Aluminosilicate-solution equilibria

Equilibrium Data. The only laboratory studies of aluminosilicate-solution equilibria in which both solid phases and aqueous solutions have been well defined seem to be those of Hemley, who has studied the K system (11) and the Na system (12) and discussed the mixed Na-K system (13). To obtain reasonable equilibration times with well-defined phases, it was necessary to work at temperatures higher than 150°C. 

Mechanisms of Sorption Processes. Kinetic studies are valuable for hypothesizing mechanisms of reactions in homogeneous solution, but the interpretation of kinetic data for sorption processes is more difficult. Recently it has been shown that the mechanisms of very fast adsorption reactions may be interpreted from the results of chemical relaxation studies (25-27). Yasunaga and Ikeda (Chapter 12) summarize recent studies that have utilized relaxation techniques to examine the adsorption of cations and anions on hydrous oxide and aluminosilicate surfaces. Hayes and Leckie (Chapter 7) present new interpretations for the mechanism of lead ion adsorption by goethite. In both papers it is concluded that the kinetic and equilibrium adsorption data are consistent with the rate relationships derived from an interfacial model in which metal ions are located nearer to the surface than adsorbed counterions. 

Fig. 2. Logarithmic activity diagram depicting equilibrium phase relations among aluminosilicates and sea water in an idealized nine-component model of tire ocean system at the noted temperatures, one atmosphere total pressure, and unit activity of H20. The shaded area represents (lie composition range of sea water at the specified temperature, and the dot-dash lines indicate the composition of sea water saturated with quartz, amotphous silica, and sepiolite, respectively. The scale to the left of the diagram refers to calcite saturation foi different fugacities of CO2. The dashed contours designate the composition (in % illite) of a mixed-layer illitcmontmorillonitc solid solution phase in equilibrium with sea water (from Helgesun, H, C. and Mackenzie, F T.. 1970. Silicate-sea water equilibria in the ocean system Deep Sea Res.).

The primary minerals of igneous rocks are all mildly basic compounds. When they react in excess with acids such as HC1 and CO2, they produce neutral or mildly alkaline solutions plus a set of altered aluminosilicate and carbonate reaction products. It is improbable that ocean water has changed through time from a solution approximately in equilibrium with these reaction products, which are clay minerals and carbonates. 

When crystallization of aluminosilica gels is fully completed, the solution appears to be in contact not with amorphous but only with crystalline aluminosilicate. Therefore, other equilibrium concentrations should be established in the mother liquor. The data (37) show that the concentration of aluminate ions in the mother liquor is always lower than that in the liquid phase of gels, whereas the concentration of silicate ions is either higher or lower than that in the gel liquid phase. However, the decrease in the product of concentrations of the silicate and aluminate ions in mother liquors is a general regularity, as visibly evidenced by the data of Table IV obtained at 20°C. 

Concentrations of silica of around 2 ppm were reached in dilute salt solution with mica and kaolin and up to 15 ppm with montmorillonite (36). When seawater was enriched with soluble silica to 25 ppm SiOa, it remained at that level for a year in the absence of these minerals, but when the latter were then added, the silica was removed from solution down to the 2-15 ppm level that was reached when the minerals alone were added. Since many ocean waters contain 2-10 ppm SiOj, it is possible that this value is reached as the equilibrium solubility of colloidal aluminosilicate in suspension. The above experiment is consistent with the fact that in pure water, pure a morphous silica dissolves to give a concentration of monosilicic acid of 100-110 ppm, but in the presence of polyvalent metal cations such as iron, aluminum, and other metals, colloidal silicates are formed with a much lower solubility with respect to monosilicic acid. Her (37) has shown that soluble aluminum reduces the solubility of amorphous silica from about 110 to less than 10 ppm. 

Baumann (68b) found that when different amounts of aluminum ion were added to a solution of monomer (420 ppm SiOj), more silica remained in the molybdate reactive state than when no aluminum was present. With no aluminum present, after 4 days there remained 130 ppm of molybdate-reactive silica as monomer in equilibrium with 290 ppm of relatively inactive high polymer. But when aluminum was present in the Al Si atomic ratio of 1 7, there remained about 200 ppm of molybdate-reactive silica. It can be interpreted that the alumina had combined with silica to form an aluminosilicate that later was decomposed by the strongly acidic molybdate reagent liberating additional active silica that appeared as monomer. 

Bovine albumin was adsorbed on quartz and amorphous silica powders at a maximum around pH 6 and was in equilibrium with that adsorbed (252). However, Her (253) found that the pH, below which enough albumin was adsorbed to cause flocculation of colloidal silica, depended on the salt concentration. With no NaCl present it was pH 4.4, but in 0.1 N solution was 6.2. It was also noted that when the surface of the silica was only 5% covered with aluminosilicate anions, no coagulation occurred in 0.1 N NaCl solution above pH 5.5. Thus colloidal aluminosilicates (clays) which bear a higher anionic charge than pure silica does in neutral solution, are less reactive with some proteins... . ... 

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