Application of Calixarenes as Macrocyclic Ligands for Uranium(VI): A Review
Hindawi Publishing Corporation
Journal of Chemistry
Volume 2013, Article ID 762819, 16 pages
http://dx.doi.org/10.1155/2013/762819
Review Article
Application of Calixarenes as Macrocyclic Ligands for
Uranium(VI): A Review
Katarzyna Kiegiel, Lukasz Steczek, and Grazyna Zakrzewska-Trznadel
Centre for Radiochemistry and Nuclear Chemistry, Institute of Nuclear Chemistry and Technology, Dorodna 16, 03-195 Warsaw, Poland
Correspondence should be addressed to Katarzyna Kiegiel;
Received 22 June 2012; Revised 15 September 2012; Accepted 8 October 2012
Academic Editor: Satoru Tsushima
Copyright © 2013 Katarzyna Kiegiel et al. is is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.
Calixarenes represent a well-known family of macrocyclic molecules with broad range of potential applications in chemical,
analytical, and engineering materials �elds. is paper covers the use of calixarenes as complexing agents for uranium(VI). e high
effectiveness of calix[6]arenes in comparison to other calixarenes in uranium(VI) separation process is also presented. Processes
such as liquid-liquid extraction (LLE), liquid membrane (L�) separation, and ion exchange are considered as potential �elds for
application of calixarenes as useful agents for binding UO2 2+ for effective separation from aqueous solutions containing other metal
components.
1. Introduction
In the last years an increasing interest in calixarenes as potential complexing agents for metals, among them actinides, is
observed. It is supported by several reviews [1–3]. e present
paper focuses on application of calixarenes for separation
of uranium(VI) from competing metal ions in aqueous
solutions.
Uranium plays an important role in generation of nuclear
power. e selective isolation of uranium is of particular
interest in the context of both energy resources and treatment
of nuclear wastes. As a key element for production of the fuel
for nuclear reactors, uranium, the more common element
in the Earth’s crust occurring in rocks, soil, and river and
ocean waters [4], has to be extracted from the raw material
in complex hydrometallurgical processes involving many
separation steps. Processes such as acidic leaching, liquidliquid extraction, or ion exchange are applied to obtain pure
triuranium octaoxide (U3 O8 ) from uranium ore. Since in
most of uranium minerals uranium is accompanied by other
heavy metals, postleaching solutions usually contain a mixture of different metallic ions that should be separated from
UO2 2+ , the uranyl ion that forms complexes with various
organic chelating agents. e separation can be achieved by
using of extracting agents that exhibit high speci�city towards
UO2 2+ and allowing selective uranium recovery.
Uranium(VI) has unique characteristics, namely, the
extreme stability of the triatomic uranyl ion OUO2+ . is
ion possesses very stable uranium(VI)-oxygen double bonds,
leaving the oxygen atoms largely unreactive [5]. In crystalline structures, UO2 2+ is linear and is capable of forming complexes of coordinative bonds with host molecules
containing �ve or six ligand groups, primarily oxygen atoms
[6]. is suggests that a macrocyclic host molecule having
a nearly coplanar arrangement of either �ve or six ligand
groups would act as a speci�c ligand for UO2 2+ (i.e., as an
uranophile).
In order to design a ligand that can selectively extract
UO2 2+ , one has to overcome a difficult problem, that is, the
ligand must discriminate strictly between UO2 2+ and other
metal ions present in great excess in water or waste solution.
Over the last three decades, a variety of studies have targeted
molecular design and implementation of various polydentate
compounds that serve effectively as uranium(VI) extracting
agents, for example, a macrocyclic hexaketone, macrocyclic
hexacarboxylic acid, and tridithiocarbamate synthesized by
Tabushi et al. [7–9]. Shinkai and coworkers [10] applied
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Journal of Chemistry
Wide rim
R
R
R
R
R
OH
OH
HO
R
HO
HO
OH
Narrow rim
examined for their chemical stability under acidic and basic
conditions, their behavior under irradiation conditions is still
under investigation. It was found that aer the exposure to the
gamma radiation the ligands could change their properties.
Mariani et al. [16] studied a derivative of calix[6]arenes.
ey observed that an absorbed dose above 100 kGy in
the presence of air decreased the distribution coefficient
for 241 Am and 152 Eu without signi�cant in�uence on the
selectivity in comparison to nonirradiated ligands. However,
an absorbed dose up to 55 kGy in the presence of air caused
an increase of the distribution coefficient for both metals.
e same absorbed dose in the presence of nitrogen caused
a decrease of the distribution coefficient. ese results indicated how important the in�uence of oxidizing environment
on radiolysis is.
F 1: Illustration of the structure of calixarenes.
2. Speciation of Uranium(VI) in Water
calixarenes for UO2 2+ complexation with efficient results in
terms of stability and selectivity. e increasing interest in
these macrocycles is not only due to their easy synthesis
through well-established and simple methodologies [11], but
also due to the possibility of shaping their basket through
functionalization at the lower (narrow) or at upper (wide)
rims (Figure 1).
Calixarenes are formed by paraphenolic units linked by
methylene bridges ortho to OH functions. In addition, they
can be easily functionalized to be more speci�c. e OR
groups (chelating groups) on the lower rim are usually chosen
for their affinity and selectivity towards a speci�c molecule
or ion. On the other hand, the groups in paraposition on the
upper rim can give hydrophilic or hydrophobic character to
the molecule. ese groups can also rigidify the conformation
of calix[n]arene.
e extraction study of lanthanides and actinides showed
that the calixarenes bearing ligands including P=O groups
were more efficient than TBP (tributyl phosphate), TOPO
(trioctylphosphine oxide), and CMPO (carbamoyl phosphonate) [12, 13]. e ligand concentration necessary to reach
a given extraction yield was 10 to 100 times lower with the
calixarenes than with the classical extractants.
Very interesting results were obtained in the study of toxicity of calixarenes [14]. e calix[6]arenes and calix[8]arenes
functionalized with sulfonate group had the same level of
toxicity as glucose. On the other hand, derivatives of psulfonato-calix[4]arenes showed slight toxicity, in contrast to
calix[4]arene phosphonic acid derivatives which exhibited no
effect on the cell growth of human �broblast. It is worthy
to remind that derivatives of p-sulfonato calix[6]arene and
calix[8]arene analogs were investigated in radiotherapy [15].
Complexation studies of 230 U with these calixarenes showed
that in vivo application of such compounds is no (...truncated)
