INTERPRETATION OF XPS MN(2P) SPECTRA OF MN OXYHYDROXIDES AND CONSTRAINTS ON THE MECHANISM OF MNO2 PRECIPITATION

Calculated Mn(2p(3/2)) X-ray photoelectron spectra (XPS) of Mn2+, Mn3, and Mn4+ free ions are strikingly similar to Mn(2p(3/2)) spectra of Mn2+-, Mn3+-, and Mn4+-oxides and oxyhydroxides, indicating that these ions adopt high spin states in MnO, manganite, and birnessite. The Mn (2p) peak structures reveal the presence of only Mn3+ in manganite, bu t Mn2+, Mn3+, and Mn4+ are present in the near-surface of synthetic bi rnessite at about 5, 25, and 70%, respectively. Precipitation of birne ssite by reaction of Mn2+(aq) with an oxidant includes two electron tr ansfer steps: (1) oxidation of Mn2+(aq) to produce Mn3+-oxyhydroxide, an intermediate reaction product that forms on the surface of syntheti c birnessite and (2) subsequent oxidation of Mn3+-oxyhydroxide surface species to produce synthetic birnessite. Some surface Mn3+, however, remains unoxidized and is incorporated into birnessite. As for this sy nthesis (KMnO4 used as oxidant), oxidation may not proceed to completi on in natural settings (as O-2 is the oxidant) leading to Mn3+ incorpo ration into Mn-oxides. The hypothesis explains the abundance of non-st oichiometric MnO2 phases in sedimentary environments. The MnO2 precipi tation scheme proposed by Stumm and Morgan (1981) includes the surface species Mn2+. MnO2. This and other studies indicate that the reactive intermediate is a Mn3+-bearing surface species. The formation rate of birnessite is probably controlled by one of these redox reactions. Th e proposed rate expression of Davies and Morgan (1989), however, needs no modification provided surface area is a reasonable measure of the surface density of the reactive intermediate.