IN SITU X-RAY DIFFRACTION STUDY OF CS+ ION EXCHANGE IN UMBITE

Umbite, as with many other heterosilicates, may be a potential ion exchanger for the separation of Cs and Sr from high level waste. The mechanisms of ion exchange and ion mobility within microporous heterosilicates and aqueous solutions are not well understood due to the high rate of diffusion and the difficulty in probing the sample in situ.

The exchange of Cs⁺ into H_1.22K_0.84ZrSi₃O₉•2.16H₂O (umbite-(HK)) was followed, in situ, using time-resolved X-ray diffraction at the National Synchrotron Light Source. The umbite framework (space group P2₁/c with cell dimensions of a = 7.2814(3) Å, b = 10.4201(4) Å, c = 13.4529(7) Å, and β = 90.53(1)°) consists of wollastonite-like silicate chains linked by isolated zirconia octahedra. Within umbite-(HK) there are two unique ion exchange sites in the tunnels running parallel to the a-axis. Exchange Site 1 is marked by an 8 member-ring (MR) window containing K⁺ cations, and Exchange Site 2 is marked by a larger 8-MR tunnel containing H₂O molecules. The occupancy of the exchanging Cs⁺ cations through these tunnels was modeled by Rietveld structure refinements of the diffraction data and demonstrated that there was a two step exchange process. The incoming Cs⁺ ions populated the larger 8-MR channel (Exchange Site 2) first and then migrate into the smaller 8-MR channel. During the exchange process a structural change occurs, transforming the exchanger from monoclinic P2₁/c to orthorhombic P2₁2₁2₁. This structural change occurs when Cs⁺ occupancy in the small cavity becomes greater than approximately 0.50. The final in situ ion exchange diffraction pattern was refined to yield umbite-(CsK) with the formula H_0.18K_0.45Cs_1.37ZrSi₃O₉•0.98H₂O possessed an orthorhombic unit cell with dimensions a = 10.6668(8) Å, b = 13.5821(11) Å, c = 7.3946(6) Å. Valence bond sums for the completely occupied Exchange Site 1 demonstrate that Cs-O bonds of up to 3.8Å contribute to the coordination of the Cs⁺ cation.