Surface reaction controlling

Reactions for common minerals fall in both categories, but many important cases tend, except under acidic conditions, to be surface controlled (e.g., Aagaard and Helgeson, 1982 Stumm and Wollast, 1990). For this reason and because of their relative simplicity, we will consider in this chapter rate laws for surface-controlled reactions. The problem of integrating rate laws for transport-controlled reactions into reaction path calculations, nonetheless, is complex and interesting (Steefel and Lasaga, 1994), and warrants further attention. 

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). 

Penn, L.S., Tesoro, G.C. and Zhou, H.X. (1988). Some effects of surface-controlled reaction of Kevlar 29 on the interface in epoxy composites. Polym. Composites 9, 184-191. 

This CVD procedure is somewhat different from that used to deposit semiconductor layers. In the latter process, the primary reaction occurs on the substrate surface, following gas-phase decomposition (if necessary), transport, and adsorption. In the fiber optic process, the reaction takes place in the gas phase. As a result, the process is termed modified chemical vapor deposition (MCVD). The need for gas-phase particle synthesis is necessitated by the slow deposition rates of surface reactions. Early attempts to increase deposition rates of surface-controlled reactions resulted in gas-phase silica particles that acted as scattering centers in the deposited layers, leading to attenuation loss. With the MCVD process, the precursor gas flow rates are increased to nearly 10 times those used in traditional CVD processes, in order to produce Ge02-Si02 particles that collect on the tube wall and are vitrified (densified) by the torch flame. 

As mentioned earlier, the dissolution of oxides and hydroxides, like feldspars and ferromagnesian minerals, appears to be a surface-controlled reaction. One indication of this is the high E values found by several investigators. Bloom and Erich (1987) obtained E values ranging from 59 4.3 to 67 0.6 kj mol"1 for gibbsite dissolution in acid solutions (pH 1.5-4.0). These values are much higher than for diffusion-controlled reactions reported earlier. 

For diffusion-controlled situations. Equation (28) shows that the driving force for mass flux to the surface is the pressure difference of species i between the main gas flow and the wall. For surface-controlled reactions, the driving force instead is the supersaturation at the surface. In other words, ps > peq, and there are more gas molecules available at the surface than can be reacted there. 

Fig. 23 Comparison between surface-controlled reactions (left 1= interstitial sites, 2= vacant sites, 3= screw dislocation, 4= jump dislocation, 5=step dislocation) and diffusion-controlled processes (right)...

Modeling hydrogeochemical processes requires a detailed and accurate water analysis, as well as thermodynamic and kinetic data as input. Thermodynamic data, such as complex formation constants and solubility products, are often provided as data sets within the respective programs. However, the description of surface-controlled reactions (sorption, cation exchange, surface complexation) and kinetically controlled reactions requires additional input data. 

It was, therefore, suggested that the rate of dissolution was surface controlled and the change in the relationship between increasing water depth and dissolution rate could be attributed to a change in surface-controlled reaction mechanisms. 

Dreybrodt, W., Eisenlohr, B., Madry, B. and Ringer, S., 1997, Precipitation kinetics of calcite in the system CaC03 - H2O - CO2 The conversion to CO2 by the slow process H" + HCO3 CO2 + H2O and the inhibition of surface controlled reactions as rate limiting steps, Geochim. Cosmochim. Acta 61 3897-3904. 

The second mechanism was put forward by Ruderhausen and Watson (ref.6) based on a surface controlled reaction as follows ... 

Table 7.2 shows the activation ena-gies determined for the various etching systems. In general, the apparent activation ena-gy, as determined from the dependence of etch rate on temperature, is 3-6 kcal/mol or 0.13-0.26eV for diffusion-limited reactions, whereas it is 10-20 kcal/mol or 0.44-0.87 eV for surface-controlled reactions. Using these criteria to evaluate the values of activation energy in Table 7.2, it appears... 

Ligand-Promoted Dissolution. In some cases, organic matter-surface associations markedly increase oxide dissolution rates. The acceleration of oxide dissolution by organic ligands exhibits saturation kinetics that is, the dissolution rate reaches a plateau at high concentration of dissolved ligand as the surface becomes saturated. This behavior is characteristic of surface-controlled reactions (in which the reaction of a surface-bound species is the rate-limiting step) and is consistent with the direct dependence of the dissolution rate on the concentration of the reactive species at the surface (22-24). 

Rate Expression of the Photochemical Reductive Dissolution of y-FeOOH by Oxalate. The kinetics of the light-induced reductive dissolution of oxide minerals obey the general rate expression of surface-controlled reactions. The rate is proportional to the concentration of the adsorbed reductant, as in the case of adsorbed oxalate ... 

A pH electrode connected to an autotitration instrument (pH meter, controller, and autoburet) is often used to maintain a constant pH (pH-stat) during the reaction. If it is desired to monitor pH changes during the reaction, then the pH meter can be connected to a recorder. It should be remembered I hat autotitration units have finite response times that are longer than many surface-controlled reactions. Thus, pH changes can occur before the autotitra-1 ion system can fully respond, and there will be some delay between the reac-lion and return to pH-stat conditions. 

In Eq. [1], protons are shown to be important in determining the equilibrium of the hydrolysis of a silicate mineral. Protons are important factors in determining dissolution rates of silicates, oxides, hydroxides, and hydrous oxides. Because of the relatively high Arrhenius activation energies of surface-controlled reactions, temperature is an especially important factor in determining dissolution rates. Anions that bind to mineral surfaces can... 

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