Spectroscopic and first-principles investigations of iodine species incorporation into ettringite: Implications for iodine migration in cement waste forms.

ABSTRACT

Low-level radioactive wastes are commonly immobilized in cementitious materials, where cement-based material can incorporate radionuclides into their crystal structure. Specifically, ettringite (Ca6Al2(OH)12(SO4)3∙26H2O) is known to stabilize anionic species, which is appealing for waste streams with radioactive iodine (129I) that persists as iodide (I-) and iodate (IO3-) in the cementitious nuclear waste repository. However, the structural information and immobilization mechanisms of iodine species in ettringite remain unclear. The present results suggested minimal I- incorporation into ettringite (0.05 %), whereas IO3- exhibited a high affinity for ettringite via anion substitution for SO42- (96 %). The combined iodine K-edge extended X-ray absorption fine structure (EXAFS) spectra and first-principles calculations using density functional theory (DFT) suggested that IO3- was stabilized in ettringite by hydrogen bonding and electrostatic forces. Substituting IO3- for SO42- was energetically favorable by -0.41 eV, whereas unfavorable substitution energy of 4.21 eV was observed for I- substitution. Moreover, the bonding charge density analysis of the substituted IO3- and I- anions into the ettringite structure revealed the interaction between intercalated ions with the structural water molecules. These results provided valuable insight into the long-term stabilization of anionic iodine species and their migration in cementitious nuclear waste repository or alkaline environments.