Mechanism of formation of delta FeOOH in a basic aqueous medium

Abstract

The mechanism of oxidation of ferrous hydroxide precipitated by mixing FeSO4 · 7H2O and NaOH solutions depends on the ratioR=[FeSO4]/[NaOH]. The study presented here concerns values ofR 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 valueR=5/12 is particular. At this value, 8FeOOH is the main product of oxidation. Its formation is assumed to take place through an intermediate compound, which formula is deduced from the value R=5/12 to be: 3Fe(OH)2 · 2Fe(OH)3 ·nH2O. This compound could be considered as a green rust one compound including excess OH− ions in theAcB i BaC j CbA k stacking, whereA, B andC are the OH− planes,a, b andc the iron planes, andi, j andk layers made of the excess OH− ions and water molecules.