Ion exchange selectivity of phillipsite for Cs+: A structural investigation using the Rietveld method

Different ion exchange selectivities for Cs of two phillipsites, one sedimentary and one synthetic, having similar framework compositions, were interpreted by means of Rietveld structural analysis. Both phillipsites were pre-exchanged to achieve Na forms throughout exhaustive elution using an NaCl solution and then enriched in Cs by contact with mixed Cs+ and Na+ solutions. X-ray diffraction data of the original and Cs-exchanged samples were refined using the Rietveld method. Four extra-framework sites labelled I, II, II′, and II″ were refined. Cation distribution in the four sites showed that, although sites I and II are readily occupied by Cs, some Na is prone to be retained in site II′, which displays therefore a reduced selectivity for Cs. The new site II″ is occupied by Cs. The synthetic sample retains much more Na than the sedimentary sample, and this is the reason for the higher selectivity of the sedimentary phillipsite for Cs compared with the synthetic one. The results of this work also suggest that phillipsite may be utilized as a selective cation exchanger for the removal of Cs+ radionuclide in nuclear waste.