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Co-condensation Synthesis of a Novel DCH18C6-functionalized Organosilica for Strontium Adsorption
Available online at www.sciencedirect.com
Procedia Chemistry 7 (2012) 616 – 621
ATALANTE 2012 International Conference on Nuclear Chemistry for Sustainable Fuel Cycles
Co-condensation synthesis of a novel DCH18C6-functionalized organosilica for strontium adsorption Gang Ye , Feifei Bai, Jichao Wei, Jianchen Wang, Jing Chen Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China
Abstract A novel kind of organosilica functionalized with dicyclohexano-18-crown-6 (DCH18C6) was synthesized in this study. The DCH18C6 molecule, well known as an excellent complexing agent of strontium, was modified for chemical bonding to the organosilica matrix via a co-condensation approach. 29Si and 13C solid-state NMR, ESEM and N2 adsorption-desorption measurement were employed to characterize the organosilica’s structure and surface properties. The functionalized organosilica showed favourable adsorption capacity to the strontium, and the distribution coefficient (Kd) of 114.5 cm3/g could be obtained in 1 mol/L HNO3 solution. This novel functionalized organosilica might be potentially used for the separation of the strontium in radioactive liquid waste. ©2012 2012Elsevier The Authors. Publishedand/or by Elsevier B.V. under responsibility of the Chairman of the ATALANTE 2012 © B.V...Selection peer-review SelectionCommittee and/or peer-review under responsibility of the Chairman of the ATALANTE 2012 Program Program Keywords: Crown ether; Strontium; Organosilica; Synthesis; HLLW
1. Introduction Partitioning of hazardous fission products has great value for the vitrification treatment of the radioactive liquid waste. As a kind of long lived nuclide and heat generator, 90Sr (T1/2=28.5 years) generally exists in the high level liquid waste (HLLW)[1]. Separating the 90Sr prior to vitrification can not only help to reduce the volume of the disposable waste, but also lower the risk of matrix deformation caused by the radiated heat during the longtime storage[2].
Corresponding author. Tel.: +86-10 89796063; fax: +86-10 62771740 E-mail address: [email protected]
1876-6196 © 2012 Elsevier B.V...Selection and/or peer-review under responsibility of the Chairman of the ATALANTE 2012 Program Committee doi:10.1016/j.proche.2012.10.094
Gang Ye et al. / Procedia Chemistry 7 (2012) 616 – 621
Due to the strong complexing ability and selectivity, the macrocyclic ether family play an important role in metal separation. It has been reported that dicyclohexano-18-crown-6 (DCH18C6), as well as its tert-butyl substituted derivative, exhibited favorable extraction ability and selectivity towards Sr(II)[3]. Based on such complexing agents, solvent extraction and extraction chromatography have been developed as two widelyaccepted methods for the separation of strontium from radioactive stream. Extraction chromatography is characterized by loading the complexing agents onto an inert support material. With the advantage of minimal organic diluents utilization and less waste accumulation, extraction chromatography is considered as a promising alternative for replacing solvent extraction toward metal separation in radioactive liquid waste[4]. Horwitz et al. prepared a novel Sr-selective extraction chromatographic resin (Sr-Spec, abbreviation for Strontium Specific) by impregnating a 1-octanol solution containing DtBuCH18C6 into an inert polymeric support (XAD-7)[5]. This innovative work opened a new period in radiostrontium separation and analysis. Zhang et al. developed the DtBuCH18C6 impregnated macroporous silica-based polymeric composite and proposed the SPEC process for chromatographic separation the Sr(II) in HLLW[6]. Compared to the previous polymeric resin, the silica-based adsorption materials have excellent thermal and hydrolytic stability. However, so far, most of the reported adsorption materials for the selective separation of Sr(II) were developed based on physical methodology, such as impregnation or inclusion. The complexing agent was immobilized into the substrate material by van der Waals force or hydrogen bond. These resins suffered from the leak of extractant during the adsorption and elution, which would lead to an obvious decrease of separation efficiency in cycle use. In this study, a novel organosilica functionalized with dicyclohexano-18-crown-6 (DCH18C6) was synthesized via a co-condensation approach. The synthesis, structure characterization and surface morphology were detailed. The adsorption behavior toward Sr(II) in HNO3 solution was evaluated. 2. Experimental 2.1. Synthesis of the DCH18C6-functionalized organosilica 4,4’-Di(aminocyclohexyl)-18-crown-6 [1]. The synthesis of cis-di(aminocyclohexyl)-18-crown-6 was started with dibenzo-18-crown-6 molecule [0] according to a three-stage procedure, namely, nitration, reduction and catalytical hydrogenation. The synthesis operation and the structural characterization can be seen in our earlier report[7]. 4,4’-Bis((3-(triethoxylsilyl)propyl)amino)dicyclohexyl-18-crown-6 [2]. 2.01 g compound [1] (5 mmol) was dissolved in 150 mL tetrahydrofuran (THF). 1.38 g potassium carbonate (10 mmol) and 1.66 g potassium iodide (10 mmol) were added into the solution. Then, with nitrogen protection and vigorous stirring, 3chloropropyltriethoxysilane (CPTES) (2.40 g, 10 mmol) was dropwise added. The mixture was heated under reflux for 5 h, the solid salt was separated by filtration. The crude product with brown color was obtained from the filtrate after removing the THF under vaccum. The product was purified via a procedure similar to that reported by Dubois et al[8]. Yield: 47%. 1H NMR (300 MHz, CDCl3): į 3.79-3.70 (q, 12H), 3.67-3.54 (m, 16H), 3.08 (m, 4H), 2.69 (m, 6H), 1.95-1.86 (m, 4H), 1.60-1.52 (m, 8H), 1.40-1.22 (m, 4H), 1.19 (t, 18H), 0.74 (t, 4H). DCH18C6-functionalized organosilica [3]. The DCH18C6-functionalized organosilica was prepared through the co-condensation reaction of [2] and tetraethoxysilane (TEOS). 0.50 mol [2] and 0.50 mol TEOS were dissolved in quantitative anhydrous ethanol with total concentration of 1 mol/L. Under stirring, a stoichiometric amount of water was slowly added with a syringe, followed by 2 wt.% di-n-butyltin dilaurate (DBTL) as catalyst. The mixture was further stirred for 10 min. Homogeneous solution was obtained and set for 3 days at ambient temperature for gelation. The product was granulated, dried in 60 ºC oven, followed by thermal treatment at 120 ºC under vacuum for 8 hours. The catalyst and the unreacted monomers were washed off with hot acetone. The resulting powder was collected and dried in 60 ºC vacuum oven for 24 hours.
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2.2. Adsorption of Sr(II) in HNO3 solution Batch adsorption experiments toward Sr(II) in nitric acid media were performed in 25 ºC thermostat. 0.05 g organosilica samples were added into 1.0~6.0 mol/L HNO3 containing 1.0×10-3 mol/L Sr(II) ion. The phase ratio was set to be 0.01 g/1 mL. After vigorous agitating for 2 h, the aqueous phase was separated with micro-pore filter. The residual amount of the Sr(II) in the solution was measured by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES). The distribution coefficient Kd was defined as shown in Equation (1) for the evaluation of the sample’s adsorption ability, (1) where C0 and Ct represented the initial and residual Sr(II) concentration in the aqueous phase, M and V were the weight of organosilicas and the volume of the stock solution, respectively. 3. Result and discussion 3.1. Synthesis and characterization The co-condensation synthesis of the DCH18C6-functionalized organosilica is illustrated in Scheme 1. O O O
O O
HNO3
N2H4 H2O
H2
O
HAc
Pd/C
Ru catalyst
H2N
O
O
O
O
NH2
O O [1]
[0] Cl
Si(OEt)3 K2CO3, KI
(EtO)3Si H N
Si(OEt)3
O O
O
O
H N
O O [2]
TEOS
EtOH, H2O
O O O
O O O
Si
Si
O HN
O
O
O
NH
O O [3]
Scheme 1. Synthesis route of the DCH18C6-functionalized organosilica by co-condensation.
Dicyclohexyl-18-crown-6 modified with amino groups (compound [1]) was prepared based on dibenzo-18crown-6 [0] by selective nitration, reduction and catalytical hydrogenation. Then, the bissilylated crown ether monomer [2] could be obtained through the nucleophilic substitution reaction between [1] and 3-
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Gang Ye et al. / Procedia Chemistry 7 (2012) 616 – 621
chloropropyltriethoxysilane (CPTES). The purified monomer [2] was condensated together with tetraethoxysilane (TEOS) following a typical sol-gel process to produce the functionalized organosilica. For comparative study, another intrinsic silica sample was synthesized only by the sol-gel process of TEOS molecules. The two samples were noted as Crown-X (X=50 or 0), where the X referred to the proportion of the crown ether monomer. Solid state 29Si MAS NMR provides information about the silica matrix of the functionalized organosilica. Fig. 1 (a) shows the 29Si NMR spectra of the Crown-X samples. The three regions of major intensity centered at 110.2 ppm (Q4), -102.2 ppm (Q3) and -67.5 ppm (T3) correspond to silicon atoms in Si(-O-)4, EtOSi(-O-)3 and RSi(-O-)3 segments. Solid state 13C CP-MAS NMR was used to identify the organic functional groups of samples. The spectra are shown in Fig. 1 (b). The signals at 17.6 and 60.3 ppm correspond to the two carbon atoms in residual ethoxy group. These two peaks refer to an incomplete hydrolysis of the ethoxy groups during the synthesis. The resonances at 77.5, 71.0, 54.1, 23.0 ppm are attributed to the carbon atoms in the ether ring and cyclohexyl group of the DCH18C6. The three carbon atoms in the spacer connecting the Si-O framework and the DCH18C6 show their resonances at 47.9, 26.4 and 10.5 ppm.
Fig. 1. Solid state NMR spectra of Crown-0 and Crown-50. (a) 29Si CP-MAS NMR spectra; (b) 13C CP-MAS NMR spectra.
3.2. Surface property Surface and pore structural properties of the organosilica resins were evaluated by N2 adsorption-desorption measurements. The results calculated by BET method were shown in Table 1. The result suggests that the employment of the crown ether monomer affects the surface property of the organosilica particles. The specific surface area of 185.03 m2/g was obtained for Crown-50, with the pore volume reaching up to 0.068 cm3/g. Compared to our previous work, there was a substantial improvement of the surface property, which could be attributed to the advantage of the co-condensation synthesis. Such one-step approach, without the post-grafting treatment, avoided the blocking of the channels in the organosilica matrix and provided the particles with better surface property. Table 1. Elemental analysis and N2 adsorption-desorption measurement of the organosilica Crown-0 and Crown-50.
wt.% C
wt.% H
wt.% N
Particle size (ȝm)
Crown-0
17.01
3.66
Null
10.40
152.24
0.044
Crown-50
37.98
6.30
2.66
5.57
185.03
0.068
Sample
Elemental analysis
BET surface area (m2/g)
Pore volume (cm3/g)
The size and distribution of the samples was examined by laser particle analyzer. The result showed that the average size of 10.40 ȝm and 5.57 ȝm was obtained for Crown-0 and Crown-50, respectively. The morphology of the organosilica particles was studied by environmental scanning electron microscopy (ESEM). The
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Gang Ye et al. / Procedia Chemistry 7 (2012) 616 – 621
micrographs are given in Fig. 2. It reveals that all the particles have a non-spherical appearance with relatively broad size distribution. In comparison, the functionalized organosilica Crown-50 is more likely to aggregate together due to the higher content of organic ingredient.
Fig. 2 ESEM micrographs of the functionalized organosilica particles. a. Crown-0; b. Crown-50.
3.3. Adsorption of Sr(II) in nitric acid solutions. The adsorption performance of the DCH18C6-functionalized organosilica toward strontium ion in HNO3 media was investigated. The contact time of the adsorption process was previously determined. The fast kinetics during the adsorption could guarantee an equilibrium state less than 1 h. It has been reported that the acidity of the solution has a dramatic influence to the adsorption behaviour, because the nitric acid can compete with Sr(II) ion for oxygen atoms in the crown ether[9]. In our study, the effect of the HNO3 concentration on the DCH18C6functionalized organosilica’s adsorption of Sr(II) was examined. It can be seen in Fig. 3 that the Crown-50 sample can effectively separate the Sr(II) in HNO3 solutions with a wide concentration range of 1.0~6.0 mol/L. The adsorption ability for Sr(II) varied with the change of HNO3 concentration, and the maximum value could be reached as 114.5 cm3/g in 1.0 mol/L HNO3 solution. The favourable adsorption of Sr(II) on the Crown-50 was reasonably attributed to the recognition ability of the crown ether groups, because the intrinsic silica Crown-0, with absence of crown ether group, had scarcely any adsorption of the Sr(II). Compared to our former report, the novel functionalized organosilica Crown-50 showed more promising performance. The upgrade of the adsorption ability is believed to benefit from the improvement of the surface properties of the functionalized organosilica.
Fig. 3 Effect of the nitric acid concentration on the DCH18C6-functionalized organosilica’s adsorption behaviour.
Gang Ye et al. / Procedia Chemistry 7 (2012) 616 – 621
4. Conclusion In this work, a novel kind of organosilica functionalized with dicyclohexano-18-crown-6 (DCH18C6) was synthesized through a co-condensation approach. The functionalized organosilicas, with large surface areaˈ showed good adsorption capacity toward Sr(II) in HNO3 solutions. This novel adsorption material might be potentially applied for the separation of the radiostrontium from the radioactive liquid waste.
Acknowledgements The financial support from the NSFC under Project 51103079 is gratefully acknowledged.
References [1] Vajda N, Kim C. Determination of radiostrontium isotopes: a review of analytical methodology. Appl Radiat Isotopes 2010; 68:230626. [2] Rawat N, Mohapatra P K, Lakshmi D S, Bhattacharyya A, Manchanda V. Evaluation of a supported liquid membrane containing a macrocyclic ionophore for selective removal of strontium from nuclear waste solution. J Membrane Sci 2006; 275:82-8. [3] Kumar A, Mohapatra P K, Pathak P N, Manchanda V K. Dicyclohexano 18 crown 6 in butanol-octanol mixture: a promising extractant of sr(ii) from nitric acid medium. Talanta 1997; 45:387-95. [4] Zhang A Y, Xiao C L, Kuraoka E, Kumagai M. Preparation of a novel silica-based dtbuch18c6 impregnated polymeric composite modified by tri-n-butyl phosphate and its application in chromatographic partitioning of strontium from high level liquid waste. Ind Eng Chem Res 2007; 46:2164-71. [5] Horwitz E P, Chiarizia R, Dietz M L. A novel strontium-selective extraction chromatographic resin. Solvent Extr Ion Exc 1992; 10:313-36. [6] 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; 1157:85-95. [7] Ye G, Bai F, Wei J, Wang J, Chen J. Novel polysiloxane resin functionalized with dicyclohexano-18-crown-6 (DCH18C): synthesis, characterization and extraction of Sr(II) in high acidity HNO3 medium. J Hazard Mater 10. 1016/j.jhazmat.2012.04.020. [8] Dubois G, Reye C, Corriu R, Chuit C. Organic-inorganic hybrid materials. Preparation and properties of dibenzo-18-crown-6 etherbridged polysilsesquioxanes. J Mater Chem 2000; 10:1091-8. [9] Horwitz E P, Dietz M L, Fisher D E. Separation and preconcentration of strontium from biological, environmental, and nuclear waste samples by extraction chromatography using a crown ether. Anal Chem 1991; 63:522-5.
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Procedia Chemistry 7 (2012) 616 – 621
ATALANTE 2012 International Conference on Nuclear Chemistry for Sustainable Fuel Cycles
Co-condensation synthesis of a novel DCH18C6-functionalized organosilica for strontium adsorption Gang Ye , Feifei Bai, Jichao Wei, Jianchen Wang, Jing Chen Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China
Abstract A novel kind of organosilica functionalized with dicyclohexano-18-crown-6 (DCH18C6) was synthesized in this study. The DCH18C6 molecule, well known as an excellent complexing agent of strontium, was modified for chemical bonding to the organosilica matrix via a co-condensation approach. 29Si and 13C solid-state NMR, ESEM and N2 adsorption-desorption measurement were employed to characterize the organosilica’s structure and surface properties. The functionalized organosilica showed favourable adsorption capacity to the strontium, and the distribution coefficient (Kd) of 114.5 cm3/g could be obtained in 1 mol/L HNO3 solution. This novel functionalized organosilica might be potentially used for the separation of the strontium in radioactive liquid waste. ©2012 2012Elsevier The Authors. Publishedand/or by Elsevier B.V. under responsibility of the Chairman of the ATALANTE 2012 © B.V...Selection peer-review SelectionCommittee and/or peer-review under responsibility of the Chairman of the ATALANTE 2012 Program Program Keywords: Crown ether; Strontium; Organosilica; Synthesis; HLLW
1. Introduction Partitioning of hazardous fission products has great value for the vitrification treatment of the radioactive liquid waste. As a kind of long lived nuclide and heat generator, 90Sr (T1/2=28.5 years) generally exists in the high level liquid waste (HLLW)[1]. Separating the 90Sr prior to vitrification can not only help to reduce the volume of the disposable waste, but also lower the risk of matrix deformation caused by the radiated heat during the longtime storage[2].
Corresponding author. Tel.: +86-10 89796063; fax: +86-10 62771740 E-mail address: [email protected]
1876-6196 © 2012 Elsevier B.V...Selection and/or peer-review under responsibility of the Chairman of the ATALANTE 2012 Program Committee doi:10.1016/j.proche.2012.10.094
Gang Ye et al. / Procedia Chemistry 7 (2012) 616 – 621
Due to the strong complexing ability and selectivity, the macrocyclic ether family play an important role in metal separation. It has been reported that dicyclohexano-18-crown-6 (DCH18C6), as well as its tert-butyl substituted derivative, exhibited favorable extraction ability and selectivity towards Sr(II)[3]. Based on such complexing agents, solvent extraction and extraction chromatography have been developed as two widelyaccepted methods for the separation of strontium from radioactive stream. Extraction chromatography is characterized by loading the complexing agents onto an inert support material. With the advantage of minimal organic diluents utilization and less waste accumulation, extraction chromatography is considered as a promising alternative for replacing solvent extraction toward metal separation in radioactive liquid waste[4]. Horwitz et al. prepared a novel Sr-selective extraction chromatographic resin (Sr-Spec, abbreviation for Strontium Specific) by impregnating a 1-octanol solution containing DtBuCH18C6 into an inert polymeric support (XAD-7)[5]. This innovative work opened a new period in radiostrontium separation and analysis. Zhang et al. developed the DtBuCH18C6 impregnated macroporous silica-based polymeric composite and proposed the SPEC process for chromatographic separation the Sr(II) in HLLW[6]. Compared to the previous polymeric resin, the silica-based adsorption materials have excellent thermal and hydrolytic stability. However, so far, most of the reported adsorption materials for the selective separation of Sr(II) were developed based on physical methodology, such as impregnation or inclusion. The complexing agent was immobilized into the substrate material by van der Waals force or hydrogen bond. These resins suffered from the leak of extractant during the adsorption and elution, which would lead to an obvious decrease of separation efficiency in cycle use. In this study, a novel organosilica functionalized with dicyclohexano-18-crown-6 (DCH18C6) was synthesized via a co-condensation approach. The synthesis, structure characterization and surface morphology were detailed. The adsorption behavior toward Sr(II) in HNO3 solution was evaluated. 2. Experimental 2.1. Synthesis of the DCH18C6-functionalized organosilica 4,4’-Di(aminocyclohexyl)-18-crown-6 [1]. The synthesis of cis-di(aminocyclohexyl)-18-crown-6 was started with dibenzo-18-crown-6 molecule [0] according to a three-stage procedure, namely, nitration, reduction and catalytical hydrogenation. The synthesis operation and the structural characterization can be seen in our earlier report[7]. 4,4’-Bis((3-(triethoxylsilyl)propyl)amino)dicyclohexyl-18-crown-6 [2]. 2.01 g compound [1] (5 mmol) was dissolved in 150 mL tetrahydrofuran (THF). 1.38 g potassium carbonate (10 mmol) and 1.66 g potassium iodide (10 mmol) were added into the solution. Then, with nitrogen protection and vigorous stirring, 3chloropropyltriethoxysilane (CPTES) (2.40 g, 10 mmol) was dropwise added. The mixture was heated under reflux for 5 h, the solid salt was separated by filtration. The crude product with brown color was obtained from the filtrate after removing the THF under vaccum. The product was purified via a procedure similar to that reported by Dubois et al[8]. Yield: 47%. 1H NMR (300 MHz, CDCl3): į 3.79-3.70 (q, 12H), 3.67-3.54 (m, 16H), 3.08 (m, 4H), 2.69 (m, 6H), 1.95-1.86 (m, 4H), 1.60-1.52 (m, 8H), 1.40-1.22 (m, 4H), 1.19 (t, 18H), 0.74 (t, 4H). DCH18C6-functionalized organosilica [3]. The DCH18C6-functionalized organosilica was prepared through the co-condensation reaction of [2] and tetraethoxysilane (TEOS). 0.50 mol [2] and 0.50 mol TEOS were dissolved in quantitative anhydrous ethanol with total concentration of 1 mol/L. Under stirring, a stoichiometric amount of water was slowly added with a syringe, followed by 2 wt.% di-n-butyltin dilaurate (DBTL) as catalyst. The mixture was further stirred for 10 min. Homogeneous solution was obtained and set for 3 days at ambient temperature for gelation. The product was granulated, dried in 60 ºC oven, followed by thermal treatment at 120 ºC under vacuum for 8 hours. The catalyst and the unreacted monomers were washed off with hot acetone. The resulting powder was collected and dried in 60 ºC vacuum oven for 24 hours.
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2.2. Adsorption of Sr(II) in HNO3 solution Batch adsorption experiments toward Sr(II) in nitric acid media were performed in 25 ºC thermostat. 0.05 g organosilica samples were added into 1.0~6.0 mol/L HNO3 containing 1.0×10-3 mol/L Sr(II) ion. The phase ratio was set to be 0.01 g/1 mL. After vigorous agitating for 2 h, the aqueous phase was separated with micro-pore filter. The residual amount of the Sr(II) in the solution was measured by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES). The distribution coefficient Kd was defined as shown in Equation (1) for the evaluation of the sample’s adsorption ability, (1) where C0 and Ct represented the initial and residual Sr(II) concentration in the aqueous phase, M and V were the weight of organosilicas and the volume of the stock solution, respectively. 3. Result and discussion 3.1. Synthesis and characterization The co-condensation synthesis of the DCH18C6-functionalized organosilica is illustrated in Scheme 1. O O O
O O
HNO3
N2H4 H2O
H2
O
HAc
Pd/C
Ru catalyst
H2N
O
O
O
O
NH2
O O [1]
[0] Cl
Si(OEt)3 K2CO3, KI
(EtO)3Si H N
Si(OEt)3
O O
O
O
H N
O O [2]
TEOS
EtOH, H2O
O O O
O O O
Si
Si
O HN
O
O
O
NH
O O [3]
Scheme 1. Synthesis route of the DCH18C6-functionalized organosilica by co-condensation.
Dicyclohexyl-18-crown-6 modified with amino groups (compound [1]) was prepared based on dibenzo-18crown-6 [0] by selective nitration, reduction and catalytical hydrogenation. Then, the bissilylated crown ether monomer [2] could be obtained through the nucleophilic substitution reaction between [1] and 3-
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Gang Ye et al. / Procedia Chemistry 7 (2012) 616 – 621
chloropropyltriethoxysilane (CPTES). The purified monomer [2] was condensated together with tetraethoxysilane (TEOS) following a typical sol-gel process to produce the functionalized organosilica. For comparative study, another intrinsic silica sample was synthesized only by the sol-gel process of TEOS molecules. The two samples were noted as Crown-X (X=50 or 0), where the X referred to the proportion of the crown ether monomer. Solid state 29Si MAS NMR provides information about the silica matrix of the functionalized organosilica. Fig. 1 (a) shows the 29Si NMR spectra of the Crown-X samples. The three regions of major intensity centered at 110.2 ppm (Q4), -102.2 ppm (Q3) and -67.5 ppm (T3) correspond to silicon atoms in Si(-O-)4, EtOSi(-O-)3 and RSi(-O-)3 segments. Solid state 13C CP-MAS NMR was used to identify the organic functional groups of samples. The spectra are shown in Fig. 1 (b). The signals at 17.6 and 60.3 ppm correspond to the two carbon atoms in residual ethoxy group. These two peaks refer to an incomplete hydrolysis of the ethoxy groups during the synthesis. The resonances at 77.5, 71.0, 54.1, 23.0 ppm are attributed to the carbon atoms in the ether ring and cyclohexyl group of the DCH18C6. The three carbon atoms in the spacer connecting the Si-O framework and the DCH18C6 show their resonances at 47.9, 26.4 and 10.5 ppm.
Fig. 1. Solid state NMR spectra of Crown-0 and Crown-50. (a) 29Si CP-MAS NMR spectra; (b) 13C CP-MAS NMR spectra.
3.2. Surface property Surface and pore structural properties of the organosilica resins were evaluated by N2 adsorption-desorption measurements. The results calculated by BET method were shown in Table 1. The result suggests that the employment of the crown ether monomer affects the surface property of the organosilica particles. The specific surface area of 185.03 m2/g was obtained for Crown-50, with the pore volume reaching up to 0.068 cm3/g. Compared to our previous work, there was a substantial improvement of the surface property, which could be attributed to the advantage of the co-condensation synthesis. Such one-step approach, without the post-grafting treatment, avoided the blocking of the channels in the organosilica matrix and provided the particles with better surface property. Table 1. Elemental analysis and N2 adsorption-desorption measurement of the organosilica Crown-0 and Crown-50.
wt.% C
wt.% H
wt.% N
Particle size (ȝm)
Crown-0
17.01
3.66
Null
10.40
152.24
0.044
Crown-50
37.98
6.30
2.66
5.57
185.03
0.068
Sample
Elemental analysis
BET surface area (m2/g)
Pore volume (cm3/g)
The size and distribution of the samples was examined by laser particle analyzer. The result showed that the average size of 10.40 ȝm and 5.57 ȝm was obtained for Crown-0 and Crown-50, respectively. The morphology of the organosilica particles was studied by environmental scanning electron microscopy (ESEM). The
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Gang Ye et al. / Procedia Chemistry 7 (2012) 616 – 621
micrographs are given in Fig. 2. It reveals that all the particles have a non-spherical appearance with relatively broad size distribution. In comparison, the functionalized organosilica Crown-50 is more likely to aggregate together due to the higher content of organic ingredient.
Fig. 2 ESEM micrographs of the functionalized organosilica particles. a. Crown-0; b. Crown-50.
3.3. Adsorption of Sr(II) in nitric acid solutions. The adsorption performance of the DCH18C6-functionalized organosilica toward strontium ion in HNO3 media was investigated. The contact time of the adsorption process was previously determined. The fast kinetics during the adsorption could guarantee an equilibrium state less than 1 h. It has been reported that the acidity of the solution has a dramatic influence to the adsorption behaviour, because the nitric acid can compete with Sr(II) ion for oxygen atoms in the crown ether[9]. In our study, the effect of the HNO3 concentration on the DCH18C6functionalized organosilica’s adsorption of Sr(II) was examined. It can be seen in Fig. 3 that the Crown-50 sample can effectively separate the Sr(II) in HNO3 solutions with a wide concentration range of 1.0~6.0 mol/L. The adsorption ability for Sr(II) varied with the change of HNO3 concentration, and the maximum value could be reached as 114.5 cm3/g in 1.0 mol/L HNO3 solution. The favourable adsorption of Sr(II) on the Crown-50 was reasonably attributed to the recognition ability of the crown ether groups, because the intrinsic silica Crown-0, with absence of crown ether group, had scarcely any adsorption of the Sr(II). Compared to our former report, the novel functionalized organosilica Crown-50 showed more promising performance. The upgrade of the adsorption ability is believed to benefit from the improvement of the surface properties of the functionalized organosilica.
Fig. 3 Effect of the nitric acid concentration on the DCH18C6-functionalized organosilica’s adsorption behaviour.
Gang Ye et al. / Procedia Chemistry 7 (2012) 616 – 621
4. Conclusion In this work, a novel kind of organosilica functionalized with dicyclohexano-18-crown-6 (DCH18C6) was synthesized through a co-condensation approach. The functionalized organosilicas, with large surface areaˈ showed good adsorption capacity toward Sr(II) in HNO3 solutions. This novel adsorption material might be potentially applied for the separation of the radiostrontium from the radioactive liquid waste.
Acknowledgements The financial support from the NSFC under Project 51103079 is gratefully acknowledged.
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