Mineralogical and Geochemical Analyses of Synthetic and Natural Birnessites

Open Access
Author:
Ling, Florence T
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 26, 2016
Committee Members:
  • Peter J. Heaney, Dissertation Advisor
  • Peter J. Heaney, Committee Chair
  • James D. Kubicki, Committee Member
  • Christopher H. House, Committee Member
  • William D. Burgos, Outside Member
Keywords:
  • birnessite
  • manganese oxide
  • FTIR
  • EXAFS
  • XRD
  • lead
Abstract:
Manganese (Mn) oxides have long been challenging to study using conventional materials characterization techniques due to their small particle size, poor crystallinity and variable structure at the nanoscale. The phyllomanganates within the birnessite family, for example, exhibit a diversity of Mn oxidation states, interlayer cations, water contents, and octahedral vacancy concentrations. These differences lead to subtle structural modifications that control the redox and cation exchange capacities of birnessite phases in soils, and they determine the usefulness of birnessites for environmental applications, such as the remediation of contaminant metals. Although powder X-ray diffraction (XRD) can offer deep insights into birnessite crystallography, spectroscopic techniques provide complementary information when structural disorder is high. This study introduces Fourier-transform infrared spectroscopy (FTIR) as a method for analyzing birnessite varieties, coupling our analyses with synchrotron X-ray diffraction and absorption spectroscopy for comparison. We found that FTIR can readily differentiate synthetic triclinic Na-birnessite and hexagonal H-birnessite. The region from 400 to 750 cm-1, which is most sensitive to Mn-O bond vibrations, displays the most prominent distinctions between the two varieties, with peaks at ~418, ~478, and ~511 cm-1 in synthetic triclinic Na-birnessite, and peaks at ~440 and ~494 cm-1 in hexagonal H-birnessite. Spectral unmixing of known mixtures of triclinic and hexagonal birnessite yielded errors comparable to results from linear combination fitting (LCF) of extended X-ray absorption fine-structure (EXAFS) data. Spectral unmixing of synthetic cation-exchanged birnessites, including K-birnessite, Ba-birnessite, Ca-birnessite, and two hexagonal birnessite samples prepared in pH 3 and HEPES-buffered pH 7 solutions, also yielded fractions of triclinic and hexagonal birnessite comparable to those calculated by LCF of EXAFS data. Our EXAFS, FTIR, and Rietveld refinements of XRD data all confirmed that as Mn3+ content increases, the triclinic character of the birnessite also increases, as manifested by an increase in the β angle and in the length of the a-axis of the unit cell. With these synthetic samples providing a baseline for comparison, natural birnessite samples next were interrogated using the same suite of techniques. Contrary to the common assumption that most natural birnessites are hexagonal, our FTIR and EXAFS investigations revealed natural birnessite varieties as either triclinic or hexagonal or, most commonly, a mixture of the two. Further, this work demonstrates that the use of biological buffers such as HEPES and MES can promote the transformation of synthetic triclinic Na-birnessite into hexagonal H-birnessite, perhaps accounting for the presumption of hexagonal symmetry in natural biogenic samples. When our EXAFS and FTIR analyses were combined with X-ray photoelectron spectroscopy (XPS) data in an allied study (Ilton et al. 2016), a relation between Mn3+ content in birnessite and deviations from hexagonal symmetry appeared, suggesting that when Mn3+ exceeds ~25 mol% in birnessite, Jahn-Teller distortions couple, and the resultant strains generate structures with triclinic symmetry. In order to connect birnessite crystal chemistry with reactivity, we examined dissolved Pb uptake into synthetic triclinic Na- and hexagonal H-birnessite at pH 3 and pH 5 using ~6 hr time-resolved XRD and 112-day batch experiments. We found that Pb sorbs onto the surface of birnessite and into the interlayer, with uptake occurring in the interlayer even after 14 days. In hexagonal H-birnessite the ratio of the (1 0 0) to (1 0 -1) peaks increased as Pb exchanged into the interlayer, as expected from our diffraction simulations. Triclinic Na-birnessite transformed into a turbostratically disordered hexagonal H-birnessite during uptake at pH 3 and pH 5 due to the relative instability of triclinic birnessite at pH < 8.2. As a consequence of this transformation, both triclinic Na-birnessite and hexagonal H-birnessite sequestered similar amounts of Pb after 112 days.