Local environment of iodine dissolved as iodate in high-pressure aluminoborosilicate glasses: A I K-edge x-ray absorption spectroscopic study

Abstract

The use of high-pressure synthesis conditions to produce I-bearing aluminoborosilicate represent a promising issue for the immobilization of 129I radioisotope. Furthermore, iodine appears to be more solubilized in glasses under its iodate (I5+) form rather than its iodide (I-) form. Currently, the local atomic environment for iodine is poorly constrained for I- and virtually unknown for I5+ or I7+. We used I K-edge X-ray Absorption Spectroscopy conducted at 20 K for determining the local atomic environment of iodine dissolved as I-, I5+, I7+ in a series of aluminoborosilicate glasses. We determined that I- is surrounded by either Na+ or Ca2+ in agreement with previous works. The signal collected from EXAFS reveals that I5+ is surrounded invariably by three oxygen atoms forming a IO3- cluster charge compensated by Na+ and/or Ca2+. The I-O distance in iodate dissolved in glass is comparable to the I-O distance in crystalline compounds at ~1.8 Å. The distance to the second nearest neighbor (Na+ or Ca2+) is also constant at ~3.2 Å. This derived distance is identical to the distance between I- and Na+ or Ca2+ in the case of iodide local environment. For one sample containing iodate and periodate, the distinction between the local environment of I5+ and I7+ could not be made suggesting that both environments have comparable EXAFS signal.

Iodine dissolution mechanisms in high-pressure aluminoborosilicate glasses and their relationship to oxygen speciation

“…This result is in agreement with the previous study by Morizet et al 34 demonstrating that I atoms release electrons to form I 5+ cations that combine with available O atoms to form IO 3 − clusters within the glass structure. 41 However, the achieved I 5+ concentrations in the present work are lower than the ones reported in Morizet et al 34 in which I 5+ can reach up to 90% of the total iodine species. We suspect that there is an experimental issue for this difference.…”

“…The peak assignment has been described in previous similar studies. 34,35,40,41 The peak for I 5+ is essentially present in the experiments with I 2 O 5 as the initial source and almost absent in the experiments with I 2 . In some cases, we observed the presence of I 0 (Fig.…”

“…23,36,37,39,40 Morizet et al 34 also identified other dissolution mechanisms under oxidizing conditions with the presence of iodate species (I 5+ ), and in some particular circumstances periodate species (I 7+ ). Under these oxidizing conditions, the I 5+ cations form IO 3 − units 41 are charge compensated by available alkali or alkaline-earth cations. In these dissolution mechanisms, both types of species (I − and IO 3 − ) are not bonded to the glass network (Si, Al or B units) but instead occupy the voids in the glass structure induced by the presence of network modifying cations (alkali or alkaline-earth cations).…”

“…23,36,37,39,40 Morizet et al 34 also identied other dissolution mechanisms under oxidizing conditions with the presence of iodate species (I 5+ ), and in some particular circumstances periodate species (I 7+ ). Under these oxidizing conditions, the I 5+ cations form IO 3 − units 41 are charge compensated by available alkali or alkaline-earth cations. In these dissolution mechanisms, both types of species (I − and IO 3…”

Review of recent developments in iodine wasteform production

“…Iodide is the primary species identified in glasses containing iodine (Muller et al, 2014;McKeown et al, 2015;Gin et al, 2017), however iodate has also been observed (Riley et al, 2014;Cicconi et al, 2019a;Jolivet et al, 2020) and is suggested to have higher stability (Morizet et al, 2022). No clear trend on the resulting iodine species in the glass based on feed species (I − and/ or IO 3 − ) has been observed (Muller et al, 2014;McKeown et al, 2015).…”