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Ultrafiltration of Dyes by Polysulfone Membranes
321
ULTRAFILTRATION OF DYES BY POLYSULFONE MEMBRANES K. MAJEWSKA, T. WINNICKI, J. WISNIEWSKI Institute of Environment Protection Engineering Technical University of Wroclaw (Poland)
ABSTRACT A method of water recovery from dye solutions by meat of ultrafiltration, is described. The process involved polysulfone membranes prepared from 15% polysulfone solution in dimethylformamjde, and formed on a glass support. The casting solutions were prepared at different initial temperatures (303, 318, 333 and 348 K) and at various solvent evaporation times (30,60,90 and 120 s). The membrane thickness was between 30 and 135 pm. The ultrafiltration process was carried out in a pressure range of 0.5 to 2.5 MPa. The molecular weight of the dye was 739. Transport properties of the membranes were found to be better with thicker membranes, shorter evaporation times and higher temperatures of the casting solution. Investigating separation properties it has been found that polysulfone membranes are characterized by a high ability (90-100%) to separate dyes from water solutions regardless of casting parameters and of pressure applied. The volume flow through the membranes of the best separation properties does not exceed a value of 0.14 m3/m*d at 2.5 MPa. The decrease in the hydraulic permeability of the membranes during ultrafiltration of dyes is insignificant (less than 5%).
INTRODUCTION
Among the various industrial wastewaters giving rise to a serious treatment problem those containing hardly degr adable dyes are especially troublesome. The main difficulty in solving this problem is that the manufactures of dyes use different technologies and often change them arbitrarily. There are also a number of industrial processes involving a variety of dyes resistant to most of the conventional treatment procedures. It is therefore very difficult to develop such a treatment method that would be quick, economic and efficient at the same time. Conventional methods do not provide a complete removal of coloured matter, so they are generally thought of as being inefficient. Wastewaters containing mainly coloured matter usually exhibit very low osmotic pressures. Thus, it seems reasonable that their treatment involves low-pressure membrane processes. Ultrafiltration has many good points such as the recovery of dyes and water or the possibiIity of reusing them - to name just a few. The application of polysulfone membranes (1) yields satisfactory dye removals. Polysulfone membranes are stable over the whole range of pH. They are resistant to oxidation by chloride (2) and can work even at a.temperature as high as 363 K (1). Hence, they show almost the same properties asdo ideal membranes but, so far, their applications have been limited to ultrafiltration alone, because all attempts t o make polysulfone membranes able to retain salts have failed (2).
322 EXPERIMENTAL Preparation of Polysulfone Membranes
The membranes were prepared from aromatic 3500 polysulfone made by Union Carbide. Dimethylformamide (DMF) was used as a solvent. The casting solution (15% wt. of polysulfone in DMF) was prepared in the way shown by Koenst and Mitchell (3). Membranes were obtained by a uniform coating of a glass support with the casting solution, using a facility for the preparation of flat membranes (4). Three series of membranes were prepared. The membranes of the first series were casted from a polysulfone solution of temperature 318 K and at different evaporation times: the solvent from the membrane surface was evaporated during 30,60,90 and 120 s. The membranes of the second series were characterized by a constant 60 s time of solvent evaporation and at different temperatures of the casting solution (303,318, 333 and 348 K). The membranes of the first two series were 65 p n thick. The membranes of the third series were characterized by the same casting parameters: 60 s evaporation time, 318 K temperature of the casting solution, but at different thickness varying from 30 to 135pm. Testing Apparatus
The membranes under study were tested in the apparatus shown in Fig. 1. An autoclav e made of acid resistant steel (1) constitutes the body of the installation. The membrane to be tested was placed in the bottom part of the liquid tank (2). Because of the
L
-t=i
.-
I
Fig. 1. Testing installation for ultrafiltration membranes (1 - apparatus with the membrane inside; 2 - liquid reservoir; 3 - outlet of product; 4 - measuring cylinder; 5 - gas inlet; 6, 7 - pressure regulator; 8 - gas cylinder).
323 very poor mechanical properties of the membranes, a support made by PVC sinter having an apparent density of 0.8.103 kg/m3 (4) was used in the experiments. Pressures were applied by means of nitrogen supplied from the cylinder (8) through the reducer (7) designed for keeping the pressure at the level required. The apparatus permitted a pressure up to 15 MPa. Investigation of Transport Properties
Transport properties were investigated by forcing distilled water through the membranes. The effective surface area of the membranes tested was 15.9.10-4m2.Membranes were conditioned prior to each experimental cycle at 2.5 MPa until a volume flow through the membrane was established. This usually took place after times between 30 and 50 h, depending on the membrane thickness. Each membrane was acted upon with the following pressures: 0.5,1,1.5,2 and 2.5 MPa. Investigation of Separation Properties
Separation properties of the membranes were studied with reference to the dye which is known in trade as direct meta black (molecular weight = 739). Three concentrations of the dye in the aqueous solution were chosen: 50, 100 and 150 g/m3. The ultrafiltration process was carried out at the following pressures: 0.5, 1, 1.5, 2 and 2.5 MPa (after the measuring conditions had been established). The concentrations of the dye were measured in a Zeiss-Jena spectrophotometer at a wavelength of 570 nm. DISCUSSION OF RESULTS Transport Properties
The relationship between some of the casting parameters and the transport properties of the membranes was determined. The experiments were run with the first two series of membranes. As can be seen from the experimental results, the time of solvent evaporation and the temperature of the casting solution have a great influence on the transport properties of the membranes. The volume flow through the membrane increases when evaporation time is decreased (Fig. 2) and the temperature of the casting solution increases (Fig. 3). The shortening of the evaporation time from 120 s to 30 s and a rise in temperature of the casting solution from 303 to 348 K yield a fivefold increase in permeability for each pressure applied. It follows that the porosity of the membrane increases with increasing temperature of the casting solution and with decreasing time of solvent evaporation. The transport properties were also studied as a function of the kind of pressure applied (increasing or decreasing pressure) and membrane thickness. For this purpose the third aeries of membranes was used. It was found that when increasing pressures were applied the volume flow was higher than at decreasing pressures (Fig. 4). This is and indication
324
0,121
I
I
I
I
I
Fig. 2. Volume flow of water versus applied pressure for membranes of various solvent evaporation times: 30 ( l ) , 60 (2), 90 (3), 120 s (4). Membranes thickness: 65 pm. Temperature of casting solution: 318 K.
pressure MPa
pressure MPa
Fig. 3. Volume flow of water versus applied pressure for membranes of 65 p m thickness prepared from casting solutions of various temperatures: 303 ( l ) , 318 (2), 333 (3), 348 K (4). Solvent evaporation time: 60 s.
Fig. 4. Volume flow of water versus applied pressure for membranes of various thicknesses: 30 (11, 65 (2), 80 (3), 110 (4), 135 p m (5). Solvent evaporation time: 60 s. Temperature 3 of casting solution: 318 K. iI - increasing, I pressure MP6I decreasing pressures.
'
325
U
cv
E
3
4
20
1 1
I 80
I
1
100 120 membrane thickness p m
60
1
140
Fig. 5. Volume flow of water versus membrane thickness at various applied pressures: 0.5 (l),1 (2), 1.5 (3), 2 (4), 2.5 MPa (5). Solvent evaporation time: 60 s. Temperature of casting solution: 318 K.
that the membrane is subject to compacting, which substantiates the well-known phenomenon of the pressure hysteresis loop. It has also been observed that the area of the pressure hysteresis is higher for thicker membranes, so that their characteristics are worse as compared to thinner membranes. The relationdup between membrane thickness and transport properties is as follows when membrane thickness increases, so does the permeability of the membrane (Fig. 5 ) . The differences in volume flow increase with increasing thickness and pressure. The experimental results show that thicker membranes are characterized by a greater porosity. Separation Properties
The experiments were run by means of three series of membranes. Their separation properties were found to be strongly influenced by the time of evaporation and by the temperature of the casting solution. There is a substantial increase in separability with increasing solvent evaporation time and decreasing temperature of the casting solution (Fig. 6 ) . In general, polysulfone membranes prepared at a solvent evaporation time of 120 s, as well as membranes casted from a 303 K solution, yield a separation of the dye which ranges between 99 and 100 percent, irrespective of the pressure and dye concentration applied. It must be considered, however that the volume flows ofthe dye solution through these membranes are very small: 0.02 and 0.045 m3/mzd, respectively, at a pressure of 2.5 MPa. This means that the usability of the membranes is insufficient from the economic point of view despite the high degree of separation. The polysulfone membranes of the third group were prepared at a relatively short time of solvent evaporation (60 s) - which gives a rather high permeability - and at a moderately high temperature of the casting solution (318 K) - which yelds a quite good de-
326
100
98
96 9L 92
s 90
time s
303 318 333 ,348 temperature of casting
solution K
Fig. 6 . Dye el imination coefficient versus solvent evaporation time and temperature of casting solution for various initial concentrations of dye solution: 50 (A), 100 (B), 150 g/m3 (C). Applied pressure: 0.5 ( l ) ,1.5 ( 2 ) , 2.5 MPa (3). Membrane thickness: 65 pm.
gree of separation. The evaporation time could not be decreased to 30 s because adequate casting facilities are lacking. Curves of Fig. 7 show that, for certain pressures, there is a maximum in the separation efficiency. It is also interesting t o observe that, as the thickness of the membranes increases, this maximum tends toward lower pressure values. No evident relationship was found between the separation coefficient and the concentration of the dye for thinner membranes. For thicker membranes the separation coefficient decreased with increasing dye concentration. As shown in Fig. 8 , the separation properties of the membranes are thickness - de-
327
100 97 94
91
88
85
pressure M I 3 Fig. 7. Dye elimination coefficient versus applied pressure for some membranes of v a d s thicknesses: 30 (A), 5 0 (B), 65 (C), 80 (D),1 1 0 (E), 135 p m (F). Initial concentration of dye solution: 5 0 (l), 100 (2), 150 g/m3 (3). Temperature of casting solution: 318 K, Solvent evaporation time: 6 0 s.
pendent, but this dependence is especially pronounced at higher thicknesses and higher pressure values. When membrane thickness increases, as well as the pressure applied, the separation coefficient decreases. The measurement results for the volume flows showed that the volume flow of the dye solution was by some 5 percent smaller than the volume flow of distilled water irrespective of the dye concentration. CONCLUSIONS
1. Polysulfone membranes formed on a glass support can be used in the ultrafiltration
3 28
100
90 80
A
70
50
100
90 80
B
s70 4-
C
.b50
C
Fig. 8. Dye elimination coefficient versus membrane thickness for various initial concentrations of dye solution: 50 (A), 100 (B), 150 g/m3 (C). Applied pressure: 1 (l), 2 (2), 2.5 MPa (3). Temperature of casting solution: 318 K. Solvent evaporation time: 60 s.
process to separate dyes (of a molecular weight equal to, or higher than, 700) from aqueous solutions. Membrane transport properties are influenced by casting parameters and membrane thickness. The permeabllity of the membrane increases with decreasing time of solvent evaporation, increasing temperature of casting solution and increasing membrane thickness. The increase in the pressure applied also increases the volume flow through the membrane. 2 . There is a strong reiationship between casting parameters and separation properties of the membrane. The coefficient of dye separation increases when the time of solvent evaporation increases and the temperature of the casting solution decreases. Membranes prepared from casting solutions of an initial temperature of 318 K, at a solvent evaporation time of 60 s, yield a dye separation between 95 and 100 percent irrespective of the pressures and dye concentrations applied, provided that the membrane thickness does not exceed 100 pm. Polysulforie membranes 90 to 100 pm thick exhibit the best transport properties (volume flow, 0.12 to 0.14 m3/m2dat 2.5 m a ) .
329 REFERENCES 1 H. A. Fremount, Verfahren zur Entfernung von Farbkorpen aus Abwassern von der Papier und Zellstoffherstellung, Ger. Offen. Pat. No. 2 (1977) 711, 072. 2 D. Spatz, and R. H. Friendlander, Rating of the chemical stability U.C. RO/UF membrane m a t e rials, Wa4er and Sewage Works, 2 (1978) 36-40. 3 J. W. Koenst, and E. Mitchell, Method of casting tabular polysulfone ultrafiltration membranes in sand modules, U.S. Pat. No. 4,038,351 (1977). 4 K. Majewska, Evaluating transport properties of membranes for the ultrafiltration process, Reports of the Institute of Environment Protection Engineering, Technical University of Wroclaw, No. D-131, 1979 (in Polish).
ULTRAFILTRATION OF DYES BY POLYSULFONE MEMBRANES K. MAJEWSKA, T. WINNICKI, J. WISNIEWSKI Institute of Environment Protection Engineering Technical University of Wroclaw (Poland)
ABSTRACT A method of water recovery from dye solutions by meat of ultrafiltration, is described. The process involved polysulfone membranes prepared from 15% polysulfone solution in dimethylformamjde, and formed on a glass support. The casting solutions were prepared at different initial temperatures (303, 318, 333 and 348 K) and at various solvent evaporation times (30,60,90 and 120 s). The membrane thickness was between 30 and 135 pm. The ultrafiltration process was carried out in a pressure range of 0.5 to 2.5 MPa. The molecular weight of the dye was 739. Transport properties of the membranes were found to be better with thicker membranes, shorter evaporation times and higher temperatures of the casting solution. Investigating separation properties it has been found that polysulfone membranes are characterized by a high ability (90-100%) to separate dyes from water solutions regardless of casting parameters and of pressure applied. The volume flow through the membranes of the best separation properties does not exceed a value of 0.14 m3/m*d at 2.5 MPa. The decrease in the hydraulic permeability of the membranes during ultrafiltration of dyes is insignificant (less than 5%).
INTRODUCTION
Among the various industrial wastewaters giving rise to a serious treatment problem those containing hardly degr adable dyes are especially troublesome. The main difficulty in solving this problem is that the manufactures of dyes use different technologies and often change them arbitrarily. There are also a number of industrial processes involving a variety of dyes resistant to most of the conventional treatment procedures. It is therefore very difficult to develop such a treatment method that would be quick, economic and efficient at the same time. Conventional methods do not provide a complete removal of coloured matter, so they are generally thought of as being inefficient. Wastewaters containing mainly coloured matter usually exhibit very low osmotic pressures. Thus, it seems reasonable that their treatment involves low-pressure membrane processes. Ultrafiltration has many good points such as the recovery of dyes and water or the possibiIity of reusing them - to name just a few. The application of polysulfone membranes (1) yields satisfactory dye removals. Polysulfone membranes are stable over the whole range of pH. They are resistant to oxidation by chloride (2) and can work even at a.temperature as high as 363 K (1). Hence, they show almost the same properties asdo ideal membranes but, so far, their applications have been limited to ultrafiltration alone, because all attempts t o make polysulfone membranes able to retain salts have failed (2).
322 EXPERIMENTAL Preparation of Polysulfone Membranes
The membranes were prepared from aromatic 3500 polysulfone made by Union Carbide. Dimethylformamide (DMF) was used as a solvent. The casting solution (15% wt. of polysulfone in DMF) was prepared in the way shown by Koenst and Mitchell (3). Membranes were obtained by a uniform coating of a glass support with the casting solution, using a facility for the preparation of flat membranes (4). Three series of membranes were prepared. The membranes of the first series were casted from a polysulfone solution of temperature 318 K and at different evaporation times: the solvent from the membrane surface was evaporated during 30,60,90 and 120 s. The membranes of the second series were characterized by a constant 60 s time of solvent evaporation and at different temperatures of the casting solution (303,318, 333 and 348 K). The membranes of the first two series were 65 p n thick. The membranes of the third series were characterized by the same casting parameters: 60 s evaporation time, 318 K temperature of the casting solution, but at different thickness varying from 30 to 135pm. Testing Apparatus
The membranes under study were tested in the apparatus shown in Fig. 1. An autoclav e made of acid resistant steel (1) constitutes the body of the installation. The membrane to be tested was placed in the bottom part of the liquid tank (2). Because of the
L
-t=i
.-
I
Fig. 1. Testing installation for ultrafiltration membranes (1 - apparatus with the membrane inside; 2 - liquid reservoir; 3 - outlet of product; 4 - measuring cylinder; 5 - gas inlet; 6, 7 - pressure regulator; 8 - gas cylinder).
323 very poor mechanical properties of the membranes, a support made by PVC sinter having an apparent density of 0.8.103 kg/m3 (4) was used in the experiments. Pressures were applied by means of nitrogen supplied from the cylinder (8) through the reducer (7) designed for keeping the pressure at the level required. The apparatus permitted a pressure up to 15 MPa. Investigation of Transport Properties
Transport properties were investigated by forcing distilled water through the membranes. The effective surface area of the membranes tested was 15.9.10-4m2.Membranes were conditioned prior to each experimental cycle at 2.5 MPa until a volume flow through the membrane was established. This usually took place after times between 30 and 50 h, depending on the membrane thickness. Each membrane was acted upon with the following pressures: 0.5,1,1.5,2 and 2.5 MPa. Investigation of Separation Properties
Separation properties of the membranes were studied with reference to the dye which is known in trade as direct meta black (molecular weight = 739). Three concentrations of the dye in the aqueous solution were chosen: 50, 100 and 150 g/m3. The ultrafiltration process was carried out at the following pressures: 0.5, 1, 1.5, 2 and 2.5 MPa (after the measuring conditions had been established). The concentrations of the dye were measured in a Zeiss-Jena spectrophotometer at a wavelength of 570 nm. DISCUSSION OF RESULTS Transport Properties
The relationship between some of the casting parameters and the transport properties of the membranes was determined. The experiments were run with the first two series of membranes. As can be seen from the experimental results, the time of solvent evaporation and the temperature of the casting solution have a great influence on the transport properties of the membranes. The volume flow through the membrane increases when evaporation time is decreased (Fig. 2) and the temperature of the casting solution increases (Fig. 3). The shortening of the evaporation time from 120 s to 30 s and a rise in temperature of the casting solution from 303 to 348 K yield a fivefold increase in permeability for each pressure applied. It follows that the porosity of the membrane increases with increasing temperature of the casting solution and with decreasing time of solvent evaporation. The transport properties were also studied as a function of the kind of pressure applied (increasing or decreasing pressure) and membrane thickness. For this purpose the third aeries of membranes was used. It was found that when increasing pressures were applied the volume flow was higher than at decreasing pressures (Fig. 4). This is and indication
324
0,121
I
I
I
I
I
Fig. 2. Volume flow of water versus applied pressure for membranes of various solvent evaporation times: 30 ( l ) , 60 (2), 90 (3), 120 s (4). Membranes thickness: 65 pm. Temperature of casting solution: 318 K.
pressure MPa
pressure MPa
Fig. 3. Volume flow of water versus applied pressure for membranes of 65 p m thickness prepared from casting solutions of various temperatures: 303 ( l ) , 318 (2), 333 (3), 348 K (4). Solvent evaporation time: 60 s.
Fig. 4. Volume flow of water versus applied pressure for membranes of various thicknesses: 30 (11, 65 (2), 80 (3), 110 (4), 135 p m (5). Solvent evaporation time: 60 s. Temperature 3 of casting solution: 318 K. iI - increasing, I pressure MP6I decreasing pressures.
'
325
U
cv
E
3
4
20
1 1
I 80
I
1
100 120 membrane thickness p m
60
1
140
Fig. 5. Volume flow of water versus membrane thickness at various applied pressures: 0.5 (l),1 (2), 1.5 (3), 2 (4), 2.5 MPa (5). Solvent evaporation time: 60 s. Temperature of casting solution: 318 K.
that the membrane is subject to compacting, which substantiates the well-known phenomenon of the pressure hysteresis loop. It has also been observed that the area of the pressure hysteresis is higher for thicker membranes, so that their characteristics are worse as compared to thinner membranes. The relationdup between membrane thickness and transport properties is as follows when membrane thickness increases, so does the permeability of the membrane (Fig. 5 ) . The differences in volume flow increase with increasing thickness and pressure. The experimental results show that thicker membranes are characterized by a greater porosity. Separation Properties
The experiments were run by means of three series of membranes. Their separation properties were found to be strongly influenced by the time of evaporation and by the temperature of the casting solution. There is a substantial increase in separability with increasing solvent evaporation time and decreasing temperature of the casting solution (Fig. 6 ) . In general, polysulfone membranes prepared at a solvent evaporation time of 120 s, as well as membranes casted from a 303 K solution, yield a separation of the dye which ranges between 99 and 100 percent, irrespective of the pressure and dye concentration applied. It must be considered, however that the volume flows ofthe dye solution through these membranes are very small: 0.02 and 0.045 m3/mzd, respectively, at a pressure of 2.5 MPa. This means that the usability of the membranes is insufficient from the economic point of view despite the high degree of separation. The polysulfone membranes of the third group were prepared at a relatively short time of solvent evaporation (60 s) - which gives a rather high permeability - and at a moderately high temperature of the casting solution (318 K) - which yelds a quite good de-
326
100
98
96 9L 92
s 90
time s
303 318 333 ,348 temperature of casting
solution K
Fig. 6 . Dye el imination coefficient versus solvent evaporation time and temperature of casting solution for various initial concentrations of dye solution: 50 (A), 100 (B), 150 g/m3 (C). Applied pressure: 0.5 ( l ) ,1.5 ( 2 ) , 2.5 MPa (3). Membrane thickness: 65 pm.
gree of separation. The evaporation time could not be decreased to 30 s because adequate casting facilities are lacking. Curves of Fig. 7 show that, for certain pressures, there is a maximum in the separation efficiency. It is also interesting t o observe that, as the thickness of the membranes increases, this maximum tends toward lower pressure values. No evident relationship was found between the separation coefficient and the concentration of the dye for thinner membranes. For thicker membranes the separation coefficient decreased with increasing dye concentration. As shown in Fig. 8 , the separation properties of the membranes are thickness - de-
327
100 97 94
91
88
85
pressure M I 3 Fig. 7. Dye elimination coefficient versus applied pressure for some membranes of v a d s thicknesses: 30 (A), 5 0 (B), 65 (C), 80 (D),1 1 0 (E), 135 p m (F). Initial concentration of dye solution: 5 0 (l), 100 (2), 150 g/m3 (3). Temperature of casting solution: 318 K, Solvent evaporation time: 6 0 s.
pendent, but this dependence is especially pronounced at higher thicknesses and higher pressure values. When membrane thickness increases, as well as the pressure applied, the separation coefficient decreases. The measurement results for the volume flows showed that the volume flow of the dye solution was by some 5 percent smaller than the volume flow of distilled water irrespective of the dye concentration. CONCLUSIONS
1. Polysulfone membranes formed on a glass support can be used in the ultrafiltration
3 28
100
90 80
A
70
50
100
90 80
B
s70 4-
C
.b50
C
Fig. 8. Dye elimination coefficient versus membrane thickness for various initial concentrations of dye solution: 50 (A), 100 (B), 150 g/m3 (C). Applied pressure: 1 (l), 2 (2), 2.5 MPa (3). Temperature of casting solution: 318 K. Solvent evaporation time: 60 s.
process to separate dyes (of a molecular weight equal to, or higher than, 700) from aqueous solutions. Membrane transport properties are influenced by casting parameters and membrane thickness. The permeabllity of the membrane increases with decreasing time of solvent evaporation, increasing temperature of casting solution and increasing membrane thickness. The increase in the pressure applied also increases the volume flow through the membrane. 2 . There is a strong reiationship between casting parameters and separation properties of the membrane. The coefficient of dye separation increases when the time of solvent evaporation increases and the temperature of the casting solution decreases. Membranes prepared from casting solutions of an initial temperature of 318 K, at a solvent evaporation time of 60 s, yield a dye separation between 95 and 100 percent irrespective of the pressures and dye concentrations applied, provided that the membrane thickness does not exceed 100 pm. Polysulforie membranes 90 to 100 pm thick exhibit the best transport properties (volume flow, 0.12 to 0.14 m3/m2dat 2.5 m a ) .
329 REFERENCES 1 H. A. Fremount, Verfahren zur Entfernung von Farbkorpen aus Abwassern von der Papier und Zellstoffherstellung, Ger. Offen. Pat. No. 2 (1977) 711, 072. 2 D. Spatz, and R. H. Friendlander, Rating of the chemical stability U.C. RO/UF membrane m a t e rials, Wa4er and Sewage Works, 2 (1978) 36-40. 3 J. W. Koenst, and E. Mitchell, Method of casting tabular polysulfone ultrafiltration membranes in sand modules, U.S. Pat. No. 4,038,351 (1977). 4 K. Majewska, Evaluating transport properties of membranes for the ultrafiltration process, Reports of the Institute of Environment Protection Engineering, Technical University of Wroclaw, No. D-131, 1979 (in Polish).