X-ray Spectroscopic and Diffraction Study of the Speciation of Uranium in Contaminated Sediments from the DOE's Hanford Site


J. G. Catalano,¹ J. M. Zachara,² S. M. Heald,³ and G. E. Brown, Jr.^1,4

¹Surface & Aqueous Geochemistry Group, Department of Geological & Environmental Sciences, Stanford University, Stanford, CA 94305-2115
²Pacific Northwest National Laboratory, Environmental Dynamics & Simulation, P.O. Box 999, MS K8-96, Richland, WA 99352
³PNC-CAT, Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439
⁴Stanford Synchrotron Radiation Laboratory, SLAC, Stanford University, Stanford, CA 94305

Uranium contamination of the subsurface at numerous locations across the DOE's Hanford site has occurred through the leakage of high-level nuclear waste. At one site of particular concern, the BX Tank Farm, leakage from the overfilling of tank BX-102 released approximately 7.5 metric tons of uranium dissolved in caustic aqueous sludge to the vadose zone. Assessment of the potential hazards posed by this leakage, as well as the development and application of accurate contaminant transport models, requires an understanding of the speciation of uranium in the sediments under tank BX-102. We have applied advanced, synchrotron-based X-ray spectroscopic and diffraction techniques to characterize the distribution, phase associations, and chemical form of uranium in samples from these sediments. X-ray absorption fine structure (XAFS) spectroscopic studies demonstrate that >95% of the uranium in all samples occurs as uranium(VI), and that the primary uranium species is likely a uranium(VI) silicate from the uranophane group of minerals. Unfortunately, XAFS cannot distinguish between the members of this group due to near identical local coordination environments of uranium in these phases. Combined micro-scanning x-ray fluorescence (mSXRF) and electron microprobe analyses show uranium distributed heterogeneously in the sediments, occurring as micron-sized particles inside cracks in feldspar grains. X-ray microdiffraction (mXRD) studies reveal the presence of sodium-boltwoodite, Na (UO₂)(SiO₃OH)·1.5H₂O; no other uranophane group mineral was observed. mXRD studies also confirm the association of uranium with feldspar. Future release of uranium from these sediments will occur through dissolution of sodium-boltwoodite, which has the lowest expected solubility of the uranophane group minerals.