Incorporation Modes of Iodate in Calcite

Abstract

Iodate (IO₃^-) incorporation in calcite (CaCO₃) is a potential sequestration pathway for environmental remediation of radioiodine-contaminated sites (e.g., Hanford Site, WA), but the incorporation mechanisms have not been fully elucidated. Ab initio molecular dynamics (AIMD) simulations and extended X-ray absorption fine structure spectroscopy (EXAFS) were combined to determine the local coordination environment of iodate in calcite, the associated charge compensation schemes (CCS), and any tendency for surface segregation. IO₃^- substituted for CO₃^2- and charge compensation was achieved by substitution of Ca²⁺ by Na⁺ or H⁺. CCS that minimized the I-Na/H distance or placed IO₃^- at the surface were predicted by density functional theory to be energetically favored, with the exception of HIO₃, which was found to be metastable relative to the formation of HCO₃^-. Iodine K-edge EXAFS spectra were calculated from AIMD trajectories and used to fit the experimental spectrum. The best-fit combination consisted of a significant proportion of surface-segregated IO₃^- and charge compensation was predominantly by H⁺. Important implications are therefore that pH should strongly affect the extent of IO₃^- incorporation and that IO₃^- accumulated at the surface of CaCO₃ particles may undergo mobilization under conditions that promote calcite dissolution. These impacts need to be considered in calcite-based iodate remediation strategies.