Solubility of Amorphous Silica

A given sample of amorphous silica exhibits a reproducible equilibrium solubility in water. However, reported solubility values for amorphous silicas range from 70 to more than 150 ppm at 25 C. Such variations are apparently due to differences in particle size, state of internal hydration, and the presence of traces of impurities in the silica or absorbed on its surface during the measurements. 

As a basis of comparison, it is of interest to note the solubility data for silica of which the particles are large enough that size has no effect. 

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Vitreous silica has the same solubility as other amorphous silica. Because of the small specific surface area of powdered silica glass in comparison with that of microamorphous or colloidal silicas, workers found it difficult to establish solubility equilibrium. Stober (144) found that at pH 8.4 in Ringer s solution (0.9% NaCI, 0.1% NaHCOj) at 25 C, at least 15 days was required to reach equilibrium when 20 m of silica surface was exposed per liter, regardless of particle size Without the use of this solution, which has an optimum catalytic effect, it would probably have been impossible to establish equilibrium. The solubility was found to be about 100 ppm. 

A highly porous, vitreous silica which equilibrated much more rapidly was used by Elmer and Nordberg (153) in a study- of solubility in nitric acid solutions. Their values for high dilution at pH 3 were 160 ppm at 36 C, 260 ppm at 65 C, and 400 ppm at 95 C. They found the solubility was identical to that of a commercial dried silica gel, provided care was taken not to abrade the gel by disturbing it during the test. 

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Chemical methods to determine the crystalline content in silica have been reviewed (6). These are based on the solubility of amorphous silica in a variety of solvents, acids or bases, with respect to relatively inert crystalline silica, and include differences in reactivity in high temperature fusions with strong bases. These methods ate qualitative, however, and fail to satisfy regulatory requirements to determine crystallinity at 0.1% concentration in bulk materials. 

Hydrated amorphous silica dissolves more rapidly than does the anhydrous amorphous silica. The solubility in neutral dilute aqueous salt solutions is only slighdy less than in pure water. The presence of dissolved salts increases the rate of dissolution in neutral solution. Trace amounts of impurities, especially aluminum or iron (24,25), cause a decrease in solubility. Acid cleaning of impure silica to remove metal ions increases its solubility. The dissolution of amorphous silica is significantly accelerated by hydroxyl ion at high pH values and by hydrofluoric acid at low pH values (1). Dissolution follows first-order kinetic behavior and is dependent on the equilibria shown in equations 2 and 3. Below a pH value of 9, the solubility of amorphous silica is independent of pH. Above pH 9, the solubility of amorphous silica increases because of increased ionization of monosilicic acid. 

Her also noted that in dilute solutions colloidal metal silicates precipitate at a pH slightly below that at which the metal hydroxide alone would be precipitated. The strong tendency for some hydroxides to react with silica is demonstrated by the fact that the addition of 300 p.p.m. of Mg (OH)2 to water will reduce the soluble silica content from 42 to 0.1 p.p.m. Aluminum oxide is capable of reducing the solubility of silica from 170 p.p.m. to 20 p.p.m. Cations capable of forming insoluble silicates will reduce the solubility of amorphous silica. Al3+ at a concentration of 100 p.p.m. reduces the solubility of silica from 120 p.p.m. to 1 p.p.m. at pH 8—9 (Okamoto et al., 1957). 

An estimate of k can be made from the solubility of amorphous silica at pH 7, that is 2x10 3mol. I"1 ( 1 9), according to... 

The rate of ciystallization of quartz is so slow in the low-temperature range that the solubility of amorphous silica represents the upper limit of dissolved aqueous silica (see Tableau 7.2). 

It is shown that the solubility of siliceous rocks was 18.6 times higher than the solubility of quartz but 2.3 times lower than the solubility of amorphous silica. The soluble form of silica, monosilicic acid, is unstable. First the concentration of silica in solution increases and reaches a maximum then it decreases because of processes of sorption and polycondensation of soluble forms. 

The solubility of amorphous silica is roughly 100 ppm at ambient temperature and neutral pH, although its solubility in pure water depends very much on the structure of the surface and the particle size of silica. The solubility increases exponentially by temperature increase, for instance to about 1000 ppm at 100°C. It also increases drastically by increasing pH as shown in the solubility-pH relation in Fig. 1. This behaviour is used for the hydrothermal reaction of silica and related compounds in industry or in the geothermal reaction in nature. 

Figure 1. The solubility of amorphous silica against pH at 303 K. The full and dotted lines show the original and alkali-treated materials, respectively.

This means that soil solutions in equilibrium with amorphous silica are oversaturated with respect to quartz. Nevertheless, because quartz crystallizes extremely slowly at ambient temperatures, the solubility of amorphous silica effectively sets the upper limit of dissolved silica in natural waters. 

In soils, aluminosilicate clays precipitate from siliceous solutions if alumina is available the solubility of these clays in terms of dissolved SilOH) is less than the solubility of amorphous silica. 

Figure 5 shows that the solubility of amorphous silica is independent of pH between 4 and 9 above pH 9 the solubility increases because of the formation of monosilicate, disilicate, and multimer ions. DS coatings are deposited at 90 °C—equilibrium constants are not available at this temperature. Silica solubility data are available (24), and hence it is... 

Figure 7,6 shows the solubilities of quartz and amorphous silica in relation to the minerals of Fig. 7.5. The solubilities of substances having the empirical formula SiC>2 or SiC>2 /1H2O have been studied for decades. These studies are much more complicated than they appear, because of the reluctance of soluble silica to reach equilibrium or even metastable equilibrium with its solid phases. Soluble silica tends to polymerize slowly in supersaturated solutions rather than to precipitate cleanly. In addition, the solid phase that precipitates is often amorphous silica instead of the most stable phase, quartz or its close relative chert. Amorphous silica is metastable and much more soluble than quartz. The solubilities of amorphous silica and quartz are often assumed to be the upper and lower limits of silica solubility in soils. Viewed from the range of soil solution compositions shown in Fig. 7.6, silica concentrations caii be less than the equilibrium solubility of quartz even though quartz is almost always present in the sand fraction of soils. The slow kinetics of silica reactions and the slow release of Si(OH)4 during weathering create wide deviations from equilibrium.

Fournier RO, Rowe JJ (1977) The solubility of amorphous silica in water at high temperatures and high pressures. Am Miner 62 1052-1056... 

Morey GW, Fournier RO, Rowe II (1967) The solubility of amorphous silica at 25 °C. Jour. Geophysical Research 69(io) i995-20o2... 

Figure L Behavior of silicate solutions at different pH values and concentrations. Solid line represents solubility of amorphous silica.

Attempts to determine the solubility of amorphous silica in salt water solutions at near neutral pH and 0 to 5 C or 22 to 25 C have yielded a wide range of values9 which results in part from aging of the silica surface in contact with solution This makes determination of an initial solubility for silica difficult. Low temperature aging in salt water solutions or seawater causes a decrease in surface area, in specific pore volxime, and in solubility. Solubilities determined at pressures to 1000 atmospheres (lx 10 pascals) indicate that aging causes an increase in density of the surface silica this data also allows calculation of the partial molal voltime of dissolved silica. Identification of specific processes involved with aging of an amorphous surface are necessary for understanding silica solubility. 

The solubility of amorphous silica in salt water solutions at 0 to 25 C has been the subject of much study in recent years, and it is interesting that determination of such an apparently simple nxunber can yield such a wide range of results (Table I). As an illustration of the problem, Willey ( 1) showed a plot of the solubility of amorphous silica in seawater at 0 C and at pressures from 1 to 1000 atmospheres pressure A later study using the same experimental apparatus ( ) reproduced the same plot. However, two months later during the next experiment with the same equipment and the same silica, a solubility decrease occurred at all pressures, and the pressure dependence became slightly different than in both previous studies. This aging effect caused a solubility decrease of approximately 20%. Two other pressure studies (3, , which used different experimental... 

Figure L Solubility of amorphous silica in salt water solution or seawater at...

Several other possible explanations in addition to short term aging were considered to explain the variation in published data (Figure 1) for the solubility of amorphous silica at elevated pressures ... 

The solubility of amorphous silica in salt water solutions (at 0-3 C or 19-26 C, and over the pressure range from 1... 

The solubility of amorphous silica in seawater or in salt water solutions similar to seawater is not affected by the extent of hydration of the solid phase, and is not limited by sepiolite formation. 

Silicon permanently enters into living organisms with food and water the solubility of amorphous silica in water attains lOOmg/1 [3]. Silicon also enters into... 

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