Rutherfordium (Rf) has atomic number 104 and is the lightest among the transactinide, or superheavy, elements . The element is positioned as a homologue in group 4 of the Periodic Table of elements, which is predicted to be in the 6d transition series. The chemical properties of such elements have attracted much attention because of possible deviations from those of their homologs as a result of enhancement of the relativistic effect for heavy elements , , . It was found difficult that the characterization is performed based on one-atom-at-a-time chemistry for their low production rates and short half-lives .
In previous studies , , , , , , our research group investigated fluoride complexation of Rf, and the lighter homologs Zr and Hf, by anion- and cation-exchange chromatography in HF and HF/HNO3 mixed solutions. The results showed that the ion-exchange behavior of Rf differs significantly from those of Zr and Hf, and that fluoride complexation with Rf is much weaker than in the cases of the homologs. An additional cation-exchange experiment on Rf in HF/0.1 M HNO3 solution  was performed to investigate consecutive fluoride complex formation with Rf and to determine the strengths of fluoride complexation with Rf, Zr, Hf, and the tetravalent pseudo-homolog Th. The results showed that at low HF concentrations the order of the strengths of the complexes formed with group 4 elements is Zr≥Hf>Rf. The Rf chemical species relevant to this behavior are still not clear.
In the present study, we used reversed-phase-chromatography with 2-thenoyltrifluoroacetone (TTA) as a chelating extractant to clarify the chemical properties of the cationic fluoride complexes of the superheavy element Rf. Unlike previous studies with cation-exchange resins, the use of TTA enables preferential extraction of metal ions in group 4, therefore the specific complex formation constant of Rf can be determined. The experiments were performed at various HF/0.01 M HNO3 concentrations, with a newly developed resin containing a solution of TTA in n-octanol. Fluoride complexation with Rf was investigated by extraction with TTA, which is sensitive to the valence of the metal complex. The Rf behavior was compared with those of Zr and Hf.
2.1 Preparation of TTA resin
TTA resin was prepared from an inert supporting material and TTA solution as follows.
In a preliminary treatment, the supporting material, namely MCI GEL™ CHP20/P20 powder of MITSUBISHI CHEMICAL, which consisted of spherical macromolecular particles of diameter 20 μm , was mixed well with methanol (100 mL) and incomplete particles of smaller size were removed by skimming. The resin was then washed with acetone (100 mL). The cleaning procedures were repeated three times, and the resin was dried in a vacuum drier at 60°C until a constant mass was reached. The resin was weighed and then mixed for 1 h with a solution of TTA in n-octanol, using a spatula. The resin prepared for column chromatography consisted of CHP20/P20 and 50 wt% TTA-octanol solution of the same weight as the supporting material.
2.2 Batch experiment and on-line experiment with Zr and Hf tracers
Prior to the experiments with Rf, batch experiments were performed with Zr and Hf to determine their distribution ratios with the TTA resin. The resin (50 mg) was prepared as described in the previous section. It was placed in a polypropylene tube and contacted with a solution (ca. 3 mL) consisting of a mixture of 2.0×10−4 to 1.0×10−2 M HF/0.01 M HNO3 for 15 min until a preliminary equilibrium was reached. Then a tracer solution (50 μL) containing 107–108 carrier-free atoms of 88Zr and 175Hf, produced by the reactions of 89Y and natLu, respectively, with protons from the AVF cyclotron at the Research Center for Nuclear Physics, Osaka University, was added to the solution. The mixture was allowed to stand for 5 min in the acid solution in the tube at room temperature, transferred to a polypropylene beaker, and then the resin was separated with a 5 mL syringe and a poly(tetrafluoroethylene) membrane filter (ADVANTEC, 13HP020AN, pore diameter 0.2 μm). An aliquot (2 mL) of the isolated solution was subjected to γ-ray spectrometry, with a Ge detector. The distribution ratios, Kd, for the resin were determined by comparing the radioactivity with that from the same procedure without the resin, as a reference:
where Ar, Aaq, and Aref are the radioactivities of the stationary phase (resin), aqueous phase (solution), and reference, respectively, Vaq is the solution volume (mL), and mr is the resin weight (g).
Off-line reversed-phase extraction chromatography of 88Zr and 175Hf was also performed in a resin-filled Teflon tube column (1.6 mm ϕ) or a micro-column (1.6 mm ϕ×7 mm). On-line reversed-phase extraction chromatography was then performed with 89mZr and 175Hf, which were simultaneously produced from 89Y (p, n) and 175Lu (p, n) reactions, respectively, at the RIKEN K70 AVF Cyclotron.
2.3 On-line experiment with Rf and No
For the Rf experiments, the nuclides 261Rf, with a half-life of 68 s , and 169Hf were simultaneously produced from the 248Cm(18O, 5n)261Rf and natGd(18O, xn)169Hf reactions with the 109.3 MeV 18O beam at the RIKEN K70 AVF cyclotron. The reaction products were rapidly transported by a KCl/He gas-jet system to the chemistry laboratory and deposited on the collection site of the on-line automated rapid chemistry apparatus (ARCA) for chromatography experiments . The deposited products were dissolved in HF/0.01 M HNO3 solution and fed into the micro-column (1.6 mm ϕ×7 mm) of the ARCA at a flow rate of 0.1 mL/min. The column was filled with resin containing 25 wt% TTA. The effluent from the column was collected on a Ta disk as fraction 1. The remaining products in the column were then stripped with 0.1 M HF/0.1 M HNO3 solution and collected on another Ta disk as fraction 2. These disks were then separately evaporated to dryness with a halogen heat lamp and heated He gas. The samples were assayed with the rapid α/SF detection system for superheavy element aqueous chemistry at RIKEN .
The 261Rf isotope used in the experiments decays to its daughter 257No. The α-particle energies of 257No (Eα=8.22, 8.32 MeV) are close to that of 261Rf (Eα=8.28 MeV), therefore these energies are hard to distinguish from each other. In the Rf experiments, two types of 257No α-events were observed. One was from 257No produced from α-decay of 261Rf after its chemical separation. This reflected the chemical behavior of Rf. The other was from 257No that was deposited during collection of 261Rf, and this reflected the chemical behavior of No. To correct for the contribution of 257No, we investigated the adsorption behavior of No in the same systems as those used for the Rf experiments.
Similarly to the Rf experiments, the isotope 255No (half-life=3.10 min) was produced from the 248Cm(12C, 5n) reaction with an 84 MeV 12C beam at the RIKEN K70 AVF cyclotron. The reaction products were rapidly transported by a KCl/He gas-jet system to the chemistry laboratory and were deposited on the collection site for 180 s. After deposition, the products were dissolved in various HF/0.01 M HNO3 solutions (85 μL) and fed into a 1.6 mm i.d.×7.0 mm TTA resin column at a flow rate of 0.1 mL/min. The effluent from the column was collected on a Ta disk as fraction 1. The remaining products in the column were then stripped with 0.1 M HF/0.1 M HNO3 solution (250 μL) at a flow rate of 1.0 mL/min and collected on another Ta disk as fraction 2. Both samples were evaporated to dryness with hot He gas and a halogen heating lamp. The samples were assayed with the rapid α/SF detection system for superheavy element aqueous chemistry at RIKEN. To determine the chemical yield, 162Yb was simultaneously produced from the Gd in the Cm target and measured by a Ge detector after 255No measurement. The average chemical yield of 162Yb in the experiments was about 17%.
From 195 cycles of the No experiments, a total of 1042 α-events of 255No were registered in the energy range 7.60–8.20 MeV.
3 Results and discussion
3.1 Results of batch experiment and on-line experiment with Zr and Hf tracers
The Kd values for Zr and Hf in the batch experiments and adsorption coefficients in the on-line experiments were determined and compared across a wide range of F−equilibrium concentrations ([F−]eq). The results are shown in Figure 1; the dashed lines and circles show data from the batch experiments and on-line experiments, respectively. The fluoride ion concentration at equilibrium is derived from the dissociation constant of HF, which is a weak acid. The results from both sets of experiments are in good agreement. This shows that the on-line experiments appropriately reflect the behaviors under the investigated conditions. In addition, the data for both Zr and Hf show sudden decreases in the range [F−]eq>4×10−5 M in both the batch and on-line experiments. This indicates that fluoride complexation with Zr and Hf proceeds sequentially to form neutral or anionic species from cationic ones in this [F−]eq range.
In a liquid–liquid extraction experiment with TTA-octanol solution performed in this study, trivalent chemical species were extracted, as shown in Figure 2. This suggests that group 4 elements tend to hydrolyze and form trivalent hydrolysis species at low fluoride ion concentrations. A theoretical calculation showed that cationic complex formation with fluoride starts with formation of trivalent hydrolysis species. If an increase in the fluoride concentration induces a decrease in the amount of trivalent species, the observed process should be fluoride complexation of such species to form bivalent cations, as
3.2 Results of on-line experiment with Rf
In the chromatography experiments, 222 α-events, including formation of 29 time-correlated α-particle pairs (8.00–8.40 MeV) from 261Rf and its daughter nuclide 257No, were observed in 1771 cycles. The adsorbed percentage (%ads) of Rf was calculated as
where Fr1 and Fr2 are the radioactivities of fraction 1 and 2, respectively. The %ads values for Rf were constant at around 60% in the [F−]eq range up to 5×10−4 M and then steeply decreased at [F−]eq=9×10−4 M. In contrast, the value for Hf decreased significantly from 100% to a few percent at around [F−]eq=1×10−4 M. This is in good agreement with the results of the Zr and Hf experiments, shown in Figure 1. This suggests that the cationic fluoride complexes of Rf are more stable than those of Hf at [F−]eq>1×10−4 M. Differences among fluoride species formation with Rf and its homologs are therefore clearly observed in these [F−]eq ranges.
3.3 Results of on-line experiment with No
Decay of No was taken into account in correction of the %ads values. The %ads values for 255No as a function of [F−]eq in the range 1.93×10−5 to 1.66×10−3 M are shown in Figure 3, together with the results for 261Rf. In the Rf experiments, the %ads values for 261Rf were constant, at around 60%, in the [F−]eq range up to 5×10−4 M and then steeply decreased at [F−]eq=9×10−4 M. In contrast, in the No experiments, the %ads values for 255No were less than 10% across the entire range of [F−]eq. The results of the present work confirm that No is adsorbed on TTA to a small extent and the effect on the %ads values of 261Rf is negligible.
To evaluate the %ads values for 261Rf, we assumed that adsorption of No was negligible. The results confirm that No was adsorbed on TTA to a small extent and the %ads values for 261Rf can be precisely determined.
3.4 Deduction of Kd values of Rf
In most column chromatography experiments performed on superheavy elements, it is not feasible to obtain an elution curve. The Kd values for such elements are therefore rarely obtained from such experiments. On the assumption that there is a universal relationship between the adsorption coefficients and corresponding Kd values among group 4 elements, which means that their elution curves show a similar dependence on the elution volume, we used the method described in reference 7 to obtain the Kd values for Rf. Elution curves were difficult to obtain in the Rf experiments, therefore it was difficult to obtain Kd values. However, the relationship between the adsorption coefficients and Kd values for Zr and Hf were determined, and it was therefore possible to estimate the former from the latter. The Kd values for Rf were estimated from the relationship between %ads and Kd observed in the present study, shown in Figure 4.
In Figure 5, the obtained Kd values for Rf are compared with those for Zr and Hf from the independent experiments in the present study. The figure shows significant differences between the behavior of Rf and those of its homologs. The Kd values for Rf appear at higher F− concentrations than those for Zr and Hf. Assuming that during extraction Rf forms species with TTA, similarly to the other elements in group 4, the decrease in extraction may be caused by additional cationic fluoride complexation. The Kd results show that the cationic complex of Rf, [RfF]3+, is more stable than those of the other elements at high fluoride concentrations. This stability means that fluoride complexation with the extracted Rf species is weaker than in the cases of the other elements. This conclusion is consistent with the experimental results obtained with a cation-exchange resin .
The observed data are in full agreement with theoretical predictions . The free-energy changes of the complex formation reactions were determined on the basis of fully relativistic density functional theory calculations of the electronic structures of various hydrated, hydrolyzed, and fluoride complexes of Zr, Hf, and Rf. The results showed that at low HF concentrations the order of the strengths of the formed complexes is Zr≥Hf>Rf.
In conclusion, the adsorption behaviors of Rf, Zr, and Hf with a TTA extractant were observed in dilute solutions of HF and HNO3. The behaviors of the No atoms produced in the decay of Rf were determined with 255No nuclides produced in runs other than the Rf experiments to assess the precise behavior of Rf, without the effects of its daughters. The results of the present study suggest that the chemical species involved in TTA extraction may be RfF3+, based on comparisons with the results for Zr and Hf. It was concluded that fluoride complexation with Rf cations is weaker than that with either Zr or Hf, which are group 4 homologs. The observed behavior of Rf is supported by theoretical calculations and agrees with the results of previous experiment on cation exchange. This study is the first comparison of complexation data for Rf cations with those for Zr and Hf, other than those based on cation-exchange experiments.
We thank the RIKEN AVF accelerator crew for supplying stable beams for the experiments. This study was supported in part by JSPS KAKENHI Grant Number 23550070 to A. Yokoyama. This work was performed at the RI Beam Factory operated by the RIKEN Nishina Center and CNS, University of Tokyo. We thank Helen McPherson, PhD, from Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript.
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Published Online: 2018-08-31
Published in Print: 2018-12-19