Characterisation of heterogeneous molybdate and chromate phase assemblages in model nuclear waste glasses by multinuclear magnetic resonance spectroscopy

Abstract

A series of sodium borosilicate glasses containing cesium, molybdenum, and chromium was prepared to investigate the partitioning of chromium amongst the glass and phase-separated crystalline molybdates. The precipitates were examined by (133)Cs, (23)Na, and (95)Mo MAS NMR, revealing a phase assemblage consisting of Na(2)MoO(4), Na(2)MoO(4)·2H(2)O, Cs(2)MoO(4), Cs(2)CrO(4), CsNaMoO(4)·2H(2)O, and Cs(3)Na(MoO(4))(2). (133)Cs MAS NMR indicates random substitution of Cr into the Mo sites of Cs(3)Na(MoO(4))(2) and provides a quantitative assessment of Cr incorporation. The sample compositions were verified by various analytical techniques and highlight the centrality of NMR in the identification and quantification of heterogeneous crystalline composites, including sensitivity to cationic substitution. The observation and facile interconversion of hydrated phases invites careful consideration of these materials for nuclear waste disposal.

133Cs and 23Na MAS NMR Spectroscopy of Molybdate Crystallization in Model Nuclear Glasses

“…The site‐specific nature of high‐resolution NMR provides additional information about the identities of the devitrification products formed with different Mo‐loading levels and quench rates. With reference to peak assignments drawn from previous work, the 133 Cs MAS NMR data reveal that the dominant phases are CsNaMoO 4 ·2H 2 O (−70 ppm), Cs 3 Na(MoO 4 ) 2 (161, −144 ppm), and Cs 2 Mo 2 O 7 (22, 9, −14, −56 ppm), with several unidentified peaks (69, 47, −9, −78 ppm) which may belong to Cs molybdate phase(s) not yet characterized. CsNaMoO 4 ·2H 2 O is the most populous compound appearing in the 133 Cs MAS NMR spectra and is prominent also in the 23 Na MAS NMR data (Fig.…”

Structure of Rhenium‐Containing Sodium Borosilicate Glass

“…This water-soluble phase can contain radionuclides such as 137 Cs/ 135 Cs and 90 Sr, which can be partitioned to the molten molybdate/ chromate/sulfate/chloride salt ("yellow phase"). 40,[42][43][44] Extended X-ray absorption fine structure spectroscopy (EXAFS) has suggested that Mo in borosilicate glass forms MoO 2À 4 tetrahedra that are unconnected with the borosilicate network but rather associated with alkali and alkaline earth regions. 45 Others have suggested that MoO 3 , at least at increasing concentrations, acts as a "reticulating agent" for the silicate network where Q 4 units are increased, and the molybdate exists in a "depolymerized region" rich in alkali and alkaline earth ions and nonbridging oxygens.…”

Network connectivity in cesium borosilicate glasses: 17O multiple-quantum MAS and double-resonance NMR

“…We have previously used various solid-state NMR techniques to detect devitrification [38][39][40] and glass-in-glass phase separation [33] in model cesium-and molybdenum-containing nuclear waste glasses.…”