Anion and Cation Order in Iodide-Bearing Mg/Zn–Al Layered Double Hydroxides

Abstract

Uptake of iodine in hydrotalcite-like minerals is a potential retardation mechanism for dose-relevant 129I in the near-field of a deep repository for radioactive waste. The location of iodide in (Zn/Mg)Al layered double hydroxides (LDH) was investigated using a combination of advanced atomic-scale techniques. Wavelet transform analysis of Zn K-edge extended X-ray absorption fine structure (EXAFS) spectra and geometry optimization based on ab initio density functional calculations allowed the distribution of Al3+ in the cationic layer to be determined. Using Rietveld refinement of synchrotron X-ray powder diffraction data (XRD) and EXAFS at the I K-edge enabled the average location of iodide in the interlayer to be established. Additional short- and medium-range structural information was also obtained from the pair distribution function analysis of the XRD data in support of the findings obtained with the long- and short-range techniques. By combining the results, a local order of Al3+ in Zn2Al–I and Zn3Al–I LDHs was shown generating hexagonal and orthorhombic supercells, respectively. Furthermore, an uncorrelated distribution between I– anions and Zn2+/Al3+ cations was demonstrated, resulting from a dynamic disorder of water and iodide position in the interlayer space.