This is a repository copy of Compositional Dependence of Solubility/Retention of Molybdenum Oxides in Aluminoborosilicate-Based Model Nuclear Waste Glasses . Solubility/Retention Oxides in Aluminoborosilicate-Based Model Nuclear

Molybdenum oxides are an integral component of the high-level waste streams being generated from the nuclear reactors in several countries. Although borosilicate glass has been chosen as the baseline waste form by most of the countries to immobilize these waste streams, molybdate oxyanions (MoO 42- ) exhibit very low solubility (~ 1 mol.%) in these glass matrices. In the past 3-4 decades, several studies describing the compositional and structural dependence of molybdate anions in borosilicate and aluminoborosilicate glasses have been reported in the literature, providing a basis for our understanding of fundamental science that governs the solubility and retention of these species in the nuclear waste glasses. However, there are still several open questions that need to be answered to gain an in-depth understanding of the mechanisms that control the solubility and retention of these oxyanions in glassy waste forms. This article is focused on finding answers to two such questions: (1) What are the solubility and retention limits of MoO 3 in aluminoborosilicate glasses as a function of chemical composition? Why is there a considerable increase in the solubility of MoO 3 with incorporation of rare-earth oxides (for example, Nd 2 O 3 ) in aluminoborosilicate glasses? Accordingly, three different series of aluminoborosilicate glasses (compositional complexity being added in a tiered approach) with varying MoO 3 concentration have been synthesized and characterized for their ability to accommodate molybdate ions in their structure (solubility) and as a glass-ceramic (retention). The contradictory viewpoints (between different research groups) pertaining to the impact of rare-earth cations on the structure of aluminoborosilicate glasses have been discussed and their implications on the solubility of MoO 3 in these glasses have been evaluated. A novel hypothesis explaining the mechanism governing the solubility of MoO 3 in rare-earth containing aluminoborosilicate glasses has been proposed. The MAS NMR spectra of 11 B and 23 Na were acquired using commercial spectrometers (VNMRs, Agilent) and MAS NMR probes (Agilent). The samples were powdered in an agate mortar, packed into 3.2 mm zirconia rotors, and spun at 20 kHz for 11 B MAS NMR and 22 kHz for 23 Na MAS NMR. 23 Na MAS NMR data was collected at 16.4 T (185.10 MHz resonance frequency ), using a 0.6 µs (~(cid:652)/12 tip angle) pulse width for uniform excitation of the resonances. A range of 400 to 1000 acquisitions were co-added and the recycle delay between scans was 2 s. 11 B MAS NMR experiments were conducted at 11.7 T (160.34 MHz resonance frequency), incorporating a 4 s recycle delay, short radio frequency (RF) pulses (0.6 µs) corresponding to a (cid:652)/12 tip angle, and signal averaging of 400 to 1000 scans. The acquired spectra were processed with minimal apodization and referenced to aqueous boric acid (19.6 ppm) and aqueous NaCl (0 ppm), respectively. Fitting of the MAS NMR spectra was performed using DMFit, 28 and the CzSimple model, accounting for distributions in the quadrupolar coupling constant, was utilized for 23 Na MAS NMR spectra. The “Q MAS ½” and Gaus/Lor functions were used to fit 3 - and 4-fold coordinated boron resonances in the 11 B MAS NMR data, respectively, and N 4 was calculated from the relative areas of these peaks, with a small correction due to the overlapping satellite transition of the 4-fold coordinated boron peak. 29