Mineral surface-controlled dissolution kinetics

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... 

W. Stumm and R. Wollast. Coordination chemistry of weathering Kinetics of the surface-controlled dissolution of oxide minerals, Rev. Geophys. 28 53 (1990). See also A. E. Blum and A. C. Lasaga, The role of surface speciation in the dissolution of albite, Geochim. Cosmochim. Acta 55 2193 (1990). 

Figure 13.5. Transport vs surface controlled dissolution. Schematic representation of concentration in solution, C, as a function of distance from the surface of the dissolving mineral. In the lower part of the figure, the change in concentration (e.g., in a batch dissolution experiment) is given as a function of time, (a) Transport controlled dissolution. The concentration immediately adjacent to the mineral reflects the solubility equilibrium. Dissolution is then limited by the rate at which dissolved dissolution products are transported (diffusion, advection) to the bulk of the solution. Faster dissolution results from increased flow velocities or increased stirring. The supply of a reactant to the surface may also control the dissolution rate, (b) Pure surface controlled dissolution results when detachment from the mineral surface via surface reactions is so slow that concentrations adjacent to the surface build up to values essentially the same as in the surrounding bulk solution. Dissolution is not affected by increased flow velocities or stirring. A situation, intermediate between (a) and (b)—a mixed transport-surface reaction controlled kinetics—may develop.

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. 

Ks do not match with each other. This is partly the result of the effects of the specific surface which was different in the two methods. However, the mechanisms of the dissolution kinetics seem to be identical. The reaction rate of the acid with carbonate mineral would be controlled by diffusion of the reactant into and the products out of the pores. Therefore, the availability of only the contact surface is not adequate. The type of surface in terms of relevant diffusion model and the closest theory to that model, such as film, penetration, or any other, should also be specified. 

The rate of calcite dissolution is known to depend on the hydrodynamic conditions of the environment and on the rate of heterogeneous reaction at the mineral surface. Numerous laboratory studies demonstrate transport and surface-controlled aspects of calcite reactions in aqueous solutions, but until recently, no study has been comprehensive enough to enable comparison of kinetic results among differing hydro-chemical systems. 

The chemical oxidation of metal sulfides is controlled in part by the dissolution of sulfide minerals under acidic conditions and by the presence of oxidants (DO, Fe ) that lead to the disruption of sulfide chemical bonds. Bacteria can have a significant effect on the rate of oxidative dissolution of sulfide minerals by controlling mineral solubility and surface reactivity. Metal-enriched waters and solutions rich in sulfuric acid that form in association with mining can be directly linked to microbial activity. The majority of studies to date have focused on the reactivity and kinetics of sulfide minerals in the presence of A. ferrooxidans and L. ferrooxidans, and in some cases A. thiooxidans (Singer and Stumm, 1970 Tributsch and Bennett, 1981a,b Sand et al., 1992, 2001 Nordstrom and Southam, 1997 Sasaki et al., 1998 Edwards et al., 1998, 1999, 2000 Nordstrom and Alpers, 1999a Banfield and Welch, 2000 Tributsch, 2001). Additional studies have been conducted on other species of bacteria and archea (Edwards et al, 1998, 1999, 2000). 

Figure 13.6. (a) Linear dissolution kinetics observed for the dissolution of 6-AI2O3, representative of processes whose rates are controlled by a surface reaction and not by a transport step. (Data from Furrer and Stumm, (1986).) (b) Linear dissolution kinetics of frame silicates. Minerals used were pyroxenes and olivines their essential structural feature is the linkage of Si04 tetrahedra, laterally linked by bivalent cat-Fe -, Ca ). Plotted ate... 

Rate Expression of the Photochemical Reductive Dissolution of y-FeOOH by Oxalate. The kinetics of the light-induced reductive dissolution of oxide minerals obey the general rate expression of surface-controlled reactions. The rate is proportional to the concentration of the adsorbed reductant, as in the case of adsorbed oxalate ... 

The dissolution of a mineral is a sum of chemical and physical reaction steps. If the chemical reactions at the surface are slow in comparison with the transport (diffusion) processes, the dissolution kinetics is controlled by one step in the chemical surface processes thus, rates of transport of the reactants from the bulk solution to the surface and of products from the surface into the solution can be neglected in the overall rate. It has been shown by Petrovic et al. (1976) and Berner and Holdren (1979) that the dissolution of many minerals, especially under conditions encountered in nature, are surface-controlled. 

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... 

A number of factors contribute to the disparity between the predictions of kinetic theory and conditions observed in the field, as discussed in Section 16.2. In this case, we might infer the dissolution and precipitation of minerals such as opal CT (cristobalite and tridymite, Si02), smectite and other clay minerals, and zeolites help control silica concentration. The minerals may be of minor significance in the aquifer volumetrically, but their high rate constants and specific surface areas allow them to react rapidly. 

Fig. 2.3 Rate-limiting steps in mineral dissolution (a) transport-controlled, (b) surface reaction-controlled, and (c) mixed transport and surface reaction control. Concentration (C) versus distance (r) from a crystal surface for three rate-controUing processes, where is the saturation concentration and is the concentration in an infinitely diluted solution. Reprinted from Sparks DL (1988) Kinetics of soil chemical processes. Academic Press New York 210 pp. Copyright 2005 with permission of Elsevier...

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