Dissolution kinetics minerals, oxides

The Al reaction remains a critical factor in the performance of these Si rich systems. Given that aluminate anions for the reaction are solely derived from the dissolution of mineral oxides under alkaline conditions, monomeric [Al(OH)4] ions are probably the only aluminate species existing under high alkaline conditions. On the other hand, silicate species come from both soluble alkaline silicates and the dissolution of mineral oxides. In the specific case of metakaolin systems, the silicate species from the dissolution of particles are difficult to predict because the hydrolysis process of amorphous silica is kinetically dependent on various factors, such as the reactivity of the particles, temperature, time, and the eoncentration and pH value of alkaline silicate solutions. 

Koretsky, E. M., Sverjensky, D. A., and Sahai, N. (1998). A model of surface site types on oxide and silicate minerals based on crystal chemistry— implications for site types and densities, multisite adsorption, surface infrared-spectroscopy, and dissolution kinetics. Amer. J. Sci. 298, 349-438. 

Rate-Limiting Steps in Mineral Dissolution 146 Feldspar, Amphibole, and Pyroxene Dissolution Kinetics 148 Parabolic Kinetics 149 Dissolution Mechanism 155 Dissolution Rates of Oxides and Hydroxides 156 Supplementary Reading 161... 

There are many reactions in soil-water systems pertaining to nutrient availability, contaminant release, and nutrient or contaminant transformations. Two processes regulating these reactions are chemical equilibria (Chapter 2) and kinetics. The specific kinetic processes that environmental scientists are concerned with include mineral dissolution, exchange reactions, reductive or oxidative dissolution, reductive or oxidative precipitation, and enzymatic transformation. This chapter provides a quantitative description of reaction kinetics and outlines their importance in soil-water systems. 

The data in Figure 7.13 show reductive-dissolution kinetics of various Mn-oxide minerals as discussed above. These data obey pseudo first-order reaction kinetics and the various manganese-oxides exhibit different stability. Mechanistic interpretation of the pseudo first-order plots is difficult because reductive dissolution is a complex process. It involves many elementary reactions, including formation of a Mn-oxide-H202 complex, a surface electron-transfer process, and a dissolution process. Therefore, the fact that such reactions appear to obey pseudo first-order reaction kinetics reveals little about the mechanisms of the process. In nature, reductive dissolution of manganese is most likely catalyzed by microbes and may need a few minutes to hours to reach completion. The abiotic reductive-dissolution data presented in Figure 7.13 may have relative meaning with respect to nature, but this would need experimental verification. 

Where might this be important As discussed above, biological activity can result in the simultaneous precipitation of mixtures of nanoscale sulfide minerals under certain conditions. Each mineral will exhibit a particular particle size distribution, dependent on the solution composition, bacterial activity, rate of crystal growth, and the nature of electrochemical interactions between the particles. These electrochemical reactions could lead to oxidation of one type of nanophase sulfide mineral of a certain size, and reduction of another type of nanophase sulfide particle or other species in the solution. In this way, a tremendous number of mineral-solution-mineral galvanic cells could develop, with potentially significant impact on dissolution kinetics, growth kinetics, and the mixture of phases observed. In addition to environmental relevance, these processes may shape the mineralogy of low-temperature ore deposits. 

Scheckel. K. G., and Sparks, D. L. (2001). Dissolution kinetics of nickel surface precipitates on clay mineral and oxide surfaces. Soil Sci. Soc. Am. J. 65, 685-694. 

The effects of pH on sorption isotherms have been studied extensively particularly with oxide surfaces (Anderson and Rubin, 1981 Sposito, 1984), but pH effects in sorption kinetic studies have not received equal attention. In contrast, pH effects in mineral-dissolution kinetic experiments have received a great deal of attention (e.g., Chou and Wollast, 1984 Stumm, 1986 Stone, 1987a,b). 

The rate-controlling step for dissolution of an oxide or primary silicate mineral generally involves a surface reaction. For surface-controlled dissolution, the rate-controlling step is either the detachment of silica or a metal ion from the surface or the attack of the surface to form precursor sites for detachment. Surface detachment controlled kinetics can be modelled using the surface complexation rate model (Wieland et al., 1988) that models rates as a function of the surface concentration of surface complexation sites that are precursors for dissolution. In this model, the formation of precursor sites is rapid compared to the rate of detachment and the concentration of sites can be described by surface complexation theory (Sposito, 1983). 

The objectives of this chapter are (1) to illustrate that the surface structure is important in characterizing surface reactivity and that kinetic mechanisms depend on the coordinative environment of the surface groups, (2) to derive a general rate law for the surface-controlled dissolution of oxide and silicate minerals and illustrate that such rate laws are conveniently written in terms of surface species, and (3) to illustrate a few geochemical implications of the kinetics nf oxide dissolution. 

The rate of formation of dissolved iron(II), d[Fe2 + ]diss/df, depends, in addition, on the efficiency of detachment of reduced surface iron ions from the crystal lattice. The detachment step is a key step in the overall dissolution kinetics of slightly soluble minerals, since it is assumed to be the rate-determining step (Stumm and Furrer, 1987). The efficiency of detachment depends primarily on the crystallinity, and thus the stability of the iron(IIJ) hydroxide phase, and also on the coordinative surrounding of the reduced surface metal centers. It has been shown that a combination of a reductant and a ligand that forms stable surface complexes in the dark is especially efficient for the thermal reductive dissolution of hydrous iron(III) oxides (Banwart, et al. 1989). The role of a ligand as an electron donor and as a detacher in the photochemical dissolution of hydrous iron(III) oxides remains to be elucidated. 

Minerals with Kinetic Dissolution Condition Minerals of this group are considered in everyday life insoluble. Ihey include mostly metal oxides, hydroxides, sulphides and aluminum sihcates. The mechanism of their dissolution is dominated by hydrolysis whose nature depends on the structure and composition of minerals. Their dissolution under any conditions has kinetic condition, i.e., it is controlled by extremely slow chemical reactions of surface complexation. The rate of their dissolution is noticeably lower than 10 ° mole m s and the solubility does not exceed 10" mole l Besides, both their dissolution rate and solubility depend on pH values. These minerals are most common in the Earth crust and often play a leading role in the formation of imderground water composition. It is convenient to subdivide minerals with kinetic dissolution regime into three groups 1- silica, 2 - oxides, hydroxides and sulphides of metals, 3-aluminum silicates. 

Most of these minerals are insoluble in water. Exceptions are oxides, hydroxides and sulphides of alkaline and alkaline-earth metals, at interaction of which with H O appear soluble bases. The remaining metals form a large series of quite stable and common in natme minerals. Studies of dissolution kinetics for some of them showed that the rate of their dissolution depends on pH values according to equation (2.227). General parameters of their dissolution rates are listed in Table 2.23. It is clear from this Table... 

U. Schwertmann and R. M. Taylor, Iron oxides, in Minerals in Soil Environments (J. B. Dixon and S. B. Weed, eds.) Soil Science Society of America, Madison, Wis., 1977. U. Schwertmann, D. G. Schulze, and E. Murad, Identification of ferrihydrite in soils by dissolution kinetics, differential X-ray diffraction, and Mossbauer spectroscopy, Soil Sci. Soc. Am. J. 46 869 (1982). 

The rate order is given by a charge balance of protons at the surface this result is, however, not observed for complicated silicate minerals. There are many potential reasons for this, including the selective leaching of cations from the surface [e.g., 41, 45, 46], self-poisoning of the reaction [e.g., 47], or migration of protons deep into the mineral. We yet have virtually no fundamental understanding of the dissolution kinetics of complicated mixed oxide and aluminosilicate polymerized structures. 

Casey WH (1991) On the relative dissolution rates of some oxide and orthosilicate minerals. J Colloid Interface Sci 146 586-589 Casey WH, Westrich HR (1992) Control of dissolution rates of orthosilicate minerals by local metal-oxygen bonds. Nature 355 157-159 Casey WH, Carr MJ, Graham RA (1988a) Crystal defects and the dissolution kinetics of rutile. Geochim Cosmochim Acta, 52 1545-1556 Casey WH, Westrich HR, Arnold GW (1988b) The surface chemistry of labradorite feldspar reacted with aqueous solutions at pH = 2, 3 and 12. Geochim Cosmochim Acta 52 2795-2807... 

Stumm, W. and R. Wollast, 1990, Coordination chemistry of weathering, kinetics of the surface-controlled dissolution of oxide minerals. Reviews of Geophysics 28, 53-69. 

The Kinetics of Surface Controlled Dissolution of Oxide Minerals an Introduction to Weathering... 

Stumm, W., and E. Wieland (1990), "Dissolution of Oxide and Silicate Minerals Rates Depend on Surface Speciation", in W. Stumm, Ed., Aquatic Chemical Kinetics, John Wiley and Sons, New York, 367-400. 

Bloesch, P.M. Bell, L.C. Hughes, J.D. (1987) Adsorption and desorption of boron by goethite. Aust. J. Soil Res. 25 377-390 Blomiley, E.R. Seebauer, E.G. (1999) New approach to manipulating and characterising powdered photo adsorbents. NO on Cl treated Ee20j. Langmuir 15 5970-5976 Bloom, P.R. Nater, E.A. (1991) Kinetics of dissolution of oxide and primary silicate minerals. In Sparks, D.L. Suarez, D.L. (eds.) Rates of soil chemical processes. Soil Sci. 

Core and valence level photoemission studies of iron oxide surfaces and the oxidation of iron. Surface Sd. 68 459—468 Bruno, J. Sturam, J.A. Wersin, P. Brand-berg, E. (1992) On the influence of carbonate on mineral dissolutions I. The thermodynamics and kinetics of hematite dissolution in bicarbonate solutions at T = 25°C. Geo-chim. Cosmochim. Acta 56 1139—1147 Brusic.V. (1979) Ferrous passivation. In Corrosion Chemistry, 153—184 Bruun Hansen, H.C. Raben-Lange, R. Rau-lund-Rasmussen, K. Borggaard, O.K. 

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