Spectroscopic evaluation of U^VI–cement mineral interactions: ettringite and hydrotalcite

Abstract

Portland cement based grouts used for radioactive waste immobilization contain high replacement levels of supplementary cementitious materials, including blast-furnace slag and fly ash. The minerals formed upon hydration of these cements may have capacity for binding actinide elements present in radioactive waste. In this work, the minerals ettringite (Ca6Al2(SO4)3(OH)12·26H2O) and hydrotalcite (Mg6Al2(OH)16CO3·4H2O) were selected to investigate the importance of minor cement hydrate phases in sequestering and immobilizing UVI from radioactive waste streams. U L III-edge X-ray absorption spectroscopy (XAS) was used to probe the UVI coordination environment in contact with these minerals. For the first time, solid-state 27Al magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy was applied to probe the Al coordination environment in these UVI-contacted minerals and make inferences on the UVI coordination, in conjunction with the X-ray spectroscopy analyses. The U L III-edge XAS analysis of the UVI-contacted ettringite phases found them to be similar (>∼70%) to the uranyl oxyhydroxides present in a mixed becquerelite/metaschoepite mineral. Fitting of the EXAFS region, in combination with 27Al NMR analysis, indicated that a disordered Ca- or Al-bearing UVI secondary phase also formed. For the UVI-contacted hydrotalcite phases, the XAS and 27Al NMR data were interpreted as being similar to uranyl carbonate, that was likely Mg-containing.

Enhanced uranium extraction selectivity from seawater using dopant engineered layered double hydroxides

“…43 The EXAFS modelling suggests, U uptake by all MgAl(X) LDH samples was through formation of surface sorbed uranyl carbonate phase along with mixing of some magnesium orthouranate character. The similar mechanism is also reported elsewhere, 71 where MgAl hydrotalcite in cement minerals was used to immobilize U(VI). Furthermore, XAS measurements were performed at the Mg, Al, O and N K-edges for U adsorbed MgAl and MgAlNd (Fig.…”

Spectroscopic identification of Ca-bearing uranyl silicates formed in C–S–H systems

“…The type of U-containing phases identified as being saturated in the systems did not vary with U concentration, but included: calcium uranate (CaUO 4 ); haiweeite (Ca(UO 2 ) 2 (Si 5 O 12 )(OH) 2 ⋅6H 2 O); soddyite ((UO 2 ) 2 (SiO 4 )·2H 2 O); uranophane (Ca(UO 2 ) 2 SiO 3 (OH) 2 ·5H 2 O); metaschoepite (UO 3 ·2H 2 O); and uranium hydroxide (UO 2 (OH) 2 ). Although the calcium uranate phase identified by the model is a high temperature phase 17 , hydrous forms of calcium uranate are known to exist and are typically solubility limiting at high pH 18 .

Figure 1 Aqueous concentrations of Ca and Si released to solution.

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Foreword to the special virtual issue on Actinide physics and chemistry with synchrotron radiation

“…Jian Sun and co-authors studied two series of uranium-doped Nd 2 Zr 2 O 7 pyrochlore materials as potential nuclear waste host matrices by the combination of the X-ray diffraction, Raman and EXAFS techniques (Sun et al, 2022). Using EXAFS at the U L 3 -edge and XRD, Antonia S. Yorkshire and co-authors investigated U(VI) -cement mineral interactions, relevant to under-standing the waste disposal of actinide-containing materials (Yorkshire et al, 2022). Hao Ding and co-workers examined synthetic Chernobyl lava specimens using micro-focus spectroscopy at the U L 3 -edge along with diffraction techniques and were able to construct U oxidation state maps (Ding et al, 2021).…”