The effect of pH on the kinetics of spontaneous Fe(II) oxidation by O2 in aqueous solution - basic principles and a simple heuristic description

Barak Morgan, Ori Lahav*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

333 Scopus citations

Abstract

The spontaneous chemical oxidation of Fe(II) to Fe(III) by O2 is a complex process involving meta-stable partially oxidized intermediate species such as green rusts, which ultimately transform into a variety of stable iron oxide end-products such as hematite, magnetite, goethite and lepidocrocite. Although in many practical situations the nature of the end-products is of less interest than the oxidation kinetics, it is difficult to find in the literature a description of all the basic steps and principles governing the kinetics of these reactions. This paper uses basic aquatic-chemistry equilibrium theory as the framework upon which to present a heuristic model of the oxidation kinetics of Fe(II) species to ferric iron by O2. The oxidation rate can be described by the equation (in units of mol Fe(II)/(l min)): - d [Fe2 +] / d t = 6 × 10- 5 [Fe2 +] + 1.7 [Fe (OH)+] + 4.3 × 105 [Fe (OH)20]. This rate equation yields a sigmoid-shaped curve as a function of pH; at pH values below ∼4, the Fe2+ concentration dominates and the rate is independent of pH. At pH > ∼5, [Fe (OH)20] determines the rate because it is far more readily oxidized than both Fe2+ and FeOH+. Between pH 5 and 8 the Fe (OH)20 concentration rises steeply with pH and the overall oxidation rate increases accordingly. At pH values > ∼8 [Fe (OH)20] no longer varies with pH and the oxidation rate is again independent of pH. The paper presents a heuristic overview of the pH dependent kinetics of aqueous ferrous oxidation by O2(aq) which we believe will be useful to professionals at both research and technical levels.

Original languageEnglish
Pages (from-to)2080-2084
Number of pages5
JournalChemosphere
Volume68
Issue number11
DOIs
StatePublished - Aug 2007
Externally publishedYes

Keywords

  • Aqueous phase
  • Ferrous iron
  • Kinetics
  • Oxidation rate
  • Oxygen
  • pH

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