The Kinetics of Redox Reactions of Mn(II) and Mn(III) in Aqueous Systems: Homogenous Autoxidation of Mn(II) and the Formation and Disappearance of Mn(III) Complexes

Citation

Klewicki, J. Kenneth (1996) The Kinetics of Redox Reactions of Mn(II) and Mn(III) in Aqueous Systems: Homogenous Autoxidation of Mn(II) and the Formation and Disappearance of Mn(III) Complexes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/40pv-rq92. https://resolver.caltech.edu/CaltechETD:etd-12192007-155028

Abstract

The kinetics of manganese redox reactions are important for understanding redox cycles in natural waters. This study examined the kinetics of the homogenous oxidation of Mn(II) and formation and disappearance of Mn(III) complexes.

The oxidation of Mn(II) was studied to determine the homogenous oxidation rate in the absence of solid surfaces and biological activity. Experiments were conducted at 35, 45, 50, and 60°C. The pH was 8.0. The reaction solution was prepared so that at no time during the experiment was the solubility product of any solid phase exceeded. Oxidized Mn was measured using leuco crystal violet dye reagent. Measurable rates were observed for the 45, 50, and 60°C experiments. An Arrhenius expression was fitted to the rates in order to extrapolate to 25°C. The second order rate constant for the rate expression

-d[Mn(II)]/dt = k⋅[Mn(II)⋅[O₂]

was calculated to be 6.9 ± 1.6 x 10^-7 M^-1s^-1.

The kinetics of disappearance of Mn(III) complexes from aqueous solution were studied. Complexes of pyrophosphate (P₂O₇^4-), ethylenediaminetetracetate (EDTA), and citrate (CIT) were synthesized from MnO_4- and a Mn(II) salt in a 1:4 ratio in the presence of excess ligand. Concentrations of Mn(III) complex were monitored spectrophotometrically. Experiments were conducted in the pH range of 6 to 9 for pyrophosphate and citrate and 3 to 9 for EDTA. The total manganese concentration was varied between 0.5 and 1.0 mM. Ligand concentrations were varied from 0.5mM to 200mM. Experiments were also conducted to examine the effects of oxygen, light, and ionic strength. Oxygen had a significant effect on only the citrate complex; ionic strength affected only the EDTA complex. Light was found to be insignificant in all cases.

The Mn(III)P₂O₇ complex was found to disappear from solution relatively slowly providing the ligand was in at least ten-fold excess. Disappearance time scales were on the order of 10⁷ s. The Mn(III)EDTA complex reacted rather rapidly with time scales on the order of 10⁴ s. There were at least two Mn(III)EDTA complexes, a protonated one more stable at low pH and an unprotonated one more stable at high pH. The pK_a of the complex appeared to be approximately 5.3. The rate of disappearance of the Mn(III)EDTA had a fractional dependence on pH, probably indicative of an unknown pH dependent intermediate in the decomposition of the complex. The rate was found to increase with increased EDTA, indicating that the rate limiting step was an outer sphere electron transfer from Mn(III)EDTA to an excess EDTA. The rate law for the reaction above pH 6 was found to be

-d[Mn(III)EDTA]/dt = k⋅[H⁺]^0.31⋅[EDTA]^1.35⋅[Mn(III)EDTA]

The Mn(III)CIT complex was found to undergo a redox cycle. The Mn(III)CIT complex was reduced, forming Mn(II). The Mn(II) was then oxidized in the presence of oxygen to re-form the Mn(III) complex. Both pH and ligand concentration were found to have fractional orders in the rate expression, largely due to the competition between the reduction and the oxidation and possibly complicated by radicals formed by the reaction.

The dissolution of MnOOH by pyrophosphate, EDTA, and citrate was studied. A MnOOH solid was synthesized by oxidizing Mn(II) with hydrogen peroxide at elevated temperatures and high pH. The solid was identified by X-ray diffraction to be β-MnOOH, with some contamination by Mn₃O₄. Throughout the dissolution process samples were removed by pipette and filtered. The filtrate was analyzed spectrophotometrically for the presence of Mn(III) complexes and total Mn. The solids captured on the filter were analyzed by an iodine titration technique, coupled with formaldoxime measurements to determine the average oxidation state of the solids. The effects of pH and ligand concentration on rates were examined.

Pyrophosphate was found to dissolve the Mn(III) solids nonreductively, producing the Mn(III) complex in solution. The dissolution reaction rate was dependent on approximately the half power of [H⁺], possibly indicative of a surface binding ligand binding on the surface. No dependence on the ligand concentration was found down to a ligand:Mn ratio of 10:1, probably indicative of surface site saturation by ligand.

EDTA was found to dissolve the solids reductively with no Mn(III) solution species being observed. The dependence on [H⁺] was approximately one half order, possibly indicative of a surface binding.

Citrate dissolved the MnOOH solids in what appeared to be two steps. There seemed to be an initial stage of nonreductive dissolution, followed by a reductive dissolution. The rate and duration of the two different stages depended on pH. The dependence was slightly greater than first order in [H⁺], possibly indicating the reaction becomes controlled by reactions of the radicals produced by oxidation of the citrate.

This study has shown that Mn(III) complexes can be formed in pH conditions relevant to natural waters. These complexes can be formed either through oxidation of Mn(II) by strong oxidants in the presence of stabilizing ligands or by dissolution of Mn(III)-containing solids by stabilizing ligands. Once formed, the lifetime of these complexes will depend on the nature of the ligand and chemical characteristics of the aquatic environment. If the ligand does not rapidly reduce Mn(III) these complexes can be powerful mobile oxidants which could significantly affect the local redox environment.

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