EDTA-promoted dissolution

Many of these oxoanions can form, depending on concentration and pH, various surface complexes. This ability may explain the different effects observed under different solution conditions. For example, Bondietti et al. (33) found that phosphate at low pH (where mononuclear complexes are probably formed) accelerated EDTA-promoted dissolution of lepidocrocite, whereas at near-neutral pH conditions (where binuclear complexes are presumably formed), phosphate was an efficient inhibitor. Furthermore, because of the several geometries involved, the extent of comer sharing or edge sharing by adsorbed oxoanions may differ with the type of oxide and with allotropic modifications of the same metal oxide. 

Case Examples. The effects of various oxoanions on EDTA-pro-moted dissolution of lepidocrocite (y-FeOOH) have been studied by Bondietti et al. (33). EDTA was chosen as a reference system because it is dissolution-active over a relatively wide pH range. Phosphate, arsenate, and selenite markedly inhibit the dissolution at near-neutral pH values. At pH <5 phosphate, arsenate, and selenite accelerate the dissolution. It is presumed that the bi-nuclear surface complexes formed at near-neutral pH values by these oxoanions (Table II) inhibit the dissolution. Figure 8a displays data on the effect of selenite on EDTA-promoted dissolution, and Figure 8b shows that calculations on surface speciation by Sposito et al. (35) support the preponderance of binuclear selenite surface complexes in the neutral-pH range. Mononuclear surface species prevail at lower pH values. 

Figure 8. The effect of selenite on the EDTA-promoted dissolution of y-FeOOH 0.5 gIL). Part a At low pH the dissolution rate is increased by selenite at pH 7 it is strongly inhibited. Concentration of the ligands is given in inol/L. Part b Surface speciation on lepidocrocite as a function of pH according to Sposito et al. (35). These data suggest that binuclear selenite surface complexes are formed in the neutral pH range (from reference 33).

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... 

Fig. 7.15 Inhibitation of reductive- and ligand-promoted dissolution of iron oxides by oxoanions. a The dissolution of goethite by H S is inhibited by borate and phosphate, b The dissolution of lepidocrocite by EDTA is inhibited by phosphate and arsenate (adopted from Stumm and Morgan 1996, original data from Biber et at, 1994).

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. 

Ligands and metal complexes present in aqueous systems in contact with natural oxides can affect their dissolution either by promoting or inhibiting it. For example, some metal—EDTA complexes react with Fe2C>3 and dissolve it, producing [Fe(III)EDTA]. Other minerals like Co(III)OOH and Mn(III)OOH reductively dissolve by oxidizing ligands and metal complexes. Dissolution rates can... 

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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