Actinde(IV) colloids at near-neutral ph due to reaction with dissolved silicic acid

Transcription

1 Actinde(IV) colloids at near-neutral ph due to reaction with dissolved silicic acid H. Zänker, S. Weiss, C. Hennig, R. Husar Institute of Resource Ecology BELBaR Annual Meeting 2014 at Meiningen Text optional: Institutsname Prof. Dr. Hans Mustermann Mitglied der Leibniz-Gemeinschaft

2 Outline The BELBaR project as the background What are actinide(iv)-silica colloids? Conceivable geochemical implications Seite 2

3 Colloids hitherto in the focus of BELBaR Bentonite errosion results in the formation of relatively stable clay mineral colloids Radionuclides may adsorb onto these clay mineral colloids Colloid-facilitated transport of radionuclides Radionuclides that are often regarded as immobile such as tetravalent actinides may become mobile Seite 3

4 Seite 4 What are actinide(iv)-silica colloids?

5 Generation of actinide(iv)-silica colloids in the laboratory Starting point: solutions of actinide(iv) carbonate complexes Uranium: Electrochemical reduction of U(VI) carbonate complexes to U(IV) carbonate complexes Neptunium: Elecrochemical reduction of Np(V) in HNO 3 to Np(III). Electrochemical oxidation of Np(III) to Np(IV) in HNO 3. Formation of Np(IV) carbonate complexes by adding the acidic solution to 1.0 M NaHCO 3 Thorium: Dissolution of Th(IV) in HClO 4. Adding of solid NaHCO 3 Seite 5

6 Generation of actinide(iv)-silica colloids in the laboratory Destruction of the actinide(iv) carbonate complexes results in the formation of actinide(iv) nanoparticles Dilution with pure water: Precipitate of actinide(iv) oxyhydroxide Dilution with water containing silicic acid: Waterborne actinide(iv)-silica colloids of surprisingly high concentration at near-neutral ph Seite 6

7 Generation of U(IV) Power supply 2 Anode (Hg) 3 Cathode (Pt wire) 4 Diaphragm 5 Nitrogen 6 Solution of U According to Ikeda-Ohno A et al. Inorg. Chem. 48 (2009) 7201 Seite 7

8 Generation of U(IV)-silica colloids Galvanostatic reduction Dilution 1:20 With water 20 mm UO 2 (ClO 4 ) M NaHCO 3 (ph ~ 8.5) 20 mm U(IV) + 1 M NaHCO 3 (ph ~ 8.5) Precipitate With colloid-free silicic acid solution (0.24 to 2.7 mm Si) (plus adjustment of ph) 1 mm U(IV) colloids Seite 8

9 U(IV)-silica colloids: Tyndall effect Colloidal stability by light scattering: Detector 90 Laser beam Cuvette sealed by melting in inert gas Measuring the intensity of the light scattered by the nanoparticles Sedimented particles are not seen by the detector Seite 9

10 Scattered light intensity [kcps] U(IV)-silica colloids: Long-term Stability by light scattering uranium(iv)-silica colloids pure water Time [d] U = M, Si = M, ph = 6.9 High scattered light intensity indicates that U(IV)-silica colloids are stable over years! Seite 10

11 Th(IV)-silica colloids: Long-term Stability by light scattering [Th] = 1 mm, ph ~ 7 Seite 11

12 U concentration in supernatant [mg/l] 21 nm 10 nm < 7 nm Si concentration in Supernatant [mg/l] U(IV)-silica colloids: Particle size by ultracentrifugation Uranium Silicon Fraction of colloidborne Si raw 1 h rpm 1 h rpm 1 h rpm 1 h rpm 5h rpm 0 raw 1 h rpm 1 h rpm 1 h rpm 1 h rpm 5h rpm Centrifugal forces Centrifugal forces U = M, Si = M, ph = 7.8 Particle diameter: < 20 nm Seite 12

13 Mononuclear wall of silicic acid Below M silicic acid occurs in a mononuclear form ( mononuclear wall ) Above this wall fractions of polysilicic acid (Stumm, W., Huper, H., Champlin, L., Environ. Sci. Technol. 1 (1967) ) Affinity of metal ions to polysilicic acid is much higher than affinity to monosilicic acid Formation of actinide(iv)-silica colloids below the mononuclear wall of silicic acid differs from that above the mononuclear wall Seite 13

14 Correlation Coefficient Intensity [%] Np(IV)-silica colloids below the mononuclear wall of silicic acid: Particle size about 150 nm (PCS) 1,0 0,8 0,6 0, Stokes Diameter [nm] 0,2 0, Delay Time [µs] Np(IV) = M, Si(IV) = M Seite 14

15 Np(IV)-silica colloids above the mononuclear wall of silicic acid: Particle size about 10 nm (a) Light-intensity weighted particle size distribution (b) Number weighted particle size distribution Np(IV) = 10-3 M, Si(IV) = M Seite 15

16 Hydrodynamic diameter / nm Dependence of PCS particle size of Np(IV)-silica colloids on the silicic acid concentration 1000 c Np(IV) = 1 mm / 0,1 M NaHCO 3 2 days of equilibration c Si in initial solution / mm Seite 16

17 zeta potential [mv] U(IV)-silica colloids: Zeta potential by Laser Doppler velocimetry without Si ph IEP I= 0.05 M NaClO 4 ph Seite 17

18 zeta potential [mv] U(IV)-silica colloids: Zeta potential by Laser Doppler velocimetry without Si 1 mm silica ph IEP I= 0.05 M NaClO 4 ph Seite 18

19 zeta potential [mv] U(IV)-silica colloids: Zeta potential by Laser Doppler velocimetry without Si 1 mm silica 3 mm silica ph IEP I= 0.05 M NaClO 4 ph Seite 19

20 Zeta Potential [mv] Th(IV)-silica colloids: Zeta potential by Laser Doppler velocimetry pure Th colloid pure SiO 2 Th/silica colloid with 1.1 x 10-2 M silica ph Seite 20

21 FT U(IV)-silica colloids: Uranium binding by EXAFS spectroscopy (ROBL, ESRF Grenoble) A B O1 U A (no Si) With increasing Si content: distance U-O1 is shortend U-U scattering is reduced C O/Si R+ [Å] Fourier transforms (FTs) of EXAFS spectra B (Si/U 0.83) C (Si/U 1.68) O/Si scattering is increased U-O-U bonds are partly replaced by U-O-Si bonds Structure similar to coffinite (USiO 4 ), but much less ordered Seite 21

22 Properties of the Actinide(IV)-silica colloids Tetravalent actindes could be stabilized in colloid-borne form at concentrations of up to 10-3 M. The colloids remain in suspension over years The colloids contain silica which stabilizes them The colloids form from two dissolved components; preexisting colloids like bentonite colloids do not play a part Seite 22

23 Properties of the Actinide(IV)-silica colloids The prevailing particle size is <20 nm Silica increases the negative charge of the An(IV) particles in the near-neutral ph range (electrostatic repulsion). Shift of the isoelectric point toward lower ph values. Furthermore: Non-DLVO forces Internal structure of the particles: An-O-Si bonds increasingly replace the An-O-An bonds of the actinide oxyhydroxide structure Seite 23

24 Questions Is there dissolved silicic acid in the near field of a nuclear waste repository? Yes. Ingressing groundwater; corrosion of glass, cement, bentonite, grout silica Can the formation of An(IV)-silica colloids be ruled out? No. Several mechanisms are conceivable Would An(IV)-silica colloids be colloidally stable (resistant to coagulation) under near-field conditions? They might be stable. Under laboratory conditions they could be stable over years Seite 24

25 Questions Can the transport of An(IV)-silica colloids through the engineered barrier system (EBS) be ruled out? No. Macromolecules of 30 kda (lignosulfate) were able to diffuse through the pores of bentonite (Wold and Eriksen 2003) Can the transport of An(IV)-silica colloids in the far-field be ruled out? No. Colloidal radionuclide transport has been shown in column and in field experiments Would An(IV)-silica colloids be chemically stable (resistant to disintegration) when they leave the nearfield of a repository? This question is largely unanswered Seite 25

26 Seite 26 Further research is needed to elucidate the potential role of actinide(iv)-silica colloids in environmental scenarios

27 Seite 27 Thank you very much!

28 Publications Uranium(IV) Dreissig I, Weiss S, Hennig C, Berhard G, Zänker H: Formation of uranium(iv)-silica colloids at near-neutral ph. Geochim. Cosmochim. Acta 75 (2011) Thorium(IV) Hennig C, Weiss S, Banerjee D, Brendler E, Honkimäki V, Cuello G, Ikeda-Ohno A, Scheinost AC, Zänker H: Solid state properties and colloidal stability of thorium(iv)-silica nanoparticles. Geochim. Cosmochim. Acta 103 (2013) Neptunium(IV) Husar R, Weiss S, Zänker H, Bernhard G: Investigations into the formation of neptunium(iv)-silica colloids. Migration Brighton, Book of Abstracts, p. 384 Seite 28

29 FT U(IV)-silica colloids: EXAFS (k)k A B C D O U U O+Si U Si U Probe A: Referenz ohne Silikat UO n (OH) 4-2n mh 2 O Probe B: Kolloide (ausgefällt) Molares Si/U Verhältnis im Feststoff = 0,83 Probe C: Kolloide (ausgefällt) Molares Si/U Verhältnis im Feststoff = 1:1,68 Probe D: Coffinit (synthetisch) k [Å -1 ] R+ [Å] Seite 29

30 Hitherto laboratory experiments concerning An(IV) colloids If colloid removal is omitted, total actinide concentrations, [An(IV)] tot, higher by a factor of about 100 are found. An(IV) oxyhydroxide colloids Altmaier, M. et al.; Radiochim. Acta (2004) 92, 537 Seite 30

31 Laboratory experiments as to An(IV) colloids Th(IV)- silica colloids: 10-3 M Th(IV)-oxyhydroxide colloids: 10-6 M Thermodyn. Solubility: 10-8 M Altmaier, M. et al.; Radiochim. Acta (2004) 92, 537 Seite 31

32 Scattered light Intensity / kcps Hydrodynamic diameter / nm Dependence of SLI and particle size of Np(IV)- silica particles on the silicic acid concentration 1000 c Np(IV) = 1 mm / 0,1 M NaHCO 3 2 days of equilibration c Si in initial solution / mm 1 Seite 32

33 FT (k)k 3 FT Np EXAFS NpCarb NpCarb 2,6 2.6 O O ,2 2.2 O dist O dist χ(k)k NpSi NpSi 12 NpOalk NpOalk 8 1,8 1, O Si O Np Si Np 1, NpOac NpOac 0,6 O 0.6 O Np Np 0 0 0, k [Å -1 ] k[å -1 ] R+ [Å] R+Δ[Å] Seite 33

34 Ideas for a work plan Formation of Pu(IV)-silica colloids is to be tested Mechanisms of An(IV)-silica nanoparticle formation are to be studied (ESI-MS, EXAFS, HEXS, 29 Si NMR) Quantification of DLVO and non-dlvo contributions to the stabilization (aggregation kinetics, influence of ionic strength and ph on stability) Passage (diffusion) of An(IV)-silica colloids through compacted bentonite (cf. C. Joseph et al.) Chemical persistency of An(IV)-silica colloids after leaving the near-field Passage (convection) of An(IV)-silica colloids through columns packed with crushed rock Seite 34

35 Ideas for a work plan Deposition of actinide(iv)-silica colloids on host rock mineral surfaces by RAXR, CTR and AFM (cf. M. Schmidt et al.) If useful and helpful, field experiment at the Grimsel Test Site (GTS) Seite 35