Rate control mineral dissolution

An important factor in controlling mineral dissolution rates are the steps which involve the hydrolysis of cations at the mineral surface and their subsequent removal into the solvent (20, and references therein). 

The rate of mineral dissolution may be mass-transport or surface-reaction controlled. Explain how the applicability of these controls relates to mineral solubility, with examples. 

The magnitudes of chemical kinetic and macroscopic transport processes, evaluated as their linear rates [linear rate=(mass flux)/(concentration or density) = F/p], indicate that great differences exist between the mineral dissolution rates, as reported from laboratory measurements, and the rates derived from river-water composition and volume flow. These differences point to an important role of the physical structure of the weathering zone and water residence time within it that control mineral dissolution fluxes and transport of the reaction products. An additional factor responsible for the faster rates of chemical weathering could be bacterial, activity which may be expected to vary from lower levels in the cold regions to the higher levels in the tropics, in parallel with the rates of net primary productivity. 

Mineral dissolution in aqueous solutions is a multi-step process. Generally, the slowest of steps in this process detennines the overall dissolution rate. If the slowest one is the chemical reaction, i.e. chemical bonding/bond breaking process, on mineral surfaces, such dissolution process is said to be surface reaction controlled . A generalized rate law for the surface controlled mineral dissolution rates Rg (mmol-cm" s ) can be expressed as (Lasaga, 1998 Jeschke et al, 2001)... 

In case of a transport controlled mineral dissolution, the dissolution rate R, (mmol-cra -s ) by molecular transport is (Jeschke et al, 2001) ... 

Under natural conditions the rates of dissolution of most minerals are too slow to depend on mass transfer of the reactants or products in the aqueous phase. This restricts the case to one either of weathering reactions where the rate-controlling mechanism is the mass transfer of reactants and products in the soHd phase, or of reactions controlled by a surface process and the related detachment process of reactants. 

Berner, R. A. (1978). Rate control of mineral dissolution under earth surface conditions. Am.. Sci. 278, 1235-1252. 

Rates of reductive dissolution of transition metal oxide/hydroxide minerals are controlled by rates of surface chemical reactions under most conditions of environmental and geochemical interest. This paper examines the mechanisms of reductive dissolution through a discussion of relevant elementary reaction processes. Reductive dissolution occurs via (i) surface precursor complex formation between reductant molecules and oxide surface sites, (ii) electron transfer within this surface complex, and (iii) breakdown of the successor complex and release of dissolved metal ions. Surface speciation is an important determinant of rates of individual surface chemical reactions and overall rates of reductive dissolution. 

The importance of "parabolic kinetics" in laboratory studies of mineral dissolution has varied as interpretations of the underlying rate-controlling mechanism have changed. Much of the research on silicate mineral weathering undertaken in the past decade or so served to test various hypotheses for the origin of parabolic kinetics. 

Observations from deep-water sediment traps have demonstrated that PIC is present in waters that are undersaturated with respect to this mineral. Thus, thermodynamic considerations are not a perfect predictor of the presence of PIC. In other words, some PIC is present out of equilibrium with the seawater it is in. This is largely a result of kinetics in which dissolution is slow enough to enable PIC to persist for some time. Much effort has been applied to determining the factors that control the rate of PIC dissolution. Marine scientists have reached agreement that the rate law for CaC03 dissolution in undersaturated waters (H < 1) can be represented as ... 

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...

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. 

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). 

Dissolution occurring by a surface reaction is often slower than by transport-controlled kinetics because the latter results from more rapid surface detachment. There appears to be a good correlation between the solubility of a mineral and the rate-controlling mechanism for dissolution. Table 7.1 lists dissolution rate-controlling mechanisms for a number of substances. The less soluble minerals all dissolve by surface reaction-controlled kinetics. Silver chloride is an exception, but its dissolution... 

TABLE 7.1 Dissolution Rate-Controlling Mechanism for Various Substances Arranged in Order of Solubilities in Pure Water (Mass of Mineral That Will Dissolve to Equilibrium)0... 

W. H. Casey and H. R. Westrich, Control of dissolution rates of orthosilicate minerals by divalent metal-oxygen bonds, Nature 355 157 (1992). 

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 rate constants are estimated by the slopes of lines fitted by a least squares method and listed in Table 1. It is concluded that a two-step first-order kinetic behavior dominates in the surface sorption of U(VI). The relatively slower second-step may be due to such effects as a diffusion-controlled sorption onto the fracture surface of micropores and a mineral dissolution of the granite surface [19], It is noticed from Table 1 that the reaction rates do not greatly depend upon pH although the amount of U(VI) sorbed onto the granite surface is greatly dependent on pH. 

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