Dissolution of fe(iii)(hydr)oxides in natural waters; laboratory assessment on the kinetics controlled by surface coordination

Abstract

Dissolution of Fe (III) (hydr)oxides, of importance in oceanographic and limnological cycles of iron, occurs (1) in anoxic environments, (2) at the oxic-anoxic boundary of the water-sediment column, and (3) as a light-induced process in oxic surface waters. The various pathways of dissolution of Fe (III) (hydr) oxides, especially by reductants in thermal and in photochemical processes, are discussed and assessed on the basis of laboratory experiments. The relevance of these mechanisms for the transformation of iron in natural waters is discussed. It is shown that surface processes and not transport processes control the dissolution kinetics and that all the dissolution reactions are critically dependent on the type of complexes formed on the surface of the 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, because of the larger lability in the crystalline lattice surface of the Fe(II)-O bond than of the Fe(III)-O bond. A catalytic dissolution that appears to be important in natural waters is accomplished by Fe (II) in the presence of a bifunctional complex former (dicarboxylic acid, hydroxycarboxylic acid, diphenol). The latter is able to form on the surface of Fe (III)(hydr)oxides a ternary complex with Fe(II): >Fe III−X−Fe II (X≠bridging ligand). Electron transfer from Fe (II) to the surface Fe (III) ions occurs through the bridging ligand. In light-induced reductive dissolution of Fe (III) (hydr) oxides as well as in thermal reductive dissolution, the inner-sphere surface coordination of the electron donor to the oxide surface is essential for the efficient occurrence of the photochemical (or thermal) redox reaction.