The formation of iron(III) oxide hydroxides from iron(II) oxalate

Abstract

FeC₂O₄,2H₂O(s) suspensions in sodium hydroxide solutions were oxidized by a fast air-bubble flow at room temperature until complete reaction had occurred. With amounts of NaOH in the range OH/Fe initial mole ratio ≤1.0, the reaction is FeC₂O₄, 2H₂(s)+ OH-(aq)+ ¼O₂(g) → ⅔γFeOOH(s)+1/3Fe(C₂O₄)₃^3-(aq)+13/6H₂O With OH/Fe mole ratio ≥ 2.0 the reaction is FeC₂O₄, 2H₂(s)+ 2OH-(aq)+ ¼O₂(g) → αFeOOH(s)+(C₂O₄)₃^3-(aq)+ 5/2 H₂O Mixtures of goethite (α-FeOOH) and lepidocrocite (γ-FeOOH) form at intermediate OH/Fe mole ratio. The oxidation process occurs in a solid-phase intermediate product. Comparisons with similar oxidations of iron(11)sulphate solutions showed that γ-FeOOH formation was favoured and α-FeOOH formation inhibited in the iron(11) oxalate oxidation. These differences are related to pH and dissolved iron concentrations.