FORMATION OF IRON OXIDES BY DECOMPOSITION OF IRON-PHENOLIC CHELATES

Iron chelates of some simple di-and trihydric phenols can be decomposed by peroxide treatment or by hydrolysis to yield crystalline ferric oxides. The species of ferric oxide obtained depends upon the precipitating conditions and the nature of the chelating ligand. In oxidative virtually anion-free conditions, decomposition of all aged chelates yields a precipitate of disordered ferric oxide, which is referred to as‘protohaematite’, since it ages to haematite in water even at room temperature. Protohaematite is considered to be a discrete form of ferric oxide similar in structure to δ-FeOOH but devoid of hydroxyl groups and may be present in freely drained soils as a precursor of haematite. Hydrolysis of all unaged chelates, except that of iron protocatechuic acid, yields lepidocrocite. The redox cycle undergone by iron in this reaction may be analogous to one prevailing in gley soils. In the presence of montmorillonite, the iron-catechol system forms a clay-metal-organic complex, which also decomposes to yield lepidocrocite provided chloride ions are present on the clay surface. The fact that various inorganic gels amorphous to X-rays can also deplete the ligand content of the chelates indicates a possible inorganic decomposition mechanism for metal-organic chelates in soils.