Showing papers in "Radiochimica Acta in 1992"

The Role of Natural Organics in Radionuclide Migration in Natural Aquifer Systems

Abstract: The wide variety of organic compounds present in natural waters is assessed for the role they may play in radionuclide and, especially, actinide migration. Some natural analog and environmental data are briefly reviewed for evaluation of the effect of organics in these systems. The binding constants and the kinetics of complexation of actinides by humics are discussed in terms of probable effects on actinide migration. The role of organics in redox, and in sorption, is also considered.

Excitation Functions of (p,xn)-Reactions on natNi and Highly Enriched 62Ni: Possibility of Production of Medically Important Radioisotope 62Cu at a Small Cyclotron

Abstract: Excitation functions were measured by the stacked-foil technique for Ni(p,xn)Cu and Ni(p,xn)Cu reactions from threshold to 18.5 MeV. Thin samples were prepared by electrolytic deposition of highly enriched Ni on Au-backing. Thick target yields of 61 'Cu were calculated from the measured excitation functions of Ni(p,xn)-processes. The Ni(p,n)Cu reaction is a very high yield process, and is a potential candidate for production of the medically important ß + emitting radioisotope Cu at a small cyclotron. Over the optimum energy range of E p = 14->10 MeV the theoretically expected Cu-yield amounts to 350 mCi (12.95 GBq)^Ah and the level of 6 0 · 1 · Οι impurities to <0.8%.

Actinide Binding and Solubilization by Microbial Siderophores

Abstract: Accurate predictions of actinide and fission product migration in the geosphere are critically dependent on identification of the biological, chemical and physical processes which affect actinide mobility in soil and water. Siderophores are low molecular weight iron chelators produced by microbes in response to low availability of soluble iron. Because of the similarities between iron(III) and tetravalent actinides, and the prevalence of siderophore-producing microbes in soil, there is strong likelihood that siderophores may also bind actinides, thereby influencing their mobility in the environment. In order to begin to assess the potential importance of siderophore-mediated actinide mobility, we have determined rate constants for solubilization of hydrous plutonium oxide by the siderophores enterobactin and desferrioxamine Β and selected carboxylate, amino polycarboxylate, and catecholate ligands. The measured rate constants for solubilization of insoluble actinide oxides show that siderophores are extremely effective in solubilizing actinides; on a per molecule basis, enterobactin is ~ 1 0 3 times more effective than the other chelators tested in increasing the rate of solubilization of hydrous plutonium oxide. Notably, ferric-siderophore complexes are more effective in solubilizing actinide oxides than the siderophores in the absence of iron. These results suggest that siderophores have the potential to mobilize actinides in the environment.

Solubility and Hydrolysis Behaviour of Neptunium(V)

Abstract: The hydrolysis behaviour of the N p O i ion has been investigated at 25 by solubility experiments in the pH ränge 7 to 14 in 0.1 M. 1.0 M and 3.0 M NaC104 solutions under pure Ar atmosphere. By measuring the time dependent equiiibrium concentration of Np(V), the chemical State of Np(V) hydroxide precipita tes in each Solution isconfirmed. In 0.1 M NaClO« the precipitate remains amorphous over several months, while in 1.0 M N a C l O i the precipitate changes f rom an amorphous to a more stable aged State within a relatively short time. In 3.0 M N a C i O t the aged modification of NPO2OH is formed f rom the beginning. Applying specific ion interaction theory (SIT), the thermodynamic constants at 1 = 0 are found to be: log/ r ,p= 8 . 7 6 ± 0 . 0 5 for amorphous N p 0 2 0 H ( s ) . l o g A : , p = 9 . 4 4 ± 0 . 1 0 for aged N p 0 2 0 H ( s ) , l o g / ? i = 2 . 7 ± 0 . 2 for the hydrolysis species N p O ^ O H and iog^2 = 4 . 3 5 ± 0 . 1 5 for Np02(0H)2\" .

Sorption of Uranyl Ions on Hydrous Silicon Dioxide

Abstract: Sorption of uranyl ions on SiO[sub 2].[chi]H[sub 2]O (silica gel) is investigated in absence and in presence of carbonate as function of pH. The curves obtained are very similar to those observed for sorption of uranyl ion on TiO[sub 2].[chi]H[sub 2]O, indicating the dominating influence of the uranium species in solution. Between pH 2 and 5 the sorption ratio R[sub s] increases with hydrolysis of uranyl ions (formation of UO[sub 2]OH[sup +]), around pH 7 it is nearly independent of pH, and at higher pH it decreases again. The equilibrium constants are calculated for these ranges. In presence of carbonate R[sub s] decreases drastically above pH 6, due to the formation of carbonato complexes in solution. Sorption of uranyl ions on SiO[sub 2].[chi]H[sub 2]O, on TiO[sub 2].[chi]H[sub 2]O, and on cryst. SiO[sub 2] and Al[sub 2]O[sub 3] is compared. The problems of 'surface complexation' modelling are discussed. (orig.).

Chemical Interactions of Actinide Ions with Groundwater Colloids in Gorleben Aquifer Systems

Abstract: Chemical interactions of actinide ions with natural aquatic colloids in Gorleben aquifer systems have been investigated, particularly with selected groundwaters rich in humic substances. Actinides studied are trivalent Am and Cm, tetravalent Th and pentavalent Np. The naturally occuring uranium is carefully analysed, as for its distribution between colloids and aqueous phase, to ascertain the behaviour of the hexavalent uranyl ions in the systems under investigation. The actinide ions of M(III), M(IV) and M(VI) are found to be strongly sorbed on the aquatic colloids rich in humic substances (termed humic colloids) and thus generate actinide pseudocolloids. The sorption and desorption of the actinide ions onto and from the aquatic colloids are reversible with respect to the pH variation. The neptunyl ion shows negligible sorption. The interaction process of Cm(III) is speciated at different pH by time resolved laser fluorescence spectroscopy (TRLFS). The húmate or fulvate complexation is found as a predominant reaction process for the generation of actinide pseudocolloids in the aquifer systems under investigation.

Actinide Speciation in the Presence of Humic Substances in Natural Water Conditions

Abstract: A review of literature data concerning complex properties of humic substances with actinides (Th, U, Np, Pu, Am) and with cations largely present in natural waters is presented. From data which have been selected according to criteria discussed in the present paper, speciation diagrams of actinides have been calculated in the most representative conditions for natural systems (pH range 4—9; [humic substances] 0.1 to lOppm). Humic substances dominate actinide (Th, U, Am) speciation up to pH 7 (or even 8). Above these pH, inorganic complexes regulate actinide speciation. The presence of competing cations (Ca or Al) modifies actinide speciation in the pH range 4—6.

Complexation Behaviour of Humic Substances towards Actinides and Lanthanides Studied by Time-Resolved Laser-Induced Spectrofluorometry

Abstract: Natural waters are the main transport medium for radionuclides in the geosphere in case of an accidental release of radioactivity from a nuclear waste disposal. Complexation studies of radionuclides with natural organic substances present in aquifers are important to perform in order to predict their behaviour in natural systems. Interactions of trivalent elements such as curium for actinides and dysprosium for lanthanides with humic acids have been investigated by Time-Resolved Laser-Induced Spectrofluorometry (TRLIS) allowing to work at low concentrations of cations (IO 6 —IO 8 M). By using a \"fluorescence titration\" method, complexing capacities and interaction constants are obtained over a broad pH range (4 to 7) at a constant ionic strength (0.1 M). The influence of a competing cation such as calcium in the system in also studied. The complexing behaviour of Cm and Dy is analysed. Data are discussed and compared with existing literature data.

A Study of Solid-Liquid Phase Equilibria of Trivalent Lanthanide and Actinide Ions in Carbonate Systems

Abstract: The solid-liquid phase equilibria of trivalent Nd, Eu and Am are investigated in carbonate systems of 0.1 M NaC104 under different C 0 2 partial pressures of 0.03%, 1.0% and 100%. The solid phase produced by precipitation in the narrow pH range from 5 to 7 under each given C 0 2 partial pressure is characterized for its chemical state by thermogravimetry in combination with differential thermal analysis, X-ray diffraction analysis and FTIR spectroscopy. Under 0.03% C 0 2 partial pressure, the solid phase found is M 0 H C 0 3 , while M2(C03)3 is produced under 1.0% and 100% C 0 2 partial pressure. The metal ion concentration at equilibrium with a given solid phase under controlled C 0 2 partial pressure is analysed by spectroscopic speciation methods supported by ICP-AES for Nd and Eu and liquid scintillation counting for Am. The solubility products of hydroxocarbonates determined under 0.03% C 0 2 partial pressure are: log A,,, = — 19.94± 0.16 f o r N d 0 H C 0 3 , -20 .18±0 .09 for EuOHCOj and -18 .70±0 .12 for A m 0 H C 0 3 . The solubility products of normal carbonates obtained under both C 0 2 partial pressures of 1.0% and 100% are: logtfsf) = -31 .35±0 .12 for Nd 2 (C0 3 ) 3 , —31.78±0.22 for Eu2(C03)3 and -29 .45 ±0.18 for Am2(C03)3 .

A Study of Hydrolysis Reaction of Curium(III) by Time Resolved Laser Fluorescence Spectroscopy

Abstract: T h e hydrolys is reac t ion of C m 3 + has been invest igated by t ime resolved laser f luorescence spec t roscopy ( T R L F S ) in 0.1 M N a C 1 0 4 a t 25 C u n d e r a r g o n a t m o s p h e r e . T h e exper iment is carr ied ou t in the p H range f r o m 6 to 10 fo r Cm ( III ) concen t ra t ions f r o m 1.21 χ 1 0 \" 7 mol/1 d o w n to 3 χ I O \" 9 mol/1, which are subs tan t ia l ly lower than solubi l i ty l imits of the cu r ium hydrox ide at each given p H . T h e p r i m a r y a n d seconda ry hydrolys is cons t an t s d e t e r m i n e d a re : log/?i ι = 6.67 ± 0 . 1 8 fo r C m O H 2 + a n d l o g 1 2 = 12.06 ± 0 . 2 8 fo r C m ( O H ) J . These values are f o u n d to be in g o o d ag reemen t with the corres p o n d i n g hydro lys i s c o n s t a n t s o f A m 3 + d e t e r m i n e d previous ly in o u r l a b o r a t o r y by the solubi l i ty expe r imen t . However , the present va lues a re o n e or m a n y o r d e r s of m a g n i t u d e smal ler t h a n the da t a k n o w n fo r Cm (III) in the l i tera ture .

Solubility Products and Complex Formation Equilibria in the Systems Uranyl Hydroxide and Uranyl Carbonate at 25°C and I = 0.1 M

Abstract: The solubility of uranyl hydroxide and uranyl carbonate and the formation constants of the complexes present in the supematant Solution were determined at 25°C in 0.1 M NaClOi solutions. The solubility products were found to be logA^,p= —22.21 and log/Csp= — 13.29. respectively. In the pH ränge from 4.5 to 5.5 the hydroxo complexes (U02)2(0H)r (lg/? = 22.16), and (U02)3(0H)5^ (lg^ = 53.05) were found. Under the condition 11 < l g ( C 0 3 \" ) < 6 three mononuclear uranyl carbonate complexes. UO2CO3 (lg/J = 8.70), U02(C03) r (lg^= 16.33), and U02(C03)r (Ig/? = 23.92) exist.

A Comparative Spectroscopic Study of the Fulvate Complexation of Trivalent Transuranium Ions

Abstract: A spectroscopic study has been carried out at pH 6 in 0.1 M NaClO« for the complexation of trivalent transuranium ions with a fulvic acid from the Gorleben aquifer system. For comparison of methods, two different spectroscopic procedures are applied, i.e. conventional UV/Vis absorption spectroscopy for Am(III) and time-resolved laser fluorescence spectroscopy (TRLFS) for Cm(III). Both absorption and emission spectroscopy result in comparable constants of the fulvate complexation; log)3 = 5.78±0.07 for Am(III) and log;S=5.90±0.11 for Cm(III). These values, being independent of pH and ionic strength, are evaluated by taking into account the loading capacity of the fulvic acid, which amounts 64.9% of the total proton exchange capacity at the given pH and ionic strength. A large concentration variety of the metal ions, i.e. from 4.6x 10\" 'mol r ' to 5.6x10\"® m o i r \\ does not show any concentration effect on the complexation reaction and hence leads to nearly the same complexation constants.

Weathering of Natural Uranyl Oxide Hydrates: Schoepite Polytypes and Dehydration Effects

Abstract: Partial dehydration of schoepite, U 0 3 · 2H 20, is reported to produce three discrete schoepite polytypes with characteristic unit cell parameters, but this has not been confirmed. The loss of structural water from the schoepite interlayer results in progressive modification to the structure; expansion parallel to schoepite cleavage planes, and extensive fracturing. Dehydration of schoepite commences at grain boundaries and progresses inward until the entire grain is converted to dehydrated schoepite, U 0 3 · 0.8H20. The volume decrease associated with dehydration results in expanded grain boundaries. These gaps can provide pathways for the access of groundwater, and uranyl silicates and uranyl carbonates have precipitated within these gaps, replacing both schoepite and dehydrated schoepite. Schoepite, however, is not observed to re-precipitate where in contact with dehydrated schoepite. Thus, while the formation of schoepite early during the corrosion of uraninite may be favored, schoepite is not a long-term solubility limiting phase for oxidized uranium in natural ground waters containing dissolved silica or carbonate.

The Solubility of TcO2 · nH2O in Neutral to Alkaline Solutions under Constant pco2

Abstract: We have investigated the solubility of Tc0 2 • n H 2 0 in aqueous solution as a function of pH and pco2· The experimental data collected shows that the solubility of Tc02 · n H 2 0 increases with increasing pH and hydrocarbonate concentration. By studying the dependence of the solubility on the pH and Pco2 w e have determinated the stoichiometry and stability of the predominant aqueous complexes: TcO(OH)2 (aq), TcO(OH)J, Tc(0H)2C03 and Tc(OH)3CO;\\ The behaviour of Tc(IV) in natural water systems is discussed in the light of these new data.

Gas Phase Chromatography Experiments with Bromides of Tantalum and Element 105

Abstract: The retention behavior of volatile bromide molecules of shortlived isotopes of tantalum and element 105 in KCl coated quartz columns was studied using continuous isothermal gas chromatography. HBr and HBr saturated with BBrs vapor were used as reactive gases. The isotopes were produced in the fusion reactions ^°Ne+\"\" Eu and ^^O + ^^^Bk, respectively. The reaction producta were transported from the accelerator to the chromatography set-up with a gas-jet system using an aerosol with He as carrier gas and KCl as transporting particles. The measured retention times are compared to those of \" ' N b bromides from a previous study. The retention times of niobium and tantalum bromides are very similar, in agreement with expectations based on the nearly identical Sublimation enthalpies of NbBrj and TaBr;. For the bromides of element 105, higher retention times are found, indicating a lower volatility of lOSBrs than of NbBrs and TaBrs. On the basis of empirical systematics, a Sublimation enthalpy of 138± 15 kJ/mol can be estimated for 105Br5 from the experimental data.

Chemical Properties of Element 105 in Aqueous Solution: Cation Exchange Separations with α-Hydroxyisobutyric Acid

Abstract: Studies of the complexation of element 105 by unbuffered ahydroxyisobutyric acid (a-HiB) and its elution from strongly acidic cation exchange resin were performed on a mixture of 34-s ^ \" H a and 27-s \" ' H a produced in the and reactions at a beam energy of 99 MeV. The Ha isotopes were detected by measuring the spontaneous fission and a-activities associated with their decay, and the a-decays of their daughters, 4-s and 6-s ^\"Lr . Time-correlated pairs of parent and daughter a-particles were also registered. The experiments were performed on a one-minute time scale with the computer-controlled hquid chromatography system ARCA II. In 0.05 M a-HiB, element 105 was found to be eluted within 4 s from 1.6x8 mm columns together with the pentavalent Nb-, Ta-, and Pa-ions, while tetravalent Zrand trivalent Eu-ions were strongly retained on the columns. This is independent proof that the most stable oxidation State of element 105 in aqueous Solution is + 5. This very efficient Separation of 262,263^^2 was also used, for the first time, to demonstrate that their aand spontaneous fission decays can be registered from a « 2 3 um thin aqueous layer in contact with a large-area passivated ionimplanted planar Silicon detector. In addition, the a-HiB separations were instrumental in the successful Identification of the hitherto undiscovered isotope produced at an '®0 beam energy of 93 MeV.

A Direct Speciation of Cm(III) in Natural Aquatic Systems by Time-Resolved Laser-Induced Fluorescence Spectroscopy (TRLFS)

Abstract: A direct speciation of Cm(III) traced in three different Gorleben groundwaters is performed by time resolved laser fluorescence spectroscopy (TRLFS) in very dilute concentrations from 7.8 χ 10~ mol 1\" to 6.3 χ IO\" mol Γ 1 , which are below the solubility of Cm(III) at the given aquatic neutral pH. As spectroscopic references, fluorescence spectra are produced for individual hydroxides, carbonates, húmate and fulvate of Cm(III). Parallel to the spectroscopic speciation, a thermodynamic speciation is also made based on complexation constants of the predominant aquatic chemical reactions, i.e. hydrolysis, carbonate complexation and húmate complexation. The speciation results from thermodynamic calculation and spectroscopy are compared with one another.

Kd Measurements of Activation, Fission and Heavy Elements in Water/Solid Phase Systems

Abstract: Batch experiments have been carried out to measure Kd values, their variations, and some other parameters for radionuclides of interest in safety assessments including long-lived β/γ fission products and α emitters, such as Ra and the light actinides. The parameters investigated were mainly the concentrations of the elements competing with the radionuclides and the pH of the aqueous phases. The systems studied were as close as possible to those that could reasonably be expected to be encountered in a surface/near surface radioactive waste repository. A number of partition mechanisms are discussed.

Possible Effects of Bacteria on Trace Element Migration in Crystalline Bed-Rock

Abstract: This work reports the total numbers of bacteria in the ground water and on surfaces exposed to slowly flowing ground water from two boreholes, KLX01,1078 m, close to the forth-coming Swedish Äspö hard rock laboratory and V2, 1240 m, in the Stripa mine. There were from 0.2 χ ΙΟ up to 12 χ 10 unattached bacteria/m groundwater and from 0.09 χ 10 up to 5.9 χ 10 attached bacteria/m. The results indicate that there can be up to 790000 attached bacteria per unattached bacterium in channels in fractured rock width a mean width of 0.1 mm. The bacterial populations significantly assimilated C0 2 , indicating an in situ production of organic carbon from carbonate, that might have a profound influence on the ground water chemistry. This work also indicates that sulphate-reducing bacteria produce complexing agents that has a significant influence on the speciation and sorption properties of trivalent lanthanides.

Studies of the Extent of Surface Diffusion in the Migration of Radionuclides through Geological Materials

Abstract: Surface diffusion has been invoked previously to explain rates of radionuclide migration which were greater than those predicted. Results were generally open to interpretation but the possible existence of surface diffusion, whereby sorbed radionuclides could potentially migrate at much enhanced rates, necessitated investigation. In the present work, results from through-diffusion experiments have indicated that for strontium and americium, surface diffusion accounted for 50 -80% and 5 6 0 % of the total diffusion respectively, depending on the geological material. However, for caesium, no surface diffusion contribution was observed. Evidence for anion exclusion of iodide was found. Therefore, although some radionuclides do appear to partake in surface diffusion, the magnitude of the phenomenon is probably not sufficient to affect repository safety assessment modelling.

Thermodynamic Properties of Actinide Aqueous Species Relevant to Geochemical Problems

Abstract: The status of our knowledge of the basic thermodynamic properties of the aqueous complexes of the actinides in their different valency states with two environmentally important ligands, namely hydroxide and carbonate, is overviewed. Even in the case of uranium which has been the most studied, gaps are found among the relative wealth of trustworthy formation constants, especially for the less stable valence states. The need for substantial improvement of our knowledge in the case of the other actinides, especially transuranium elements, is outlined. The relative scarcity of enthalpy of formation data is also stressed.

Migration of Radioelements Around the New Nuclear Reactors at Oklo: Analogies with a High-Level Waste Repository

Abstract: Samples from a new natural reactor of Oklo (zone 10) have been investigated with the aim of specifying possible elemental migrations into its close surroundings. The mineralogy demonstrates the occurrence of a redox front at the boundary of the reactor. We show first that significant migration of rare earths and uranium took place on a decimetric scale around the reactor and that secondary phases (both clay minerals, carbonates filling fissures and accessory minerals such as zircon) are responsible for their subsequent immobilization within the rock.

Interaction of Uranyl(VI) with Silicic Acid

Abstract: The interaction of uranyl(VI) with silicic acid was investigated by a solvent extraction method. The equilibrium constant of the reaction υ Ο Γ + Si(OH)4 = U020Si(0H)î + H + was determined to be (3.6 + 0.5) χ I O 3 in 6.7 χ Ι Ο 2 M silicic acid and pH 3 — 4.5. The extraction behavior of uranyl(VI) was also studied from silicic acid-acetate medium. The results are discussed in relation to metal-silicate interaction and silica polymerization.

Hydrolysis and Colloid Formation of Thorium in Water and Consequences for its Migration Behaviour – Comparison with Uranium

Abstract: The different behaviour of Th and U in aqueous solutions is discussed. Hydrolysis, colloid formation and retention of Th are sludied by paper chromatography and by filtration and ultrafiltration as function of pH and time. The behaviour of Th in ihree natural groundwaters is investigated as function of time by filtration and ultrafiltration and by means of a scanning electron microscope with microprobe. Autoradiography is also applied. — Evidence for the formation of colloids of Th is presented. In groundwaters silicic acid serves as carrier. After filtration and ultrafiltration new fine (colloid) and coarse particles are formed carrying Th. The percentage of Th in the coarse particle fractions increases with the concentration of Th and with the salinity of the groundwater.

Hydrolysis and Carbonate Complexation of Dioxoplutonium(V)

Abstract: The formation constants for the reactions P U O 2 + H 2 O PU02(0H) + H^ and PuO^ + C O r •^PUOjCCOj ) ' weredetermined in aqueous sodium Perchlorate solutions by laser-induced photoacoustic spectroscopy. The molar absorptivity of the PUO2 band at 569 nm decreased with increasing hydroxide concentration. Similarly, spectral changes occurred between 540 and 580 nm as the carbonate concentration was increased. The absorption data were analyzed by the non-linear least-squares program SQUAD to yield complexation constants. For the hydrolysis reaction at 0.1 M ionic strength, log*^i,(0.1) = — 9.73 ±0.10, and for the first carbonate complexation constant at 0.5 M ionic strength, \\ogßn(0.5) = 4 .60±0.03. Using the specific ion interaction theory, both complexation constants were extrapolated to zero ionic strength: for hydrolysis, log*^,i(0) = —9.73±0.10, and for carbonate complexation, logi?u(0) = 5.12±0.07.

Factors Affecting the Position of Y and Actinides(III) with Respect to Lanthanides in the NH4SCN – Adogen-464SCN Extraction System

Abstract: The extraction of La, Eu, Lu, Y, Am and Cf by Adogen-464SCN from ammonium thiocyanate solutions has been studied as a Function of extractant and thiocyanate concentration. It has been shown that the degree of complex formation in the aqueous phase decreases across the lanthanide series and increases between Am and Cf. whereas in the organic phase it increases for both series. With respect to complexation in the aqueous phase, Y is midway between Eu and Lu, Am is near to La, and Cf is a pseudolanthanide lighter than La. On the other hand, it has been shown that with respect to complexation in the organic phase Am and Cf are out of the heavy end of the lanthanide series, and Y is near to La. Because the organic term prevails, the distribution ratio increases in the order La < Y < Eu < Lu < Am < Cf, except for very high concentrations of thiocyanate ions, where Am and Cf are less extracted than Lu.