Predicting iodine solubility at high pressure in borosilicate nuclear waste glasses using optical basicity: an experimental study

Abstract

Constraining the systematic evolution of iodine solubility in borosilicate glasses is required for the formulation of adequate glass matrices able to immobilize 129I representing a major troublesome radioisotopes produced by nuclear anthropic activities. We investigated experimentally the change in iodine solubility in a large series of borosilicate glasses synthesized under high-pressure conditions (1.5 GPa) and at 1350 °C. The nature of network modifying cation (Na, K, Rb, Ca, Ba and Sr) and the concentration of B2O3 (~ < 10 mol.% and ~ > 10 mol.%) have been tested. The XPS measurements showed that iodine speciation in glasses is mostly represented by iodide (I−) for a range of iodine solubility from 0.3 to 3.1 mol.% as determined by SEM EDS and LA-ICP-MS analyses. The iodine solubility is enhanced in glasses with lower B2O3 content and with a higher concentration in network modifying cation. Regardless of the cation nature: alkali or alkaline-earth; increasing the cation size appears to induce a decrease in iodine solubility. We used the optical basicity (ΛGlass) and iono-covalent parameter (ICPGlass) to express the large variety of investigated glass compositions; both relating to the electron donor capability of the glass. We show that iodine solubility is positively correlated with ΛGlass and accordingly negatively correlated with ICPGlass. It implies that more iodine will be dissolved in glass compositions having a stronger electron-donating capability. Evidence of the relationship between iodine solubility and oxygen network is shown by the iodine solubility positive trend with equilibrium constant of the oxygen speciation. Future work should take these parameters into consideration for modeling iodine solubility in borosilicate glasses providing additional information are collected such as the boron, aluminum and silicon speciation in glasses.