Light-induced dissolution of hematite

Light-induced dissolution of hematite in the presence of oxalate at pH 3. The deaerated hematite suspension was irradiated with light that had passed a monochromator (X = 375 nm I0 = 4 W nr2). Initial oxalate concentration = 3.3 mM. 

Siffert, Ch., and B. Sulzberger (1991), "Light-Induced Dissolution of Hematite in the Presence of Oxalate A Case Study", Langmuir 7,1627-1634. 

Shi JP, Khan A A, Harrison RM (1999) Measurements of ultrafine particle concentration and size distribution in the urban atmosphere. Sci Total Environ 235 51-64 Siefert RL, Pehkonen SO, Erel Y, Hoffmatm MR (1994) Iron photochemistry of aqueous suspensions of ambient aerosol with added organic-acids. Geochim Cosmochim Acta 58 3271-3279 Sievering H, Boatman J, Gorman E, Kim Y, Anderson L, Ennis G, Luria M, Pandis S (1992) Removal of sulphur from the marine boimdaiy layer by ozone oxidation in sea-salt aerosols. Nature 360 571-573 Siffert C, Sulzberger B (1991) Light-induced dissolution of hematite in the presence of oxalate-A case-study. Langmuir 7 1627-1634... 

CASE STUDY LIGHT-INDUCED DISSOLUTION OF HEMATITE IN THE PRESENCE OF OXALATE... 

Light-induced dissolution of hematite in the presence of oxalate 413... 

Influence of Oxygen on the Light-Induced Dissolution of Hematite in the... 

Figure 9, Light-induced dissolution of hematite in the presence of oxalate under nitrogen atmosphere, Experimental conditions 0.5 gL-1 hematite initial oxalate concentration ImmolL-1 pH = 3 /0 = 4kWm"2 white light from a high-pressure xenon lamp after passing a Pyrex glass filter irradiated surface 50 cm2 reaction volume 250 mL (Siffert, 1989).

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. ... 

Figure 12.10. Schematic representation of the various steps involved in the light-induced reductive dissolution of hematite in the presence of oxalate. (Adapted from Sulzberger et al., 1989.)...

In addition to the dark oxidation of S(IV) on surfaces, there may be photochemically induced processes as well. For example, irradiation of aqueous suspensions of solid a-Fe203 (hematite) containing S(IV) with light of A > 295 nm resulted in the production of Fe(II) in solution (Faust and Hoffmann, 1986 Faust et al., 1989 Hoffmann et al., 1995). This reductive dissolution of the hematite has been attributed to the absorption of light by surface Fe(III)-S(IV) complexes, which leads to the generation of electron-hole pairs, followed by an electron transfer in which the adsorbed S(IV) is oxidized to the SO-p radical anion. This initiates the free radical chemistry described earlier. 

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