Ligand-promoted dissolution

The surface concentration of the particular surface species, Cj, corresponds to the concentration of the precursor of the activated complex. Note that we use braces ( and brackets [ ] to indicate surface concentrations [mol nr2] and solute concentrations [M], respectively. 

We will first describe a relatively simple scenario for the enhancement of the dissolution of Al203 by a (complex-forming) ligand. As we have seen ligands tend to become adsorbed specifically and to form surface complexes with the AI(III) Lewis acid centers of the hydrous oxide surface. They also usually form complexes with AI(III) in solution. Complex formation in solution increases the solubility. This has no direct effect on the dissolution rate, however, since the dissolution is surface-controlled. 

In nature, ligands that enhance the dissolution reaction such as oxalate, citrate, diphenols, hydroxy carboxylic acids are formed, as a byproduct of biodegradation of organic matter. Such ligands are also among the exudation products of plants and trees released through the roots. 

The trans influence is a ground state property it is attributable to the fact that ligands trans to each other both participate in the orbital of the metal ion (in our example, Al) the more one ligand preempts this orbital, the weaker will the bond to the other ligand be. 

In a similar way a sigma bond exerted by a functional surface OH-group to a metal ion causes a trans effect on the ligands bound to the metal ions (cf. Chapter 9.1). 

---

The scheme of Fig. 5.5a corresponds to steady state conditions (Table 5.1). We can now apply the general rate law (Eqs. 5.7, 5.8), the rate of the ligand-promoted dissolution, Rl, is proportional to the concentration of surface sites occupied by L (metal-ligand complex, >ML) or to the surface concentration of ligands, C (mol nr2) ... 

Ligand-promoted dissolution of iron(III)(hydr)oxides, where the iron(II) catalyzed dissolution plays the major role. 

Amrhein, C., and D. L. Suarez (1988), "The Use of a Surface Complexation Model to Describe the Kinetics of Ligand-Promoted Dissolution of Anorthite", Geochim. Cosmochim. Acta 52, 2795-2807. 

Hering, J., and W. Stumm (1991), "Fluorescence Spectroscopic Evidence for Surface Complex Formation at the Mineral-Water Interface Elucidation of the Mechanism of Ligand-Promoted Dissolution," Langmuir7, 1567-1570. 

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 ... 

Ludwig, C., Casey, W. H. Rock, P. A. (1995). Prediction of ligand-promoted dissolution rates from the reactivities of aqueous complexes. Nature, 375,44-7. 

Here Lj refers to a hgand, K refers to the rate constant for ligand-promoted dissolution, and... 

Amrhein C. and Suarez D. L. (1988) The use of a surface complexation model to describe the kinetics of ligand-promoted dissolution of anorthite. Geochim. Cosmochim. Acta 52, 2785-2793. 

Stillings L., Drever J. I., and Poulson S. R. (1998) Oxalate adsorption at a plagioclase (An(47)) surface and models for ligand-promoted dissolution. Environ. Sci. Technol. 32, 2856-2864. 

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.

As shown in Figure 13.19a, phosphate and borate inhibit the dissolution of goethite by H2S. Similarly, the dissolution of lepidocrocite (7-FeOOH) by EDTA (Y" ) is inhibited by phosphate and arsenate (Figure 13.19b). Both in the reductive dissolution (by H2S) and the ligand-promoted dissolution (by... 

X. Carrier, J.F. Lambert, and M. Che, Ligand-Promoted Dissolution in the Preparation of MoOx/y-AkOs Catalysts Evidence for the Formation and Deposition of an Anderson-type Alumino Heteropolymolybdate,/. Am. Chem. Soc., 119 (1997) 10137-10146. 

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). 

The proposed mechanism for the ligand-promoted dissolution of aluminum oxide involves three general steps (1) ligand adsorption and surface complex formation, (2) slow detachment of a surface metal center (as a complex with the ligand), and (3) regeneration of the surface (shown schematically in Figure 1). 

Figure 2. Ligand-promoted dissolution ofh-Al2Os (2.2 g/L). Part a Dissolution rates as a function of adsorbed ligand concentrations for a series of organic ligands. Part b Dissolution rate constants as a function of pH. Symbols (Q) oxalate, (A) malonate, (V) citrate, fD) salicylate, and (<>) benzoate. (Adapted with permission from reference 22. Copyright 1986 Pergamon Press.)...

---