Dissolution reactions proton-promoted

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, . 

Figure 13.10. Schematic representation of the oxide dissolution processes [exemplified for Fe(III) (hydr)oxides] by acids (H ions), ligands (example oxalate), and reductants (example ascorbate). In each case a surface complex (proton complex, oxalato and ascorbato surface complex) is formed, which influences the bonds of the central Fe ions to O and OH on the surface of the crystalline lattice, in such a way that a slow detachment of a Fe(III) aquo or a ligand complex [in case of reduction an Fe(ll) complex] becomes possible. In each case the original surface structure is reconstituted, so that the dissolution continues (steady-state condition). In the redox reaction with Fe(III), the ascorbate is oxidized to the ascorbate radical A . The principle of proton-promoted and ligand-promoted dissolution is also valid for the dissolution (weathering) of Al-silicate minerals. The structural formulas given are schematic and simplified they should indicate that Fe(III) in the solid phase can be bridged by O and OH.

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... 

Binding of complex-forming ligands to oxide and hydroxide surfaces increases dissolution rates. Stumm and Furrer (1987) suggest that in acidic solutions the measured rate of dissolution of an oxide or hydroxide can be treated as the sum of the rate of the proton-promoted reaction (/ h) plus the rate of the ligand-promoted reaction (7 l )... 

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. 

This approach seems to fit the data of Stumm and coworkers for dissolution of oxides and hydroxides in dilute acids, but Carroll-Webb and Walther (1988) suggest for corundum that 7 = 1, and that the required number of protons adjacent to an Al to promote detachment from the surface need not be equal to the number required to convert the Al in corundum to aP . At sufficiently low pH, the dissolution of Al(0H)3 is first-order with respect to protons in solution (Pulfer et al., 1984 Bloom and Erich, 1987) and dissolution cannot be explained by the surface complexation model. For this reaction, protonation rather than detachment of surface sites appears to be rate controlling. 

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. 

Metals and metalloids on the surface of silicate minerals are also connected by oxide or hydroxide ion bridges, which undergo acid-base reactions with the adjacent aqueous solution. The results of these acid-base reactions are measurable as surface charge, which varies in concentration with solution pH. (The situation is a little more complicated than this, especially for minerals that have a structural charge due to uncompensated cation substitutions. The reader is directed to, for example, Schindler and Stumm 1988.) The enhancement of dissolution rates via adsorption of hydrogen ions is referred to as the proton-promoted pathway for dissolution (Furrer and Stumm 1986) and is analogous to the proton-promoted pathway for the dimer dissociation discussed above. 

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