Iodine valence and local environments in borosilicate waste glasses using X-ray absorption spectroscopy

Abstract

a b s t r a c tThe radioisotope 129 I, a fission product in spent nuclear fuel, has a long half-life, and can be highly mobile in the environment. Iodine K-edge X-ray absorption spectra were collected to characterize the iodine valence and coordination environment in simulated Hanford low activity waste glasses. Both iodine XANES and EXAFS data for eleven borosilicate glasses indicate iodide-like environments within the glass structure, where I À has Na or Li nearest-neighbors, and where the nearest-neighbor cation-type correlates to the most common network-modifying cation in the glass. This is further supported by the systematic increase of iodine incorporation with the combined Na 2 O + Li 2 O content in the glass. EXAFS analyses determined I-Na distances near 3.04 Å with coordination numbers near 4.0 and I-Li distances near 2.80 Å with coordination numbers near 3.0. I-Na environments determined for the glasses are similar to the tetrahedral INa 4 coordination found in NaI-sodalite. These weakly bound iodine-alkali configurations may be the only pathways for iodine to be retained in the glass. These environments may be precursors to NaI-sodalite crystallization in Na-rich glass. Iodine also shows distinct differences from chlorine in terms of the preferred sites in the glass structure.

Mechanism of iodine release from iodoapatite in aqueous solution

“…Iodine cannot be efficiently incorporated into the borosilicate glasses developed to immobilize high-level nuclear waste by a vitrication process 1,2 due to its low solubility and volatile nature. 3,4 Therefore, other durable waste forms need to be developed to immobilize iodine-129 for disposal. In addition, iodine release behaviour from these waste forms in disposal environments needs to be established to predict their long-term performance.…”

“…In the iodoapatite structure, Pb cations occupy two crystallographic sites with 8 and 9 coordination numbers and V forms tetrahedra with oxygen. Together, VO 4 and Pb sites form a more covalently bonded framework structure while the iodide anions, at the channel site in the crystal structure, are more ionically bonded to the cations due to a smaller ionic potential (charge to radius ratio) than the lead ion. 19 The results of previous durability tests with iodoapatite vary signicantly.…”

Bromine speciation and partitioning in slab-derived fluids and melts: Implications for halogens recycling in subduction zones

“…General similarities between Cl, Br and I speciation in aqueous solutions and silicate glasses (Evans et al, 2008;McKeown et al, 2011McKeown et al, ,2015Shermann et al, 2010) HBr at volcanic arcs, ~ 3-22 x10 3 kt/yr Cl and 5-15 kt/yr Br. (Barnes et al, 2018;Chavrit et al, 2016;Kendrick et al, 2013;Pyle and Mather, 2009).…”

Incorporation et rôle des halogènes dans les silicates vitreux et fondus

“…Le XANES (X-ray Absorption Near-Edge Spectrosopy) aux seuils K du chlore et de l'iode (K pour iode ou L3) a notammentété utilisé pour déterminer la spéciation de ces halogènes et leurs plus proches voisins (e.g. [28,29]). La RMN aété principalement utilisée pour déterminer le voisinage du fluor dans les verres (e.g.…”

“…Très récemment, uneétude de la spéciation du brome dans des liquides silicatés hydratés a montré que le brome est entouré des ions sodium et que sa coordinence augmente avec la pression (passage de 3à 6 ions sodium autour du brome) [37]. L'iode s'associe sous forme iodure avec les alcalins et principalement les ions sodium au sein du réseau vitreux [18,29].…”