Designing Wasteforms for Technetium Anion sorption with precursors for ceramic phases

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

2. Background Common form: 99Tc with a half life of 2.13x105 years. Tc is a low energy beta emitter, with no accompanying gamma rays. 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.

3. 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-.

4. 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.

5. Brucite - Mg(OH)2 / Portlandite - Ca(OH)2 • Mg cations: coordination 6 • Edge sharing of octahedra forming large sheets Hydroxide Group Magnesium/ Calcium

6. Brucite/Portlandite Sheets Mg,Ca M(II)

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

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

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

10. Production of Layered Double Hydroxides Coprecipitation Method • Nitrate Precursors • Rapid stirring in NaOH • Aging Benefits • Scalable • Rapid Production • Flexible Ca(1-x) (AlyFe(1-y))x(OH)2 (NO3)x

11. 750 500 Intensity(Counts) 250 0 10 20 30 40 50 60 2-Theta(°) X-Ray Diffraction Pattern and SEM (003) (006) (110) (116) CaCO3 (119) (113) (300) (033) 10μm

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. Thermogravimetric Analysis & Differential Scanning Calorimetry H2O loss NO3- - NO2- NO2- Loss CO2Loss

15. Anion capture with LDHs • 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

17. Intensity(a.u) 5 10 15 20 25 30 35 2-Theta(°) Thermal Conversion • Temperatures associated with the Tc system: • Tc2O7 = MP 119.5°C BP 311°C • TcO2 = sub ~900°C • Calcining to 950°C results in partial loss of the memory effect. 400°C x O O B Ca2(Fe,Al)2O5 B B B B = Brownmillerite O = Calcium Oxide X = Calcium Carbonate *ICSD, Vanpeteghem et al, 2008

18. Summary • Ca based LDHs have been produced with a composition of x=0.3-0.33 • Currently limited stability in terms of pH and temperature • Limited carbonate contamination • The memory effect exists in CaAlFe LDHs up to 600°C, and is only partially lost as high as 950°C. • Thermal conversion to Brownmillerite possible at T as a low as 400°C

19. Future Work • Mechanism and efficiency of adsorption. • Effect of pH and competing anions on the ability to capture specific anions • Durability of phases produced. • Alternative compositions, e.g. MgTiO4

20. Questions

21. Calcium Cation Ordering H2O or anion Oxygen Hydrogen Calcium