Razvan Buda

Studies of the ternary systems humic substances – kaolinite – Pu(III) and Pu(IV)

Abstract The behaviour of plutonium with respect to its migration in the aquifer has been studied under conditions close to nature. Most relevant under these conditions are Pu(III) and Pu(IV) in contact with humic substances (HS) and minerals. As a model for the host rock, kaolinite (KGa-1b) was chosen. The complexation of Pu(III) and Pu(IV) with Aldrich humic acid (AHA) in aqueous solution at ionic strength 0.1 M was investigated by the ultrafiltration method. The sorption of Pu(III) and Am(III) onto kaolinite (K) as a function of pH and metal-ion concentration was studied under aerobic and anaerobic conditions. The pH edge was found at pH∼5.5 independent of the metal-ion concentration and working atmosphere. The influence of HS on the sorption of Pu(III) and Pu(IV) onto kaolinite was investigated in the ternary systems Pu(III)-K-HS and Pu(IV)-K-HS and for comparison, in the system Th(IV)-K-HS. The dependence on pH, contact time, concentration of HS (for Pu(IV)-K-HS) was studied as well as the sequence in which the components were added. Generally, it was found that HS tend to enhance the sorption onto kaolinite below pH 6 and to decrease sorption at higher pH depending in detail on the sequence in which the components were added. An identification of the species sorbed on the surface of the kaolinite by X-ray absorption spectroscopy, as well as chemically, was attempted and preliminary results are discussed.

Sorption of tetravalent plutonium and humic substances onto kaolinite

The sorption of tetravalent plutonium onto kaolinite, a clay mineral, has been studied as a function of pH. The sorption studies have been performed by batch experiments under aerobic and anaerobic conditions (glove box). A pH range of 0–11 has been investigated with plutonium concentrations of 3.5 × 10-7−6.9 × 10-9 M and a solid phase concentration of 4 g/L. A sorption edge at about pH=1 and maximum sorption around pH=8.5 has been found under aerobic and anaerobic conditions. In the presence of CO2 at pH > 8.5, the sorption of plutonium is decreased due to the formation of soluble carbonate complexes. This is supported by speciation calculations for Pu(IV)-hydroxo-carbonate species in aqueous solution. Depending on the pH, 1%−10% of the sorbed plutonium is desorbed from the kaolinite and released into the fresh solution. For comparison with the behavior of Pu(IV), the sorption of the redox-stable Th(IV) onto kaolinite has also been investigated. Furthermore, the sorption of humic substances (HS) onto kaolinite has been studied as a function of pH and for varying concentrations of HS as a prerequisite to understand the more complex ternary system: plutonium, humic substances, and clay. It has been found that the sorption of Aldrich humic acid onto kaolinite is generally higher than that for Gorleben fulvic acid.

Application of XAFS Spectroscopy to Actinide Environmental Science

The use of XAFS Spectroscopy and related synchrotron radiation techniques for the molecular‐level speciation of environmental contaminants including actinides has led to an improved understanding of the fundamental chemical and biological processes determining their behavior in complex systems. Several recent applications of XAFS spectroscopy to actinides in model systems and more complex environmental samples are reviewed to highlight the impact these studies have on our knowledge about the bioavailability of actinides, the development of remediation strategies, and predictive models for risk assessment. XAFS studies of actinide ion sorption at solid/aqueous solution interfaces are presented in greater detail. Representative examples include XAFS studies in combination with batch‐type experiments of U(VI), Np(V), Pu(III), and Pu(IV) sorption on kaolinite.

Speciation of the oxidation states of plutonium in aqueous solutions by UV/Vis spectroscopy, CE-ICP-MS and CE-RIMS

For the speciation of the plutonium oxidation states in aqueous solutions, the online coupling of capillary electrophoresis (CE) with inductively coupled plasma mass spectrometry (ICP-MS) has been developed. Depending on the radius/electrical charge ratio, the oxidation states III, IV, V, and VI of plutonium are separated by CE, based on the different migration times through the capillary and are detected by ICP-MS. The detection limit is 20 ppb, i.e. 109–1010 atoms (10-12–10-13 g) for one oxidation state with an uncertainty of the reproducibility of the migration times of ≤1% and ≤5% for the peak area. The redox kinetics of the different plutonium oxidation states in the presence of humic substances (humic and fulvic acid) have been studied. A relatively rapid reduction of Pu(VI) (10 to 1000 h) in contact with Gorleben fulvic or Aldrich humic acid could be observed, depending on the pH of the solution. Furthermore, at pH=1, a reduction to Pu(III) and Pu(IV) in a mixture of all four oxidation states in contact with Gorleben fulvic acid after one month has been observed. In order to improve the sensitivity of the CE method, the offline coupling of CE to resonance ionization mass spectrometry (RIMS) has been explored. First applications of this new speciation method are presented.

Electrodeposition methods in superheavy element chemistry

Summary To prepare electrodeposition experiments with superheavy elements (SHE), their homologs were investigated. In the experiments, various electrode materials and electrolytes were used. Critical potentials (Ecrit) where the electrodeposition starts and potentials for the deposition of 50% of the atoms in solution (E50%) were determined. Underpotential deposition was observed in most cases. An electrolytic cell for a fast electrochemical deposition was developed and the time for the deposition of 50% of the atoms in solution (t50%) was determined. Short lived α-emitting isotopes were produced at Gesellschaft für Schwerionenforschung (GSI), Darmstadt, transferred to the aqueous phase with ALOHA (Automated Liquid Online Heavy element Apparatus), transported to an electrolytic cell and deposited on a palladinated Ni tape. It was shown that the coupling of devices for collection, electrodeposition, and α-spectroscopy is feasible and might be of great use in SHE chemistry.

Isotope selective ultratrace analysis of plutonium by resonance ionisation mass spectrometry

Abstract Resonance ionisation mass spectrometry (RIMS) is a sensitive and selective method for isotopically resolved ultratrace analysis of long-lived radionuclides. For the routine analysis of plutonium three titanium–sapphire lasers pumped by a pulsed Nd:YAG laser in combination with a time-of-flight mass spectrometer are used. The detection limit of this system is as low as 106–107 atoms for the plutonium isotopes 238Pu to 244Pu. The RIMS technique was applied to investigate the isotopic composition and the content of plutonium in a depleted uranium penetrator as used during the Balkan conflict delivering important information on the origin of the depleted uranium in this type of ammunition. Furthermore, groundwater samples from an in-situ experiment performed at the Grimsel Test Site, Switzerland, have been analysed with the aim to study the migration behaviour of plutonium under natural conditions.

Uranium from German Nuclear Power Projects of the 1940s— A Nuclear Forensic Investigation

Here we present a nuclear forensic study of uranium from German nuclear projects which used different geometries of metallic uranium fuel.3b,d, 4 Through measurement of the 230Th/234U ratio, we could determine that the material had been produced in the period from 1940 to 1943. To determine the geographical origin of the uranium, the rare-earth-element content and the 87Sr/86Sr ratio were measured. The results provide evidence that the uranium was mined in the Czech Republic. Trace amounts of 236U and 239Pu were detected at the level of their natural abundance, which indicates that the uranium fuel was not exposed to any major neutron fluence.