Rapid photo-oxidation of Mn(II) mediated by humic substances

The oxidation of Mn(II) by O 2 to Mn(III) or Mn(IV) is thermodynamically favored under the pH and pO 2 conditions present in most near surface waters, but the kinetics of this reaction are extremely slow. This work investigated whether reactive oxygen species, produced through illumination of humic substances, could oxidize Mn at an environmentally relavent rate. The simulated sunlight illumination of a solution containing 200 μM Mn(II) and 5 mg/L Aldrich humic acid buffered at pH 8.1 produced ∼19 μM of oxidized Mn (MnO x where x is between one and two) after 45 minutes. The major oxidants reponsible for this reaction appear to be photoproduced superoxide radical anion, O 2 −, and singlet molecular oxygen, 1O 2. The dependencies of MnO x formation on Mn(II), humic acid, and H + concentration were characterized. A kinetic model based largely on published rate constants was established and fit to the experimental data. As expected, analysis of the model indicates that the key reaction rate controlling MnO x production is the rate of decomposition of a MnO 2 + complex formed from the reaction of Mn(II) with O 2 −. This rate is strongly dependent on the Mn(II) complexing ligands in solution. The MnO x production in the seawater sample taken from Bodega Bay, USA and spiked with 200 μM Mn(II) was well reproduced by the model. Extrapolations from the model imply that Mn photo-oxidation should be a significant reaction in typical surface seawaters. Calculated rates, 5.8 to 55 pM h −1, are comparable to reported rates of biological Mn oxidation, 0.07 to 89 pM h −1. Four fresh water samples that were spiked with 200 μM Mn(II) also showed significant MnO x production. Based on these results, it appears that Mn photo-oxidation could constitute a significant, and apparently unrecognized geochemical pathway in natural waters.