Lepidocrocite oxalate

Ambe et aL, 1986) and phosphate adsorbed on lepidocrocite raised the adsorption of zinc (Madrid et aL, 1991a). Adsorption of Cd on goethite is increased by sulphate (Hoins et aL, 1993) and by oxalate (Lamy et aL, 1991). 

Leland and Bard (1987) found that the different iron oxides induced photooxidation of oxalate and sulphite at rates that varied by up to two orders of magnitude. For oxalate, the rate was greater for maghemite than for hematite, but this order was reversed for sulphite. Lepidocrocite (layer structure) induced faster oxidation of both compounds that did the other polymorphs of FeOOH (tunnel structures) the authors considered that the rate differences were probably associated with structural differences between the adsorbents. 

Fig. 12.11 Reaction schemes suggested for the photochemical reduction of lepidocrocite in the presence of citrate (left) (Waite Morel, 1984, with permission) and ofgoethite in the presence of oxalate (right) (Cornell Schindler, 1987, with permission).

Recently we presented (23) the results of an experimental study on the kinetics and mechanisms of the reaction of lepidocrocite (y-FeOOH) with H2S. With respect to the interaction between iron and sulfur, lepidocrocite merits special attention. It forms by reoxidation of ferrous iron under cir-cumneutral pH conditions (24), and it can therefore be classified as a reactive iron oxide (19). The concept of reactive iron was established by Canfield (19), who differentiated between a residual iron fraction and a reactive iron fraction (operationally defined as soluble in ammonium oxalate). The reactive iron fraction is rapidly reduced by sulfide or by microorganisms. 

How precisely the above methods will separate ferrihydrite from better crystalline oxides depends on the form and crystallinity of the latter. A positive relationship was found between Fco/Fct on the one hand and the surface area and XRD line width on the other for 14 synthetic goethites (range of FCo/Fct = 0.003-0.05) and 15 synthetic lepidocrocites (range of Foo/Fet = 0.06-077) (Schwertmaim, 1973). This shows that whereas goethites (and hematites) are essentially insoluble in oxalate irrespective of their crystal size, the method can only be used for well crystalline lepidocrocites. The same applies to the use of dilute strong acids. 

Carbonates Phosphates Silica Calcite Aragonite Vaterite Monohydrocalcite Amorphous Dahllite Francolite Amorphous calcium phosphate hydrogel Amorphous ferric phosphate hydrogel Opal Iron oxides Sulfates Halides Oxalates Magnetite Goethite Lepidocrocite Amorphous hydrates Celestite Barite Gypsum Fluorite Weddellite Whewellite... 

The kinetics of the photochemical reductive dissolution of lepidocrocite (y-FeOOH) with oxalate as the reductant depends strongly on pH both the rate and the overall rate constant, k> decrease with increasing pH. This behavior means that the pH dependence of the rate does not simply reflect the pH dependence of oxalate adsorption at the lepidocrocite surface. Between pH 3 and 5, the log k() values can be fitted with a straight line. The dependence of k on the concentration of surface protons, >FeOH2+, can be estimated from the slope of this line and from the protonation curve of lepidocrocite k0 >FeOHf I6. The value of 1.6, which can be considered only a rough estimate, is not too different from the theoretically expected value of 2 for the proton-catalyzed detachment of reduced surface iron centers (i.e., of surface metal centers with the formal oxidation state of II). 

The concentrations of ferrous iron were determined colorimetrically with ferrozene by measuring the absorbance of the Fe(II)-ferrozene complex at 562 nm according to a modified method described by Stookey (22). The concentrations of dissolved oxalate were determined by measuring the P counts of 14C-labeled oxalate. The ionic medium used for the adsorption and dissolution experiments was 5mM NaCl04. The pH values were established with HC104 and NaOH solutions. The lepidocrocite concentration used in the adsorption and dissolution experiments was 0.5 g/L. 

In deaerated lepidocrocite suspensions, the photochemical reductive dissolution with oxalate as the reductant occurs according to the following overall stoichiometry (Figure 1) ... 

Figure 2 shows the concentration of dissolved Fe(II) as a function of time at various pH values upon the photochemical reductive dissolution of lepidocrocite (deaerated suspensions) with oxalate as the reductant. The rate of dissolved Fe(II) formation (i.e., the slope of the straight lines through the experimental points) decreases strongly with increasing pH. 

According to equation 1, the overall rate constant is the rate of the photochemical reductive dissolution of lepidocrocite divided by the surface concentration of oxalate. The overall rate constant as a function of pH is shown in Figure 4. The three experimental points between pH 3 and 5 can be fitted reasonably well with a straight line ... 

Under our experimental conditions, the overall rate constant of the photochemical reductive dissolution of lepidocrocite in the presence of oxalate is pH-dependent. Thus, the pH dependence of the rate reflects more than the pH dependence of oxalate adsorption at the lepidocrocite surface. Various pH effects may account for this observed pH dependence of ka. One possibility is catalysis of detachment of the reduced surface iron centers by protonation of their neighboring hydroxo and oxo groups. The following question then arises How does the observed rate constant, ka, depend on surface protonation The general rate expression of the proton-catalyzed dissolution of oxide... 

Figure 3. Surface concentration of oxalate as a function of pH upon the specific adsorption of oxalate at the lepidocrocite surface. (Reproduced with permission from reference 10. Copyright 1994 Lewis Publishers.)...

Figure 4. Overall rate constant of the photochemical reductive dissolution of lepidocrocite with oxalate as the reductant, as a function of pH.

With the experimentally determined values n = 0.27 and m 0.17, j can be estimated j = n/m 1.6. This result indicates that, between pH 3 and 5, catalysis of detachment of the reduced surface iron centers by protons may be the predominant pH effect in this heterogeneous photoredox process, because the estimated j value of 1.6 is not too different from the theoretically expected value of 2 for reduced surface iron centers. However, readsorption of the photochemically formed Fe(II), blocking surface sites for the adsorption of oxalate, also has to be taken into account. Because the extent of adsorption of Fe(II) at the surface of lepidocrocite is expected to increase with increasing pH (11), this effect becomes increasingly important. Thus, at pH 6 a large fraction of the photochemically formed Fe(II) may become readsorbed at the reconstituted lepidocrocite surface before oxalate gets adsorbed. This process may explain the relatively low value of log k0 at pH 6. 

Figure 5. Qualitative representation of the energetics of the photochemical reductive dissolution of lepidocrocite with oxalate as the electron donor. >Fe uOx is the iron(III) oxalato surface complex (i.e., the precursor complex) in its electronically ground state and >FeOx is the precursor complex in its electronically excited state. AG is the free energy of the overall reductive dissolution process AGE7/ is the free energy of activation of formation of a reduced surface iron, >Fe(Il), and the oxidized oxalate, C204 and AGDE1 is the free energy of activation of detachment of the reduced surface iron from the crystal lattice. For the sake of simplicity, the oxidized product is omitted in this figure. (Adapted from reference 9. Copyright 1991 American Chemical Society.)...

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