Speciation of Radionuclides in the Environment

Transcription

1 Speciation of Radionuclides in the Environment Tobias Reich Institut für Kernchemie GRK Trace Analysis of Elemental Species Ringvorlesung WS 05/

2 Outline Sources of radioactivity Mobility of radionuclides (RN) in the geosphere Reactions of RN with components of natural waters Interactions of RN with solid components of the geosphere Examples

3 Sources of Radioactivity Natural Anthropogenic Primordial RN 40 K, 232 Th, 235 U, 238 U Produced continuously T, 14 C Nuclear power stations Nuclear weapon tests Nuclear accidents Fission products, transuranium elements

4 Double-Wall Tanks for Liquid Waste of the Plutonium Production in Hanford, Washington, USA K.D. Crowley, Physics Today, 50 (1997) 32-39

5 Sources of Radioactivity Radiation dose and possible hazards depend on local concentrations and radiotoxicity of the RN. High local concentrations Natural sources U and Th ores Anthropogenic sources Nuclear reactors Reprocessing plants High-level nuclear waste Low local concentrations Dispersed natural RN (T, 14 C, 40 K, U, Th, daughter nuclides) Off-gas and effluents from nuclear installations Nuclear fall-out (except Chernobyl)

6 Sources of Radioactivity Radiation dose and possible hazards depend on local concentrations and radiotoxicity of the RN. Low radiotoxicity High radiotoxicity T, 14 C, 40 K Many fission products Actinides

7 RN Mobility in the Geosphere 40 K K + easily soluble 232 Th Th(IV) sparingly soluble in natural waters Decay products 228 Ra, 224 Ra, 220 Rn mobile 238 U U(IV) sparingly soluble in natural waters U(VI) easily soluble in natural waters Decay products 226 Ra, 222 Rn mobile Ra 2+, UO 2+ 2 are leached from ores or minerals by groundwater

8 RN Mobility in the Geosphere Mining of uranium ores Ra Rn 1 GBq of 222 Rn released per ton of ore containing 1% U 3 O 8 Oil production Global activity of ~10 13 Bq Ra isotopes per year Burning of coal in thermal power stations Global release ~10 14 Bq Rn per year Waste gas, ash contains U, Th, 210 Pb, 210 Po

9 RN Mobility in the Geosphere Nuclear explosions and weapon tests ( ) 4.2 tons Pu 2.8 tons Pu dispersed in the atmosphere 1.4 tons Pu deposited locally 1.5 tons Pu in underground explosions Nuclear reactors and reprocessing plants Bq tritium per GW e per year ~10 12 Bq 14 C per GW e per year 90 Sr, 99 Tc, 129 I, 137 Cs, and actinides

10 RN Mobility in the Geosphere Behavior of RN in the environment depends primarily on their chemical and physicochemical form (species). 137 Cs +, 90 Sr 2+ easily dissolved in water, independently of ph 129 I 2 quiet mobile species, reacts easily with organic substances 85 Kr, 133 Xe stay predominantly in air Lanthanides (Ln) ( 144 Ce, 147 Pm, 151 Sm) only sparingly soluble in water (hydrolysis of the cations); however, colloids may be formed

11 RN Mobility in the Geosphere An(III), An(IV) similar solubility as Ln AnO 2+, AnO 2 2+ relatively high solubility in water in the presence of CO 3 2-, HCO 3 - Zr(IV), Tc(IV) similar mobility as An(IV) Pronounced influence of the redox potential in case of Tc Tc(IV) not dissolved in water and immobile Tc(VII) easily dissolved in the form of TcO 4- and very mobile Oxidation of PuO 2 in moist air PuO 2+x => unexpected solubility of PuO 2, influence on the migration behavior of Pu

12 Reactions of RN with Components of Natural Waters Reactions to be taken into account Hydration (aquo complexes) Hydrolysis (hydroxo complexes) Condensation (polynuclear hydroxo complexes) Complexation (complexes with inorganic or organic ligands) Formation of radiocolloids (intrinsic or carrier colloids)

13 Reactions of RN with Components of Natural Waters Groundwater, rivers, lakes, and oceans contain a great variety of substances that may interact with RN: Water Inorganic compounds Organic compounds

14 Reactions of RN with Components Inorganic compounds of Natural Waters Dissolved gases (O 2, CO 2 ) Salts (NaCl, NaHCO 3 ) Inorganic colloids (polysilicic acid, iron hydroxide, hydrous iron oxide, clay minerals) Inorganic suspended matter (coarse particles)

15 Reactions of RN with Components Organic compounds of Natural Waters Low molecular mass (organic acids, amino acids, other metabolites) High molecular mass (humic and fulvic acids, colloids, degradation products of organic matter) Suspended coarse particles Microorganisms

16 Reactions of RN with Components of Natural Waters Thermodynamic equilibrium conditions not applicable to colloids and microorganisms Calculations very difficult Laboratory experiments with model waters difficult to relate to natural waters

17 Reactions of RN with Components of Natural Waters Great influence of the redox potential Eh on oxidation state of I, Tc, U, Np, Pu: Aerobic, oxidizing conditions (O 2 ) Anaerobic, reducing conditions (H 2 S) I 2 volatile, reacts with organic compounds, in contrast to I -, IO 3 - U(IV)/U(VI) UO 2 Np(V), NpO 2+ great stability range, differs markedly from U, Pu Tc(IV)/Tc(VII) TcO - 4

18 Reactions of RN with Components of Natural Waters ph of 6-8 in natural waters Hydrolysis tendency MO 2+ < M 3+ < MO 2 2+ < M 4+

19 Reactions of RN with Components Inorganic salts of Natural Waters High ionic strength Colloid formation is hindered Colloids already present are coagulated ph buffer Seawater, ph 8.2, NaHCO 3 Influence on hydrolysis, complexation, solubility, colloid formation, sorption

20 Reactions of RN with Components Organic compounds of Natural Waters Dissolved organic carbon (DOC) in waters 0.1 mg/l groundwater mg/l oceans 50 mg/l swamp water Relatively high stability constants for complexes with An

21 Reactions of RN with Components Microorganisms of Natural Waters Uptake, incorporation Food chain

22 Reactions of RN with Components of Natural Waters Precipitation and coprecipitation At low concentrations coprecipitation by isomorphous substitution most important CaCO 3 Fe(OH) 3 Formation of solid solutions BaSO 4, SrSO 4 / An(IV), An(III)

23 RN Interactions with Solid Components of the Geosphere Main components of the geosphere Consolidated rocks Granite, volcanic tuff Unconsolidated rocks Sand, clays Soils

24 RN Interactions with Solid Components of the Geosphere Other important minerals Clay minerals Kaolinite Montmorillonite Vermiculite Illite Chlorite

25 1 st Example Discovery of a new uranium species in mine waters

26 Uranium Mining by the Wismut AG Tons of Metallic Uranium produced Rocks: U < 1 mg/kg Seepage water: 120 µg/l Photos: Wismut AG

27 Calculated Uranium Speciation in Seepage Water 1,1 x 10-5 M U mmol/l Component Seepage water Schlema Ca 11,45 Mg 16,46 Na 0,572 K 0,465 U 0, SO 4 25,5 - CO 2 /HCO 3 2- /CO 3 1, PO 4 3- AsO 4-0,012 Cl - 0,121 TOC (mg l -1 ) - ph 8,14 [CO 2 3 ] TOT = 0.45 mm [UO 2+ 2 ] TOT = µm Fraction UO 2 2+ (UO 2 ) 2 (OH) 3 CO 3 UO 2 CO UO 3 2 (CO 3 ) 2 UO 2 OH + 2 I= M [Ca 2+ ] TOT = 0.00 UO 2 (CO 3 ) 3 4 UO 2 (OH) 3 (UO 2 ) 2 (OH) 2+ 2 (UO 2 ) 3 (OH) ph UO 2 (OH) 4 2 t= 25 C G. Bernhard, et al., J. Alloys Compounds (1998) 201, Radiochim. Acta 74 (1996) 87

28 Time-Resolved Laser-Induced Fluorescence Spectroscopy Institute of Radiochemistry, Forschungszentrum Rossendorf Energy (Wavenumber cm -1 x10 3 ) (470,1 nm) (585,4 nm) 0

29 Limits of Detection and Speciation by TRLFS Element Detection (M) Speciation a (M) Uranium(VI) Curium(III) Americium(III) Lanthanide(III) a: Depending on the system C. Moulin, Radiochim. Acta 91 (2003) 651

30 Laser Fluorescence Spectroscopy (TRLFS) Fluorescence intensity 6x10 1 5x10 1 4x10 1 3x10 1 2x10 1 1x [UO 2 2+ ] = 10-5 mol/l ph = 1.0, I = 0.1 M Fluorescenc intensity τ = 1.7 ± 0.5 µs Emission wavelength (nm) 0 1x10 3 2x10 3 3x10 3 4x10 3 5x10 3 6x10 3 7x10 3 Time (ns) Characteristic Values Fluorescence bands Speciation Fluorescence lifetime

31 Fluorescence Spectra of Uranium in Carbonate Medium 10-5 M U(VI), ph = 8,0, I = 0,1 M ClO 4 - Fluorescence intensity a.u. ohne CO 3 2-3,0 x 10-4 M CO 3 2-2,5 x 10-3 M CO Emission wavelength (nm) G. Bernhard, et al., Radiochim. Acta 74 (1996) 87

32 Time-Resolved Fluorescence Spectrum of Seepage Water (Rockpile No.66, Schlema) Wavelength / nm Time / ns Intensity / A.U. Lifetime: 64 ± 17 ns Emission maxima (nm): 463,9 483,6 502,8 524,3 555,4 G. Bernhard, et al., Radiochim. Acta 74 (1996) 87

33 Intensität a.u. 3.5x x x x x x x Seepage water Wellenlänge (nm) Ca 2+ UO 2 2+ CO 3 2- Intesität a.u Synthetic solution for determination of complexation constants 100 2x10-5 M UO 2+ 2, 83 2x10-2 M HCO - 3 /CO 2-3, ph 8, Wellenlänge (nm) [Ca 2+ ] 9,3 mm 4,7 mm 2,8 mm 0,9 mm Emission wavelength / nm 466,5; 484,5; 504,4; 527,2; 550,8; 572,0 Lifetime 28 ± 5 ns Emission wavelength / nm 466,0; 484,6; 504,1; 525,4; 549,1; 573,4 Lifetime 43 ± 12 ns

34 Complex Stability Constant 2 Ca 2+ + UO CO 3 2- Ca 2 UO 2 (CO 3 ) 3 (aq.) log β = 30,90 ± 0,20 G. Bernhard, G. Geipel, T. Reich, V. Brendler, S. Amayri, H. Nitsche, Radiochim. Acta 89 (2001) 511

35 Speciation in the System Ca 2+ UO 2 2+ CO 3 2- [CO 2 3 ] TOT = 0.45 mm [UO 2+ 2 ] TOT = µm 1.0 UO 2 2+ I= M [Ca 2+ ] TOT = Ca 2 UO 2 (CO 3 ) 3 (aq) 8.00 mm UO 2 (OH) 4 2 Fraction (UO 2 ) 2 (OH) 3 CO 3 UO 2 (OH) 3 UO 2 OH + UO 2 CO UO 2 (CO (UO 2 ) 2 (OH) 2+ 3 ) (UO 2 ) 3 (OH) ph t= 25 C

36 Recipients of the Kurt-Schwabe Award 2005 G. Bernhard, G. Geipel S. Amayri

37 2 nd Example Sorption of neptunium on kaolinite

38 Kaolinite Structure gibbsite surface edge surface 7.14 Å [AlO 6 ] siloxane surface [SiO 4 ] Kaolinite (Al 2 Si 2 O 5 (OH) 4 ) KGa-1b Cation exchange capacity: 2.0 meq/100 g Specific surface (BET-N 2 ) :10.0 m 2 /g

39 Mechanism of the Interaction of Metal Ions at a Mineral Surface Metal Ion Outer-sphere Sorption Multi-nuclear Species Colloids Inner-sphere Sorption Incorporation Surface Precipitation Manceau et al., Rev. Mineral. Geochem., 49, 344 (2002)

40 Speciation Calculation for 8 µm Np(V), I = 0.1 M pco 2 = atm CO 2 -free 100 NpO 2 + NpO 2 CO 3 - NpO 2 (CO 3 ) NpO 2 + NpO 2 (OH) 2 - Np(V) species (%) NpO 2 (CO 3 ) 2 3- NpO 2 OH NpO 2 (CO 3 ) 2 (OH) 4 - Np(V) species (%) NpO 2 OH ph ph R. J. Lemire, J. Fuger, H. Nitsche, e al., Chemical Thermodynamics of Neptunium and Plutonium, Elsevier Science, Amsterdam (2001) L. Rao, T.G. Srinivasan, A.Yu. Garnov, et al. Geochim. Cosmochim. Acta, Vol. 68, No. 23, (2004)

41 Result of Batch Experiments 8 µm Np(V) 100 pco 2 : atm Np(V)-Sorption (%) CO 2 -free 1-5 EXAFS samples ph

42 Experimental EXAFS Measurements Samples (200 mg) prepared as wet paste Np L II -edge EXAFS spectra measured at Rossendorf Beamline ROBL, BM20, ESRF in fluorescence mode (5-12 scans) at room temperature EXAFSPAK and FEFF 8.20 used for analysis ESRF BM20 W. Matz, N. Schell, G. Bernhard, et al., J. Synchrotron Rad. 6, 1076 (1999) G.N. George, I.J. Pickering, SSRL (1995) A.L. Ankudinov, C.E. Bouldin, J.J. Rehr, et al., Phys. Rev. B65, 104 (2002)

43 Neptunium L II -edge EXAFS and FT 8 µm Np(V), ph 9.0, Influence of CO 2 12 NpO 2 + (aq.) 3 FT Data Fit 8 2 χ(k) k pco 2 = atm CO 2 -free 1 0 O ax O eq C O dist Sample 2` Np(V)-Sorption (%) pco 2 : atm CO 2 -free ph k (Å -1 ) R+ (Å)

44 Sample NpO 2 (CO 3 ) 5-3 (aq.) [1] NpO 2+ (aq.) [2] ph Np(V) Sorption on Kaolinite Interatomic Distances in Å CO 2 yes yes yes yes no no yes O ax O eq C O dis Np 4.86 Np(V)-Sorption (%) 100 pco 2 : atm CO 2 -free EXAFS samples ph [1] D.L. Clark, S.D. Conradson, S.A. Ekberg, et al., J. Am. Soc. 118, 2089 (1996) [2] T. Reich, G. Bernhard, et al., Radiochim. Acta 88, 633 (2000) C C Np O ax O dis O eq C

45 3 rd Example Ultratrace and isotope selective detection of plutonium in dust samples

46 Principal of Resonance Ionization Mass Spectrometry Ionization of atoms by resonant absorption of laser light Element selectivity Electric field Energy Real state Ground state Isotope shift of the excited levels Isotope sensitivity N. Trautmann, G. Passler, K.D.A. Wendt, Anal. Bioanal. Chem. 378 (2004) 348

47 Principal of Resonance Ionization Mass Spectrometry N d :Y A G T i:s a 3 T i:s a 2 T i:s a 1 B B O F lu g ze itm a ss e n s p e k tro m e te r D e te kto r 2 D e te kto r 1 R e fle ktio n sg itte r F a s e r F ila m e n t Nd:YAG pump laser Frequency doubled λ = 532 nm P = 50 W ν Rep = 1 25 khz Titanium-sapphire laser λ = nm P = 2 W Width of laser line 3 5 GHz ν Rep = 6,6 khz B e sch le u n ig u n g s- g itte r TOF mass spectrometer with reflectron m/ m = 600 at A = 240 amu

48 Sample Preparation Addition of a Pu tracer isotope Dissolution of the sample Iron hydroxide precipitation Separation of Pu with anion exchange column (TEVA Resin SPS) Electrolytic deposition of Pu(OH) 4 on tantalum Filament Titan Pu(OH) 4 Tantal Sputtering of a thin (~1 µm) titanium layer Chemical yield: %

49 Efficiency measurement with 240 Pu 10-4 Effizienz 10-5 Nachweisgrenze [Atome] Probe Probe Efficiency: ε Detection limit: ~ atoms

50 Dust Samples with Fallout and Reactor Plutonium 500 a) Elbmarsch Tracer b) WA Karlsruhe Ereignisse Ereignisse m [amu] Tracer m [amu] a) Isotope ratio: measured: 240 Pu : 239 Pu = 0,16(2) Fallout plutonium: 0,18 b) Isotopic composition: 238 Pu [%] 239 Pu [%] 240 Pu [%] 241 Pu [%] 242 Pu [%] 1,1(1) 61,6(50) 27,9(12) 4,8(4) 4,7(4)