III-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

Solvent extraction of Zr and Hf in trioctylamine/H2 SO4 system
T. Yokokita∗1 and H. Haba∗2
The chemical properties of superheavy elements(SHEs) with atomic number Z ≥ 104 are expected todeviate from the periodicity of their lighter homologues
in the periodic table. Thus, chemical studies on SHEs
are interesting. SHEs are produced at accelerators using heavy-ion-induced nuclear reactions. The production rates of SHEs are low, and their half-lives are short(T1/2 ≤ 1 min). Thus, the chemical studies of SHEs are
conducted on a single-atom basis, which makes it diﬃcult to perform chemical experiments with SHEs. Thusfar, the chemical properties of SHEs have been discussed
by comparing their behavior with those of homologous
elemenets.1)
We performed anion-exchange experiments of Rf andits homologous elements, Zr and Hf, to study its sulfate complexation.2) From the previous study, we obtained the distribution coeﬃcients of Rf, Zr and Hf in
0.060–0.46 M H2 SO4 .2) We need to identify the chemical species of the sulfate complexes of Zr and Hf under
the studied conditions to discuss the sulfate complexation of Rf based on the comparison of the anion-exchange
behavior of Rf and its homologous elements. In thiswork, we performed solvent extraction with an anion exchanger, trioctylamine (TOA), from H2 SO4 and ESI-MS
spectroscopy to deduce the anionic sulfate complexes ofZr and Hf under the experimental condition of Rf.
Radiotracers 88 Zr and 175 Hf were produced in89
Y(d, 3n)88 Zr and nat Lu(d, xn)175 Hf reactions, respectively, using the RIKEN AVF cyclotron. Then, thesetracers were puriﬁed by an anion-exchange method. Thesolvent extractions of Zr and Hf were performed by employing the batch method with 1.6–20 mM trioctylaminein CH2 Cl2 from 0.11 M H2 SO4 . The distribution ratios(D) were obtained according to D = [M]org /[M]aq ; here,[M]org and [M]aq denote the metal concentration in theorganic and aqueous phases, respectively.
For ESI-MS spectroscopy, Zr and Hf samples in the organic phases were obtained in the solvent extraction of9.4 mM Zr and 7.6 mM of Hf, respectively, from 0.15 M
H2 SO4 into 3.6 mM TOA-CH2 Cl2 solutions. Further,
we made blank samples, i.e., TOA-CH2 Cl2 solutions
without Zr and Hf. They were the organic phases obtained in the solvent extraction of 0.15 M H2 SO4 without the metal ions and 3.6 mM TOA-CH2 Cl2 solutions.
All samples were diluted with CH2 Cl2 before ESI-MS
measurement using LTQ Orbitrap XL mass spectrometer (Thermo Fisher Scientiﬁc).
The slopes of log D and log [TOA] of Zr and Hf wereobtained: the slope is related to the number of the protonated TOA cation (HTOA+ ) of the extracted species.
The slope values of both Zr and Hf were 1.9 ± 0.1, which
implies two HTOA+ cations are paired with the anionic∗1∗2Department of General Education, Salesian Polytechnic
RIKEN Nishina Center

Fig. 1. ESI-MS spectrum (negative mode) of the Zr sample.
The m/z values marked by the squares are the distinguishable peaks of the Zr sample.

sulfate complexes of Zr and Hf with a net charge −2 tobe extracted into the organic phase from the aqueousphase.
In ESI-MS spectroscopy, the peaks of the ionized Zr
and Hf species in the organic phase were assigned bycomparing the mass spectra of Zr and Hf samples in
the organic phase with those of the blank samples. The
mass spectrum of the Zr samples is shown in Fig. 1. 7
distinguishable peaks were observed at m/z = 189.0,
238.0, 378.9, 396.8, 476.7, 830.0, and 1280.8, whichcorresponded to [Zr(SO4 )3 ]2− , [Zr(HSO4 )2 (SO4 )2 ]2− ,[Zr(OH)(HSO4 )2 (SO4 )]− ,[Zr[Zr(HSO4 )(SO4 )2 ]− ,−(HSO4 )3 (SO4 )] , (HTOA)[Zr(HSO4 )2 (SO4 )2 ]− , and(HTOA)2 [Zr(HSO4 )3 (SO4 )2 ]− , respectively. The extracted Zr species in organic phase contains the two
HTOA+ cations, and therefore, the Zr species before ionization are (HTOA)2 [Zr(SO4 )3 ], (HTOA)2 [Zr(HSO4 )2(SO4 )2 ],(HTOA)2 [Zr(OH)(HSO4 )(SO4 )2 ],and(HTOA)2 [Zr(OH)(HSO4 )3 (SO4 )].
For the Hf sample, 4 distinguishable peaks were observed at m/z = 234.0, 486.8, 919.6, and 1371.2, whichcorrespond to [Hf(SO4 )3 ]2− , [Hf(OH)(HSO4 )2 (SO4 )]− ,(HTOA)[Hf(HSO4 )2 (SO4 )2 ]− , and (HTOA)2 Hf(HSO4 )3(SO4 )2 ]− , respectively.
The extracted Hf species
in organic phase contains the two HTOA+ cations,and therefore, the Hf species before ionizationare (HTOA)2 [Hf(SO4 )3 ], (HTOA)2 [Hf(OH)(HSO4 )(HTOA)2 [Hf(OH)(HSO4 )3 (SO4 )],and(SO4 )2 ],(HTOA)2 [Hf(HSO4 )2 (SO4 )2 ].
These results indicate that the proposed Zr andHf species at 0.1 M H2 SO4 are [M(SO4 )3 ]2− ,[M(HSO4 )2 (SO4 )2 ]2− , [M(OH)(HSO4 )(SO4 )2 ]2− , and[M(OH)(HSO4 )3 (SO4 )]2− (M = Zr and Hf). However,
the main species of Zr and Hf in this studied condition
cannot be determined from the present study. A combination of other analytical methods must be performedto identify the main chemical species.
References
1) A. Türler, V. Pershina, Chem. Rev. 113, 1273 (2013).
2) T. Yokokita et al., RIKEN Accel. Prog. Rep. 54, 148(2021).

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