Calcite As a Host for a Variety of Contaminant Oxyanions.

Iodine, chromium, and uranium are three contaminants of concern at the Hanford site in southeastern Washington and are part of the clean-up effort of the legacy waste at the site. Geochemically aided remediation by chemically trapping of I, Cr, and/or U in naturally occurring mineral lattices can help expedite and reduce costs involved in the cleanup and may prove to be a reliable method for similar remediation efforts at other DOE sites and globally.

Due to its crystal structure, calcium carbonate is able to uptake numerous contaminants, including I, Cr, and U, and is already present in the subsurface at the Hanford site. This study is an effort to revisit and refine our understanding on the controls responsible for the uptake of I, Cr, U by calcium carbonate (calcite and other polymorphs that may be unstable). The objective is to determine mechanisms and measure the extent of contaminant oxyanion interaction with calcite.

Calcite has been precipitated in the presence of I, Cr, and U, in the Hanford-relevant forms of IO₃^-, CrO₄^-, and uranyl carbonate species, with variable concentrations and combinations. Current results have shown that I and Cr are removed from the aqueous phase during precipitation of carbonates; U results are pending. Solid phase characterization (SEM/EDS, TEM, QXRD) is underway to quantify the amount of contaminant incorporated into the structure versus the amount adsorbed to the surface of the carbonate mineral. In addition, geochemical modeling, alkaline fusion and calcite stability testing are all ongoing to increase understanding of the capability and capacity of calcium carbonates to incorporate these contaminants as part of a long term remediation strategy.