Manganese oxides, typically similar to d-MnO2, form in the aquatic environment at near neutral pH via bacterially-promoted oxidation of Mn(II) species by O2, as the reaction of [Mn(H2O)6]2+ with O2 alone is not thermodynamically favorable below pH of ~9. We present field data from the oxic-anoxic interface of the Chesapeake Bay, the hemipelagic St. Lawrence Estuary (water column and sediments) and the Broadkill River estuary surrounded by salt marsh wetlands that demonstrate the oxidation of Mn(II) species occurs at O2 concentrations from below 3 μM and above. As manganese oxide species react with leucoberbelein blue (LBB) to form the colored oxidized form of LBB (λmax = 623 nm), their concentration in the aquatic environment can be determined in aqueous samples and their reaction progress followed in kinetic studies. In some aquatic environments, the reduction of these manganese oxides leads to the formation of Mn(III)-ligand complexes [Mn(III)L)] at significant concentrations. Key reductants are hydrogen sulfide, Fe(II) and organic ligands, including the siderophore desferioxamine-B. We present laboratory data on the reaction of colloidal MnO2 (λmax~ 370 nm) with these reductants. In marine waters, colloidal forms of Mn oxides have not been detected, and Mn oxides are quantitatively trapped on 0.2 μm filters. Thus, the reactivity of Mn oxides with reductants depends on surface reactions and possible surface defects. In the case of MnO2, Mn(IV) is an inert cation; thus, an inner sphere process is required for electrons to go into the empty eg band of its orbitals. We discuss these surface reactions and possible surface defects [including the replacement of inert Mn(IV) with labile Mn(III)] that may promote its reduction using laboratory and field data for the reaction of MnO2 with hydrogen sulfide and other reductants.

Formation and reactivity of manganese oxides in the environment: field and laboratory results

255th National Meeting (2018)

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