Dissolution rate of silicates

Perera, G., Doremus, R.H. and Lanford, W. (1991). Dissolution rates of silicate glasses in water at pH 7. Journal of the American Ceramic Society 74 1269-1274. 

Since the work of Daubree in 1867 ( 2), workers have experimented to define the dissolution rates of silicate minerals. This paper will briefly review the types of reaction rates that have been observed, followed by more detailed discussion concerning the dependence of these rates on constituents in the aqueous media, and the implications for solute modeling. 

In Eq. [1], protons are shown to be important in determining the equilibrium of the hydrolysis of a silicate mineral. Protons are important factors in determining dissolution rates of silicates, oxides, hydroxides, and hydrous oxides. Because of the relatively high Arrhenius activation energies of surface-controlled reactions, temperature is an especially important factor in determining dissolution rates. Anions that bind to mineral surfaces can... 

Reference [198] reports dissolution rates of silicate minerals. The kinetics of for-sterite (MgSiO4) dissolution was studied in [199]. 

The pH dependence of the dissolution rates of silicates is a subject of intensive theoretical interest, based on transition-state and surface-reaction rate theories (e.g., Schott and Petit, 1987 Wollast and Chou, 1988 Stumm and Wieland, this volume). The features of the pH dependence of the silicate dissolution rates that are relevant to this section are the reported dependence of the rate in the acidic solution range (pH < 5.5) on a power of the hydrogen ion concentration, Roc[H + ]0 5 to [H + ]10, and its dependence in the alkalilne range (pH >7.5) on Roc[H + ]-°3. 

Perera, G., R.H. Doremus, and W. Lanford, Dissolution Rates of Silicate Glasses in Water at pH 7. Journal of the American Ceramic Society, 1991. 74(6) p. 1269-1274. Andersson, P., A.-P. NikkilS, and P. Lintula, Wear chartutieristics of water-lubricated SiC journal bearings in intermittent motion. Wear, 1994.179 p. 57-62. 

Goldich (1938) examined the mineral assemblages present in soil (Appendix, Plate 5) under a variety of environmental conditions and established a stability series for sand and silt-sized particles that illustrates the relative stability of primary silicate minerals (Goldich s weathering series) (Fig. 1.8). For example, Ca-plagioclase, olivine (Appendix, Plate 6) and pyroxene (Appendix, Plate 7) tend to be most easily suffered chemical weathering and quartz and mica are most resistant to the weathering. This order is quite consistent with calculated solubility (Fig. 1.7) and experimentally determined dissolution rate of silicate minerals (Fig. 1.10). The solubility and dissolution rate of silicate minerals are related to the crystal structures, which is described below. 

The rate constants depend on temperature. Wood and Walther (1983) summarized the experimental results of dissolution rate of silicates as a function of temperature (Fig. 3.3). For the simple case of surface reaction the reaction rate is expressed as km (k rate constant, m concentration in aqueous solution), and for the diffusion-controlled mechanism, it is (D/x)m where D is diffusion coefficient and x is effective distance of diffusion. For the surface reaction mechanism, rate constant, k is Zexp (—E/kT) where E is activation energy and Z is constant value. Thus, reaction rate is Zexp (—E/kT)m. 

If lake water reacts with lake bottom carbonate (limestone, dolomite), pH increases rapidly because of fast dissolution rates of carbonates (Sect. 6.3.1.1). In contrast dissolutions of silicates (granite, metamorphic rocks etc.) do not proceed considerably because of slow dissolution rate of silicates, and pH of lake water does not change quickly. Figure 6.15 shows that pH of lake water depends on lithology of lake bottom. Examples of ion exchange reactions controlling pH are given by... 

Fukui et al. [33] have investigated the effects of NaOH concentration on the crystal stracture and the rate of reaction of the synthesized zeolite from fly ash with a hydrothermal treatment method. They have reported that fly ash or the mixture of fly ash and silica powder results in an increase in the reaction rates with the increase of NaOH concentration due to increase of the dissolution rate of silicate ion and aluminate ion. It has been clarified that the NaOH concentration also affects the crystal structure of synthesized zeolites. It has been concluded that the proportion of Phillipsite continues to be lower than the increasing proportion of Hydroxy-sodalite in the product with the increase in the concentration of NaOH. 

Blum, A. E. and Stillings, L. L. (1995). Feldspar dissolution kinetics. In "Chemical Weathering Rates of Silicate Minerals" (A. F. White and S. L. 

Blum, A.E. Stillings, L.L. 1995. Felsdpar dissolution kinetics. In White, A.F. Brantley, S.L. (ed.), Reviews in mineralogy, 31 Chemical weathering rates of silicate minerals, Mineralogical Society of America, USA, 291-352. 

As was mentioned in the introduction to this chapter "diffusion-controlled dissolution" may occur because a thin layer either in the liquid film surrounding the mineral or on the surface of the solid phase (that is depleted in certain cations) limits transport as a consequence of this, the dissolution reaction becomes incongruent (i.e., the constituents released are characterized by stoichiometric relations different from those of the mineral. The objective of this section is to illustrate briefly, that even if the dissolution reaction of a mineral is initially incongruent, it is often a surface reaction which will eventually control the overall dissolution rate of this mineral. This has been shown by Chou and Wollast (1984). On the basis of these arguments we may conclude that in natural environments, the steady-state surface-controlled dissolution step is the main process controlling the weathering of most oxides and silicates. 

For the calculation of convective dissolution rate of a falling crystal in a silicate melt, the diffusion is multicomponent but is treated as effective binary diffusion of the major component. The diffusivity of the major component obtained from diffusive dissolution experiments of the same mineral in the same silicate melt is preferred. Diffusivities obtained from diffusion-couple experiments or other types of experiments may not be applicable because of both compositional effect... 

Experimental investigations will continue to play a critical role in understanding heterogeneous reaction kinetics, such as mineral dissolution rates in silicate melts and in aqueous solutions, the melting rates at the interfaces of two... 

Most of the dissolved calcium in groundwater in northern Wisconsin is the result of silicate hydrolysis of the aquifer materials. The assumption of conservancy is accurate only because of the relatively slow rates of silicate dissolution. The presence of more soluble calcium-containing minerals, such as calcite or gypsum, would invalidate assumptions of conservancy and would lead to significant errors in solute budgets. 

When fine powders of vitreous silica, quartz, tridymite, cristobalite, coesite, and stishovite of known particle-size distribution and specific surface area are investigated for their solubility in aqueous suspensions, final concentrations at and below the level of the saturated concentration of molybdate-active silicic acid are established. Experimental evidence indicates that all final concentrations are influenced by surface adsorption of silicic acid. Thus, the true solubility, in the sense of a saturated concentration of silicic acid in dynamic equilibrium with the suspended silica modification, is obscured. Regarding this solubility, the experimental final concentration represents a more or less supersaturated state. Through adsorption, the normally slow dissolution rates of silica decrease further with increasing silicic acid concentrations. Great differences exist between the dissolution rates of the individual samples. 

With the addition of bentonite to a crushed basalt backfill, aqueous diffusion would be the most effective mass transfer process (31). Aagaard and Helgeson (32) state that at temperatures <200°C, aqueous diffusion rates are orders of magnitude greater than rates of silicate hydrolysis even in acid solutions. Therefore, the dissolution rate of backfill phases and the overall mass transfer process could be controlled by reactions at the mineral-fluid interface. As stated earlier, dissolution of basalt phases in the sampling autoclave experiments may also be controlled by interface reactions. 

Casey, W. H. Ludwig, C. (1995). Silicate mineral dissolution as a ligand-exchange reaction. In Chemical Weathering Rates of Silicate Minerals, Reviews in Mineralogy,... 

The removal of silica from a siliceous iron ore, such as the taconites found in Minnesota and Wisconsin, has been studied by Tiemann (T7, T9). Caustic concentrations from 25-500 gm/liter were used to digest the ore in a bomb at temperatures from 312 to 408°F. The leaching pressures in the bomb correspond closely to the equilibrium vapor pressures of the sodium hydroxide solutions used. A residual concentrate containing around 65% iron was obtained with —200 mesh material in 60 min of contact time. The high rate of dissolution of the silica was attributed to its occurrence in the form of microcrystalline (chalcedonic) varieties with high specific surface. The dissolution rate of pure quartz is directly proportional to the surface area and an average rate of 17 X 10 gm moles/cm sec was obtained for a 100 gm/liter NaOH solution at 312°F for the —400 mesh fraction. 

The effect of organic anions on silicates other than quartz and feldspar has also been investigated. Dissolution rates of kaolinite are enhanced more by oxalate than salicylate, while malonate and phthalate show little effect (CarroU-Webb and Walther, 1988 Chin and Mills, 1991 Wieland and Stumm, 1992). Dissolution of hornblende is accelerated at pH 4 in the presence of organic acids at 2.5 mM rate enhancement was observed... 

Oelkers E. H. and Schott J. (1995a) Dissolution and crystallization rates of silicate minerals as a function of chemical affinity. Pure Appl. Chem. 67, 903-910. 

Experimental rates of silicate dissolution decrease as solutions approach thermodynamic equilibrium (Burch et ai, 1993 Taylor et al., 2000). The saturation state fl is defined as the product of the solute activities (lAP) divided by the saturation constant of the specific mineral and is related to the net free energy of reaction AG (kJ mol ) by the relationship... 

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