Mn (II) Oxidation in the Presence of Metal Oxides

Citation

Davies, Simon Henry Richard (1985) Mn (II) Oxidation in the Presence of Metal Oxides. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/512V-8A90. https://resolver.caltech.edu/CaltechETD:etd-03212005-111003

Abstract

The oxidation of Mn(II) by oxygen in the presence of goethite (α-FeOOH), lepidocrocite (γ-FeOOH), silica and alumina was studied. All the solids, except perhaps alumina,enhanced the rate of Mn(II) oxidation. The degree of enhancement was as follows:

lepidocrocite > goethite > silica > alumina.

At constant pO₂ Mn(II) oxidation on goethite, lepidocrocite and silica can be described by the following equation

[Equation; see abstract in scanned thesis for details.]

where <≡SOH> is the concentration of the surface hydroxyl group and a is the solids concentration.

Mn(II) oxidation in the presence of goethite or lepidocrocite is first order in pO₂. Both these reactions are strongly temperature dependent (apparent activation energy ~100 kJ/mol). Normal laboratory lighting has no effect on the rate of these reactions.

The rate of Mn(II) oxidation in the presence of lepidocrocite is about 4 times slower in 0.7M NaClO₄, than in 0.1M NaClO₄. This reaction is inhibited by the following ions; Mg²⁺, Ca²⁺, silicate, salicylate, phosphate, chloride, and sulfate. Phthalate has little or no effect on the rate of this reaction.

The adsorptive behaviour of Mn(II) on the metal oxides studied could be described using a surface complexation model. Using this model it was shown that the rate of Mn(II) oxidation on the metal oxides studied is described by the equation

[Equation; see abstract in scanned thesis for details.]

where (≡SOH)₂Mn is a bidentate surface complex. It is possible that a hydrolyzed surface complex (≡SOMnOH) rather than the bidentate complex is involved in the reaction.

The results of the laboratory studies indicate that in natural waters the important factors which influence Mn(II) on metal oxides are pH, iron oxide concentration, temperature, [Mg²⁺], [Cl^-], and ionic strength. These studies predict that at pH < 7.5 the rate of Mn(II) oxidation in natural waters is slow (t_1/2 < 100 days). Mn(II) may be completely oxidized within a few days in iron-rich, high pH waters.

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