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Calculated Mn(2p 3/2 ) X-ray photoelectron spectra (XPS) of Mn 2 , Mn 3 , and Mn 4 free ions are strikingly similar to Mn(2p 3/2 ) spectra of Mn 2 -, Mn 3 -, and Mn 4 -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 Mn 3 in manganite, but Mn 2 , Mn 3 , and Mn 4 are present in the near-surface of synthetic birnessite at about 5, 25, and 70%, respectively. Precipitation of birnessite by reaction of Mn 2 (aq) with an oxidant includes two electron transfer steps: (1) oxidation of Mn 2 (aq) to produce Mn 3oxyhydroxide, an intermediate reaction product that forms on the surface of synthetic birnessite and (2) subsequent oxidation of Mn 3 -oxyhydroxide surface species to produce synthetic birnessite. Some surface Mn 3 , however, remains unoxidized and is incorporated into birnessite. As for this synthesis (KMnO 4 used as oxidant), oxidation may not proceed to completion in natural settings (as O 2 is the oxidant) leading to Mn 3 incorporation into Mn-oxides. The hypothesis explains the abundance of non-stoichiometric MnO 2 phases in sedimentary environments. The MnO 2 precipitation scheme proposed by Stumm and Morgan (1981) includes the surface species Mn 2 MnO 2 . This and other studies indicate that the reactive intermediate is a Mn 3 -bearing surface species. The formation rate of birnessite is probably controlled by one of these redox reactions. The proposed rate expression of
Nesbitt et al. (1998) studied this question.