Contaminant sequestration and phase transformation properties of birnessite-like phases (delta-mno2)

Open Access
- Author:
- Fleeger, Claire Rene
- Graduate Program:
- Geosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 30, 2011
- Committee Members:
- Peter J Heaney, Dissertation Advisor/Co-Advisor
James David Kubicki, Committee Member
Matthew Scott Fantle, Committee Member
William D Burgos, Committee Member - Keywords:
- birnessite
cesium sequestration
phase transformation
manganese oxide - Abstract:
- For the first time, time-resolved X-ray diffraction (TR-XRD) and inductively coupled plasma-mass spectrometry (ICP-MS) have been coupled to monitor contaminant (e.g., Cs and Mn) cation exchange reactions into two birnessite-like phases, which exhibit triclinic and hexagonal symmetry. Cs is a principal contaminant at several nuclear waste sites, including the DOE Hanford Site, where high-level, high pH nuclear waste is leaking from tens of underground storage tanks. Our results indicate that Cs uptake by hexagonal H-birnessite increases as pH increases from pH 3 to pH 11; the increase in Cs uptake (wt%) correlates to an incremental increase in unit-cell volume as a function of pH. As Cs adsorption increases and pH increases, the rate of Cs cation exchange reaction decreases. On the other hand, triclinic Na-birnessite adsorbs less Cs at a neutral pH than at high or low pH. Again, greater uptake of Cs correlates to a decrease in Cs cation exchange rate as the pH becomes more acidic or basic. During the course of these variable pH reactions, we observed that a phase transformation occurred between the triclinic and hexagonal phases as a function of pH even in the presence of competing cations (i.e., Cs). At pH 3, triclinic Na-birnessite transformed to hexagonal H- or Cs-birnessite, and at pH 13, hexagonal H-birnessite transformed to triclinic Na- or Cs-birnessite. At both low and high pH, cation exchange with the dissolved cation (H, Na, or Cs) preceded the symmetry-changing transformation. The interlayer cation remained within the structure during the phase transformations, indicating that these materials act as sequestering agents no matter the pH or crystalline phase. For the last set of experiments, we exchanged Mn2+ into triclinic Na-birnessite to assess the possibility of a redox-driven phase transformation to hexagonal H-birnessite. We determined that aqueous Mn2+ catalyzed the phase transformation at pH 3, and the rate of phase transformation increased as [Mn2+] increased. However, at pH 4.5 and 6, Mn2+(aq) promoted the breakdown of the manganese oxide octahedral sheets. Initially the Mn2+ exchanged into the interlayer region, indicated by a decrease in unit-cell volume, followed by an increase in the Mnoct-Ooct bond distances to 2.01 Å, suggesting that the octahedral Mn4+(s) was reduced to Mn3+(s). We supported this observation by completing the same reaction in a batch reactor, where the final solid-state products were a mixture of triclinic birnessite, hausmannite, and three Mn3+OOH products (manganite, groutite, and feitknechtite). This investigation reveals the adaptability of birnessite as a function of chemical environment. Mn oxides control heavy metal mobility not only by high sorption and cation exchange capacities but by redox reactivity, and these properties vary with the particular phase of Mn oxide. Thus, each phase needs to be characterized individually to fully understand how mixtures will interact in natural systems.