[PPT] - Uranium(VI ) Uptake by Synthetic Calcium Silicate Hydrates Jan Tits PowerPoint Presentation

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Uranium(VI ) Uptake by Synthetic Calcium Silicate Hydrates

Jan Tits(1), N. Macé(1), M. Eilzer(2), E. Wieland(1), G. Geipel(2)

Paul Scherrer Institut(1) Forschungszentrum Dresden - Rossendorf(2) 2nd International Workshop

MECHANISMS AND MODELLING OF WASTE/CEMENT INTERACTIONS

Le Croisic, October 12-16 , 2008

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Lay-out

Introduction
Batch sorption studies:

Sorption isotherms

Spectroscopic investigations:

Time-resolved Laser Fluorescence Spectroscopy

Conclusions

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Safety barrier systems of cementitious repositories

Disposal of Low- and intermediate level radioactive waste

Cementitious materials are used for conditioning of the waste and for the construction of the engineered barrier system

Waste solidification Container: concrete, mortar, steel

Mortar Mortar Construction concrete Shotcrete liner

Deep geological repository

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10 10

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Incongruent dissolution

f CSH phases

Solution saturated w.r.t Ca(OH)2 (Na,K)OH saturated w.r.t. Ca(OH)2)

Region III Region II Region I

pH Total volume of water per unit mass

f anhydrous cement (L kg
1)

C-S-H phases in cement

Fresh cement Altered cement Gypsum Portlandite (40%) Monosulfo– aluminate Ettringite Aluminate Ferrite CSH gels (50)% Monosulfo- aluminate Ettringite Aluminate Ferrite CSH gels Ettringite Ferrite CSH gels CSH gels Silica gel rich in Fe/Al

(Atkinson et al., 1988, Berner, 1990; Adenot & Richet, 1997)

ACW Alkali-free

Calcium Silicate Hydrate (C-S-H) phases play an important role throughout the evolution of cement

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Structure of C-S-H phases

Garbev et al., 2008

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Recrystallisation of C-S-H phases from 45Ca uptake

[ ]

45 45 recryst.solid sol recryst.solid sol

Ca Ca Ca Ca ⎡ ⎤ ⎣ ⎦ =

Assumption:

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N2 atmosphere C:S ratio: 0.5 – 1.60 S:L ratio: 5.0 g/L (batch sorption tests) 1.0 g/L (TRLFS measurements) [U(VI)] added: 10-3 M – 10-7 M Ageing time: 2 weeks Equilibration time: 2 weeks (batch sorption tests) 1 – 14 days (TRLFS measurements) TRLFS / measurements Alpha counting / ICP-OES analysis centrifugation

UO2

2+

ageing supernatant sampling of

Aerosil-300 CaO

H2O ACW equilibration

Batch sorption experiments

Sorption tests Experimental set-up

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Batch sorption experiments

Sorption isotherms 10

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C:S = 0.75; alkali-free, pH=10.1

C:S = 1.07; alkali-free, pH=12.1

C:S = 1.65; alkali-free, pH=12.5 C:S = 0.74 ACW, pH=13.3 C:S = 1.07 ACW, pH=13.3 C:S = 1.25 ACW, pH=13.3

UO2 (sorbed) [mol kg

1]

UO2 equilibrium concentration [M]

Non-linear sorption: 2-site langmuir isotherm: Site 1: 1.5x10-3 mol/kg; site 2: > 0.6 mol/kg Effect of U(VI) speciation (pH) and C:S ratio (aqueous Ca concentration)

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Batch sorption experiments

Sorption isotherms

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Ca; C:S=1.65 Si; C:S=1.65 Ca; C:S=1.07 Si; C:S=1.07 Ca; C:S=0.65 Si; C:S=0.65

Cation equilibrium concentration(M)

UO2+ 2 equilibrium concentration (M)

Alkali-free conditions

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Ca; C:S = 1.07 Si; C:S = 1.07 Ca; C:S = 0.75 Si; C:S = 0.75

Cation concentration (M)

UO

2+ 2 equilibrium concentration (M)

in ACW

Solution composition is independent of the U(VI) sorption

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C-S-H phases have a high recrystallisation rate providing
pportunities for incorporation (SS formation)
U(VI) sorption on C-S-H phases:

– Is non-linear – Depends on the U(VI) aqueous speciation (influence of pH) – Depends on the C-S-H composition (Ca concentration?)

Batch sorption experiments

summary of the observations

Can these observations be described by a solid solution model?

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Batch sorption experiments

Requirements to model solid – solutions :

Mixing model: ideal or non-ideal Amount of recrystallized solid

From recrystallisation experiments with 45Ca

End-members and end-member stoichiometries:

C-S-H end-members: (see e.g. presentations of D. Kulik and B.

Lothenbach, S. Churakov,…)

U(VI) containing end-members ??

Indications from spectroscopic investigations (EXAFS, TRLFS,…)

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Time Time-

resolved laser fluorescence spectroscopy

resolved laser fluorescence spectroscopy of uranyl

f uranyl

Fluorescence process Fluorescence process

Vibrational relaxation Fluorescence emission Excitation λ = 266 nm

ligand σμ (axial oxygen 2p orbital)- to-metal δu (5f orbital) charge- transfer

Non-radiative relaxation e.g. via O-H stretch vibrations

ν=1 ν=n ν=2

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The position (↓ ), spacing (Δ), relative intensities (Pi/Pi+1) of the vibronic bands, lifetime, are sensitive to geometry of the uranyl and local chemical environment O=U=O axial bond length, RUO: RUO = 10650·[Δ]-2/3+57.5 (Bartlett &

Cooney, 1989)

Uranyl compounds fluoresce above

470 nm with characteristic vibronic progressions originating mainly from the symmetric stretch vibration of the O= U= O moiety (minor contributions from assymetric stretch- and bending vibration)

460 480 500 520 540 560 580 600

Time-resolved laser fluorescence spectroscopy

P6 P5 P4 P3 P2

Luminescence intensity (A.U.) Wavelength (nm)

P1

Δ Δ Δ Δ Δ

Aqueous uranyl 0.001 M Ca(OH)2

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Time-resolved laser fluorescence spectroscopy

Comparison of spectra from sorbed and aqueous uranyl species

Increasing red shift (lower energy): Indication of weakening of the axial U= O bond, (lower stretch frequency ) Stronger interaction between U(VI) and the equatorial ligands Change in geometry of uranyl moiety

λex= 266 nm, T= 150 K 480 500 520 540 560 580 600 620

U(VI)-CSH; C:S= 1.07 low loading U(VI) in ACW Free Uranyl in 1 M HClO4 U(VI)-CSH; C:S= 1.07 high loading

Luminescence intensity (A.U.) Wavelength (nm)

Red shift

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Time-resolved laser fluorescence spectroscopy

Sorption isotherm

450 480 510 540 570 600 630 Wavelength (nm)

1.0 mol kg

1

0.1 mol kg

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5x10

2 mol kg
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2 mol kg
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2x10

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3 mol kg
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3x10

4 mol kg
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4 mol kg
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Fluorescence emission (A.U.)

S:L = 1.0 g L-1; equilibration time = 1 day U(VI) sorbed on CSH at pH 12.0; C:S = 1.07

λex= 266 nm, T= 150 K

U(VI) loading

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Time-resolved laser fluorescence spectroscopy

Comparison with spectra of reference compounds

440 480 520 560 600

α α β β

Wavelength (nm)

β α

Luminescence intensity (A.U.)

Room temperature T=150 K

Uranophane α and β

480 500 520 540 560 580 600 620

U(VI )-CSH; C:S= 1.65 alkali-free; all loadings U(VI )-CSH; ACW C:S= 1.07; all loadings Soddyite U(VI )-CSH; C:S= 1.07 alkali-free; low loading Uranophane (α and β) U(VI )-CSH; C:S= 0.75; alkali-free; all loadings

Luminescence intensity (A.U.) Wavelength (nm)

λex= 266 nm, T= 293 K or 150 K

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Time-resolved laser fluorescence spectroscopy

Comparison with spectra of reference compounds

Axial U – O distance (Å) XRD EXAFS TRLFS Soddyite K-boltwoodite Uranophane α Uranophane β C-S-H (alkali-free) C-S-H (ACW) 1.78 (Demartin et al. 1983) 1.80 (Burns et al. 1998) 1.80 (Ginderow, et al. 1988) 1.82 (Viswanathan et al. 1986) 1.77(2) 1.80(2) 1.83(2) 1.81(2) 1.80(5)

1.86(5)

1.84(5) 1.9(1) 1.9(1)

TRLFS: RUO = 10650·[Δ] -2/ 3+ 57.5 (Bartlett & Cooney, 1989)

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Summary

C-S-H phases have a high recrystallisation rate
U(VI) sorption onto C-S-H phases is non-linear (at least 2 sorbed species)
increases with increasing C:S ratio
decreases with increasing pH
TRLFS can give indications about possible U(VI) containing end-members:
Luminescence spectra of U(VI) sorbed on C-S-H phases are all similar

similar geometry of the uranyl moiety (1 sorbed species)

In contrast to information from batch sorption experiments

Geometry of the sorbed uranyl is similar to the uranyl geometry in

α-uranophane (derived from spectral shape and peak position)

Uncertainies on axial oxygen distances is still high

(Future experiments at 4 K may improve the quality of this kind of information)

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Acknowledgements

Partial financial support was provided by the Swiss National Cooperative for the Disposal

f Radioactive Waste (Nagra) and by the European Communities (Actinet and MISUC)