Soluble Silica—Monosilicic Acid

The soluble form of silica is monomeric, containing only one silicon atom and generally formulated as Si(OH)4. This is often called monosilicic acid or orthosilicic acid. The state of hydration is not known, although at high pressure there is some indication that one water molecule is linked to each OH group, probably by hydrogen bonding, so the hydrated molecule is represented by Willey (20) as Si(OH OH, 4. 

The structure of monosilicic acid is assumed to involve silicon coordinated with four oxygen atoms as in amorphous vitreous silica and in crystalline quartz. Although there are rare minerals such as the stishovite form of SiOj (21) or thau-masite (22), in which silicon is coordinated with six o.xygen atoms, silicon in most oxides and silicates is surrounded by only four oxygen atoms. If the monomer had the structure HjSKOH), one would expect it to be a strong acid like the analogous HaSiF, but in fact it is a very weak acid. 

It is essentially nonionic in neutral and weakly acidic solution and is not transported by electric current unless ionized in alkaline solution.,It is not salted out of water nor can it be extracted by neutral organic solvents. 

It remains in the monomeric state for long periods in water at 25 C. as long as the concentration is less than about 2 x 10M. but polymerizes, usually rapidly, at higher concentrations, initially forming polysilicic acids of low molecular weight and then larger polymeric species recognizable as colloidal particles. 

The question often arises as to whether the term soluble silica should include the low polymers such as tetramer or deciimer. which are classed as oligomers. It becomes a matter of definition. Soluble materials have been recognized as those that pass through a dialysis membrane, whereas colloids do not but even though membranes can now be made with pores sufficiently small to separate dextrose from sucrose, we think of sucrose as being soluble. On the other hand, sucro.se is certainly not colloidal. 

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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. 

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. 

Vitreous silica does not react significantly with water under ambient conditions. The solution process involves the formation of monosilicic acid, Si(OH)4. Solubility is fairly constant at low pH but increases rapidly when the pH exceeds 9 (84—86). Above a pH of 10.7 silica dissolves mainly as soluble silicates. Solubility also increases with higher temperatures and pressures. At 200—400°C and 1—30 MPa (<10"300 atm), for example, the solubility, S, of Si02 in g/kg H20 can be expressed as follows, where dis the density of the vapor phase and Tis the absolute temperature in Kelvin. 

Richardson and Waddams (31) found the same behavior for silica. Ground quartz particles agitated with water release some monosilicic acid molecules which form a true solution and, in addition, some very small quartz crystals which form a dispersion in water until a hydrated equilibrium surface is established. Analogous to the ferric oxide, heating the quartz to a temperature above 600° C. regenerates the anomalous solubility behavior. 

We determined that vicinal dihydroxy organic reagents stabilize the soluble forms of silica. The stability of monosilicic acid solution is determined by the structure of the stabilizer ethylene glycol and humic acids do not stabilize soluble forms of silica enough, but glycerin and catechol stabilize the silica solution when they are added at 5 - 7% to the solution. This fact is connected with the formation of hydrogen bonds and stable penta- and hexacoordinated compounds, preventing the processes of polycondensation of silica in solution. 

Figure 4.8 (A) Variations in silica solubility with pH (after Williams and Crerar, 1985 Dove and Rimstidt, 1994). (B) The release and sorption of monosilicic acid by a black earth soil under varying pH (after Beckwith...

Beckwith, R.S. Reeve, R. (1964) Studies on soluble silica in soils. II. The release of monosilicic acid from soils. Australian Journal of Soil Research 2, 33-45. 

A) Variations in silica solubility with pH. (B) The release and sorption of monosilicic acid by a black... 

In alkaline solutions, silica exists in the form of silicate ions e.g., Si03 ). In dilute solutions (up to 0.1 mg of Si per ml) between pH 1 and 8, water-soluble monomeric silicic acid is the stable form. In more concentrated solutions of the same acidity, monosilicic acid condenses to disilicic acid and polysilicic acids which can be transformed into colloidal species. 

Silica is converted into monosilicic acid by fusing with sodium carbonate and acidifying the alkaline solution produced when the melt is dissolved in water. Alternatively, an acidic sample solution may be made alkaline with sodium hydroxide and heated to convert colloidal silica into silicate. Soluble monosilicic acid is formed after appropriate dilution and acidification. 

On heating with dilute hydrofluoric acid, silica is transformed into the soluble hexafluorosilicic acid, H2Sip6. Aluminium chloride or boric acid introduced subsequently mask the excess of HF and decompose H2S1F6 to monosilicic acid (the more stable AlFe or BF4 complexes being produced). 

The solubility of silica can be characterized by the following equilibria at 25 C. Monosilicic acid has been written H2Si02(0H)2, rather than Si(OH) or H SiO in order to emphasize its diabasic character, and the tendency of silicon, like other metalloids, to coordinate with hydroxo and oxo ligands. 

Stumm(52) used these equilibria to construct the diagram in Figure 3 which describes the speciation of silica in aqueous solution. His data indicate that at normal environmental pH values (pH 9) dissolved silica exists exclusively as mono-silicic acid. This conclusion is supported by the finding that soluble silica has a diffusion coefficient of 0.53 indicating a molecular size about equivalent to monosilicic acid(53). 

Fig. 14 shows the elution curves for silicic acid in solutions of pH 9.5 on a Sephadex G-lOO column. The peaks on the right are due to monosilicic acid and those on the left to polysilicic acid. The elution curves for the polysilicic acids indicate a symmetrical distribution of particle sizes, and the elution volume of the polymers obtained after standing the sample solution for 250 h is almost the same as that obtained after 100 h. This suggests that when the concentration of monosilicic acid is close to the solubility of amorphous silica, the growth rate of the particles of the polymers becomes very low. It was thus assumed that the growth of the particles is mainly due to the polymerization between monomer and polymer, and the polymerization between polymer species... 

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. 

Soluble silica is found in essentially all plants and animals. For e. ample, human blood contains I ppm. Ingested monosilicic acid as an undersaturated solution rapidly penetrates all tissues and body fluids and is excreted apparently without any effect (48). Plants, especially grasses, including the grains and rice, take up silica and deposit it in the tissues as characteristic microscopic amorphous opaline particles, which are later found in the soil and in the intestinal tracts of grazing animals (49). The widespread occurrence and possible role of silica in living systems are more fully discussed in Chapter 7. 

Stober investigated the solubility behavior of several modifeations of silica and developed a theory and equation for the behavior of adsorbed monosilicic acid in retarding or preventing approach to true solubility equilibrium (144). 

Although monosilicic acid, Si(OH)4, is believed to be the material actually deposited, it is possible to use active silica in the form of low molecular weight polysilicic acids (including extremely small colloidal particles) as a source of silica. Such small particles are highly soluble and are in equilibrium with a concentration of Si(OH>4 that is highly supersaturated with respect to larger particles or a flat surface. 

The chemistry of silica in aqueous systems (e.g., sodium silicate) is discussed in detail by Her (10). Silicon is hydrolyzed even in dilute acid and as shown in Fig. 5.4, silicic acid [Si(OH)4], often referred to as monosilicic acid, orthosilicic acid, or soluble silica, is the dominant mononuclear species in solution below pH values of 7 (11). The Si-OH group is called a silanol group, indicating that Si(OH)4 contains four silanol groups. Above pH 7, further hydrolysis produces anionic species ... 

There is some evidence to suggest that monosilicic acid is the principal form of silica in the soil solution (Alexander et al. [1954]), and McKeague and Cline [1963] argue that since the amounts of soluble silica recorded in the soil solution are considerably less than the solubihty of amorphous silica, the stability of silica sol in soils is highly suspect. The heat of... 

Sorption reactions of silica in soils were postulated many years ago (Sreenivasan [1935]). In the early work, however, somewhat high concentrations of sodium and potassium silicates were used, and such systems would be subject to hydrolysis and polymerization reactions and also to pH changes. Thus, in recent studies on the sorption of soluble silica by soils (Eliassaf [1962] Beckwith and Reeve [1963] McKeague and Cline [1963]), dilute solutions (100 to 135 ppm) of monomeric silicic acid have been employed, and results indicate that the residual concentration of monosilicic acid is controlled by an adsorption equilibrium which is pH dependent. Sesquioxides make a considerable contribution to the capacity of soils to sorb soluble silica (Nejegebauer [1958] Beckwith and Reeve [1963]), and the apparent increase in solubility of silica in soil suspensions with increased acidity has been discussed in terms of... 

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