Immobilizing Pertechnetate in Ettringite via Sulfate Substitution

Technetium-99 immobilization in low-temperature nuclear waste forms often relies on additives that reduce environmentally mobile pertechnetate (TcO 4 - ) to insoluble Tc(IV) species. However, this is a short-lived solution unless reducing conditions are maintained over the hazardous life cycle of radioactive wastes (some ∼10,000 years). Considering recent experimental observations, this work explores how rapid formation of ettringite [Ca 6 Al 2 (SO 4 ) 3 (OH) 12 ·26(H 2 O)], a common mineral formed in cementitious waste forms, may be used to directly immobilize TcO 4 - . Results from ab initio molecular dynamics (AIMD) simulations and solid-phase characterization techniques, including synchrotron X-ray absorption, fluorescence, and diffraction methods, support successful incorporation of TcO 4 - into the ettringite crystal structure via sulfate substitution when synthesized by aqueous precipitation methods. One sulfate and one water are replaced with one TcO 4 - and one OH - during substitution, where Ca 2+ -coordinated water near the substitution site is deprotonated to form OH - for charge compensation upon TcO 4 - substitution. Furthermore, AIMD calculations support favorable TcO 4 - substitution at the SO 4 2- site in ettringite rather than gypsum (CaSO 4 ·2H 2 O, formed as a secondary mineral phase) by at least 0.76 eV at 298 K. These results are the first of their kind to suggest that ettringite may contribute to TcO 4 - immobilization and the overall lifetime performance of cementitious waste forms.