Time-Resolved Structural Analyses of Cation Exchange Reactions in Synthetic Birnessite

Open Access
Author:
Lopano, Christina Lynn
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 19, 2007
Committee Members:
  • Peter J Heaney, Committee Chair
  • Sridhar Komarneni, Committee Member
  • Susan Louise Brantley, Committee Member
  • James David Kubicki, Committee Member
  • C Enid Martinez, Committee Member
Keywords:
  • birnessite
  • cation exchange
  • X-ray diffraction
  • synchrotron
  • manganese oxides
Abstract:
Rietveld refinements of time-resolved synchrotron X-ray diffraction data have documented real-time changes in unit-cell parameters in response to cation substitution in synthetic Na-birnessite. K-, Cs-, and Ba-birnessite, like Na-birnessite, were all found to have a triclinic unit-cell. Rietveld analyses of the X-ray diffraction patterns for K- and Ba-exchanged birnessite revealed decreases in the a, c, and  unit-cell parameters in unit-cell volume relative to Na-birnessite. Cs-exchange resulted in increases in c and unit-cell volume. Fourier electron difference syntheses revealed that the changes in the configuration of the interlayer species, and the charge, size, and hydration of the substituting cations, serve as the primary controls on changes in unit-cell parameters. Further TEM, SEM, and EDS analyses of batch Cs-exchange samples suggests that Na-birnessite and Cs-birnessite may co-exist in individual grains of birnessite, with both Na+ and Cs+ homogeneously distributed throughout individual incompletely exchanged grains. This suggests that the Cs-exchange reaction pathway likely occurred as part of a diffusion process, possibly aided by partial delamination of the Mn-O octahedral layers. Cation exchange rates for K-, Cs-, and Ba-exchange in birnessite were calculated for the first time from the time-resolved XRD results. The exchange rates for Na in birnessite decreased in the order: Cs >> K > Ba. These results are likely a function of hydration energy differences of the cations and the preference of the solution phase for the more readily hydrated cation. The kinetic modeling supports a two-stage reaction pathway for cation exchange, and these kinetic steps correlate with changes in crystal structure. Upon variation of solution pH, the K-exchange results quantitatively demonstrate that decreasing pH leads to faster cation exchange reaction kinetics. It is hypothesized that the enhanced K+ exchange with increased H+ concentration is caused mechanistically by the role that H+ plays during the diadochic substitution. These state-of-the-art time-resolved synchrotron studies have revealed for the first time reaction rates and possible mechanisms for cation exchange in birnessite, a prominent Mn-oxide soil constituent.