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TECHNICAL REPORTS

Vadose Zone Processes and Chemical Transport

Aluminum Effect on Dissolution and Precipitation under Hyperalkaline Conditions

II. Solid Phase Transformations

^a O.S. Qafoku, Pacific Northwest National Lab., 902 Battelle Blvd., P.O. Box 999, MSIN: K3-61, Interfacial Geochemistry Group, Richland, WA 99352
^b Los Alamos National Lab., MS D469 EES-1: Geology and Geochemistry, Los Alamos, NM 87545

^* Corresponding author (nik.qafoku{at}pnl.gov).

Received for publication January 3, 2003. The high-level radioactive, Al-rich, concentrated alkaline and saline waste fluids stored in underground tanks have accidentally leaked into the vadose zone at the Hanford Site in Washington State. In addition to dissolution, precipitation is likely to occur when these waste fluids contact the sediments. The objective of this study was to investigate the solid phase transformations caused by dissolution and precipitation in the sediments treated with solutions similar to the waste fluids. Batch experiments at 323 K were conducted in metal- and glass-free systems under CO₂ and O₂ free conditions. Results from X-ray diffraction (XRD), quantitative X-ray diffraction (QXRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and energy dispersive X-ray fluorescence spectroscopy (EDXRF) indicated that significant solid phase transformations occurred in the sediments contacted with Al-rich, hyperalkaline, and saline solutions. The XRD and QXRD analyses confirmed that smectite and most likely biotite underwent dissolution. The SEM and the qualitative EDS analyses confirmed the formation of alumino-silicates in the groups of cancrinite and probably sodalite. The morphology of the alumino-silicates secondary phases changed in response to changes in the Si/Al aqueous molar ratio. The transformations in the sediments triggered by dissolution (weathering of soil minerals) and precipitation (formation of secondary phases with high specific surface area and probably high sorption capacities) may play a significant role in the immobilization and ultimate fate of radionuclides and contaminants such as Cs, Sr, and U in the Hanford vadose zone.

Abbreviations: DOE, Department of Energy • EDS, energy dispersive spectroscopy • EDXRF, energy dispersive X-ray fluorescence spectroscopy • FESEM, Field Emission Scanning Electron Microscopy • HLWF, high-level radioactive waste fluids • IS, ionic strength • QXRD, quantitative X-ray diffraction • SEM, scanning electron microscopy • XRD, X-ray diffraction

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