Photochemical reductive dissolution

In heterogeneous photoredox systems also a surface complex may act as the chromophore. This means that in this case not a bimolecular but a unimolecular photoredox reaction takes place, since electron transfer occurs within the lightabsorbing species, i.e. through a ligand-to-metal charge-transfer transition within the surface complex. This has been suggested for instance for the photochemical reductive dissolution of iron(III)(hydr)oxides (Waite and Morel, 1984 Siffert and Sulzberger, 1991). For continuous irradiation the quantum yield is then ... 

Faust and Hoffmann (1986) and Litter and Blesa (1988) who investigated the wavelength-dependence of the rate of photochemical reductive dissolution of iron(III)(hydr)oxides using hematite-bisulfite and maghemite-EDTA as model systems, respectively. 

Rate of the photochemical reductive dissolution of hematite, = d[Fe(II)]/dt, in the presence of oxalate as a function of the wavelength at constant incident light intensity (I0 = 1000 peinsteins "1 lr1). The hematite suspensions were deaerated initial oxalate concentration = 3.3 mM pH = 3. (In order to keep the rate of the thermal dissolution constant, a high enough concentration or iron(II), [Fe2+] = 0.15 mM, was added to the suspensions from the beginning. Thus, the rates correspond to dissolution rates due to the surface photoredox process). 

If the surface complex is the chromophore, then the photochemical reductive dissolution occurs as a unimolecular process alternatively, if the bulk iron(III)(hydr)-oxide is the chromophore, then it is a bimolecular process. Irrespective of whether the surface complex or the bulk iron(IIl)(hydr)oxide acts as the chromophore, the rate of dissolved iron(II) formation depends on the surface concentration of the specifically adsorbed electron donor e.g. compare Eqs. (10.11) and (10.18). It has been shown experimentally with various electron donors that the rate of dissolved iron(II) formation under the influence of light is a Langmuir-type function of the dissolved electron donor concentration (Waite, 1986). 

The Iron Cycle in the Photic Zone of Surface Waters In the photic zone the formation of iron(II) occurs as a photochemical process. The photochemical iron II) formation proceeds through different pathways 1) through the photochemical reductive dissolution of iron(III)(hydr)oxides, and 2) through photolysis of dissolved iron(lll) coordination compounds, Fig. 10.16. 

As discussed in previous subchapters, the rate of the photochemical reductive dissolution of iron(III)(hydr)oxides depends on the concentration and type of surface complexes present and on the light intensity and its energy. Because the light intensity varies diurnally, also a diurnal variation in the iron(II) concentration can be expected in surface waters. This has been observed in acidic waters (McKnight and Bencala, 1988 Sulzberger et al., 1990). Fig. 10.17 shows such a diurnal variation in the concentration of dissolved Fe(II) in a slightly acidic alpine lake (Lake Cristallina) of Switzerland. 

When oxides in soils or sediments dissolve, substances adsorbed to the oxide surfaces will also be released into solution. Thus, for example, phosphate release into sediment pore waters accompanies the reductive dissolution of iron oxides in anoxic sediments. Release of phosphate into the overlying (oxic) water column is limited by phosphate adsorption on freshly precipitated amorphous iron oxides at the oxic-anoxic interface (9, 10). A similarly coupled cycle of phosphate and iron is observed in surface waters where photochemical reductive dissolution of iron oxides results in increased dissolved concentrations of ferrous iron and phosphate during the day (11). 

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 efficiency of detachment is a key parameter in the overall photochemical reductive dissolution kinetics, because in oxic environments detachment of reduced surface metal centers may be in competition with their oxidation (9, 10). 

Rate Expression of the Photochemical Reductive Dissolution of y-FeOOH by Oxalate. The kinetics of the light-induced reductive dissolution of oxide minerals obey the general rate expression of surface-controlled reactions. The rate is proportional to the concentration of the adsorbed reductant, as in the case of adsorbed oxalate ... 

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

Figure 1. Concentration of dissolved Fe(II) and of oxalate as a function of time upon photochemical reductive dissolution of y-FeOOH in a deaerated suspension at pH 3. Initial oxalate concentration was 1 mM. (Reproduced with permission from reference 10. Copyright 1994 Lewis Publishers.)...

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.)...

Sherman, D.M., Electronic structures of iron(lll) and magnanese(IV) (hydr)oxidc minerals Thermodynamics of photochemical reductive dissolution in aquatic environments, Geochim. Cosmochim. Acta, 69, 3249, 2005. 

Since the reduction of adsorbed molecular oxygen competes with detachment of the reduced surface iron from the crystal lattice, it is the efficiency of detachment that decides to what extent oxygen inhibits the photochemical reductive dissolution of hydrous iron(III) oxides. The efficiency of detachment depends above all on the crystallinity of the iron(III) hydroxide phase and is expected to be much higher with iron(IIl) hydroxide phases less crystalline and thus less stable than hematite. Not only does the efficiency of the light-induced dissolution of iron(III) hydroxides depend on their crystal and surface structure, but so does the efficiency of photoxidation of electron donors. Leland and Bard (1987) have reported that the rate constants of photooxidation of oxalate and sulfite varies by about two orders of magnitude with different iron(III) oxides. From their data they concluded that this appears to be due to differences in crystal and surface structure rather than to difference in surface area, hydro-dynamic diameter, or band gap. ... 

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