Surface protonation Ligand-promoted dissolution

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.

However, the major recent advance in the modeling of mineral dissolution has been the presentation by Stumm and coworkers (Furrer and Stumm, 1986 Zinder et al., 1986 Stumm arid Furrer, 1987) of a surface coordination approach to explain the proton- and ligand-promoted dissolution of simple oxides (A1203, BeO, a-FeOOH). This approach, which relies on surface titrations and double layer concepts for characterizing the chemical speciation on mineral surfaces, offers several advantages ... 

The ligand-promoted dissolution is illustrated by the effect of bidendate chelate formers seen in Figure 5. An example of the acid-promoted dissolution of A1203 and the dependence of some types of oxides on surface protonation is given in Figures 6 and 7. 

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. 

There is clear evidence that the dissolution of oxide minerals is promoted by the specific sorption of solutes at the mineral-solution interface. Moreover, it has been found that comparatively simple rate laws are obtained if the observed rates are plotted against the concentrations of adsorbed species and surface complexes (Pulfer et al., 1984 Furrer and Stumm, 1986). For example, in the presence of ligands (anions and weak acids) surface chelates are formed that are strong enough to weaken metal-oxygen bonds and thus to promote rates of dissolution proportional to their surface concentrations. Simple rate laws have been also observed with H+—or OH —promoted dissolution of oxides in a manner that can be predicted from knowledge of the oxide composition and the surface concentrations of protons and hydroxyl radicals. 

Fig. 7.14 Pathways of Fe(III)(hydr)oxide dissolution. From the left to right Proton-, ligand- (here oxalate), and reductant- (here ascorbate) promoted dissolution is initiated by surface complexation. The subsequent step of detachment (Fe, Fe -ligand, Fe ) is rate determining. Note that the shown pathways of dissolution are fundamental for the described extractions (chapter 7.5) (adopted from Stumm and Morgan, 1996).

---