Extraction of Cesium and Some Typical Metals with a Supramolecular

Dec 10, 2012 - A macrocyclic supramolecular recognition agent, 1,3-bis(1-nonyloxy)-2,4-crown-6-calix[4]arene (NonCalix[4]C6), was synthesized...
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Extraction of Cesium and Some Typical Metals with a Supramolecular Recognition Agent 1,3-Bis(1-nonyloxy)-2,4-crown-6calix[4]arene Wenwen Zhang,† Anyun Zhang,*,† Lei Xu,† and Zhifang Chai†,‡ †

Department of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, P. R. China Institute of High Energy Physics, Chinese Academy of Sciences, P.O. Box 918, Beijing 100049, P. R. China

‡

ABSTRACT: A macrocyclic supramolecular recognition agent, 1,3-bis(1-nonyloxy)2,4-crown-6-calix[4]arene (NonCalix[4]C6), was synthesized. The extraction of Cs(I), a heat generator, and some typical fission and nonfission products, La(III), Y(III), Rb(I), Mo(VI), Zr(IV), Sr(II), Ba(II), Ru(III), Na(I), K(I), and Pd(II), with NonCalix[4]C6/1-octanol in HNO3 medium was investigated. The effects of contact time, the concentration of HNO3 in the range of (0.4 to 5.0) mol·kg−1, and temperature were evaluated. NonCalix[4]C6 showed excellent extraction ability and high selectivity for Cs(I) over the tested metals except Rb(I), which was ascribed to the effective recognition of NonCalix[4]C6 for Cs(I). The optimum acidity in the extraction of Cs(I) was 4.0 mol·kg−1 HNO3. The extraction property and mechanism of Cs(I) were discussed. The composition of the extracted species of Cs(I) with NonCalix[4]C6 was determined as CsNO3·NonCalix[4]C6. Valuable chemical engineering parameters in the extraction of Cs(I) were obtained. It is beneficial to effectively partition Cs(I) from highly active liquid waste with the NonCalix[4]C6/1-octanol extraction system.

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INTRODUCTION One of the purposes in the reprocessing of nuclear spent fuel is to partition and recover uranium and plutonium with the tri-nbutyl phosphate/kerosene extraction system by Purex or its modified extraction process.1 As a result, an extremely complicated solution with a strong radioactivity and high HNO3 concentration, so-called highly active liquid waste (HLW), was produced. Fission and nonfission products, such as the long-lived minor actinides MAs(III), rare earths REs(III), heat generators Cs(I) and Sr(II), noble metals, Mo(VI), Tc(VII), and Zr(IV), and others are contained in HLW.1 To significantly reduce the potential risk of HLW to the environment, elimination of these metals is valuable. However, effective partitioning of the related metals has always been one of the most challenging works in recent years.2 Most attention has been focused on the separation of MAs(III), such as Am(III) and Cm(III), based on the partitioning and transmutation (P/T) strategy.3−11 Insufficient attention is concentrated on the separation of heat-emitting nuclides, Cs(I) and Sr(II).12−14 It is reported that Cs-137 and Sr-90 are harmful to the safety of vitrified HLW in final geological disposal. Cs-135 might have long-termed potential risks for the environment. In addition, it is said that the removal of Cs-135 and Cs-137 from HLW is beneficial for the significant reduction of the need for cooling fission products containing solutions and the amount of time of vitrified waste storage. On the other hand, Cs-137 and Sr-90 are soft β-emitters; they can be used as the resources of β-radiation and energy generators in industrial determination or hospital examination. The separation and recovery of the heat emitting © 2012 American Chemical Society

nuclides from HLW are therefore meaningful. However, similar to the separation of the long-lived minor actinides, the partitioning of Cs(I) and Sr(II) has also been a challenge up to now. The calix[4]arene-monocrown as shown in Figure 1 is a kind of supramolecular recognition agent. It is composed of a

Figure 1. Molecular structure of supramolecular recognition agent calix[4]crown−crown.

calix[4]arene, a crown ether, and two alkyl substituting groups. They are bonded together through the phenolic oxygens of the calix[4]arene and a polyether chain by the p-basic phenolic cavities, CH−p or p−p-stacking interactions.15−18 Based on its molecular interactions, calix[4]arene-monocrown has different conformations, for example, cone, partial cone, 1,2-alternate, 1.3-alternate, and others. This makes it possible for the calix[4]crown compounds such as some derivatives of 1.3Received: September 19, 2012 Accepted: November 27, 2012 Published: December 10, 2012 167

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Figure 2. SPEC process for strontium/cesium partitioning from HLW by extraction chromatography using two novel macroporous silica-based supramolecular recognition materials.

alternate calix[4]arene-monocrown-6 to have high selectivity for Cs(I), Sr(II), Am(III), Cm(III), and the other special metals. As a result, it is quite possible to remove Cs(I), a heat generator, from nuclear waste.19,20 Based on the special molecular structure of the calix[4]arenecrown compounds, a novel macroporous silica-based supramolecular recognition material (SSRM) has been developed by impregnation and immobilization techniques.21,22 This was performed through induction of calix[4]arene-crown into the pores of the macroporous SiO2−P particles by means of the molecular modification. In terms of the novel SSRMs, the advanced partitioning technology entitled the SPEC (Strontium/cesium Partitioning from HLW by Extraction Chromatography) process as shown in Figure 2 has been proposed.23 Cs(I) and Sr(II) were able to separate by the calix[4]arenecrown/SiO2−P and 4,4′,(5′)-di(tert-butyl-cyclohexano)-18crown-6 (DtBuCH18C6)/SiO2−P.24−28 The long-lived MAs(III) and others showed no adsorption onto these silica-based materials and then flowed into effluent along with Pd(II), Zr(IV), Mo(VI), Rh(III), Ru(III), REs(III) (Y and La to Lu), Na(I), K(I), Fe(III), MAs(III) (Am and Cm), and 2.0 mol·kg−1 HNO3 except Rb(I) and Ba(II).29 The satisfactory partitioning of Cs(I) and Sr(II) from HLW by extraction chromatography was achieved. However, some fundamental investigations such as the complexation, the composition of the complex of Cs(I) with these novel macroporous SSRMs, and the others have not been understood yet. Seeking an effective pathway to make the complexation of Cs(I) with the calix[4]arene-crown compounds clear is valuable. To understand the complexation of Cs(I), the objective of the present work was focused on as follows: (1) a new supramolecular recognition agent, 1,3-bis(1-nonyloxy)-2,4crown-6-calix[4]arene (NonCalix[4]C6), was synthesized. It was prepared by a complicated synthesis technical route. (2) The extraction of some typical fission and nonfission products Rb(I), Cs(I), Na(I), K(I), La(III), Y(III), Mo(VI), Sr(II), Ba(II), Zr(IV), Ru(III), and Pd(II) by NonCalix[4]C6/1-

octanol was investigated by examining the effects of contact time, the HNO3 concentration in the range of (0.4 to 5.0) mol·kg−1, and temperature. (3) The extraction mechanism and the composition of the extracted species of Cs(I) with NonCalix[4]C6 were discussed. The technical possibility and feasibility of application of the NonCalix[4]C6/1-octanol extraction system in effective partitioning of Cs(I) from HLW were evaluated.

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EXPERIMENTAL PROCEDURE Materials. The materials used were as follows: alkali metal nitrates MINO3 (MI = Na, K, Cs, and Rb), alkaline earth nitrates MII(NO3)2 (MII = Sr and Ba), RE(NO3)3·nH2O (RE = La and Y, n = 3 or 6), ZrO(NO 3 ) 2 ·2H 2 O, and (NH4)6Mo7O24·4H2O; all of analytical grade. Palladium nitrate solution was provided by the Tanaka Noble Metal Co. Inc. (Omuta City, Japan) and ruthenium nitrosyl nitrate solution by Strem Chemicals (Newburyport, MA, USA). The ionic strength in aqueous phase in the determination of the complex composition of Cs(I) was maintained by addition of a stock solution of NaNO3. The concentrations of the metals in HNO3 solution were around 5.0·10−3 mol·kg−1.23,25,28,30−32 A supramolecular recognition agent, 1,3-bis(1-nonyloxy)-2,4crown-6-calix[4]arene (NonCalix[4]C6) as shown in Figure 3, was synthesized quantitatively through a complicated technical route.28 The diluent, 1-octanol, was an available commercial

Figure 3. Structure of 1,3-bis(1-nonyloxy)-2,4-crown-6-calix[4]arene (NonCalix[4]C6). 168

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Table 1. Purity of the Tested Chemical Reagents chemical name palladium nitrate ruthenium nitrosyl nitrate NonCalix[4]C6 a

source

initial mole fraction purity

Tanaka Noble Metal Co. Inc. (Omuta City, Japan) Strem Chemicals (Newburyport, MA, USA)

purification method

1.127·10−2

none

2.972·10−3

none

Synthesis

column chromatography

final mole fraction purity

analysis method

0.9998

GCa

Gas−liquid chromatography. The expanded uncertainty of the results has a level of confidence of 0.95.

product. It was used to dissolve the extractant NonCalix[4]C6 in organic phase. Because of the excellent solubility of NonCalix[4]C6 in 1-octanol, no other additive or modifier was introduced. The other organic and inorganic reagents were of analytical grade and were used without further treatment. The purities of the tested chemical reagents such as palladium nitrate, ruthenium nitrosyl nitrate, and NonCalix[4]C6 are listed in Table 1. Synthesis of NonCalix[4]C6. The quantitative preparation of NonCalix[4]C6 was carried out according to the following steps: (1) The synthesis of 5,11,17,23-tetrabutyl-25,26,27,28tetra-hydroxy-calix[4]arene (TBTHCalix[4]arene) by the polymerization of p-tert-butylphenol in formaldehyde was carried out by catalyzation with sodium hydroxide in diphenyl ether under refluxing in nitrogen atmosphere for 5 h at 533 K. (2) For the preparation of 25,26,27,28-tetrahydroxy-calix[4] arene (THCalix[4]arene) in toluene and phenol, the purpose was to remove tetrabutyl group from TBTHCalix[4]arene. An anhydrous AlCl3 was used as dryer for 2 h to eliminate the microamount of water in the reaction system. (3) A 1,3alternate intermediate, 25,27-di(1-nonyloxy)-26,28-dihydroxycalix[4]arene (NonCalix[4]arene), was synthesized by a substitutional reaction between THCalix[4]arene with 1CH(I)(CH2)7CH3 in acetonitrile through addition of potassium carbonate conducted at 355 K under refluxing with a nitrogen atmosphere. Following the elimination of acetonitrile, dissolution with dichloromethane, extraction with 10 % hydrochloric acid solution, and separation and purification by column chromatography packed with silica gel, the product NonCalix[4]arene was obtained. (4) Pentaethylene glycol ditosylate was prepared via the reaction of 1,2-bis[2-(2hydroxyethoxy)ethoxy]ethane and toluenesulfonyl chloride in tetrahydrofuran (THF) for 4 h below 273 K. After treatment by 2.0 mol·kg−1 sodium hydroxide under stirring, extraction with dichloromethane, and washing with 2.0 mol·kg−1 hydrochloric acid and water, the target product was obtained. (5) NonCalix[4]C6 was synthesized through the 1,3-alternate cyclization of NonCalix[4]arene with pentaethylene glycol ditosylate in acetonitrile. It was catalyzed by cesium carbonate under refluxing in nitrogen atmosphere for 24 h at 355 K. The resultant NonCalix[4]C6, a white product, was then characterized by elementary analysis, 1H NMR, and EI-MS. The appearance of NonCalix[4]C6 is shown in Figure 4. The total synthesis technical route of NonCalix[4]C6 is illustrated in Figure 5. Measurement of the Distribution Ratio. The solvent extraction of the tested metals such as Sr(II), Ba(II), Na(I), K(I), Rb(I), Cs(I), Zr(IV), Ru(III), La(III), Y(III), Mo(VI), and Pd(II) with NonCalix[4]C6/1-octanol was performed at 298.1 K except that of the experiment of the temperature effect. It was carried out and controlled by TAITEC MM-10 model thermostatted water bath.

Figure 4. Appearance and scanning electron microscope (SEM) photos of NonCalix[4]C6.

Equal volumes (5 mL) of a HNO3 solution containing the tested metals as an aqueous phase and of a NonCalix[4]C6/1octanol solution as an organic phase were mixed into a ground glass-stopped equilibration tube, which was then shaken mechanically at 120 rpm. Prior to the extraction performance the organic phase was pre-equilibrated with HNO3 of the same concentration without containing metals. The preliminary studies showed that the extraction equilibrium was established within 5 min. To ensure that the full equilibrium was reached, the contact time was extended to 30 min. The concentrations of NonCalix[4]C6 and the tested metal and the ratio of aqueous phase to organic one were 2.0·10−2 mol·kg−1, 5.0·10−3 mol·kg−1, and 1:1. The concentration of HNO3 in the aqueous phase was in the range of (0.4 to 5.0) mol·kg−1. The ionic strength in aqueous phase was kept constant by addition of a stock solution of sodium nitrate. After the phases were centrifuged and separated, the measurement of the contents of the tested metals in the aqueous phase proceeded, using a Varian 700-ES model simultaneous inductively coupled plasmaoptical emission spectrometer (ICP-OES, Varian, Inc., Palo Alto, CA, USA) with the exception of the concentrations of Cs(I), Na(I), Rb(I), and K(I), which were analyzed using a Varian AA 240 FS model atomic adsorption spectroscope, similar to a method reported previously. Distribution ratios (DM) of the tested elements were calculated using the following equation:23,25,28,30−32 DM = C(o)/C(a)

(1)

In eq 1, C(o) and C(a) represent the equilibrium concentrations of the tested metals in organic phase and the aqueous phase, respectively.

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RESULTS AND DISCUSSION Dependence of the Tested Metal Extraction on Contact Time. It is found that the structure of the 1,3alternate supramolecular recognition agent NonCalix[4]C6 contains a cavity capable of recognizing metal ions, which is constructed by the synergetic interaction of calix[4]arene and crown ether. It was reported that, in the match of the size of 169

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Figure 5. Total technical route of the NonCalix[4]C6 synthesis.

Table 2. Comparison of the Distribution Ratio of the Tested Metals with the Change in Contact Timea distribution ratio, DM

time min

Cs(I)

Rb(I)

Na(I)

K(I)

Sr(II)

Ba(II)

La(III)

Y(III)

Pd(II)

Zr(IV)

Mo(VI)

Ru(III)

1 3 5 7 10 20 30 60 90

3.40 7.14 7.19 7.08 7.06 7.08 7.30 7.21 7.43

0.78 1.36 1.21 1.30 1.25 1.28 1.34 1.33 1.34

0.60 0.51 0.56 0.55 0.16 1.52 1.21 1.19 1.04

0.01 0.02 0.01 0.04 0.05 0.02 0.04 0.06 0.02

0.01 0.02 0.01 0.01 0.03 0.01 0.02 0.02 0.01

0.01 0.01 0.01 0.02 0.02 0.02 0.01 0.01 0.01

0.00 0.00 0.01 0.01 0.02 0.01 0.01 0.00 0.01

0.01 0.01 0.00 0.00 0.01 0.01 0.00 0.02 0.01

0.05 0.08 0.10 0.14 0.13 0.11 0.11 0.14 0.05

0.01 0.02 0.01 0.01 0.02 0.02 0.02 0.01 0.01

0.29 0.43 0.43 0.43 0.48 0.46 0.40 0.44 0.45

0.04 0.10 0.11 0.14 0.13 0.10 0.14 0.14 0.13

a Conditions: T = 298.1 K, [NonCalix[4]C6](o) = 1.0·10−2 mol·kg−1, [Metal](a) = 5.0·10−3 mol·kg−1, [HNO3](o) = 4.0 mol·kg−1, phase ratio = 1:1. The expanded uncertainty of the results has a level of confidence of 0.95.

excess of 3 min was always greater than 7. It was ascribed to the effective complexation of NonCalix[4]C6 for Cs(I). Meanwhile, the tested Sr(II), Ba(II), and K(I) showed almost no extraction with NonCalix[4]C6. Their distribution ratios were less than 0.01 except Na(I) and Rb(I). It was attributed to the weak complexation of the tested metals with NonCalix[4]C6. The high supramolecular recognition ability of NonCalix[4]C6 for Cs(I) in solvent extraction was confirmed. The high distribution ratio (DM) of Cs(I) with NonCalix[4]C6/octanol to the effective molecular recognition of NonCalix[4]C6 based on the size matching between host and guest was examined. It was found to make a strong chemical complexation of NonCalix[4]C6 for Cs(I). No extraction of Sr(II) and Ba(II) revealed that, in the tested HNO3 solution, all alkaline earths might have no adverse impact on the Cs(I) extraction with NonCalix[4]C6. Namely, these metals might be no adverse impact on the extraction separation of Cs(I). It is of great beneficial to recognition and extraction of Cs(I) with NonCalix[4]C6/octanol system in HLW. In comparison with the high extraction of Cs(I), it was found that, in the whole tested contact time range, the tested Zr(IV), Pd(II), Ru(III), La(III), Y(III), and Mo(VI) showed weak or almost no extraction with NonCalix[4]C6, while their distribution ratios (DM) were always smaller than 0.4. It

metal ion and the cavity of the calix[4]arene-crown, the supramolecular recognition agent usually had strong complexation for the metal. This makes the separation and concentration of a certain metal ion from others possible. In addition, the alkyloxy group, −OR with a long carbon chain, in calix[4]arene-crown is helpful for significantly increasing its lipophilicity in the organic phase. As a result, it increases the recognition ability and selectivity of calix[4]arene-crown for the certain metal ion. To understand the supramolecular recognition property of calix[4]arene-crown well, the dependence of the extraction of some typical fission and nonfission products such as Na(I), K(I), Rb(I), Cs(I), Sr(II), Ba(II), Zr(IV), Pd(II), Ru(III), La(III), Y(III), and Mo(VI) with NonCalix[4]C6/1-octanol in 4.0 mol·kg−1 HNO3 with a change in contact time was investigated at 298.1 K. The results are listed in Table 2. Table 2 shows the dependence of the extraction behavior of some typical alkali metal and alkaline earths with the NonCalix[4]C6/1-octanol system on contact time in 4.0 M HNO3. As can be seen, the extraction of Cs(I) with NonCalix[4]C6 increased at initial 3 min and then kept constant with increasing contact time, reflecting that the extraction equilibrium of Cs(I) was reached. This implied that, in HNO3 medium, NonCalix[4]C6 had fast extraction dynamics for Cs(I). The distribution ratio (DM) of Cs(I) in 170

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Table 3. Comparison of the Distribution Ratio of the Tested Metals with the Change in the HNO3 Concentration in the Aqueous Phasea distribution ratio, DM

HNO3 concentration mol·kg 0.4 1.0 2.0 3.0 4.0 5.0

−1

Cs(I)

Rb(I)

Na(I)

K(I)

Sr(II)

Ba(II)

La(III)

Y(III)

Mo(VI)

Ru(III)

1.88 3.11 5.37 6.14 7.19 5.90

0.17 0.40 0.86 1.21 1.33 1.04

0.21 0.33 0.45 0.67 1.10 0.65

0.04 0.01 0.00 0.06 0.11 0.02

0.01 0.00 0.00 0.23 0.00 0.00

0.02 0.01 0.04 0.06 0.07 0.04

0.00 0.00 0.01 0.03 0.05 0.03

0.01 0.03 0.02 0.05 0.06 0.04

0.05 0.12 0.30 0.40 0.44 0.51

0.00 0.00 0.03 0.08 0.25 0.28

a Conditions: T = 298.1 K, [NonCalix[4]C6](o) = 1.0·10−2 mol·kg−1, [Metal](a) = 5.0·10−3 mol·kg−1, phase ratio = 1:1. The expanded uncertainty of the results has a level of confidence of 0.95.

indicated that, in 4.0 mol·kg−1 HNO3, these fission products were not effectively recognized with NonCalix[4]C6. Therefore, it could be concluded that these earths may have no adverse impact on the extraction separation of Cs(I). It is known that, due to the lanthanide contraction, the chemical properties of rare earths are very similar. From this viewpoint, the tested La(III) and Y(III) were used to understand the extraction behavior of all of trivalent rare earths, REs(III). In comparison with Cs(I), the weak or almost no extraction of La(III) and Y(III) showed that, in HNO3 medium, NonCalix[4]C6 may have no extraction for all REs(III). In reprocessing of nuclear spent fuel, REs(III) are usually contained in acidic HLW, which implies that the 16 species of REs(III) including Y(III) and La(III) to Lu(III) may be not extracted with the NonCalix[4]C6/octanol extraction system. As a result, it is quite possible that REs(III) have no adverse impact on the extraction of Cs(I) with NonCalix[4]C6. On the other hand, the long-lived minor actinides such as Am(III) and Cm(III) usually exist in HLW. A survey of literature shows that “the ionic radii of the actinide elements decrease with an increase in atomic number, indicative of decreased shielding by f electrons of the outer valence electrons nuclear charge”. This process demonstrates the similiarity of the actinide and lanthanide contractions.23,27,30−32 Because of weak or almost no extraction of REs(III) with NonCalix[4]C6, it is possible that the NonCalix[4]C6/octanol extraction system shows weak or almost no extraction ability for the long-lived minor actinides. As a result, all rare earths and the minor actinides might show no adverse impact on the solvent extraction of Cs(I), which is valuable for the elimination of Cs(I) from the acidic HLW containing these elements. Based on the above discussion, the difference in the extraction of the tested metal ions exhibited that, in 4.0 mol·kg−1 HNO3, NonCalix[4]C6/octanol had an excellent extraction ability and high selectivity for Cs(I) over all of the tested elements except Na(I) and Rb(I). Dependence of the Tested Metal Extraction on the HNO3 Concentration. Calix[4]crown combines the advantages of calix[4]arene and crown ether. The O atom in the hydrophilic crown ether usually has strong affinity for nitric acid through hydrogen bonding. It was reported that, in the Purex process, the concentration of HNO3 in HLW generated is usually about 3.0 mol·kg−1. This concentration may have a certain effect on the recognition ability of calix[4]arene for metal ions. As a result, it is possible that the solvent extraction of the tested metals with NonCalix[4]C6 caused by the chemical complexation and the protonation of NonCalix[4]C6 with HNO3 would be two competing reactions.

To understand the effect of the HNO3 concentration, the extraction of a few typical FPs and non-FPs with NonCalix[4]C6 was performed in a wide HNO3 concentration range of (0.4 to 5.0) mol·kg−1. The impact of the change in the HNO3 concentration on the solvent extraction of the tested metals with NonCalix[4]C6 was evaluated. The corresponding results are listed in Table 3. Table 3 shows the dependence of the extraction of some typical fission and nonfission products with NonCalix[4]C6 on the HNO3 concentration in the range of (0.4 to 5.0) mol·kg−1. As can be seen, with increasing the HNO3 concentration, the distribution ratios (DM) of Mo(VI), Pd(II), La(III), Y(III), Ru(III), Na(I), K(I), Sr(II), Ba(II), and Zr(IV) with NonCalix[4]C6 were always less than 0.5, showing weak or no extraction. It reflected the coexistent metals might have no adverse impact on the solvent extraction and separation of Cs(I). The tested metals La(III) and Y(III) also showed almost no extraction with NonCalix[4]C6. Because the chemical properties of La(III) and Y(III) are very close to these of the other rare earths and trivalent minor actinides, it is possible that the trivalent minor actinides and all of lanthanides have no adverse impact on the Cs(I) extraction and separation. Opposite to the metals mentioned above, it was found that the distribution ratios (DM) of Cs(I) with NonCalix[4]C6 increased obviously from (0.4 to 4.0) mol·kg−1 HNO3 and then decreased to 5.0 mol·kg−1 HNO 3. The corresponding distribution ratios were 1.888 in 0.4 mol·kg−1 HNO3, 7.193 in 4.0 mol·kg−1 HNO3, and 5.940 in 5.0 mol·kg−1 HNO3. The optimum acidity of the Cs(I) extraction in the experimental conditions was therefore determined to be 4.0 mol·kg−1 HNO3. NonCalix[4]C6 showed an excellent extraction ability and high selectivity for Cs(I) over all of the tested metals. Of the alkali metals, Cs(I) usually shows comparative difficulty in forming a complex with the conventional complexing agents.23,25,27,32 The difference in the distribution ratios (DM) of the tested elements with NonCalix[4]C6 reflected that, in 4.0 mol·kg−1 HNO3 solution, Cs(I) was capable of being recognized, complexed by a hard-atom O contained in NonCalix[4]C6, and then formed the stable complex.33 The selectivity of NonCalix[4]C6 for Cs(I) was considered to match the size between the calix[4]arene cavity and Cs(I) ion as well as π-bonding interactions with the arene groups and structural reorganization of the molecule. Extraction Mechanism of Cs(I) with NonCalix[4]C6. If one assumes that, in HNO3 medium, the complex composition of the extracted species of Cs(I) with NonCalix[4]C6 is mCsNO3·nNonCalix[4]C6, then, the extraction equilibrium of Cs(I) with the supramolecular recognition agent NonCalix[4]C6 is expressed as: 171

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mCs+(a) + m NO3−(a) + n NonCalix[4]C6(o) ⇌ mCsNO3 ·n NonCalix[4]C6(o)

(2)

The extraction constant (Kex) can be described as: Kex = C Ex(o)/(C M m(a)C N m(a)C L n(o))

(3)

In eq 3, the symbols show that Ex = mCsNO3·nNonCalix[4]C6(o), M = Cs+, N = NO3−, and L = NonCalix[4]C6, respectively. The distribution ratio (DCs) of Cs(I) is DCs = mC Ex(o)/CM m(a)

(4)

According to eqs 3 and 4, the relationship between the distribution ratio and the extraction constant is described as: log DCs = n log C L(o) + (m − 1)log C M(a) + m log C N(a) + log mKex

Figure 7. Relationship between log DCs and log CM(a). M = Cs(I). Conditions: T = 298.1 K, [HNO3](a) = 4.0 mol·kg−1, [NonCalix[4]C6](o) = 1.0·10−2 mol·kg−1, phase ratio = 1:1.

(5)

In terms of eq 5, the distribution ratios (DCs) of Cs(I) were measured by varying the concentrations of NonCalix[4]C6 in organic phase at fixed the concentrations of Cs(I) and the other experimental conditions. A plot of log DCs versus log CL(o) yielded a straight line with a linear slope close to 1, that is, n = 1, as shown in Figure 6. Similarly, the impact of the Cs(I)

Figure 8. Relationship between log DCs and log CN(a). N = NO3−. Conditions: T = 298.1 K, [H+](a) = 4.0 mol·kg−1, [NonCalix[4]C6](o) = 1.0·10−2 mol·kg−1, [Cs(I)](a) = 5.0·10−4 mol·kg−1, phase ratio = 1:1. Figure 6. Relationship between log DCs and log CL(o). L = NonCalix[4]C6. Conditions: T = 298.1 K, [HNO3](a) = 4.0 mol·kg−1, [Cs(I)](a) = 5.0·10−4 mol·kg−1, phase ratio = 1:1.

On the basis of the above discussion, the reaction equation of the extraction of Cs(I) with NonCalix[4]C6 in 1-octanol is represented as follows: Cs+(a) + NO3−(a) + NonCalix[4]C6(o)

concentration on the distribution ratio (DCs) of Cs(I) was examined at constant concentrations of NonCalix[4]C6 and the others, and a plot of log DCs versus log CM(a) is shown in Figure 7. The slope of the resultant straight line is close to 0. It showed that the extracted species of Cs(I) and NonCalix[4]C6 was a kind of mononuclear complex in metal under the experimental conditions, that is, m = 1. The effect of the concentration of NO3− on the distribution ratio of Cs(I) was investigated at fixed the other experimental conditions. A plot of log DCs versus log CN(a) got a straight line with a linear slope close to 1, that is, m = 1, as shown in Figure 8. A 1:1 type of the extracted species between NonCalix[4]C6 and Cs(I) was formed. The composition of the complex of Cs(I) with NonCalix[4]C6 was therefore determined to be CsNO3·NonCalix[4]C6.

⇌ CsNO3·NonCalix[4]C6(o)

(6)

The equilibrium constant (Kex) of the extraction of Cs(I) was calculated, and the corresponding value logKex was 1.661. The structure of the extracted species, CsNO3·NonCalix[4]C6, is shown in Figure 9. To evaluate the extraction capacity (QCs(I)) of Cs(I), the dependence of the extraction of Cs(I) with NonCalix[4]C6 on the Cs(I) concentration in 3.0 mol·kg−1 HNO3 was investigated with the other experimental conditions fixed. The corresponding results are depicted in Figure 10. The extraction capacity of Cs(I) was determined to be 0.751 mmol·g−1. Dependence of the NonCalix[4]C6 Extraction on Temperature. The dependence of the extraction of NonCalix[4]C6 for the tested metals in 4.0 mol·kg−1 HNO3 172

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Cs(I). In terms of the linear slope of the plot of ln DCs vs 1/T as shown in Figure 11, the change in enthalpy of the extraction of Cs(I) with NonCalix[4]C6 was calculated to be −28.3 kJ·mol−1 through the Arrhenius law d ln D/dT = ΔH°/RT2.

Figure 9. Complex formation of the extracted species CsNO3·NonCalix[4]C6.

Figure 11. Relationship between ln DCs and 1/T. Conditions: [HNO3](a) = 4.0 mol·kg−1, [NonCalix[4]C6](o) = 1.0·10−2 mol·kg−1, phase ratio = 1:1.

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CONCLUSION The effective treatment and safety disposal of HLW have always been one of the most challenging processes due to the partitioning and transmutation (P/T) strategy. Most research attention has been focused on the partitioning of the long-lived MAs(III) such as Am(III) and Cm(III), while insufficient attention is concentrated on the separation of heat-emitting nuclides. In the SPEC process developed recently, Cs(I) and/ or Sr(II) were effectively separated utilizing two novel macroporous silica-based supramolecular recognition materials (SSRMs). However, the complexing mechanism and the composition of the complex formation of Cs(I) with the supramolecular recognition agent have not been understood. To reduce the potential risk of HLW to the environment, understanding the complexation of Cs(I) with the supramolecular recognition agent in the separation of Cs(I) is valuable. For this purpose, NonCalix[4]C6, a macrocyclic supramolecular recognition compound, was prepared quantitatively under the framework of the SPEC and MPS processes. The extraction of some typical fission and nonfission products, La(III), Y(III), Rb(I), Na(I), Mo(VI), Zr(IV), Sr(II), Ba(II), Ru(III), K(I), and Pd(II), with NonCalix[4]C6/1octanol was investigated. The effect of contact time, the HNO3 concentration in the range of (0.4 to 5.0) mol·kg−1, and

Figure 10. Dependence of the Cs(I) extraction with NonCalix[4]C6/ 1-octanol on the concentration of Cs(I) at 298.1 K. Qcs(I): quantity of Cs(I) extracted. Ccs(I): concentration of Cs(I). Conditions: [HNO3](a) = 4.0 mol·kg−1, [NonCalix[4]C6](o) = 1.0·10−2 mol·kg−1, phase ratio = 1:1.

with a change in temperature from (288.1 to 308.1) K was investigated. The results are listed in Table 4. The extraction curve of Cs(I) decreased gradually with increasing the temperature. It indicated that, in 4.0 mol·kg−1 HNO3, the extraction of NonCalix[4]C6/1-octanol for Cs(I) was an exothermic reaction. In other words, increasing the operation temperature in experimental conditions was adverse to the Cs(I) extraction. Therefore, the extraction temperature of Cs(I) with NonCalix[4]C6/1-octanol performed better at room temperature. Meanwhile, the other tested metals had weak or almost no adsorption with NonCalix[4]C6/1-octanol except Rb(I), which extraction behavior was similar to that of

Table 4. Comparison of the Distribution Ratio of the Tested Metals with the Change in Temperaturea T

distribution ratio, DM

K

Cs(I)

Rb(I)

Na(I)

K(I)

Sr(II)

Ba(II)

La(III)

Y(III)

Pd(II)

Zr(IV)

Mo(VI)

Ru(III)

288.1 293.1 298.1 303.1 308.1

12.5 8.45 7.20 6.51 5.45

2.04 1.51 1.34 1.20 1.12

0.08 0.19 0.09 0.09 0.08

0.21 0.15 0.11 0.10 0.08

0.10 0.09 0.05 0.03 0.02

0.09 0.09 0.07 0.06 0.05

0.09 0.08 0.06 0.05 0.03

0.09 0.08 0.07 0.04 0.02

0.23 0.18 0.14 0.09 0.07

0.06 0.04 0.04 0.03 0.01

0.58 0.47 0.44 0.42 0.41

0.25 0.13 0.13 0.14 0.13

a Conditions: [NonCalix[4]C6](o) = 1.0·10−2 mol·kg−1, [Metal](a) = 5.0·10−3 mol·kg−1, [HNO3](o) = 4.0 mol·kg−1, phase ratio = 1:1. The expanded uncertainty of the results has a level of confidence of 0.95.

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temperature on the extraction of these metals was examined. NonCalix[4] C6 showed excellent extraction ability and high selectivity for Cs(I) over all of the tested metals. The composition of the extracted species, the optimum extraction acidity, and the extraction property and mechanism of Cs(I) with NonCalix[4]C6 were discussed. Some valuable parameters such as the change in the thermodynamic enthalpy, the saturated capacity of the Cs(I) extraction, and others were obtained. The results of the strong complexation of Cs(I) with NonCalix[4]C6 are beneficial to the partitioning of Cs(I) with the NonCalix[4]C6-containing extraction system. Meanwhile, it is valuable to understand the adsorption mechanism and the adsorption-separation cycle of Cs(I) from an acidic HLW in the SPEC process.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (A.Z.). Tel./fax: +86 571 87953919. Funding

The present work was financially supported by the National Natural Science Foundation of China under contract No. 91126021, Zhejiang Provincial Natural Science Foundation of China under contract No. Y4110002, and Zhejiang Provincial Commonweal Technology Applied Research Project under contract No. 2012C23032. Notes

The authors declare no competing financial interest.

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NOTE ADDED AFTER ASAP PUBLICATION This article posted ASAP on December 10, 2012. In several places within the article (including the title), 1,3-[(2,4nony)oxy]-2,4-crown-6-calix[4]arene has been changed to 1,3-bis(1-nonyloxy)-2,4-crown-6-calix[4]arene. The correct version was posted on January 10, 2013.

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