Silica in solution

The boiler water pH must be raised to about 9 or over to avoid corrosion, to maintain silica in solution, and to reduce the release of CO2 into the steam. 

As a second example, we choose quartz (or any silica polymorph) as a component for a system containing an aqueous fluid and quartz. Now the mole number for the quartz component includes not only the silica in the quartz mineral, the real quartz, but the silica in solution in species such as SiC>2(aq) and IGSiO. Again, the mole numbers of component quartz and real quartz are not the same. A common mistake in geochemical modeling is confusing the components used to describe the composition of a system with the species and phases that are actually present. 

A further step in the "activation" of the chemical species present might result in the active migration of silica in solution. Then a diagram such as that of Figure 46c would be applicable. In this system the pH is still controlled by the mineral species present, all of which are stable at some a-alkali, a-Si values. A further intensive variable can be conceived as being due to the dissociation of H O and considering the activity of the H+.ion in solution. This is the most commonly used type of representation, as mentioned previously. Here only one-phase... 

It is clearly established that, in these solutions, the silicon is in tetrahedral coordination (13). The simplest form of silica in solution is the monosilicic acid Si(0H)4. Polymerization results from the condensation of two silanol groups with the elimination of a water molecule. A whole series of silicate anions, of various polymerization and ionization degrees, are thus connected through dynamic equilibria. The equilibria are governed by the normal chemical parameters, namely the silica concentration, the pH and the cation type. 

The contents of total (Si02cou + Si02,on) and ionic silica in solution were determined after three months. The results obtained are presented in a graph (Fig. 50) and can be discussed briefly as follows ... 

Fig. 52. Coagulation of mixed colloidal solutions of iron and silica. Environment A—slightly acid B—slightly alkaline. Concentration of recent waters indicated at right. / = total precipitation of iron and silica with formation of banded sediments // = same, with formation of mixed sediments /// = total precipitation of iron and partial of silica / F= total precipitation of iron, silica in solution V = incomplete precipitation of iron, silica in solution VI— no precipitation observed.

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. 

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 4.16. Rate of dissolution of Vycor 7900 at 80 °C versus total silica in solution. After Hooley. ... 

Silica precipitation is controlled by the concentration of silica in solution, itself related to silica availability and, to a lesser extent, the duration of wetting/drying cycles (Knauth, 1994). The most soluble silica species will be precipitated first from any supersaturated solution (Millot, 1960, 1970). This is usually amorphous silica, which is both poorly ordered and has a higher solubility under neutral conditions (60-130ppm) compared with opal-C (20-30ppm) and quartz (6-10ppm) (Williams et al., 1985). Silica monomers polymerise and aggregate to form colloids from solutions, and these colloids may precipitate to form opal-A (Iler, 1979 Williams et al., 1985). If solutions are supersaturated with respect to quartz then... 

Although the solubilities of quartz, its polymorphs, and amorphous silica are fixed and pH-in-dependent in most natural waters, the dissociation of silicic acid at alkaline pH s leads to substantial increases in their solubilities above pH 9 to 10. The following calculations show how we can predict this effect. First, the solubility of any silica solid must equal the sum of the concentrations of all species of silica in solution at equilibrium. This summation is given by the mass-balance equation... 

Jones, L. H. P., and K. A. Handreck. 1963. Effect of iron and aluminum oxides on silica in solution in soils. Nature 198 852-53. 

Eremenko, B.V., Surface charge of silica in solutions of polyoxyethylene. Point of zero charge and surface potentials of silica in aqueous solutions of polyoxyethylene. Colloid J. USSR, 47, 892, 1985. 

Soil-leaching studies indicate that some silica is released from soil rather rapidly. McKeague and Cline (20) have shown that in soil—water mixtures at 100% water saturation, the silica in solution after 5 minutes was approximately half as great as that after 10 days. After the first day or two the silica concentration increased very slowly. They also demonstrated that pH has a marked effect on silica concentrations in soil solutions (21). These authors attributed the control of silica concentration to pH-dependent adsorption and indicated that, of the common soil minerals, iron and aluminum oxides have appreciable adsorption capacity. Jones and Handreck (22, 23) studied the effects of iron and aluminum oxides on silica concentrations in soil solutions and concluded that both caused a significant reduction in dissolved silica, with aluminum oxides being most effective. Minimum silica concentrations occurred at pH 9-10 in solutions in contact with iron and aluminum oxides. Harder and Flehmig (24) reported that the hydroxides of iron, aluminum, and other elements could remove silica from solutions containing as little as 0.5 mg/liter SiOo. 

Separate splits of the 1966 sample of suspended sediment were stirred with deionized water at concentrations of 2000 and 14,000 mg/liter for 31 hours, and the water was then removed and analyzed. After 31 hours, the silica in solution corresponded to 0.63, 1.80 (from extrapolation, Figure 16), and 2.84 mg/liter for the 2000, 6000, and 14,000 mg/liter suspensions, respectively. Silica in solution per 1000 mg/liter suspended sediment was 0.31, 0.30, and 0.20 for the 2000, 6000, and 14,000 mg/liter suspensions, respectively. Above about 6000 mg/liter, therefore, a linear relation between sediment concentration and silica released may not exist. This is not surprising if there is a back reaction owing to uptake of silica on mineral surfaces, as suggested by Wollast (17) and others. 

In considering the probable effect of suspended sediment on silica in solution, one must take into account not only the concentration of the... 

We present some dissolved silica data here for waters from various environments in northern Algeria subsurface waters 5-10 ppm, thermal waters 5-20 ppm, shallow nearshore water 1-2 ppm, fluvial surface water 11-12 ppm. Meteoric waters with 10-15 ppm dissolved silica are known to circulate to depths of many hundred of meters and therefore could present a cement source (Blatt 1979). Although surface water contains some silica in solution which might promote quartz cementation at shallow depth, it is clear that this silica content is too little to account for the amount of authigenic quartz present in Saharan oil-field sandstones at depths of several thousand meters. In the Hassi Messaoud oil field formation water from the Cambro-Ordo-vician reservoirs contains about 150 ppm dissolved silica, formation water from the Albian reservoirs in the same field contains 15 ppm, while in the Berkaoui oil field Albian formation water contains 10 ppm, Triassic water 3.6 ppm and Hassi R Mel (gas-condensate) Triassic formation water contains 5-7 ppm silica. It is obvious therefore that the 150 ppm of dissolved silica found today in Cambro-Ordovician reservoirs could... 

In alkali metal aqueous solutions containing more than 2.5 mols of Si02 per mole of M2O, it is found by ultralil-tration, according to a procedure referred to herein as the Gore procedure, that at the concentrations used in this invention part of the silica in solution is ionic and part of it is colloidal, the colloidal fraction being retained by the ultrafilter while the ionic silicate passes through. 

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