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- autocatalysis of the oxidation of Fe(II) by ferric hydroxide

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A review of the scientific literature reveals that the rates at which dissolved Fe(II) oxidises to Fe(III) and the Fe(OH)3 precipitate (Skeaff 2004):

  • exhibit linear reaction kinetics at Fe(II) loadings less than ~50 μm (~3 mg/L);
  • increase 100-fold with a unit increase in pH;
  • decrease with increasing ionic strength of the aqueous medium;
  • depend to some extent on the anions present in solution;
  • increase 10-fold for a 15 °C increase in temperature;
  • exhibit a linear dependence on the partial pressure of oxygen; and
  • exhibit negative deviations from linear reaction kinetics at Fe(II) loadings greater than ~50 μm (~3 mg/L) due to the autocatalytic oxidation of Fe(II) on the ferric hydroxide precipitate.

Based on empirical data reported in the literature, calculated half-times for the oxidation Fe(II) under an atmosphere of air range from 79 h at pH 6, 20.5°C and an ionic strength of 0.034 to 8 seconds at pH 8, 25 °C and an ionic strength of 0.009, which is close to that for the OECD TG 203 toxicity testing medium. The autocatalysis of the oxidation of Fe(II) by ferric hydroxide sets in at Fe(II) loadings greater than ~50 μm (~3 mg/L) and increases the reaction rate to levels greater than predicted by linear homogeneous reaction kinetics. Based on literature data and empirical reaction kinetics, it was calculated that, at pH 6 in the dilute 10x OECD TG 203 medium, the half-times are 11, 9 and 3.6 h, respectively, for the oxidation of Fe(II) as 1, 10 and 100 mg/L loadings of FeSO4.

  • Skeaff JM (2004). Review of the Oxidation of Ferrous Ion in Aqueous Media. Work Performed for Arcelor (Luxembourg, Luxemburg), CEFIC (Brussels, Belgium), EUROFER (Brussels, Belgium), Rio Tinto plc. (London, SW1Y 4LD, U.K.). Rev 10; Aug 17 2004. Self-published CANMET-MMSL 04-035 (CR)/Contract No. 602866 Natural Resources Canada. August 2004. 34 p.