Oxalate-promoted dissolution

Figure 1. Schematic diagram of oxalate-promoted dissolution of an oxide mineral for M = Al(III) or Fe(III) based on the surface-controlled dissolution model. (Reproduced with permission from reference 22. Copyright 1986 Per-...

Figure 10. The oxalate-promoted dissolution of kaolinite. Both A1 and Si detachment is promoted in the presence of oxalate. The linear increase of A1 and Si concentrations represents the zero-order kinetics indicating the homogenity of surface sites. Figure 10c displays a schematic representation of the kaolinite structure. It reveals the 1 1 structure due to the alternation of silica-type (black) ami gibbsite-type layers (white).

Figure 11 gives the correlations between the rate of oxalate promoted dissolution and the surface oxalate concentrations. It indicates that there is a linear correlation between the detachment of Si from the kaolinite surface and (he oxalate concentration hence, a pH-independent rate constant kOXiSi can be evaluated (Table 2). Although the rates of the oxalate-promoted detachment of Al, R0xA1, are linearly proportional to increasing surface oxalate concentrations, I lie evaluated rate constants k0x A1 significantly depend on pH (Fig. 11). The ability of oxalate to remove Al from the kaolinite surface decreases with decreasing H+ activity in solution. It is interesting to notice that the apparent rate constant kQx of oxalate-promoted dissolution of <5-Al203 takes a pH-mdependent value of 10.8 x 10"3 h-1 (Table 2) (Furrer and Stumm, 1986).

Figure 11. The rate of oxalate-promoted dissolution, / 0j A), depends linearly on the surfad concentration of oxalate. The pH-independent rate constant, kDx Si, determined from the oxalate promoted detachment of Si, is 1.71 x 10 3 0.22 x 10 3 h . The rate constant,/c0liAI, increases with increasing H+ activity 0.34x 10 3 h 1 (pH 5), 0.88 x 10 3 h 1 (pH 4), 2.31 x 10 3 h (pH 3).

I-inure 12. The proton- and oxalate-promoted dissolution of muscovite. The slow weathering kinetics is a characteristic of micas. Oxalate affects the stoichiometry of A1 and Si release, but has not i significant catalytic effect. Figure 12c displays a schematic representation of the muscovite structure. It reveals the 2 1 structure. For example, an A1 layers (black) exists in an octahedral sheet between two tetrahedral sheets (white) whose cations are composed or25% A1 and 75% Si. Siioxane and edge surfaces are exposed lo solution. 

Figure 13. The dependence of the rates of proton- and oxalate-promoted dissolution on solution pH. The rate of the proton-promoted A1 release linearly decreases to higher pH. Three sequences of pH dependence may be identified for the proton-promoted detachment of Si, Proton- and oxalate ...

Examples include the oxalate (10 M) promoted dissolution of goethite, hematite and ferrihydrite over the pH range 3-5 (Fig. 12.2) in the absence of the ligand, dissolution at this pH is essentially zero (Stumm et al., 1985). 

Reductive dissolution may be more complex than the two previous mechanisms in that it involves electron transfer processes. Formation of Fe" via reductive dissolution can be effected by adsorption of an electron donor, cathodic polarization of an electrode supporting the iron oxide and by transfer of an electron from within a ternary surface complex to a surface Fe ". Addition of Fe" to a system containing a ligand such as EDTA or oxalate promotes electron transfer via a surface complex and markedly accelerates dissolution. 

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.

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...

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.)...

TABLE 2. Rate Constants of Oxalate- and Salicylate-Promoted Dissolution... 

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).

Promotion of the dissolution of an oxide by a ligand. The ligand illustrated here, in a short hand notation, is a bidentate ligand with two oxygen donor atoms (such as in oxalate, salicylate, citrate or diphenols). 

Pathway (d) in Fig. 9.3 provides a possible explanation for the efficiency of a combination of a reductant and a complex former in promoting fast dissolution of Fe(III) (hydr)oxydes. In this pathway, Fe(II) is the reductant. In the absence of a complex former, however, Fe2+ does not transfer electrons to the surface Fe(III) of a Fe(III) (hydr)oxide to any measurable apparent extent. The electron transfer occurs only in the presence of a suitable bridging ligand (e.g., oxalate). As illustrated in Fig. 9.3d, a ternary surface complex is formed and an electron transfer, presumably inner-sphere, occurs between the adsorbed Fe(II) and the surface Fe(III). This is followed by the rate-limiting detachment of the reduced surface iron. In this pathway, the concentration of Fe(U)aq remains constant while the concentration of dissolved Fe(III) increases thus, Fe(II)aq acts as a catalyst to produce Fe(II)(aq) from the dissolution of Fe(III)(hydr)oxides. 

Specific adsorption of ligands can enhance or inhibit dissolution rates by altering the strength and lability of Me-0 lattice bonds. Salicylate, oxalate, and citrate promote the dissolution of alumina (40). In the presence of ligand (L) the dissolution rate becomes (7 ) , ... 

When particles or large molecules make contact with water or an aqueous solution, the polarity of the solvent promotes the formation of an electrically charged interface. The accumulation of charge can result from at least three mechanisms (a) ionization of acid and/or base groups on the particle s surface (b) the adsorption of anions, cations, ampholytes, and/or protons and (c) dissolution of ion-pairs that are discrete subunits of the crystalline particle, such as calcium-oxalate and calcium-phosphate complexes that are building blocks of kidney stone and bone crystal, respectively. The electric charging of the surface also influences how other solutes, ions, and water molecules are attracted to that surface. These interactions and the random thermal motion of ionic and polar solvent molecules establishes a diffuse part of what is termed the electric double layer, with the surface being the other part of this double layer. 

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