Competitive TcO₄^-, IO₃^-, and CrO₄^2- Incorporation into Ettringite

Ettringite is a naturally occurring mineral found in cementitious matrices that is known for its ability to incorporate environmentally mobile oxyanion contaminants. To better assess this immobilization mechanism for contaminants within cementitious waste forms intended for nuclear waste storage, this work explores how mixed oxyanion contaminants compete for ettringite incorporation and influence the evolving mineralogy. Ettringite was precipitated in the presence of TcO₄^-, IO₃^-, and/or CrO₄^2-, known contaminants of concern to nuclear waste treatment, over pre-determined precipitation periods. Solution analyses quantified contaminant removal, and the collected solid was characterized using bulk and microprobe X-ray diffraction coupled with pair distribution function and microprobe X-ray fluorescence analyses. Results suggest that ≥96% IO₃^- is removed from solution, regardless of ettringite precipitation time or the presence of TcO₄^- or CrO₄^2-. However, TcO₄^- removal remained <20%, was not significantly improved with longer ettringite precipitation times, and decreased to zero in the presence of IO₃^-. When IO₃^- is co-mingled with CrO₄^2-, calcite and gypsum are formed as secondary mineral phases, which allows for oxyanion partitioning, e.g., IO₃^- incorporation into ettringite, and CrO₄^2- incorporation into calcite. Results from this work exemplify the importance of competitive immobilization when assessing waste form performance and environmental risk of contaminant release.