UC Irvine

To enhance the long-term immobilization of radioactive wastes, ferrihydrite nanoparticles were incorporated into cementitious materials. The effects of ferrihydrite nanoparticles on the physicochemical and mechanical properties of cementitious materials and the immobilization of uranium (U), strontium (Sr) and cesium (Cs) were investigated. Adding ferrihydrite nanoparticles at 0.65%, 1.30%, 3.90% and 6.50% of cement weight slightly improved compressive strength by 5-11%, but dramatically reduced U leaching by 50-57%. The enhanced U immobilization was attributed to the strong adsorption of U by ferrihydrite nanoparticles, and the structural incorporation of U into hematite formed during ferrihydrite recrystallization. Although ferrihydrite nanoparticles had weaker effect than hematite nanoparticles on improving cement hydration and reducing permeability, they exhibit stronger U immobilization capacity. In contrast, incorporating ferrihydrite nanoparticles into cementitious materials had no significant effects on Cs and Sr leaching and no detectable adsorption of Sr and Cs. This study elucidated the fundamental differences in the interactions between ferrihydrite nanoparticles and U, Sr or Cs within cementitious systems that led to the distinctive immobilization mechanisms for these radionuclides. It generated new mechanistic understandings of U, Sr and Cs leaching from cementitious barriers modified by Fe-based nanoparticles, and proposed a new approach for enhancing long-term immobilization of U.