Removal of Contaminants (I or U) From Solution During Calcium Carbonate Precipitation Following CO2-induced Dissolution of Calcite

Studies have shown that carbonates can incorporate a variety of contaminants (e.g., Cr, I, Sr, U) and may be an effective approach for sequestering contaminants in the subsurface. Contaminant sequestration via co-precipitation with carbonates is being investigated as a potential in situ remedy at the Hanford Site located in southeastern Washington State. One such technology involves an initial injection of CO₂ gas into contaminated groundwater or vadose zone porewater to dissolve carbonates in the surrounding sediments, followed by a period of re-equilibration during which calcium carbonates will re-precipitate and incorporate co-contaminants. In this investigation, Hanford sediments containing natural calcite were added to batch reactors with double deionized (DDI) water containing either IO₃^- or U. After CO₂ injection, pH, Ca, and I or U measurements were recorded over a 28-day period. The IO₃^--spiked experiments indicated IO₃^- incorporation into precipitating carbonates, with up to a 75% decrease in aqueous IO₃^- concentrations for high calcite-containing sediments. Preliminary data from the U-spiked batch tests showed that decreasing U concentrations in the control reactor (no CO₂ injection), but not in the CO₂-injected reactors, could be a pH effect. Solid phase characterization (e.g., SEM/EDS and/or QXRD) will further elucidate the mechanisms of IO₃^- removal from solution for both the IO₃^- and U batch experiments. Work is ongoing to test IO₃^- and U removal in different solutions; previous experiments have shown greater IO₃^- removal in synthetic vadose zone porewater (VZPW) when compared to DDI water due to the constituents present. For example, aragonite formation due to the presence of Mg in the VZPW may influence contaminant incorporation. Results from these experiments will be used to further develop in situ sequestration treatments in both the vadose zone and groundwater.