Lepidocrocite photochemical dissolution

Similar photo-induced reductive dissolution to that reported for lepidocrocite in the presence of citric acid has been observed for hematite (a-Fe203) in the presence of S(IV) oxyanions (42) (see Figure 3). As shown in the conceptual model of Faust and Hoffmann (42) in Figure 4, two major pathways may lead to the production of Fe(II)ag i) surface redox reactions, both photochemical and thermal (dark), involving Fe(III)-S(IV) surface complexes (reactions 3 and 4 in Figure 4), and ii) aqueous phase photochemical and thermal redox reactions (reactions 11 and 12 in Figure 4). However, the rate of hematite dissolution (reaction 5) limits the rate at which Fe(II)aq may be produced by aqueous phase pathways (reactions 11 and 12) by limiting the availability of Fe(III)aq for such reactions. The rate of total aqueous iron production (d[Fe(aq)]T/dt = d [Fe(III)aq] +... 

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). 

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. 

To calculate the overall rate constants of the photochemical reductive dissolution of lepidocrocite, we experimentally determined the surface con-... 

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

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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