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 (TcO₄^-) 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 [Ca₆Al₂(SO₄)₃(OH)₁₂·26(H₂O)], a common mineral formed in cementitious waste forms, may be used to directly immobilize TcO₄^-. 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 TcO₄^- into the ettringite crystal structure via sulfate substitution when synthesized by aqueous precipitation methods. One sulfate and one water are replaced with one TcO₄^- and one OH^- during substitution, where Ca²⁺-coordinated water near the substitution site is deprotonated to form OH^- for charge compensation upon TcO₄^- substitution. Furthermore, AIMD calculations support favorable TcO₄^- substitution at the SO₄^2- site in ettringite rather than gypsum (CaSO₄·2H₂O, 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 TcO₄^- immobilization and the overall lifetime performance of cementitious waste forms.