MECHANISM OF FORMATION OF DELTA-FEOOH IN A BASIC AQUEOUS-MEDIUM

The mechanism of oxidation of ferrous hydroxide precipitated by mixing FeSO4.7H(2)O and NaOH solutions depends on the ratio R = [FeSO4]/[NaO H]. The study presented here concerns values of R ranging from 0.38 to 0.43, i.e. Fe(OH)(2) is precipitated in the presence of an excess of caustic soda. The analysis made by Mossbauer spectroscopy of the final compounds of oxidation shows that the value R = 5/12 is particular. A t this value, delta FeOOH is the main product of oxidation. Its format ion is assumed to take place through an intermediate compound, which f ormula is deduced from the value R = 5/12 to be: 3Fe(OH)(2).2Fe(OH)(3) .nH(2)O. This compound could be considered as a green rust one compoun d including excess OH- ions in the AcB i BaC j CbA k stacking, where A , B and C are the OH- planes, a, b and c the iron planes, and i, j and k layers made of the excess OH- ions and water molecules.