2, Pacific Northwest National Laboratory, Richland, Washington, United States
Designing porous materials that can uptake target ions from contaminated effluents in a continuous flow process is a desired way to efficiently contain the ions. Unfortunately, the typical ion-exchange resins suffer from mechanical instability and swelling under flow. Silica and aluminosilicate supports, such as zeolites, are promising because they do not swell and allow for binding specific radionuclides by functionalizing with selective organic chelating complexes. Ultimate confinement can be achieved using a multi-scale silica-based support containing sequestered elements within the pores, potentially forming a precursor for a containment matrix. In fact, mesoporous silica-based materials exhibit properties very similar to high-activity waste glass containment systems.
An aluminosilicate based system of interest is LTA zeolite. We performed density functional theory (DFT) calculations of the binding energies of different ions being incorporated at the 3 distinct doping sites in LTA zeolite. In our study we are used pure silica LTA (ps-LTA), and we introduced 3 different ions, Na+, Sr2+ and Ba2+, in the cavity of the ps-LTA. We considered the system to be in vacuum and in a solvent (water), simulating a more realistic case of the LTA being submerged in a water solution. We show that each of the ions will preferably occupy the same doping site in the ps-LTA, with the binding energy increasing from Ba2+ to Sr2+ to Na+. In addition, we observe that the Na+ ion becomes the stable dopant at a high electron chemical potential (Fermi level), < 4.5 eV.
This work was supported as part of the Center for Hierarchical Waste Form Materials, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0016574.