Proton-promoted dissolution

A weakening of the critical metal-oxygen bonds occurs as a consequence of the protonation of the oxide ions neighboring a surface metal center and imparting charge to the surface of the mineral lattice. The concentration (activity) of D should reflect that three of such oxide or hydroxide ions have to be protonated. If there is a certain numer of surface-adsorbed (bound) protons whose concentration (mol nr2) is much lower than the density of surface sites, S (mol 2), the probability of finding a metal center surrounded with three protonated oxide or hydroxide ions is proportional to (CJ/S)3. Thus, as has been derived from lattice statistics by Wieland et al. (1988), the activity of D is related to (C )3, and the rate of proton-promoted dissolution, Rh (mol nrr2 lr1), is proportional to the third power of the surface protonation ... 

The first of these reactions is a hydrolysis process, the second is a carbonic acid-promoted dissolution, and the third is a proton-promoted dissolution. Equations 3.59b and 3.59c are the forward reactions in Eqs. 3.17 and 3.15, respectively. They provide a mechanistic underpinning for the dependence of kd in Eq. 3.14 on pH or pc0, as discussed in Section 3.1. Indeed, if Eq. 3.7 is applied to the forward reaction in Eq. 3.14 and rate laws for Eq. 3.59 are developed consistently with the hypothesis leading to Eq. 3.7, the result is7,33,34... 

Proton-promoted dissolution reactions are exemplified for carbonates, silicates, and metal oxyhydroxides by Eqs. 3.15, 3.18-3.20, 3.25, 3.39, 3.46, 3.53, 3.56, and 3.59c. The typical response of the rate of dissolution to varying pH is illustrated in Fig. 3.2, and this response is often hypothesized to be a result of the proton adsorption-bond-weakening structural detachment sequence described in connection with Eq. 3.60.36 This sequence can be represented by the following generic reaction scheme ... 

The dissolution reaction in Eq. 3.59b can be regarded as an example of a ligand-promoted process, in that adsorbed bicarbonate species are likely to play a role as intermediates in the kinetic analysis of the reaction.5 Ligand-promoted dissolution reactions are a principal basis for the reductive dissolution processes described in Section 3.4 (see Eq. 3.46). The sequence of steps is analogous to that in proton-promoted dissolution ... 

These three concentration-time equations describe the proton-promoted dissolution reaction in Eq. 4.51 under the assumption that the selenate detachment reaction is already at equilibrium. The case K < < K corresponds to the approximate equality in Eq. 4.52f. Note that the rate of mineral dissolution, d[Mn2+]/dt, will be constant for observation times much smaller than 1/k, . 

The literature data for ortho-, soro-, ino-, and phyllosilicate dissolution at 25 °C derived from long duration dissolution experiments (> a month except for wollastonite and forsterite. Tables 3 and 5) bracket the value of the order with respect to H, n (see Equation (17)), between 0 and 0.85 at 25 °C. The higher values of n from the literature tend to be for silicates containing iron. Extrapolating the rate constant for the proton-promoted dissolution rate constant to other temperatures, h(T), can be accomplished with the Arrhenius equation ... 

Figure 13.11. Ligand- and proton-promoted dissolution of AI2O3. (a) The ligand-catalyzed dissolution of a trivalent metal (hydr)oxide. (b) Measurement of Al(UI)(aq) as a function of time at constant pH at various oxalate concentrations. The dissolution Idnetics are given by a reaction of zero order. The dissolution rate, / l, is given by the slope of the (Al(III)(aq)] versus time curve, (c) Dissolution rate as a function of the surface ligand concentration for various ligands. The dissolution is proportional to the surface concentration of the ligand, <=MeL> or C(. (/ l = (d) Proton-promoted...

To exemplify inhibition effects, we choose a few case studies with Fe(III)(hydr)oxides because these oxides are readily dissolved with protons, ligands, and reductants and are of great importance in the iron cycles in natural waters. The reductive dissolution of Fe(III) minerals by a reductant such as H2S is much faster than ligand- or proton-promoted dissolution. The dissolution reaction, as shown by Dos Santos-Afonso and Stumm (1992), is initiated by the formation of =FeS and =FeSH surface complexes the subsequent electron transfer within the complex leads to the formation of Fe(II) centers in the... 

Figure 13.21. Effect of 10 M Cr(III) on the proton-promoted dissolution (pH = 3) of a-FeOOH (0.5 g liter ) in 0.1 M KNO3. (From Bondietti et al., 1993.)...

The rate law for proton-promoted dissolution of the AI13 molecule has been determined through experiments by Furrer and colleagues (Wehrli et al. 1990 Furrer et al. 1999 Amirbahman et al. 2000), but no data yet exist for the GaAli2 or GeAli2 molecules. These data allow geochemists to compare dissolution of the molecule with the rates of dissociation of some of the Al-0 bonds, which cannot be done with any other system. 

Figure 10 illustrates that Crw effectively inhibits the proton-promoted dissolution of goethite. Cr(III) adsorbs even at low pH and, as bi- or polynuclear surface complexes, blocks surface sites from being protonated. Furthermore, isomorphically substituted Cr3+ ions, characterized by an extremely low water-exchange rate, impart inertness to the surface lattice bonds. 

The apparent dependence of PO4-promoted dissolution on solution PO4 rather than adsorbed P contrasts with the results of Stumm et al. (1985) for organic ligands. Stumm et al. (1985), however, studied the reaction rates at higher pH values (3-6). As with proton-promoted dissolution, the reaction mechanism at higher pH may be different from mechanisms occurring at low pH. The rate-controlling step for phosphate- and possibly fluoride-mediated dissolution may not be the detachment of the complex from the surface. If surface detachment is sufficiently rapid, surface complex formation may be rate limiting. 

For proton-promoted dissolution, the adsorption of protons can be predicted using the constant capacitance model for the formation of protonated sites on oxide surfaces (Sposito, 1983 Hayes and Leckie, 1987). The rate can then be related to the quantity of protonated sites, [= MOH2 ], by the equation... 

In acid (proton)-promoted dissolution of oxides the probability of finding a surface site in the form of a precursor configuration is proportional to the surface concentration of protonated sites to the power j... 

Figure 6 schematically depicts the proton-promoted dissolution of hydrous oxides (e.g., A1203). In fast initial steps, the protons become bound to the surface hydroxyl groups or to the oxide ions closest to the metal center at the surface of the lattice. Subsequent to surface protonation, the detachment of the metal ion from the surface is the slowest of the consecutive steps. Therefore, the rate of the proton-promoted dissolution, RH, is proportional to the concentration (activity) of the surface species D(Fig. 6). 

Figure 6. (a) Schematic representation of the proton-promoted dissolution process at a M203 surface site. Three preceding fast protonation steps are followed by a slow detachment of the metal from the lattice surface, (b) The reaction rate derived from individual experiments is proportional to the surface protonation to the third power. 

The pH-dependent rates of kaolinite dissolution are presented in Figure 13. Within the experimental time range (t= 10-15 days), the proton-promoted dissolution of kaolinite occurs nonstoichiometrically, that is, the detachment of Si is faster than the release of A1 from the kaolinite surface (jRh.s h.ai)-... 

Generally, the protonation of Al sites promotes the dissolution process with increasing H+ activity in acid solution (A1203, kaolinite, muscovite), whereas the rate of silica dissolution even decreases or remains constant (pH < 3). Obviously (lie more Al centers are exposed per unit surface area, the higher the proton-promoted dissolution rate and the more effective are surface chelates in catalyzing the weathering process. 

Figure 15. The pH dependence of the proton-promoted dissolution rates ofkaolinite, muscovite, and their constitutent oxides of A1203 and amorphous Si02 or quartz, respectively. With increasing H4 activity, the rate of A1 detachment is promoted whereas the rate of Si detachment is slowed down.

Observations indicate that Ca-rich feldspars are more susceptible to organo-ligand promoted dissolution than are the Na-or K-feldspars in experiments, soils and lithified sediment (32.70.73). although the proton-promoted dissolution rates of albite, anorthite and K-feldspar are approximately equal (22). The rate of feldspar dissolution promoted by organo-ligands is proposed to increase when inner-sphere adsorption of organo-ligands on aluminosilicates weakens critical crystal lattice bonds at the site of adsorption (77). 

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