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Centrifugal casting of ceramic membrane tubes and the coating with chitosan
Separation and Purification Technology 25 (2001) 407– 413 www.elsevier.com/locate/seppur
Centrifugal casting of ceramic membrane tubes and the coating with chitosan G.C. Steenkamp a, H.W.J.P. Neomagus b, H.M. Krieg a, K. Keizer a,b,* a
School for Chemistry and Biochemistry, Potchefstroom Uni6ersity for CHE, Pri6ate Bag X6001, Potchefstroom 2520, South Africa b School of Chemical and Mineral Engineering, Potchefstroom Uni6ersity for CHE, Pri6ate Bag X6001, Potchefstroom 2520, South Africa
Abstract In this paper, the centrifugal casting of ceramic membrane tubes is presented using different powder particle sizes and powder mixtures. The inner surface of the tubes has a very regular pore structure and is also very smooth. The strength of the tubes increases with increasing sintering temperature and decreasing particle size up to stress values of 1500 MPa. For the strongest materials, the pores sizes and porosities are in the order of 50 nm and 30%, respectively. The water permeability varies between 5 and 50 l/m2.h.bar. The inner surface of the tubes was coated with a highly porous chitosan biopolymer layer of 30 – 50 mm thickness. The chitosan layer is prepared by a phase-inversion method using water with low and high pH values as a solvent/non-solvent system. Silica, which dissolves at high pH values, is used as porogen and creates the pores. This biopolymer can adsorb heavy metals like copper. With this membrane system, Cu2 + (50 ppm as CuSO4) can be removed almost completely with a membrane capacity of 0.1 g Cu2 + /g chitosan. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Centrifugal casting; Chitosan coating; Heavy metal adsorption; Porous alumina tubes
1. Introduction Usually, ceramic membranes have a composite structure consisting of a support for mechanical strength and a number of layers coated on the support. These layers decrease in thickness and pore size towards the top of the membrane. In manufacturing these composite membranes, it is important that the surface quality of the support * Corresponding author. Tel.: + 27-18-2992359; fax: +2718-2992350. E-mail address: [email protected] (K. Keizer).
and supporting layer is sufficient to be suitable for a next layer. Surface roughness, defects and irregular pore-size distributions can cause defects and irregular structure in the top layers. Therefore, a high-quality surface is a necessity. If pastes are used for support manufacturing, defects and irregular packing are almost unavoidable. The distribution of particles in suspensions is much better and a good packing can be achieved. Then, the porous structure of the support is very regular and the surface can be very smooth. A new method for such a support preparation is centrifugal casting [1,2]. Although this method
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is rather more expensive than extrusion, it is very suitable for manufacturing high-quality tubes. For this method, a suspension of particles is poured into a mould and is subjected to a high centrifugal force. The largest particles move firstly to the mould wall followed by the smaller particles. The quality of the outside surface of the tube is determined by the surface quality of the mould, while the inside surface quality is determined by the suspension quality and consists of the smallest particles in the suspension. This high-quality support can be coated directly with thin, microporous layers of silica or zeolites, for instance. In this paper, the preparation of a layer of chitosan, a biopolymer, is presented. Chitosan, poly(2-amino-2-deoxy-D-glucose), is the deacetylated form of chitin, and chitin, poly(Nacetyl-D-glucosamine), is the second most abundant biopolymer after cellulose. Chitosan membranes have already been prepared [3,4]. Much attention has been paid to the porous structure of these membranes since the material is very suitable for removal of heavy metals from polluted water. Heavy metals like copper, chromium, mercury and others are a serious pollution problem in South Africa. The objectives of this paper are therefore related to the following points: 1. preparation, structure and water permeability of centrifugal casted alumina tubes; 2. surface quality and strength of the tubes; 3. can the tubes be provided with a gradual pore structure by using powder mixtures? 4. optimal structure of the chitosan layer for adsorption purposes; and 5. adsorption performance of the composite membrane.
2. Experimental
2.1. Preparation of tubular supports by centrifugal casting The starting a-Al2O3 powders for the tube manufacture were AKP-30 and AKP-15 (Sumitomo Chemical Company, Ltd., Japan) with a mean particle size of 0.40 and 0.62 mm and a BET
surface of 6.2 and 3.5 m2/g, respectively. A 50– 50% w/w mixture of the two types of particles was also used. Both powders have narrow particle-size distributions and a chemical purity of \ 99.99%, as stated by the producer. To obtain tubes with 2 mm wall thickness, 120 g of the starting powder were mixed with different amounts of APMA (ammonium polymethacrylate aqueous solution, Darvan C, R.T. Vanderbilt Company, Inc., Norwalk, USA) and distilled water. The mixture of powder, water and APMA, 120 ml in total, was brought to a pH of 9.5 by adding (1.5 ml) concentrated ammonia. The resulting suspension was ultrasonically treated for 15 min using a frequency of 20 kHz and a transducer output power of 100 W (Model 250 Sonifier, Branson Ultrasonics Corporation, Danbury, USA). With this suspension, tubes were prepared with 6 cm lengths in a home-built apparatus, using steel moulds. Before pouring the suspension into the mould, the mould was coated on the inside with a solution of Vaseline in petroleum ether (boiling range 60–80°C) to ensure easy mould release. The tubes were centrifuged for 20 min at 20 000 rpm, and the remaining liquid was poured out of the moulds afterwards. The green tubes were horizontally dried inside the moulds for 1 day at 30°C. After drying, the green tubes were removed from the moulds and sintered horizontally on a flat support at 1050, 1150 and 1200°C respectively, for 1 h with a heating/cooling rate of 1°C/min.
2.2. Characterisation of the tubes 2.2.1. Water permeability The liquid permeability of the ceramic membrane supports was tested using a liquid permeation set-up, consisting of a N2 gas cylinder, a storage vessel and the permeation cell. The nitrogen gas provides the driving force for the Q2-water (feed) in the storage vessel. The outlet of the storage vessel is connected to the permeation cell in which a tubular support is placed. The supports are sealed off with two O-rings at the bottom and top of the support inside the cell. The permeation module has two inlets and two outlets. This is to ensure that the module can be used in dead-end
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mode as well as in cross-flow mode. The cell was only used in dead end mode, and for these experiments, one inlet and one outlet were always closed. The volumetric flow rate was calculated at five different pressures, and an average value was taken for each support
M=
2.2.2. Strength tests To ensure that the support will be strong enough to withstand the pressures exerted on it, strength tests had to be carried out on the supports. A force is exerted on the outside of the tube by means of a metal object, and the tension before breakage is calculated. A schematic presentation of the break tests is given in Fig. 1. In the test, a porous ceramic tube is placed inside the tensiometer apparatus. Metal objects B and C are used to support the support, while metal object A is used to exert a force on the tube. The force on the tube is increased until breakage occurs. The following equations were used to calculate the tension a tube could withstand until rupture or breakage occurred. The schematic presentation in Fig. 1 serves as a reference for the equations.
P(N) is the force exerted on the support by A before breakage; L (m) is the distance between the two objects B and C; M (Nm) is the momentum exerted on the tube by metal object A; Ixx (m4) is the moment of inertia of the area of the ceramic support and is the tension the support could withstand before breakage; the parameters ravg, t, do, di and c are given in Fig. 1.
.Ixx = yr 3avt rav = |=
do − di 2
Mc . Ixx
(1)
2.2.3. Mercury porosimetry, SEM Mercury porosimetry (Autopore III, Micromeretics) tests were done to determine the median pore radius as well as the porosity of the supports. An SEM type Philips XL30 was used to investigate the structure of the tubular support and coated layer. 2.3. Chitosan coating
Fig. 1. Schematic representation of the strength test for the porous tube.
2.3.1. Preparation of a macroporous chitosan membrane The chitosan coating of the membrane is based on the phase-inversion method using silica as a porogen to create pores in the layer [3]. One gram of chitosan (Aldrich Company) was dissolved in 100 ml of 1 vol.% aqueous acetic acid solution. Silica particles [Silica gel 60 (0.063– 0.200 mm), Merck, Darmstadt, Germany] were added to this solution, followed by vigorous stirring in order to disperse them uniformly. An AKP-30 ceramic support sintered at 1050°C was filled with this solution. The largest part of the solution was then removed so that only a thin chitosan layer remained on the inside of the support. This procedure was repeated, and then the liquid inside the support was allowed to evaporate. The dried membrane was immersed in a 5 wt% aqueous NaOH solution and kept for 2 h at 80°C in order to dissolve the silica particles and generate a porous membrane. The heat treatment
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accelerated the dissolution of silica and also improved the mechanical properties of the membrane. Finally, the porous membrane was washed with distilled water to remove the remaining NaOH. In order to prevent its shrinkage during drying, the membrane was immersed in a 20 vol.% aqueous glycerol solution (softening agent) for 30 min and, after the excess glycerol solution was removed, the support/chitosan system was dried at room temperature. In this way, a strong and flexible macroporous chitosan membrane without shrinkage was obtained. The Cu2 + adsorption and water permeation through the support and chitosan membrane were measured with the same set-up as for the waterpermeation measurements. Then, the feed vessel was filled with a 50 ppm Cu2 + (as CuSO4.5H2O) solution. Permeation and adsorption tests were performed at a pressure of 6 bar. The Cu2 + concentration was analyzed with atomic adsorption (Varian SpectrAA 250) on the permeate collected during the permeation and adsorption test.
3. Results and discussion
3.1. Preparation and properties of the centrifugal casted tubes Porous alumina tubes were prepared with a length of about 6 cm, an inner diameter of about 16 mm and an inner surface area of about 25 cm2. The inner surface area of an AKP-15 tube sintered at 1050°C is shown in Fig. 2. This SEM photograph shows a typical example of the regular and smooth surface of centrifugal casted tubes. In Table 1, a number of characterizations of the centrifugal casted tubes are shown for different starting powders and at different sintering temperatures ranging from 1050 to 1200°C. The pore radius at 1050°C, as measured by mercury porosimetry, is 51, 57 and 100 nm for the AKP30, AKP-15 and AKP15/30 mixture, respectively. The difference between AKP-30 and AKP-15 is easily explained because the powder particle size of AKP30 (0.4 mm) is about twice as small as that for AKP-15 powders (0.7 mm). The porosity of
Fig. 2. SEM photograph of the inner surface of AKP-15 support sintered at 1050°C.
AKP-15 is also somewhat larger because shrinkage during sintering is smaller. The structure of the tubes prepared from the AKP15/30 mixture (50/50 mass ratio) is more comparable with that of the AKP-30 tube. It seems that the addition of finer particles affects the sintering behaviour considerably. The pore size does not change much as the sintering temperature increases. Only for AKP-30 can a clear decrease with increasing temperature be observed. The porosity decrease with increasing temperature for all tubes is clearer and is greatest for the tubes with the smallest pore size. The water permeability of these tubes was measured using different pressures. Here, there is a direct relation between the used particle size and sintering temperature. The permeability decreases with higher sintering temperatures and a smaller particle size. The permeability can also be calculated using the extended Hagen–Poisseuille equation: J=
m r 2 DP ~ 8p Dx
(2)
where J is water flux (m/s), m is porosity (-), ~ is tortuosity (-) is 2.5 for sphere particle packing, r is pore radius (m), p is water viscosity is 10 − 3 Pa.s, P is pressure difference (Pa), and x is membrane thickness (m). The calculated water flux is remarkably close to the experimental water flux. The deviation is
Tube type
Sintering temperature (°C)
Pore radius (nm)
Porosity (%)
Water flux (l/h.bar.m2)
Water flux (Poiseuillea) (l/h.bar.m2)
Mechanical strength (MPa)
AKP-15 AKP-15 AKP-15 AKP-30 AKP-30 AKP-30 AKP 15/30 AKP-15/30 AKP 15/30
1050 1150 1200 1050 1150 1200 1050 1150 1200
100 90 99 51 50 44 57 50 55
41 36 36 37 36 30 38 32 31
44 38 28 9 8.3 7.3 20 18 10
54 26 32 9 8 5 11 7 8
320 230 830 470 840 1580 440 860 1110
a
Extended Poiseuille equation; tortuosity t =2.5.
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Table 1 Structure, mechanical and transport properties of the centrifugal casted tubes
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larger only for the AKP15/30 powder mixture. This can be due to the fact that the gradual decrease of pore size towards the inner side of the tube is larger for powder mixtures than for monosized powders, as is shown by Biesheuvel et al. [5]. SEM photographs show some indications of this gradual structure, but there is no clear segregation of the larger-sized powder to the outside of the tube. It does not seem possible to obtain a twolayer support in this way with large pores at the outer side and small pores at the inner side. This should be the ideal combination of a membrane with a high flux and a good selectivity. A two-step centrifugal casting is probably necessary to obtain such an ideal structure. The mechanical strength of the tubes increases with increasing sintering temperature, decreasing powder particle size or increasing water flux of the tubes. One can imagine that at higher sintering temperatures, the neck area between the particles, and therefore also the strength of the porous compact, increases. The strength is also larger for a small particle packing compared to a larger particle packing because the number of necks per volume is larger. For practical applications, both water flux and strength should be as high as possible. From the presented results, the best choice for a porous ceramic support is a combination of a coarser powder and a somewhat higher sintering temperature.
Fig. 3. SEM photograph of the structure of a chitosan layer on top of an AKP-30 support prepared by using the phase inversion method and silica as a porogen.
metal adsorption. The layer thickness of the chitosan layer is about 30–50 mm, which results in about 50 mg of chitosan per tube, assuming a porosity of the chitosan layer of 50%. The adsorption of Cu2 + by the coated chitosan is shown in Figs. 4 and 5. The starting Cu2 + concentration was 50 ppm. In Fig. 4, the amount of adsorbed copper per amount of chitosan is shown as a function of the copper concentration in the permeate solution. The shape of the curve resembles a Langmuir type of adsorption according to the equation: C Cu ads =
3.2. Chitosan coating A photograph of a chitosan coating is shown in Fig. 3. This chitosan layer is prepared according to the recipe described in Ref. [3], which is in principle a phase inversion method using silica as a porogen. In an acid solution (solvent), chitosan dissolves, but the silica particles do not dissolve. After the formation of a layer on a porous ceramic support, the pH is increased (non-solvent). Now, chitosan precipitates, but the silica particles dissolve. This means that a regular pore structure is formed as an imprint from the silica particle distribution. In this way, highly porous and regular structures can be obtained, and the inner surface area of the chitosan is easily available for
KlC Cu eq 1+ bC Cu eq
Fig. 4. Weight ratio of adsorbed Cu2 + over chitosan as a function of the amount of copper in the permeate [the starting concentration Cu2 + (as CuSO4) is 50 ppm].
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4. Conclusions
Fig. 5. Linearized Langmuir adsorption isotherm for copper adsorption in a chitosan/alumina membrane.
C Cu max =
Kl . b
(3)
C Cu ads is amount of adsorbed copper per amount 2+ chitosan (g/g), C Cu (mg/ eq is concentration of Cu Cu l) in permeate, C max is maximum amount of copper per amount of chitosan adsorbed according to the Langmuir model, and b and Kl are Langmuir constants. For testing Langmuir adsorption behavior, the linearized form of the Langmuir isotherm can be used according to the equation C Cu bC Cu 1 eq eq = + . Cu C ads Kl Kl
(4)
This linearized form of Langmuir adsorption is shown in Fig. 5. The line is more or less linear with an R 2 = 0.966. From both graphs, the C Cu max can be calculated and is of the order of 0.2– 0.22 g Cu/g chitosan. These values are somewhat higher than those found for batch adsorption processes (0.08–0.1 g/g) using particles and flakes [6]. However, one should expect that the adsorption for chitosan membranes is more effective than for particles or flakes. The porous structure of the chitosan membranes is much more open than of the particles and flakes, so the number of available adsorption sites will be larger.
Tubular ceramic membranes have been prepared by the centrifugal casting method using different types of powders and powder mixtures. The inner pore structure was very regular. Pore sizes varied between 50 and 100 nm, and porosities varied between 30 and 50%. As expected, the tubes with small pores and a low porosity were strongest (1500 MPa), which could be achieved using higher sintering temperatures (1200°C) and a smaller particle size (0.40 mm). For particle mixtures, the structure and sintering behavior resembled that of the smallest particles used. The biopolymer chitosan was coated as a layer on the inner side of the tubes with a highly porous and very regular structure. A phase-inversion method was used for the coating with water at low and high pH values as a solvent/ non-solvent system. Silica particles were used as a porogen, which created the pores when it was dissolved and removed at the high pH values. The chitosan layer could remove Cu2 + almost completely in a permeation experiment until a capacity of 0.1 g Cu2 + /g chitosan with a maximum capacity of 0.2 g Cu2 + /g chitosan. The adsorption behavior was described with a Langmuir isotherm.
References [1] A. Nijmeijer, C. Huiskes, N.G.M. Sibelt, H. Kruidhof, H. Verweij, Am. Ceram. Soc. Bull. 77 (1998) 95. [2] W. Huisman, T. Graule, L.J. Gauckler, J. Eur. Ceram. Soc. 15 (1995) 811. [3] X. Zeng, E. Ruckenstein, Ind. Eng. Chem. Res. 35 (1996) 4169. [4] W. Kaminski, Z. Modrezejewska, Sep. Sci. Technol. 32 (1997) 2659. [5] P.M. Biesheuvel, H. Verweij, J. Membr. Sci. 156 (1999) 141. [6] R. Schmuhl, H.M. Krieg, K. Keizer, Water SA 27 (2001) 1.
Centrifugal casting of ceramic membrane tubes and the coating with chitosan G.C. Steenkamp a, H.W.J.P. Neomagus b, H.M. Krieg a, K. Keizer a,b,* a
School for Chemistry and Biochemistry, Potchefstroom Uni6ersity for CHE, Pri6ate Bag X6001, Potchefstroom 2520, South Africa b School of Chemical and Mineral Engineering, Potchefstroom Uni6ersity for CHE, Pri6ate Bag X6001, Potchefstroom 2520, South Africa
Abstract In this paper, the centrifugal casting of ceramic membrane tubes is presented using different powder particle sizes and powder mixtures. The inner surface of the tubes has a very regular pore structure and is also very smooth. The strength of the tubes increases with increasing sintering temperature and decreasing particle size up to stress values of 1500 MPa. For the strongest materials, the pores sizes and porosities are in the order of 50 nm and 30%, respectively. The water permeability varies between 5 and 50 l/m2.h.bar. The inner surface of the tubes was coated with a highly porous chitosan biopolymer layer of 30 – 50 mm thickness. The chitosan layer is prepared by a phase-inversion method using water with low and high pH values as a solvent/non-solvent system. Silica, which dissolves at high pH values, is used as porogen and creates the pores. This biopolymer can adsorb heavy metals like copper. With this membrane system, Cu2 + (50 ppm as CuSO4) can be removed almost completely with a membrane capacity of 0.1 g Cu2 + /g chitosan. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Centrifugal casting; Chitosan coating; Heavy metal adsorption; Porous alumina tubes
1. Introduction Usually, ceramic membranes have a composite structure consisting of a support for mechanical strength and a number of layers coated on the support. These layers decrease in thickness and pore size towards the top of the membrane. In manufacturing these composite membranes, it is important that the surface quality of the support * Corresponding author. Tel.: + 27-18-2992359; fax: +2718-2992350. E-mail address: [email protected] (K. Keizer).
and supporting layer is sufficient to be suitable for a next layer. Surface roughness, defects and irregular pore-size distributions can cause defects and irregular structure in the top layers. Therefore, a high-quality surface is a necessity. If pastes are used for support manufacturing, defects and irregular packing are almost unavoidable. The distribution of particles in suspensions is much better and a good packing can be achieved. Then, the porous structure of the support is very regular and the surface can be very smooth. A new method for such a support preparation is centrifugal casting [1,2]. Although this method
1383-5866/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 3 - 5 8 6 6 ( 0 1 ) 0 0 0 6 9 - 7
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is rather more expensive than extrusion, it is very suitable for manufacturing high-quality tubes. For this method, a suspension of particles is poured into a mould and is subjected to a high centrifugal force. The largest particles move firstly to the mould wall followed by the smaller particles. The quality of the outside surface of the tube is determined by the surface quality of the mould, while the inside surface quality is determined by the suspension quality and consists of the smallest particles in the suspension. This high-quality support can be coated directly with thin, microporous layers of silica or zeolites, for instance. In this paper, the preparation of a layer of chitosan, a biopolymer, is presented. Chitosan, poly(2-amino-2-deoxy-D-glucose), is the deacetylated form of chitin, and chitin, poly(Nacetyl-D-glucosamine), is the second most abundant biopolymer after cellulose. Chitosan membranes have already been prepared [3,4]. Much attention has been paid to the porous structure of these membranes since the material is very suitable for removal of heavy metals from polluted water. Heavy metals like copper, chromium, mercury and others are a serious pollution problem in South Africa. The objectives of this paper are therefore related to the following points: 1. preparation, structure and water permeability of centrifugal casted alumina tubes; 2. surface quality and strength of the tubes; 3. can the tubes be provided with a gradual pore structure by using powder mixtures? 4. optimal structure of the chitosan layer for adsorption purposes; and 5. adsorption performance of the composite membrane.
2. Experimental
2.1. Preparation of tubular supports by centrifugal casting The starting a-Al2O3 powders for the tube manufacture were AKP-30 and AKP-15 (Sumitomo Chemical Company, Ltd., Japan) with a mean particle size of 0.40 and 0.62 mm and a BET
surface of 6.2 and 3.5 m2/g, respectively. A 50– 50% w/w mixture of the two types of particles was also used. Both powders have narrow particle-size distributions and a chemical purity of \ 99.99%, as stated by the producer. To obtain tubes with 2 mm wall thickness, 120 g of the starting powder were mixed with different amounts of APMA (ammonium polymethacrylate aqueous solution, Darvan C, R.T. Vanderbilt Company, Inc., Norwalk, USA) and distilled water. The mixture of powder, water and APMA, 120 ml in total, was brought to a pH of 9.5 by adding (1.5 ml) concentrated ammonia. The resulting suspension was ultrasonically treated for 15 min using a frequency of 20 kHz and a transducer output power of 100 W (Model 250 Sonifier, Branson Ultrasonics Corporation, Danbury, USA). With this suspension, tubes were prepared with 6 cm lengths in a home-built apparatus, using steel moulds. Before pouring the suspension into the mould, the mould was coated on the inside with a solution of Vaseline in petroleum ether (boiling range 60–80°C) to ensure easy mould release. The tubes were centrifuged for 20 min at 20 000 rpm, and the remaining liquid was poured out of the moulds afterwards. The green tubes were horizontally dried inside the moulds for 1 day at 30°C. After drying, the green tubes were removed from the moulds and sintered horizontally on a flat support at 1050, 1150 and 1200°C respectively, for 1 h with a heating/cooling rate of 1°C/min.
2.2. Characterisation of the tubes 2.2.1. Water permeability The liquid permeability of the ceramic membrane supports was tested using a liquid permeation set-up, consisting of a N2 gas cylinder, a storage vessel and the permeation cell. The nitrogen gas provides the driving force for the Q2-water (feed) in the storage vessel. The outlet of the storage vessel is connected to the permeation cell in which a tubular support is placed. The supports are sealed off with two O-rings at the bottom and top of the support inside the cell. The permeation module has two inlets and two outlets. This is to ensure that the module can be used in dead-end
G.C. Steenkamp et al. / Separation/Purification Technology 25 (2001) 407–413
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mode as well as in cross-flow mode. The cell was only used in dead end mode, and for these experiments, one inlet and one outlet were always closed. The volumetric flow rate was calculated at five different pressures, and an average value was taken for each support
M=
2.2.2. Strength tests To ensure that the support will be strong enough to withstand the pressures exerted on it, strength tests had to be carried out on the supports. A force is exerted on the outside of the tube by means of a metal object, and the tension before breakage is calculated. A schematic presentation of the break tests is given in Fig. 1. In the test, a porous ceramic tube is placed inside the tensiometer apparatus. Metal objects B and C are used to support the support, while metal object A is used to exert a force on the tube. The force on the tube is increased until breakage occurs. The following equations were used to calculate the tension a tube could withstand until rupture or breakage occurred. The schematic presentation in Fig. 1 serves as a reference for the equations.
P(N) is the force exerted on the support by A before breakage; L (m) is the distance between the two objects B and C; M (Nm) is the momentum exerted on the tube by metal object A; Ixx (m4) is the moment of inertia of the area of the ceramic support and is the tension the support could withstand before breakage; the parameters ravg, t, do, di and c are given in Fig. 1.
.Ixx = yr 3avt rav = |=
do − di 2
Mc . Ixx
(1)
2.2.3. Mercury porosimetry, SEM Mercury porosimetry (Autopore III, Micromeretics) tests were done to determine the median pore radius as well as the porosity of the supports. An SEM type Philips XL30 was used to investigate the structure of the tubular support and coated layer. 2.3. Chitosan coating
Fig. 1. Schematic representation of the strength test for the porous tube.
2.3.1. Preparation of a macroporous chitosan membrane The chitosan coating of the membrane is based on the phase-inversion method using silica as a porogen to create pores in the layer [3]. One gram of chitosan (Aldrich Company) was dissolved in 100 ml of 1 vol.% aqueous acetic acid solution. Silica particles [Silica gel 60 (0.063– 0.200 mm), Merck, Darmstadt, Germany] were added to this solution, followed by vigorous stirring in order to disperse them uniformly. An AKP-30 ceramic support sintered at 1050°C was filled with this solution. The largest part of the solution was then removed so that only a thin chitosan layer remained on the inside of the support. This procedure was repeated, and then the liquid inside the support was allowed to evaporate. The dried membrane was immersed in a 5 wt% aqueous NaOH solution and kept for 2 h at 80°C in order to dissolve the silica particles and generate a porous membrane. The heat treatment
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accelerated the dissolution of silica and also improved the mechanical properties of the membrane. Finally, the porous membrane was washed with distilled water to remove the remaining NaOH. In order to prevent its shrinkage during drying, the membrane was immersed in a 20 vol.% aqueous glycerol solution (softening agent) for 30 min and, after the excess glycerol solution was removed, the support/chitosan system was dried at room temperature. In this way, a strong and flexible macroporous chitosan membrane without shrinkage was obtained. The Cu2 + adsorption and water permeation through the support and chitosan membrane were measured with the same set-up as for the waterpermeation measurements. Then, the feed vessel was filled with a 50 ppm Cu2 + (as CuSO4.5H2O) solution. Permeation and adsorption tests were performed at a pressure of 6 bar. The Cu2 + concentration was analyzed with atomic adsorption (Varian SpectrAA 250) on the permeate collected during the permeation and adsorption test.
3. Results and discussion
3.1. Preparation and properties of the centrifugal casted tubes Porous alumina tubes were prepared with a length of about 6 cm, an inner diameter of about 16 mm and an inner surface area of about 25 cm2. The inner surface area of an AKP-15 tube sintered at 1050°C is shown in Fig. 2. This SEM photograph shows a typical example of the regular and smooth surface of centrifugal casted tubes. In Table 1, a number of characterizations of the centrifugal casted tubes are shown for different starting powders and at different sintering temperatures ranging from 1050 to 1200°C. The pore radius at 1050°C, as measured by mercury porosimetry, is 51, 57 and 100 nm for the AKP30, AKP-15 and AKP15/30 mixture, respectively. The difference between AKP-30 and AKP-15 is easily explained because the powder particle size of AKP30 (0.4 mm) is about twice as small as that for AKP-15 powders (0.7 mm). The porosity of
Fig. 2. SEM photograph of the inner surface of AKP-15 support sintered at 1050°C.
AKP-15 is also somewhat larger because shrinkage during sintering is smaller. The structure of the tubes prepared from the AKP15/30 mixture (50/50 mass ratio) is more comparable with that of the AKP-30 tube. It seems that the addition of finer particles affects the sintering behaviour considerably. The pore size does not change much as the sintering temperature increases. Only for AKP-30 can a clear decrease with increasing temperature be observed. The porosity decrease with increasing temperature for all tubes is clearer and is greatest for the tubes with the smallest pore size. The water permeability of these tubes was measured using different pressures. Here, there is a direct relation between the used particle size and sintering temperature. The permeability decreases with higher sintering temperatures and a smaller particle size. The permeability can also be calculated using the extended Hagen–Poisseuille equation: J=
m r 2 DP ~ 8p Dx
(2)
where J is water flux (m/s), m is porosity (-), ~ is tortuosity (-) is 2.5 for sphere particle packing, r is pore radius (m), p is water viscosity is 10 − 3 Pa.s, P is pressure difference (Pa), and x is membrane thickness (m). The calculated water flux is remarkably close to the experimental water flux. The deviation is
Tube type
Sintering temperature (°C)
Pore radius (nm)
Porosity (%)
Water flux (l/h.bar.m2)
Water flux (Poiseuillea) (l/h.bar.m2)
Mechanical strength (MPa)
AKP-15 AKP-15 AKP-15 AKP-30 AKP-30 AKP-30 AKP 15/30 AKP-15/30 AKP 15/30
1050 1150 1200 1050 1150 1200 1050 1150 1200
100 90 99 51 50 44 57 50 55
41 36 36 37 36 30 38 32 31
44 38 28 9 8.3 7.3 20 18 10
54 26 32 9 8 5 11 7 8
320 230 830 470 840 1580 440 860 1110
a
Extended Poiseuille equation; tortuosity t =2.5.
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Table 1 Structure, mechanical and transport properties of the centrifugal casted tubes
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larger only for the AKP15/30 powder mixture. This can be due to the fact that the gradual decrease of pore size towards the inner side of the tube is larger for powder mixtures than for monosized powders, as is shown by Biesheuvel et al. [5]. SEM photographs show some indications of this gradual structure, but there is no clear segregation of the larger-sized powder to the outside of the tube. It does not seem possible to obtain a twolayer support in this way with large pores at the outer side and small pores at the inner side. This should be the ideal combination of a membrane with a high flux and a good selectivity. A two-step centrifugal casting is probably necessary to obtain such an ideal structure. The mechanical strength of the tubes increases with increasing sintering temperature, decreasing powder particle size or increasing water flux of the tubes. One can imagine that at higher sintering temperatures, the neck area between the particles, and therefore also the strength of the porous compact, increases. The strength is also larger for a small particle packing compared to a larger particle packing because the number of necks per volume is larger. For practical applications, both water flux and strength should be as high as possible. From the presented results, the best choice for a porous ceramic support is a combination of a coarser powder and a somewhat higher sintering temperature.
Fig. 3. SEM photograph of the structure of a chitosan layer on top of an AKP-30 support prepared by using the phase inversion method and silica as a porogen.
metal adsorption. The layer thickness of the chitosan layer is about 30–50 mm, which results in about 50 mg of chitosan per tube, assuming a porosity of the chitosan layer of 50%. The adsorption of Cu2 + by the coated chitosan is shown in Figs. 4 and 5. The starting Cu2 + concentration was 50 ppm. In Fig. 4, the amount of adsorbed copper per amount of chitosan is shown as a function of the copper concentration in the permeate solution. The shape of the curve resembles a Langmuir type of adsorption according to the equation: C Cu ads =
3.2. Chitosan coating A photograph of a chitosan coating is shown in Fig. 3. This chitosan layer is prepared according to the recipe described in Ref. [3], which is in principle a phase inversion method using silica as a porogen. In an acid solution (solvent), chitosan dissolves, but the silica particles do not dissolve. After the formation of a layer on a porous ceramic support, the pH is increased (non-solvent). Now, chitosan precipitates, but the silica particles dissolve. This means that a regular pore structure is formed as an imprint from the silica particle distribution. In this way, highly porous and regular structures can be obtained, and the inner surface area of the chitosan is easily available for
KlC Cu eq 1+ bC Cu eq
Fig. 4. Weight ratio of adsorbed Cu2 + over chitosan as a function of the amount of copper in the permeate [the starting concentration Cu2 + (as CuSO4) is 50 ppm].
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4. Conclusions
Fig. 5. Linearized Langmuir adsorption isotherm for copper adsorption in a chitosan/alumina membrane.
C Cu max =
Kl . b
(3)
C Cu ads is amount of adsorbed copper per amount 2+ chitosan (g/g), C Cu (mg/ eq is concentration of Cu Cu l) in permeate, C max is maximum amount of copper per amount of chitosan adsorbed according to the Langmuir model, and b and Kl are Langmuir constants. For testing Langmuir adsorption behavior, the linearized form of the Langmuir isotherm can be used according to the equation C Cu bC Cu 1 eq eq = + . Cu C ads Kl Kl
(4)
This linearized form of Langmuir adsorption is shown in Fig. 5. The line is more or less linear with an R 2 = 0.966. From both graphs, the C Cu max can be calculated and is of the order of 0.2– 0.22 g Cu/g chitosan. These values are somewhat higher than those found for batch adsorption processes (0.08–0.1 g/g) using particles and flakes [6]. However, one should expect that the adsorption for chitosan membranes is more effective than for particles or flakes. The porous structure of the chitosan membranes is much more open than of the particles and flakes, so the number of available adsorption sites will be larger.
Tubular ceramic membranes have been prepared by the centrifugal casting method using different types of powders and powder mixtures. The inner pore structure was very regular. Pore sizes varied between 50 and 100 nm, and porosities varied between 30 and 50%. As expected, the tubes with small pores and a low porosity were strongest (1500 MPa), which could be achieved using higher sintering temperatures (1200°C) and a smaller particle size (0.40 mm). For particle mixtures, the structure and sintering behavior resembled that of the smallest particles used. The biopolymer chitosan was coated as a layer on the inner side of the tubes with a highly porous and very regular structure. A phase-inversion method was used for the coating with water at low and high pH values as a solvent/ non-solvent system. Silica particles were used as a porogen, which created the pores when it was dissolved and removed at the high pH values. The chitosan layer could remove Cu2 + almost completely in a permeation experiment until a capacity of 0.1 g Cu2 + /g chitosan with a maximum capacity of 0.2 g Cu2 + /g chitosan. The adsorption behavior was described with a Langmuir isotherm.
References [1] A. Nijmeijer, C. Huiskes, N.G.M. Sibelt, H. Kruidhof, H. Verweij, Am. Ceram. Soc. Bull. 77 (1998) 95. [2] W. Huisman, T. Graule, L.J. Gauckler, J. Eur. Ceram. Soc. 15 (1995) 811. [3] X. Zeng, E. Ruckenstein, Ind. Eng. Chem. Res. 35 (1996) 4169. [4] W. Kaminski, Z. Modrezejewska, Sep. Sci. Technol. 32 (1997) 2659. [5] P.M. Biesheuvel, H. Verweij, J. Membr. Sci. 156 (1999) 141. [6] R. Schmuhl, H.M. Krieg, K. Keizer, Water SA 27 (2001) 1.