmorillo-nite and illite, Proc. Royal Soc. (London) 278A 35 (1964). 

A detailed model of the interfacial region requires the specification of the position of the plane where the diffuse ion swarm begins, A popular choice in the literature of soil chemistry has been jc = 0, which means that outer-sphere surface complexes are neglected entirely and inner-sphere surface complexes are ignored if they would protrude beyond the plane to which (Tin, the intrinsic surface charge density, refers. (See Secs. 1.5 and 3.1 for a discussion of trjn ) That this choice is not reasonable physically, however, can be seen from a simple calculation involving Eq. 5.16. Consider a 1 1 electrolyte at the concentration Cq == 100 mol m" and suppose that /r(0) = SRT/F, a value that is not unrealistic for a smectite siloxane surface. Then k = = 1.04 x 10 m" at 298 K, a =... 

I. Nicotera, A. Enotiadis, K. Angjeh, L. Coppola, D. Goumis, Evaluation of smectite clays as nanofiUers for the synthesis of nanocomposite polymer electrolytes for fuel ceU applications, Int. J. Hydrogen Energy 37 (2012) 6236-6245. 

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