SREL Reprint #3379

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

J. Podder1, J. Lin1, W. Sun1, S. M. Botis1, J. Tse2, N. Chen3, Y. Hu3, D. Li4, J. Seaman5, and Y. Pan1

1Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK S7N 5E2, Canada
2Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, SK S7N 5E2, Canada
3Canadian Light Source, University of Saskatchewan, Saskatoon, SK S7N 5E2, Canada
4Savannah River National Laboratory, Aiken, SC 29808, USA
5Savannah River Ecology Laboratory, University of Georgia, Aiken, SC 29802, USA

Abstract: 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 (129I and 131I) 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.

Keywords: Radionuclides; Calcite; Vaterite; Iodate; Synchrotron XAS; First-principles calculations

SREL Reprint #3379

Podder, J., J. Lin, W. Sun, S. M. Botis, J. Tse, N. Chen, Y. Hu, D. Li, J. C. Seaman, and Y. Pan. 2017. Iodate in calcite and vaterite: Insights from synchrotron X-ray absorption spectroscopy and first-principles calculations. Geochimica et Cosmochimica Acta 198(2017): 218-228.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).