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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian


IMPACT FACTOR 2018: 2.631

CiteScore 2018: 2.55

SCImago Journal Rank (SJR) 2018: 1.355
Source Normalized Impact per Paper (SNIP) 2018: 1.103

Online
ISSN
1945-3027
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Volume 83, Issue 3-4

Issues

Interpretation of XPS Mn(2p) spectra of Mn oxyhydroxides and constraints on the mechanism of MnO2 precipitation

H.W. Nesbitt / D. Banerjee
Published Online: 2015-11-13 | DOI: https://doi.org/10.2138/am-1998-3-414

Abstract

Calculated Mn(2p3/2) X-ray photoelectron spectra (XPS) of Mn2+, Mn3+, and Mn4+ free ions are strikingly similar to Mn(2p3/2) 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, but Mn2+, Mn3+, and Mn4+ are present in the near-surface of synthetic birnessite at about 5, 25, and 70%, respectively. Precipitation of birnessite by reaction of Mn2+(aq) with an oxidant includes two electron transfer steps: (1) oxidation of Mn2+(aq) to produce Mn3+- oxyhydroxide, an intermediate reaction product that forms on the surface of synthetic 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 synthesis (KMnO4 used as oxidant), oxidation may not proceed to completion in natural settings (as O2 is the oxidant) leading to Mn3+incorporation into Mn-oxides. The hypothesis explains the abundance of non-stoichiometric MnO2 phases in sedimentary environments. The MnO2 precipitation scheme proposed by Stumm and Morgan (1981) includes the surface species Mn2+·nO2. 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. The 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

About the article

Received: 1997-05-28

Accepted: 1997-11-05

Published Online: 2015-11-13

Published in Print: 1998-03-01


Citation Information: American Mineralogist, Volume 83, Issue 3-4, Pages 305–315, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am-1998-3-414.

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© 2015 by Walter de Gruyter Berlin/Boston.

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