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

Laboratory for Waste Management Nuclear Energy and Safety Department 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

Laboratory for Waste Management Nuclear Energy and Safety Department Lay-out • Introduction • Batch sorption studies: Sorption isotherms • Spectroscopic investigations: Time-resolved Laser Fluorescence Spectroscopy • Conclusions

Laboratory for Waste Management Nuclear Energy and Safety Department 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 Container: system Waste concrete, mortar, solidification steel Mortar Mortar Shotcrete liner Construction concrete Deep geological repository

Laboratory for Waste Management Nuclear Energy and Safety Department C-S-H phases in cement 14 Altered cement Fresh Region I Region II Region III cement 13 Gypsum Incongruent dissolution ACW Portlandite of CSH phases (40%) (Na,K)OH Monosulfo– Monosulfo- pH saturated Solution saturated 12 w.r.t. aluminate aluminate w.r.t Ca(OH)2 Ca(OH)2 ) Ettringite Ettringite Ettringite Aluminate Aluminate Alkali-free Ferrite Ferrite Ferrite 11 Silica gel rich in CSH gels CSH gels CSH gels CSH gels Fe/Al (50)% 0 1 2 3 4 10 10 10 10 10 (Atkinson et al., 1988, Berner, 1990; Adenot & Richet, 1997) Total volume of water per unit mass -1 ) of anhydrous cement (L kg Calcium Silicate Hydrate (C-S-H) phases play an important role throughout the evolution of cement

Laboratory for Waste Management Nuclear Energy and Safety Department Structure of C-S-H phases Garbev et al., 2008

Laboratory for Waste Management Nuclear Energy and Safety Department Recrystallisation of C-S-H phases from 45 Ca uptake Assumption: ⎡ ⎤ 45 Ca 45 ⎣ ⎦ Ca recryst.solid = sol [ ] Ca Ca recryst.solid sol

Laboratory for Waste Management Nuclear Energy and Safety Department Batch sorption experiments Sorption tests Experimental set-up Aerosil-300 CaO 2+ UO 2 N 2 atmosphere C:S ratio: 0.5 – 1.60 S:L ratio: 5.0 g/L (batch sorption tests) 1.0 g/L (TRLFS measurements) ageing H 2 O [U(VI)] added : 10 -3 M – 10 -7 M ACW Ageing time: 2 weeks Equilibration time: equilibration 2 weeks (batch sorption tests) 1 – 14 days (TRLFS measurements) sampling of supernatant TRLFS / measurements centrifugation Alpha counting / ICP-OES analysis

Laboratory for Waste Management Nuclear Energy and Safety Department Batch sorption experiments Sorption isotherms -1 10 -1 ] UO 2 (sorbed) [mol kg -2 10 -3 10 -4 10 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 -5 10 C:S = 0.74 ACW, pH=13.3 C:S = 1.07 ACW, pH=13.3 C:S = 1.25 ACW, pH=13.3 -6 10 -11 10 -10 10 -9 10 -8 10 -7 10 -6 10 -5 10 -4 10 UO 2 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)

Laboratory for Waste Management Nuclear Energy and Safety Department Batch sorption experiments Sorption isotherms 1 Cation equilibrium concentration(M) 10 -1 10 Ca; C:S=1.07 Ca; C:S=1.65 Ca; C:S = 1.07 Cation concentration (M) Si; C:S=1.07 Si; C:S=1.65 0 10 Si; C:S = 1.07 Ca; C:S=0.65 Ca; C:S = 0.75 Si; C:S=0.65 -2 -1 10 10 Si; C:S = 0.75 -2 10 -3 10 -3 10 -4 10 -4 10 -5 10 Alkali-free conditions in ACW -6 10 -5 -11 10 10 -10 -9 -8 -7 -6 -5 10 10 10 10 10 10 -9 -8 -7 -6 -5 -4 10 10 10 10 10 10 UO2+ 2+ 2 equilibrium concentration (M) UO 2 equilibrium concentration (M) Solution composition is independent of the U(VI) sorption

Laboratory for Waste Management Nuclear Energy and Safety Department Batch sorption experiments summary of the observations • C-S-H phases have a high recrystallisation rate providing opportunities 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?) Can these observations be described by a solid solution model?

Laboratory for Waste Management Nuclear Energy and Safety Department Batch sorption experiments Requirements to model solid – solutions : � Mixing model: ideal or non-ideal � Amount of recrystallized solid � From recrystallisation experiments with 45 Ca � 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 ,…)

Laboratory for Waste Management Nuclear Energy and Safety Department Time- -resolved laser fluorescence spectroscopy resolved laser fluorescence spectroscopy of uranyl of uranyl Time Fluorescence process Fluorescence process Vibrational relaxation Non-radiative relaxation e.g. via O-H stretch Excitation λ = 266 nm vibrations ligand σ μ (axial oxygen 2 p orbital)- ν =n to-metal δ u (5 f orbital) charge- � transfer � � Fluorescence emission ν =2 ν =1

Laboratory for Waste Management Nuclear Energy and Safety Department Time-resolved laser fluorescence spectroscopy Luminescence intensity (A.U.) � Uranyl compounds fluoresce above 470 nm with characteristic vibronic Aqueous uranyl 0.001 M Ca(OH) 2 progressions originating mainly from P3 P2 the symmetric stretch vibration of the O= U= O moiety (minor contributions P4 from assymetric stretch- and bending vibration) � The position ( ↓ ) , spacing ( Δ ), P1 P5 relative intensities ( P i /P i+1 ) of the P6 vibronic bands, lifetime, are sensitive to geometry of the uranyl and local Δ Δ Δ Δ Δ chemical environment 460 480 500 520 540 560 580 600 Wavelength (nm) � O=U=O axial bond length, R UO : R UO = 10650·[ Δ ] -2/3 +57.5 (Bartlett & Cooney, 1989)

Laboratory for Waste Management Nuclear Energy and Safety Department Time-resolved laser fluorescence spectroscopy Comparison of spectra from sorbed and aqueous uranyl species Red shift Increasing red shift (lower energy): U(VI)-CSH; C:S= 1.07 Indication of weakening of the axial high loading U= O bond, (lower stretch Luminescence intensity (A.U.) frequency ) U(VI)-CSH; C:S= 1.07 low loading Stronger interaction between U(VI) U(VI) in ACW and the equatorial ligands Free Uranyl in 1 M HClO4 480 500 520 540 560 580 600 620 Change in geometry of uranyl moiety Wavelength (nm) λ ex = 266 nm, T= 150 K

Laboratory for Waste Management Nuclear Energy and Safety Department Time-resolved laser fluorescence spectroscopy Sorption isotherm U(VI) sorbed on CSH at pH 12.0; C:S = 1.07 S:L = 1.0 g L -1 ; equilibration time = 1 day Fluorescence emission (A.U.) U(VI) loading -1 1.0 mol kg -1 0.1 mol kg -2 mol kg -1 5x10 -2 mol kg -1 10 -3 mol kg -1 2x10 -3 mol kg -1 10 -4 mol kg -1 3x10 -4 mol kg -1 10 450 480 510 540 570 600 630 Wavelength (nm) λ ex = 266 nm, T= 150 K

Laboratory for Waste Management Nuclear Energy and Safety Department Time-resolved laser fluorescence spectroscopy Comparison with spectra of reference compounds β β Luminescence intensity (A.U.) α α U(VI )-CSH; C:S= 0.75; β Luminescence intensity (A.U.) alkali-free; all loadings α U(VI )-CSH; C:S= 1.65 Room alkali-free; all loadings temperature U(VI )-CSH; C:S= 1.07 alkali-free; low loading T=150 K U(VI )-CSH; ACW C:S= 1.07; all loadings 440 480 520 560 600 Wavelength (nm) Uranophane ( α and β) Soddyite Uranophane α and β 480 500 520 540 560 580 600 620 Wavelength (nm) λ ex = 266 nm, T= 293 K or 150 K

Laboratory for Waste Management Nuclear Energy and Safety Department Time-resolved laser fluorescence spectroscopy Comparison with spectra of reference compounds Axial U – O distance (Å) XRD EXAFS TRLFS Soddyite 1.78 (Demartin et al. 1983) 1.77(2) 1.80(5) K-boltwoodite 1.80 (Burns et al. 1998) 1.80(2) - Uranophane α 1.80 (Ginderow, et al. 1988) 1.86(5) Uranophane β 1.84(5) 1.82 (Viswanathan et al . 1986) C-S-H (alkali-free) 1.83(2) 1.9(1) C-S-H (ACW) 1.81(2) 1.9(1) TRLFS: R UO = 10650 · [ Δ ] -2/ 3 + 57.5 (Bartlett & Cooney, 1989)

Laboratory for Waste Management Nuclear Energy and Safety Department 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)