Investigation of Metatorbernite Phase Relations in Uranium-contaminated Vadose-zone Soils
Bereston, Margaux A.
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Uranyl phosphates of the autunite/meta-autunite group are important sinks for uranium in contaminated soils. Basic understanding of uranium migration and planning for remediation depend on recognition and understanding of these materials. The Cu meta-autunite mineral, metatorbernite [Cu(UO2)2(PO4)2 • 8H2O] comprises nearly half the solid-phase uranium budget in the contaminated vadose zone in the 300 Area of the Hanford site in Washington State, USA. In situ, X-ray diffraction investigation shows that temperature and activity of water (aH2O) are controlling parameters in the occurrence of metatorbernite. Data were collected in reflection mode with a Bruker D-8 Focus laboratory X-ray Diffractometer at the Johns Hopkins University and in transmission mode at beam line 13 BMC of the Advanced Photon Source synchrotron, Argonne National Laboratory. Temperature and aH2O control and monitoring were utilized in the synchrotron experiments. Room temperature studies (~24°C) revealed intensity peaks with d- spacings 8.66 Å, 6.4 Å, and 5.4 Å. During dehydration experiments, metatorbernite heated to greater than or equal to 64°C dehydrated to a phase with d-spacings 8.3 Å, 6.9 Å, and 5.3Å. When heated above 84°C, the 6.9 Å peak increased in intensity, while the lesser hydrates of metatorbernite and its associated 8.3 Å and 5.3 Å intensity peaks lost intensity. Dehydration experiments were followed by rehydration-cooling experiments. The results indicate that lesser hydrates of metatorbernite undergo a phase change to rehydrate, as evidenced by the reappearance of intensity peaks 8.66 Å, 6.4 Å, and 5.4 Å. This phase change occurred when the temperature was lowered and the relative humidity was progressively increased.