Kaolinite solubility

Figure 2.53 Kaolinite solubility vs. pH of water. Solid curve is total solubility, straight lines are concentrations of individual compounds in solution.

Dang et al. (1994) observed that the experimentally determined solubility lines for Zn2+ in 14 soil solutions from southern Queensland with soil pH from 7.45-8.98 and 0.08-2.07% CaC03 were not undersaturated with respect to the solubility of any known mineral form of Zn. Therefore, they suggested that Zn2+ activity was mainly controlled by adsorption-desorption reactions in these soils. Similar observation on solubility of Cr(VI) in arid soils was reported by Rai et al. (1989). In the absence of a solubility controlling solid phase, Cr(VI) aqueous concentrations under slightly alkaline conditions may be primarily controlled by adsorption/desorption reactions (Rai et al., 1989). Chromuim(VI) is adsorbed by iron and aluminum oxides, and kaolinite and its adsorption decreases with increasing pH. 

Fig. 12.1. Solubility diagram for aluminum species in aqueous solution as a function of pH at 200 °C in the presence of boehmite (solid lines) and kaolinite plus quartz (dashed lines). Aluminum is soluble at a specific activity (horizontal line) either under acidic conditions as species Al(OH), A10H++, or A1+++ (e.g., point A), or under alkaline conditions as Al(OH)4 (point B).

In acids soils, particularly those with kaolinite clay minerals, soluble Fe + concentrations tend to rise to high levels because of low CEC and because conditions do not favour precipitation of Fe(II) oxides or carbonates or synthesis of silicates. 

No interaction between ferrihydrite and kaolinite was found at pH 9 because both compounds are negatively charged at this pH (Fig. 16.20b, c). Boiling kaolinite and montmorillonite in a Fe(N03)3 solution for 8 min resulted in clays containing up to ca. 55 mg oxalate soluble Fe/g clay. The BET surface area of kaolinite increased from 18 to 34 m /g and that of montmorillonite from 11 to 62 m g . Whereas kaolinite shows only a small decrease in > 10 pm pores, montmorillonite lost about half of its >10 pm pores even with the lowest Fe oxide content (6.6 mg ECq g clay). It has been speculated that in contrast to kaolinite, the Fe oxide, in the presence of montmorillonite, remained highly disorderd and active due to A1 and Si dissolved from... 

Eordham, A.W. (1970) Sorption and precipitation of iron on kaolinite. III. The solubility of iron(III) hydroxides predpitated in the presence of kaolinite. Aust. J. Soil Res. 8 107— 122... 

Although gibbsite and kaolinite are important in quantity in some soils and hydrothermal deposits, they have diminishing importance in argillaceous sediments and sedimentary rocks because of their peripheral chemical position. They form the limits of any chemical framework of a clay mineral assemblage and thus rarely become functionally involved in critical clay mineral reactions. This is especially true of systems where most chemical components are inert or extensive variables of the system. More important or characteristic relations will be observed in minerals with more chemical variability which respond readily to minor changes in the thermodynamic parameters of the system in which they are found. However, as the number of chemical components which are intensive variables (perfectly mobile components) increases the aluminous phases become more important because alumina is poorly soluble in aqueous solution, and becomes the inert component and the only extensive variable. 

Figure 11.6 Sorption isotherms for two kinds of nonionic organic compounds from aqueous solutions to suspended kaolinite (a) slightly monopolar compound, pyrene, showing a linear isotherm up to its solubility (Backhus, 1990), and (b) monopolar compound, 1,3,5-trinitrobenzene, showing a hyperbolic isotherm (Had-erlein et al., 1996).

Alternatively, several workers have shown that not only is the soluble, zero-charged hydrolysis product considerably more surface active than the free (aquo) ion but also a polymeric charged or uncharged hydrolysis product may be formed at the solid-liquid interface at conditions well below saturation or precipitation in solution. Hall (5) has considered the coagulation of kaolinite by aluminum (III) and concluded that surface precipitates related to hydrated aluminum hydroxide control the adsorption-coagulation behavior. Similarly Healy and Jellett (6) have postulated that the polymeric, soluble, uncharged Zn(OH)2 polymer can be nucleated catalytically at ZnO-H20 interfaces and will flocculate the colloidal ZnO via a bridging mechanism. 

Chemical dissolution techniques indicate kaolinite from Cornwall contains 3.1-4.9% of easily soluble Si02 and 1.5-5.9% of easily soluble A1203 (Follett et al., 1965). Most of this material is presumably present as amorphous material. Experiments (by the senior author) with Georgia kaolinite indicate the amount of amorphous material varies as a function of particle size and preparation (Table LX). Amorphous silica and alumina is a common constituent of kaolinite and considerable care must be taken in determining and interpreting the significance of the Si/Al ratio of kaolinites. 

Most kaolinites contain appreciable amounts of MgO (range 0.01—1.0% modal value between 0.2—0.3%). Bundy et al. (1965) found that MgO as well as total iron and soluble iron were directly related to the C.E.C. and suggested that the Mg and Fe were present in montmorillonite which they believe is commonly present, in amounts less than 5%, in kaolinites. This may be true in part, but electron probe studies (Weaver,1968) indicate that some of the MgO is related to the Ti02-Fe203 material and some is present in biotite. 

May H.M., Kinniburgh D.G., Helmke P.A. and Jackson M.L. (1986) Aqueous dissolution, solubilities and thermodynamic stabilities of common aluminosilicate clay minerals Kaolinite and smectites. Geochim. Cosmochim Acta 50, 1667-1677. 

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