Solubility amorphous silica

Fig. 13. Quartz ai>d amorphous silica solubility vs. temperature along the vapour saturation curve. The dashed lines show the silica concentration in water initially in equilibrium with quartz during adiabatic boiling to 100 C and subsequent cooling. The increase in aqueous silica concentrations during boiling is the consequence of steam formation. Amorphous silica saturation (shown by the dots) is attained at 188 C in the case of the 300 C aquifer water, but at 94 C in the case of the 200 C aquircr water. It was assumed that the pH of the water is not raised sufficiently during boiling to cause significant ionization of the aqueous silica. If some ionization had occurred, amorphous silica saturation would be reached at lower temperatures than those indicated in Fig. 13.

At this point (the same pH as that of Wollast, t al., 1968) sepiolite begins to precipitate. In experiments maintaining pH at values above 9, montmorillonoids and talc were formed. Chemical analysis of the precipitates reveal a greater proportion of magnesium as the pH of the experiment is increased. Recalling the information on amorphous silica solubility, a two-fold increase in solubility of SiC>2 occurs between pH 8 and 10.5 (Krauskopf, 1959)—and thus at higher pH it could be expected that relatively less silicious phases would precipitate where the masses of Mg and Si are fixed. Final concentrations of Mg-Si in solution were not determined by Siffert and therefore thermodynamic calculations of mineral stabilities cannot be made. 

The synthesis of MCM-41 was extended into very wide reaction conditions and various reactants. The silicon resource may be either organic silicon compounds (e.g., TEOS, TMOS, TBOS) or inorganic compounds (such as amorphous silica, soluble silicate). Synthesis temperature can be from lower than room temperature to high temperature 150 °C). Reaction time may vary from several minutes to a few weeks. The synthesis media can be from very basic to near neutral. The long-chain quaternary ammonium (C TMA) surfactant is the best template. 

MarshAt.i, W. L., and C. A. Chen. 1982. Amorphous silica solubilities. V. Prediction of solubility behavior in aqueous mixed electrolyte solutions to 300°C. Geochim. Cos-mochim. Acta 46 289-91. 

This solubility change makes amorphous silica solubility difficult to determine, and contributes to the scatter in published solubility values. 

Figure 2.45 Quartz and amorphous silica solubility vs. temperature (Krauskopf and Bird D. K., 1995).

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. 

Microscopic sheets of amorphous silica have been prepared in the laboratory by either (/) hydrolysis of gaseous SiCl or SiF to form monosilicic acid [10193-36-9] (orthosihcic acid), Si(OH)4, with simultaneous polymerisation in water of the monosilicic acid that is formed (7) (2) freesing of colloidal silica or polysilicic acid (8—10) (J) hydrolysis of HSiCl in ether, followed by solvent evaporation (11) or (4) coagulation of silica in the presence of cationic surfactants (12). Amorphous silica fibers are prepared by drying thin films of sols or oxidising silicon monoxide (13). Hydrated amorphous silica differs in solubility from anhydrous or surface-hydrated amorphous sdica forms (1) in that the former is generally stable up to 60°C, and water is not lost by evaporation at room temperature. Hydrated sdica gel can be prepared by reaction of hydrated sodium siUcate crystals and anhydrous acid, followed by polymerisation of the monosilicic acid that is formed into a dense state (14). This process can result in a water content of approximately one molecule of H2O for each sdanol group present. 

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. 

Silica is normally present in most sources of MU water, although not to any significant level as soluble Si02 but rather as both colloidal amorphous silica and silicate salts. In any particular water, such... 

Solubilities of quartz and amorphous silica in aqueous solutions increase with increasing of temperature (Holland and Malinin, 1979). Solubility of barite depends on salinity and temperature (Blount, 1977). The solubility of barite in hydrothermal solution having more than 1 molal NaCl concentration increases with increasing temperature, while a solubility maximum exists in the solution with NaCl concentration less than ca. 0.2 molal (Blount, 1977). 

A conveniently prepared amorphous silica-supported titanium catalyst exhibits activity similar to that of Ti-substituted zeolites in the epoxidation of terminal linear and bulky alkenes such as cyclohexene (22) <00CC855>. An unusual example of copper-catalyzed epoxidation has also been reported, in which olefins are treated with substoichiometric amounts of soluble Cu(II) compounds in methylene chloride, using MCPBA as a terminal oxidant. Yields are variable, but can be quite high. For example, cis-stilbene 24 was epoxidized in 90% yield. In this case, a mixture of cis- and /rans-epoxides was obtained, suggesting a step-wise radical mechanism <00TL1013>. 

Fig. 26.3. Silica concentration (bold lines) in a fluid packet that cools from 300 °C as it flows along a quartz-lined fracture of 10 cm aperture, calculated assuming differing traversal times At. Fine lines show solubilities of the silica polymorphs quartz, cristobalite, and amorphous silica.

The rate of silicate sol and gel formation is pH and water-alcohol-sensitive as is the solubility of the amorphous silica that is formed. Silica networks are based on (Si04) " tetrahedra modified by (O3 Si-O, M+) units and often addition of boron oxide, aluminum oxide, titanium IV oxide, or zirconium IV oxide. 

Geothermal aquifer waters are close to saturation with some scale-forming minerals (calcite, pyrite) but undersaturated with others (amorphous silica, amorphous metallic sulphides). Only the slightest degassing suffices to produce calcite oversaturated water. By contrast, extensive cooling may be required to produce amorphous-silica oversaturation. As solubility constants are... 

Reliable determination of the solubility of silica in water has been complicated by the effects of impurities and of surface layers that may affect attainment of equilibrium. The solubility behavior of silica has been discussed (9,27). Reported values for the solubility of quartz, as Si02, at room temperature are in the range 6—11 ppm. Typical values for massive amorphous silica at room temperature are around 70 ppm for other amorphous silicas, 100—130 ppm. Solubility increases with temperature, approaching a maximum at about 200°C. Solubility appears to be at a minimum at about pH 7 and increases markedly above pH 9 (9). 

Results obtained at high temperatures indicate that the solubilities of the crystalline modifications of silica are in the order tridymite > cristobalite > quartz, an order that parallels to some extent the chemical reactivity of these forms. Lower values for solubility of crystalline as compared to amorphous silica are consistent with the free-energy differences between them. 

Silica dissolves in water at high temperatures and pressures. For amorphous silica up to 200°C, the solubility in liquid water is given as follows (28) ... 

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