Anna-Maria Jakobsson

Sorption of Pu(VI) onto TiO2

The sorption of Pu(VI) onto TiO(2) was studied as a function of pH (2-10) and Pu concentration (10(-8)-10(-4) M) under an N(2) atmosphere, in 0.016 and 0.1 M NaClO(4). A batch-wise method was used, in which pH was measured in separate experimental containers after removal of a sample to determine the amount of Pu that had been sorbed. As Pu is radioactive, it was used as a tracer and measured by liquid scintillation counting. No ionic strength dependence was discerned, which was taken as an indication of inner sphere complex formation. In the interval of pH 2-7 the system could be described by the formation of two positively charged surface complexes using a 1-pK Stern model. Sorption of the plutonyl ion (PuO(2)(2+)) and the first hydrolysis species (PuO(2)(OH)(+)) was estimated using FITEQL to logK(1)=6.9 and logK(2)=1.4, respectively.

Autoradiographic comparisons of radionuclide adsorption between subsurface anaerobic biofilms and granitic host rocks

In high level nuclear waste repositories, the host rock is considered to be an important barrier to radionuclide migration by adsorbing metals at fluid rock interfaces. In granitic rock environments the surfaces of hydraulically conductive fractures are covered with mixed community biofilms. Biofilms were grown in situ on glass and rock surfaces in high pressure flow cells using groundwater sourced from a borehole 450 meters below sea level in the Äspö hard rock laboratory, Sweden. Scanning electron microscopy (SEM), epifluorescence microscopy and energy dispersive X-Ray spectroscopy (EDS) revealed monolayer biofilms consisting of up to 2 × 104 bacteria/mm2 surrounded by an extensive extracellular matrix and carbonate precipitates that covered <10% of the total surface area of each glass slide. The total organic carbon (TOC) associated with each slide was 11 μg ± 1.1 μg of which approximately 40% was attributed to bacteria and the rest to the extracellular matrix. The adsorption capacity of these biofilms was compared to the capacity of granite rock from the same environment. Surfaces were immersed for 42 days in an anaerobic synthetic groundwater containing either 60Co (II), 147Pm (III), 241Am (III), 234Th (IV), 237Np (V) or 99Mo (VI) as radioactive tracers. Adsorption and distribution of the tracers was investigated using 2D autoradiography. Compared to the biofilm, the rock adsorbed 88% more 60Co, 87% more 99Mo, 40% more 241Am and 237Np and 12% more 234Th per unit area. The biofilm adsorbed 57% more 147Pm than the rock. The β-emitting daughter products 233Pa and 234Pa also adsorbed to both rock and biofilms. Comparisons between rock surfaces and rock surfaces covered in biofilms indicate that the biofilms form a barrier between the rock and the groundwater and slow down radionuclide diffusion to the rock. These results suggest that the differences in adsorption are related to the surface functional groups available and the chemical properties of the radionuclides. Biological suppression of subsurface radionuclide adsorption should be accounted for in performance safety assessment models.

A method for preparation and purification of 234Th

An improved method to recover 234Th from depleted uranium has been developed. The method is based on solvent extraction and ion-exchange separations. The final thorium fraction has a high specific activity, about 1-3 PBq/mol Th, which makes it well suited for investigations, where a low thorium concentration is essential. The method is comparably fast, with a total processing time of 2 days. Another advantage is that the uranium fraction can be used as a 234Th generator for several years.

Measurements and modelling of Pm sorption onto TiO2 and goethite

Summary The sorption of Pm (1×10-9 M) onto TiO2 and goethite has been studied as a function of pH and ionic strength (0.01, 0.1 M NaClO4) at ambient temperature under N2 atmosphere. For both minerals studied there is no effect of the different ionic strengths and the sorption (log Kavs. pH) increases with a slope of ∼2. At the same pH sorption is lower on the more positively (or less negatively) charged goethite (pHpzc=8.4) than on the TiO2 (pHpzc=6.2). A comparison to other trivalent cations sorbed on different substrates (made by extracting values from %-sorbed curves and calculating these to Ka) indicate a similar pH-dependence. The results have been fitted using a 1-pK basic Stern model with FITEQL. An a priori restriction was that only one sorption reaction stoichiometry at a time was to be used to keep the number of parameters as small as possible. Outer-sphere and mono-, bi- and tri-dentate inner-sphere complexes were fitted. For each fit the value of the capacitance was varied to find the value that provided the best fit. Two different site densities were used: 1 site/nm2 and 2.31 sites/nm2. For the low site density the reaction which provides the best fit was: ≡SOH0.5-+Pm+3↔≡SOHPm2.5+ for both minerals. At the high site density the same reaction fits for TiO2 while there are several reactions that are equivalent in the fit for goethite.

Sorption of trivalent plutonium onto UO2 and the effect of the solid phase on the Pu oxidation state

Summary A problem with plutonium in sorption studies is its tendency to occur in a mix of oxidation states. This work was a study of the sorption of plutonium on the solid phase UO2 — in a perchlorate medium as a function of pH — where plutonium is kept in its trivalent oxidation state. Experiments also showed that uranium(IV) oxide can reduce Pu(IV) to Pu(III) in an acidic solution and maintain it in that state. It was observed at the same time that the chemically inert solid phases TiO2 and ThO2 possibly increase the rate of disproportionation of Pu(IV) at a pH of about 0.5. In accordance with previous studies MnO2 was found to have an oxidizing effect, converting Pu(IV) into Pu(VI). A comparison is made between the sorption of Th(IV), Pu(III) and Co(II) on UO2 and TiO2, and Pu(VI) on TiO2.

Sorption of Pu(III-IV) onto TiO2: A Preliminary Study

A preliminary study of the sorption of reduced plutonium (III-IV) onto TiO2 has been done under anoxic conditions at varying pH and ionic strength. The first approach was to study tetravalent Pu, but the sorption decreases with time in this case. This is probably due to disproportionation of Pu(IV) at the TiO2 surface, which then acts as a catalyst. A further observation that supports this explanation is that the sorption of Pu(IV) onto the walls of the experimental equipment is in some cases higher than the sorption of (disproportioned) Pu onto the oxide. Pu(III) sorption onto the oxide was also investigated and showed similarities with the previously investigated sorption of Pm(III) in a similar system.