Etch pits dissolution kinetics

The morphology of weathered feldspar surfaces, and the nature of the clay products, contradicts the protective-surface-layer hypothesis. The presence of etch pits implies a surface-controlled reaction, rather than a diffusion (transport) controlled reaction. Furthermore, the clay coating could not be "protective" in the sense of limiting diffusion. Finally, Holdren and Berner (11) demonstrated that so-called "parabolic kinetics" of feldspar dissolution were largely due to enhanced dissolution of fine particles. None of these findings, however, addressed the question of the apparent non-stoichiometric release of alkalis, alkaline earths, silica, and aluminum. This question has been approached both directly (e.g., XPS) and indirectly (e.g., material balance from solution data). 

Dissolution kinetics at etch pits. If an etch pit opens up at a... 

Discrepancies between reactive and adsorption surface area may also be related to the presence of deep etch pits or pore outcrops which can constitute transport-limited micro-environments for dissolution (Jeschke and Dreybrodt, 2002). Much of the BET surface area for some alkali feldspars used for dissolution in the laboratory has been attributed to grinding-induced microporosity (Hodson et al, 1999), and such pore outcrops are candidates for transport limitation. If such induced surfaces react dilferently than surfaces of weathered samples, then the BET surface area may be an inappropriate parameter to use for extrapolating interface-limited kinetics from laboratory to field (Lee et al, 1998 Brantley and Mellott, 2000 Jeschke and Dreybrodt, 2002) and consideration may need to be given to length and extent of grinding for laboratory samples (Hodson, 1999). It may be more appropriate to use geometric rather than BET surface area to extrapolate kinetics for samples where etch pits or pore outcrops are important contributors to BET surface area (Gautier et al, 2001 Jeschke and Dreybrodt, 2002 Mellott et al, 2002). 

We have seen above that the kinetics of mineral dissolution is well explained by transition-state theory. The framework of this theory and kinetic data for minerals have shown that dissolution is initiated by the adsorption of reactants at active sites. Until now these active sites have been poorly characterized nevertheless, there is a general consensus that the most active sites consist of dislocations, edges, point defects, kinks, twin boundaries, and all positions characterized by an excess surface energy. Also these concepts have been strongly supported by the results of many SEM observations which have shown that the formation of crystallographically controlled etch pits is a ubiquitous feature of weathered silicates. 

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... 

In this section, we briefly provide preliminary results on the morphology of etch pits obtained on cleaved calcite crystals by dissolution in unbuffered aqueous solutions of maleic acid (3 mM) under both stationary conditions and through channel electrode experiments. Kinetic and morphological studies on the effect of maleic acid on the dissolution of calcium carbonate... 

Most kinetics data in the literature suggest that gypsum dissolution is diffusion transport controlled. However, the work by Raines and Dewers (1997) shows that a mixed suiface reaction/transport control mechanism can operate over a range of hydrodynamic conditions and chemical saturation states for gypsum dissolution. Additionally, very little work can be found on gypsum dissolution kinetics at near equilibrium conditions where surface reaction controlled dissolution could be the dominated mechanism. Research on surface behavior of gypsum during dissolution is consistent with the conclusion that dissolution on the 010 surface is a lay-by-lay process and is not characterized by the formation of deep etch pits, even at conditions far fi om equilibrium. 

It is well known that dislocation etch pits on the surfaces of metals are produced in solutions of salts of other metals as a result of contact displacement reactions (53)(54). The size of pits formed by these solutions depends on the concentration of a salt and the time of etching. However, prolonged etching often leads to the precipitation of mono- or polycrystalline displaced metal at relatively more active sites where dissolution is faster than that at the rest of the surface. Subsequent etching can yield etch hillocks, as observed In the case of etching white tin in acidic solutions of CuSO. Whether etch hillocks or etch pits will be formed at dislocation sites Is determined by the exchange kinetics at the electric double layer and by the diffusion kinetics. 

Hebert s model for tunnel growth predicted the tuimel shape on the basis of the repassivation model just described." Starting from the initial condition of a cubic etch pit, the model calculated the evolution of the pit shape resulting from dissolution and sidewall passivation. The dissolution rate was taken directly from experimental measurements." Since the passivation kinetics were potential-dependent, it was necessary to accmately predict the potential at the tunnel tip. This required the use of concentrated solution transport equations, for the first time in a pitting model. All transport and kinetic parameters used in the model were taken from independent sources. The calculations showed that pits growing at the bulk solution repassivation potential spontaneously transformed into tunnels by sidewall passivation (Fig. 6). The tuimels then grew with parallel walls until the concentration at the tip approached saturation, at... 

In addition to the visualization of topographic transformations, sequences of in-situ images yield a measure of the local kinetics of the reaction. The etch rate of Si has been evaluated in [20] by using the expression R = (AS/S) /i/Af, with AS/S the surface area of terraces removed per cm of electrode in one sequence, h the step height (3.14 A) and At the time elapsed. The quantity (AS/S)h in fact represents the volume of material which has been removed per cm of electrode, because the dissolution occurs layer by layer. The experimental determination of AS is sketched in Fig. 22 f, in which the hatched area represents AS. In other sequences AS includes the surface of eventual pits. The bias dependence of the etch rate and the current voltage curve are shown in Fig.26 for n-Si(lll) in a 2M NaOH solution [20]. 

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