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Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Uptake of Cesium and Some Typical Metals onto Hybrid Calix[4]crown Adsorbent with Silica Carrier by Host−Guest Recognition Anyun Zhang,† Chunmei Chen,† Yanqin Ji,*,‡ Shaorong Liu,† and Surong Guo† †

College of Chemical and Biological Engineering, Zhejiang University, No.38 Zheda Road, Hangzhou 310027, P.R. China China CDC Key Laboratory of Radiological Protection and Nuclear Emergency, National Institute for Radiological Protection, Chinese Center for Disease Control and Prevention, No. 2 Xinkang Street, Beijing 100088, P.R. China

‡

ABSTRACT: A new silica adsorbent, C7C6CalixSi, was prepared by intermolecular hybridization. It was achieved by introducing and immobilizing a macrocyclic C7C6Calix into the pores of the hybrid silica carrier specially developed. FT-IR, SME, TG-DSC, BET, and XAR were utilized to characterize the structure of the adsorbent. C7C6Calix was successfully introduced into the macroporous silica carrier by self-assembly. The adsorption of Cs and other typical metals toward C7C6CalixSi was investigated in the range of 0.4 to 5.0 M HNO3. C7C6CalixSi had an excellent adsorption ability and selectivity for Cs over others, except for Rb. The adsorption capacity of Cs was 0.1233 mM/ g at the optimal adsorption conditions of 1.0 M HNO3, 298 K, and a contact time of 120 min. The experimental results fitted well with the Langmuir model. The thermodynamic parameters ΔH°, ΔG°, and ΔS° of Cs uptake were obtained. The stability of C7C6CalixSi was evaluated by TOC content. Some valuable adsorption data of the silica adsorbent in the chemical engineering were obtained. The results revealed that C7C6CalixSi is promising for Cs removal.

1. INTRODUCTION Cs and Sr, heat emitting nuclides, are considered to be one of the major sources of strong radioactivity in high level liquid waste (HLLW).1−3 This makes experimental operation in the separation process inconvenient. Taking into account the thermal output caused by decay, it would also have a potential risk to the safety of the vitrified-HLLW in geological disposal. For effective treatment and disposal of HLLW, removing the two nuclides would be favorable.4,5 Some technical methods, such as liquid−liquid solvent extraction, ion exchange, and others, have been studied.6−9 But the satisfactory separation results have not been achieved yet. Calix[n]crown with a cavity is a derivative of macrocyclic calix[n]crown, a so-called third-generation supramolecular agent.10,11 It contains a calix[n]arene and one or two crown ether cycles. As a host, it recognizes the guest, such as inorganic metal ions or organic molecules, through noncovalent bond interactions.12 In most cases, the molecular recognition or the complexation occurs through a host−guest size matching principle. This recognition style is different from traditional complexing agents, which react with metals by the coordination bond and with the organic molecule by hydrogen bonding.13−15 Based on the structural characteristics of calix[n]crown, it is possible to select an appropriate calix[n]crown derivative with a perfect cavity to recognize Cs ions. The investigations have shown that the cavity of some calix[4]arene-crown-6 derivatives and the size of the Cs ion match well.16−20 It provides a promising technical method in applying calix[4]arene-crown-6 derivatives or corresponding functional materials in Cs separation by host−guest molecular recognition.21−26 © XXXX American Chemical Society

We have developed a separation process entitled SPEC (strontium/cesium partitioning from HLW by extraction chromatography) for the removal of Cs and Sr from HLLW.27 It was achieved by a silica composite material composed of 1,3-[(2,4-diethylheptyl ethoxy)oxy]-2,4-crown-6calix[4]arene (1) for Cs and 4,4′,(5′)-di(tert-butylcyclohexano)-18-crown-6 (2) for Sr by the intermolecular modification.28−35 Cs and Sr were efficiently removed by extraction chromatography. However, the agent (1) exhibited a few disadvantages, such as low affinity, difficult to synthesize, low solubility in preparation, and others. This makes the application of the functional adsorbent containing 1 in the GPSC process restricted. Seeking a more excellent adsorption material for Cs removal is therefore necessary. The objective of this work is focused on the efficient removal of Cs by a new silica adsorbent, C7C6CalixSi. The introduction and immobilization of a calix[4]monocrown derivative, di(1-hexyloxy)-2,4-crown-6calix[4]arene (C7C6Calix), into the pores of the silica carrier were conducted through vacuum sucking for hybridization. The adsorption of Cs and other typical metals onto C7C6CalixSi was investigated. Some valuable adsorption data and thermodynamic parameters in chemical engineering were obtained. The technical feasibility of applying the new silica adsorbent, C7C6CalixSi, in Cs removal was evaluated. Received: December 17, 2017 Accepted: February 28, 2018

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DOI: 10.1021/acs.jced.7b01092 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Figure 1. SEM images (A and B). Nitrogen adsorption−desorption isotherms and BJH pore size distributions (C). TG-DSC curves (D) of C7C6CalixSi adsorbent.

Figure 2. FT-IR spectra (A) and powder X-ray diffraction patterns (B) of the materials.

2. EXPERIMENTAL SECTION 2.1. Reagents. The HLLW was simulated using HNO3 solutions of different concentrations containing 17 representative metal ions, such as Pd(II), Mo(VI) Ru(III), Zr(IV), Na(I), K(I), Sr(II), Ba(II), La(III), Y(III), Sm(III), Yb(III), Rb(I), Cs(I), Co(II), Fe(III), and Ni(II). The concentrated nitric acid was used to dissolve their nitrates, which were of analytical grade. Four trivalents, La(III), Y(III), Sm(III), and Yb(III), were used to understand all of trivalent rare earths REs(III) and trivalent minor actinides MAs(III) based on similar chemical properties. The silica carrier, SiO2−P, with a mean pore diameter of 60−90 μm and coated with a thin layer of copolymer, was prepared by polymerization. Other reagents were of analytical grade and were used without further purification. C7C6Calix was quantitatively prepared in laboratory scale and characterized by elemental analysis, FT-IR, TG-DSC, 1H NMR, ESI-MS, and others. The characterization results were as follows: MP: 95.62 °C. Element Analysis: Cal.: C, 75.88%; H, 8.57%. Exp.: C, 75.95%; H, 8.42%. FT-IR νmax (KBr): 3022, 2919, 2844, 1643, 1464, 1392, 1122, 1089, 1037, 762 cm−1. 1H NMR (CDCl3, δ): 0.9088−0.9372 ppm (t,

OCH2CH2CH2CH2CH2CH2CH3, 6H), 1.2264−1.3342 ppm (m, OCH2CH2CH2CH2CH2CH2CH3, 20H), 3.3789−3.4432 ppm (m, ArOCH2CH2OCH2CH2OCH2, 8H), 3.4772−3.4859 ppm (t, ArOCH2CH2OCH2CH2OCH2, 4H), 3.5810−3.6049 ppm (t, OCH2CH2CH2CH2CH2CH2CH3, 4H), 3.6445− 3.6633 ppm (t, ArOCH2CH2OCH2CH2OCH2, 4H), 3.7063 ppm (s, ArO(CH2CH2O)2CH2, 4 H), 3.7734 ppm (d, ArCH2Ar, 8H), 6.7567−6.7865 ppm, 6.8106−6.8405 ppm (t, ArH para, 4H), 7.0064−7.0214 ppm, 7.0719−7.0863 ppm, (d, ArH meta, 8H). ESI-MS: m/z 822.5. [M+NH4+] 840.8, [M +H3O+] 841.7. The purity of C7C6Calix was greater than 99.3%. It was measured by microanalysis technology coupled with high-performance liquid chromatography (HPLC) and mass spectrometry (MS). Compared with 1, C7C6Calix had more excellent physical and chemical properties. 2.2. Preparation of C7C6CalixSi. Prior to synthesis, the SiO2−P carrier was treated using a method of surface active modification. It was done at least two times at ambient temperature. C7C6Calix dissolved in dichloromethane was mixed with the silica carrier at a given mass ratio. After shaking mechanically for 2 h, it was operated in a silicon oil bath at 320 K for 360 min, while a hydrophilic additive was introduced to modify C7C6Calix based on the similar compatibility principle. B



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3. RESULTS AND DISCUSSION 3.1. Dependence of the Adsorption on HNO 3 Concentration. The C7C6Calix containing a calix[4]arene and a crown ether is a derivative of macrocyclic calix[4]crown. It has eight oxygen atoms with a high affinity to the hydrogencontaining compound through hydrogen bonding. High-level liquid waste, an acidic medium, is a complicated multicomponent system. Its acidity is usually around 3.0 M HNO3. In the host−guest recognition of C7C6Calix to Cs(I), the HNO3 molecule would be probably recognized at the same time. In this case, the uptake of Cs(I) and other metals onto C7C6CalixSi and the intermolecular association of C7C6CalixSi with HNO3 might form two competing reactions. To evaluate the influence of the HNO3 concentration, the staticstate uptake of Cs(I), Sr(II), and other representative metals in the range of 0.4−5.0 M HNO3 was investigated using C7C6CalixSi as a solid phase. It was evaluated using the distribution coefficients (Kd) of the metals. The corresponding adsorption results are illustrated in Figures 3, 4, and 5.

Physically immobilizing the agents into the pores surface of the carrier was achieved by molecular self-assembly. Following drying in a vacuum drying oven, a new colorless silica adsorbent, C7C6CalixSi, was obtained. It was then characterized by a scanning electron microscope (SEM) for morphological analysis, N2 adsorption−desorption isotherm for pore structure analysis, thermogravimetric-differential scanning calorimeter analysis (TG-DSC) in a flowing oxygen atmosphere from room temperature to 800 °C at a heating rate of 10 °C/min for thermal stability analysis, Fourier transforminfrared (FT-IR) spectra in a range of 4000−500 cm−1, and powder X-ray diffraction (XRD) within the 2θ range of 5−40° for structure and composite mechanism analysis. The results of Figure 1 indicated that the SiO2−P particles carrier was porous, and the corresponding pore volume decreased after impregnation with C7C6Calix, reflecting that C7C6Calix was successfully impregnated and immobilized into the pores of the silica carrier. C7C6CalixSi had a uniform spherical shape surface, which demonstrated abundant channels, smooth surfaces filled with plenty of pores, and no destruction of the original porous structures. The most probable diameter, pore volume, and the specific surface area are 70.93 nm, 0.8171 cm 3 /g, and 153.2 m 2 /g. The decomposition temperature of the material was higher than 343.3 °C, indicating its excellent thermal stability. The composition of C7C6CalixSi was 70.90% for inorganic silica, 10.10% for copolymers, and 19.00% for C7C6Calix. Figure 2 showed that the structures of the porous silica carrier and supramolecular recognition agents had no significant change before and after impregnation, revealing the physical composite mechanism through hydrogen bonding, π−π stacking, and others, rather than chemical reaction. To understand the microstructures well, C7C6CalixSi are being characterized by X-ray absorption fine structure (XAFS) technology, X-ray photoelectron spectroscopy (XPS), and others. 2.3. Measurement of the Distribution Coefficient. The adsorption of the representative metal ions was operated at 298 K. It was achieved by a TAITEC MM-10 Model autothermostated water bath shaker. The acidity of the aqueous phase was in the range of 0.4−5.0 M HNO3, while a phase ratio of solid phase C7C6CalixSi to the aqueous one was 0.25 g to 5 cm3. The phase separation was performed by a filter membrane with a pore size of 0.45 μm. The contents of the metal ions and the total organic carbon (TOC) in the aqueous phase were measured using a Varian 700-ES model simultaneous inductively coupled plasma-optical emission spectrometer (ICP-OES), or a Varian AA 240 FS model atomic adsorption spectroscopy and a Shimadzu 5000 model TOC-VCPN analyzer. All the experimental data were determined three times in parallel. The distribution coefficient (Kd) of each metal was calculated by eq 1. Kd =

Co − Ce V × (cm 3/g) Ce W

Figure 3. Dependence of typical alkali metals and alkaline earths adsorption toward C7C6CalixSi on HNO3 concentration in the wide range of 0.4−5.0 M at 298 K. [Metal]: 5 mM; phase ratio: 0.25 g/5 cm3; shaking speed: 120 rpm; and contact time: 120 min.

As can be seen form Figure 3, C7C6CalixSi showed an excellent uptake ability and high selectivity for Cs(I) over all of the other metals, except for Rb(I) in the tested HNO3 concentration range. The Cs(I) uptake increased obviously at the beginning stage and subsequently decreased to 5.0 M HNO3. The maximum absorption of Cs(I) toward C7C6CalixSi appeared at 1.0−2.0 M HNO3. For the uptake of Cs(I) in the range of 0.4−2.0 M, it was ascribed to the efficient complexation of Cs(I) with C7C6CalixSi. The effective host− guest recognition of C7C6Calix to Cs(I) resulted in the excellent adsorption of Cs(I) because of the matched metal ion size and the C7C6Calix cavity. Following higher HNO3 concentration, especially from 2.0 to 5.0 M HNO3, the obvious decrease of the distribution coefficient (Kd) of Cs(I) indicated probably that the host−guest recognition of C7C6CalixSi to Cs(I) was insignificant, while the intermolecular association of C7C6CalixSi and HNO3 through hydrogen bonding was dominant. In this case, C7C6CalixSi was easily protonated by an interaction with HNO3. As a result, the protonation would

(1)

Where two physical symbols, Co and Ce, exhibit the initial and equilibrium concentrations of the metals in the aqueous phase, respectively. Two other symbols, W and V, present the mass of the solid phase and the volume of the aqueous phase, respectively. C



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hexano-18-crown-6 (CH18C6), 4,4′,(5′)-di(tert-butylcyclohexano)-18-crown-6 (DtBuCH18C6), and others, could be protonated with some inorganic and/or organic acids, for example, HNO3, HCOOH, HCl, H2SO4, CH3COOH, and others, through hydrogen bonding and formed to the 1:1 type of the associated species.39−41 Therefore, it is logical for C7C6Calix to be capable of associating with the HNO3 molecule. In this case, the uptake of Cs(I) toward C7C6CalixSi by the host−guest supramolecular recognition and the protonation of C7C6CalixSi with the HNO3 molecule through hydrogen bonding constituted two competing reactions. In terms of the above-mentioned analysis, the uptake mechanism of C7C6CalixSi for Cs(I) was described as follows: (1) In the HNO3 concentration range of 0.4−2.0 M, based on the effective host−guest molecular recognition, the uptake of Cs(I) toward the silica-based supramolecular material, C7C6CalixSi, was dominant, while the protonation of C7C6CalixSi might be insignificant. If assuming Cs(I) is adsorbed and forms to a 1:1 type of the species, then, the complex composition would be [CsC7C6CalixSi]NO3. The uptake equation could be described as below:

Figure 4. Dependence of some representative transition elements adsorption toward C7C6CalixSi on HNO3 concentration at 298 K. [Metal]: 5 mM; phase ratio: 0.25 g/5 cm3; shaking speed: 120 rpm; and contact time: 120 min.

Cs+ + NO−3 + C7C6CalixSi ⇌ [CsC7C6CalixSi]NO3

(2)

(2) From 2.0 to 5.0 M HNO3, the intermolecular association of C7C6CalixSi with HNO3 was dominant, while the host−guest supramolecular recognition of C7C6CalixSi to Cs(I) was insignificant. The protonation equation would probably be proposed as follows: HNO3 + C7C6CalixSi ⇌ HNO3 ·C7C6CalixSi (3)

On the basis of the analysis mentioned above, the adsorption mechanism of Cs(I) onto C7C6CalixSi is shown in Figure 6.

Figure 5. Dependence of a few representative REs(III) adsorption toward C7C6CalixSi on HNO3 concentration at 298 K. [Metal]: 5 mM; phase ratio: 0.25 g/5 cm3; shaking speed: 120 rpm; and contact time: 120 min.

result in a significant decrease in the available content of C7C6Calix inside C7C6CalixSi that is efficiently capable of recognizing Cs(I). The higher HNO3 concentration was adverse to the effective uptake of Cs(I) toward C7C6CalixSi. From this viewpoint, selecting 1.0 M HNO3 as the optimum acidity for the Cs(I) adsorption was more reasonable. For practice, the cavity inside the C7C6Calix molecule capable of recognizing Cs(I) is composed of a calix[4]arene formed by four benzene rings with the phenolic oxygens and a 18-crown-6 cycle moiety bonded together. In almost all of the studies, alkali metal ions, such as Cs(I) and Rb(I), were recognized with Calix[4]monocrown and formed to a 1:1 type of complex.21,36 It is therefore reasonable for C7C6Calix to be capable of complexing Cs(I) in the form of the 1:1 type of complex.37,38 On the other hand, some investigations showed that some 18-crown-6 derivatives, such as dicyclohexano-18crown-6 (DCH18C6), dibenzo-18-crown-6 (DB18C6), cyclo-

Figure 6. Competitive adsorption mechanism of C7C6CalixSi with Cs(I) by molecular recognition and HNO3 by protonation through hydrogen bonding.

Figures 4 and 5 showed the adsorption of some representative REs(III) and other fission products toward C7C6CalixSi in the tested HNO3 concentration range. These metal ions showed very weak or almost no uptake of C7C6CalixSi, with the exception of a little bit of palladium. It reflected that these metals had almost no influence on the removal of Cs(I) from HHLW. In addition, it is well-known that all of the REs(III), including Y(III) and from La(III) to Lu(III), have extremely similar chemical properties because of the similar structure of the valence electron shell. No adsorption of trivalent La(III), Y(III), Sm(III), and Yb(III) D



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Table 1. Separation Factors (SFs) of Cs with Others in the Range of 0.4−5.0 M HNO3 Sr(II)

Rb(I)

Ba(II)

K(I)

Na(I)

Ru(III)

Mo(VI)

Zr(IV)

0.4 1.0 2.0 3.0 4.0 5.0

M M M M M M

110.1 115.3 118.1 9793 319.9 573.8 Pd(II)

6.430 9.033 8.148 7.221 6.101 5.980 Fe(III)

124.8 98.56 128.6 1992 287.3 220.7 Co(II)

40.91 247.5 33.76 22.48 45.21 Ni(II)

132.5 335.7 2537 1653 356.8 258.9 La(III)

68.76 53.72 53.46 73.00 40.34 31.72 Sm(III)

4.463 50.75 59.33 414.5 155.9 90.89 Y(III)

26.99 36.37 32.78 815.8 114.6 65.02 Yb(III)

0.4 1.0 2.0 3.0 4.0 5.0

M M M M M M

32.45 13.54 11.24 11.35 10.44 22.09

139.6 115.3 927.3 1350 5673

88.43 113.0 145.98 673.5 914.3 861.9

78.64 115.1 159.3 1511 693.4 674.8

185.2 133.2 202.5 701.6 2537 321.2

93.65 89.03 127.2 565.1 1562 271.1

158.2 111.0 118.1 673.5 2083 291.3

90.90 84.95 143.1 353.7 513.2 203.3

might reveal that all of the REs(III) would have no impact on Cs(I) adsorption toward C7C6CalixSi. The high selectivity of Cs(I) removal by C7C6CalixSi as an efficient solid adsorbent via the host−guest molecular recognition was confirmed. The separation factors (SFs) of Cs with others in the range of 0.4− 5.0 M HNO3 are listed in Table 1. As can be seen, C7C6CalixSi had a high separation factor for Cs(I) over others, except for Rb(I), revealing efficient removal of Cs(I) along with part of Rb(I) from the simulated HLW solution. 3.2. Dependence of the Adsorption on Contact Time. The influence of the contact time on the adsorption of Cs(I), Sr(II), and other representative fission and nonfission products onto C7C6CalixSi was studied in 1.0 M HNO3 medium. The corresponding results are shown in Figures 7, 8, and 9, respectively.

Figure 8. Dependence of some representative transition elements adsorption toward C7C6CalixSi on contact time at 298 K. [Metal]: 5 mM; [HNO3]: 1.0 M; phase ratio: 0.25 g/5 cm3; and shaking speed: 120 rpm.

Figure 7. Dependence of typical alkali metals and alkaline earths adsorption toward C7C6CalixSi on contact time at 298 K. [Metal]: 5 mM; [HNO3]: 1.0 M; phase ratio: 0.25 g/5 cm3; and shaking speed: 120 rpm.

Figure 7 revealed the contact time influence on the uptake of Cs(I) and other representative alkali metals and alkaline earths in 1.0 M HNO3. It was found that the distribution coefficient (Kd) of Cs(I) quickly increased in the initial stage and then kept almost constant up to 125 min, indicating that the adsorption equilibrium of Cs(I) was reached. Rb(I) showed a similar adsorption behavior to Cs(I), while others had very

Figure 9. Dependence of a few representative REs(III) adsorption toward C7C6CalixSi on contact time at 298 K. [Metal]: 5 mM; [HNO3]: 1.0 M; phase ratio: 0.25 g/5 cm3; and shaking speed: 120 rpm.

E



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in Table 2. Sr(II), one of the heat generators, had no impact on Cs(I) removal, while Rb(I) with a similar chemical property to Cs(I) showed a certain degree of influence. These results make the application of the silica adsorbent, C7C6CalixSi, in Cs removal from an acidic HLLW promising. 3.3. Dependence of the Adsorption on Temperature. The influence of temperature on the Cs(I) adsorption onto C7C6CalixSi in 1.0 M HNO3 was studied. It was operated in the range of 293−333 K utilizing an autothermostated water bath shaker. The experimental data showed that the adsorption of C7C6CalixSi for Cs(I) is an exothermic reaction. In terms of two equations, ln Kd = −ΔHo/(RT) + ΔS°/R and ΔG° = ΔH° − TΔS°, the thermodynamic parameter of Cs(I) adsorption was calculated, and the corresponding results are listed in Table 3. Where Kd shows the distribution coefficient, cm3/g, R presents the universal gas constant, ΔH° denotes the standard enthalpy change, ΔS° is the standard entropy change, and ΔG° is the standard Gibbs free energy change. Table 3 shows that ΔG° is a negative value, while ΔH° and ΔS° are also negative. ΔG° < 0 revealed that the adsorption of Cs(I) toward C7C6CalixSi is spontaneous, while the adsorption reaction of Cs(I) is exothermic due to ΔH° < 0. This means that increasing the temperature is harmful to Cs(I) adsorption onto C7C6CalixSi. ΔS° < 0 reflected that Cs(I) was really adsorbed toward C7C6CalixSi by forming a stable 1:1 type of complex through the host−guest supramolecular recognition. It makes Cs(I) adsorption on C7C6Calix@SiO2 go from being disordered to ordered. 3.5. Adsorption Isotherms. Under the optimal adsorption conditions of 1.0 M HNO3, 298 K, and a contact time of 120 min, the adsorption of Cs(I) toward C7C6CalixSi was evaluated using the Langmuir model as described by eq 4, which is suitable for monolayer adsorption onto a homogeneous surface with a limited number of adsorption sites, and the Freundlich model as described by eq 5.42 The results are illustrated in Figure 11.

weak or almost no uptake. It reveals that all alkali metals and alkaline earths might have no influence on Cs(I) removal by C7C6CalixSi, except for Rb(I). In other words, C7C6CalixSi had a high uptake ability and selectivity for Cs(I) over others, except for Rb(I). Compared to other macroporous adsorbents containing agent 1 with inorganic SiO2 and polymer XAD-7 as carriers, C7C6CalixSi exhibited much better adsorption kinetics for Cs(I). The adsorption equilibrium time of Cs(I) was 20 min for C7C6CalixSi, 70 min for 1/SiO2−P, and 120 min for 1/XAD-7. Figures 8 and 9 denoted the contact time influence of some representative metals, such as Co(II), Fe(III), Ni(II), Pd(II), Mo(VI), Ru(III), Zr(IV), and a few typical La(III), Sm(III), Y(III), and Yb(III) on the Cs(I) adsorption. All of the tested fission products had an extremely weak or almost no uptake onto C7C6CalixSi, and the corresponding distribution coefficients of these metals in 1.0 M HNO3 medium were always below 1.0 × 10−2 cm3/g, except for 4.82 cm3/g of Pd(II), indicating that the host−guest molecular recognition of C7C6Calix to these metals was ignorable due to the unmatched cavity of C7C6Calix and metal ion size. It reflects that the tested fission products would also have no impact on Cs(I) removal by C7C6CalixSi, except for a little bit of Pd(II). On the other hand, it is well-known that the chemical properties of all REs(III) are very similar, this made the application of four La(III), Sm(III), Y(III), and Yb(III) to simulate all REs(III) reasonable. The extremely weak or almost no adsorption of the four metals showed that in 1.0 M HNO3 medium, C7C6CalixSi would have no adsorption ability for all 16 species of REs(III), including Y(III) and from La(III) to Lu(III). In addition, the chemical properties of the trivalent minor actinides MAs(III) are always similar to those of trivalent REs(III). Therefore, C7C6CalixSi could have no uptake ability for all MAs(III). These results are of a great benefit to Cs(I) removal from HLLW by C7C6CalixSi. Considering 16 species of REs(III) and two species of MAs(III), almost 28 metals was evaluated for the adsorption of Cs(I) onto C7C6CalixSi. The results represented by 3D are shown in Figure 10. On the other hand, the separation factors of Cs(I), Rb(I), and Sr(II) to others in 1.0 M HNO3 are listed

Ce C 1 = + e qe KLqm qm

lnqe = ln KF +

1 lnCe n

(4) (5)

Where Ce (mM) shows the equilibrium concentrations of Cs(I). qe (mM/g) represents the equilibrium adsorption capacity. Both KL and KF demonstrate the constants of Langmuir and Freundlich, respectively. qm (mM/g) denotes the monolayer adsorption capacity. 1/n shows the heterogeneity factor. The results showed that the equilibrium data fitted with the Langmuir model well with a correlation coefficient, R2, of 0.9989, which was higher than that of 0.8709 fitted by the Freundlich model. The maximum adsorption capacity, qmax, of Cs(I) is 0.1233 mM/g,43 which is close to the experimental value of 0.1287 mM/g. Lower deviations between the experimental data and the calculated value reflected that the Langmuir isotherm model is well acceptable. The relatively lower R2 value (0.8709) obtained from the Freundlich model suggests an inadequate adsorption. That is, the adsorption of Cs(I) into the pores of C7C6CalixSi is assumed to lead mostly to monolayer formation.44 On the other hand, the Langmuir adsorption can be classified using a separation factor, RL, as described by eq 6.45

Figure 10. Relationship between the distribution coefficients, ionic radius, and atomic number of more than 28 elements in 3D. F



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Table 2. SFs of Cs(I), Rb(I), and Sr(II) with Others in 1.0 M HNO3 Cs(I) SFCs/X SFRb/X SFSr/X

0.1384 0.6842 Pd(II)

Sr(II)

Rb(I)

Ba(II)

116.3 20.23

9.131

98.72 11.02 13.63

Na(I)

Fe(III)

Co(II)

116.4 99.91 92.41

112.9 97.53 56.73

116.2 101.5 10.34

0.4942

14.33 0.3360 0.0776

SFCs/X SFRb/X SFSr/X

K(I) 246.3 2.357 0.2311 Ni(II)

Ru(III)

Mo(VI)

Zr(IV)

334.9 87.91 0.1211 La(III)

53.83 2.162 0.4996 Sm(III)

51.91 12.28 2.836 Y(III)

37.25 24.16 5.583 Yb(III)

134.3 43.03 9.943

90.15 38.11 8.338

112.3 36.78 3.673

85.12 30.21 1.167

Table 3. Thermodynamic Parameters of the Adsorption of Cs(I) toward C7C6CalixSi ΔG° (KJ·mol−1) −1

−1

−1

ΔS° (KJ·mol ·K )

ΔH° (KJ·mol )

288 K

293 K

298 K

303 K

308 K

−0.073

−31.12

−10.10

−9.731

−9.366

−9.001

−8.636

Figure 11. Langmuir (a) and Freundlich (b) adsorption isotherms of Cs(I) toward C7C6CalixSi as well as Langmuir (c) and Freundlich (d) model plots of Cs(I).

Table 4. Adsorption Parameters of Cs(I) toward C7C6CalixSi Freundlich model

Langmuir model

metal ions

qm,exp (mM/g)

KF

1/n

R2

KL

qm,cal(mM/g)

R2

Cs(I)

0.1287

1.540

0.4969

0.8709

272.1

0.1233

0.9972

RL =

1 1 + KLC0

understand the adsorption property, the TOC content of C7C6CalixSi leakage from the HNO3 solution was studied. The corresponding results are described in Figures 12 and 13. The TOC content in the aqueous phase increased slowly with the increasing HNO3 concentration in terms of a linear equation, y = 8.971x + 187.4, with a correlation coefficient of 0.9907. It was always in the range of 197−238 ppm with an average value of 210.5 ppm. This was attributed to the protonation of C7C6CalixSi with HNO3 through hydrogen bonding. A little bit of C7C6Calix leaked into the aqueous phase in the form of a 1:1 type of the associated species. While, the TOC content in Figure 13 was in the range of 203−260 ppm with an average value of 222.3 ppm, close to that of the HNO3 concentration effect experiments. The bleeding percentage of C7C6Calix in 1.0 M HNO3 was less than 0.11%, which was lower than that of

(6)

Where KL is the Langmuir constant related to the energy of adsorption (mg−1), and C0 is the initial concentration (mg/L). RL = 0 indicates the irreversible adsorption isotherms. RL < 1 and RL > 1 show a favorable and an unfavorable adsorption, respectively. RL = 1 denotes the linear adsorption isotherms. The results calculated by eq 6 showed that the RL value was in the range of 0−1, reflecting a favorable adsorption of Cs toward C7C6CalixSi. The corresponding adsorption parameters are listed in Table 4. 3.6. Stability. The concentration of the TOC in the aqueous phase is one of the main indicators to chemically evaluate the stability of the functional adsorption material.46 To G



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range of 0.4−5.0 M, the contact time, and the temperature from 293 to 333 K on the adsorption of C7C6CalixSi, which exhibited an excellent adsorption ability and selectivity for Cs(I) over the other metals, except for Rb(I). The optimum acidity for the Cs(I) adsorption was 1.0 M HNO3. The separation factors of Cs(I) with others; the thermodynamic parameters ΔG°, ΔH°, and ΔS°; and the Langmuir and Freundlich models for Cs(I) adsorption toward C7C6CalixSi were evaluated. Valuable chemical engineering data for the Cs(I) adsorption were obtained. The resultant adsorption data and the thermodynamic parameters are beneficial to Cs(I) removal from an acidic HLLW by C7C6CalixSi based on the GPSC process.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Figure 12. TOC content of C7C6CalixSi bleeding in 0.3−5.0 M HNO3 at 298 K. [Metal]: 5 mM; phase ratio: 0.25 g/5 cm3; and shaking speed: 120 rpm.

ORCID

Anyun Zhang: 0000-0001-5816-3812 Funding

This work was financially supported by the National Natural Science Foundation of China (No. U1407115), the Fund of China CDC Key Laboratory of Radiological Protection and Nuclear Emergency (NIRP, China CDC), and the Zhejiang Provincial Natural Science Foundation of China (No. LY17B060004). Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 13. TOC content of C7C6CalixSi bleeding at different contact times. [Metal]: 5 mM; [HNO3]: 1.0 M; phase ratio: 0.25 g/5 cm3; shaking speed: 120 rpm; and temperature: 298 K.

Calix[4]arene-R14 containing materials. This is beneficial to the application of C7C6CalixSi in Cs(I) being efficiently removed from the HNO3 medium.

4. CONCLUSIONS In efficient treatment and disposal of an acidic HLLW, one of the heat generators, Cs(I) with a half-life of 30 yr, is a β-emitter. It can pose a serious radiation hazard to the public health and the environment due to the mobility in final repository, while it has the potential to be harmful to artificial solidified bodies, such as the vitrified, cemented, or bituminized-HLLW, in the final geological disposal. On the other hand, Cs(I) is an important source of β-radiation and an energy generator in hospital examinations. This makes effective removal of Cs(I) valuable. But the satisfactory removal of Cs(I) from HLLW has not been solved. For this purpose, a new supramolecular adsorbent, C7C6CalixSi, was prepared and characterized. The adsorption of Cs(I), Rb(I), Sr(II), and more than ten other representative metals was investigated. It was studied by examining the influence of the HNO3 concentration in the H



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