Adsorption Property of Cesium onto Modified Macroporous Silica

Apr 6, 2012 - Ali F. Tag El-Din , Mohamed E. El-Khouly , Emad A. Elshehy , Asem A. Atia , Waleed A. El-Said. Microporous and Mesoporous Materials 2018...
0 downloads 0 Views 465KB Size
Download PDF
Article pubs.acs.org/IECR

Adsorption Property of Cesium onto Modified Macroporous Silica− Calix[4]arene-crown Based Supramolecular Recognition Materials Anyun Zhang*,† and Zhifang Chai†,‡ †

Department of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, People's Republic of China ‡ Institute of High Energy Physics, Chinese Academy of Sciences, P.O. Box 918, Beijing 100049, People's Republic of China ABSTRACT: The adsorption behavior of Cs(I), one of the heat generators, in HNO3 medium was investigated at 298 K. The impact of U(VI), some typical elements, and temperature on the adsorption of Cs(I) was evaluated. It was performed by macroporous silica-based 1,3-[(2,4-diethylheptylethoxy)oxy]-2,4-crown-6-calix[4]arene (Calix[4]arene-R14) impregnated supramolecular recognition materials (Calix[4] + M)/SiO2−P. They were modified with tri-n-butyl phosphate (TBP), octanol (Oct), and methyloctyl-2-dimethylbutanemide (MODB). The excellent adsorption ability and high selectivity of (Calix[4] + M)/SiO2− P for Cs(I) except Rb(I) and U(VI) were confirmed. The adsorption ability of Cs(I) onto the modified supramolecular recognition materials was (Calix[4] + Oct)/SiO2−P > (Calix[4] + TBP)/SiO2−P > (Calix[4] + MODB)/SiO2−P. The chromatographic separation of Cs(I) from a 4.0 M HNO3 solution containing La(III), Sr(II), Ru(III), Cs(I), Rb(I), U(VI), Mo(VI), Zr(IV), Gd(III), and Pd(II) was carried out with a (Calix[4] + TBP)/SiO2−P packed column. Cs(I) was eluted effectively with water. The possibility and feasibility of effective partitioning of Cs(I) from highly active liquid waste by the (Calix[4] + M)/SiO2−P materials was evaluated.

1. INTRODUCTION It is reported that, in reprocessing of nuclear spent fuel, both radioactive nuclides 135Cs and 137Cs with half-lives of 2 × 106 and 30 years produced through the fission of uranium in a nuclear reactor are contained in highly active liquid waste (HLW).1 135Cs is considered to be a potential risk to the environment because of its long half-life and mobility in the repository, while 137Cs is one of the heat generators. It decays by emitting β particles of high energy. As a result, it might be harmful to vitrified HLW in final geological disposal. In addition, radiocesium is taken up in the fluid electrolytes of living organisms instead of potassium. Exposure to radiocesium will cause subtle and transient central nervous modifications at the electrophysiological level. This makes it possible that radiocesium can pose a serious radiation hazard to health and the environment. Consideration of environmental protection and resource reuse, partitioning, and recovery of cesium from HLW to a great extent is valuable. However, the effective separation of cesium has always been one of the most challenging works. Liquid−liquid solvent extraction is one of the traditional separation technologies. It usually has high extraction capacity and separation efficiency of some highly specific extractants. In the studies of Cs(I) separation, the supramolecular recognition agents calix[4]arenes, which are basket-shaped compounds of potential interest for host−guest complexation studies, have received considerable attention over the past 30 years.2,3 Calix[4]arene-crown compounds, the popular derivatives of calix[4]arenes, show high affinity for the complexation of alkali and alkaline-earth metals.4−7 Especially, the derivatives of the 1,3-alternate calix[4]arene-18-crown-6 exhibit high extraction selectivity for cesium over all of the other elements, which is exemplified by the following solvent extraction processes. A so© 2012 American Chemical Society

called cesium separation by calix-crown extraction (CCCEX) by 1,3-(dioctyloxy)-2,4-crown-6-calix[4]arene or 1,3-[(2,4diethylheptylethoxy)oxy]-2,4-crown-6-calix[4]arene (Calix[4]arene-R14) was developed by the French Atomic Energy Commission (CEA).8,9 A caustic-side solvent extraction (CSSX) process for removal of cesium from alkaline solution was proposed utilizing calix[4]arene-bis(tert-octylbenzo-crown6) (BOB CalixC6) as an extractant at Oak Ridge National Laboratory (ORNL).10,11 The fission product solvent extraction (FPEX) process integrating two extractants, 4,4′(5′)di(tert-butylcyclohexano)-18-crown-6 (DtBuCH18C6) and BOB CalixC6, was reported to simultaneously separate cesium and strontium together from acidic media.12,13 All the processes showed high selectivity and excellent extraction ability for cesium. However, some adverse diluents and phase modifiers such as tri-n-octylamine (TOA), 1-(2,2,3,3-tetrafluoropropoxy)3-(4-sec-butylphenoxy)-2-propanol (Cs-7SB), and 1-(1,1,2,2tetrafluoroethoxyl)-3-[4-(tert-octyl)phenoxyl]-2-propanol (Cs3) were introduced to avoid the formation of the third phase,14−16 which might induce a large quantity of secondary wastes. In comparison with liquid−liquid solvent extraction, extraction chromatography seems promising for effective partitioning of cesium because of some advantages such as minimal organic solvent utilization, less waste accumulation, compact equipment scale, and simple operating procedure. Therefore, from the viewpoint of technology, applying Received: Revised: Accepted: Published: 6196

November 8, 2011 April 6, 2012 April 6, 2012 April 6, 2012 dx.doi.org/10.1021/ie202540d | Ind. Eng. Chem. Res. 2012, 51, 6196−6204

Industrial & Engineering Chemistry Research

Article

Figure 1. Advanced SPEC process for Cs/Sr partitioning from HLW by extraction chromatography utilizing two novel macroporous silica-based supramolecular recognition materials.

capacity, the evaluation of the supramolecular recognition materials modified with different organic reagents, and the effect of some factors such as temperature and the concentration of uranium on the adsorption of Cs(I) have not been understood yet. The objectives of the present work were as follows: (1) The first objective was the synthesis of the macroporous silica-based supramolecular recognition material, (Calix[4] + M)/SiO2−P. It was modified with tri-n-butyl phosphate (TBP), octanol (Oct), and methyloctyl-2-dimethylbutanemide (MODB). (2) The second objective was the investigation of the effect of U(VI), Cs(I), Sr(II), the other tested metals, and temperature on the adsorption of (Calix[4] + TBP)/SiO 2−P, the determination of the adsorption capacity of Cs(I), and comparison of the adsorption properties of the (Calix[4] + M)/SiO2−P materials for Cs(I). (3) The third objective was the evaluation of the adsorption character of some typical fission and nonfission products such as Ru(III), Mo(VI), Pd(II), Rh(III), Zr(IV), Na(I), K(I), La(III), Rb(I), Sr(II), Ba(II), Cs(I), and Y(III) onto (Calix[4] + Oct)/SiO2−P, and the chromatographic separation of Cs(I) from a 4.0 M HNO3 containing Sr(II), La(III), Ru(III), Cs(I), Rb(I), U(VI), Mo(VI), Zr(IV), Gd(III), and Pd(II) by the (Calix[4] + TBP)/SiO2−P packed column. The possibility and feasibility of the partitioning of Cs(I) from HLW by the macroporous silicabased supramolecular recognition materials were demonstrated.

extraction chromatography in the separation of cesium from HLW is feasible. Extraction chromatography provides an effective means by which the separation and preconcentration of any of a variety of radionuclides can be accomplished.17 Extraction chromatography combines the selectivity of the solvent extraction process with the simplicity and multistage character of a column chromatographic system.18−20 This allows the use of much simpler equipment, easier materials handling, and reduced capital cost in the construction of process equipment compared to liquid−liquid solvent extraction. Moreover, extraction chromatography can significantly reduce the quantity of liquid and solid wastes in processing highly radioactive solutions and the harmful impact of radioactive waste on the environment. These characteristics of extraction chromatography are therefore beneficial to the partitioning of cesium and some specific fission products in the reprocessing process. We have recently developed an advanced partitioning process entitled SPEC (strontium/cesium partitioning from HLW by extraction chromatography), as shown in Figure 1, to effectively separate cesium and strontium from HLW.21 It was performed utilizing two novel macroporous silica-based supramolecular recognition materials: Calix[4]arene-R14/SiO2−P and DtBuCH18C6/SiO2−P.22,23 In the first column packed with Calix[4]arene-R14/SiO2−P, all of the tested metals were separated to (1) Na(I), Sr(II), Ba(II), K(I), Rh(III), Pd(II), Mo(VI), Ru(III), and Zr(IV), etc. (nonadsorptive group) and (2) Cs(I) and Rb(I) (Cs group) using 4.0 M HNO3 and water as eluents, respectively.24−29 Subsequently, the Sr-containing effluent in the second one packed with DtBuCH18C6/SiO2−P was separated to (1) Na(I), Ru(III), Zr(IV), Rh(III), Pd(II), Mo(IV), and K(I) (nonadsorptive group) and (2) Sr(II) and Ba(II) (Sr group) utilizing 2.0 M HNO3 and water as eluents.30−34 Chromatographic partitioning of Sr(II) and Cs(I) from a HNO3 medium was achieved preliminarily. However, the adsorption characteristics, the adsorption

2. EXPERIMENTAL SECTION 2.1. Reagents. Alkali metal nitrates MINO3 (MI = Na, K, Rb, and Cs), MII(NO3)2 (MII = Sr and Ba), ZrO(NO3)2·2H2O, RE(NO3)3·nH2O (RE = La, Y, and Gd, n = 3 or 6), (NH4)6Mo7O24·4H2O, and UO2(NO3)2·6H2O used were of analytical grade. The liquid nitrate reagents, such as ruthenium nitrosyl nitrate solution with 1.5 wt % Ru(III), were provided by Strem Chemicals, USA. Palladium nitrate solution with 4.5 wt % Pd(II) was provided by Tanaka Noble Metal Co. Inc., 6197

dx.doi.org/10.1021/ie202540d | Ind. Eng. Chem. Res. 2012, 51, 6196−6204

Industrial & Engineering Chemistry Research

Article

Figure 2. Molecular structures of cis- and trans-Calix[4]arene-R14.

μm. The schematic diagram of the synthesis of the (Calix[4] + M)/SiO2−P materials is shown in Figure 3.

Japan. The concentrations of all the tested metals in HNO3 solution were about 5.0 × 10−3 M. The macrocyclic supramolecular recognition agent, 1,3-[(2,4diethylheptylethoxy)oxy]-2,4-crown-6-calix[4]arene (Calix[4]arene-R14), with a purity greater than 97%, was provided by Innovation & Chimie Fine, France. Because in high HNO3 concentration medium trans-Calix[4]arene-R14 had better supramolecular recognition ability and greater selectivity for Cs(I) than cis-Calix[4]arene-R14, trans-Calix[4]arene-R14 was selected to use in the experiments. The structures of cis- and trans-Calix[4]arene-R14 are shown in Figure 2. The molecular modifiers, tri-n-butyl phosphate (TBP), octanol (Oct), and methyloctyl-2-dimethyl butanemide (MODB) containing −PO, −OH, and −NH functional groups, were available commercial products. They were used to modify Calix[4]areneR14 through intermolecular interaction force. The purpose was to boost the chemical stability of the silica-based Calix[4] arene-R14 impregnated materials in HNO3 medium. The other organic and inorganic reagents were of analytical grade and were used without further treatment. 2.2. Synthesis of Silica-Based Material. The macroporous silica-based supramolecular recognition materials (Calix[4] + M)/SiO2−P modified with Oct, MODB, and TBP, respectively, were synthesized quantitatively.21,33 Synthesis was performed through impregnating and immobilizing the supramolecular recognition agent Calix[4]arene-R14 and modifier into a macroporous SiO2−P particle support with a mean diameter of ∼50 μm. It was carried out using a method of vacuum suction. The letter “P” in SiO2−P means the styrene− divinylbenzene copolymer. The high molecular weight styrene−divinylbenzene copolymer inside the macropore SiO2−P particle support is a kind of nonpolar organic compound. It has low reactivity with the other organic compounds. Therefore, prior to the synthesis of (Calix[4] + M)/SiO2−P, it was actively pretreated using methanol and acetone respectively at least three times at room temperature. The purpose was to increase significantly its affinity to Calix[4]arene-R14. After shaking mechanically for 60 min and filtering by a membrane quartz filter of 0.45 μm pore, the resultant product was dried in a vacuum drying oven at 323 K for 24 h. The surface area, pore volume, and mean pore size of the silica-based materials were 3.36 m2/g, 1.1 cm3/g, and 0.6

Figure 3. Synthesis schematic diagram of (Calix[4] + M)/SiO2−P materials.

2.3. Adsorption of Tested Metals onto the Materials. The adsorption performance of U(VI), Sr(II), Cs(I), and the other tested metals onto the (Calix[4] + M)/SiO2−P materials by the static-state experiments was carried out at 298 K except during the experiment of the temperature effect. The temperature was controlled using a TAITEC MM-10 thermostatted water bath. A 5 cm3 volume of HNO3 solution containing the tested metals as an aqueous phase and the weighed quantity of (Calix[4] + M)/SiO2−P as a solid phase were mixed in a 50 cm3 ground glass stoppered flask and shaken mechanically at 120 rpm. The concentration of each metal and the ratio of solid phase to aqueous phase were ∼5.0 × 10−3 M and 0.25 g to 5 cm3, respectively. The concentration of HNO3 in the aqueous phase was in the range 0.3−7.0 M. After the phase separation by a membrane filter, the concentrations of the metals in the aqueous phase were measured using a Varian 700-ES simultaneous inductively coupled plasma optical emission spectrometer (ICP-OES; Varian, Inc., USA), while the contents of Na(I), K(I), Rb(I), and Cs(I) were analyzed using a Varian AA 240 FS atomic adsorption spectrometer (AAS; Varian, Inc., USA). The distribution coefficients (Kd) and the adsorbed quantities (Q) of the tested metals onto the (Calix[4] + M)/ SiO2−P materials were calculated by the following equations: Kd = 6198

Co − Ce V Ce W

(cm 3/g)

(1)

dx.doi.org/10.1021/ie202540d | Ind. Eng. Chem. Res. 2012, 51, 6196−6204

Industrial & Engineering Chemistry Research Q = (Co − Ce)

V W

(mmol/g)

Article

ratio of 0.25 g/5 cm3, shaking rate of 120 rpm, and contact time of 120 min. The corresponding results are shown in Figure 4.

(2)

where Co and Ce represent the initial and equilibrium concentrations of metals in aqueous phase, respectively. W and V denote the weight of dry (Calix[4] + M)/SiO2−P and the volume of aqueous phase used in the experiments. On the basis of the experiments above, the comparison of the adsorption properties of different silica-based Calix[4]areneR14 materials modified with Oct, TBP, and MODB and the effect of the concentrations of U(VI) and the others on the adsorption of Cs(I) were evaluated. 2.4. Chromatographic Partitioning of Cs(I). The Cs(I) separation from a HNO3 solution containing some typical fission and nonfission products was performed utilizing a (Calix[4] + TBP)/SiO2−P packed column, which was 10 mm in inner diameter and 300 mm in length. The (Calix[4] + TBP)/SiO2−P materials was packed under 0.25−0.35 MPa of N2 gas pressure. The mass, density, and volume of the adsorbent in the column were about 14.2 g, 0.62 g/cm3, and 22.8 cm3, respectively. Prior to the separation it was equilibrated by 4.0 M HNO3. No significant pressure drop was found through the adsorption column because of the monodisperse and rigid silica-based support, which is different from the traditional polymer-based one. The temperature, 298 K, in the loading and elution cycles of Cs(I), was controlled using an EYELA NTT-1200 (Tokyo Rikakikai Co. Ltd., Japan) water jacket. The feed solution prepared in advance was composed of 5.0 mM Sr(II), La(III), Ru(III), Cs(I), U(VI), Mo(VI), Zr(IV), Pd(II), Rb(I), and Gd(III) and 4.0 M HNO3. The flow rate was 1.0 cm3/min, which was controlled using a NPG-20UL pressure gage (Nihon Seimitsu Kagaku Co. Ltd., Japan) and a PDB-FT 4602 pressure limiter (Jing-Wei Friendship (Beijing) Technical Development Co., Ltd., China). When a 4 M HNO3 solution containing the tested metals was supplied to the (Calix[4] + TBP)/SiO2−P packed column, the given volumes of 4.0 M HNO3 and water as eluents were pumped subsequently flowing down through the adsorption column. Aliquots of 10 cm3 each of effluent fraction were collected using an EYELA DC-1500 autofractional collector (Tokyo Rikakikai Co. Ltd., Japan). The concentrations of P and the tested metals in effluent were then analyzed utilizing ICPOES or AAS as mentioned above. The content of total organic carbon (TOC) in effluent was determined using a TOC-V CPN analyzer (Shimadzu, Japan). The bleeding of Calix[4]arene-R14 and TBP from (Calix[4] + TBP)/SiO2−P in effluent were calculated based on the contents of P and TOC.

Figure 4. Dependence of U(VI), Cs(I), and Sr(II) adsorption onto (Calix[4] + TBP)/SiO2−P materials on HNO3 concentration in the range 0.3−6.0 M at 298 K.

Figure 4 shows the dependence of the U(VI), Cs(I), and Sr(II) adsorption onto the (Calix[4] + TBP)/SiO2−P materials on the HNO3 concentration in the range 0.3−6.0 M. It was found that, with an increase in the concentration of HNO3, Sr(II) showed almost no adsorption onto (Calix[4] + TBP)/ SiO2−P and its distribution coefficient (Kd) was below 0.2826 cm3/g. The maximum distribution coefficient (Kd) of U(VI) in the experimental conditions was 10.69 cm3/g, reflecting weak adsorption. The low value in the distribution coefficient of Sr(II) was due to the weak complexation of Sr(II) and Calix[4]arene-R14. It might result from the lack of affinity of oxygen atoms in the Calix[4]arene-R14 molecule for Sr(II). In other words, it was ascribed to the unmatched size of Sr(II) with the cavity of Calix[4]arene-R14. Contrary to the adsorption of Sr(II), the adsorption of Cs(I) onto (Calix[4] + TBP)/SiO2−P increased with increasing HNO3 concentration from 0.3 to 4.0 M and then decreased with further increase of HNO3 concentration to 6.0 M. The distribution coefficient (Kd) of Cs(I) was 0.6477 cm3/g at 0.3 M HNO3, 22.66 cm3/g at 2.0 M HNO3, 61.10 cm3/g at 4.0 M HNO3, and 28.53 cm3/g at 6.0 M HNO3. The optimum acidity for the Cs(I) adsorption was therefore determined to be 4.0 M HNO3. The weak adsorption of U(VI) onto the (Calix[4] + TBP)/ SiO2−P materials implied that, in 4.0 M HNO3 medium, U(VI) might have adverse impact on the separation of Cs(I). 3.2. Dependence of Tested Elements Adsorption on Temperature. Temperature is one of the important parameters in the adsorption of Cs(I) in the SPEC process. To make the effect of temperature clear, the adsorption of 13 typical fission and nonfission products such as Ru(III), Mo(VI), Pd(II), Rh(III), Zr(IV), Na(I), K(I), Sr(II), Ba(II), Rb(I), Cs(I), La(III), and Y(III) onto (Calix[4] + TBP)/SiO2−P in 4.0 M HNO3 was studied. The operating temperature was controlled in the range 293−333 K. The relevant results are presented in Figure 5. Figure 5 shows the effect of temperature on the adsorption of Cs(I) and the others onto (Calix[4] + TBP)/SiO2−P in 4.0 M HNO3. As can be seen, the adsorption curve of Cs(I) onto the (Calix[4] + TBP)/SiO2−P materials decreased obviously with increasing temperature. This indicates that in 4.0 M HNO3

3. RESULTS AND DISCUSSION 3.1. Adsorption of U(VI) onto (Calix[4] + TBP)/SiO2−P. It was reported that, in the reprocessing of nuclear spent fuel, more than 99% of uranium was effectively separated and recovered by liquid−liquid solvent extraction in the Purex or its modified process. As a result, there is still a small quantity of uranium in the liquid HLW. It is in doubt whether it has an effect on the chromatographic separation of Cs(I) in the SPEC process. To understand the adsorption behavior of U(VI) well, the static-state adsorption of Cs(I), Sr(II), and U(VI) onto the (Calix[4] + TBP)/SiO2−P materials in the HNO3 concentration range 0.3−6.0 M was investigated at 298 K. It was carried out at the metal concentration of ∼5.0 × 10−3 M, phase 6199

dx.doi.org/10.1021/ie202540d | Ind. Eng. Chem. Res. 2012, 51, 6196−6204

Industrial & Engineering Chemistry Research

Article

materials in 4.0 HNO3 medium was determined at 298 K. The relevant results are illustrated in Figure 6.

Figure 5. Dependence of tested metals adsorption onto (Calix[4] + TBP)/SiO2−P materials on temperature in the range 293−333 K in 4.0 M HNO3.

Figure 6. Dependence of Cs(I) adsorption onto (Calix[4] + TBP)/ SiO2−P materials on Cs(I) concentration in 4.0 M HNO3 at 298 K.

solution the adsorption of (Calix[4] + TBP)/SiO2−P for Cs(I) was an exothermic reaction. In other words, increasing the operating temperature in experimental conditions was adverse to the adsorption of Cs(I). Therefore, the reasonable temperature in the Cs(I) adsorption onto (Calix[4] + TBP)/ SiO2−P was considered to be at room temperature or at 298 K. Similar to the adsorption of Cs(I), the adsorption of the other metals also decreased with increasing temperature and their distribution coefficients (Kd) were always below 1.752 cm3/g except for 9.263 cm3/g for Rb(I). The fact that the tested metals except Rb(I) had weak or almost no adsorption onto the (Calix[4] + TBP)/SiO2−P materials was confirmed. This implies that in HNO3 solution the adsorption of (Calix[4] + TBP)/SiO2−P for the tested metals was also the exothermic reaction. Their weak or almost no adsorption contributed to the weak complexation of these metals with the Calix[4]areneR14 molecule; i.e., the size of these metals was unmatched with the cavity of Calix[4]arene-R14. In terms of the Arrhenius law d(ln D)/dT = ΔH°/RT2, the change in enthalpy (ΔH°) in the adsorption of Cs(I) onto (Calix[4] + TBP)/SiO2−P was calculated to be −30.67 kJ·mol−1. Based on the adsorption behavior of (Calix[4] + TBP)/ SiO2−P, it is predicted that in 4.0 M HNO3 the macroporous silica-based supramolecular recognition materials, (Calix[4] + Oct)/SiO2−P and (Calix[4] + MODB)/SiO2−P, might also have similar adsorption character for these metals. That is, the adsorption process of (Calix[4] + Oct)/SiO2−P or (Calix[4] + MODB)/SiO2−P for Cs(I) and the others is also an exothermic reaction. 3.3. Determination of the Adsorption Capacity of Cs(I). It was found that, in the SPEC process, the effective partitioning of Cs(I) depended on the supramolecular recognition of Calix[4]arene-R14 in (Calix[4] + M)/SiO2−P for Cs(I). Moreover, the molecular recognition ability and complexing selectivity of Calix[4]arene-R14 for Cs(I) in 4.0 M HNO3 would have a significant effect on the adsorption capacity of Cs(I), which was a valuable parameter in the evaluation of the adsorption property of (Calix[4] + M)/SiO2− P. To understand the possibility and feasibility of application of (Calix[4] + M)/SiO2−P in the SPEC process, the saturated adsorption capacity of Cs(I) onto (Calix[4] + TBP)/SiO2−P

Figure 6 shows the effect of the concentration of Cs(I) on the adsorption of (Calix[4] + TBP)/SiO2−P in 4.0 M HNO3. It was found that in the aqueous phase, with an increase in the Cs(I) concentration, the quantity of Cs(I) adsorbed by (Calix[4] + TBP)/SiO2−P increased rapidly and then reached equilibrium. The saturated adsorption capacity of (Calix[4] + TBP)/SiO2−P for Cs(I) in 4.0 M HNO3 was determined to 0.22 mmol/g dry adsorbent. Such a value in the adsorption capacity reflected that (Calix[4] + TBP)/SiO2−P had excellent adsorption ability for Cs(I). It is beneficial to application of the (Calix[4] + TBP)/SiO2−P materials in the separation of Cs(I) in the SPEC process. 3.4. Comparison of the Adsorption Behavior of (Calix[4] + M)/SiO2−P. The molecular modifiers TBP, Oct, and MODB used in the experiments are organic compounds containing −PO, −OH, and −NH functional groups. It was out of question that Cs(I) would have different adsorption properties onto (Calix[4] + M)/SiO2−P. The effect of the modifier on the Cs(I) adsorption was investigated in the HNO3 concentration range 0.3−7.0 M at 298 K. The results are depicted in Figure 7. Figure 7 shows the comparison of the adsorption properties of (Calix[4] + M)/SiO2−P (M = TBP, Oct, and MODB) for Cs(I) in the range 0.3−7.0 M HNO3. It was clear that, with an increase in the HNO3 concentration, the silica-based supramolecular recognition material, either (Calix[4] + Oct)/SiO2− P, (Calix[4] + TBP)/SiO2−P, or (Calix[4] + MODB)/SiO2− P, showed high adsorption ability for Cs(I). The adsorption of Cs(I) on the (Calix[4] + M)/SiO2−P materials increased gradually from 0.3 M HNO3 to 3.0 or 4.0 M HNO3 and then decreased obviously to 7.0 M HNO 3 . The optimum concentration of HNO3 in the adsorption of Cs(I) was 4.0 M for (Calix[4] + Oct)/SiO2−P and (Calix[4] + TBP)/SiO2− P as well as 3.0 M for (Calix [4] + MODB)/SiO2−P. The adsorption order of Cs(I) onto the materials in either 3.0 or 4.0 M HNO3 was followed as 6200

dx.doi.org/10.1021/ie202540d | Ind. Eng. Chem. Res. 2012, 51, 6196−6204

Industrial & Engineering Chemistry Research

Article

Figure 7. Comparison of adsorption of Cs(I) onto (Calix[4] + M)/ SiO2 −P modified with Oct, MODB, and TBP in the HNO3 concentration range 0.3−7.0 M at 298 K.

(Calix[4] + Oct)/SiO2 −P

Figure 8. Evaluation of adsorption of (Calix[4] + Oct)/SiO2−P for 13 typical metals with a change in HNO3 concentration from 0.3 to 7.0 M at 298 K.

> (Calix[4] + TBP)/SiO2 −P > (Calix[4] + MODB)/SiO2 −P

the materials is valuable for separation of Cs(I) in the SPEC process. This implies that 16 species of trivalent rare earth metals (REs(III)) including Y(III) and La(III) to Lu(III) have weak or almost no adsorption onto (Calix[4] + Oct)/SiO2−P because of the similarities among the rare earths in chemical properties. Namely, all RE(III) metals might have no adverse impact on the adsorption of Cs(I). On the other hand, it is known that the chemical properties of the trivalent minor actinides (MA(III)) such as Am(III) and Cm(III) are very close to those of RE(III) metals. It is reasonable to assume that MA(III) would also have no adverse impact on the Cs(I) adsorption. 3.6. Partitioning of Cs(I) by Extraction Chromatography. To understand the possibility and feasibility of Cs(I) separation, the chromatographic partitioning of Cs(I) from a simulated 4.0 M HNO3 HLW solution containing La(III), Ru(III), U(VI), Gd(III), Mo(VI), Zr(IV), Cs(I), Rb(I), Sr(II), and Pd(II) was performed. It was conducted utilizing the (Calix[4] + TBP)/SiO2−P materials packed column. Prior to the partitioning performance the (Calix[4] + TBP) /SiO2−P materials were pre-equilibrated using 4.0 M HNO3. The concentrations of the tested metals in feed solution were about 5.0 × 10−3 M. La(III) and Gd(III) were used to represent all the RE(III) elements based on the similarity in chemical properties. Following the automatic collection of the effluent, the mass balance of the tested metals was calculated. The partitioning results utilizing 4.0 M HNO3 and water as eluents are illustrated in Figure 9. As can be seen, during loading of the feed solution to the column, La(III), Sr(II), Ru(III), Mo(VI), Zr(IV), Gd(III), and Pd(II) showed no adsorption onto (Calix[4] + TBP)/SiO2−P and quickly leaked out the column along with 4.0 M HNO3. Such a weak adsorption behavior of these metals in the separation operation was quite similar to that in the case of batch experiments. It resulted from the weak complexation of the supermolecular recognition agent Calix[4]arene-R14 with these metals. Taking into account the adsorption behavior of La(III) and Gd(III), it is predicted that in 4.0 HNO3 solution almost all

The (Calix[4] + Oct)/SiO2−P materials showed the maximum adsorption for Cs(I). The increase in the distribution coefficient (Kd) of Cs(I) onto (Calix[4] + Oct)/SiO2−P in the range 0.3−4.0 M HNO3 was ascribed to the effective matching of size between the calixarene cavity and Cs(I) as well as πbonding interactions with the arene groups, and structural reorganization of the molecule. It resulted in the effective complexation of Cs(I) with (Calix[4] + Oct)/SiO2−P. The decrease in the distribution coefficient (Kd) of Cs(I) in the HNO3 concentration range 4.0−7.0 M revealed that the protonation of Calix[4]arene-R14 with HNO3 was dominative. Namely, in the high HNO3 concentration, Calix[4]arene-R14 was associated with HNO3 and formed a 1:1 type of complex through hydrogen bonding.34−37 It might result in a significant decrease in the available concentration of Calix[4]arene-R14 inside (Calix[4] + Oct)/SiO2−P being capable of complexing Cs(I). As a result, the decrease in the distribution coefficient (Kd) of Cs(I) onto (Calix[4] + Oct)/SiO2−P in 4.0−7.0 M HNO3 occurred. 3.5. Evaluation of Adsorption Property of (Calix[4] + Oct)/SiO2−P. Based on the above results, to evaluate the adsorption property of (Calix[4] + Oct)/SiO2−P, the effect of the HNO3 concentration in the range 0.3−7.0 M on the adsorption of Cs(I) and the other tested metals onto the materials was investigated at 298 K. The results drawn in three dimensions are shown in Figure 8. Figure 8 shows the dependence of the adsorption of 13 species of the typical fission and nonfission products Cs(I), Na(I), La(III), Y(III), Ru(III), Pd(II), Rh(III), Zr(IV), Sr(II), Ba(II), K(I), Rb(I), and Mo(VI) onto the (Calix[4] + Oct)/ SiO2−P materials on the HNO3 concentration in the range 0.3−7.0 M at 298 K. It was clear that in 4.0 M HNO3 the tested (Calix[4] + Oct)/SiO2−P showed strong adsorption ability and high selectivity for Cs(I) over all of the tested metals except Rb(I). This means that elements such as Na(I), K(I), Sr(II), and Ba(II) in groups IA and IIA in the periodic table of the elements show almost no adsorption onto (Calix[4] + Oct)/ SiO2−P. Moreover, no adsorption of La(III) and Y(III) onto 6201

dx.doi.org/10.1021/ie202540d | Ind. Eng. Chem. Res. 2012, 51, 6196−6204

Industrial & Engineering Chemistry Research

Article

Figure 10. Adsorption and elution mechanism of Cs(I) in chromatographic partitioning by (Calix[4] + M)/SiO2−P materials. Figure 9. Chromatographic partitioning of Cs(I) from a simulated 4.0 M HNO3 solution by (Calix[4] + TBP)/SiO2−P packed column at 298 K.

trivalent RE(III) including Y(III) and from La(III) to Lu(III) as well as trivalent MA(III) such as Am(III) and Cm(III) contained in HLW have no adsorption onto (Calix[4] + TBP)/ SiO2−P. They might flow into effluent along with La(III), Gd(III), and 4.0 M HNO3. Namely, 16 species of REs(III), Am(III), and Cm(III) might have no adverse impact on the separation of Cs(I). As water was supplied to the adsorption column, Cs(I) adsorbed onto (Calix[4] + TBP)/SiO2 −P was eluted efficiently. The elution band appearing in the elution curve of Cs(I) was narrow and sharp, and showed no elution tailing. Cs(I) was desorbed completely from the loaded (Calix[4] + TBP)/SiO2−P materials and quickly flowed into effluent. Such excellent elution behavior of Cs(I) in the column operation reflected that the macroporous silica-based supermolecular recognition agent impregnated materials had rapid elution kinetics for Cs(I). This was ascribed to the quick decomposition of the complex formation of Cs(I) and (Calix[4] + TBP)/SiO2−P with a rapid decrease in the NO3− concentration in the resin bed. Meanwhile, the adsorption and elution behavior of Rb(I) were very similar to those of Cs(I). Rb(I) adsorbed by (Calix[4] + TBP)/SiO2−P was eluted efficiently with water and flowed into the effluent along with Cs(I). On the other hand, U(VI) showed weak adsorption onto the (Calix[4] + TBP)/SiO2−P materials. A small portion of U(VI) was eluted and flowed into effluent along with Cs(I) and Rb(I). This showed the slightly adverse impact of U(VI) on the Cs(I) separation. The adsorption and elution mechanism of Cs(I) in the loading and elution cycles is shown in Figure 10. According to the mass balance, the recovery percent of the tested metals was calculated to be 99.2% for Cs(I) and in the range 98.5−102.3% for the other tested metals. The satisfactory partitioning and recovery of Cs(I) from the tested metals was achieved. The contents of P and TOC in effluent were determined by ICP-OES and TOC analyzers. The corresponding concentrations of Calix[4]arene-R14 and TBP leaked from the (Calix[4] + TBP)/SiO2−P materials were calculated. The relevant bleeding results are shown in Figure 11. With supply of the feed solution, 4.0 M HNO3, and water to the adsorption column, the concentration of Calix[4]arene-R14 in effluent almost remained constant and the average value was 123.1 ppm, while an obvious change in the concentration of

Figure 11. Bleeding of Calix[4]arene-R14 and TBP from (Calix[4] + TBP)/SiO2−P in loading and elution of Cs(I) at 298 K.

TBP in effluent was found. As the feed solution and 4.0 M HNO3 were supplied to the column, the concentration of TBP in this range remained constant and the corresponding value was 153.5 ppm. However, when water as an eluent was subsequently supplied to the column, the content of TBP in the effluent rapidly increased and the average value in this range was 263.1 ppm. This might be attributed to the intermolecular association of TBP with HNO3; in other words, it was related to the solubility of TBP in 4.0 M HNO3 and water. In the adsorption and washing step in 4.0 M HNO3, TBP was associated with HNO3 and formed a 1:1 type of complex . In the elution step with water, the association of TBP with HNO3 was destroyed because of the formation of hydrogen bonding between TBP and water. Because the solubility of TBP was higher in water than in 4.0 M HNO3, the bleeding of TBP from (Calix[4] + TBP)/SiO2−P in effluent remained constant basically in the adsorption and washing step and increased obviously in the elution step. The corresponding average concentration of organic compounds in the loading and elution of Cs(I) was 123.1 ppm for Calix[4]arene-R14 and 218.5 ppm for TBP. Recently, the preliminary adsorption behavior of U(VI), Pu(IV), Pu(III), and Np(V) onto (Calix[4] + TBP)/SiO2−P was studied. It showed that in radioactive experiments Pu(III), Np(V), and almost all of U(VI) had no adsorption in the loading and elution of Cs(I). Only Pu(IV) and a little of U(VI) had an adverse impact on the Cs(I) separation. This reveals that, in HNO3 medium, applying the (Calix[4] + TBP)/SiO2− P materials in effective partitioning of Cs(I) from HLW by 6202

dx.doi.org/10.1021/ie202540d | Ind. Eng. Chem. Res. 2012, 51, 6196−6204

Industrial & Engineering Chemistry Research

Article

(3) Ludwing, R.; Dzung, N. T. K. Calixarene-Based Molecules for Cation Recognition. Sensors 2002, 2, 397. (4) Delmau, L. H.; Lefranc, T. J.; Bonnesen, P. V.; Bryan, J. C.; Presley, D. J.; Moyer, B. A. Fundamental Studies Regarding Synergism Between Calix[4]arene-Bis(tert-Octylbenzo-Crown-6) and Alcohol Modifiers in the Solvent Extraction of Cesium Nitrate. Solvent Extr. Ion Exch. 2005, 23, 23. (5) Baron, P.; Lecomte, M.; Boullis, B.; Simon, N.; Warin, D. Separation of the Long Lived Radionuclides: Current Status and Future R&D Program in France. Proceedings of Global 2003, New Orleans, LA, USA; 2003; pp 565−574. (6) Ji, H.-F.; Dabestani, R.; Brown, G. M.; Hettich, R. L. Synthesis and Sensing Behavior of Cyanoanthracene Modified 1,3-Alternate Calyx[4]benzocrown-6: A New Class of Cs+ Selective Optical Sensors. J. Chem. Soc., Perkin Trans.2 2001, 585. (7) Sachleben, R. A.; Urvoas, A.; Bryan, J. C.; Haverlock, T. J.; Hay, B. H.; Moyer, B. A. Dideoxygenated Calix[4]arene Crown-6 Ethers Enhanced Selectivity for Caesium over Potassium and Rubidium. Chem. Commun. 1999, 17, 1751. (8) Simon, N.; Tournois, B.; Eymard, S.; Volle, G.; Rivalier, P.; Leybros, J.; Lanoe, J. Y.; Tronche, N. R.; Ferlay, G.; Dozol, J. F. Cs Selective Extraction from High Level Liquid Waste With Crown Calixarene: Where Are Today? International Conference Atalante 2004, Advances for Future Nuclear Fuel Cycles, Nimes, France, June 21−25, 2004; 2004; pp 1−5. (9) Simon, N.; Eymard, S.; Tournois, B.; Dozol, J. F. Caesium Extraction from Acidic High Level Liquid Wastes with Functionalized Calixarenes. International Conference Atalante 2000, Scientific Research on the Back-End of the Fuel Cycle for 21st Century, Avignon, France, Oct 24−26, 2000; 2000; pp 1−8. (10) Walker, D. D.; Norato, M.; Campbell, S. G.; Crowder, M. L.; Fink, S. D.; Fondeur, F. F.; Geeting, M. W.; Kessinger, G. F.; Pierce, R. A. Cesium Removal from Savannah River Site Radioactive Waste Using the Caustic Side Solvent Extraction (CCSX) Process. WSRCMS-2003-00317, 2003, pp 1−14. (11) Moyer, B. A.; Bonnesen, P. V.; Bryan, J. C.; Engle, N. L.; Levitskaia, T. G.; Sachleben, R. A.; Bartsch, R. A.; Talanov, V. S.; Gibson, H. W.; Jason, J. W. (Chemical and Analytical Science Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA). Next Generation Extractants for Cesium Separation from High-Level Waste: From Fundamental Concepts To Site Implementation. DEAC05-00OR22725, 2001, pp 3−10. (12) Riddle, C. L.; Baker, J. D.; Law, J. D.; McGrath, C. A.; Meikrantz, D. H.; Mincher, B. J.; Peterman, D. R.; Todd, T. A. Fission Product Extraction (FPEX): Development of a Novel Solvent for the Simultaneous Separation of Strontium and Cesium from Acidic Solutions. Solvent Extr. Ion Exch. 2005, 23, 449. (13) Mincher, B. J.; Mezyk, S. P.; Bauer, W. F.; Elias, G.; Riddle, C.; Peterman, D. R. FPEX γ-Radiolysis in the Presence of Nitric Acid. Solvent Extr. Ion Exch. 2005, 25, 593. (14) Bonnesen, P. V.; Delmau, L. H.; Moyer, B. A.; Lumetta, G. J. Development of Effective Solvent Modifiers for the Solvent Extraction of Cesium from Alkaline High-Level Tank Waste. Solvent Extr. Ion Exch. 2003, 21, 141. (15) Bazelarire, E.; Gobunova, M. G.; Bonnesen, P. V.; Moyer, B. A.; Delmau, L. H. pH-Switchable Cesium Nitrate Extraction with Calix[4]arene Mono and Bis(benzo-Crown-6) Ethers Bearing Amino Functionalities. Solvent Extr. Ion Exch. 2004, 22, 637. (16) Bonnesen, P. V.; Delmau, L. H.; Moyer, B. A.; Leonard, R. A. A Robust Alkaline-Side CSEX Solvent Suitable for Removing Cesium from Savannah River High Level Waste. Solvent Extr. Ion Exch. 2000, 18, 1079. (17) Horwitz, E. P.; Chiarizia, R.; Dietz, M. L. A Novel StrontiumSelective Extraction Chromatographic Resin. Solvent Extr. Ion Exch. 1992, 10, 313. (18) Chiarizia, R.; Horwitz, E. P.; Dietz, M. L. Acid Dependency of the Extraction of Selected Metal Ions by a Strontium-Selective Extraction Chromatographic Resin: Calculated vs. Experimental Curves. Solvent Extr. Ion Exch. 1992, 10, 337.

extraction chromatography is promising. The relevant investigations are being carried out.

4. CONCLUSIONS Cs(I) and Sr(II), two fission products contained in HLW, are heat generators. Because they are harmful to the vitrified HLW in final geological disposal, the effective partitioning of them from acidic HLW in advance is valuable. In the SPEC process developed recently, Cs(I) and Sr(II) are separated through two kinds of macroporous silica-based supramolecular recognition materials. To understand the adsorption character of Cs(I), silica-based 1,3-[(2,4-diethylheptylethoxy)oxy]-2,4-crown-6calix[4]arene (Calix[4]arene-R14) impregnated supramolecular recognition materials, (Calix[4] + M)/SiO2−P, were synthesized by modification with TBP, Oct, and MODB. The adsorption behavior of Cs(I) onto (Calix[4] + M)/ SiO2−P in HNO3 was investigated. It was performed by examining the effects of U(VI), the other typical fission and nonfission products, the HNO3 concentration, and temperature. (Calix[4] + M)/SiO2−P showed excellent adsorption ability and selectivity for Cs(I) except for Rb(I) and a little of U(VI). The adsorption order of Cs(I) onto the modified materials was (Calix[4] + Oct)/SiO2−P > (Calix[4] + TBP)/ SiO2−P > (Calix[4] + MODB)/SiO2−P. The optimum HNO3 concentration in the Cs(I) adsorption was 4.0 M for (Calix[4] + Oct)/SiO2−P and (Calix[4] + TBP)/SiO2−P and 3.0 M for (Calix[4] + MODB)/SiO2−P. Based on the results, the chromatographic separation of Cs(I) from 4.0 M HNO3 containing La(III), Sr(II), Ru(III), U(VI), Mo(VI), Cs(I), Rb(I), Zr(IV), Gd(III), and Pd(II) was conducted by (Calix[4] + TBP)/SiO2−P packed column. The effective separation of Cs(I) from the others was achieved. The results demonstrated that the macroporous silica-based supramolecular recognition materials (Calix[4] + M)/SiO2−P are promising to apply in chromatographic partitioning of Cs(I) in the SPEC process.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 571 8795 3919. Fax: +86 571 8795 3919. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge financial support for the project sponsored by the National Natural Science Foundation of China under Contract Nos. 20671081 and 91126021, the Specialized Research Fund for the Doctoral Program of Higher Education under Contract No. 20070335183, and the Zhejiang Provincial Natural Science Foundation of China under Contract Nos. Y406022 and Y4110002.

■

REFERENCES

(1) Madic, C. Overview of the Hydrometallurgical and Pyrometallurgical Process Studied World-Wide for the Partitioning of High Active Nuclear Wastes. International Symposium NUCEF 2001, Oct 31−Nov 2, 2001, Japan Atomic Energy Research Institute; Conf 2002-004; Japan Atomic Energy Research Institute: Toaki, Ibaraki, Japan, 2002; pp 27− 37. (2) Asfari, Z.; Wenger, S.; Vicens, J. Calixcrowns and Related Molecules. J. Inclusion Phenom. Mol. Recognit. Chem. 1994, 19, 137. 6203

dx.doi.org/10.1021/ie202540d | Ind. Eng. Chem. Res. 2012, 51, 6196−6204

Industrial & Engineering Chemistry Research

Article

(19) Dietz, M. L.; Horwitz, E. P.; Chiarizia, R.; Diamona, H. Novel Extraction Chromatographic Materials for the Separation and Preconcentration of Radionuclides. Proceedings of the International Solvent Extraction Conference (ISEC’93). University of York, England, Sept 9−15, 1993; 1993; Vol. 3, p 1587. (20) Arena, G.; Casnati, A.; Contino, A.; Mirone, L.; Sciotto, D.; Ungaro, R. Synthesis of New Calixcrowns and Their Anchoring to Silica Gel for the Selective Separation of Cs+ and K. Chem. Commun. 1996, 19, 2277. (21) Zhang, A.; Hu, Q.; Chai, Z. SPEC: A New Process for Strontium and Cesium Partitioning Utilizing Two Macroporous Silica-Based Supramolecular Recognition Agents Impregnated Polymeric Composites. Sep. Sci. Technol. 2009, 44, 2146. (22) Zhang, A.; Wei, Y. Z.; Hoshi, H.; Koma, Y.; Kamiya, M. Chromatographic Separation of Cesium and Strontium from High Level Liquid Waste Using Silica-Based Calixarene-Crown Extraction Resins. Proceedings of Global 2005, Japan, Oct 9−13, 2005; 2005; pp 25−27. (23) Zhang, A.; Wang, W.; Chai, Z.; Kumagai, M. Separation of Strontium Ions From a Simulated Highly Active Liquid Waste Using a Composite of Silica-Crown Ether in a Polymer. J. Sep. Sci. 2008, 31, 3148. (24) Zhang, A.; Kuraoka, E.; Kumagai, M. Development of the Chromatographic Partitioning of Cesium and Strontium Utilizing Two Macroporous Silica-Based Calix[4]arene-Crown and Amide Impregnated Polymeric Composites: PREC Partitioning Process. J. Chromatogr., A 2007, 1157, 85. (25) Zhang, A.; Xiao, C.; Xue, W.; Chai, Z. Chromatographic Separation of Cesium by a Macroporous Silica-Based Supramolecular Recognition Agent Impregnated Material. Sep. Purif. Technol. 2009, 66, 541. (26) Zhang, A.; Hu, Q.; Chai, Z. Chromatographic Partitioning of Cesium by a Macroporous Silica-Calix[4]arene-Crown Supramolecular Recognition Composite. AIChE J. 2010, 56, 2632. (27) Zhang, A.; Hu, Q.; Chai, Z. Synthesis of a Novel Macroporous Silica-Calix[4]arene-Crown Polymeric Composite and Its Adsorption for Alkali Metal and Alkaline Earths. Ind. Eng. Chem. Res. 2010, 49, 2047. (28) Zhang, A.; Xiao, C.; Hu, Q.; Chai, Z. Synthesis of a Novel Macroporous Silica-Calix[4]arene-Crown Supramolecular Recognition Material and Its Adsorption for Cesium and Some Typical Metals in Highly Active Liquid Waste. Solvent Extr. Ion Exch. 2010, 28, 526. (29) Zhang, A.; Hu, Q. Chromatography Partitioning of Cesium by a Modified Macroporous Silica-Based Supramolecular Recognition Material. Chem. Eng. J. 2010, 159, 58. (30) Zhang, A.; Chen, C.; Wang, W. Adsorption Behavior of Sr(II) and Some Typical Co-Existent Metals Contained in High Level Liquid Waste onto a Modified Macroporous Silica-Based Polymeric DtBuCH18C6 Composite. Solvent Extr. Ion Exch. 2008, 26, 624. (31) Zhang, A.; Wang, W.; Chai, Z.; Kuraoka, E. Modification of a Novel Macroporous Silica-Based Crown Ether Impregnated Polymeric Composite with 1-Dodecanol and Its Adsorption for Some Fission and Non-Fission Products Contained in High Level Liquid Waste. Eur. Polym. J. 2008, 44, 3899. (32) Zhang, A.; Chen, C.; Chai, Z.; Kumagai, M. SPEC Process III. Synthesis of a Macroporous Silica-Based Crown Ether Impregnated Polymeric Composite Modified with 1-Octanol and Its Adsorption Capacity for Sr(II) Ions and Some Typical Co-Existent Metal Ions. Adsorpt. Sci. Technol. 2008, 26, 705. (33) Zhang, A.; Xiao, C.; Chai, Z. SPEC process II. Adsorption of Strontium and Some Typical Co-Existent Elements Contained in High Level Liquid Waste onto a Macroporous Silica-Based Crown Ether Impregnated Functional Composite. J. Radioanal. Nucl. Chem. 2009, 280, 181. (34) Dietz, M. L.; Bond, A. H.; Clapper, M.; Finch, J. W. Isomer Effects in the Extraction of Metal Ions from Acidic Nitrate Media by Dicyclohexano-18-Crown-6. Radiochim. Acta 1999, 85, 119.

(35) Kriz, J.; Dybal, J.; Makrlik, E.; Budka, J. Interaction of Hydronium Ion with Dibenzo-18-Crown-6: NMR, IR, and Theroretical Study. J. Phys. Chem. A 2008, 112, 10236. (36) Dybal, J.; Makrlik, E.; Vanura, P.; Budka, J. Protonation of 25,27-Bis(1-Octyloxy) Calix[4]arene-Crown-6 in the 1,3-Alternate Conformation. Monatsh. Chem. 2008, 139, 1175. (37) Michael, M. N. The Ionic Hydrogen Bond. 2. Intramolecular and Partial Bonds. Protonation of Polyethers, Crown Ethers, and Diketones. J. Am. Chem. Soc. 1983, 105, 4906.

6204

dx.doi.org/10.1021/ie202540d | Ind. Eng. Chem. Res. 2012, 51, 6196−6204