Reduction of Manganese Oxides: Thermodynamic, Kinetic and Mechanistic Considerations for One- Versus Two-Electron Transfer Steps

Manganese oxides, typically similar to δ-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. As manganese oxide species are reduced by the triphenylmethane compound leucoberbelein blue (LBB) to form the colored oxidized form of LBB (λ_max = 623 nm), their concentration in the aquatic environment can be determined in aqueous environmental samples (e.g., across the oxic–anoxic interface of the Chesapeake Bay, the hemipelagic St. Lawrence Estuary and the Broadkill River estuary surrounded by salt marsh wetlands), and their reaction progress can be followed in kinetic studies. The LBB reaction with oxidized Mn solids can occur via a hydrogen atom transfer (HAT) reaction, which is a one-electron transfer process, but is unfavorable with oxidized Fe solids. HAT thermodynamics are also favorable for nitrite with LBB and MnO₂ with ammonia (NH₃). Reactions are unfavorable for NH₄ ⁺ and sulfide with oxidized Fe and Mn solids, and NH₃ with oxidized Fe solids. In laboratory studies and aquatic environments, the reduction of manganese oxides leads to the formation of Mn(III)-ligand complexes [Mn(III)L] at significant concentrations even when two-electron reductants react with MnO₂. 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 solutions (λ_max ~ 370 nm) with these reductants. In marine waters, colloidal forms of Mn oxides (< 0.2 µm) have not been detected as 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 in octahedral coordination; thus, an inner-sphere process is likely for electrons to go into the empty e_g ^* conduction band of its orbitals. Using frontier molecular orbital theory and band theory, we discuss aspects of these surface reactions and possible surface defects that may promote MnO₂ reduction using laboratory and field data for the reaction of MnO₂ with hydrogen sulfide and other reductants.