IN SITU X-RAY DIFFRACTION STUDY OF CS ION EXCHANGE INTO H-TS

H-TS (Ti₂(OH)₂OSiO₄∙1.5H₂O) with mineral sitinakite topology is one of the ion-exchange materials being tested in field trials for the selective removal of ¹³⁷Cs and ⁹⁰Sr from nuclear waste solutions. In order to better understand the mechanisms of these ion exchange processes, time resolved in situ X-ray diffraction was used to monitor the exchange pathways in real time at beam line X7B of the National Synchrotron Light Source using a MAR 345 imaging plate.

The structures of the initial H-TS and final Cs-TS forms are well established from both X-ray and neutron powder diffraction investigations from published literature. Cs is disordered in two closely spaced sites along the [001] 8-member ring channel. Cs1 is located in the center of the 8-member rings while Cs2 is located on either side of the 8-member rings. The 8-member rings in the H-TS form are elliptical with a longest diameter (L) of approximately 6.98Å and a shortest diameter (S) of 5.45Å (L:S=1.28). However, the 8-member rings in Cs-TS are more circular with an L:S=1.0.

The initial H-TS and Cs-TS models were used as the starting and ending points for time-resolved Rietveld structure refinements. Upon Cs exchange, the Cs2 site selectively filled first outside the 8 member ring along the [001] channel to approximately 18% occupancy. While the Cs2 site is being filled, the H-TS (P 42/m b c) structure distorts to the Cs-TS structure (P 42/m c m). Once the Cs2 site is filled to 18%, Cs1 began to fill and ended at approximately 20% occupancy. These observations suggest that the structural distortion of the H-TS structure during the ion exchange of Cs2 allows access to the geometrical symmetric site at the center of the 8 member rings. Hydrogen exchange into Cs-TS was attempted, however no changes in the diffraction patterns were observed illustrating the selectivity for Cs in TS. Bond valance calculations further confirm the ion exchange selectivity for Cs.