Sequestering Uranium and Technetium through Co-Precipitation with Aluminum in a Contaminated Acidic Environment

This research evaluated a method of controlled base addition for immobilizing uranium (U) and technetium (Tc) through co-precipitation with aluminum (Al) and other metal ions which co-exist in a highly contaminated acidic environment. The batch and column experiments indicate that the addition of strong base (NaOH) provided a rapid yet effective means of sequestering U, Tc, and toxic metal ions such as nickel (Ni²⁺) and cobalt (Co²⁺) in the sediment and groundwater. Greater than 94% of soluble U (as UO₂²⁺) and >83% of Tc (as TcO₄⁻) can be immobilized at pH above 4.5 by co-precipitation with Al-oxyhydroxides. The presence of sediment minerals appeared to facilitate co-precipitation of these contaminants at lower pH values than those in the absence of sediments. The immobilized U and Tc were found to be stable against dissolution in Ca(NO₃)₂ solution (up to 50 mM) because of the formation of strong surface complexes between U or Tc and Al-oxyhydroxides. This research concludes that, as long as a relatively high pH (>5) and a low carbonate concentration are maintained, both U and Tc can be effectively immobilized under given site-specific conditions.