EDTA-promoted dissolution of lepidocrocite

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. 

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... 

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. 

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).

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