Research and Development Relating to the Accident at the TEPCO's Fukushima Daiichi NPS

1-15 Development of a Highly Cesium-Selective Adsorbent for Volume Reduction=Molecular Design of Crown Ether Using New Chemical Bond Property=
Cs

4.0 Å

expectation 4.8 ÅComplex structure

Cs

6.0 Å

extending thecrown size ofcrown ether

DB18C65.0 Å

expectation

Complex structure

DB20C6

Fig.1-30 Molecular design concept of crown ether
Design of new crown ether was conducted with the aim of extending crown size in order to unite crown size withthe ion radius of cesium.
0.005

B3LYP of DET th eory
Lan L2DZ for Cs
6-311G(d) for H. C. O

00-5-5-10-10-15-15-20-20-25-25-30-30

0.004
0.003
0.002

FTKCs0.001
0-0.001

Binding energy Eo:‫ ޓ‬E iot (Cs+-ether) = -244.036568 Ha‫ ޓޓ‬E iot (ether) = -223.993667 Ha‫ ޓޓޓ‬E iot (Cs+)= -19.985162 Ha‫ ޓ‬E o = -1.57 eV = -36.23 kcal/mol‫ޓޓޓ‬c.f. E o (Li+-H2O) = -33.6 kcal/mol‫ޓޓޓޓޓ‬Eo (Cs+-H2O) = -13.2 kcal/mol-0.003012345678R/AFig.1-31 The Cs(I)-DB20C6 complex is formed by electron donationfrom benzene π to Cs+ 5d(and 4f) orbitalAccording to the results of MO calculation, dibenzo-20-crown-6-ether(DB20C6) offers the capability to identify the ionic size of Cs. Furthermore, thecalculated binding energy of Cs+-ether is smaller than that of Li+-H2O, whichindicates that the Cs-DB20C6 complex is more stable than the Li hydratedcomplex.

Radioactive cesium (Cs) contamination occurred in theenvironment owing to the accident at the TEPCO s
Fukushima Daiichi NPS. Clay minerals adsorbed radioactiveCs; in order to recover this radioactive Cs, continued
development of new materials and new methods will berequired. Previously, it was established that crown ether is
one of the most popular absorbents for alkaline metals suchas Cs.
In this study, we conducted the following: (1) developmentof new crown ether, (2) evaluation of absorption properties of
new crown ether for alkaline metals, and (3) field investigationof the new crown ether in Fukushima. The development of anew Cs-selective adsorbent used potassium (K)-selective 18crown-6-ether as a starting compound. The design of the newcrown ether was conducted with the aim of extending thecrown size of the crown ether in order to unite it with the ion
radius of Cs and promoting the complexation properties of
the crown ether over those of Cs (Fig.1-30). Moreover,chemical bonding properties between Cs and six oxygen
atoms of the crown ether were evaluated by MO calculation.
The results of the MO calculations indicate that dibenzo-20crown-6-ether (DB20C6) is capable of identifying the ionicsize of Cs. Furthermore, it was also established that the pi
electron of the benzene ring of DB20C6 and the d-f hybridorbital electron of Cs were combined (Fig.1-31). Although

RCs-H2O: 2.92Å
DWF: 0.001Å2-0.002

Fig.1-32 Radial structural function ofCs-DB20C6 complex and Cs-crown ether
complex
From the results of EXAFS measurements,
the bond distance of Cs-O(OH2) in theDB20C6-Cs complex was determined.

Cs made an orbit of d-f hybrid orbital, it did not exist insodium (Na) or K, which are some of the most common
alkaline metals in the environment. For the interaction
between the d-f hybrid orbital electron of Cs and the pi
electron of the benzene ring of DB20C6, DB20C6 isolated
Cs when high concentrations of Na and K existed in the samesystem. According to the results obtained by EXAFS
measurements in SPring-8, the bond distance of Cs-O(OH2)in the DB20C6-Cs complex is 0.05~0.1 shorter than that ofCs-O(OH2) in a CsCl hydrated complex (Fig.1-32). This
result indicates that the DB20C6-Cs complex exhibits astrong capability to draw water molecules compared to Cs.
As part of the field examination in Fukushima, we also
developed a tornado adsorber tower and conducted samplingin Iitate village. The combined use of DB20C6 and theadsorber tower allowed removal of the water-soluble
radioactive Cs from agricultural water with about 100%efficiency. The adsorbent consisting of DB20C6 can beincinerated completely, in contrast to inorganic materials
such as zeolite. Therefore, the volume of radioactive waste(such as contaminated soil) could be drastically reduced by
the use of an organic adsorbent based on DB20C6.
The present study was sponsored by the Ministry ofEducation, Culture, Sports, Science and Technology of Japan(MEXT).

Reference
Seko, N., Suzuki, S. et al., Countermeasures Against Radioactive Materials Pollution after the Great East Japan Earthquake, NTS Inc, 2012, p.204-210 (inJapanese).

26 JAEA R&D Review 2012