Rate-Limiting Steps in Mineral Dissolution

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 basically three, rate-limiting mechanisms for mineral dissolution assuming a fixed degree of undersaturation. They are (1) transport of solute away from the dissolved crystal or transport-controlled kinetics. 

In transport-controlled kinetics, the dissolution ions are detached very rapidly and accumulate to form a saturated solution adjacent to the surface. Then the dissolution is controlled by ion transport by advection and diffusion into the undersaturated solution. The rate of transport-controlled kinetics is affected by stirring and flow velocity. As they increase, transport and dissolution both increase (Berner, 1978). 

With surface reaction-controlled kinetics, ion detachment is slow and ion accumulation at the crystal surface cannot keep up with advection and diffusion. In this type of phenomenon, the concentration level next to the crystal surface is tantamount to the surrounding solution concentration. Increased flow rate and stirring have no effect on the rate of surface reaction-controlled rate processes (Berner, 1978, 1983). 

The third type of rate-limiting mechanism for mineral dissolution— mixed or partial surface reaction-controlled kinetics—exists when the surface detachment is fast enough that the surface concentration builds up to levels greater than the surrounding solution concentration but lower than that expected for saturation (Berner, 1978). 

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The rate of chemical weathering of minerals in the subsurface depends on a number of factors, including mineralogy, temperature, flow rate, surface area, presence of ligands and CO, and H+ concentrations in the subsurface water (Stumm et al. 1985). Figure 2.3 shows the rate-limiting steps in mineral dissolution consisting of... 

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...

Previous work with de-ionised water (van Eldik Palmer 1982), albeit at relatively low pressures, has shown that about 99% of the total dissolved CO2 is in the form of the dissolved gas rather than as true carbonic acid. As a consequence, reaction (1) is of key interest in understanding the dissolution of CO2. This reaction is also likely to be slower than reactions involving ionic species, and is possibly the overall rate-limiting step in CO2 dissolution. However, the rate of dissolution is still much faster than most fluid-mineral reactions, with equilibrium being obtained within 24 hours for a wide range of temperatures and pressures below 90 bar (Czernichowski-Lauriol et al. 1996 Ellis Golding 1963 Stewart Munjal 1970). 

Despite the authority apparent in its name, no single rate law describes how quickly a mineral precipitates or dissolves. The mass action equation, which describes the equilibrium point of a mineral s dissolution reaction, is independent of reaction mechanism. A rate law, on the other hand, reflects our idea of how a reaction proceeds on a molecular scale. Rate laws, in fact, quantify the slowest or rate-limiting step in a hypothesized reaction mechanism. 

Several refinements of our experiments could test these theories further. By measuring etch pit densities as well as pit dimensions on sequentially-etched crystals, nucleation rate data and pit growth data could be collected, yielding information about the rate-limiting steps and mechanisms of dissolution. In addition, since the critical concentration is extremely dependent on surface energy of the crystal-water interface (Equation 4), careful measurement of Ccrit yields a precise measurement of Y. Our data indicates an interfacial energy of 280 + 90 mjm- for Arkansas quartz at 300°C, which compares well with Parks value of 360 mJm for 25°C (10). Similar experiments on other minerals could provide essential surface energy data. 

Whether solid solutions actually do form in soils is probably limited more by slow rates of soil mineral dissolution—a necessary prehminary step for co-precipitation— than by lack of thermodynamic favorability. Although the overall impact of solid solution formation on metal solubility in soils remains to be determined, certain features of metal sorption are consistent with (but do not prove) sohd solution formation. These include a sorption capacity that is ill-defined and increasing with time, decreasing reversibility of sorption with time that follows a decreasing labihty of the... 

It has been proposed that the extent to which mixed-cation hydroxide compounds actually do form in aquatic and terrestrial environments is limited more by slow rates of soil mineral dissolution, a necessary preliminary step, than by lack of thermodynamic favorability (57). Because the dissolution rates of clays and oxide minerals are fairly slow, the possibility of mixed-cation hydroxide formation as a plausible "sorption mode" in 24 hour-based sorption experiments (and also most long-term studies) containing divalent metal ions such as Mg, Ni, Co, Zn, and Mn and Al(III)-, Fe(III)-, and Cr(III)-(hydr)oxide or silicate minerals has been ignored in the literature 16,17). This study and others recently published (77), however, suggests that metal sorption onto mineral surfaces can significantly destabilize surface metal ions (A1 and Si) relative to the bulk solution, and therefore lead to an enhanced dissolution of the clay and oxide minerals. Thus, predictions on the rate and the extent of mixed-cation hydroxide formation in aquatic and terrestrial environments based on the dissolution rate of the mineral surface alone are not valid and underestimate the true values. 

This point is important because a dissolving mineral surface can be thought of as a polymer that detaches monomers from monomolecular steps. In the limiting case of pure induction by adsorbed protons, the rate order for dissolution comes from a charge balance at the surface (8) and, in acid solutions, commonly equals the formal metal valence in the oxide. In these cases, for example, rates of dissolution of BeO(s) increase with the square of adsorbed proton concentrations likewise the dissolution rates of Al203(s) increase with the cube of adsorbed proton concentration... 

Acidic and basic dissolution of uraninite (UO is depicted below as an example of U(IV) leaching. Sitmlar reactions may be written for pitchblende (UaO. Uraninite and pitchblende are the two most impoitant uranium minerals. The leaching of oxidized [U(VI)] uranium minerals such as camotite, K20 2U03 V 0) 3H20, is readily achieved in both acid and basic circuits since oxidation is not required. Oxidation-leaching processes are rate limited by the oxidation step. 

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. 

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