Jens Volker Kratz

Humic colloid-borne Np migration: influence of the oxidation state

The migration behavior of Np(IV/V) is investigated by column experiments using a sandy sediment with groundwater rich in humic substances. NIR spectroscopy, redox speciation by TTA extraction and ultrafiltration are used for the Np speciation in the groundwater prior to its introduction into the column and after elution. Np is found to be transported as humic colloid-bound species which are eluted slightly faster than the water flow velocity. The fraction of humic colloid-borne Np increases with the reduction progress of Np(V) to Np(IV), because the interaction of Np(IV) with humic substances is much stronger than Np(V). The results demonstrate the importance of the speciation of redox sensitive actinides in natural aquifers for the assessment of the humic colloid facilitated migration.

Superheavy Element Flerovium (Element 114) Is a Volatile Metal

The electron shell structure of superheavy elements, i.e., elements with atomic number Z ≥ 104, is influenced by strong relativistic effects caused by the high Z. Early atomic calculations on element 112 (copernicium, Cn) and element 114 (flerovium, Fl) having closed and quasi-closed electron shell configurations of 6d(10)7s(2) and 6d(10)7s(2)7p1/2(2), respectively, predicted them to be noble-gas-like due to very strong relativistic effects on the 7s and 7p1/2 valence orbitals. Recent fully relativistic calculations studying Cn and Fl in different environments suggest them to be less reactive compared to their lighter homologues in the groups, but still exhibiting a metallic character. Experimental gas-solid chromatography studies on Cn have, indeed, revealed a metal-metal bond formation with Au. In contrast to this, for Fl, the formation of a weak bond upon physisorption on a Au surface was inferred from first experiments. Here, we report on a gas-solid chromatography study of the adsorption of Fl on a Au surface. Fl was produced in the nuclear fusion reaction (244)Pu((48)Ca, 3-4n)(288,289)Fl and was isolated in-flight from the primary (48)Ca beam in a physical recoil separator. The adsorption behavior of Fl, its nuclear α-decay product Cn, their lighter homologues in groups 14 and 12, i.e., Pb and Hg, and the noble gas Rn were studied simultaneously by isothermal gas chromatography and thermochromatography. Two Fl atoms were detected. They adsorbed on a Au surface at room temperature in the first, isothermal part, but not as readily as Pb and Hg. The observed adsorption behavior of Fl points to a higher inertness compared to its nearest homologue in the group, Pb. However, the measured lower limit for the adsorption enthalpy of Fl on a Au surface points to the formation of a metal-metal bond of Fl with Au. Fl is the least reactive element in the group, but still a metal.

Cold fusion reactions with 48Ca

Abstract Fusion reactions of 48 Ca projectiles with 180 Hf, 184 W, 197 Au, 208 Pb and 209 Bi targets were studied. Highest cross sections were found in the 208 Pb( 48 Ca,2n) 254 No channel with σ max 3.2 ± 0.3 μb. The results of the heavier systems are discussed in terms of the extra — push model. For the lighter systems missing cross section is observed if compared to estimates made by the evaporation code HIVAP.

Complexation behaviour of neptunium with humic acid

The complexation of Np(V) with humic acid (HA) was studied under environmental conditions (pH≥6, concentration of HA 1-150 mg/L) over a wide range of metal ion concentrations (10-14 to 10-4 mol/L). The experimental methods used for the determination of the complexation constants were continuous electrophoretic ion focusing, anion exchange, and ultrafiltration in combination with radiometric measurements, and, as a direct speciation method, UV/Vis-spectroscopy. The interaction between Np(V) and humic acid has been described with the charge neutralization model including a correction for different loading capacities (LC) which makes possible the comparison of results obtained at various experimental conditions. Furthermore, the degree of deprotonation has also been used to calculate the complexation constants. The complexation constants calculated for Np(V) with the LC-concept vary with the metal ion concentration in the range between 10-14 and 10-4 mol/L. At metal concentrations below 10-11 mol/L, a complexation constant logβLC=5.0±0.3 was obtained. In the concentration range 10-7 to 5×10-4 mol/L, the logβLC values decrease continuously to 3.7±0.4 (10-5-5×10-4 mol/L). The latter is in agreement with direct measurements (UV/Vis-spectroscopy) at this concentration and data from the literature.

Attempts to chemically investigate element 112

Summary Two experiments aiming at the chemical investigation of element 112 produced in the heavy ion induced nuclear fusion reaction of 48Ca with 238U were performed at the Gesellschaft für Schwerionenforschung (GSI), Darmstadt, Germany. Both experiments were designed to determine the adsorption enthalpy of element 112 on a gold surface using a thermochromatography setup. The temperature range covered in the thermochromatography experiments allowed the adsorption of Hg at about 35 °C and of Rn at about -180 °C. Reports from the Flerov Laboratory for Nuclear Reactions (FLNR), Dubna, Russia claim production of a 5-min spontaneous fission (SF) activity assigned to 283112 for the 238U(48Ca,3n) 283112 reaction. Hence, Experiment I was designed to detect spontaneously fissioning (SF) isotopes of element 112 with half-lives (t1/2) longer than about 20 s. 11 high-energy events were detected. 7 events exhibit a deposition pattern resembling a chromatographic peak in the vicinity of Rn deposition. However, the energy of the events observed in Experiment I was lower than expected for a SF-decay of 283112. Therefore, these events could not be unambiguously attributed to the decay of 283112. In contradiction with earlier publications newer reports from FLNR Dubna claim that 283112 decays by α-particle emission (Eα = 9.5 MeV) with t1/2 = 4 s followed by a SF-decay of 279Ds (t1/2 = 0.2 s). Therefore, Experiment II was designed to be sensitive to both claimed decay properties of 283112. However, during this experiment neither short α-SF correlations nor SF coincidences were detected. The conclusion is that 283112 was not unambiguously detected, neither in Experiment I nor in Experiment II.

Theoretical predictions of hydrolysis and complex formation of group-4 elements Zr, Hf and Rf in HF and HCl solutions

Summary Fully relativistic molecular density–functional calculations of the electronic structures of hydrated, hydrolyzed and fluoride/chloride complexes have been performed for group–4 elements Zr, Hf, and element 104, Rf. Using the electronic density distribution data, relative values of the free energy change for hydrolysis and complex formation reactions were defined. The results show the following trend for the first hydrolysis step of the cationic species: Zr > Hf > Rf in agreement with experiments. For the complex formation in HF solutions, the trend to a decrease from Zr to Hf is continued with Rf, provided no hydrolysis takes place. At pH > 0, further fluorination of hydrolyzed species or fluoro–complexes has an inversed trend in the group Rf ≥ Zr > Hf, with the difference between the elements being very small. For the complex formation in HCl solutions, the trend is continued with Rf, so that Zr > Hf > Rf independently of pH. A decisive energetic factor in hydrolysis or complex formation processes proved to be a predominant electrostatic metal–ligand interaction. Trends in the Kd (distribution coefficient) values for the group–4 elements are expected to follow those of the complex formation.

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.

Fluoride complexation of rutherfordium (Rf, element 104)

The fluoride complexation of the group-4 elements Zr, Hf and Rf, and of the pseudo-homolog Th, has been investigated in mixed HNO3/HF solutions by studying Kd values on both cation exchange resins (CIX) and anion exchange resins (AIX) using the automated rapid chemistry apparatus ARCA. On the CIX, the four elements are strongly retained as cations below 10-3 M HF. For Zr and Hf, the decrease of the Kd values due to the formation of fluoride complexes occurs between 10-3 M HF and 10-2 M HF. For Rf and Th, this decrease is observed at one order of magnitude higher HF concentrations. On the AIX, for Zr and Hf, a rise of the Kd values due to the formation of anionic fluoride complexes is observed between 10-3 M HF and 10-2 M HF, i.e. in the same range of HF concentrations where the decrease of the Kd values on the CIX is observed, yielding a consistent picture. For Rf and Th, on the AIX, no rise of the Kd values is observed even if the HF concentration is increased up to 1 M. By varying the concentration of the counter ion NO3- which is competing for the binding sites on the AIX resin, it could be shown, nevertheless, that Rf does form anionic fluoride complexes. Apparently, there is a more specific competition of NO3- with respect to [RfFx](x-4)- than with [ZrFy](y-4)- and [HfFz](z-4)-.

Determination of the first ionization potential of actinide elements by resonance ionization mass spectroscopy

Abstract Resonance ionization mass spectroscopy (RIMS) in the presence of an external static electric field has been used for the determination of photoionization thresholds. Extrapolation of the thresholds obtained with different electric field strengths to zero field strength directly leads to the first ionization potential (IP). The ionization potentials of the transplutonium elements americium, curium, berkelium and californium could be measured for the first time. Due to the high sensitivity of RIMS, samples of only 1012 atoms have been used. The results are: IPAm = 5.9738(2)eV, IPCm = 5.9915(2)eV, IPBk = 6.1979(2)eV and IPCf = 6.2817(2)eV. The same technique was applied to thorium, neptunium and plutonium, yielding IPTh = 6.3067(2)eV, IPNp = 6.2655(2)eV and IPPo = 6.0258(2)eV. Plotted as a function of the number of electrons N, the actinide ionization potentials can be approximated by two straight lines joining at the half-filled shell when normalized to ionization from the lowest fN s2 level to the lowest fN s level.

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.