Clays structural charge

The Li ions were introduced in two different ways either before or after Zr intercalation. The montmorillonite (Weston L-Eccagun) was first exchanged with NaCl (IN) and washed. Two montmorillonites with reduced charge were prepared following the Brindley and Ertem method (13). Part of the Na+ montmorillonite was first saturated with LiCl (IN) and washed. The Li+ clay thus obtained and Na+ clay suspension were stirred for 24 hours at 25°C and dried on glass plate. The films were then heated at 220°C for 24 h in order to allow Li diffusion in the clay structure. Two different Li concentrations (F=0.4 and F=0.6) were used. The Na Li+ modified montmorillonite were dispersed in water acetone solution (1/1). The ZrOCla, 8H2O solution was added to the Na+Li+ montmorillonite (0.02g.l l Zr/Clay=5.CEC). The suspension was stirred with NaOH solution (0.1 N) up to a OH/Zr ratio of 0.5. The final pH of the suspension was 1.85. After two hours of reaction at 40°C the Zr pillared clay was washed up to constant conductivity of the solution, freeze-dried and calcined at different temperatures up to 700°C (Eni-02 and EIII-03). 

From the discussion above, it can be seen how the atomic structure of phyllosilicate clays plays a key role in determining the final state of clay particles in aqueous media. The presence of structural charges, neutralizing cations, and the capacity of forming hydrogen bonds between different layers produces a system that can be completely delaminated, completely flocculated, or in an intermediate state having floes mixed with isolated layers. Whether the more stable situation corresponds to isolated layers, floes, or a mixture depends on the type of clay, its concentration, pH, concentration and type of supporting electrolyte, and so on. 

Presence of structural charges and their effects on the electric potential at both the basal and edge surfaces. This is the main difference between a clay-water system and a metal oxide-water system, which in principle contains no structural charges. 

The main difference between the proton adsorption behavior of clays carrying structural charge and metal oxides with no structural charge is the effect of the electrolyte concentration on the proton adsorption curves. In clays, the pH where proton adsorption is zero decreases as the electrolyte concentration increases, whereas in metal oxides this pH value does not change by changing the electrolyte concentration and a crossing point is observed (PZC). 

The model presented here succeeds in predicting the behavior of clay samples because it allows the structural charge to affect the potential at the location of protonating sites. Consider equation 4.12. Using the logarithm and rearranging it results in... 

The clay-water interface is represented in Figure 4.14. The structural charge density... 

Finally, clays such as the smectites almost invariably have a net negative structural charge because of isomorphous substitution of cations of lower charge than would be present in a balanced structure. In kaolinite, the amphoteric nature of the hydrated aluminum and silica surface contributes more to surface charge than does substitution. As a result of either substitution or surface dissociation, a region of counter ions (exchangeable and... 

Figure 2.16. Common groups of layer silicate clay structures found in soils, pictured terms of their tetrahedral (iHk) and octahedral ( ) sheets. The usual locations of - /uctural charge and exchange cations are indicated by — and + signs.

Unlike layer silicate clays, the oxides of Fe and A1 are not inclined to develop structural charge as a result of isomorphous substitution. Consequently they have very low cation exchange capacities despite sometimes possessing impressively large surface areas. The surfaces do, however, develop limited charge (negative or positive) in response to the pH of the surrounding solution, and this process will be discussed... 

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