Environmental redox cycling of manganese

I have investigated the rates and mechanisms of several redox reactions involving Mn. One half of my research has been to study the reaction rates of three different compounds, Cr(III), S2−, and phenol with δ-MnO_1.98, which is the synthetic analog of a common natural Mn-oxide. Manganese occurs naturally in three redox states. The soluble Mn(II) form is an essential element. The Mn(III) and Mr(IV) forms exist as mixed valent solid oxyhydroxides (MnO_x). Since MnO_x are powerful oxidizers, they can affect the fate, transport, and toxicity of other compounds. For example, the oxidation of Cr(III) to Cr(VI) by δ-MnO _1.98 significantly increases its mobility and toxicity and therefore its environmental hazard. The goal of this project was to determined how important Mn(III) centers are to the reactions of this predominately Mn(IV) oxide. The results show that while less that 10 percent of the surface sites are Mn(III), they are extremely important to the rate of each of the three reactions. In fact, when these sites are blocked by complexation with pyrophosphate, the reaction rate of Cr(III), S2−, or phenol with δ-MnO _1.98 is inhibited by greater than 60, 40, and 30 percent, respectively.; The other half of my research has been to examine the oxidation of Mn(II). Currently it is accepted that without catalysis by microbes or oxide surfaces, the oxidation of Mn(II) by O₂ is so slow as to be unimportant in natural systems. I investigated whether light could also catalyze this reaction because if it did Mn photo-oxidation would constitute a significant, and apparently unrecognized, source of MnO_x. Illumination with simulated sunlight of solutions of Mn(II) and Aldrich humic acid produced significant quantities of MnO_x. The major oxidants responsible for this reaction appear to be photoproduced superoxide radical anion, O₂− , and singlet oxygen, 1O₂. A kinetic model based largely on published rate constants was established and fit to the experimental data. Extrapolations from the model imply that Mn photooxidation could 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.