Kinetics of reaction between O2 and Mn(II) species in aqueous solutions

The objective of this research is to assess critically the experimental rate data for O₂ oxidation of dissolved Mn(II) species at 25°C and to interpret the rates in terms of the solution species of Mn(II) in natural waters. A species kinetic rate expression for parallel paths expresses the total rate of Mn(II) oxidation as Σk_i a_ij, where k_i is the rate constant of species i and a_ij is the species concentration fraction in solution j. Among the species considered in the rate expression are Mn(II) hydrolysis products, carbonate complexes, ammonia complexes, and halide and sulfate complexes, in addition to the free aqueous ion. Experiments in three different laboratory buffers and in seawater yield an apparent rate constant for Mn(II) disappearance, k_app,j ranging from 8.6 × 10⁻⁵ to 2.5 × 10⁻² (M⁻¹s⁻¹), between pH 8.03 and 9.30, respectively. Observed values of k_app exceed predictions based on Marcus outer-sphere electron transfer theory by more than four orders of magnitude, lending strong support to the proposal that Mn(II) + O₂ electron transfer follows an inner-sphere path. A multiple linear regression analysis fit of the observed rates to the species kinetic rate expression yields the following oxidation rate constants (M⁻¹s⁻¹) for the most reactive species: MnOH⁺, 1.66 × 10⁻²; Mn(OH)₂, 2.09 × 10¹; and Mn(CO₃)₂²⁻, 8.13 × 10⁻². The species kinetic rate expression accounts for the influence of pH and carbonate on oxidation rates of Mn(II), through complex formation and acid-base equilibria of both reactive and unreactive species. At pH ∼8, the greater fraction of the total rate is carried by MnOH⁺. At pH greater than ∼8.4, the species Mn(OH)₂ and Mn(CO₃)₂²⁻ make the greater contributions to the total rate.