Immobilizing Pertechnetate in Ettringite via Sulfate Substitution

Abstract

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.

Spectroscopic evaluation of UVI–cement mineral interactions: ettringite and hydrotalcite

“…1(a) and 1(b)] ( Clark et al, 2008), shows potential for anion-exchange and incorporation within its structure. This has been demonstrated previously for anionic radionuclide species, such as pertechnetate (Tc VII O 4 À ), which exchanges in for sulfate in the ettringite channels (Saslow et al, 2020). Hydrotalcite-type phases (e.g.…”

“…While it has been shown that AFt-SO 4 (i.e. ettringite) phases have capacity for the uptake of anionic species such as pertechnetate (TcO 4 À ) (Saslow et al, 2020), evidence for the same behaviour in AFm-SO 4 phases is more limited. One example is that of iodate (I À ), which has been shown to incorporate into the interlayer of AFm-SO 4 to form an AFm phase with a mixed sulfate and iodate interlayer (Aimoz et al, 2012).…”

Enhancing Arsenic Solidification/Stabilisation Efficiency of Metallurgical Slag-Based Green Mining Fill and Its Structure Analysis

“…Anion substitution in ettringite can occur via reaction with surface sites (ligand exchange), replacing Al/Ca coordinated surface OH − or substituting for sulphate inside channels (isomorphic substitution) [35]. This occurs as the ettringite surfaces are negatively charged under alkaline pH range conditions and due to the column and channel-like structure of ettringite, which is composed of columns of Ca 6 [36]. The extent of channel substitution of ettringite may be inversely proportional to the difference in size and electronegativity of the oxyanion compared with SO 4 2− (0.29 Å), resulting in As(V) (0.47 Å) being likelier to be channelsubstituted with sulphur than As(III) (0.69 Å) [37].…”

Use of WetSEM® capsules for convenient multimodal scanning electron microscopy, energy dispersive X-ray analysis, and micro Raman spectroscopy characterisation of technetium oxides

“…In the safety case for geological disposal of radioactive wastes, 99 Tc makes a significant contribution to the long term dose risk, due its long half-life (t 1/2 = 2.1 × 10 5 years), and the high solubility and poor sorption of the pertechnetate species, TcO 4 − [1][2][3] In the context of some advanced nuclear fuel reprocessing flowsheets, such as the UREX process [4], an objective is to separate and immobilise Tc in a durable glass, ceramic or metal alloy wasteform for geological disposal [5,6]. Consequently, there is considerable research directed at the synthesis and characterisation of Tc wasteforms and understanding the sorption and migration behaviour of Tc in engineered barrier systems relevant to their geological disposal [3,7].…”