Designing Wasteforms for Technetium Anion sorption with precursors for ceramic phases

1. Designing Wasteforms for TechnetiumAnion sorption with precursors for ceramic phases Jonathan Phillips Centre for Advanced Structural Ceramics Department of Materials, Imperial College London Prince Consort Road, London, SW7 2AZ Supervisor Dr Luc Vandeperre

2. Overview

3. Background Common form: 99Tc with a half life of 2.13x105 years. Tc is a low energy beta emitter. It is produced with sufficient yield (6.1%) to be a concern for the environment. Technetium compounds generally do not bind well with soils and are highly mobile in the environment.

4. Background In the UK, Tc was formerly discharged to the sea by BNFL however it is now separated using a process involving tetraphenylphosphonium bromide (TPPB). The TPPB enables Tc to be disposed of by cement encapsulation. In alkaline environments TPPB is known to degrade releasing the pertechnetate anion TcO4-.

5. Aim The aim is to capture the pertechnetate anion from solution using layered double hydroxide materials with a suitable composition to be thermally converted to stable ceramic phases.

6. Portlandite - Ca(OH)2 • Ca cations: coordination 7 (with additional water/anion in interlayer) • Edge sharing of octahedra forming large sheets Hydroxide Group Calcium

7. Layered Double Hydroxides Mg,Ca M(II) Isomorphous Substitution + + + M(III) Al,Fe(III) + + + +

8. Charge Balance + Anions + + + + + + + + + + - - - - - - + M2+(1-x) M3+x (OH)2 (Az+)x/z.nH2O H2O

9. Materials and Methods 1M Total Ca(NO3)2.4H2O Al(NO3)3.9H2O Fe3+(NO3)3.9H2O Ca(1-x) (Al(1-y)Fey )x(OH)2 (NO3)x NaOH + NaNO3 pH >12.5 Stirrer bar Phillips, J. and L.J. Vandeperre, Production of Layered Double Hydroxides for Anion Capture and Storage, in Materials Research Needs to Advance Nuclear Energy, Mater. Res. Soc. Symp. Proc., Vol. 1215, G. Baldinozzi, et al., Editors. 2010, MRS: Warrendale, PA. p. V11-04.

10. X-Ray Diffraction Pattern and SEM

11. Characterisation of product

13. Anion Exchange Mechanism Topotactic Exchange Dissolution Reprecipitation LDH dissolves, increasing the solution pH and then reprecipitates with new anion Preference for to be intercalated therefore exchange with

14. Anion Exchange Method • 1g of LDH powder (NO3 intercalated) was added to a solution containing the desired interlayer anions • The composition of the anionic solution were varied in the following molar ratios (balanced for charge differences of the anions) • 0.1 : 0.9 0.5 : 0.5 0.9 : 0.1 • The exchange was allowed to occur for a period of 1hr and for 14 days. • The solids were separated by vacuum enhanced filtration before being dried in an oven.

15. Results : Anion Exchange Cl:NO3 • Formation of two distinct interlayer spacings in the short term. • Prolonged exposure to high [CO3] solutions deleterious. J.D. Phillips, L.J. Vandeperre, J. Nucl. Mater.(2010),doi:10.1016/j.jnucmat.2010.11.101

16. Results : Anion Exchange NO3:CO3 CO3 effect Cl:CO3

17. XRD- Memory effect Untreated LDH Powder O B B O x Calcined LDH Powder B B Rehydrated -Calcined LDH Powder B = Brownmillerite O = Calcium Oxide X = Calcium Carbonate Calcine Capture

18. Anion Capture with LDHs • Competition with other anions. • Capture of pertechnetate or other anions with calcined LDH, taking advantage of the memory effect • Adsorption efficiency for surrogates of TcO4- - ICP OES Wang Y. et al Jour. Coll and Int. Sci. 301 (2006) 19-26

20. Thermal Conversion • Temperatures associated with the Tc system: • Tc2O7 = MP 119.5°C BP 311°C • TcO2 = sub ~900°C • Conversion at as low a temperature as possible desirable. • The aim is to convert these LDH phases to Brownmillerite Ca2(Fe,Al) 2O5 which are compositions commonly found in cements Ca Fe,Al O Ca2(Fe,Al)2O5 *ICSD, Vanpeteghem et al, 2008

21. Intensity(a.u) 5 10 15 20 25 30 35 2-Theta(°) Thermal Conversion • A sample of LDH-NO3 was calcined to 400°C for 1 hour • Browmillerite and Calcium Oxide have formed. 400°C x O O B B B B B = Brownmillerite O = Calcium Oxide X = Calcium Carbonate

22. Results : Thermal Analysis NO3

23. Thermal Product - NO3 B: Brownmillerite Ca2AlFeO5 P: Calcium Hydroxide Ca(OH)2 C: Calcium Oxide CaO

24. Results : Thermal Analysis Cl

25. Thermal Product – Cl B: Brownmillerite Ca2AlFeO5 P: Calcium Hydroxide Ca(OH)2 C: Calcium Oxide CaO

26. Thermal Product – Cl B: Brownmillerite Ca2AlFeO5 P: Calcium Hydroxide Ca(OH)2 C: Calcium Oxide CaO

27. Conclusions • Layered double hydroxides with a composition suitable for thermal conversion to ceramic phases have been produced. • The absorption capacity of these materials for the perhenate anion is significantly reduced due contamination with CO3 from equilibrium with the atmosphere. • Capture of Cl- is favourable even in the presence of CO3, these materials may be applicable to the remediation of 36Cl- from the processing of graphitic wastes. • Thermal conversion product dependent on interlayer anion.

28. Acknowledgements This project is funded by the UK Engineering and Physical Sciences Research Council through the DIAMOND consortium Thank you for your attention