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Synthesis and characterization of a NaA zeolite membrane and its applications for desalination of radioactive solutions
Desalination 225 (2008) 199–208
Synthesis and characterization of a NaA zeolite membrane and its applications for desalination of radioactive solutions A. Malekpoura*, M.R. Millanib, M. Kheirkhaha,b a
Jaber ben Hayyan Research Labs and Esfahan Research Center, AEOI, Tehran, Iran Tel. +98 (311) 2228820; Fax +98 (311) 2223902; email: [email protected] b Chemistry Department, Iran University of Science and Technology, Tehran, Iran
Received 12 October 2006; accepted 2 February 2007
Abstract NaA zeolite membranes were prepared by hydrothermal synthesis on a porous α-alumina support with secondary growth crystallization. These membranes were characterized by XRD, SEM, AFM and permeation tests and were used for desalination of simulated radioactive wastes through pervaporation process. The influence of several operating parameters such as synthesis time, number of zeolite layers and seeding procedure was investigated. Based on our results the performance of NaA zeolite membranes can be improved by employing a multi-stage synthesis method. Results showed that membranes which were prepared in a four-stage process (each step 3 h) can yield best separation. At first the membranes were evaluated by dehydration of water/isopropanol solutions. Results showed that a separation factor equal to 5041 with 1.2 kg/m2.h total flux can be obtained. For ionic solutions including 0.001 M of Cs+, Sr2+ and MoO42– very high rejection factors were obtained (more than 99%). This work showed that zeolitic membranes can be used for treatment of low level radioactive wastes as well as desalination and volume reduction of these solutions especially through the pervaporation process. Keywords: Zeolite A; Membrane; Pervaporation; Dehydration; Ion separation
1. Introduction Zeolites are crystalline aluminosilicates with a microporous structure and high chemical and radiation stability. They can withstand high temperatures and chemically harsh conditions, and are used as selective adsorbents and catalysts. Due *Corresponding author.
to their uniform pore sizes of molecular dimensions and their adsorption properties, they have a potential for membrane applications. Zeolite membranes are capable of separating compounds by a combination of molecular sieving, selective adsorption, and differences in diffusion rates. Zeolite membranes have been studied extensively for more than 15 years, mainly focusing on gas
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separation and liquid pervaporation processes [1– 15]. The use of natural zeolites to clean-up different types of wastewaters, especially nuclear wastewaters and hazardous industrial wastes, has received a considerable attention over the last decades [16–19]. In nuclear waste treatment most of research on zeolites has been concerned with removal of cationic species such as cesium and strontium [20,21]. There are some works showing the use inorganic membranes for concentration of radioactive solutions [22–24] In this scientific work, we prepared a zeolite A membrane on a porous alumina support by using a secondary growth method and applying the membranes initially for separation of water/isopropanol mixtures and after that for removal of some important ions from simulated nuclear waste. The results demonstrate the possibility of applying this type of membranes to desalination and volume reduction of nuclear waste. 2. Experimental A porous α-Al 2O 3 disk (20 mm diameter, 1.2 mm thickness, 150 nm pore radius, about 30% porosity) was used as the support. In some cases, one side of the support was coated with NaA zeolite crystals as nucleation seeds before synthesis. Thin zeolite membrane layers were grown hydrothermally over the external surface of the porous supports. Synthesis solution was prepared by mixing aluminate and silicate solutions. 0.723 g of NaOH was dissolved in 80 ml of distilled water. The solution was divided into two equal volumes and kept in polyethylene bottles. Aluminate solution was prepared by adding 6.212 g sodium aluminate (Riedel-deHean 54% Al2O3) to one part of the NaOH solution. It was well mixed until cleared. Silicate solution was prepared by adding 6.952 g sodium silicate solution (Riedel-deHean 63% SiO2) to another part of the NaOH solution. Silicate solution was then poured into aluminate solution and well mixed until a thick homogenized
gel was formed. The composition of the homogeneous solution for the zeolite NaA membrane is represented by the following molar ratio: 3.165 Na2O: Al2O3: 1.926 SiO2: 128 H2O The seeded support was placed vertically with a Teflon holder in a polyethylene autoclave. The synthesis solution was added into the autoclave carefully without hitting the support. Then the autoclave was sealed. The crystallization was carried out in an air-circled oven at 100°C for the required time. In order to improve the perfection of the NaA zeolite membrane, multi-stage synthesis was carried out. After synthesis, the as-synthesized zeolite membranes were washed several times with deionizer water until the pH valve of the washings became neutral, then dried in air at 373 K for 24 h. The phase of the as-synthesized membrane was determined by X-ray diffraction (XRD) patterns. XRD measurements were performed using a D8 advanced diffractometer (BRUKER). The Cu kα (λ = 1.54 Å) was used under 40 kV and 40 mA. The morphology and the thickness of the as-synthesized membrane were examined by scanning electron microscope (SEM). A Phillips scanning electron microscope XL30 was used. Measurement were performed on the membrane synthesized using the static method with evaporated gold on the surface. The zeolite membrane was dried in vacuum before evaporation because it adsorbed water in the pores. A DualScope Scanner, DS 9550 scanning probe microscope was used for taking AFM photographs with using non-contact mode scanner. The experimental system used for pervaporation experiments is shown in Fig. 1. In these experiments, the permeate side was evacuated and permeate vapor was condensed by liquid nitrogen. For initial experiments isopropanol and water mixture was used as feed solution and separation factor was calculated by the following equation: α = ( y2 / y1 ) / ( x2 / x1 )
(1)
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Fig. 1. A schematic layout of the laboratory pervaporation system.
where (x2/x1) is the molar ratio of water to isopropanol in feed and (y2/y1) is that for permeate. For ionic solutions, rejection factors were obtained as:
r = ( Ci ) f − ( Ci ) p / ( Ci ) f × 100
(2)
where (Ci)f and (Ci)p are concentrations of each ion in the feed and permeate solutions respectively. Fluxes were obtained by:
Flux =
Permeate (kg) (3) Surface of membrane (m 2 ) time (h)
Distilled water and 90% of isopropanol were used as feed solution and gas chromatography was used for measuring alcohol concentration. In the other way for desalination experiments, solutions with 1.0×10–3 mol.dm–3 of ionic species were used and the concentration of each ion was obtained by inductively coupled plasma or AAS (for Cs) in appropriate wavelength. All experiments were performed at 25°C.
3. Results and discussion Table 1 lists the prepared membranes with their synthesis conditions and anonyms. The XRD patterns of some synthesized membranes with different synthesis times and synthesis stages are shown in Fig. 2 and they are compared with standard NaA zeolite crystals. All patterns well exhibit that NaA zeolite is formed on the porous alumina support. From these pattern it is obvious that 3-h synthesis is the best one because the pick and relative intensities are very compatible with the standard pattern. Table 1 Synthesized membranes and their conditions
Membrane
Synthesis time No. of synthesis (h) steps
M1 M2 M3 M4 M5 M6
2 3 5 3 3 3
4 4 4 1 2 3
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(a)
4500
4000
3500
3000
2500
2000
1500
1000
500
0 5
(b)
10
20
30
40
50
10
20
30
40
50
60
70
9000
8000
7000
6000
5000
4000
3000
2000
1000
0
6
Fig. 2. XRD patterns of NaA zeolite membranes on alumina supports a) M1 b) M2.
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(c)
4500
4000
3500
3000
2500
2000
1500
1000
500
0
5
10
20
30
40
50
60
70
Fig. 2. XRD patterns of NaA zeolite membranes on alumina supports c) M3.
The SEM images from different membranes from top and side view were obtained. Based on the SEM images, the crystals are typically in the morphology of NaA zeolite. Figs. 3a, b show that there was no continuous NaA zeolite membrane formed on the alumina after one 3-h stage. In order to synthesize a continuous NaA zeolite membrane on the support surface, multi-stage synthesis was carried out. After a 4-stage synthesis a continuous and perfect layer of NaA zeolite membrane, including good crystals, was formed on the alumina support (Figs. 3c, d). Based on these images, NaA crystals are about 2 μm, and from the cross section view, it can be seen that the thickness of the membrane prepared after the 4-stage synthesis is about 3–4 μm. The AFM photographs (Fig. 4) can confirm the formation of the continuous layer. The crystals were all in the morphology of NaA zeolite
type and did not transform into other type of zeolites after the 4-stage synthesis. The thickness of the NaA zeolite membrane is about 10 μm when the membrane is perfected and that is without some cracks. When the support was coated with nucleation seeds, not only the formation of NaA zeolite on the support surface was accelerated, but also the transformation of NaA zeolite into other types of zeolites was inhibited. For evaluation of the membranes prepared, separation of different solutions of water/isopropanol by the pervaporation process was performed and the effect of some parameters for optimization of membrane performance was also investigated. Table 2 shows that the best separation factors are obtained by M2 membrane. However total flux is decreased to 1.3 kg.m–2h–1 due to thickness increase. Based on results, a mixture of 90% solution of isopropanol can be concentrated by
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c)
d)
Fig. 3. SEM images of NaA zeolite membrane after a one stage synthesis: (a) top view and (b) cross-section and after 4stage synthesis (c) top view and (d) cross-section.
selective removal of water by the used membranes (99.82% by M2, 99.51% by M3 and 99.17% by M1 membrane respectively). However fluxes of water transfer were decreased from 4.70 to 1.04 kg.m–2.h–1. The effects of the synthesis stages on different experimental parameters such as separation factor and solution flux are also shown in Fig. 5. As expected, with increasing the number of stages the separation factor was increased but the fluxes were decreased. Rejection of some important radioactive spe-
Table 2 Flux and separation factor of membranes for pervaporation of 10% water/isopropanol Experiment
Flux (kg/m2 h)
Separation factor
Permeate water content
M1 M2 M3 M4 M5 M6
1.04 1.3 2.2 4.7 3.02 1.55
1073.2 5041.4 1846.0 8.8 9.1 120.6
99.17 99.82 99.51 49.49 50.25 93.15
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Fig. 4. AFM images of zeolite A membrane in two different spot (M2).
3
6000
(a)
Separation factor
-1
4
-2
Flux (kg.m .h )
5
2 1 0 0
1
2
3
4
5
Synthesis stage
(b)
5000 4000 3000 2000 1000 0 0
1
2
3
4
5
Synthesis stage
Fig. 5. Effect of synthesis stages on a) flux and b) separation factor of water/isopropanol.
cies from dilute solutions was achieved by a highquality membrane of zeolite A (M2). Table 3 lists the results obtained. Based on these results our prepared membrane can be used for concentration of simulated nuclear solutions containing Cs+, . All rejection factors are above Sr2+ and MoO2– 4 99% and it can be shown that zeolite membranes with their high stabilities to radiations are a good candidate for concentrating radioactive solutions. Fig. 6 shows the variation of concentrationd of
different ions after solution passing from the membrane as a function of time. It is obvious that almost all ions are rejected from the membranes and only water molecules are allowed to pass from the membrane. As can be seen, with increasing the time, the water flux is decreased. This fact is related to partial fouling of membranes. In our idea, the nonzeolitic pores can be obstructed by precipitation of some salts that can be passed from nonzeolitic pores and in the other side
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Table 3 Flux and rejection factor of membranes toward Cs, Sr and MoO42– ions Analyte
Time (min)
Feed concentration (ppm)
Permeate concentration (ppm)
Flux (kg/m2h)
Rejection factor (%)
Cs+ Cs+ Cs+ Cs+ Cs+ MoO2– 4 MoO2– 4 MoO2– 4 MoO2– 4 MoO2– 4 Sr2+ Sr2+ Sr2+ Sr2+
0 30 75 105 165 0 45 60 135 165 0 45 90 120
132.9 135 170 177 210 95.94 94.1 95 95.1 95.21 87.62 100.7 105.4 117
0 1.09 1 0.8 0.8 0 0.171 0.15 0.17 0.16 0 0.2 0.25 0.24
5.3 5.256 4.85 3.49 2.24 1.56 1.43 1.14 1.05 0.86 3.01 2.86 2.15 1.37
99.44
of the membrane due to evacuation it became solid. That is the major reason for increasing the rejection factor and decreasing fluxes vs. experiment time. The lowest amount of flux was obtained for molybdate solutions although their rejection factor was high. Molybdate anions seem to have smaller diffusion because of the bigger kinetic radius relative to cesium and strontium ions. 4. Conclusion This work reports an experimental study on the desalination of ionic solutions, including the most important radioactive species, using NaAtype membrane. NaA zeolite membrane was successfully synthesized on a porous ˜-Al2O3. A continuous NaA zeolite membrane formed on the alumina support after a four–stage synthesis. The membrane thickness was about 10 μm. Initial experiments were performed for separation of isopropanol/water mixtures through pervaporation process. These experiments were achieved by high
99.83
99.78
separation factor (more than 5000). After that, some ionic solutions were passed from membrane and the ionic concentration was reduced at least 99% in permeates relative to feed. This high degree of desalination can be introducing zeolite membrane such as good candidate for removal of ionic species from different industrial aqueous solutions. Based on results four-stage synthesis of zeolite A is the best ones. Further increasing the synthesis–stage to five stages deteriorated the quality of the NaA zeolite membrane. These results can be show that Zeolite A membrane is a good candidate for separation of ions from solutions in other techniques such as reverse osmosis. In the future work we have intend to consider this subject. Acknowledgements The authors gratefully acknowledge funding from the Jaber Ebne Hayyan Research Labs, Atomic Energy Organization of Iran.
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References
Fig. 6. Variation of ionic species vs. time of experiment in feed and permeate a) Cs+ b) Sr2+ c) MoO42–.
[1] T.C. Bowen, R.D. Noble and J.L. Falconer, Fundamentals and applications of pervaporation through zeolite membranes, J. Membr. Sci., 245 (2004) 1– 33. [2] X. Xu, W. Yang, J. Liu and L. Lin, Synthesis of NaA zeolite membrane by microwave heating, Sep. Purif. Tech., 25 (2001) 241–249. [3] J. Coronas, J.L. Falconer and R.D. Noble, Characterisation and permeation properties of ZSM-five tubular membranes, AIChE J., 43 (1997) 1797. [4] Y. Yan, M.E. Davis and G.R. Gavalas, Preparation of zeolite ZSM-5 membranes by in situ crystallization on porous-Al2O3, Ind. Eng. Chem. Res., 34 (1995) 1652. [5] D. Shah, K. Kissick, A. Ghorpade, R. Hannah and D. Bhattacharyya, Pervaporation of alcohol–water and dimethylformamide–water mixtures using hydrophilic zeolite NaA membranes: mechanisms and experimental results, J. Membr. Sci., 179 (2000) 185. [6] X.M. He, W.H. Chan and C.F. Ng, Water–alcohol separation by pervaporation through zeolite-modified poly(amidesulfonamide), J. Appl. Polym. Sci., 82 (2001) 1323. [7] Z. Gao, Y. Yue and W. Li, Application of zeolitefilled pervaporation membrane, Zeolites, 16 (1996) 70. [8] J.P. Boom, D. Bargeman and H. Strathmann, Zeolite filled membranes for gas separation and pervaporation, Stud. Surf. Sci. Catal., 84 (1994) 1167. [9] J. Coronas and J. Santamaria, Separations using zeolite membranes, Sep. Purif. Meth., 28 (1999) 127. [10] H. van Bekkum, E.R. Geus and H.W. Kouwenhoven, Supported zeolite systems and applications, Stud. Surf. Sci. Catal., 85 (1994) 509. [11] K. Kusakabe, T. Kuroda, K. Uchino, Y. Hasegawa and S. Morooka, Gas permeation properties of ionexchanged Faujasite-type zeolite membranes, AIChE J. 45 (1999) 1220. [12] K. Aoki, K. Kusakabe and S. Morooka, Gas permeation properties of A-type zeolite membrane formed on porous substrate by hydrothermal synthesis, J. Membr. Sci., 141 (1998) 197. [13] H. Kita, K. Horii, Y. Ohtoshi, K. Tanaka and K.I. Okamoto, Synthesis of a zeolite NaA membrane for pervaporation of water–organic liquid mixtures, J. Mater. Sci. Lett., 14 (1995) 206. [14] A.J. Burggraaf, Z.A.E.P. Vroon, K. Keizer and H. Verweij, Permeation of single gases in thin zeolite
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MFI membranes, J. Membr. Sci., 144 (1998) 77. [15] T. Sano, H. Yanagishita, Y. Kiyozumi, F. Mizukami and K. Haraya, Separation of ethanol–water mixture by silicalite membrane on pervaporation, J. Membr. Sci., 95 (1994) 221. [16] D. Petruzzelli, M. Pagano, G. Tiravanti and R. Passino, Lead removal and recovery from battery wastewaters by natural zeolite clinoptilolite, Solvent Extrac. Ion Exch., 17(3) (1999) 677–694. [17] M.I. Panayotova, Kinetics and thermodynamics of copper ions removal from wastewater by use of zeolite, Waste Manag., 21(7) (2006) 71–76. [18] H. Kazemian, Ph D. Thesis, Isfahan University, Isfahan, 1999. [19] S.X. Wang, L.M. Wang and R.C. Ewing, Electron and ion irradiation of zeolites. J. Nucl. Mater., 278 (2000) 233. [20] A. Dyer and S. Aggarwal, Removal of fission products from mixed solvents using zeolites II. Caesium
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Synthesis and characterization of a NaA zeolite membrane and its applications for desalination of radioactive solutions A. Malekpoura*, M.R. Millanib, M. Kheirkhaha,b a
Jaber ben Hayyan Research Labs and Esfahan Research Center, AEOI, Tehran, Iran Tel. +98 (311) 2228820; Fax +98 (311) 2223902; email: [email protected] b Chemistry Department, Iran University of Science and Technology, Tehran, Iran
Received 12 October 2006; accepted 2 February 2007
Abstract NaA zeolite membranes were prepared by hydrothermal synthesis on a porous α-alumina support with secondary growth crystallization. These membranes were characterized by XRD, SEM, AFM and permeation tests and were used for desalination of simulated radioactive wastes through pervaporation process. The influence of several operating parameters such as synthesis time, number of zeolite layers and seeding procedure was investigated. Based on our results the performance of NaA zeolite membranes can be improved by employing a multi-stage synthesis method. Results showed that membranes which were prepared in a four-stage process (each step 3 h) can yield best separation. At first the membranes were evaluated by dehydration of water/isopropanol solutions. Results showed that a separation factor equal to 5041 with 1.2 kg/m2.h total flux can be obtained. For ionic solutions including 0.001 M of Cs+, Sr2+ and MoO42– very high rejection factors were obtained (more than 99%). This work showed that zeolitic membranes can be used for treatment of low level radioactive wastes as well as desalination and volume reduction of these solutions especially through the pervaporation process. Keywords: Zeolite A; Membrane; Pervaporation; Dehydration; Ion separation
1. Introduction Zeolites are crystalline aluminosilicates with a microporous structure and high chemical and radiation stability. They can withstand high temperatures and chemically harsh conditions, and are used as selective adsorbents and catalysts. Due *Corresponding author.
to their uniform pore sizes of molecular dimensions and their adsorption properties, they have a potential for membrane applications. Zeolite membranes are capable of separating compounds by a combination of molecular sieving, selective adsorption, and differences in diffusion rates. Zeolite membranes have been studied extensively for more than 15 years, mainly focusing on gas
0011-9164/08/$– See front matter © 2008 Elsevier B.V. All rights reserved doi:10.1016/j.desal.2007.02.096
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separation and liquid pervaporation processes [1– 15]. The use of natural zeolites to clean-up different types of wastewaters, especially nuclear wastewaters and hazardous industrial wastes, has received a considerable attention over the last decades [16–19]. In nuclear waste treatment most of research on zeolites has been concerned with removal of cationic species such as cesium and strontium [20,21]. There are some works showing the use inorganic membranes for concentration of radioactive solutions [22–24] In this scientific work, we prepared a zeolite A membrane on a porous alumina support by using a secondary growth method and applying the membranes initially for separation of water/isopropanol mixtures and after that for removal of some important ions from simulated nuclear waste. The results demonstrate the possibility of applying this type of membranes to desalination and volume reduction of nuclear waste. 2. Experimental A porous α-Al 2O 3 disk (20 mm diameter, 1.2 mm thickness, 150 nm pore radius, about 30% porosity) was used as the support. In some cases, one side of the support was coated with NaA zeolite crystals as nucleation seeds before synthesis. Thin zeolite membrane layers were grown hydrothermally over the external surface of the porous supports. Synthesis solution was prepared by mixing aluminate and silicate solutions. 0.723 g of NaOH was dissolved in 80 ml of distilled water. The solution was divided into two equal volumes and kept in polyethylene bottles. Aluminate solution was prepared by adding 6.212 g sodium aluminate (Riedel-deHean 54% Al2O3) to one part of the NaOH solution. It was well mixed until cleared. Silicate solution was prepared by adding 6.952 g sodium silicate solution (Riedel-deHean 63% SiO2) to another part of the NaOH solution. Silicate solution was then poured into aluminate solution and well mixed until a thick homogenized
gel was formed. The composition of the homogeneous solution for the zeolite NaA membrane is represented by the following molar ratio: 3.165 Na2O: Al2O3: 1.926 SiO2: 128 H2O The seeded support was placed vertically with a Teflon holder in a polyethylene autoclave. The synthesis solution was added into the autoclave carefully without hitting the support. Then the autoclave was sealed. The crystallization was carried out in an air-circled oven at 100°C for the required time. In order to improve the perfection of the NaA zeolite membrane, multi-stage synthesis was carried out. After synthesis, the as-synthesized zeolite membranes were washed several times with deionizer water until the pH valve of the washings became neutral, then dried in air at 373 K for 24 h. The phase of the as-synthesized membrane was determined by X-ray diffraction (XRD) patterns. XRD measurements were performed using a D8 advanced diffractometer (BRUKER). The Cu kα (λ = 1.54 Å) was used under 40 kV and 40 mA. The morphology and the thickness of the as-synthesized membrane were examined by scanning electron microscope (SEM). A Phillips scanning electron microscope XL30 was used. Measurement were performed on the membrane synthesized using the static method with evaporated gold on the surface. The zeolite membrane was dried in vacuum before evaporation because it adsorbed water in the pores. A DualScope Scanner, DS 9550 scanning probe microscope was used for taking AFM photographs with using non-contact mode scanner. The experimental system used for pervaporation experiments is shown in Fig. 1. In these experiments, the permeate side was evacuated and permeate vapor was condensed by liquid nitrogen. For initial experiments isopropanol and water mixture was used as feed solution and separation factor was calculated by the following equation: α = ( y2 / y1 ) / ( x2 / x1 )
(1)
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Fig. 1. A schematic layout of the laboratory pervaporation system.
where (x2/x1) is the molar ratio of water to isopropanol in feed and (y2/y1) is that for permeate. For ionic solutions, rejection factors were obtained as:
r = ( Ci ) f − ( Ci ) p / ( Ci ) f × 100
(2)
where (Ci)f and (Ci)p are concentrations of each ion in the feed and permeate solutions respectively. Fluxes were obtained by:
Flux =
Permeate (kg) (3) Surface of membrane (m 2 ) time (h)
Distilled water and 90% of isopropanol were used as feed solution and gas chromatography was used for measuring alcohol concentration. In the other way for desalination experiments, solutions with 1.0×10–3 mol.dm–3 of ionic species were used and the concentration of each ion was obtained by inductively coupled plasma or AAS (for Cs) in appropriate wavelength. All experiments were performed at 25°C.
3. Results and discussion Table 1 lists the prepared membranes with their synthesis conditions and anonyms. The XRD patterns of some synthesized membranes with different synthesis times and synthesis stages are shown in Fig. 2 and they are compared with standard NaA zeolite crystals. All patterns well exhibit that NaA zeolite is formed on the porous alumina support. From these pattern it is obvious that 3-h synthesis is the best one because the pick and relative intensities are very compatible with the standard pattern. Table 1 Synthesized membranes and their conditions
Membrane
Synthesis time No. of synthesis (h) steps
M1 M2 M3 M4 M5 M6
2 3 5 3 3 3
4 4 4 1 2 3
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(a)
4500
4000
3500
3000
2500
2000
1500
1000
500
0 5
(b)
10
20
30
40
50
10
20
30
40
50
60
70
9000
8000
7000
6000
5000
4000
3000
2000
1000
0
6
Fig. 2. XRD patterns of NaA zeolite membranes on alumina supports a) M1 b) M2.
60
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(c)
4500
4000
3500
3000
2500
2000
1500
1000
500
0
5
10
20
30
40
50
60
70
Fig. 2. XRD patterns of NaA zeolite membranes on alumina supports c) M3.
The SEM images from different membranes from top and side view were obtained. Based on the SEM images, the crystals are typically in the morphology of NaA zeolite. Figs. 3a, b show that there was no continuous NaA zeolite membrane formed on the alumina after one 3-h stage. In order to synthesize a continuous NaA zeolite membrane on the support surface, multi-stage synthesis was carried out. After a 4-stage synthesis a continuous and perfect layer of NaA zeolite membrane, including good crystals, was formed on the alumina support (Figs. 3c, d). Based on these images, NaA crystals are about 2 μm, and from the cross section view, it can be seen that the thickness of the membrane prepared after the 4-stage synthesis is about 3–4 μm. The AFM photographs (Fig. 4) can confirm the formation of the continuous layer. The crystals were all in the morphology of NaA zeolite
type and did not transform into other type of zeolites after the 4-stage synthesis. The thickness of the NaA zeolite membrane is about 10 μm when the membrane is perfected and that is without some cracks. When the support was coated with nucleation seeds, not only the formation of NaA zeolite on the support surface was accelerated, but also the transformation of NaA zeolite into other types of zeolites was inhibited. For evaluation of the membranes prepared, separation of different solutions of water/isopropanol by the pervaporation process was performed and the effect of some parameters for optimization of membrane performance was also investigated. Table 2 shows that the best separation factors are obtained by M2 membrane. However total flux is decreased to 1.3 kg.m–2h–1 due to thickness increase. Based on results, a mixture of 90% solution of isopropanol can be concentrated by
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c)
d)
Fig. 3. SEM images of NaA zeolite membrane after a one stage synthesis: (a) top view and (b) cross-section and after 4stage synthesis (c) top view and (d) cross-section.
selective removal of water by the used membranes (99.82% by M2, 99.51% by M3 and 99.17% by M1 membrane respectively). However fluxes of water transfer were decreased from 4.70 to 1.04 kg.m–2.h–1. The effects of the synthesis stages on different experimental parameters such as separation factor and solution flux are also shown in Fig. 5. As expected, with increasing the number of stages the separation factor was increased but the fluxes were decreased. Rejection of some important radioactive spe-
Table 2 Flux and separation factor of membranes for pervaporation of 10% water/isopropanol Experiment
Flux (kg/m2 h)
Separation factor
Permeate water content
M1 M2 M3 M4 M5 M6
1.04 1.3 2.2 4.7 3.02 1.55
1073.2 5041.4 1846.0 8.8 9.1 120.6
99.17 99.82 99.51 49.49 50.25 93.15
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Fig. 4. AFM images of zeolite A membrane in two different spot (M2).
3
6000
(a)
Separation factor
-1
4
-2
Flux (kg.m .h )
5
2 1 0 0
1
2
3
4
5
Synthesis stage
(b)
5000 4000 3000 2000 1000 0 0
1
2
3
4
5
Synthesis stage
Fig. 5. Effect of synthesis stages on a) flux and b) separation factor of water/isopropanol.
cies from dilute solutions was achieved by a highquality membrane of zeolite A (M2). Table 3 lists the results obtained. Based on these results our prepared membrane can be used for concentration of simulated nuclear solutions containing Cs+, . All rejection factors are above Sr2+ and MoO2– 4 99% and it can be shown that zeolite membranes with their high stabilities to radiations are a good candidate for concentrating radioactive solutions. Fig. 6 shows the variation of concentrationd of
different ions after solution passing from the membrane as a function of time. It is obvious that almost all ions are rejected from the membranes and only water molecules are allowed to pass from the membrane. As can be seen, with increasing the time, the water flux is decreased. This fact is related to partial fouling of membranes. In our idea, the nonzeolitic pores can be obstructed by precipitation of some salts that can be passed from nonzeolitic pores and in the other side
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Table 3 Flux and rejection factor of membranes toward Cs, Sr and MoO42– ions Analyte
Time (min)
Feed concentration (ppm)
Permeate concentration (ppm)
Flux (kg/m2h)
Rejection factor (%)
Cs+ Cs+ Cs+ Cs+ Cs+ MoO2– 4 MoO2– 4 MoO2– 4 MoO2– 4 MoO2– 4 Sr2+ Sr2+ Sr2+ Sr2+
0 30 75 105 165 0 45 60 135 165 0 45 90 120
132.9 135 170 177 210 95.94 94.1 95 95.1 95.21 87.62 100.7 105.4 117
0 1.09 1 0.8 0.8 0 0.171 0.15 0.17 0.16 0 0.2 0.25 0.24
5.3 5.256 4.85 3.49 2.24 1.56 1.43 1.14 1.05 0.86 3.01 2.86 2.15 1.37
99.44
of the membrane due to evacuation it became solid. That is the major reason for increasing the rejection factor and decreasing fluxes vs. experiment time. The lowest amount of flux was obtained for molybdate solutions although their rejection factor was high. Molybdate anions seem to have smaller diffusion because of the bigger kinetic radius relative to cesium and strontium ions. 4. Conclusion This work reports an experimental study on the desalination of ionic solutions, including the most important radioactive species, using NaAtype membrane. NaA zeolite membrane was successfully synthesized on a porous ˜-Al2O3. A continuous NaA zeolite membrane formed on the alumina support after a four–stage synthesis. The membrane thickness was about 10 μm. Initial experiments were performed for separation of isopropanol/water mixtures through pervaporation process. These experiments were achieved by high
99.83
99.78
separation factor (more than 5000). After that, some ionic solutions were passed from membrane and the ionic concentration was reduced at least 99% in permeates relative to feed. This high degree of desalination can be introducing zeolite membrane such as good candidate for removal of ionic species from different industrial aqueous solutions. Based on results four-stage synthesis of zeolite A is the best ones. Further increasing the synthesis–stage to five stages deteriorated the quality of the NaA zeolite membrane. These results can be show that Zeolite A membrane is a good candidate for separation of ions from solutions in other techniques such as reverse osmosis. In the future work we have intend to consider this subject. Acknowledgements The authors gratefully acknowledge funding from the Jaber Ebne Hayyan Research Labs, Atomic Energy Organization of Iran.
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References
Fig. 6. Variation of ionic species vs. time of experiment in feed and permeate a) Cs+ b) Sr2+ c) MoO42–.
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