- Email: [email protected]
Synthesis and water ultrafiltration properties of silver membrane supported on porous ceramics
DESALINATION Desalination 114 (1997) 203-208
ELSEVIER
Synthesis and water ultrafiltration properties of silver membrane supported on porous ceramics* Antonio P6rez Padilla**, Jorge A. Rodriguez, Hugo A. Saitfa INTEQUI, Instituto de lnvestigaciones en Tecnologia Quimica (CONICET), Universidad Nacional de San Luis, Casilla de Correo 290, 5700 San Luis, Argentina Fax +54 (6) 522-6711; E-mail: [email protected]
Received 29 December 1996; accepted 1 December 1997
Abstract Studies were conducted on the synthesis, characterization and water ultrafiltration properties of a composite membrane consisting of a thin silver film supported on the outer surface of a porous ceramic cylinder. The support was obtained by slip-casting using materials easier to prepare and less expensive than commercial supports. Ag films were coated on the surface of support using electroless plating and cathodic electrodeposition techniques. Cross-flow ultrafiltration was used to treat water from a public water supply. The experimental results indicated the efficiency of the composite membrane. Treatment quality was evaluated on removal turbidity, heterotroph plate counts and humics as determined by removal of substances absorbing UV light at 254 nm.
Keywords: Metallic membrane; Composite membrane; Water ultrafiltration; Membrane preparation
I. Introduction The use of inorganic membranes in separation technologies is relatively new and has recently become relevant. Advantages of inorganic membranes over organic ones are their chemical, thermal and pH stability. They do not become deformed during operation, they have a long life-
*Patent submitted.
**Corresponding author.
time, they can be sterilized at 120°C and they are resistant to high temperature and pressure and to corrosive solutions [1 ]. They can also be cleaned and regenerated in situ. Ceramic membranes represent a distinct class of inorganic membranes. The most important kind of supported ceramic membranes are refractory oxides such as alumina, titania or zirconia. However, the disadvantage of these membranes is their brittle character. To overcome this defective behavior, the active membrane can be
0011-9164/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved Pll S0011-9164(98)0001 2-5
204
A. Pdrez Padilla et al. / Desalination 114 (1997) 203-208
coated on a macroporous ceramic support. Concerning the shape of ceramic membranes two main geometries have largely been investigated: tubular and flat membranes. The best geometry is tubular, and this gives the best mechanical resistance to the structure. This is a well adapted geometry for cross-flow filtration in which the feed stream is circulated across the surface of the membrane and the permeated flux goes through the membrane in a perpendicular direction. The cross-flow operation prevents accumulation of the retained products on the membrane surface and slows down fouling phenomena. Although the use of inorganic membranes for filtrating surface water to produce potable water is one of the most prominent industrial applications, the published literature is rather scanty. However, it is well known that the major barrier to the use of membrane technologies for potable water treatment by ultrafiltration (UF) is the cost of produced water. To keep it as low as possible, the most important factors are the flux that can be maintained continuously and the cost and life time of the membrane [2]. In this paper, a composite membrane is proposed consisting of a thin silver film deposited on a porous ceramic (PC) tube, and the characteristics of a composite membrane, called "Ag/PC composite membrane" are discussed with respect to water UF. The objectives of the present contribution are to report the synthesis and characterization of thin silver membranes on ceramic support by electroless plating and conventional cathodic electrodeposition [3] methods and to evaluate the quality of treatment in natural water by crossflow UF.
2. Experimental 2.1. M e m b r a n e fabrication
Ag membranes were prepared in our laboratory by electroless plating and cathodic electrodeposition techniques on a ceramic support. The support consisted of a PC tube. The
PC tube was prepared by slip-casting. Suspensions of 5-9% A1203, 12-17% kaolin, 3-5% bentonite, and 40-45% clay mixtures in water were prepared, and the suspension pH adjusted to 7.5 with HCI. They were agitated vigorously to break up agglomerates, and then they were stirred slowly to remove air bubbles. Supports were slipcast into gypsum mold from 5 to 10 min. After being slip-cast, the supports were dried by air for 5 days with a high humidity to prevent crack formation during drying and in the early stage of firing. The supports were then dried at 110°C and fired to 1100°C at 2°C/min and held there for 60 min. In all cases, sodium carbonate and sodium silicate were added to the suspensions to avoid flocculation and to adjust density. To prepare supports, all suspensions were hydrolyzed for 7 days, sieved (200 mesh with magnetic trap) and hydrolyzed for at least one additional day after sieving. The specific surface area of the supports determined from krypton adsorption isotherms at 77K by the BET method was 5 m2.g 1. The average pore radius determined from mercury intrusion porosimetry was 4.4 #m. Silver was supported on a PC tube (OD 27.5 mm, thickness ca 3 mm). Earlier the substrates were ultrasonically cleaned and dried at I10°C. Silver was deposited on a PC tube by using the electroless-plating technique since the tube is electrically nonconductive. The electroless bath contained silver nitrate, EDTA, ammonium hydroxide and hydrazine. Subsequently, the support with silver deposit was burnt in reductive flame. Then a new silver film was supported on the outer surface of the tube by a conventional cathodic electrodeposition technique. Membranes placed on the outside of such support showed an optimum compressive strength and less tendency to shear off the supports during filtration. The effective area of the membrane was 104 cm 2. 2.2. M e m b r a n e characterization
The surface morphology of the deposited films was inspected by using a scanning electron
205
A. POrezPadilla et al. /Desalination 114 (1997) 203-208 (
RETEklTATE 0
HOUg~G
1 ) MEMm~AM[
)
t
PERMEATE
Fig. 1. Schematic diagram of the experimental apparatus. microscope (ETEC Mod. Autoscan). The composition and phase structure of the metal films were obtained by using an X-Ray diffractometer (RIGAKU Mod. D-MAX III C) operated at 35 kV and 30mA employing Ni-filtered Cu Kcc radiation. In order to measure the permeability to pure water and rejection characteristic of Ag/PC composite membranes, a conventional system of cross-flow with recycle of retentate was designed and manufactured (Fig. 1). The pure water flux through a membrane is directly proportional to the applied pressure acording to Eq. (1) AP T J = .-Rm
In order to determine if this type of behavior is also characteristic of composite membranes, the permeability of pure water through supported silver membrane was measured at temperatures between 10 and 40°C (Fig. 2). Water fluxes through these membranes were measured with the membranes stored in pure water between measurements. The effect of applied pressure on permeate flux was also studied over that temperature range (see Fig. 3). The convention established by the solute passage testing is based on the retention of globular proteins [4]. The percent of retention is defined as: 50 4,846 44 42 40
i 32
3i3
i 34
i 35
36
1/T (Kt x103)
Fig. 2. Arrhenius plot of permeability data. Variation of permeate flux with temperature.
(1) 2OO
where J is the flux, A P r the transmembrane pressure and Rm the intrinsic membrane resistance. Rm values are useful not only for modeling purposes but also for evaluating the effectiveness o f the cleaning procedures and for charting the long-term stability of the membrane. The intrinsic resistance of the composite mem-brane (Rm) obtained was 5.97 x 1(}7 S2 cm-2. Permeabilities of polymeric membranes have been found to exhibit Arrhenius behavior with temperature J = Jo exp (-Ea IRT~
(2)
~
180t t 1601 140~ t
20
. • • •
Templ0C Temp20C Temp25C Temp30C
J J =~
~1, Zemp.40C
40
.
/
.
.
60
.
80
.
/
.
100
.
.
/
.
1~0
140
Pressure (kPa)
Fig. 3. Effect of pressure on permeate flux at different temperatures.
2O6
A. P~rez Padilla et al. / Desalination 114 (1997) 203-208
(3) where Cp is the concentration of solute in the permeate and C r is the concentration of solute in the retentate. Convention states that the molecular weight cut-off (MWCO) of the membrane is equal to the molecular weight of globular proteins which are 90% retained by the membrane. In this work the following conditions were used: (a) pressure of 100kPa, (b) temperature of 25°C, (c) 0.1% w/v concentration of solute in 1% saline solution, (d) not more than 10% of the permeate removal to avoid concentration effect and (e) washing steps until a reproducible pure water flux is obtained prior to passage testing. Aqueous solutions of gamma globulin (160 kD), albumin (69kD) and cytochrome C (13 kD) were used as test solutions.
3. Results and discussion 3.1. M e m b r a n e characteristics
Fig. 4 shows SEM photograph of the outer surface of the cylinder after cathodic electrodeposition. It can be seen that the surface morphology of the coating appears uniform and without visible "pores". The coated films show good adhesion to the support. The XRD pattern of a porous ceramic supported Ag membrane and target Ag foil are compared in Fig. 5. In the membrane sample, reflection peaks of the Ag film are clear. The XRD pattern of the coated Ag film is identical to that of the Ag foil used as the target. The reflection position of a complete solid solution generally depends on the composition. Taking this into consideration, one can deduce that the Ag film is pure silver. The permeabilities of the membrane to pure water are observed to decline as time goes by. The loss in permeate flux is little. Ninety percent of the initial water permeability was obtained even after 30 h of operation. Membrane permea-
--
Fig. 4. SEM photograph of the porous ceramic tube after cathodic electrodepositionof silver. bilities to water at different temperatures follow an Arrhenius behavior, with an activation energy of 0.016kJ/mole. This value is similar, but slightly lower than, those observed for polymeric membranes which are typically about 0.021 kJ/ mole [5]. According to Fig. 3, the flux is directly proportional to the applied pressure (pressurecontrolled region). Thus, increasing the temperature the flux increases. The rejection coefficients obtained with the test solutions were of 97% for gammaglobulin, of about 90% for albumin and 20% for cytochrome C. The results indicate that the membrane has a MWCO of approximately 70kD. Since the effective pore size is estimated from the molecular diameter of globular proteins, the membrane should have an estimated effective pore size of 0.01 # [6]. 3.2. Application in the treatment o f raw water
Batch UF was used to evaluate the appropriateness of Ag/PC composite membrane
207
A. P~rez Padilla et aL / Desalination 114 (1997) 203-208 90 JCPDS N° 4-783 (A9) 6O to 30 I
i
,
,i
,
,
,
•_. 90° XRD pattern coated Ag film a:
60 30
L 20
40
8O
60
Fig. 5. XRD patterns of the Ag foil used as target and ofa Ag membrane coated on a ceramic support.
20 (o) for the treatment of water from Cruz de Piedra, Lake San Luis, a public water supply in the midwest of Argentina. The membrane filtration system used was the same as previously reported in Fig. 1. Very high turbidity removal is achieved with UF. The turbidity of the water fed to the membrane was 20 NTU while permeate turbidity was between 0.1 and 0.2 NTU. Heterotroph plate counts removal was total. Removal of humics as determined by removal of substances absorbing UV light at 254nm was approximately 35%. Usually, most organic carbon in natural waters is present as humic substances that represent a variety of molecules with different functional groups and with different molecular weights [7]. It has been reported that a large fraction of the organic carbon in the waters of the lake has an average molecular weight around I kD, while a somewhat smaller fraction has a molecular weight larger than 100kD [8]. Therefore, a lower MWCO membrane should not be really expected to be more efficient than a 70kD MWCO membrane, since both membranes would pass the lower molecular weight fraction. Besides, both membranes would retain the higher molecular weight fraction. Evidently, pretreatment before UF is necessary to achieve significant organic carbon removals. Fig. 6 illustrates the effect of applied pressure on permeate flux during UF of raw water and pure water at 20oC. The results obtained indicate that the UF performance of the composite
140 120 .c=
100
~
8o
•
raw
•
pure
402o
20
4'0
6'0
8'0
160
I~0
i~0
~o
Pressure (kPa)
Fig. 6. Effect of pressure on permeate flux at 20°C. membrane is similar despite the large difference in the composition of the water samples. The flow of raw water is limited by accumulation of particles on the membrane surface during the course of filtration. 4. Conclusion
Electroless plating and conventional cathodic electrodeposition has been successfully used to deposit thin silver films on porous ceramic support. The methods are found to be faster and not so difficult. The porous ceramic support is prepared by slip-casting. The choice of support material has been given by several factors including ease of preparation, cost of materials and reproducibility. The composite membrane
208
A. P~rez Padilla et al. / Desalination 114 (1997) 203-208
obtained show good stability. The permeation of water through composite membrane at different temperatures follow an Arrhenius behavior, with an activation energy which is similar to that obtained for several polymeric membranes. Measured water permeabilities of the membrane also decrease to a small degree as the membrane is exposed to water for longer times. The production of potable water by cross-flow UF is used to evaluate the efficiency of the composite membrane. The excellent performance of membrane for the complete removal of total heterotroph plate counts from their high feed levels is indeed noteworthy. The results also shows the turbidity reduction in raw water from 20 NTU to values below 0.2NTU in the filtered water. Removal of humics in approximately 35% is observed.
Acknowledgments We wish to express our thanks to Lic. Jorge GonzAlez for their technical assistance with XRD studies.
References [1] R.R.Bhave, Inorganic Membranes Synthesis: Characteristics and Applications, Van Nostrand, New York, 1991. [2] P. Aptel, Applications of Membranes in Drinking Water Treatment. in: Membranes Processes and Applications, ESMST, X Summer School on Membranes, Valladolid, Spain, 1993, p. 47. [3] E. Bertorelle, Galvanot6cnica, Ulrico Hoepli, Milano, 1951. [4] M. Cheryan, Ultrafiltration Handbook, Technomic Publishing, Lancaster, 1986. [5] H.K. Lonsdale, U. Merten and R. L. Riley, J. Appl. Polym. Sci., 9 (1965) 1341. [6] M.C. Porter, Handbook of Industrial Membrane Technology,Noyes Publications, New Jersey, 1990, p.156. [7] J.M. Montgomery, Water Treatment Principles and Design, John Wiley, New York, 1985. [8] M.M. Clark and K. Heneghan, Desalination, 80 (1991) 143.
ELSEVIER
Synthesis and water ultrafiltration properties of silver membrane supported on porous ceramics* Antonio P6rez Padilla**, Jorge A. Rodriguez, Hugo A. Saitfa INTEQUI, Instituto de lnvestigaciones en Tecnologia Quimica (CONICET), Universidad Nacional de San Luis, Casilla de Correo 290, 5700 San Luis, Argentina Fax +54 (6) 522-6711; E-mail: [email protected]
Received 29 December 1996; accepted 1 December 1997
Abstract Studies were conducted on the synthesis, characterization and water ultrafiltration properties of a composite membrane consisting of a thin silver film supported on the outer surface of a porous ceramic cylinder. The support was obtained by slip-casting using materials easier to prepare and less expensive than commercial supports. Ag films were coated on the surface of support using electroless plating and cathodic electrodeposition techniques. Cross-flow ultrafiltration was used to treat water from a public water supply. The experimental results indicated the efficiency of the composite membrane. Treatment quality was evaluated on removal turbidity, heterotroph plate counts and humics as determined by removal of substances absorbing UV light at 254 nm.
Keywords: Metallic membrane; Composite membrane; Water ultrafiltration; Membrane preparation
I. Introduction The use of inorganic membranes in separation technologies is relatively new and has recently become relevant. Advantages of inorganic membranes over organic ones are their chemical, thermal and pH stability. They do not become deformed during operation, they have a long life-
*Patent submitted.
**Corresponding author.
time, they can be sterilized at 120°C and they are resistant to high temperature and pressure and to corrosive solutions [1 ]. They can also be cleaned and regenerated in situ. Ceramic membranes represent a distinct class of inorganic membranes. The most important kind of supported ceramic membranes are refractory oxides such as alumina, titania or zirconia. However, the disadvantage of these membranes is their brittle character. To overcome this defective behavior, the active membrane can be
0011-9164/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved Pll S0011-9164(98)0001 2-5
204
A. Pdrez Padilla et al. / Desalination 114 (1997) 203-208
coated on a macroporous ceramic support. Concerning the shape of ceramic membranes two main geometries have largely been investigated: tubular and flat membranes. The best geometry is tubular, and this gives the best mechanical resistance to the structure. This is a well adapted geometry for cross-flow filtration in which the feed stream is circulated across the surface of the membrane and the permeated flux goes through the membrane in a perpendicular direction. The cross-flow operation prevents accumulation of the retained products on the membrane surface and slows down fouling phenomena. Although the use of inorganic membranes for filtrating surface water to produce potable water is one of the most prominent industrial applications, the published literature is rather scanty. However, it is well known that the major barrier to the use of membrane technologies for potable water treatment by ultrafiltration (UF) is the cost of produced water. To keep it as low as possible, the most important factors are the flux that can be maintained continuously and the cost and life time of the membrane [2]. In this paper, a composite membrane is proposed consisting of a thin silver film deposited on a porous ceramic (PC) tube, and the characteristics of a composite membrane, called "Ag/PC composite membrane" are discussed with respect to water UF. The objectives of the present contribution are to report the synthesis and characterization of thin silver membranes on ceramic support by electroless plating and conventional cathodic electrodeposition [3] methods and to evaluate the quality of treatment in natural water by crossflow UF.
2. Experimental 2.1. M e m b r a n e fabrication
Ag membranes were prepared in our laboratory by electroless plating and cathodic electrodeposition techniques on a ceramic support. The support consisted of a PC tube. The
PC tube was prepared by slip-casting. Suspensions of 5-9% A1203, 12-17% kaolin, 3-5% bentonite, and 40-45% clay mixtures in water were prepared, and the suspension pH adjusted to 7.5 with HCI. They were agitated vigorously to break up agglomerates, and then they were stirred slowly to remove air bubbles. Supports were slipcast into gypsum mold from 5 to 10 min. After being slip-cast, the supports were dried by air for 5 days with a high humidity to prevent crack formation during drying and in the early stage of firing. The supports were then dried at 110°C and fired to 1100°C at 2°C/min and held there for 60 min. In all cases, sodium carbonate and sodium silicate were added to the suspensions to avoid flocculation and to adjust density. To prepare supports, all suspensions were hydrolyzed for 7 days, sieved (200 mesh with magnetic trap) and hydrolyzed for at least one additional day after sieving. The specific surface area of the supports determined from krypton adsorption isotherms at 77K by the BET method was 5 m2.g 1. The average pore radius determined from mercury intrusion porosimetry was 4.4 #m. Silver was supported on a PC tube (OD 27.5 mm, thickness ca 3 mm). Earlier the substrates were ultrasonically cleaned and dried at I10°C. Silver was deposited on a PC tube by using the electroless-plating technique since the tube is electrically nonconductive. The electroless bath contained silver nitrate, EDTA, ammonium hydroxide and hydrazine. Subsequently, the support with silver deposit was burnt in reductive flame. Then a new silver film was supported on the outer surface of the tube by a conventional cathodic electrodeposition technique. Membranes placed on the outside of such support showed an optimum compressive strength and less tendency to shear off the supports during filtration. The effective area of the membrane was 104 cm 2. 2.2. M e m b r a n e characterization
The surface morphology of the deposited films was inspected by using a scanning electron
205
A. POrezPadilla et al. /Desalination 114 (1997) 203-208 (
RETEklTATE 0
HOUg~G
1 ) MEMm~AM[
)
t
PERMEATE
Fig. 1. Schematic diagram of the experimental apparatus. microscope (ETEC Mod. Autoscan). The composition and phase structure of the metal films were obtained by using an X-Ray diffractometer (RIGAKU Mod. D-MAX III C) operated at 35 kV and 30mA employing Ni-filtered Cu Kcc radiation. In order to measure the permeability to pure water and rejection characteristic of Ag/PC composite membranes, a conventional system of cross-flow with recycle of retentate was designed and manufactured (Fig. 1). The pure water flux through a membrane is directly proportional to the applied pressure acording to Eq. (1) AP T J = .-Rm
In order to determine if this type of behavior is also characteristic of composite membranes, the permeability of pure water through supported silver membrane was measured at temperatures between 10 and 40°C (Fig. 2). Water fluxes through these membranes were measured with the membranes stored in pure water between measurements. The effect of applied pressure on permeate flux was also studied over that temperature range (see Fig. 3). The convention established by the solute passage testing is based on the retention of globular proteins [4]. The percent of retention is defined as: 50 4,846 44 42 40
i 32
3i3
i 34
i 35
36
1/T (Kt x103)
Fig. 2. Arrhenius plot of permeability data. Variation of permeate flux with temperature.
(1) 2OO
where J is the flux, A P r the transmembrane pressure and Rm the intrinsic membrane resistance. Rm values are useful not only for modeling purposes but also for evaluating the effectiveness o f the cleaning procedures and for charting the long-term stability of the membrane. The intrinsic resistance of the composite mem-brane (Rm) obtained was 5.97 x 1(}7 S2 cm-2. Permeabilities of polymeric membranes have been found to exhibit Arrhenius behavior with temperature J = Jo exp (-Ea IRT~
(2)
~
180t t 1601 140~ t
20
. • • •
Templ0C Temp20C Temp25C Temp30C
J J =~
~1, Zemp.40C
40
.
/
.
.
60
.
80
.
/
.
100
.
.
/
.
1~0
140
Pressure (kPa)
Fig. 3. Effect of pressure on permeate flux at different temperatures.
2O6
A. P~rez Padilla et al. / Desalination 114 (1997) 203-208
(3) where Cp is the concentration of solute in the permeate and C r is the concentration of solute in the retentate. Convention states that the molecular weight cut-off (MWCO) of the membrane is equal to the molecular weight of globular proteins which are 90% retained by the membrane. In this work the following conditions were used: (a) pressure of 100kPa, (b) temperature of 25°C, (c) 0.1% w/v concentration of solute in 1% saline solution, (d) not more than 10% of the permeate removal to avoid concentration effect and (e) washing steps until a reproducible pure water flux is obtained prior to passage testing. Aqueous solutions of gamma globulin (160 kD), albumin (69kD) and cytochrome C (13 kD) were used as test solutions.
3. Results and discussion 3.1. M e m b r a n e characteristics
Fig. 4 shows SEM photograph of the outer surface of the cylinder after cathodic electrodeposition. It can be seen that the surface morphology of the coating appears uniform and without visible "pores". The coated films show good adhesion to the support. The XRD pattern of a porous ceramic supported Ag membrane and target Ag foil are compared in Fig. 5. In the membrane sample, reflection peaks of the Ag film are clear. The XRD pattern of the coated Ag film is identical to that of the Ag foil used as the target. The reflection position of a complete solid solution generally depends on the composition. Taking this into consideration, one can deduce that the Ag film is pure silver. The permeabilities of the membrane to pure water are observed to decline as time goes by. The loss in permeate flux is little. Ninety percent of the initial water permeability was obtained even after 30 h of operation. Membrane permea-
--
Fig. 4. SEM photograph of the porous ceramic tube after cathodic electrodepositionof silver. bilities to water at different temperatures follow an Arrhenius behavior, with an activation energy of 0.016kJ/mole. This value is similar, but slightly lower than, those observed for polymeric membranes which are typically about 0.021 kJ/ mole [5]. According to Fig. 3, the flux is directly proportional to the applied pressure (pressurecontrolled region). Thus, increasing the temperature the flux increases. The rejection coefficients obtained with the test solutions were of 97% for gammaglobulin, of about 90% for albumin and 20% for cytochrome C. The results indicate that the membrane has a MWCO of approximately 70kD. Since the effective pore size is estimated from the molecular diameter of globular proteins, the membrane should have an estimated effective pore size of 0.01 # [6]. 3.2. Application in the treatment o f raw water
Batch UF was used to evaluate the appropriateness of Ag/PC composite membrane
207
A. P~rez Padilla et aL / Desalination 114 (1997) 203-208 90 JCPDS N° 4-783 (A9) 6O to 30 I
i
,
,i
,
,
,
•_. 90° XRD pattern coated Ag film a:
60 30
L 20
40
8O
60
Fig. 5. XRD patterns of the Ag foil used as target and ofa Ag membrane coated on a ceramic support.
20 (o) for the treatment of water from Cruz de Piedra, Lake San Luis, a public water supply in the midwest of Argentina. The membrane filtration system used was the same as previously reported in Fig. 1. Very high turbidity removal is achieved with UF. The turbidity of the water fed to the membrane was 20 NTU while permeate turbidity was between 0.1 and 0.2 NTU. Heterotroph plate counts removal was total. Removal of humics as determined by removal of substances absorbing UV light at 254nm was approximately 35%. Usually, most organic carbon in natural waters is present as humic substances that represent a variety of molecules with different functional groups and with different molecular weights [7]. It has been reported that a large fraction of the organic carbon in the waters of the lake has an average molecular weight around I kD, while a somewhat smaller fraction has a molecular weight larger than 100kD [8]. Therefore, a lower MWCO membrane should not be really expected to be more efficient than a 70kD MWCO membrane, since both membranes would pass the lower molecular weight fraction. Besides, both membranes would retain the higher molecular weight fraction. Evidently, pretreatment before UF is necessary to achieve significant organic carbon removals. Fig. 6 illustrates the effect of applied pressure on permeate flux during UF of raw water and pure water at 20oC. The results obtained indicate that the UF performance of the composite
140 120 .c=
100
~
8o
•
raw
•
pure
402o
20
4'0
6'0
8'0
160
I~0
i~0
~o
Pressure (kPa)
Fig. 6. Effect of pressure on permeate flux at 20°C. membrane is similar despite the large difference in the composition of the water samples. The flow of raw water is limited by accumulation of particles on the membrane surface during the course of filtration. 4. Conclusion
Electroless plating and conventional cathodic electrodeposition has been successfully used to deposit thin silver films on porous ceramic support. The methods are found to be faster and not so difficult. The porous ceramic support is prepared by slip-casting. The choice of support material has been given by several factors including ease of preparation, cost of materials and reproducibility. The composite membrane
208
A. P~rez Padilla et al. / Desalination 114 (1997) 203-208
obtained show good stability. The permeation of water through composite membrane at different temperatures follow an Arrhenius behavior, with an activation energy which is similar to that obtained for several polymeric membranes. Measured water permeabilities of the membrane also decrease to a small degree as the membrane is exposed to water for longer times. The production of potable water by cross-flow UF is used to evaluate the efficiency of the composite membrane. The excellent performance of membrane for the complete removal of total heterotroph plate counts from their high feed levels is indeed noteworthy. The results also shows the turbidity reduction in raw water from 20 NTU to values below 0.2NTU in the filtered water. Removal of humics in approximately 35% is observed.
Acknowledgments We wish to express our thanks to Lic. Jorge GonzAlez for their technical assistance with XRD studies.
References [1] R.R.Bhave, Inorganic Membranes Synthesis: Characteristics and Applications, Van Nostrand, New York, 1991. [2] P. Aptel, Applications of Membranes in Drinking Water Treatment. in: Membranes Processes and Applications, ESMST, X Summer School on Membranes, Valladolid, Spain, 1993, p. 47. [3] E. Bertorelle, Galvanot6cnica, Ulrico Hoepli, Milano, 1951. [4] M. Cheryan, Ultrafiltration Handbook, Technomic Publishing, Lancaster, 1986. [5] H.K. Lonsdale, U. Merten and R. L. Riley, J. Appl. Polym. Sci., 9 (1965) 1341. [6] M.C. Porter, Handbook of Industrial Membrane Technology,Noyes Publications, New Jersey, 1990, p.156. [7] J.M. Montgomery, Water Treatment Principles and Design, John Wiley, New York, 1985. [8] M.M. Clark and K. Heneghan, Desalination, 80 (1991) 143.