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, 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.

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. Coprecipitation Method: • Addition of nitrate precursors in desired stoichiometric ratio to a solution of NaNO3 maintained at pH 14. • Rapid stirring during production and subsequent aging Benefits • Scalable • Rapid Production • Flexible Ca(NO3)2 + Al(NO3)3 + Fe(NO3)3 NaOH + NaNO3 pH 14 Ca(1-x) (Al(1-y)Fey )x(OH)2 (NO3)x Stirrer bar

10. 750 500 Intensity(Counts) 250 0 10 20 30 40 50 60 2-Theta(°) X-Ray Diffraction Pattern and SEM (003) (00x) (006) (110) (116) CaCO3 (119) (113) (300) (033)

11. Effect of Aging Time at elevated temperature C: Ca(OH2) X: Ca3Al1.54Fe0.46[(OH)4]3 L:LDH phase

12. Variation in trivalent content, x Ca(1-x)(Al(1-y)Fey )x(OH)2 (NO3)x Where C is Ca(OH)2 L is a NO3-LDH L2 is thought to be a drying effect

13. Variation in Al/Fe Ratio, y Ca(1-x) (Al(1-y)Fey )x(OH)2 (NO3)x L is a NO3-LDH L2 is thought to be a drying effect Y=0 CaAl LDH Ternary Compositions Y=1 CaFe LDH

14. Compositional Variation –ICP OES Ca(1-x) (Al(1-y)Fey )x(OH)2 (NO3)x

16. 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

17. Test Methodology Method 1 Method 2 Direct Exchange Uncalcined material added to a solution containing desired anion 1M NaCl Solution Memory Effect Calcined material added to a solution containing desired anion 0.1M NaCl Solution

18. XRD – Method 1 NO3-Cl exchange Initial Low pH (7) Initial High pH (14) Red: LDH - NO3 Blue: Cl Exchanged1M NaCl

19. XRD – Method 1 NO3-CO3 exchange Initial Low pH (7) Initial High pH (14) Red: LDH - NO3 Blue: CO3 Exchanged 0.1M NaCO3

20. Test Methodology Method 1 Method 2 Direct Exchange Uncalcined material added to a solution containing desired anion 1M NaCl Solution Memory Effect Calcined material added to a solution containing desired anion 0.1M NaCl Solution

21. XRD- Method 2 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

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

23. Thermogravimetric Analysis & Differential Scanning Calorimetry Rehydrated Powder H2O loss CO2Loss ?

24. XRD – Method 2 NO3-Cl - Calcined Material Initial High pH (14) Red: Calcium Carbonate Blue: Cl Exchanged 0.1M NaCl

25. 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

27. 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

28. 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

29. Durability of converted product • Calcining to 950°C results in partial loss of the memory effect. • Rehydration to cement phases.

30. Summary • Ca based LDHs have been produced with a composition of x=0.3-0.33 • 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 • Anion exchange is possible on both uncalcined and calcined material (memory effect)

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

32. Acknowledgements This project is funded by the UK Engineering and Physical Sciences Research Council through the DIAMOND consortium

33. Questions