Quantum Beam Science Research

5-9

Self-Assembled Structure of Coordination Species Clarified by Quantum Beam Techniques－ Development of Reprocessing Techniques by Elucidation of Solution States －Absolute intensity (cm-1)(a) X-ray scattering (TBP/Octane-h18)
1

Ellipsoidal TBP Assembly
PeakⅠ

Coordination spacefor heavy metal ions

φTBP =1.00.1(Volume fraction of TBP)

0.50

PO
0.70

0.15

0.85

O

0.05
00.01

1q(nm-1)

OO
Inner core

OPOO

101

Difference inscattering lengthdensity0.40

ρinner core

Inner core

ρouter shell~ρoctane -h18

0.65
0.10

0.1

0.90

0.010.001(b) Neutronscattering
φTBP =1.0

Difference inscattering lengthdensity(Volume fractionof TBP)

1q(nm-1)

10

TBP molecule(ρ:Scattering lengthdensity)(a) X-rayscattering(b) Neutron scattering (TBP/Octane-d18)
Absolute intensity (cm-1)

OOuter shellρoctane -d18
Overall structure of TBP assemblyρinner core~ρouter shell
Length scale

Fig.5-19 X-ray and neutron scattering profiles obtainedfor TBP/octane mixtures
Scattering maxima (peak Ⅰ) arising from the intermolecularinteractions of two TBPs were observed in (a), whereas in(b) the scattering contribution from the ellipsoidal orderedstructure was observed at lower q.

Fig.5-20 Diagram for scattering contrast and partial
structure of ellipsoidal TBP assembly in X-ray and neutron
scattering experiments
Schematic of TBP assembly, consisting of a hydrophilic innercore (light blue) and a hydrophobic outer shell (yellow), inoctane, and schematic of the spatial distribution of scattering
contrast (a) for X-rays across a TBP assembly in octane-h18and (b) for neutrons across a TBP assembly in octane-d18.

Tri-n-butyl phosphate (TBP) is an important extractant
for separating hexavalent uranium, U(VI), and tetravalent
plutonium, Pu(IV), from used nuclear fuel by solvent
extraction. However, little was known on the structure
and characteristic features of the TBP assembly and its
coordination species with the heavy metal ions; thus, further
study is needed to identify them. We aim to address these
problems using scattering techniques; therefore, we collected
X-ray and neutron scattering data for the TBP/octane mixturesas the first step of this study. As a result, we clarified theordered structure formed by TBPs in octane.
In Fig.5-19(a), scattering maxima (peak I) due to theintermolecular interactions of two TBPs were clearlyobserved. It is evident that TBP forms a self-assembly inoctane. In Fig.5-19(b), on the other hand, scattering from theoverall structure of the assembly was observed at q < 6.0 nm-1.
This indicates that TBPs form an ellipsoidal ordered structure,
as illustrated in Fig.5-20, where the inside core is capable ofincorporating a heavy metal ion.
The diagrams in Fig.5-20 indicate that (a) with X-rays thescattering length densities of the hydrophobic butyl groups

in TBP (ρbutyl) and octane-h18 (ρoctane-h18) are roughly equal toeach other, but that of the hydrophilic phosphate ester of TBP(ρhydrophilic) is larger than those of both hydrocarbon chains;and (b) with neutrons, the difference in the scattering lengthdensity between the hydrophobic and hydrophilic parts of
TBP (ρbutyl and ρhydrophilic) is negligible as compared with thescattering length density of octane-d18 (ρoctane-d18). Note that
X-rays and neutrons, respectively, enable the scattering to beeffectively observed from the inner core of the TBP assemblyand from its overall structure, respectively. In fact, the qdependence of the obtained scattering profiles differs greatly
between X-rays and neutrons, elucidating the structural andcharacteristic features of TBP assemblies.
The quantum beam technique is a powerful tool forinvestigating the microscopic structures of the various organic
molecules and their coordination species with heavy metal
ions. Anticipated applications include the development of not
only fuel reprocessing but also diverse research topics such as
the recycling of valuable metals from the waste products or
the polluted solutions.

Reference
Motokawa, R. et al., Microscopic Structures of Tri-n-Butyl Phosphate/n-Octane Mixtures by X-ray and Neutron Scattering in a Wide q Range, The Journalof Physical Chemistry B, vol.116, issue 4, 2012, p.1319-1327.

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JAEA R&D Review 2013