Research Group for Bioactinides ChemistryGroup Leader : Toshihiko Ohnuki
Members : Naofumi Kozai, Fuminori Sakamoto, Shinya Yamasaki, Shousuke Igarashi, KazuhiroShiina, Takashi Kanno

The research objectives of the group are to elucidate achemical states change of actinides and lanthanides including
nano-particles formation in the biological reaction environments.
In the year of 2012, characterization of uranium and iodine insaline groundwater by SEC-UV-ICP-MS and adsorption oflanthnides ions on liposome have been studied.
Characterization of uranium and iodine in salinegroundwater by SEC-UV-ICP-MS
We have elucidated that nano-particles of rare earth elements
were developed on the cell surface of microorganisms [1,2]. Thesize and characteristics of nano-particles in the solution should
be revealed. As a technique for investigating the size andcharacteristics of nano-particles in solutions, size exclusion
chromatography (SEC) coupled on-line to ultravioletevisible(UV-Vis) detection and multi-element-specific detection
methods such as inductively coupled plasma mass spectrometry(ICP-MS) has been used recently [3]. The advantage of thistechnique is that elements attached to or presented within thenano-particles are monitored through a simple procedure. In
SEC-UV-Vis-ICP-MS analysis, nano-particles are separated
through an SEC column on the basis of their size andconsequent rate of elution; the separated colloids are thendetected with a UV-Vis detector, and the elements associatedwith the nano-particles are detected by ICP-MS. In principle, allnano-particles are eluted with mobile phases from an SEC
column. In practice, most packing materials (stationary phase)
have a negative charge on the surface, and thus cations andcationic paricles adsorb on the packing materials while anions
and anionic colloids are repelled. The anions and anionic nanoparticles are also separated through an SEC column and appear
on an SEC chromatograph. Using SEC-UV-Vis-ICP-MS
analysis, we have investigated the chemical species of uranium
and iodine in the saline groundwater collected near Horonobe,
Japan, where an underground research laboratory (URL) has
been under construction in sedimentary rock for research anddevelopment of technologies for the geological disposal of highlevel radioactive waste (HLW) [4]. Uranium is a representativeactinide.
The saline groundwater was collected at a depth of about 500
m in Horonobe, Japan, where is rich in saline (Na 4900 ppm, Cl7600 ppm), iodine (42 ppm), and methane gas. We analyzed thecolloids and ions of this groundwater mainly by employing a
SEC-UV-Vis-ICP-MS technique and focused on the speciationof uranium and iodine, both of which are of particularimportance for radioactive waste disposal. For this purpose, thegroundwater sample was introduced to SEC columns after being
passed through a 0.45 mm filter but without further pretreatment,such as isolation of colloids.
The chromatographic profiles obtained with two different SEC
columns were compared. We revealed that uranium present in
the groundwater at several tens of ppt was associated with low
molecular weight silica species with a neutral charge. The silica
species were virtually free of metal elements such as Na, K, Mg,
Ca, and Al. This study also found that almost all of the iodine in

the groundwater was iodide (I-). The groundwater contained anunidentified organic colloid that was not a carrier for theradioactive waste-relevant elements Se, Sr, I, Cs, Th, and U.
Adsorption of lanthnides ions on liposome
The surface of microorganisms, i.e. the biomembrane, iscomposed mainly of phospholipids and membrane proteins.
The phospholipids have a phosphate group as shown in Fig. 1.
Here we used a simplified model system, liposome toinvestigate the understanding the adsorption behavior oflanthanide ions on a phospholipid membrane without any
biological activity.

Fig. 1 SEM image of the liposome in ionic liquid. The oval balls inthe image are the liposome.

The adsorption behavior of lanthanide ions (except for Pm) on
liposomes composed of phosphatidylcholine and cholesterol
was examined to understand the interaction between lanthanide
ions and the phosphoryl moiety of phospholipids [5]. The
adsorption amount of lanthanide ions enhanced with an ascent in
pH in the weakly acidic condition. Selective adsorption with thelocal maximum at the Eu3+ ion and local minimum at the Er3+
ion was observed, similar to the selective adsorption of thebacterial cell surface, but different from that of synthetic Lnresin, which is composed of di(2-ethylhexyl)orthophosphoricacid. These results indicate that the adsorption of lanthanide on
the phospholipid is not derived by simple adsorption on
orthophosphate functional groups, but by the composition andmolecular structure of the phospholipid. Our results strongly
suggest that liposomes can be used as a simple biomembranemodel without any biological activity for the study of adsorptionof lanthanide ions.
References[1] M. Jiang, et al., Chem. Geol., 277, 61(2010).
[2] M. Jiang, et al., Geochim. Cosmochim. Act., 93, 30(2012).
[3] B. B. M. Sadi, et al., J. Enviorn. Monit., 4, 1010(2002).
[4] N. Kozai, et al., Water Res., 47, 1570 (2013).
[5] S. Yamasaki, et al., Chem. Lett., in press.