THE CYCLING OF IRON IN NATURAL ENVIRONMENTS - CONSIDERATIONS BASED ON LABORATORY STUDIES OF HETEROGENEOUS REDOX PROCESSES

The various pathways for the oxygenation of ferrous iron and for the dissolution of Fe(III) (hydr)oxides, especially by reducing ligands with oxygen donor atoms in thermal and photochemical processes, are assessed on the basis of laboratory experiments for application to natural systems. The typically large specific surface area of Fe-bearing solids in natural systems and the ability of these surfaces to interact chemically (surface complexation, ligand exchange) with reductants and oxidants facilitates electron transfer as well as dissolution and precipitation. Adsorption of Fe(II) to particle surfaces (complexation with surface hydroxyl groups) enhances the oxygenation rate of Fe(II) in a similar way as hydrolysis in solution (complexation with OH- ions). Surface processes (and not transport processes) control the dissolution kinetics. The rate of dissolution is proportional to the surface complexes formed on the surface of Fe(III) (hydr)oxides. Thus, a reductant, such as ascorbate, exchanges electrons with a surface Fe(III) ion subsequent to its inner-sphere coordination to the oxide surface. The Fe(II) thereby formed becomes more easily detached from the surface. Complex formation reactions of Fe(III) and Fe(II) with organic and inorganic ligands to form solute and solid complexes makes it possible that electron cycling of Fe(III) - Fe(II) transformations can occur over the entire E(H) range within the stability of water (E(H) from -0.5 V to +1.1 V). Solid and solute Fe(II) complexes with silicates, with hydrous oxides (e.g., Fe3O4), and with sulfides are very efficient reductants from a thermodynamic as well as from a kinetic point of view. Photosynthetic processes occurring on some inorganic Fe-bearing surfaces (semiconductors) and with iron species may be looked at as "primitive" alternatives or precursors to biological photosynthesis. In light-induced reductive dissolution of Fe(III)-(hydr)oxides, dissolved Fe(II) (example: reduction of solid Fe(III) phases with an organic ligand such as oxalate) is formed. In the heterogeneous photoredox reaction, the inner-sphere surface coordination of the electron donor to the oxide surface is essential for the efficiency of the electron transfer.