Iodate in calcite and vaterite: Insights from synchrotron X-ray absorption spectroscopy and first-principles calculations

Calcium carbonates such as calcite are the dominant hosts of inorganic iodine in nature and are potentially important for the retention and removal of radioactive iodine isotopes (I-129 and I-131) in contaminated water. However, little is known about the structural environment of iodine in carbonates. In this study, iodate (IO3-) doped calcite and vaterite have been synthesized using the gel-diffusion method at three NaIO3 concentrations (0.002; 0.004; 0.008 M) and a pH value of 9.0, under ambient temperature and pressure. Inductively coupled plasma mass spectrometry (ICP-MS) analyses show that iodine is preferentially incorporated into calcite over vaterite. Synchrotron iodine K-edge X-ray absorption near-edge structure (XANES) spectra confirm that IO3- is the dominant iodine species in synthetic calcite and vaterite. Analyses of iodine Kedge extended X-ray absorption fine structure (EXAFS) data, complemented by periodic first-principles calculations at the density functional theory (DFT) levels, demonstrate that the I5+ ion of the IO3- group in calcite and vaterite is bonded by three and two additional O atoms (i.e., coordination numbers = 6 and 5), respectively, and is incorporated via the charged coupled substitution I5+ + Na+ <-> C4+ + Ca2+, with the Na+ cation at a nearest Ca2+ site being the most energetically favorable configuration. (C) 2016 Elsevier Ltd. All rights reserved.