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.

Incorporation of iodate into calcite (CaCO3) may be used as an in situ treatment strategy for radioiodine in contaminated soils and groundwater, but the presence of other contaminants may inhibit its efficiency. To this end, the potential for chromate to interfere with iodate incorporation into CaCO3 was investigated as an example of how co-located contaminants may impact in situ remediation efficacy. Here, batch precipitation experiments were periodically sub-sampled over 21 days to determine the kinetic effects of chromate on iodate removal and incorporation into calcite. From these experiments, a decrease in iodate removal from >60 to <40% was observed upon chromate addition (1–112 ppm of chromate) and ≤11% of chromate was removed with a minor dependence upon the initial chromate concentration. Analysis of the solid phase using extended X-ray absorption fine structure (EXAFS) spectroscopy informed by ab initio molecular dynamics simulations revealed that the iodate incorporation mode remains unchanged by the presence of chromate. Iodate readily substitutes for carbonate (CO3 2–), and the calcite structure is charge-balanced primarily by substituting H+ for Ca2+. Furthermore, chromate incorporated as a nearest neighbor to iodate did not contribute to the EXAFS fit; therefore, iodate and chromate clustering is unlikely when co-incorporated into calcite.

The evolution of sulfur chemistry in cements is best known in the bailiwick of failure mechanisms via sulfate attack, but is equally important for its contributions to the reduction capacity of cementitious materials often used for immobilizing nuclear waste streams destined for long‐term storage, for example, cementitious waste forms (CWF). The total reduction capacity of CWFs, encompassing contributions from both S and Fe reductants, and its implications toward radionuclide immobilization is most often studied by destructive wet chemistry methods requiring acid digestion in the presence of Ce(IV) and subsequent titration and colorimetric interpretation. Here, we investigate a similarly analytical but nondestructive alternative, benchtop high resolution wavelength‐dispersive X‐ray fluorescence spectroscopy, most commonly known as X‐ray emission spectroscopy (XES), for probing the bulk sulfur oxidation state distribution. We present here an initial investigation of S XES, including an improved experimental protocol for lab XES of inhomogeneous samples, both as a complement to the Ce(IV) test and for new scientific opportunities that it enables for observing changes in sulfur chemistry. We discuss future improvements and opportunities, including: (1) the practical challenges associated with coordinating XES and Ce(IV) liquid extraction for a more comprehensive perspective on reduction capacity and for a high‐precision evaluation of uncertainties in the Ce(IV) test; and (2) new opportunities, due to the nondestructive nature of XES, for controlled evolution studies aimed at elucidating specific chemical responses of CWFs exposed to invasive gas or liquid species or to accelerated aging by radiative dose or thermal treatment.

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