A study of the Synthesis of Natisite and its Zirconium Doped Analogues as Solid Ion-Exchangers for the Remediation of Nuclear Waste

Abstract

Unlike zeolites which contain SiO4 and AlO4 tetrahedra, microporous titanium silicates display SiO4 in tetrahedral coordination and TiO6 octahedra and therefore display a wider range of possible structures which can result in different properties. Doping different transition metals into existing titanium silicate structures can lead to significant changes in the structure which lead to differences in the materials ion-exchange chemistry. Sitinakite, KNa2Ti4Si2O13(OH)·4H2O, and the synthetic niobium doped analogue (IONSIV-911) is one example of a doped titanium silicate which is used as an ion-exchanger for the removal of Cs+ and Sr2+ from nuclear waste.
Natisite, Na2TiSiO5, crystallises in the tetragonal space group P4/n m m and is a layered titanium silicate with titanium in an unusual 5 coordinate square pyramidal environment. Little previous work has been conducted on this material, with any published work focusing on synthesis and characterisation of the structure. No work to date has been conducted on the modification of the framework or any tests on natisites ion-exchange chemistry. Preliminary results have found that increasing the levels of zirconium doping has significant effects on the materials ion-exchange chemistry. Such differences include a faster rate of exchange for Co, increased affinity for Sr and Cs and, a slight increase in the level of exchange with respect to Ce and Nd (used as inactive surrogates for Pu and U respectively).
Ti Edge XAS data has been investigated and shown significant changes in the XANES region upon ion-exchange. The presence of Zr within the natisite framework has a significant effect on the materials ion-exchange chemistry but the changes in the Ti XAS data suggests that the exchanged cations prefer sites closer to titanium rather than zirconium.
Cerium ion-exchanges shows the formation of CeO2 nanoparticles between the layers. X-Ray diffraction shows that the layers themselves are still intact but the stacking has been disrupted by the introduction of these CeO2 nanoparticles with X-Ray Absorption Spectroscopy (XAS) used to identify the Ce phase.
The work presented here will focus on the differences between 0, 10% and 20% Zr doped natisite and the effect this has on the materials ion-exchange chemistry both with single and competitive ion-exchanges. These results will be compared to clinoptilolite which is used as the industrial standard material for treatment of radioactive liquid waste via the SIXEP process. Clinoptilolite sourced from Sellafield has undergone ion-exchanges under controlled conditions with the capacity directly compared to that of natisite, 10% Zr natisite and 20% Zr natisite.
Results have shown that natisite has superior performance towards Ce, Nd, Co, both in the single exchanges and in the presence of competing ions such as Ca, Mg and K. Also the zirconium doped analogues of natisite has a comparable exchange capacity towards Cs and Sr to that of clinoptilolite.