Application of Calixarenes as Macrocyclic Ligands for Uranium ( VI ) : A Review

Calixarenes represent a well-known family of macrocyclic molecules with broad range of potential applications in chemical, analytical, and engineeringmaterials �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 othermetal components.


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][2][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 (U 3 O 8 ) 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 UO 2 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 UO 2 2+ and allowing selective uranium recovery.Uranium(VI) has unique characteristics, namely, the extreme stability of the triatomic uranyl ion OUO 2+ .is ion possesses very stable uranium(VI)-oxygen double bonds, leaving the oxygen atoms largely unreactive [5].In crystalline structures, UO 2 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 UO 2 2+ (i.e., as an uranophile).
In order to design a ligand that can selectively extract UO 2 2+ , one has to overcome a difficult problem, that is, the ligand must discriminate strictly between UO 2 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][8][9].Shinkai and coworkers [10] applied calixarenes for UO 2 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 not possible.e complexation of uranium(VI) was efficient, but serum proteins and carbonate led to the destruction of the desired complexes.
e ligands that can be used in chemical process of radioactive waste treatment should be resistant to chemical and radiolytic conditions.Although calixarenes were well examined for their chemical stability under acidic and basic conditions, their behavior under irradiation conditions is still under investigation.It was found that aer 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.

Speciation of Uranium(VI) in Water
e speciation of uranium(VI) in aqueous solution is very important to understand the mechanism of extraction and to choose the appropriate extraction system.In aqueous solution, uranium(VI) exists as a linear UO 2 2+ , and it forms stable complexes with both organic and inorganic ligands [17,18].Simulations [19,20], calculations [21], and experiments [22,23] suggest that in water, UO 2 2+ is coordinated to �ve water molecules.e hydrolysis of uranium(VI) has been investigated by different techniques, for example, Raman spectroscopy [24], calorimetry, and potentiometry [25].Moll et al. [26] investigated the structure of UO 2 2+ as a function of pH with the aid of U L III -edge EXAFS (extended Xray absorption �ne structure) spectroscopy.UO 2 2+ forms strong complexes with OH − .In slightly acidic solutions (pH 3 to 4), there are two dominated polynuclear complexes: (UO 2 ) 2 (OH) 2 2+ and (UO 2 ) 3 (OH) 5 + .In the pH region between 6-11, the uranium(VI) speciation is dominated by the precipitation of schoepite phases UO 2 (OH) 2 ⋅H 2 O.In the presence of carbonate or atmospheric carbon dioxide, in the basic pH region uranyl-carbonate complexes: UO 2 CO 3 , UO 2 (CO 3 ) 2 2− , and UO 2 (CO 3 ) 3 4− are formed [27].e complexes of uranium(VI) with NO 3 − are the most fundamental species in the PUREX process (plutonium and uranium recovery by extraction), which is a liquid-liquid extraction method used to reprocess spent nuclear fuel.is is the most developed and widely used process in commercial reprocessing plants.Most of the unit operations in the PUREX reprocessing process are carried out in aqueous nitrate media.In aqueous nitrate the water molecules in the �rst hydration shell are replaced by the bidentate species NO 3 − as a function of increasing NO 3 − concentration at low pH [28].With increasing HNO 3 concentration, coordination numbers (CN) increase from 5 to 6. e U(VI)-NO 3 − complexation system in HNO 3 undergoes the formation of a 5-fold bidentate coordination mononitrato complex, can be also present [32].
Uranyl phosphates and uranyl arsenate minerals constitute about one-third of the approximately two hundreds known uranium minerals [33].e speciation in the UO 2 2+water-phosphate system was also studied [34].In the pH range from 2 to 5, two forms of species: UO 2 H 2 PO 4 + and neutral (UO 2 )(HPO 4 ) exist.At pH above 6.5, negatively charged species UO 2 (PO 4 ) − is also formed.

Structures and Conformational Analysis of
Calix [6]arenes e conformational �exibility of calix [6]arenes and presence of functional groups facilitates the binding of metal centers [36].ere are eight conformations of calix [6]arenes described in the literature [37].ey are called as distorted cone, compressed cone, pinched cone, double partial cone, winged, 1,2,3-alternate, 1,3,5-alternate, and distorted 1,2,3alternate.Until now few crystal structures of UO 2 2+ with calix [6]arenes have been described.Calix [6]arenes similar to calix [8]arenes show smaller ability to form inclusion complexes than their lower homologies [38].e �rst crystal structures of the uranium(VI) complex with calix [6]arene were reported by uéry et al. [39].In the complex whose formula was [(UO [6]arene, each UO 2 2+ was bound to four oxygen atoms (two from each calixarene moiety).e mean value of O-U-O angle is 90 ∘ , suggesting that atom of uranium(VI) had octahedral centre of coordination.e conformation of the calix [6]arene was slightly distorted with respect to the usual pinched cone conformation of calix [6]arenes.e minus charge of the dimer was compensated by two protonated triethylamine molecules and two hydronium ions.Triethylamine molecules were outside of the calixarene in comparison to hydronium ions, which were inside the cavity de�ned by each calixarene.In the complex hydrogen, bonds were formed between the hydronium ions and two phenolic oxygen atoms nonbonded to the UO 2 2+ and with the nitrogen atom of one acetonitrile molecule, which was included in the hydrophobic cavity of the calixarene.
Later, uéry and Masci [40] studied the hetero(triand tetra-)nuclear complexes of UO 2 2+ and alkali metal (Li + and K + ) with p-tert-butylhexahomotrioxacalix [6]arene (L).e authors synthesized in the presence of lithium or potassium hydroxide the following complexes: . e complex, which was obtained in the presence of lithium hydroxide, contained the trimetallic dianionic species, in which three cations were complexed in internal fashion.e two UO 2 2+ were bound to three phenoxide groups and the central hydroxide ion.e lithium ion was bound to two phenoxide groups and one other oxygen atom and the pyridine molecule.e geometry of coordination center of uranium(VI) could be described as distorted square bipyramidal environment.
e complex, which was obtained in the presence of potassium hydroxide, was built of an asymmetric unit, which contains [UO 2 K(LH 3 )(H 2 O) 2 ] 2 heterobinuclear unit and pyridine solvent.UO 2 2+ was bound to three phenoxide groups and the potassium ion to two phenol groups and one of ether groups of p-tert-butylhexahomotrioxacalix [6]arene.To obtain the usual square bipyramidal geometry around the uranium atom, the UO 2 2+ completed its coordination sphere with a water molecule.e conformation of macrocycle could be described as a double partial cone.In comparison to the lithium complex, the potassium complex was dimeric.e next crystal structure described in the literature was [(HO){UO 2 (calix [6]H 4 )(dmso)} 3 H]⋅11MeCN⋅6H 2 O synthesized by Delaigue et al. [41].e conformation of calixarene ligand in this complex could be described as distorted cones.e complex was trinuclear.Uranium atoms were linked in their equatorial planes symmetrically by a "hydroxyl" oxygen atom, disposed on a crystallographic 3-axis in cubic space group P2 1 3. e equatorial plane of uranium(VI) was of �ve coordinates; one of these sites was occupied by DMSO oxygen.Next two sites were occupied by adjacent phenoxyoxygen of calixarene ligand.In this case, UO 2 2+ was bound to the calixarene in an external manner.
ere have been well-known structures of uranium(VI) complexes with calixarenes in a solid state, but not in a solution.e two spectroscopic methods described below may overcome the analytical problems, providing insight into the structure of complexes studied.
TRLFS is a very selective and sensitive method for actinide and lanthanide analysis because it can offer the spectral and temporal resolution together.is method provides information on lifetime and spectral characteristic of species, which can be used to obtain the number of different species and their spectral identity [42].TRLFS has been widely used to investigate the speciation of the �uorescent metal ions, extracting quantitative and structural information from multiple TRLFS data measured as a function of chemical and physical parameters.e distinguishing ability of TRLFS relies on the fact that different chemical species of a �uorescent metal ion have different lifetimes of �uorescent and spectral shapes.e estimation of the number of different species, their concentrations, and their chemical or structural information from TRLFS data is a difficult task, due to overlapped spectra and similar �uorescence lifetimes.e development of statistical techniques such as two-way factor analysis (FA), evolving factor analysis (EFA) combined with multivariate curve resolution (MCR), or parallel factor analysis (PARAFAC) can overcome such difficulties [43].TRLFS was used by Schmeide et al. [44] to study the complexation of uranium(VI) by calix [6]arenes in water and organic solvents.EXAFS spectroscopy is a very useful method for structural studies of organometallic complexes in solution.is method can be used to probe a local structure providing information on the number of the adjacent atoms, their type, and their lengths from the absorbing atom.Using EXAFS measurements in the investigation of the complexes of calixarenes with metals was reported [45,46].
It seems that the studies with spectroscopic techniques (EXAFS, TRLFS) could provide insight into the coordination models of novel calixarene complexes with uranium(VI).

Kinetic Studies of the Complexation Uranium(VI) with Calixarenes
Despite of very high selectivity  uranyl / M  and remarkably large stability constants, calixarenes have a very slow binding rate with uranium(VI) [47].is is because uranium(VI) is a linear guest cation.In order to permit UO 2 2 to penetrate the calixarene ring �rst the exocomplex [16] is created and �nally the endocomplex [48,49] (Figure 2) is formed.
Nagasaki et al. [49] investigated the difference in the kinetics between the calixarene and noncyclic analog pentasodium 2,6-bis{[2-hydroxy-3-(2-hydroxy-3-methyl-5-sulfonate-(phenyl))methyl-5-sulfonate(phenyl)]methyl}-4-sulfonate(phenol) (1) (Table 1).In case of the noncyclic ligand, the rate constants were greater than those for the cyclic ones.However, the examination of kinetic parameters revealed that rapid equilibration in case of noncyclic analogs was not only due to the fast forward complexation rate but also due to the more enhanced reverse decomplexation rate.e surprising effect was observed with pentasodium 31,32,33,34,35-pentahydroxycalix [5] (3).e authors reported that in contrast to calix [6]arene where at pH 6 all six OH groups were dissociated, in calix [5]arene only four groups were dissociated [49].e remained groups dissociated scarcely at pH 9 to form the fully saturated pentacoordinate complex.In conclusion, the initial state and �nal state in the binding to UO 2 2 were both destabilized, and the calix [5]arene ring was a priori distorted.In this connection, Nagasaki et al. [49] pointed out that calix [5]arenes and the noncyclic analog are better extraction agents for UO 2 2 than calix [6]arenes.Later studies showed stable complexes of uranium(VI) with calix [6]arenes in exoconformation [39].

Application of Calixarenes in Separation of Uranium(VI)
5.1.Liquid-Liquid Extraction.e separation of metal ions from a solution and selective removal of particular cations are of interest in a variety of �elds such as recovery of precious metal ions from the waste and hydrometallurgy.e separation methods that are oen used include solvent extraction, chromatography, and membrane separation.Two-phase solvent extraction of uranium(VI) with calixarene derivatives has been reported by several groups.is review concerns the uranophile properties of calix [5]arene and calix [6]arene derivatives.
Shinkai et al. [50] noticed that calix [5]arene and calix [6]arene had an ideal architecture for the design of uranophiles because the introduction of ligand groups into each benzene unit of these calixarenes exactly provided the required pseudoplanar penta-and hexacoordinate structures.ey synthesized several water-soluble calixarene derivatives from calix[n]arenes (    ) (4, 2, and 3) (Table 1).ey found that the
e selectivity of uranophiles can be evaluated by competitive binding with other metal cations.e results are presented in Table 3.
Examination of Table 4 reveals that 16 and 18 (entries 3, 4, 7, and 8) exhibited excellent extractability (Ex%).e slight decrease in extractability at higher pH was attributed to the hydrolysis of UO 2 (CH 2 CO 2 ) 2 .e low extractability for 15 and 17 was thus attributed to the p  of the OH groups, which is sensitive to the nature of parasubstituents.e values were lowered by electron-withdrawing groups for example, sulfonate group, so ligands such as 2 and 3 were able to associate well with UO 2 2+ at neutral to slightly basic pH [50].In contrast, the p  values of 15 and 17 were too high to use them as uranophiles in this pH region.Also in contrast, as the p  values of 16 and 18 were scarcely affected by the nature of the parasubstituents, they behaved as excellent uranophiles at pH 8-10 [57].
On the other hand, the compound 19, bearing hydroxamate groups on the lower rim, was an excellent uranophile even in the acidic pH region (Table 4, entry 10).is pH dependence was correlated with the dissociation of the hydroxamic groups and binding to UO 2 2+ .It is worth noting that calixarene-based uranophiles from 15 to 18 did not leak out into the aqueous phase.It was con-�rmed on the basis of spectrophotometric method [57].e main difference between the homogeneous aqueous system and two-phase solvent-extraction system is that the species extracted into the organic phase must be "neutral" [59].e above �ndings suggest, therefore, that calix [6] 2+ adsorbed as countercations [57,58].
In Table 5, the selectivity of hydrophobic calix [6]arenes is shown.Although calix [6]arenes 15, 16, and 19 could efficiently extract UO 2 2+ from aqueous solution to organic phase, the selectivity factors in two-phase solvent extraction were not so excellent as those obtained with 3 in aqueous system (Table 3 ) with an apparent extraction constant equal to 7.1 × 10 −5 M (  0.04 M).Further, the compound 21 has been found as an extractant molecule for selective separation of plutonium(IV) from uranium(VI) [62].Both elements were selectively extracted depending on the aqueous phase pH (Table 6).At the �rst step, 93% of plutonium was extracted at pH 2 by 21  a Organic solvent: 1,2, without extraction of uranium.en, increasing the pH of the aqueous phase to pH 5 allowed the quantitative extraction of uranium(VI) by 21.However, it should be noted that at this pH, more than 8% of the remaining plutonium(IV) was also extracted.It is necessary to note that the Pu(IV) is a spherical cation of Pu 4 , not a trans-dioxo cation like PuO 2 2 which is isostructural with the UO 2 2 .e extraction mechanism of UO 2 2 by calixarene 20 has been studied in a two-phase solvent-extraction system [63].e competition between calix [6]arene and other complexing anions, such as phosphate or sulfate, has been investigated.e results showed that UO 2 2 extraction was independent of the cation concentration, whatever the pH and the ionic strength were.e UO 2 2 -calixarene extraction constant was higher than that of the UO 2 2 -anion complexation.
e strong ability of calixarene nanoemulsion to extract UO 2 2+ was found by Spagnul et al. [65,66].ey designed an oil-in water emulsion, taking advantage of the small droplet size offering a large contact surface with contaminated aqueous medium.e calixarene 20 nanoemulsion extracted up to 80% of uranium.It is important that the calixarene nanoemulsion effect was observed aer a very short time of contact with uranium-contaminated solution.e calixarene nanoemulsion appeared to be a very promising system for uranium skin decontamination [65].
Aer adsorption of UO 2 2+ onto goethite, its extraction with the compound 24 in aqueous environment was studied.e results indicated that UO 2 2+ was almost completely extracted around pH 10.5 when U(VI) : calixarene ratio was 1 : 3.

Extraction with Liquid Membranes.
In order to selectively remove and recover metals from aqueous solution, the membrane-based processes are still being developed.
e selective transport of UO 2 2+ across the liquid membrane with hydrophobic derivative of calix [6]arene 16 as the carrier was studied by Shinkai et al. [57].e authors examined the transport rate from H 2 O (source phase)organic solvent-H 2 O (receiving phase).ey found that the rate determining step was the UO 2 2+ release from organic solvent to the receiving aqueous phase.e extent of UO 2

2+
transport was efficiently controlled by the changes in pH and temperatures.Later Kondo et al. [70] investigated selective transport of UO 2 2+ through a bulk liquid membrane (BLM) containing hydrophobic ion associate of methyltrioctylammonium and hydroxycalix [6]arene-p-sulfonates (25) diluted in chloroform.e authors observed that the extraction rate of UO 2 2+ increased with increasing pH in the source phase as well as with increasing temperature in the temperature range 15-55 ∘ C. is effect could be explained by increasing the concentration of dissociated carrier 25. e developed system was not suitable for temperatures higher than 55 ∘ C since bubbles were generated in the membrane and sometimes the boiling solvent broke the layer structure of the interfaces of both aqueous phases with the membrane.e other ions, such as Na + , K + , Mg 2+ , Ca 2+ , Sr 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Fe 3+ , La 3+ , Eu 3+ , and Lu 3+ , are not transported through the BLM, and therefore UO 2 2+ can be separated selectively.
A facilitated selective transport of UO 2 2+ across a BLM with the compound 15 in chloroform as ion carrier was studied by Ramkumar et al. [71] as well.With this system uranium(VI) with crown ethers as UO 2 2+ -18C6 was effectively separated from Cu 2+ , Co 2+ , Ni 2+ , and Zn 2+ .ey con�rmed that the transport of UO 2 2+ increased with the increase of the acidity of the receiving solution.e use of excess of crown ether 18C6 in proportion to calixarene (100 : 1) allowed decreasing pH in the receiving phase without loose of the extraction yield.It was caused by better solubility of uranyl ion in the membrane phase.e hydrophobic crown ether 18C6 plays a role in the absorption of uranyl ion into hydrophobic phase by bonding to coordination sphere of the metal ion.e authors [71] suggested that crown ether is involved in the secondary coordination sphere of the uranylcalixarene complex.

Ion Exchange and Chelating
Resin.e separation of  4+ , Ce 4+ , and UO 2 2+ by polymer supported calix [6]arene hydroxamic acid 26 as a novel chelating resin was investigated by Trivedi et al. [72].e novel resin was stable against light, air, water, and temperature up to 210 ∘ C. With this system,  4+ , Ce 4+ , and UO 2 2+ were effectively separated by changing pH of sorption or phase elution.e yield of the recovery at pH 6.5 of the sorption was (99.1 ± 0.3)% for  4+ (in this pH region thorium ions were not sorbed), (98.2 ± 0.2)% for Ce 4+ with 0.1 M HCl as an eluent, (97.6 ± 0.2)% for UO 2 2+ with 2 M HCl as an eluent.

2+
from aqueous solution by mono-p-nonyl-penta-p-tert-butylcalix [6]arene hexacarboxylic acid (28) modi�ed textiles.e best results were obtained for separation of UO 2 2+ at pH 7 (97.8%)and pH 5 (94.9%).ey studied also the in�uence of other ions such as sodium, potassium, magnesium, and sulfate on the separation of UO 2 2+ by calixarene modi�ed textiles.ey found that neither at pH 5 nor at pH 7 a change of the separation yield of UO 2 2+ was observed.A different situation was observed in the case of calcium-and carbonate-rich waters.At pH 5 there was no signi�cant change, but at pH 7 UO 2 2+ separation was reduced from 92% to 9%.is was due to the formation of aqueous complex Ca 2 UO 2 (CO 3 ) 3 .
e best adsorption of uranium(VI) was observed in the pH range from 5 to 7. At lower pH, the adsorption was low because of the competition of hydronium ions with the UO 2 2+ for the adsorption sites.At higher pH, uranium(VI) becomes hydrolysed to form oligomeric species (see Section 2).

The Potential Industrial Application of
Calix [6]arenes for Uranium(VI) e well-known high affinity of the calixarenes to UO 2 2+ make base for industrial use of them as reagents for the recovery of uranium from water solutions or removal from liquid waste generated from uranium mining and processing.e patent from 1988 [77] concerns the use of calixarenes as adsorbent for UO 2 2+ .is adsorbent consisted of polyethyleneimine, which had calixarene residue 31 bonded to the side chain by SO 2 Cl groups.It was characterized by an excellent affinity and selectivity.
In 1992, the patent for a calixarene derivative 32 [78] was published.is new compound has been useful as a host compound.Another ion or neutral compound could be bound in its cavity.Such compounds have shown excellent adsorption properties even at low pH which indicates that they can be used for adsorption and selective recovery of uranium(VI) from sea water or waste water, and so forth.
e invention described in American patent [79] was related to supported liquid membranes.e supporting materials contain the novel calixarenes with formula 33 and 34, which can be used for analytical measurements of uranium, americium, and plutonium.e main function of these materials is to extract the above-mentioned actinides from complex matrices, such as biological media.

Conclusions
Calix [6]arenes represent a family of macrocyclic molecules with a broad range of potential applications in chemical, analytical, and engineering materials �elds.Low toxicity of these compounds makes them useful in applications of green chemistry and eco-friendly industrial processes.
e present paper demonstrated that calix [6]arenes act as very good uranophiles in different �elds including separation processes (e.g., solvent extraction, membrane transport, and chromatographic process).ey may be useful for extracting UO 2 2+ from solutions aer leaching of uranium ores, sea water, radioactive waste, or natural soil.It was revealed that calixarenes bearing ligands including P=O groups are more efficient than TBP (tributyl phosphate), TOPO (trioctylphosphine oxide), and CMPO (carbamoylphosphonate) for extraction of lanthanides and actinides.Application of such extracting agents enables the use of 10 to 100 times lower ligand concentrations necessary to reach an assumed extraction yield than with the other existing extractants.Extensive literature studies showed that extraction ability and selectivity of calixarene derivatives is closely related to their structural arrangement.Easy functionalization of these compounds enables simple engineering of the calixarene derivatives and preparation of the materials of good affinity to speci�c forms of uranium like UO 2 2+ .
Journal of Chemistry e concept of uranophile calixarenes for liquid-liquid extraction and BLMs was studied with different calixarenes modi�ed at the narrow and wide rims.e high effectiveness of calix [6]arenes in the process of separation of uranium(VI) from aqueous solutions was shown.It is necessary to notice that many factors, such as pH, temperature, presence of other metal ions and counter ions, and the kind of solvent applied, affect the performance of the extraction of UO 2 2+ .In most of the processes described above, the halogenated solvents were used.ey are not acceptable in the industrial chemistry due to their cost and toxicity.
Ion exchange seems powerful technique, which can be used to separate uranium from other chemical substances.Similar to solvent extraction and BLMs transport, it uses the reversible reaction causing all UO 2 2+ to be reextracted.e advantages of ion exchange technique are lack of the organic solvent and the possibility of the control of the composition of the solutions by change of the eluent pH.

F 1 :
Illustration of the structure of calixarenes.

Table 2 ,
entries 16 and 17) T 5: Solvent extraction of UO 2 2+ in the presence of competing metal cations at 30 ∘ C and at pH 5.9.
T 7: Extraction of UO 2 2+ from nitric acid solutions into -NBTF by phosphorylated calixarenes.Distribution coefficients  a .SF: synergistic factor, SF =  syn /( HTTA +  CALIX ), where  syn is the distribution coefficient of the synergistic mixture. c