Applied Radiation and Isotopes 85 (2014) 34–38

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Nanostructured zirconium phosphate as ion exchanger: Synthesis, size dependent property and analytical application in radiochemical separation Rajesh Chakraborty a, Koustava Bhattacharaya b, Pabitra Chattopadhyay a,n a b

Department of Chemistry, The University of Burdwan, Golapbag, Burdwan-713104, West Bengal, India Chemistry Division, Bhabha Atomic Research Center, Mumbai, India

H I G H L I G H T S

    

Synthesis and characterization of nanostructured zirconium phosphates (ZPs) of different sizes using Tritron X-100. Establishment of morphology and sizes by spectroscopic and electron microscopic techniques, and DLS study. Study of variation of ion exchange capacity (IEC) of the nanomaterials towards metal ions. Ability of the nanomaterial (size of ca. 21.04 nm) for separation of no-carrier-added 137mBa from 137Cs at pH¼ 5. Possibility of fabrication of a new radionuclide generator system in nanoscale.

art ic l e i nf o

a b s t r a c t

Article history: Received 7 September 2013 Received in revised form 27 October 2013 Accepted 28 October 2013 Available online 9 November 2013

Nanostructured zirconium phosphates (ZPs) of different sizes were synthesized using Tritron X-100 (polyethylene glycol-p-isooctylphenyl ether) surfactant. The materials were characterized by FTIR and powdered X-ray diffraction (XRD). The structural and morphological details of the material were established by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM study was followed by energy dispersive spectroscopic analysis (EDS) for elemental analysis of the sample. The particle sizes were determined by dynamic light scattering (DLS) method. Ion exchange capacity of these nanomaterials towards different metal ions was measured and size-dependent ion exchange property of the materials was investigated thoroughly. The nanomaterial of the smallest size (ca. 21.04 nm) was employed to separate carrier-free 137mBa from 137Cs in column chromatographic technique using 1.0 M HNO3 as eluting agent at pH ¼5. & 2013 Elsevier Ltd. All rights reserved.

Keywords: Nanostructure Tritron X-100 Ion exchange capacity Radioanalytical separation

1. Introduction Syntheses of nanostructured materials along with different size distribution have emerged as an important field of investigation in materials chemistry because of their several non conventional physical properties like catalytic, optic, electric and magnetic and so on. Apart from their various applications in some areas such as proton conduction, catalysis (Liu et al., 2010; Matsuda and Kato, 1983), photosensitization (Bavykin et al., 2006; Varghese et al., 2003), ultrasensitive detection of proteins, DNA sequencing, clinical diagnostics (Raschke et al., 2003; Hodneland and Mrksich, 2000; Patolsky et al., 2000; Paunesku et al., 2003; Chakraborty et al., 2012) etc, nanomaterials are expected to provide unprecedented opportunities in developing a new

n

Corresponding author. Tel.: þ 91 9434473741; fax: þ91 342 2530452. E-mail address: [email protected] (P. Chattopadhyay).

0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.10.018

class of sorbents for chromatographic applications due to their unique surface and morphological features. While the structural features of such materials are in between of those of atoms and the bulk materials, the properties of materials with nanometer dimensions are significantly different from those of atoms and bulks materials. Due to their small dimensions, nanomaterials have extremely large surface area to volume ratio, which makes a large fraction of atoms of the materials to be the surface or interfacial atoms, resulting in more ‘surface-dependent’ material properties. There are few reports available in the literature on the exploitation of nanomaterial based sorbents in the chromatographic separation of metal ions (Cumbal et al., 2003; Okuyama and Lenggoro, 2003; Chakraborty et al., 2013). However, the potential of using such materials as column matrices in radionuclide generators is yet to be explored. Owing to their high surface area and intrinsic surface reactivity, nanomaterial based sorbents has much higher sorption capacity and selectivity compared to the conventional sorbents.

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Fig. 1. (a) SEM image of ZP1. (b) STEM image of ZP1 for EDS analysis.

Fig. 2. EDS spectrum of ZP1.

The present work deals with the synthesis, characterization and successful employment of ZP nanoparticles in radioanalytical separation of the no-carrier-added 137mBa from 137Cs. Zirconium phosphate (ZP) is an established ion exchange material belonging to the class of multivalent metaloacids (Bhattacharyya and Basu, 1978) which have been employed in the separation of heavy metal ions, transuranium elements, rare earth elements and a several number of parent-daughter radionuclide pairs. 137mBa is a shortlived radionuclide 62 (T1/2 ¼2.55 min) and is in secular equilibrium with the long-lived parent, 137Cs [T1/2 ¼ 30.07 years, 63 β-decay to 137m Ba (94.4%) with single photon emission (662 keV. Radiochemical analysis is required, not only for processing radioactive waste samples in the laboratory but also for at-site or in situ applications of no-carrier-added radioactive nuclides produced in the nuclear reactions for radio-labeling of the pharmaceuticals in view of getting potent radiopharmaceuticals. 137Cs/137mBa generator has great advantage because of fast growth of radioactive 137m Ba, safe and frequent in-site elution, better image quality, specially PET transmission scans using point source of 137Cs show little or no artifacts (Schrhröder et al., 2004; Kubica et al., 1996; Mandal and Lahiri, 2011; Roy et al., 2002; Maji and Basu, 2007).

2. Experimental 2.1. Apparatus and reagents The powder X-ray diffraction (XRD) data were recorded from a PANalytical X'pert Pro diffractometer with CuKα radiation. The morphology of the nanosized materials was studied by using of a JEOL-2003 analstation scanning electron microscope (SEM). IR spectra were obtained by JASCO FT-IR model 420 using KBr disc. Radioactivity was measured with a scintillation counter equipped

with a well type NaI(Tl) detector. Size distribution measurements of the nanoparticles were made by dynamic light scattering (Model DLS-nano ZS, Zetasizer, Nanoseries, Malvern Instruments). Samples were filtered several times through a 0.22 mm Millipore membrane filter prior to measurements. The radiotracer 137Cs in equilibrium mixture of daughter 137mBa was obtained from Board of Radiation Isotope and Technology (BRIT), India. ZrOCl4, 8H2O (AR grade) was purchased from Merck (Mumbai, India). Triton X-100 was purchased from Himedia (Mumbai, India). The reagents for the synthesis of the ion-exchange material were obtained from commercial sources and used without further purification.

2.2. Synthesis of ZPs using TX-100 Zirconium phosphate (ZP) of different nanosizes was synthesized using zirconyl chloride (ZrOCl2, 8H2O) and phosphoric acid (H3PO4) in presence of aqueous TX-100 of different concentrations considering the reported procedure as described below (Varshney et al., 2008; Ganguli et al., 2008). At first, solution of 0.05 M ZrOCl2, 8H2O in 0.5 M H2SO4, aqueous solution of 6.0 M H3PO4 and 0.1 M triton X-100 using demineralized water were prepared. To a 50 mL solution 0.05 M ZrOCl2, 8H2O solution, 100 mL solution 6.0 M H3PO4 was added drop wise with continuous stirring at room temperature. During this addition of H3PO4 acid solution, 100 mL diluted aqueous solution of triton X-100 of appropriate concentration (10  5 M, 10  4 M, 10  3 M, 10  2 M and 10  1 M) was added drop wise to the solution mixture. The resulting slurry, was stirred for 3.0 h at this temperature, filtered and then washed with demineralized water for removal of the adhering ions (chloride and sulphate) till pH E4.0 before drying at room temperature. The resulting dried material was treated with 50 mL of 1.0 M HNO3 for 24 h and this treatment was repeated at least three times, and finally after washing by demineralized water the material was dried inside oven at 50 1C. Thus, the use of triton X-100 as micelle affords a favorable microenvironment for controlling the chemical reaction. As a result, the reaction rate can be easily controlled and it is possible to obtain a narrow nanomaterial size distribution (Ganguli et al., 2008). The ratio metric variation of water and triton X-100 has been utilized to produce ZP nanomaterials of different sizes. The synthesis was repeated to obtain the ZPs of different sizes using different concentration of triton X-100 with varying water to surfactant ratio. ZP1, ZP2, ZP3, ZP4 and ZP5 were prepared using 10  1 M, 10  2 M, 10  3 M, 10  4 M and 10  5 M of triton X-100 as a micelle, respectively.

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40

40

ZP 1

ZP 2

35

30

Number density (%)

Number density (%)

35

25 20 15 10 5

30 25 20 15 10 5 0

0 10

20

30

40

50

60

70

10

20

30

40

50

60

70

80

90 100

40

ZP 3

35

ZP 4

35

30

Number density(%)

Number density (%)

40

Particle size (nm)

Particle size (nm)

25 20 15 10 5

30 25 20 15 10 5

0 0

0

10 20 30 40 50 60 70 80 90 100

50

Particle size (nm)

60

70

80

90

100 110

120

Particle size (nm)

40

ZP 5

Number density (%)

35 30 25 20 15 10 5 0 60

80

100

120

140

Particle size (nm) Fig. 3. DLS study for the measurement of average size distribution of nanoparticles of ZP1, ZP2, ZP3, ZP4 and ZP5 using 10  1 M, 10  2 M, 10  3 M, 10  4 M and 10  5 M of triton X-100 as a micelle, respectively

Table 1 Exchange capacities of ZTPs of different particle sizes towards metal ions. IEC in Meq/g onto ZP of different size (nm) Ions

ZP1 (21.04)

ZP2 (41.6)

ZP3 (65.06)

ZP4 (78.82)

ZP5 (91.28)

Na þ Kþ Cs þ Ca2 þ Ba2 þ

1.375 1.875 2.593 3.299 4.129

1.340 1.836 2.491 3.157 3.932

1.308 1.801 2.445 3.102 3.856

1.277 1.768 2.399 3.046 3.763

1.237 1.737 2.358 2.995 3.710

2.3. Determination of size-dependent ion exchange capacity (IEC) of ZPs The hydrogen ion capacity was determined by the previously reported batch method (Chakraborty and Chattopadhyay, 2012). An accurately weighed (0.5 g) portion of the ion exchange materials, ZP were treated with 2.0 M HCl and then filtered off, washed with distilled water, and dried at 50 1C for 2–3 h to remove free HCl.

The acidic form of the material was equilibrated with 20.0 mL of 0.1 M NaOH solution for 1 h at room temperature with stirring, and then the excess alkali was titrated with 0.1 M HCl solution to determine the total acidic hydrogen content. The IEC of the ion exchangers for different alkali and alkaline-earth metal ions was determined by the batch method. To a glass-stoppered centrifuge tube (diameter 2.0 cm) containing 0.5 g of the dry solid ion exchangers, 50.0 mL of 2.0 M solutions of different alkali and alkaline-earth metal ions were added to the tube in each case; then the mixture was shaken for 1 h. The ion exchangers was subsequently filtered off and washed with bi-distilled water to remove the adhering H þ ions. The exchange capacities for the metal ions were determined by measuring the liberated acid by titration with a standard alkali solution. The IEC of each metal ion was determined repeatedly for each concentration of triton X-100 and the effect size on IEC was observed. 2.4. Studies on radio analytical separation using nanosized ZP1 Separation of 137mBa from 137Cs radionuclide was carried out following the column chromatographic technique reported in

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the literature (Chakraborty et al., 2011 a,b; Chakraborty and Chattopadhyay, 2012). In this method, a glass column of 5.0 cm length and 1.0 cm inner diameter was taken and it was packed with the 0.5 g of ZP1 to be used as the column bed which was preconditioned with a 10  5 M (pH¼ 5) HCl solution. A 2.0 mL 0.1 M aqueous sample solution containing the measured amount of cesium and barium radionuclides, where 137Cs is in secular equilibrium with its daughter nuclide 137mBa was allowed to pass through the column at flow rate of 1.0 mL min  1. After absorption of the mixture, 10.0 mL of a 10  5 M HCl solution was moved across the column to make sure the total absorption of the mixture. Finally, the daughter fraction was eluted with 1.0 mL 2.0 M nitric acid, which was followed by collecting the effluent in 10 successive counting tubes (1.0 mL each). The γ-activity in each tube was measured with a NaI (Tl) γ-ray spectrometer several times with a time gap of 30 s.

3. Results and discussion 3.1. Characterization of ZPs The FTIR spectra of ZPs (Fig. S1) display a broad band in the region  3289 cm  1 which is assignable to asymmetric and symmetric hydroxy OH stretches (νOH). A sharp medium band at 1638 cm  1 for aqua (H–O–H) bending and a characteristic band for P¼O stretching in the region  1093 cm  1 were also obtained. The powder X-ray diffraction of all the ZPs was carried out. The X-ray diffraction pattern of one ZP (ZP1) is shown in Fig. S2. To investigate the structural morphology of ZPs, the SEM and STEM images of the material with different magnification were also acquired (viz. Fig. 1a and b). Sample does not have specific morphology but particles (blocks) consist of small grains with the size range of nanometers. EDS analysis (shown in Fig. 2) shows that the dominant chemical elements present in these samples are P, Zr and O. The DLS study for measurement of average size distribution of ZPs is shown in Fig. 3. Variation of the average size distributions of ZPs obtained by varying the water to surfactant ratio from 10  1 M to 10  5 M concentration of TX-100 as a micelle was tabulated in Table 1. From this study, it clearly indicates that the nanomaterial of the smallest size of ca. 21.04 nm (ZP1) was obtained when high concentration (10  1 M) of triton X-100 as a micelle was used, where as the large sized nanomaterials of average size of ca. 91.28 nm (ZP5) by using low concentration of triton X-100 (10  5 M). Thus the use of high concentration of TX-100 (10  1 M) produce ZP1 of the relatively smallest size of around 21 nm, which also confirmed by TEM. TEM image depicted in Fig. 4a shows that the particles are in agglomerated form with size ranging from 10 to 25 nm. Further SAED analysis in Fig. 4b indicates that the material has partly crystalline nature. 3.2. Study of sorption behavior of ZPs towards the metal ions Exchange capacity of ZPs towards different metal ions increases with decreasing size of the nanosized ZPs (viz. Table 1). This is simply because of the fact of the increasing of the surface of the material with decreasing of the size of the particles. As the size of the particles becomes smaller, the percentage of atoms on the surface of the exchanger becomes higher. Consequently, smallsized particles show better sorption behavior than the larger ones. Again the alkali metals show a decreasing trend for the ion exchange capacity (IEC) (Cs þ 4K þ 4Na þ ) while the alkaline earth metal ions follow the order Ba2 þ 4Ca2 þ . The size and charge of the exchanging ions affect the IEC of exchanger material (ZPs). This sequence is in accordance with the hydrated radii of the

Fig. 4. (a) TEM image of ZP1, (b) SAED pattern of ZP1.

exchanging ions. Ions with the smaller hydrated radii easily enter the pores of the exchanger, which results in higher adsorption. 3.3. Radioanalytical separation Separation of 137mBa from 137Cs radionuclides onto the column bed of the smallest sized ZP1 was carried out by elution with a 1.0 M nitric acid solution. With the help of nitric acid used, Ba2 þ ions form stable water-soluble compound Ba(NO3)2 (aq.) and thus it is eluted out of the column matrix, whereas Cs þ remains absorbed on the column bed. Counts of the eluted fractions were taken at different time intervals to plot the decay curve (Fig. 5), from which it is found that the half-life of the daughter is 2.63 min. This half life obtained from the decay curve of the eluted portion is in close proximity to the actual the half-life of 137mBa (T1/2 ¼2.55 min) and the radioactivity measured 1.0 h later (far longer than the half of 137mBa but very short compared to 137Cs) is equal to the background level which indicates that the eluate does not contain observable amounts of the parent 137Cs. Here, the elution of 137mBa from the column equilibrated with the 100 μL solution of 137Cs-137mBa radioactive mixture was carried out using

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9.0

Appendix A. Supplementary material

8.5

Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.apradiso.2013.10.018.

ln (counts)

8.0 7.5

References 7.0 6.5 6.0 0

1

2

3

4

5

6

7

8

9

Time (min) Fig. 5. Decay curve of points).

137m

Ba eluted from ZP ion exchanger (R¼ 0.99768 for eight

1.0 M HNO3 solution by adjusting the rate of the elution to 1 drop/min. Each 5-drop fraction was collected, and its radioactivity was measured with certain periodicity with a precise verification of the purity of the eluted solution based on the half-life of 137mBa. From this study it was observed that only 2.0 mL of a 1.0 M HNO3 solution was sufficient to remove 137mBa completely at a given moment. Therefore, all the 137mBa was eluted from the 137Csloaded generator, the 137mBa was allowed to grow until secular equilibrium was established, so that the subsequent fractions of the daughter nuclide could be recovered from the parent nuclide adsorbed on the column by repeated elution with a 1 M HNO3 solution, and thus the overall system can be considered as a radionuclide generator.

4. Conclusion The design of the conventional exchanger zirconium phosphte in nanoscale range and its characterization has demonstrated its applicability towards attaining a novel separation of short-lived 137m Ba from the long lived 137Cs of 137Cs–137mBa equilibrated radioactive mixtures with enhanced efficiency. This radionuclide generator system in nanoscale will carry great importance in fulfilling the ongoing demand for the radioactive waste management for environmental protection as well as hemodynamic studies for diagnosis in radiopharmaceuticals. The variation of ion exchange capacity (IEC) of the nanomaterials with size points out that nanoscale materials are different from bulk materials in physicochemical properties and size-dependent property in ion exchange studies is thus adjudged. Moreover, it is inexpensive, low cost and easy to synthesize following eco-friendly procedure, which is in good agreement of today's worldwide campaign of green chemistry. Acknowledgement Financial assistance from UGC-DAE Center for Scientific Research, Kolkata is gratefully acknowledged. The authors are obliged to Dr. Suresh Valiyaveettil, Associate Professor, Materials Research Laboratory (S5-01-03), Department of Chemistry, National University of Singapore for his technical support in performing SEM and EDS experiments.

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Nanostructured zirconium phosphate as ion exchanger: Synthesis, size dependent property and analytical application in radiochemical separation.

Nanostructured zirconium phosphates (ZPs) of different sizes were synthesized using Tritron X-100 (polyethylene glycol-p-isooctylphenyl ether) surfact...
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