Surface-controlled dissolution

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... 

Activated complex theory for the surface-controlled dissolution of a mineral far from equilibrium. A is the precursor, i.e., a surface site that can be activated to A. The latter is in equilibrium with the precursor. The activation energy for the conversion of the precursor into the product is given by AG. ... 

The postulate of steady state during dissolution reaction (Table 5.1) implies a continuous reconstitution of the surface with the maintenance of a constant distribution of the various surface sites and the steady state concentration of the surface complexes. Fig. 5.7 presents experimental evidence that the concentration of the surface ligand - in line with Fig. 5.5a - remains constant during the surface controlled dissolution reaction. 

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. 

The concentration of constituent B becomes negligible at the surface of the mineral grain. Gradually, the rate of mass diffusion of B (Eq. 5.21) through an increasing depleted layer (y) becomes slower and is equal to the rate of surface-controlled dissolution of A (Eq. 5.22). Thus, a pseudosteady state is attained and the depleted layer thickness stabilizes. The rates of reaction of solid layer diffusion (Eq. 5.21) and of surface controlled dissolution become equal ... 

Congruent surface-controlled dissolution follows after the initial incongruent period. 

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. 

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). 

There are several studies that have been successful in determining the dissolution rate at conditions near seawater saturation. Acker et al. (1987) was able to employ very precise determinations of pH to measure the rate of dissolution of a single pteropod shell at different pressures from 15 atm to 300 atm. Because his measurements were at different pressures and is a function of pressure, he was able to determine whether the rate constant is indeed a function of K p. He found that Equation (9) fit his data better than (10), suggesting that the constant is not pressure dependent and the former is a more accurate universal rate law. An exponent oin= 1.9 was obtained for this surface-controlled dissolution reaction and a partial molal volume. Ay, of —39 cm mol (very close to the mean of the values determined in laboratory experiments for calcite) best fit the data. 

Other common, though volumetrically minor, feldspar-replacing minerals include titanite, ana-tase, sphalerite, barite, ankerite, siderite, and fluorite. With the exception of replacement driven by force of crystallization, feldspar replacements have intracrystalline distributions that are strongly localized at sites of surface-controlled dissolution. Interestingly, replacement of detrital feldspars by authigenic clays is rarely observed in late diagenesis. 

SURFACE-CONTROLLED DISSOLUTION OF OXIDE MINERALS AN INTRODUCTION TO WEATHERING... 

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.

Although each sequence may consist of a series of smaller reaction steps, the rate law of surface-controlled dissolution is based on the idea... 

Figure 1. Schematic diagram of oxalate-promoted dissolution of an oxide mineral for M = Al(III) or Fe(III) based on the surface-controlled dissolution model. (Reproduced with permission from reference 22. Copyright 1986 Per-...

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. 

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