PNNL

Abstract

Iodate incorporation in calcite is a potential sequestration pathway for environmental remediation of radioiodine-contaminated sites (e.g., Hanford Site, WA), but the mechanisms of iodate incorporation are still unknown. Molecular dynamics simulations were performed to determine the local coordination environment, the associated charge compensation schemes, and the tendency for iodate to segregate at calcite surfaces. Charge compensation by substitution of a calcium ion by either a sodium ion or a proton (i.e., the effect of pH), was considered. For charge compensation by sodium ions, a nearest-neighbor position was determined to be energetically favored, whereas for charge compensation by protons, the HIO3 molecule was found to be metastable and the formation of a bicarbonate ion was energetically favored. The iodate ion also showed a preference for the topmost atomic layer of the (104) calcite surface, which dominates the morphology of calcite crystals. Simulated trajectories were used to calculate iodine K-edge extended X-ray absorption fine structure spectra, which matched experimental data of iodate co-precipitated with calcite. The best-fit combination consisted of a significant proportion of surface-segregated iodate ions and included charge compensation by both sodium ions and protons. Therefore, an important implication is that the iodate accumulated at the surface regions of calcite crystals, may undergo mobilization under conditions that promote calcite dissolution. In addition, surface area and pH of the surrounding solution are both expected to strongly affect the extent of iodate incorporation in calcite. Hence, calcite-based iodate remediation strategies need to consider these impacts.