Fig.1: Sorption of Zr, Hf, Th and Rf on a CIX resin (Aminex A6) at various HF concentrations.
Fluoride Complexation of Rutherfordium (Rf, Element 104)
E. Strub, J.V. Kratz, A. Kronenberg, A. Nähler, P. Thörle, S. Zauner (Universität Mainz), W. Brüchle, E. Jäger, M. Schädel, B. Schausten, E. Schimpf (GSI Darmstadt), D. Schumann, U. Kirbach (TU Dresden), D. Jost, A. Türler (PSI), H.W. Gäggeler (Univ. Bern & PSI), R. Eichler, A. Grund, M. Gärtner (Univ. Bern), Y. Nagame, K. Tsukada (JAERI, Tokai), J.P. Glatz (Transuraninstitut Karlsruhe)
Previous studies had shown that Rf behaves differently from Hf and Zr and resembles Th on a cation exchanger in HNO3/HF solutions. These studies were extended to higher HF concentrations. Further experiments using an anion exchanger indicate that Rf forms neutral complexes at HF concentrations of about 0.05 M at which it starts to be eluted from the cation exchanger.
It had been shown experimentally [1] that 261Rf is not eluted from the cation exchange (CIX) column under the condition of previously performed seaborgium (Sg, element 106) experiments [2,3] and can only appear in the Sg sample as a result of the decay of 265Sg. Further, the distribution coefficient (Kd) of 261Rf on a cation exchange resin at a higher HF concentration (0.1 M HNO3/0.01 M HF) had been studied [1]. At this concentration, despite Zr and Hf being eluted, Rf is still retained on the column.
Now, the Kd values of Rf at HF concentrations of 0.03 M and 0.05 M (both in 0.1 M HNO3) have been determined to establish at which concentration the Kd drops down indicating that neutral or anionic complexes of Rf are being formed.
In addition, experiments using an anion exchanger (AIX) have been performed. It is expected that Rf will form anionic complexes resulting in high Kd values at high HF concentrations, while Th does not form anionic fluoride complexes.
261
Rf was produced in the 248Cm(18O,5n) reaction at the PSI Philips Cyclotron. A 730 mg/cm2 248Cm target was bombarded with a 0.5 mApart 18O5+ beam. The target contained 10% Gd thus producing simultaneously short-lived Hf isotopes. Rf and Hf were transported by a He(KCl) gas jet and collected for 90 s by impaction on a slider in the Automatic Rapid Chemistry Apparatus ARCA II. The residue was dissolved in 200 m l 0.1 M HNO3/x M HF (x variable) and fed onto the ion exchange column. The effluent was evaporated to dryness as sample 1. In order to elute remaining Rf from the column, a second fraction (200 m l) was collected which is known to elute group 4 elements from the column (i.e. 0.1 M HNO3/0.1 M HF (CIX) or 5 M HNO3/0.01 M HF (AIX)). This fraction was prepared as sample 2. 78-s 261Rf was detected by a-spectroscopy. The counting time was 12 min. Every 8th pair of samples was monitored by additional g-spectroscopy to determine the distribution of Hf.The data (see figures) show that Rf behaves differently from Zr and Hf and seems to resemble Th on both AIX and CIX (dotted lines). On the CIX, the fall of the Kd values for Rf occurs between 0.01 M and 0.1 M HF, i.e. at one order of magnitude higher HF concentrations than for Zr and Hf. It is remarkable, that the Kd values of Zr and Hf on the AIX rise in the same HF concentration range where they fall on the CIX indicating that the formation of anionic complexes takes place simultaneously with the decrease of cationic complexes. Conversely, the Rf data suggest that Rf forms neutral complexes (type [RfF4]).
A first attempt of interpretation could be that the ionic radii of Zr4+ and Hf4+ are very similar due to the lanthanide contraction resulting in a similar complexation behaviour, while in Rf, the actinide contraction is surpassed by the relativistic expansion of the d orbitals. A third beam time at the PSI will be used to verify and complete the AIX data.
Fig.1: Sorption of Zr, Hf, Th and Rf on a CIX resin (Aminex A6) at
various HF concentrations.
Fig.2: Sorption of Zr, Hf, Th and Rf on a AIX resin
(Riedel de Haen) at various HF concentrations.
REFERENCES
[1] E. Strub et al., Labor für Radio- und Umweltchemie der Universität Bern und des Paul Scherrer Instituts, Annual Report 1997, p.6