Ascorbate reductive dissolution

Redox reactions are of importance in the dissolution of Fe-bearing minerals (Table 9.1). Reductive dissolution of Fe(III)(hydr)oxides can be accomplished with many reductants, especially organic and inorganic reductants, such as ascorbate, phenols, dithionite, HS, etc. Fe(II) in presence of complex formers can readily dissolve... 

The first two pathways (a) and (b) show, respectively, the influence of H+ and of surface complex forming ligands on the non-reductive dissolution. These pathways were discussed in Chapter 5. Reductive dissolution mechanisms are illustrated in pathways (c) - (e) (Fig. 9.3). Reductants adsorbed to the hydrous oxide surface can readily exchange electrons with an Fe(III) surface center. Those reductants, such as ascorbate, that form inner-sphere surface complexes are especially efficient. The electron transfer leads to an oxidized reactant (often a radical) and a surface Fe(II) atom. The Fe(II)-0 bond in the surface of the crystalline lattice is more labile than the Fe(III)-0 bond and thus, the reduced metal center is more easily detached from the surface than the original oxidized metal center (see Eqs. 9.4a - 9.4c). 

The following reaction sequence in reductive dissolution is plausible and is exemplified (Suter et al., 1991) here for the reaction of Fe(III)(hydr)oxide with ascorbate. It follows the general scheme given in Eqs. (9.4a) - (9.4c). 

Reactivity of Fe(III)(hydr)oxide as measured by the reductive dissolution with ascorbate. "Fe(OH)3" is prepared from Fe(II) (10 4 M) and HCO3 (3 10 4 M) by oxygenation (po2 = 0.2 atm) in presence of a buffer imidazd pH = 6.7 (Fig. a) and in presence of TRIS and imidazol pH = 7.7 (Fig. b). After the formation of Fe(III)(hydr)oxide the solution is deaerated by N2, and ascorbate (4.8 10 2 M) is added. The reactivity of "Fe(OH)3 differs markedly depending on its preparation. In presence of imidazole (Fig. a) the hydrous oxide has properties similar to lepidocrocite (i.e., upon filtration of the suspension the solid phase is identified as lepidocrocite). In presence of TRIS, outer-sphere surface complexes with the native mononuclear Fe(OH)3 are probably formed which retard the polymerization to polynuclear "Fe(OH)3" (von Gunten and Schneider, 1991). 

In view of its importance, reductive dissolution of Fe oxides has been widely studied. Reductants investigated include dithionite, thioglycolic acid, thiocyanate, hydrazine, ascorbic acid, hydroquinone, H2S, H2, Fe ", tris (picolinato) V", fulvic acid, fructose, sucrose and biomass/bacteria (Tab. 12.3). Under the appropriate conditions, reductive dissolution may also be effected photochemically. As with protonation, the extent of reduction may be strongly influenced by ligand and proton adsorption on the oxide surface. 

Dos Santos Afonso, M. Morando, P.J. Blesa, M.A. Banwart, S. Stumm.W. (1990a) The reductive dissolution of iron oxides by ascorbate. J. Colloid Interface Sci. 138 74-82... 

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... 

The surface reactions in reductive dissolution processes are illustrated for a Fe(III) oxyhydroxide solid phase in Eqs. 3.53-3.55, with data shown in Fig. 3.10 for the organic ligand ascorbic acid. Other examples include Mn(IV) or Mn(III) oxyhydroxide solids and ligands, such as quinones, phenols, and inorganic oxyanions. Taking bimessite (<5-Mn02) and selenite (SeOj ) as a case in point, one can adapt Eq. 3.53 in the form27... 

Reductive Dissolution of Fenl(hydr)oxlde Ascorbate, HA ... 

If these weak-field ligands are readily oxidizable (e.g., ascorbate), they initiate reductive dissolution (eq 14). Oxalate and EDTA in the presence of Fe(III) are thermodynamically metastable. Fe(II) complexed to these ions can reductively catalyze the dissolution of Fe(III) hydroxide (24). 

Figure 16 illustrates the reaction scheme that accounts for the reductive dissolution of Fe(III) (hydr)oxides by ascorbate. Figure 17 gives experimental results illustrating the zero-order dissolution rate with varying ascorbate... 

Itanwart, S., S. Davies, and W. Stumm (in press), The role of Oxalate in Accelerating the Reductive Dissolution of Hematite (a-Fe203) by Ascorbate, Colloids Surf. 

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).

Schematic representation of the various reaction modes for the dissolution of Fe(III)(hydr)oxides a) by protons b) by bidentate complex formers that form surface chelates. The resulting solute Fe(III) complexes may subsequently become reduced, e.g., by HS c) by reductants (ligands with oxygen donor atoms) such as ascorbate that can form surface complexes and transfer electrons inner-spheri-cally d) catalytic dissolution of Fe(III)(hydr)oxides by Fe(II) in the presence of a complex former e) light-induced dissolution of Fe(III)(hydr)oxides in the presence of an electron donor such as oxalate. In all of the above examples, surface coordination controls the dissolution process. (Adapted from Sulzberger et al., 1989, and from Hering and Stumm, 1990.)...

Fig. 12.16 Comparison of the dissolution of hematite at pH 3 by protonation (HNO3), complexation (50 xM oxalate), reduction (100 iM ascorbic acid) and combined complexation-reduction (Banwart et al., 1989, with permission).

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 16. Reaction sequence for the dissolution of Fe(III) (hydr)oxides by a reductant such as ascorbate. The fast adsorption of the reductant is followed by steps that involve the electron transfer Fe(III) is reduced to >Fe(II) and ascorbate oxidized to a radical that is desorbed form the surface. The resulting Fe II)—O bond at the surface is more labile than the Fe(III)—O bond. Then Fe(II) becomes detached from the surface and the original surface structure is reconstituted.

Much work has been published on the dissolution of iron oxides in connection with the iron cycle in geochemistry, decontamination processes or the clean-up of industrial facilities. We have already seen that strong chelating agents such as EDTA or amino acids can adsorb on the surface of oxides and promote their dissolution because they can form anion complexes that are more stable than the oxide [52,63,64], Citrates and oxalates, among others, act in a similar way [65], Dissolution of oxides is markedly accelerated if oxidation-reduction processes occur in conjunction with anion adsorption [66]. The adsorption of ascorbate on hematite is a good example [67] (Figure 9.16). The reduction of ferric ions is shown... 

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