Designing Wasteforms for Technetium Anion sorption with precursors for ceramic phases

DIAMOND Decommissioning, Immobilisation and Management of Nuclear Waste for DisposalDIAMOND Decommissioning, Immobilisation and Management of Nuclear Waste for Disposal

Designing Wasteforms for TechnetiumAnion sorption with precursors for ceramic phases

Jonathan PhillipsCentre for Advanced Structural CeramicsDepartment of Materials, Imperial College LondonPrince Consort Road, London, SW7 2AZ

SupervisorDr Luc Vandeperre

DIAMOND Decommissioning, Immobilisation and Management of Nuclear Waste for Disposal

Overview


•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.

Background


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

-.


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.


•Ca cations: coordination 7 (with additional water/anion in interlayer)

•Edge sharing of octahedra forming large sheets

Hydroxide GroupCalcium

Portlandite - Ca(OH)2


+

+ +

+

+

+

M(II)

+

Isomorphous Substitution

Al,Fe(III) M(III)

Mg,Ca

Layered Double Hydroxides


+

+ +

+

+

+-

-

-

-

-

-

H2O

Anions

+

+ +

+

+

+

M2+(1-x) M3+

x (OH)2 (Az+)x/z.nH2O

Charge Balance


NaOH + NaNO3

pH 14

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

Stirrer bar

Ca(NO3)2 + Al(NO3)3 + Fe(NO3)3

Ca(1-x) (Al(1-y)Fey )x(OH)2 (NO3)x


10 20 30 40 50 602-Theta(°)

0

250

500

750

Inte

nsity

(Cou

nts)

(003)

(006)

CaCO3

(110)

(113)(116)

(119) (300)(033)

X-Ray Diffraction Pattern and SEM

(00x)


Position [°2Theta]10 20 30 40 50

140 Hours

70 Hours

40 Hours

20 Hours

L

L L LLL

C

L L LLL

LL

CC

C

C

CC

CC

C

X X

XX X

XX

XX

XX

X

X X

X

X X

XX

X X

XX

X

Inte

nsity

(a.u

)

X XX X X X

X XX

XX X XXX

L

LL

C: Ca(OH2)X: Ca3Al1.54Fe0.46[(OH)4]3

L:LDH phase

Effect of Aging Time at elevated temperature


Where C is Ca(OH)2

L is a NO3-LDHL2 is thought to be a drying effect


Variation in trivalent content, x



Y=0 CaAl LDH

Y=1 CaFe LDH

Ternary Compositions

Variation in Al/Fe Ratio, y

L is a NO3-LDH

L2 is thought to be a drying effect



Compositional Variation –ICP OES



Topotactic Exchange Dissolution Reprecipitation

Preference for to be intercalated therefore exchange with

LDH dissolves, increasing the solution pH and then reprecipitates with new anion

Anion Exchange Mechanism


Test Methodology

Method 1 Method 2

Direct Exchange

Uncalcined material added to asolution containing desired anion

1M NaCl Solution

Memory Effect

Calcined material added to asolution containing desired anion

0.1M NaCl Solution


0

100

400

900

Counts

Position [°2Theta]10 15

040210A 010210

Red: LDH - NO3

Blue: Cl Exchanged1M NaCl

0

100

400

900

Counts

Position [°2Theta]8 10 12 14

040210B 010210

XRD – Method 1

Initial Low pH (7) Initial High pH (14)

NO3-Cl exchange


Red: LDH - NO3

Blue: CO3 Exchanged 0.1M NaCO3

XRD – Method 1

Initial Low pH (7) Initial High pH (14)

NO3-CO3 exchange

0

100

400

Counts

Position [°2Theta]

5 10 15

b260210 010210

0

100

400

Counts

Position [°2Theta]

6 8 10 12 14

a260210 010210


Test Methodology

Method 1 Method 2

Direct Exchange

Uncalcined material added to asolution containing desired anion

1M NaCl Solution

Memory Effect

Calcined material added to asolution containing desired anion

0.1M NaCl Solution


Position [°2Theta]10 20 30 40 50

Counts

0

100

400

900

0

100

400

900

0

100

400

900

939ATT2

P3HTFUR

P3H2O

Untreated LDH Powder

Calcined LDH Powder

Rehydrated -Calcined LDH Powder

BB

xB B

OO

XRD- Method 2

B = BrownmilleriteO = Calcium OxideX = Calcium Carbonate

Calcine Capture


H2O loss

NO3- - NO2

-

NO2- Loss CO2

Loss

Thermogravimetric Analysis & Differential Scanning Calorimetry

Untreated Powder


H2O loss

CO2 Loss ?

Thermogravimetric Analysis & Differential Scanning Calorimetry

Rehydrated Powder


Red: Calcium CarbonateBlue: Cl Exchanged 0.1M NaCl

0

100

400

900

Counts

Position [°2Theta]10 20 30 40 50 60

S2 CALCINE

XRD – Method 2

Initial High pH (14)

NO3-Cl - Calcined Material


Wang Y. et al Jour. Coll and Int. Sci. 301 (2006) 19-26

•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

Anion Capture with LDHs



•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

Thermal Conversion

Ca2(Fe,Al)2O5

*ICSD, Vanpeteghem et al, 2008

CaFe,AlO


5 10 15 20 25 30 352-Theta(°)

Inte

nsity

(a.u

)

400°C

BB

x

B B

OO

B = BrownmilleriteO = Calcium OxideX = Calcium Carbonate

Thermal Conversion

•A sample of LDH-NO3 was calcined to 400°C for 1 hour•Browmillerite and Calcium Oxide have formed.


Durability of converted product

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


• 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)

Summary


•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

Future Work


This project is funded by the UK Engineering and Physical Sciences Research Council through the DIAMOND

consortium

Acknowledgements


Questions

Documents

Designing Wasteforms for Technetium Anion sorption with precursors for ceramic phases