Dissolution of amorphous silica

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. 

Figure 2. Normalized rate of dissolution of amorphous silica gel in alkali metal hydroxides as determined from the initial, integrated peak area of dissolved species (5 wt % silica suspensions M20 3SiC>2 I8OH2O). Other experimental details are given in the caption of Figure 1.

The reaction of interest is the dissolution of amorphous silica in aqueous solution in the temperature range from 0 to 25 C, the pressure range from one to 1000 atmospheres, and the pH range from 6 to 8.5 In this pH region, dissolved silica occurs primarily as undissociated silicic acid O) above this pH range, the solubility increases due to the Increased dissociation of silicic acid. The dissolution can be written several ways, depending upon how water is indicated, as follows ... 

Pressure is of interest as a variable both because it is a factor in many geochemical reactions and because information about a pressure dependency allows calculation of the volume change for a reaction. The relevant equations are given below. Equation 4 is the fundamental relationship for pressure dependency from thermodynamics Equation 5 is this equation as solved by Owen and Brinkley (O Equation 6 is a useful rearrangement of Equation 5 and Equation 7 defines the volume change for this dissolution of amorphous silica as written in Equation 3. 

In these equations, K is the equilibrium constant (for the dissolution of amorphous silica the solubility an be used in place of K (2))> P is pressure in atmospheres, AV is the change in voltime caused by the reaction, R is the universal gas constant,... 

In a study [6] of the dissolution of amorphous silica gels in aqueous alkali metal hydroxides, the rate of dissolution was found to depend on the cation used in the dissolution reaction. A maximum in dissolution rate was found for potassium hydroxide solutions, whereas both intrinsically smaller and larger cations (lithium-sodium and rubidium-cesium) showed slower dissolution rates, as can be concluded from the concentration of dissolved silicate species (normalized peak areas) as a function of alkali metal cation (Figure 45.2). This result is contradictory to the expectation that a monotonic increase or decrease in dissolution rate is to be observed for the different cations used. One major effect that occurs at the high pH values of this study is that the majority of silanol... 

Figure 18-3. Dissolution of amorphous silica gel in aqueous tetramethylammonium hydroxide (TMA2O 3Si02 IO8H2O) after a) 83 b) 250 c) 833 d) 1083 e) 1250 f)...

In determining the rate of dissolution of amorphous silica powders, the possible existence of a porous, rapidly soluble layer must be considered. Yates and Healy (212a) have shown that BDH-precipitated silica, widely used as a standard for study, has a surface layer that is impermeable to nitrogen but permeable to alkali. This gel layer dissolves more rapidly than the remainder of the silica and must be taken into account when the rate of dissolution is being measured. 

The high surface area and rate of dissolution of amorphous silica permits reactions at much lower temperatures than required with pulverized crystalline silica. The high chemical reactivity of colloidal silicas thas been reviewed in Chapter 4. Transparent fused silica can be formed at 2000 psi and 1200 C from silica powder of 15 nm ultimate particles, whereas 2000 C is required for blown or cast material (655). 

According to Her [1], the dissolution of amorphous silica above pH 2 is catalyzed by OH ions that are able to increase the coordination of silicon above four weakening the surrounding siloxane bonds to the network. This general nucleophilic mechanism could presumably occur via Sjv2-Si, S/ 2 Si, or S/ 2 -Si transition states or intermediates and could equally well explain alkoxide ion- or fluorine ion-catalyzed depolymerization mechanisms. 

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