The relationship between Mn oxidation state and structure in triclinic and hexagonal birnessites

Journal Article · Wed Jan 17 00:00:00 EST 2018 · Chemical Geology

DOI:https://doi.org/10.1016/j.chemgeo.2018.01.011· OSTI ID:1773815

Ling, Florence T.  ^[1]; Post, Jeffrey E. ^[2]; Heaney, Peter J. ^[1]; Ilton, Eugene S. ^[3]

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1. Pennsylvania State Univ., University Park, PA (United States)
2. Smithsonian Inst., Washington, DC (United States)
3. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)

Because of their nanocrystallinity and high cation exchange capacities, birnessite phases can control the cycling of heavy metals in soils and groundwaters, and they also are implicated in the oxidation of transition metals in natural environments. Birnessite reactivity is determined by crystal structure and composition. Because birnessites typically are poorly crystalline, synchrotron-based absorption spectroscopy (EXAFS, XANES) often is utilized for structural characterization. For example, linear combination fitting (LCF) of X-ray absorption spectra typically is applied to quantify mixed triclinic and hexagonal birnessite phases. This approach is challenged, however, because the structures of the standards are not always plainly apparent. Moreover, it is difficult to distinguish birnessites with nanoscale intergrowths of hexagonal and triclinic endmembers from homogeneous birnessite structures of “intermediate triclinicity”. In this study, we explored these issues by synthesizing a host of cation-exchanged birnessite specimens whose long-range symmetrical character could be determined by X-ray diffraction without ambiguity. Through a combination of Fourier transform infrared spectroscopy (FTIR), extended X-ray absorption fine structure (EXAFS), and X-ray photoelectron spectroscopy (XPS), we have examined the relationships among structural symmetry, Mn oxidation state, and interlayer composition. Our results confirm prior models that as the concentration of Mn³⁺ increases, the departure from hexagonal symmetry also increases. Rietveld refinements indicate that the Jahn-Teller distortions associated with Mn³⁺ induce systematic variations in unit-cell parameters, particularly an increase in the α-axis and the β angle of the unit cell. Interlayer cation composition also controls structural distortions, and Ca-rich birnessites showed less deviation from hexagonality than did Na-, K-, and Ba-birnessites. Our linear combination fits of X-ray absorption spectra sometimes yielded misleading results, reinforcing the difficulty and importance of selecting appropriate standards.

• Research Organization: Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL); Argonne National Lab. (ANL), Argonne, IL (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences & Biosciences Division; USDOE Office of Science (SC), Biological and Environmental Research (BER); National Science Foundation (NSF); USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: AC05-76RL01830; AC02-06CH11357; EAR-1147728; EAR-1552211; AC06-76RLO-1830
• OSTI ID: 1773815
• Alternate ID(s): OSTI ID: 1506722
• Report Number(s): PNNL-SA-135583
• Journal Information: Chemical Geology, Vol. 479; ISSN 0009-2541
• Publisher: Elsevier
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
58 GEOSCIENCES
birnessite
manganese oxide
triclinic
hexagonal
Mn oxidation state
FTIR
EXAFS
XRD
XPS