Silica dissolution reactions

The observed trend inverse between pH and silica content of the mixtures suggests pH reduction primarily arises from silica dissolution reactions. 

During caustic waterflooding the alkali can be consumed by the dissolution of clays and is lost in this way. The amount lost depends on the kinetics of the particular reaction. Several studies have been performed with kaolinite, using quartz as a yardstick, because the kinetic data are documented in the literature. The initial reaction rate has been found pH independent in the pH range of 11 to 13 [517]. The kinetics of silica dissolution could be quantitatively described in terms of pH, salinity, ion-exchange properties, temperature, and contact time [1549]. 

The Transition State Theory (TST) treatment of reaction kinetics accounts for the fact that the true (microscopic) behavior of a dissolution reaction may involve the production of an intermediate complex as an essential step of the bulk process. Consider, for instance, the dissolution of silica in water. The macroscopic reaction may be written as... 

Ionic Medium. Silica dispersions were freshly prepared for each experiment in solutions buffered with 10"3M HC03"/C02. The amount of species dissolved from the amorphous silica surface during the experiment was negligible because of the small rate of dissolution reactions. The ionic medium in which coagulation and adsorption studies were carried out was kept constant I = 1.0 to 2.0 X 10 3M. The conditions in all agglomeration and adsorption experiments were such that no Al(OH)3 precipitated within the period of observation. 

At present, many studies are ongoing to identify a means of enhancing the carbonation chemistry of magnesium silicates in aqueous systems, using weak acids and additives that will improve silica dissolution, such as citrates, oxalates, and EDTA [105, 111]. In this case, a near-complete recovery and reuse, thereby minimizing the losses of such chemicals, will be essential for viable process economics. Likewise, there is much to improve with regards to the reaction rates and/ or times. 

As an example of these ideas, an activity-ratio diagram for control of Al(III) solubility by secondary minerals in an acidic soil solution will be constructed. The Jackson-Sherman weathering scenario14 indicates that when soil profiles are leached free of silica with fresh water, 2 1 layer-type clay minerals are replaced by 1 1 layer-type clay minerals, and ultimately these are replaced by metal oxyhydroxides. This sequence of clay mineral transformations can be represented by the successive dissolution reactions of smectite, kaolinite, and gibbsite ... 

That nucleation and growth rate are the limiting steps in quartz cementation has no particular imphcations with respect to the ultimate source of the sihca or the mechanism of transport. Potential sources of silica for quartz cementation are numerous (McBride, 1989) and include all documented silicate dissolution reactions in sandstones and shales. 

Figure 2,5 (a) An Arrhenius plot of log k versus I/TXK) for the dissolution rates of various silicate rocks and minerals. The data points and curves for rhyolite, basalt glass, and diabase are from Apps (1983), as is the curve labeled silicates, which Apps computed from the results of Wood and Walther (1983). Curves for the S1O2 polymorphs are based on Rimstidt and Barnes (1980). Modified from Langmuir and Mahoney (1985). Reprinted from the National Well Water Assoc. Used by permission, (b) An Arrhenius plot of log k versus 1 /T(K) for the precipitation of quartz and amorphous silica based on Rimstidt and Barnes (1980). Reprinted from Geochim. Cosmochim. Acta, 44, J.D. Rimstidt and H.L. Barnes, The kinetics of silica water reactions, 1683-99, 1980, with permission from Elsevier Science Ltd, The Boulevard. Langford Lane. Kidlington OXS 1GB, U.K. 

In our weathering example (Table 7,2) kaolinite, rather than the aluminum oxyhydroxides, was the chief weathering product of the feldspars. This reflects the fact that the silica present in soil moisture and natural waters, generally, is high enough to stabilize kaolinite relative to the aluminum oxyhydroxides. This observation is better understood if we write the kaolinite dissolution reaction ... 

The dissolution reactions for quartz and amorphous silica are both written... 

In addition to ion exchange with rock surfaces, alkali can react directly with specific rock minerals. When divalents, Ca and Mg ", exist, alkali will react with them and precipitation can occur. One example is the incongruent dissolution of anhydrite or gypsum in the rock to produce the less soluble calcium hydroxide (CaS04(s) -F NaOH Ca(OH)2(s) + Na2S04). Another simple example is Ca -F COs " CaC03(s). Alkali can also dissolve other minerals from a rock, for example, silica. These reactions could cause plugging. 

In Eq. 10.30, the first term corresponds to accumulation in the fluid and the surfaces, the second term describes convective transport, and the third term indicates the loss by the kinetic dissolution reaction defined by Eq. 10.28. Equation 10.30 applies to any chemical transport process that includes fast and reversible ion-exchange, and slow and irreversible dissolution of the mth-order kinetics. In reservoir sands, both fine silica and clay minerals dissolve under attack by the alkali, yielding a complex distribution of soluble solution products... 

The basic framework and procedures to simulate alkaline-related processes are presented in Example 10.4. When a particular case is specified, the details of the reaction chemistry and input data set required must conform to that particular case. This section briefly provides one more case that includes clay and silica dissolution/precipitation based on the description by Bhuyan (1989). This section presents only the elements, species, reactions, and equilibria that are not listed or different from those in Example 10.4. We add this case because clay and silica dissolution or precipitation is a common problem. For more cases, see Mohammadi (2008). 

Quartz, a crystalline form of silica, is much less soluble than amorphous silica, as shown by the K value of its dissolution reaction ... 

The dissolution is controlled by the detachment of Al. Since the dissolution of silica is not promoted in presence of oxalate and salicylate (Bennett et al., 1988 Wieland, 1988), we may conclude that Si centers do not form stable surface complexes with these ligands. Hence, the siloxane layer of kaolinite and muscovite is not reactive with respect to dissolution reactions. Therefore, the detachment of both Al and Si is a consequence of the formation of surface complexes with Al sites. 

This phenomenon was confirmed by the introduction of symmetric tetraalkylammonium hydroxides in the dissolution of silica gel. In TMAOH the observed rate of dissolution was slow compared to the rate observed for cesium hydroxide dispersions, and cesium hydroxide has the lowest rate for the different alkali metal hydroxides. Results in Figure 3 clearly reveal an inhibition time between mixing of the silica gel with the aqueous TMAOH and the onset of dissolution. This observation is attributed to the strong interaction of the rather apolar TMA cation with the negatively charged silica gel surface. Because in this case no hydration shell is present, dissolution only occurs very slowly. The observed inhibition period of the dissolution reaction can be related to specific interactions of TMA cations with relatively large oligomeric species of the monomeric... 

However, the orthosilicic acid is a protonated polybasic oxoanion SiO and is capable of losing hydrogen when pH increases. At pH > 9 it easily loses H+ forming HjSiO" or even HjSiO " These losses are replenished through additional dissolution of silica. The final general equation of the dissolution reaction acquires the following format ... 

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... 

At low pH, acid catalysis promotes hydrolysis but hinders both condensation and dissolution reactions, " leading to small and homogeneous particles. Base catalysis of sol-gel hydrolysis and condensation reactions, in contrast, promotes fast condensation and dissolution. This leads to the production of an inhomogeneous system due to rapid condensation of the hydrolyzed precursor monomers and to dense silica particles formed by the ripening of aggregates during the collision of droplets. As a result, the microparticles show essentially no porosity, with the particles being stabilized by a water/surfactant layer on the particle surface that prevents particle precipitation. ... 

It can be noted that the brucitic layer of Ni(II) bonded to silica acts as nuclei for the growth of supported 1 1 nickel phyllosilicate or supported nickel hydroxide. The heterocondensation reaction is faster than the olation one, but it is limited by the concentration and diffusion in solution of silicic acid arising from silica dissolution, which itself depends on the silica surface area, i.e., on the extent of support-solution interface. 

A companion paper to Lasaga and Gibbs (1990) is the experimental and ab initio work of Casey et al. (1990). The study was conducted to examine the causes of the kinetic isotope effect in silica dissolution by combining careful experimentation using D2O and H2O as solvents, and results from ab initio calculations. The reaction investigated was... 

More elaborate and ambitious studies on the dissolution reactions of silica were conducted by Xiao and Lasaga (1994, 1996). Their objective was to provide full descriptions of the reaction pathway of quartz dissolution in acidic and basic solutions, from the adsorption of H2O or OH on a site, the formation of possible reaction intermediates and transition states, to the hydrolysis of the Si-O-Si bonds. Also, their aim was to extract kinetic properties such as changes in activation energy, kinetic isotope effects, catalytic and temperature effects, and the overall rate law form. The reaction mechanisms investigated were... 

For instance, the dissolution reaction of amorphous silica can be written as a synthesis of orthosilicic acid Si02(s) + 2 H2O ++ H4S104 (pKdigs = 2.7, Gunnarsson and Amorsson 2000). At pH-values above 8, the H4Si04 molecules dissociate into H3Si04 and H2Si04 and, thus, affect the dissolution equilibrium. Hence, the solubility of silica in a basic environment exceeds that of the solubility in neutral milieu considerably (approx, by factor 2 at pH 10 and by factor 50 at pH 11, cf. Fig. 3.11 Her 1979, pp. 47 and 123). 

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