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ZnAl2O4 thin films for ultrafiltration of ionic solutions
Separation and Purification Technology 32 (2003) 111 /116 www.elsevier.com/locate/seppur
Elaboration of Nasicon/ZnAl2O4 thin films for ultrafiltration of ionic solutions R. Mouazer, M. Persin *, M. Cretin, A. Larbot Institut Europeen des Membranes de Montpellier, UMR 5635 CNRS-ENSCM-UMII, 1919 Route de Mende, Place Euge`ne Bataillon CC047, F-34095 Montpellier, Cedex 5, France
Abstract In the goal to obtain high rejection rate of anions in electrolyte solutions and to filter concentrated solutions, the sol / gel route was used to prepare ceramic membranes using the Nasicon (Na3Zr2Si2PO12) material, which presents a high charge density. Confronted to the difficulties to synthesise a Nasicon membrane by slipcasting, we have used mixed Nasicon /ZnAl2O4 sols of different compositions to prepare the membranes. The first filtration test results are in agreement with the measurements of the electrophoretic mobility of the material powders. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Nasicon; Membrane; Filtration; Interactions
1. Introduction The preparation of various ultra or nanofiltration ceramic membranes by sol gel route is widely described in the literature [1,2]. The selectivity of these membranes towards the filtration of electrolytes is controlled mainly by the steric and electric interactions between the filtered species and the membrane surface. For the electrolyte solutes filtration, the membrane performances in term of rejection depend on the filtered salts. We have focused here our attention to prepare ceramic membranes, which are able to present high ionic species rejection. Nasicon material of general * Corresponding author. E-mail address: [email protected] (M. Persin).
formula Na1x Zr2Six P3 x O12 [3 /5] is known for its high sodium ionic conductivity obtained for x /2 and a negative surface charge which should lead to high rejection for divalent anion salt. As for others ceramic material, the sign of the surface charge of the material depend on the pH of the solution and on the electrolyte concentration. The attempts to prepare pure Nasicon membranes according Shimizu et al. [6] method failed due to the presence of cracks at the membrane surface. To prevent this drawback, we present here the preparation of new ceramic membrane based on the Nasicon /ZnAl2O4 spinel mixture of different composition from 50 to 75% in Nasicon. After the characterisation of the material charge properties, we will present here the filtration results obtained by means of the 75/25 Nasicon membrane for different electrolyte solutions.
1383-5866/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1383-5866(03)00075-3
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R. Mouazer et al. / Separation and Purification Technology 32 (2003) 111 /116
2. Experimental 2.1. Powders The powder of the Nasicon /ZnAl2O4 mixtures was prepared by a sol /gel method at room temperature according to the method described by Shimizu et al. [6], we have just used the citric acid instead of the tartaric acid to improve the sol stability. The aqueous solution of ZrO(NO3)2; NH4H2PO4; Na2SiO3 ×/ 9H2O and the citric acid in a ratio of 2/1/2/6 with a Na excess was prepared [7]. We obtained a Nasicon stable sol at 0.045 M. The ZnAl2O4 stable sol was then prepared according to the method described by Elmarraki et al. [8,9]. A suspension of Al2O3 ×/ H2O powder (Condea Pural SB) in deionised water is peptised by a nitric acid solution (the molar ratio [HNO3]/ [Al2O3] is 4.8). A zinc nitrate solution prepared from Zn(NO3)2 ×/ 6H2O and concentrated HNO3 was then added to the peptised alumina to obtain a 0.0455 mol l 1 stable sol. The mixed sols of Nasicon/ZnAl2O4 were prepared by addition of the two sols in the correct ratio to obtain the exact composition, the final sol is then dried at 100 8C and fired at different temperatures. The material powders were characterised by the classical methods: (X-Ray diffractometry, BET, . . .zetametry). The electrophoretic mobility of the particles in suspension was measured by means of a Coulter Delsa 440 device, the current was fixed at 0.1 mA and the frequency at 250 Hz. 2.2. Membranes The different membranes (50/50 and 75/25) were prepared from the previous sols after adding of a hexamethyl cellulose (HEC) solution at 2% to the final sol. After stirring, the sol is then deposited on the inner surface of a support tube in alumina by
Fig. 1
Fig. 1. XRD pattern of mixed Nasicon /ZnAl2O4 powder calcined at 1000 8C. *: Nasicon; o: ZrO2.
R. Mouazer et al. / Separation and Purification Technology 32 (2003) 111 /116
113
Table 1 BET area and pore size distribution measured by N2 adsorption desorption of the powder particle materials BET area (m2 g 1) Temperature (8C) Nasicon powder 75/25 powder 50/50 powder
500 17 60 68
Pore size distribution (nm) 650 13.7 7.6 6
slipcasting for 2 h. The support tubes used were in g alumina, they were treated at 1200 8C for 1 h to open the porosity up to 200 nm, during the temperature treatment of the support, the g alumina is changed to a alumina. The layer has
750 3.1 1.15 2.1
1000 0.82 0.1 0.6
650 4 8 4
been fired at 650 8C for 2 h to obtain the membrane. The laboratory pilot used for the filtration experiments was equipped with a 15 cm length membrane, the velocity flow of the solution was
Fig. 2. SEM micrography of the support calcined for 1 h at 1200 8C (a) and of the membrane 75/25 (b).
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R. Mouazer et al. / Separation and Purification Technology 32 (2003) 111 /116
this low specific area value is probably due to the Nasicon effect. As for the other amphoteric materials, the electrophoretic mobilities of the Nasicon / ZnAl2O4 powders prepared depend on the used electrolyte and on the pH of the solution. We observed also a displacement of the isoelectric point, which depends on the Nasicon composition of the powder, for example, it moves from 5.7 for the 50% Nasicon powder to 4.2 for the 75% Nasicon powder in the presence of NaCl electrolyte. 3.2. Membranes characterisation
Fig. 3. Pore diameter of the membranes 75/25 fired at 650 8C obtained by Hg porosity.
2.5 m s 1, the working pressure was fixed at 5 bars and the temperature was regulated at 25 8C.
3. Results and discussion
3.1. Powder characterisation In the Nasicon /ZnAl2O4 powders, the characteristic XRD peak of Nasicon structure (Fig. 1) have been observed only for Nasicon percent higher than 60%, nevertheless an impurity phase of monoclinic and tetragonal zirconia was observed, the ZrO2 impurity phase is also present in pure Nasicon material. The powders of pure Nasicon and two mixtures Nasicon/ZnAl2O4 (Table 1) were analysed to obtain the specific area. The BET specific area of the different Nasicon /ZrAl2O4 powder decrease hardly as the firing temperature increases. Table 1 shows that the pore diameter is centred on 4 nm for the 50/50 powders obtained from the mixtures fired at 650 8C. The average specific area of the material is around 6 m2 g 1 at this temperature,
The two membranes prepared were characterised after firing at 650 8C. The SEM crosssection view of the membrane 75/25 shows the thickness of the membrane is about 200 nm (Fig. 2). The pore size of the membrane 50/50 and 75/25 measured by mercury porosimetry is, respectively, centred on 4 and 7 nm. For example, the pore size repartition are given for the membrane 75/25 on Fig. 3. The dynamic characterisations of the membranes confirm these data. The measured fluxes through the membranes depend on the Nasicon membrane composition and the water permeability increases from 11 l h 1 m2 bar 1 for the membrane 50/50 to 30 l h 1 m2 bar 1 for the membrane 75/25. This is a probe of the increase of the pore size of membranes, nevertheless these high fluxes may be also due to the low thickness of the filtering layer. The cut-off of the membrane 50/50 measured from the rejection of uncharged molecules of polyethylene glycol (PEG) is about 6000 Da whereas it is higher for the 75/25 membrane because the pore diameter of this membrane is more important. 3.3. Filtration of electrolyte solution We have characterised the membranes towards the filtration of four classical electrolyte solutions, NaCl; Na2SO4, CaCl2, CaSO4 at a concentration fixed at 10 3 M for different pH between 2 and 10. The measurements of the rejection rates were
R. Mouazer et al. / Separation and Purification Technology 32 (2003) 111 /116
115
Fig. 4. Evolution of the electrolyte rejection rates and mobilities of material powders (75/25) vs. pH at a concentration of 10 3 mol l 1. (a) NaCl, (b) Na2SO4, (c) CaCl2, (d) CaSO4; x: electrophoretic mobility, o: R% salt rejection rate.
performed at a pressure of 5 bar after the stabilisation of the flux and of the rejection rate value for 1 h. The rejection rates observed using the Nasicon 75/25 membrane are plotted on Fig. 4a /d, they depend mainly on the salt nature and on the pH of the filtered solution. The rejection obtained are rather high for CaCl2 in acid medium and for Na2SO4 for basic medium. This behaviour is characteristic of the existence of strong interactions between the electric charge of the divalent ion and the surface charge of the membrane. The evolution of the electrophoretic mobility of a suspension of material powder in the considered
electrolyte are in agreement with the rejection rates observed. In the case of CaCl2, the highest rejection is obtained for low pH and then it decreases as the pH increases. On the contrary, the best rejection observed for Na2SO4 at high pH corresponds to the strong interactions developed between the negative membrane and the divalent SO2 anion. 4 The case of the rejection of NaCl versus pH is also in good agreement with the electrophoretic mobilities of the powder, the rejection is close to 0 at the isoelectric point of the filtering layer where the material in not charged. The rejection of this
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salt is controlled by the exclusion of Na at low pH whereas the exclusion of Cl is responsible of the high rejection observed for the pH higher than the pH of the isoelectric point. The rejection rates of CaSO4 seem to follow also the evolution of the powder electrophoretic mobilities, they are more important in acid medium and reach 0 for the high pH values where the membrane seems uncharged.
4. Conclusion The filtering properties of the membranes based on Nasicon /ZnAl2O4 mixtures are in agreement with the electrophoretic mobility measurement of the material powders. For the 75% Nasicon membrane tested, the rejection rate of electrolytes depends strongly on the pH of the filtered solution. The filtration of concentrate solutions need to prepare membrane with higher composition in Nasicon to improve the surface charge density of the membrane which is responsible of the ion
rejections. Such membranes, which contain more than 80% in Nasicon will be tested soon.
References [1] C. Guizard, in: A.J. Burggraff, L. Cot (Eds.), Fundamentals of Inorganic Membrane Science and Technology, Elsevier, Amsterdam, The Netherlands, 1996, pp. 227 /258. [2] A. Larbot, S. Alami-Younsi, M. Persin, J. Sarrazin, L. Cot, J. Membr. Sci. 97 (1994) 167. [3] H.Y.P. Hong, Mater. Res. Bull. 11 (1976) 173. [4] J.P. Boilot, G. Collin, P. Colomban, J. Solid State Chem. 73 (1998) 160. [5] H. Khireddine, Etude des performances de capteurs potentiome´triques a` ions sodium utilisant des membranes de NASICON. Thesis ENSEEG-INPG Grenoble, France 1992. [6] Y. Shimizu, T. Ushijima, Solid State Ionics 132 (2000) 143. [7] Y. Shimizu, S. Michista, T. Murata, Jpn. J. Appl. Phys. 34 (1995) 833. [8] Y. Elmarraki, M. Cretin, M. Persin, J. Sarrazin, A. Larbot, Mater. Res. Bull. 36 (2001) 227. [9] Y. Elmarraki, M. Persin, J. Sarrazin, M. Cretin, A. Larbot, Sep. Purif. Technol. 25 (2001) 493.
Elaboration of Nasicon/ZnAl2O4 thin films for ultrafiltration of ionic solutions R. Mouazer, M. Persin *, M. Cretin, A. Larbot Institut Europeen des Membranes de Montpellier, UMR 5635 CNRS-ENSCM-UMII, 1919 Route de Mende, Place Euge`ne Bataillon CC047, F-34095 Montpellier, Cedex 5, France
Abstract In the goal to obtain high rejection rate of anions in electrolyte solutions and to filter concentrated solutions, the sol / gel route was used to prepare ceramic membranes using the Nasicon (Na3Zr2Si2PO12) material, which presents a high charge density. Confronted to the difficulties to synthesise a Nasicon membrane by slipcasting, we have used mixed Nasicon /ZnAl2O4 sols of different compositions to prepare the membranes. The first filtration test results are in agreement with the measurements of the electrophoretic mobility of the material powders. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Nasicon; Membrane; Filtration; Interactions
1. Introduction The preparation of various ultra or nanofiltration ceramic membranes by sol gel route is widely described in the literature [1,2]. The selectivity of these membranes towards the filtration of electrolytes is controlled mainly by the steric and electric interactions between the filtered species and the membrane surface. For the electrolyte solutes filtration, the membrane performances in term of rejection depend on the filtered salts. We have focused here our attention to prepare ceramic membranes, which are able to present high ionic species rejection. Nasicon material of general * Corresponding author. E-mail address: [email protected] (M. Persin).
formula Na1x Zr2Six P3 x O12 [3 /5] is known for its high sodium ionic conductivity obtained for x /2 and a negative surface charge which should lead to high rejection for divalent anion salt. As for others ceramic material, the sign of the surface charge of the material depend on the pH of the solution and on the electrolyte concentration. The attempts to prepare pure Nasicon membranes according Shimizu et al. [6] method failed due to the presence of cracks at the membrane surface. To prevent this drawback, we present here the preparation of new ceramic membrane based on the Nasicon /ZnAl2O4 spinel mixture of different composition from 50 to 75% in Nasicon. After the characterisation of the material charge properties, we will present here the filtration results obtained by means of the 75/25 Nasicon membrane for different electrolyte solutions.
1383-5866/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1383-5866(03)00075-3
112
R. Mouazer et al. / Separation and Purification Technology 32 (2003) 111 /116
2. Experimental 2.1. Powders The powder of the Nasicon /ZnAl2O4 mixtures was prepared by a sol /gel method at room temperature according to the method described by Shimizu et al. [6], we have just used the citric acid instead of the tartaric acid to improve the sol stability. The aqueous solution of ZrO(NO3)2; NH4H2PO4; Na2SiO3 ×/ 9H2O and the citric acid in a ratio of 2/1/2/6 with a Na excess was prepared [7]. We obtained a Nasicon stable sol at 0.045 M. The ZnAl2O4 stable sol was then prepared according to the method described by Elmarraki et al. [8,9]. A suspension of Al2O3 ×/ H2O powder (Condea Pural SB) in deionised water is peptised by a nitric acid solution (the molar ratio [HNO3]/ [Al2O3] is 4.8). A zinc nitrate solution prepared from Zn(NO3)2 ×/ 6H2O and concentrated HNO3 was then added to the peptised alumina to obtain a 0.0455 mol l 1 stable sol. The mixed sols of Nasicon/ZnAl2O4 were prepared by addition of the two sols in the correct ratio to obtain the exact composition, the final sol is then dried at 100 8C and fired at different temperatures. The material powders were characterised by the classical methods: (X-Ray diffractometry, BET, . . .zetametry). The electrophoretic mobility of the particles in suspension was measured by means of a Coulter Delsa 440 device, the current was fixed at 0.1 mA and the frequency at 250 Hz. 2.2. Membranes The different membranes (50/50 and 75/25) were prepared from the previous sols after adding of a hexamethyl cellulose (HEC) solution at 2% to the final sol. After stirring, the sol is then deposited on the inner surface of a support tube in alumina by
Fig. 1
Fig. 1. XRD pattern of mixed Nasicon /ZnAl2O4 powder calcined at 1000 8C. *: Nasicon; o: ZrO2.
R. Mouazer et al. / Separation and Purification Technology 32 (2003) 111 /116
113
Table 1 BET area and pore size distribution measured by N2 adsorption desorption of the powder particle materials BET area (m2 g 1) Temperature (8C) Nasicon powder 75/25 powder 50/50 powder
500 17 60 68
Pore size distribution (nm) 650 13.7 7.6 6
slipcasting for 2 h. The support tubes used were in g alumina, they were treated at 1200 8C for 1 h to open the porosity up to 200 nm, during the temperature treatment of the support, the g alumina is changed to a alumina. The layer has
750 3.1 1.15 2.1
1000 0.82 0.1 0.6
650 4 8 4
been fired at 650 8C for 2 h to obtain the membrane. The laboratory pilot used for the filtration experiments was equipped with a 15 cm length membrane, the velocity flow of the solution was
Fig. 2. SEM micrography of the support calcined for 1 h at 1200 8C (a) and of the membrane 75/25 (b).
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R. Mouazer et al. / Separation and Purification Technology 32 (2003) 111 /116
this low specific area value is probably due to the Nasicon effect. As for the other amphoteric materials, the electrophoretic mobilities of the Nasicon / ZnAl2O4 powders prepared depend on the used electrolyte and on the pH of the solution. We observed also a displacement of the isoelectric point, which depends on the Nasicon composition of the powder, for example, it moves from 5.7 for the 50% Nasicon powder to 4.2 for the 75% Nasicon powder in the presence of NaCl electrolyte. 3.2. Membranes characterisation
Fig. 3. Pore diameter of the membranes 75/25 fired at 650 8C obtained by Hg porosity.
2.5 m s 1, the working pressure was fixed at 5 bars and the temperature was regulated at 25 8C.
3. Results and discussion
3.1. Powder characterisation In the Nasicon /ZnAl2O4 powders, the characteristic XRD peak of Nasicon structure (Fig. 1) have been observed only for Nasicon percent higher than 60%, nevertheless an impurity phase of monoclinic and tetragonal zirconia was observed, the ZrO2 impurity phase is also present in pure Nasicon material. The powders of pure Nasicon and two mixtures Nasicon/ZnAl2O4 (Table 1) were analysed to obtain the specific area. The BET specific area of the different Nasicon /ZrAl2O4 powder decrease hardly as the firing temperature increases. Table 1 shows that the pore diameter is centred on 4 nm for the 50/50 powders obtained from the mixtures fired at 650 8C. The average specific area of the material is around 6 m2 g 1 at this temperature,
The two membranes prepared were characterised after firing at 650 8C. The SEM crosssection view of the membrane 75/25 shows the thickness of the membrane is about 200 nm (Fig. 2). The pore size of the membrane 50/50 and 75/25 measured by mercury porosimetry is, respectively, centred on 4 and 7 nm. For example, the pore size repartition are given for the membrane 75/25 on Fig. 3. The dynamic characterisations of the membranes confirm these data. The measured fluxes through the membranes depend on the Nasicon membrane composition and the water permeability increases from 11 l h 1 m2 bar 1 for the membrane 50/50 to 30 l h 1 m2 bar 1 for the membrane 75/25. This is a probe of the increase of the pore size of membranes, nevertheless these high fluxes may be also due to the low thickness of the filtering layer. The cut-off of the membrane 50/50 measured from the rejection of uncharged molecules of polyethylene glycol (PEG) is about 6000 Da whereas it is higher for the 75/25 membrane because the pore diameter of this membrane is more important. 3.3. Filtration of electrolyte solution We have characterised the membranes towards the filtration of four classical electrolyte solutions, NaCl; Na2SO4, CaCl2, CaSO4 at a concentration fixed at 10 3 M for different pH between 2 and 10. The measurements of the rejection rates were
R. Mouazer et al. / Separation and Purification Technology 32 (2003) 111 /116
115
Fig. 4. Evolution of the electrolyte rejection rates and mobilities of material powders (75/25) vs. pH at a concentration of 10 3 mol l 1. (a) NaCl, (b) Na2SO4, (c) CaCl2, (d) CaSO4; x: electrophoretic mobility, o: R% salt rejection rate.
performed at a pressure of 5 bar after the stabilisation of the flux and of the rejection rate value for 1 h. The rejection rates observed using the Nasicon 75/25 membrane are plotted on Fig. 4a /d, they depend mainly on the salt nature and on the pH of the filtered solution. The rejection obtained are rather high for CaCl2 in acid medium and for Na2SO4 for basic medium. This behaviour is characteristic of the existence of strong interactions between the electric charge of the divalent ion and the surface charge of the membrane. The evolution of the electrophoretic mobility of a suspension of material powder in the considered
electrolyte are in agreement with the rejection rates observed. In the case of CaCl2, the highest rejection is obtained for low pH and then it decreases as the pH increases. On the contrary, the best rejection observed for Na2SO4 at high pH corresponds to the strong interactions developed between the negative membrane and the divalent SO2 anion. 4 The case of the rejection of NaCl versus pH is also in good agreement with the electrophoretic mobilities of the powder, the rejection is close to 0 at the isoelectric point of the filtering layer where the material in not charged. The rejection of this
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salt is controlled by the exclusion of Na at low pH whereas the exclusion of Cl is responsible of the high rejection observed for the pH higher than the pH of the isoelectric point. The rejection rates of CaSO4 seem to follow also the evolution of the powder electrophoretic mobilities, they are more important in acid medium and reach 0 for the high pH values where the membrane seems uncharged.
4. Conclusion The filtering properties of the membranes based on Nasicon /ZnAl2O4 mixtures are in agreement with the electrophoretic mobility measurement of the material powders. For the 75% Nasicon membrane tested, the rejection rate of electrolytes depends strongly on the pH of the filtered solution. The filtration of concentrate solutions need to prepare membrane with higher composition in Nasicon to improve the surface charge density of the membrane which is responsible of the ion
rejections. Such membranes, which contain more than 80% in Nasicon will be tested soon.
References [1] C. Guizard, in: A.J. Burggraff, L. Cot (Eds.), Fundamentals of Inorganic Membrane Science and Technology, Elsevier, Amsterdam, The Netherlands, 1996, pp. 227 /258. [2] A. Larbot, S. Alami-Younsi, M. Persin, J. Sarrazin, L. Cot, J. Membr. Sci. 97 (1994) 167. [3] H.Y.P. Hong, Mater. Res. Bull. 11 (1976) 173. [4] J.P. Boilot, G. Collin, P. Colomban, J. Solid State Chem. 73 (1998) 160. [5] H. Khireddine, Etude des performances de capteurs potentiome´triques a` ions sodium utilisant des membranes de NASICON. Thesis ENSEEG-INPG Grenoble, France 1992. [6] Y. Shimizu, T. Ushijima, Solid State Ionics 132 (2000) 143. [7] Y. Shimizu, S. Michista, T. Murata, Jpn. J. Appl. Phys. 34 (1995) 833. [8] Y. Elmarraki, M. Cretin, M. Persin, J. Sarrazin, A. Larbot, Mater. Res. Bull. 36 (2001) 227. [9] Y. Elmarraki, M. Persin, J. Sarrazin, M. Cretin, A. Larbot, Sep. Purif. Technol. 25 (2001) 493.