Spectroscopic and Microscopic Characterization of Contaminant Uptake and Retention by Carbonates in Soils and Vadose Zone Sediments

Historical releases and subsequent migration of toxic metals and radionuclides within the soil and vadose zone at various sites over the DOE Complex pose serious technological challenges, including the design and implementation of cost-effective remediation and/or monitoring strategies. This requires knowledge of the fundamental processes that govern contaminant mobility and reactivity with their host materials. The main research objective of this project is to assess the effectiveness of uptake and retention of selected contaminants (Co, Cs, Pb, Sr, U, and Cr) on calcium carbonate (primarily calcite), with a focus on conditions and sorbent materials relevant to the Hanford Site. This includes detailed microscopic and spectroscopic characterization of carbonate coatings and caliche from the Hanford Site, combined with experimental determination of the uptake behavior of selected contaminants on calcite surfaces and consideration of factors influencing uptake behavior and retention. The methodology relies on spectroscopic, microscopic, mineralogical, and geochemical characterization of natural and analog materials, including spatially resolved X-ray fluorescence (micro- XRF) and X-ray absorption fine-structure (XAFS) spectroscopy of uptake products. A primary goal is to determine the reaction mechanisms that govern contaminant uptake by carbonates. The expected results will have direct application for assessing the role that calcium carbonate phases play in sequestering toxic metals and radionuclides at the Hanford Site and other locations with calcium carbonate, and will also serve as a baseline for similar studies in more complex systems