Iodine local environment in high pressure borosilicate glasses: An X-ray photoelectron spectroscopy and X-ray absorption spectroscopy investigation

Abstract

The 129 I radioactive isotope is a by-product of nuclear plants activity. Owing to its strong volatility, there is currently no ideal protocol to immobilize 129 I in nuclear waste borosilicate glasses. Recently, we have proposed the use of high-pressure syntheses to dissolve iodine in various glass compositions; however, I speciation and dissolution mechanism could not be determined.We have adopted an approach combining X-ray Photoelectron Spectroscopy (XPS) and X-ray Absorption Spectroscopy (XAS) methods to determine I speciation and molecular environment in glasses containing from 0.5 to 2.5 mol.% I. The XPS spectra reveal that I is mostly dissolved as iodide (>85% I -) with a small proportion of elemental iodine (<15% I 0 ) and the absence of iodate species (I 5+ ). For borosilicate glasses, the XAS results and subsequent spectrum simulations suggested that Na and Ca are involved in the Ivicinity with averaged derived coordination number (CN) of 3.6 and 2.0 and bond length to the nearest neighbour (r X-I ) 2.98 and 2.85 Å, respectively. These results suggest that the coexistence of both Iand I 5+ species is not requested for electric neutrality but instead, we explain the I speciation by the possible interplay with oxygen species from the borosilicate matrix. In addition, the results imply that the borosilicate network is affected by the I dissolution.

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

“…The coordination number initialization was set as follow: the CNI-O, CNI-Na, CNI-Ca and CNI-I are 3, 4, 2 and 1, respectively. The coordination number for Na and Ca is consistent with the results reported in previous XAS investigations 17 36 . We applied a CNI-O = 3 in agreement with the local environment for iodate species, nonetheless we apply the same CNI-O for C35N0-33 in which periodate species have been identified and would therefore require a CNI-O = 4.…”

“…Although there is a large dispersion in the derived CNI-Na or Ca values, we observe that the CNI-Na is systematically higher than the CNI-Ca, probably because of the lower positive charges on sodium. Comparable results have been found in Morizet et al 36 for glasses of different compositions demonstrating that Irequires more charge compensating cations in its surrounding in the case of Na + rather than Ca 2+ . Invariably, the iodine coordination to oxygen in iodate configuration is close to 3 with CNI-O = 3.0±0.2 in adequate comparison to the structure of crystalline iodate.…”

“…XAS acquisitions at the I K-edge (~33169 eV) were preferred instead of the I L3-edge (~4557 eV) as in Morizet et al 36 owing to the possible interferences provoked by the Ca K-edge line at ~4038 eV. The energy calibration was performed using a Sb reference foil with a first derivative absorption edge at 30491 eV.…”

“…The position of the edge ranges from 33173.6 to 33177.6 eV but does not exhibit systematic change as function of the glass XNa or I content. To a first level of knowledge, the I K-edge XANES spectra can be used to determine the iodine oxidation state in glasses, for instance using linear combination analyses with spectra of model compounds 36 53 ; however, this approach appears relatively inaccurate in determining the iodine speciation dissolved in glasses using model spectra from crystalline compounds (see Morizet et al 36 ).…”

Incorporation of chlorine in nuclear waste glasses using high-pressure vitrification: Solubility, speciation, and local environment of chlorine

“…For instance, it has been demonstrated that Cl solubility is strongly dependent on CaO content (Carrol and Webster 1994, Signorelli and Carroll 2002, Evans et al 2008 and that Cl has strong structural affinities with Ca in glasses (McKeown et al 2011). Cl solubility is negatively affected by the Al content , which may be ascribed to a rivalry for the use of charge balancing cations, as observed for iodine (Jolivet et al 2020, Morizet et al 2021b.…”