Dissolution reactions ligand-promoted

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 ... 

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.

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... 

Hypothesis Mononuclear Ligand Surface, Complexes Enhance and BInuclear Surface Complexes Inhibit the Dissolution. Binuclear surface complexes most likely are inert in promoting the dissolution reaction much more energy is needed to detach two center metal ions from the surface lattice simultaneously. Because binuclear surface com-... 

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). 

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 dissolution is controlled by the detachment of Al. Since the dissolution of silica is not promoted in presence of oxalate and salicylate (Bennett et al., 1988 Wieland, 1988), we may conclude that Si centers do not form stable surface complexes with these ligands. Hence, the siloxane layer of kaolinite and muscovite is not reactive with respect to dissolution reactions. Therefore, the detachment of both Al and Si is a consequence of the formation of surface complexes with Al sites. 

Precisely the last condition explains the fact that mainly ICC have been obtained by the immediate interaction of ligands and zero-valent metals. Thus, a large series of metal p-diketonates was synthesized in the absence of a solvent [513,634-638], for example, iron bis- and tra-acetylacetonates [635]. It was shown that other ligands can serve as activators or promoters in these processes. In particular, the introduction of a,a or y,y -bipy into the reaction mixture gives the possibility of isolating copper acetylacetonates and adducts of similar complexes of cobalt and nickel [636], meanwhile the p-diketonates of the metals above are not formed under conditions similar to those reported in Ref. 635. Under dissolution of more active metallic barium in the mixture of another p-dikctone - dipivaloyl-methane (DPM) - with dyglime (DG) or tetraglime (TG) in absolute pentane, the mononuclear complex [Ba(DPM)2(TG)] and binuclear complex [Ba2(DPM)4 ( t-H20)(DG)] were isolated and structurally characterized [637]. 

These concepts are illustrated in Fig. 3.10 for the reductive dissolution of hematite (a-Fe,03) in the presence of ascorbic acid at pH 3.26 In this example, Mox = Fe(III), MRed = Fe(II), and LRed = HA, where A2 is the ascorbate anion (log K = -4 for the dissociation of H2A°, but dissociation is invoked nonetheless to promote a ligand-exchange reaction). Equation 3.46 becomes... 

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...

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. 

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