Dissolution mechanisms

Minerals dissolve into aqueous solution and precipitate from aqueous solution by the water-rock interaction process. The rate of dissolution of minerals for one component system is simply expressed as 

Shikazono, Environmental and Resources Geochemistry of Earth System Mass Transfer Mechanism, Geochemical Cycle and the Influence of Human Activity, DOI 10.1007/978-4-431-54904-8 3, Springer Japan 2015 

Rate constant (k) depends on mineral surface area/mass of water ratio, porosity, density of aqueous solution and temperature. Therefore, Eq. (3.2) converts into 

In general surfaces of mineral crystals do not homogeneously dissolve by the interaction of aqueous solution. The reactive surfaces at dislocation and imperfection sites tend to dissolve easily. The density of activated site significantly influences on dissolution and precipitation (Blum et al. 1990 Meike 1990). 

It is likely that the rate of dissolution is simply proportional to A/M. However it has been reported that silicates in natural environment have higher A/M than that theoretically estimated, implying higher dissolution rate (Wells and Ghiorso 1991 Anbeek 1992). 

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Studies on hot water tank enamelsin media of varying pH demonstrate a minimum corrosion rate at pH value of 4. In citric acid (pH 2), IR measurements indicate that ion exchange is the principal mode of corrosion. Distilled water (pH 7) showed evidence of a bulk dissolution mechanism with no silica enrichment of the surface layer. In neutral solutions, the first stage of attack is leaching of alkali ions, raising the pH of solution, which subsequently breaks down the glass network of the acidic oxides. 

Buildings, Monuments and Materials. Many materials used in man-made structures are subject to deterioration from normal weathering such as dissolution, mechanical fracture, erosion, and photochemical reactions. However, as shown by Amoroso and Fassina (43 the rates of deterioration have increased drastically since the advent of industrial pollution. Losses to Canadian heritage sites such as the federal parliament buildings has been significant and have been described by Weaver (44). 

Kuhn, A. T. Wilson, A. D. (1985). The dissolution mechanisms of silicate and glass-ionomer dental cements. Biomaterials, 6, 378-82. 

Potential cycling has been found to accelerate Pt dissolution compared with poten-tiostatic conditions. The dissolution mechanisms and dissolved species involved in this process are unclear [Johnson et al., 1970 Kinoshita et al., 1973 Ota et al., 1988 Rand and Woods, 1972]. Darling and Meyers have developed a mathematical model based on (9.5)-(9.7) to smdy Pt dissolution and movement in a PEMFC during potential cycling from 0.87 to 1.2 V [Darling and Meyers, 2003, 2005]. Severe Pt dissolution occurs when the potential switches to the upper limit potential (1.2 V), and then stops once a monolayer of PtO has formed. The charge difference between the anodic and cathodic cycles was found to be consistent with the amount... 

Conditions in the dissolution medium, which, together with the nature of the dissolving solid, determine the dissolution mechanism (see Theories and Mechanisms of Dissolution, pp. 355-358). 

For another oxide, for which the dissolution mechanism requires only two preceding protonation steps, the rate would be proportional to (Cf,)2. Generally, for all oxides, which dissolve by an acid-promoted surface controlled process, the following rate equation may be postulated (see Fig. 5.9)... 

Surface Reactions. As we have seen from the dissolution of oxides the surface-controlled dissolution mechanism would have to be interpreted in terms of surface reactions in other words, the reactants become attached at or interact with surface sites the critical crystal bonds at the surface of the mineral have to be weakened, so that a detachment of Ca2+ and C03 ions of the surface into the solution (the decomposition of an activated surface complex) can occur. 

The first two pathways (a) and (b) show, respectively, the influence of H+ and of surface complex forming ligands on the non-reductive dissolution. These pathways were discussed in Chapter 5. Reductive dissolution mechanisms are illustrated in pathways (c) - (e) (Fig. 9.3). Reductants adsorbed to the hydrous oxide surface can readily exchange electrons with an Fe(III) surface center. Those reductants, such as ascorbate, that form inner-sphere surface complexes are especially efficient. The electron transfer leads to an oxidized reactant (often a radical) and a surface Fe(II) atom. The Fe(II)-0 bond in the surface of the crystalline lattice is more labile than the Fe(III)-0 bond and thus, the reduced metal center is more easily detached from the surface than the original oxidized metal center (see Eqs. 9.4a - 9.4c). 

Schott, J., and R. A. Berner (1985), "Dissolution Mechanism of Pyroxenes and Olivines During Weathering", in J.l. Drever, Ed, The Chemistry of Weathering, NATO ASI SERIES C 149, 35-53. 

The kinetic behavior of the reductive dissolution mechanisms given in Figure 2 can be found by applying the Principle of Mass Action to the elementary reaction steps. The rate expression for precursor complex formation via an inner-sphere mechanism is given by ... 

This fast removal of Si-F species can be ascribed to the weakening of the Si backbonds induced by the strong polarizing effect of F [Ubl], The weak back-bonds are then attacked by HF or H20. This reaction scheme for the dissolution process is supported by quantum-chemical calculations [Trl]. The observed dissolution valence of two for Jelectron injection current and Si-F bond density [Be22] are experimental findings that are in support of the divalent dissolution mechanism, as shown in Fig. 4.3 [Lei, Ge7, Ho6]. 

The analysis proposed by Gabrlelll (25) does not take diffusion effects into consideration. However, this model together with the dissolution mechanism proposed by Bockrls show how relative variations in the values of rate constants can give rise to different types of Nyqulst diagrams. In other words, it is possible to evaluate rate constants for a particular system by looking at the Nyqulst diagram if the experiment has properly been designed. 

For instance, a quite marked effect on the structure of the silicate melt is produced by the basic oxide H2O. Addition of water causes a drastic decrease in the viscosity of silicate melts adding 6.4 weight % of H2O to a granitic melt at T = 1000 °C causes a decrease in viscosity of about six orders of magnitude (i.e., from about 10 to about 10 poise cf Burnham, 1975). The dissolution mechanism of H2O is important in magma rheology and will be discussed more extensively in section 9.6.1. 

A computer model has been produced that Incorporates the solubility, growth and dissolution mechanisms. It predicts the behaviour of a single crystallizer or a crystallization section, given Input conditions. 

A third mechanism by which the structural bonds between Fe atoms in iron oxides may be weakened involves reduction of structural Fe to Fe". In natural environments, reductive dissolution is by far the most important dissolution mechanism. It is mediated both biotically and abiotically. The most important electron donors, particularly in near surface ecosystems result from metabolic oxidation of organic compounds under O2 deficient conditions. In anaerobic systems, therefore, the availability of Fe oxides i. e. the electron sink, may control the degradation of dead biomass and organic pollutants in the ground water zone (see chap. 21). Reductive dissolution is also often applied to the removal of corrosion products from piping in industrial equipment and the bleaching of kaolin. 

Table 1-4 Dissolution mechanism for some substances Table 2-1 Relaxation timescale and concentration evolution for reversible reactions Table 2-2 Decay steps in decay chains...

In the remainder of this article, discussion of surfactant dissolution mechanisms and rates proceeds from the simplest case of pure nonionic surfactants to nonionic surfactant mixtures, mixtures of nonionics with anionics, and finally to development of myehnic figures during dissolution, with emphasis on studies in one anionic surfactant/water system. Not considered here are studies of rates of transformation between individual phases or aggregate structures in surfactant systems, e.g., between micelles and vesicles. Reviews of these phenomena, which include some of the information summarized below, have been given elsewhere [7,15,29]. 

The corrosion behavior and dissolution mechanism of nickel in acid solutions with hydrogen sulfide (H2S) was studied. It was found that the dissolution of nickel is influenced by both the nickel sulfide layer formed on the electrode surface and the acceleration effect of H2S [56]. 

S. C. Kohn, M. E. Smith, P. J. Dirken, E. R. H. van Eck, A. P. M. Kentgens and R. Dupree, Sodium environments in dry and hydrous albite glasses improved Na solid state NMR data and their implications for water dissolution mechanisms. Geochim. Cosmochim. Acta, 1998, 62, 79-87. 

Turner, P. S., Jones, C. F., Myhra, S., Neall, F. B., Pham, D. K. Smart, R. St. C. 1989. Dissolution mechanisms of oxides and titantate cermics - electron microscope and surface analytical studies. In Dufour, L.-C., Monty, C. Petot-Ervas, G. (eds) Surfaces and Interfaces of Ceramic... 

Some of these difficulties can be traced to unproven, and perhaps incorrect, assumptions implicit within the Aagaard Helgeson (1982) model. For example, reaction rates at the glass/ water interface are modelled as sequential. As we discuss below, there is good reason to consider reactions as taking place concurrently. In a system that is close to equilibrium, the identity of the dominant dissolution mechanism among a set of concurrent elementary reactions may change as conditions shift. Another potential pitfall is the assumption that the principle of... 

Inorganic Chemistry of the Dissolution Phenomenon The Dissolution Mechanism of Calcium Apatites at the Atomic (ionic) Level (Dorozhkin, 1999)... 

The purpose of this chapter is to present a brief overview of the geochemistry of mineral surfaces, including their (1) dissolution mechanisms, (2) development of electrical charge when in contact with aqueous solutions, and (3) uptake of aqueous cations and anions, and to discuss some of the factors that control their chemical reactivity, including (1) defect density, (2) cooperative effects among adsorbates,... 

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