US EPA

5063588 

Journal Article 

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

Luther, GW III; de Chanvalon, AT; Oldham, VE; Estes, ER; Tebo, BM; Madison, AS 

2018 

Yes 

Aquatic Geochemistry
ISSN: 1380-6165
EISSN: 1573-1421 

24 

4 

257-277 

10.1007/s10498-018-9342-1 

WOS:000446499500001 

Manganese oxides, typically similar to -MnO2, form in the aquatic environment at near neutral pH via bacterially promoted oxidation of Mn(II) species by O-2, as the reaction of [Mn(H2O)(6)](2+) with O-2 alone is not thermodynamically favorable below pH of similar to 9. As manganese oxide species are reduced by the triphenylmethane compound leucoberbelein blue (LBB) to form the colored oxidized form of LBB ((max)=623nm), 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 ((max)similar to 370nm) with these reductants. In marine waters, colloidal forms of Mn oxides (<0.2 mu m) have not been detected as Mn oxides are quantitatively trapped on 0.2-mu m 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* 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. 

Manganese oxide reduction; Electron transfer; Hydrogen atom transfer; Mn(III) ligands; Band theory