Transition state theory, dissolution rate

Following transition state theory, we can write a rate law giving the dissolution... 

A general definition for the net rate of precipitation minus dissolution, r et, derived from transition state theory (Lasaga, 1981 Aagaard and Helgeson, 1982 Helgeson et al, 1984 Lasaga, 1984), yields a rate law that is linear with respect to AG near equilibrium ... 

Here is the forward rate constant, aj is the activity of species j in the rate-determining reaction, mj and are constants, and R and T are the gas constant and absolute temperature, respectively. The sign of the rate indicates whether the reaction goes forward or backward. The relationship of this equation to transition state theory and irreversible kinetics has been discussed in the literature (Lasaga, 1995 Alekseyev et al., 1997 Lichtner, 1998 Oelkers, 2001b). The use of this equation with = 1 is generally associated with a composite reaction in which all the elementary reactions are near equilibrium except for one step which is ratedetermining. This step must be shared by both dissolution and precipitation. 

Transition-state theory may be useful in testing the dissolution mechanisms presented above. According to TST, for any elementary chemical reaction the reactants should pass through a free-energy maximum, labeled the activated complex , before they are converted to products. It is assumed that the reaction rate-determining step is related to the decomposition of this activated complex ... 

Rate Laws. The principal kinetic rate laws included in the E( code are the transition-state theory form (e.g., 2 and the Plummer et al. Q2) rate law proposed for the dissolution and growth of carbonate minerals. Less important forms are discussed in ref. 25. Generally speaking, these models may include an implicit model of speciation on the surface of the dissolving or growing mineral. However, no explicit models for speciation on mineral surfaces are presently accounted for in EQ3/6. Further development of kinetics theory may require the inclusion of such models for coupling with future rate law models. 

Recent advances in applying transition state theory to geochemical kinetics (SQ, SD have emphasized the interaction of the activated complex with specific surface reaction sites. The rate of reaction is assumed to be a function of the surface reaction site density. A correspondence is also observed between surface dissolution features such as etch pits, and crystallographically controlled extended defect features such as edge and screw dislocations (S2). Based on these lines of evidence, the reactive surface area has been proposed to be proportional to the defect density within minerals... 

For the explanation of hydrolytic dissolution processes currently is widely used activated-complex theory, which is also called transition state theory or absolute reaction rate theory. According to this theory, at hydration and protonation on the surface of the mineral form functional groups X-OH, X-OH and X-0 , which have acid-alkali properties dependent on pH of the solution. However, not the entire specific surface of the mineral participates in dissolution reactions but only its effective portion, which is taken by the... 

The pH dependence of the dissolution rates of silicates is a subject of intensive theoretical interest, based on transition-state and surface-reaction rate theories (e.g., Schott and Petit, 1987 Wollast and Chou, 1988 Stumm and Wieland, this volume). The features of the pH dependence of the silicate dissolution rates that are relevant to this section are the reported dependence of the rate in the acidic solution range (pH < 5.5) on a power of the hydrogen ion concentration, Roc[H + ]0 5 to [H + ]10, and its dependence in the alkalilne range (pH >7.5) on Roc[H + ]-°3. 

In theory, the lysocline records the sedimentary expression of the saturation horizon, that is the depth-dependent transition from waters oversaturated to waters undersaturated with respect to carbonate solubility (Figure 4). The lysocline thus marks the top of a depth zone, bounded at the bottom by the CCD, over which the bulk of carbonate dissolution in the ocean is expected to occur in response to saturation state-driven chemistry. The thickness of this sublysocline zone, as indicated by the vertical separation between the lysocline and CCD, is variable and is governed by the rate of carbonate supply, the actual dissolution gradient, and... 

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