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

The light-induced dissolution of hematite in the presence of oxalate occurs (a) through the photochemical reductive dissolution of hematite resulting in formation of dissolved Fe(II) and oxidized oxalate and (b) through the Fe(II)-catalyzed thermal dissolution of hematite, resulting in formation of dissolved Fe(III). These two dissolution pathways are coupled via the photochemical reduction of dissolved iron(III), i.e. photolysis of iron(III) oxalato complexes, leading to an autocatalytic dissolution of hematite. The rate of the photochemical reductive dissolution of hematite is strongly wavelength-dependent, and the reaction occurs only in the near-UV (lambda < 400 nm). From this wavelength-dependence we conclude that the electronically excited state involved in this heterogeneous photoredox reaction is a ligand-to-metal charge-transfer state, either of the iron(III) oxalato surface complex or of the bulk hematite (or both). Oxygen strongly inhibits the reductive dissolution of hematite under the influence of light. This phenomenon is interpreted in terms of competition between reoxidation of reduced surface iron by oxygen and phase transfer of surface Fe(II) into solution. These observations indicate that in this system detachment of surface Fe(II) from the crystal lattice is the rate-determining step of the overall dissolution process rather than electron transfer. In the presence of oxygen hematite acts as a photocatalyst for the oxidation of oxalate by oxygen. The rate of this photocatalytic oxalate oxidation exhibits a similar wavelength-dependence as does the photochemical reductive dissolution of hematite.