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Rejection in Pumping Permeate Microfiltration
0957–5820/01/$10.00+0.00 # Institution of Chemical Engineers Trans IChemE, Vol 79, Part B, November 2001
REJECTION IN PUMPING PERMEATE MICROFILTRATION S. S. MADAENI Chemical Engineering Department, Razi University, Kermanshah, Iran
T
he pumping permeate regime is a recently developed technique for minimization of fouling. However this procedure, which maintains the ux, may affect rejection. In this study a 0.22 mm micro ltration membrane was challenged with 50 nm gold sol in the pumping permeate regime below and beyond the critical ux with pre ltration of 1 mm latex beads. The rejection of colloidal gold was complete in all experiments. Above the critical ux a cake is formed and prevents the particles from passing through the membrane. Below the critical ux, with no cake formation, the particles move away from the membrane due to back-transport which is signi cant in the pumped permeate regime. Without movement towards the membrane, the particles do not reach it to pass through. Some of the particles which enter the membrane pores remain there due to adsorption. Keywords: micro ltration; pumping permeate; critical ux; rejection; gold sol.
INTRODUCTION
used throughout the experiments. Potassium tetrachloroaurarte (KAuCl4) and tri-sodium citrate (Na3C6H5O7 2H2O) were supplied by Aldrich Chemical Company (Wi, USA) and BDH Chemicals (Australia) respectively. The water was puri ed through a Milli-Q Water Puri cation System with a quality of 18 MO cm.
In micro ltration, fouling is a serious problem which reduces ux and affects rejection. Fouling is caused by internal or external deposition of suspended solids. Recently a new procedure, pumping permeate, has been introduced to minimize fouling1–4. In this technique a pump is employed on the permeate side. A constant ux is maintained and transmembrane pressure is variable, increasing if fouling occurs5. Use of a suitable pump6 and optimum conditions7 are key factors in obtaining appropriate results. The pumping permeate regime controls cake formation on the membrane surface8 due to back diffusion of the rejected particles9, providing that ux does not exceed the critical value. The non-formation of a cake layer on the membrane surface is a characteristic of this regime. However this property may affect membrane characteristics such as rejection. Rejection depends on many parameters including the cake layer which is formed on the membrane surface or in the membrane matrix10–12. This study describes the rejection of small particles (compared to the membrane pores) using a micro ltration membrane in the pumping permeate regime. Non-formation of a cake layer and the effect of this phenomenon, together with other characteristics such as back diffusion, are combined to explain the results.
Colloidal Gold Particles Gold sol was prepared by reduction of aqueous gold (III) using citrate ions13,14. A quantity of 475 ml of KAuCl4 (2.7 10 4 M, equivalent to 50 mg l 1 Au) containing hydrochloric acid (2.7 10 4 M) was heated to boiling point and 5.0 ml of 0.5 wt% (1.7 10 2 M) tri-sodium citrate solution was added to the boiling solution with vigorous mechanical stirring. For the experiments the sol (50 mg l 1 Au, pH 3.3) was diluted (10 mg l 1 Au, pH 4.0) with distilled water immediately before the experiments. Particle size of the gold sol was measured using a Hitachi H-7000 Transmission Electron Microscope. The particles had a mean diameter of 50 nm. The absorption spectrum of the sol was measured using a Varian Super Scan 3 UVVisible Spectrophotometer. Gold sol showed a maximum absorption at 530 nm, which is characteristic of gold particles15. Using a laser-based multiangle particle electrophoresis analyser (Delsa 440), the zeta potential of Au sol was measured as 50 mV at pH 4.
EXPERIMENTAL Membranes and Chemicals
Polystyrene Latex Particles
Polyvinylidene uoride (PVDF) hydrophobic Millipore membrane with nominal pore size of 0.22 mm (GVHP) was
Polystyrene latex beads (Cat. No. LB-11) were obtained from Sigma Chemical Company. Reported particle diameter 352
REJECTION IN PUMPING PERMEATE MICROFILTRATION
by the manufacturer is 1.020 mm (std. dev. <0.004 mm). Using the Delsa 440, the zeta potential of latex was measured as 60 mV at pH 4.
Filtration Procedure
353
provides a better understanding of the nature of the process of cake formation. A similar calculation for Au sol suspension with 8 109 particles per millilitre for 10 ppm (10 mg l 1) with a particle diameter of 50 nm results in the ltration of 100 ml suspension producing a monolayer cake on the membrane surface. After the pre ltration of latex suspension, Au sol (50 nm) suspension (10 ppm) was ltered below or beyond the critical ux for 2 or 5 hours. This results in formation (if the ux is beyond the critical ux) or non-formation (if the ux is below the critical value) of a cake layer on the membrane surface. The retention of gold was determined by R(%) 100[1 7 (Cp=Cb)], where Cp and Cb are absorbances of gold sol at 520 for the permeate and bulk respectively.
The equipment set-up for pumped permeate tests is shown in Figure 1. Pressure transducers from Labom Company supplied by Tempress Controls (E1301 for Pin, Pout and E2313 for Ppermeate) were used for precise measurement of transmembrane pressure. Cross ow rate was controlled by the recirculation pump, while ux was xed by the permeate pump. Transmembrane pressure was monitored as the independent variable. All pressure elements, as well as permeate pump and balance, were connected to a computer. Each parameter was monitored automatically. The ‘critical ux’ for both gold and latex colloids were measured by monitoring the changing of DP for different permeate uxes. The deviation from linearity indicates the critical ux. Prior to the cross ow ltration of colloidal gold, 1000 ml of very dilute (10 mg l 1) latex suspension with a cross ow velocity of 0.5 m s 1 was ltered for one hour. The permeate ux was xed above the critical ux of latex. If this process is carried out in a dead-end mode, i.e. in a stirred cell, a cake of latex particles is formed on the membrane surface. This can be achieved by a simple calculation as follows. The number of latex particles in a commercial 10% sample is 1.7 10(11), the latex diameter is 1.0 mm and the projected area of each particle is 0.817 mm2. Membrane surface is 15 cm2 or 15 108 mm2. If it is assumed that the particles reaching the membrane are distributed regularly on the membrane surface, in total 1.835 109 particles are needed to form a monolayer cake. This means that the ltration of 100 ml of a 10 ppm (10 mg l 1) suspension provides a monolayer cake of latex particles on the membrane surface. However this is oversimpli ed and only provides a rough estimation of the minimum volume of suspension needed for cake formation. A picture of the membrane surface taken with an electron microscope
The step by step technique5,7 was used to nd the critical ux. In this technique ux is increased stepwise and remains constant for a short time (around half an hour). Transmembrane pressure is measured independently. The highest ux with constant transmembrane pressure is the critical ux. The critical ux for 10 ppm of 1 mm latex suspension using GVHP membrane with a cross ow velocity of 0.5 m s 1 was 160 l m 2 h 1. For 50 nm Au sol with the same conditions the critical ux was 140 l m 2 h 1. In the rst set of experiments, the latex suspension was ltered for one hour with the ux higher than the critical value (275 l m 2 h 1). After pre ltration of latex suspension, Au sol was ltered for two hours without changing the ux, i.e. the ux was higher than the critical value. Figure 2 shows the variation of ux and transmembrane pressure. The transmembrane pressure is increased indicating that fouling occurs due to the formation of a cake layer. This is clearly seen in the SEM micrograph Figure 3. The
Figure 1. Equipment set-up for pumped permeate tests. 1 Pump, 3 Valve, 4 Pressure element (transducer), 5 6 Permeate, 7 Balance, 8 Computer.
Figure 2. Flux and DP pro les of Au sol using GVHP membrane after ltration of latex (10 mg l 1, 1000 ml, 1 hour, 0.5 m s 1) for uxes higher than Au sol critical ux (2 hours).
Feed tank, 2 Cross ow cell,
Trans IChemE, Vol 79, Part B, November 2001
Scanning Electron Microscopy of Membranes Surfaces of the used membrane specimens were thinly coated with 2–3 nm of chromium (Cr) using a Dynavac Xenosput 2000 and imaged with a Hitachi S-900 Field Emission Scanning Electron Microscope (FESEM) at 2 kV. RESULTS AND DISCUSSION
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Figure 3. SEM micrograph of GVHP membrane surface after ltration of latex (10 mg l 1, 1000ml, 1 hour, 0.5 m s 1) for uxes higher than Au sol critical ux (2 hours).
Figure 5. SEM micrograph of GVHP membrane surface after ltration of latex (10 mg l 1, 1000ml, 1 hour, 0.5 m s 1) for uxes higher than Au sol critical ux (5 hours).
Figure 4. Flux and DP pro les of Au sol using GVHP membrane after ltration of latex (10 mg l 1, 1000 ml, 1 hour, 0.5 m s 1) for uxes higher than Au sol critical ux (5 hours).
Figure 6. Flux and DP pro les of Au sol using GVHP membrane after ltration of latex (10 mg l 1, 1000ml, 1 hour, 0.5 m s 1) for uxes lower than Au sol critical ux (2 hours).
rejection of Au sol was complete in this set of experiments. In the same conditions but with a longer ltration time ( ltration of Au sol for ve hours) (Figure 4) a thicker cake is formed (Figure 5). The rejection was 100%, i.e. all particles were retained by the membrane. In the next set of experiments, after pre ltration of latex suspension with a ux higher than the critical value, i.e. 275 l m 2 h 1, Au sol was ltered for two hours with a ux lower than the critical ux (75 l m 2 h 1). Figure 6 shows that the transmembrane pressure is constant for ltration of Au sol. This means that a cake layer is not formed on the membrane surface during the course of ltration. The SEM micrograph (Figure 7) shows no formation of a cake on the membrane surface. The rejection was complete in this set of experiments.
If Au sol is ltered for a longer time ( ve hours), the transmembrane pressure is again constant (Figure 8) indicating that fouling does not occur during the ltration time. The SEM micrograph Figure 9 shows some particles on the membrane surface, which are not a cake. Rejection was also 100%. The rejection of small (50 nm) gold colloids using micro lter GVHP membrane with a nominal pore size of 0.22 mm is complete in the dead-end mode (stirred cell). The result was not changed with pre ltration of latex suspension. However, if Au sol is mixed with 1 mm latex particles, rejection is reduced (Figure 10). The reasons for this phenomenon have been given elsewhere11,16. The complete rejection of small gold colloids with micro ltration membrane has been attributed to the formation of a cake layer without the presence of latex particles. This selfTrans IChemE, Vol 79, Part B, November 2001
REJECTION IN PUMPING PERMEATE MICROFILTRATION
355
Figure 7. SEM micrograph of GVHP membrane surface after ltration of latex (10 mg l 1, 1000 ml, 1 hour, 0.5 m s 1) for uxes lower than Au sol critical ux (2 hours).
Figure 9. SEM micrograph of GVHP membrane surface after ltration of latex (10 mg l 1, 1000 ml, 1 hour, 0.5 m s 1) for uxes lower than Au sol critical ux (5 hours).
rejecting cake layer may form on the membrane surface or even on the top of the pre ltered latex particles. If Au sol is mixed with large latex particles the self-rejecting cake layer is not formed and the new cake layer of mixed particles is not able to retain the colloidal gold particles. Electron microscopy con rms this conclusion. For details refer to the original papers11,16. The rejection of small particles in pumping permeate micro ltration is now discussed. The results obtained in the rst two sets of experiments, i.e. ltration of gold sol with a ux higher than the critical value which results in formation of a cake layer, were anticipated. In this process, similar to the dead-end mode, a self-rejecting cake layer is formed on the membrane surface and acts as a secondary barrier to retain the particles attempting to pass through the membrane. However the complete rejection of Au sol in the last two sets of experiments, i.e. ltration of gold sol with a
ux lower than the critical value, requires more attention. In this process, cake is not formed so the complete rejection cannot be explained by the self-rejecting cake layer or secondary barrier. To explain the complete rejection requires more detailed understanding of the pumping process regime. As explained elsewhere17, the particles move towards the membrane due to diffusion and convection, settle on the membrane surface and form a cake. However, back-transport of particles away from the membrane may also occur. In the pumping permeate regime the back-transport is signi cant. The critical ux occurs when convective transport balances back-transport. The main mechanisms for back-transport are lateral migration and diffusivity. Potentially stable uxes are where convection is less than back-transport. This means that the particles are moved away from the membrane instead of going towards it. If the particles do not move
Figure 8. Flux and DP pro les of Au sol using GVHP membrane after ltration of latex (10 mg l 1, 1000 ml, 1 hour, 0.5 m s 1) for uxes lower than Au sol critical ux (5 hours).
Figure 10. Rejection of Au sol in different conditions.
Trans IChemE, Vol 79, Part B, November 2001
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towards the membrane, they do not reach it and cannot pass through it. This means complete rejection. However a small proportion of the particles reach the membrane and pass through it. Another important mechanism in micro ltration is adsorption of particles on the membrane matrix or in the membrane pores. This means that the particles which enter the membrane may stay in the membrane and do not appear in the permeate. The dilute nature of the gold sol suspension in this study leads to the complete removal of gold sol particles due to either back-transport or adsorption in the pores.
5. 6.
7. 8.
CONCLUSIONS
9.
The rejection of small colloidal gold particles with micro ltration membrane in the pumping permeate regime was complete. The experiments carried out both below and above the critical ux showed the same results, i.e. complete rejection. Above the critical ux a cake layer of Au sol particles is formed on the membrane surface and rejects the other particles. This secondary barrier acts similarly if it is formed on the membrane surface or on the top of pre ltered latex beads. While working below the critical ux, a cake is not formed and therefore complete rejection may not be attributed to the self-rejecting cake layer. Below the critical ux a cake is not formed because the particles move away from the membrane due to back-transport. When most of the particles do not move towards the membrane, they may not reach the membrane to pass through. However the small proportion of the particles that go through the membrane adsorb on the membrane matrix and do not appear in the permeate.
10.
REFERENCES 1. Madaeni, S. S., 1995, Membrane Filtration of Biological and Nonbiological Colloids, PhD Thesis (The University of New South Wales, Sydney). 2. Howell, J. A., 1995, Sub-critical ux operation of micro ltration, Journal of Membrane Science, 107: 165–171. 3. Field, R. W., Wu, D., Howell, J. A. and Gupta, B. B., 1995, Critical ux concept in micro ltration fouling, Journal of Membrane Science, 100: 259–272. 4. Chen, V., Fane, A. G., Madaeni, S. S. and Wenten, I. G., 1997, Particle deposition during membrane ltration of colloids: transition between
11. 12.
13. 14. 15.
16. 17.
concentration polarization and cake formation, Journal of Membrane Science, 125: 109–122. Madaeni, S. S., 1997, The effect of operating conditions on critical ux in membrane ltration of latexes, Trans IChemE, Part B, Process Safety and Environmental Protection, 75(B6): 266–269. Madaeni, S. S., Shu, Q. L., Wiley, D. E. and Fane, A. G., 1994, The effect of transmembrane pressure regimes on ux for the cross ow micro ltration of activated sludge, Proceedings of Chemical Engineering Conference (CHEMECA ’94), Perth, Australia, 309–316. Madaeni, S. S. and Fane, A. G., 1999, Factors in uencing critical ux in membrane ltration, Journal of Chemical Technology and Biotechnology, 74: 539–543. Madaeni, S. S., 1997, An investigation of the mechanism of critical ux in membrane ltration using electron microscopy, Journal of Porous Materials, 4: 239–244. Madaeni, S. S., 1997, Mechanism of critical ux in membrane ltration of silica, Iranian Polymer Journal, 6(3): 177–183. Madaeni, S. S., Fane, A. G. and Grohmann, G. S., 1995, Virus removal from water and wastewater using membranes, Journal of Membrane Science, 102: 65–75. Madaeni, S. S. and Fane, A. G., 1996, Micro ltration of very dilute colloidal mixtures, Journal of Membrane Science, 113: 301–312. Hodgson, P. H., Leslie, G. L., Schneider, R. P., Fane, A. G., Fell, C. J. D. and Marshall, K. C., 1993, Cake resistance and solute rejection in bacterial micro ltration: the role of the extracellular matrix, Journal of Membrane Science, 79: 35–53. Turkevich, J., Stevenson, P. C. and Hillier, J., 1951, A study of the nucleation and growth processes in the synthesis of colloidal gold, Discussions of The Faraday Society, 11: 55–75. Frens, G., 1973, Controlled nucleation for the regulation of the particle size in monodisperse gold suspensions, Nature Physical Science, 241: 20–23. Heard, S. M., Grieser, F. and Barraclough, C. G., 1983, The characterization of Ag sols by electron microscopy, optical absorbance, and electrophoresis, Journal of Colloid and Interface Science, 93(2): 545– 555. Kim, K. J., Madaeni, S. S., Chen, V., Fane, A. G. and Brown, P. L., 1994, The micro ltration of very dilute colloidal suspensions, Journal of Colloid and Interface Science, 166: 462–471. Madaeni, S. S., 1997, Modelling of critical ux in membrane ltration, ACTA Polytechnica Scandinavica, 247: 73–81.
ADDRESS Correspondence concerning this paper should be addressed to Dr S. S. Madaeni, Chemical Engineering Department, Razi University, Daneshgah Road, Tagh Bostan 67149, Kermanshah, Iran. E-mail: [email protected] The manuscript was received 19 April 2001 and accepted for publication after revision 22 October 2001.
Trans IChemE, Vol 79, Part B, November 2001
REJECTION IN PUMPING PERMEATE MICROFILTRATION S. S. MADAENI Chemical Engineering Department, Razi University, Kermanshah, Iran
T
he pumping permeate regime is a recently developed technique for minimization of fouling. However this procedure, which maintains the ux, may affect rejection. In this study a 0.22 mm micro ltration membrane was challenged with 50 nm gold sol in the pumping permeate regime below and beyond the critical ux with pre ltration of 1 mm latex beads. The rejection of colloidal gold was complete in all experiments. Above the critical ux a cake is formed and prevents the particles from passing through the membrane. Below the critical ux, with no cake formation, the particles move away from the membrane due to back-transport which is signi cant in the pumped permeate regime. Without movement towards the membrane, the particles do not reach it to pass through. Some of the particles which enter the membrane pores remain there due to adsorption. Keywords: micro ltration; pumping permeate; critical ux; rejection; gold sol.
INTRODUCTION
used throughout the experiments. Potassium tetrachloroaurarte (KAuCl4) and tri-sodium citrate (Na3C6H5O7 2H2O) were supplied by Aldrich Chemical Company (Wi, USA) and BDH Chemicals (Australia) respectively. The water was puri ed through a Milli-Q Water Puri cation System with a quality of 18 MO cm.
In micro ltration, fouling is a serious problem which reduces ux and affects rejection. Fouling is caused by internal or external deposition of suspended solids. Recently a new procedure, pumping permeate, has been introduced to minimize fouling1–4. In this technique a pump is employed on the permeate side. A constant ux is maintained and transmembrane pressure is variable, increasing if fouling occurs5. Use of a suitable pump6 and optimum conditions7 are key factors in obtaining appropriate results. The pumping permeate regime controls cake formation on the membrane surface8 due to back diffusion of the rejected particles9, providing that ux does not exceed the critical value. The non-formation of a cake layer on the membrane surface is a characteristic of this regime. However this property may affect membrane characteristics such as rejection. Rejection depends on many parameters including the cake layer which is formed on the membrane surface or in the membrane matrix10–12. This study describes the rejection of small particles (compared to the membrane pores) using a micro ltration membrane in the pumping permeate regime. Non-formation of a cake layer and the effect of this phenomenon, together with other characteristics such as back diffusion, are combined to explain the results.
Colloidal Gold Particles Gold sol was prepared by reduction of aqueous gold (III) using citrate ions13,14. A quantity of 475 ml of KAuCl4 (2.7 10 4 M, equivalent to 50 mg l 1 Au) containing hydrochloric acid (2.7 10 4 M) was heated to boiling point and 5.0 ml of 0.5 wt% (1.7 10 2 M) tri-sodium citrate solution was added to the boiling solution with vigorous mechanical stirring. For the experiments the sol (50 mg l 1 Au, pH 3.3) was diluted (10 mg l 1 Au, pH 4.0) with distilled water immediately before the experiments. Particle size of the gold sol was measured using a Hitachi H-7000 Transmission Electron Microscope. The particles had a mean diameter of 50 nm. The absorption spectrum of the sol was measured using a Varian Super Scan 3 UVVisible Spectrophotometer. Gold sol showed a maximum absorption at 530 nm, which is characteristic of gold particles15. Using a laser-based multiangle particle electrophoresis analyser (Delsa 440), the zeta potential of Au sol was measured as 50 mV at pH 4.
EXPERIMENTAL Membranes and Chemicals
Polystyrene Latex Particles
Polyvinylidene uoride (PVDF) hydrophobic Millipore membrane with nominal pore size of 0.22 mm (GVHP) was
Polystyrene latex beads (Cat. No. LB-11) were obtained from Sigma Chemical Company. Reported particle diameter 352
REJECTION IN PUMPING PERMEATE MICROFILTRATION
by the manufacturer is 1.020 mm (std. dev. <0.004 mm). Using the Delsa 440, the zeta potential of latex was measured as 60 mV at pH 4.
Filtration Procedure
353
provides a better understanding of the nature of the process of cake formation. A similar calculation for Au sol suspension with 8 109 particles per millilitre for 10 ppm (10 mg l 1) with a particle diameter of 50 nm results in the ltration of 100 ml suspension producing a monolayer cake on the membrane surface. After the pre ltration of latex suspension, Au sol (50 nm) suspension (10 ppm) was ltered below or beyond the critical ux for 2 or 5 hours. This results in formation (if the ux is beyond the critical ux) or non-formation (if the ux is below the critical value) of a cake layer on the membrane surface. The retention of gold was determined by R(%) 100[1 7 (Cp=Cb)], where Cp and Cb are absorbances of gold sol at 520 for the permeate and bulk respectively.
The equipment set-up for pumped permeate tests is shown in Figure 1. Pressure transducers from Labom Company supplied by Tempress Controls (E1301 for Pin, Pout and E2313 for Ppermeate) were used for precise measurement of transmembrane pressure. Cross ow rate was controlled by the recirculation pump, while ux was xed by the permeate pump. Transmembrane pressure was monitored as the independent variable. All pressure elements, as well as permeate pump and balance, were connected to a computer. Each parameter was monitored automatically. The ‘critical ux’ for both gold and latex colloids were measured by monitoring the changing of DP for different permeate uxes. The deviation from linearity indicates the critical ux. Prior to the cross ow ltration of colloidal gold, 1000 ml of very dilute (10 mg l 1) latex suspension with a cross ow velocity of 0.5 m s 1 was ltered for one hour. The permeate ux was xed above the critical ux of latex. If this process is carried out in a dead-end mode, i.e. in a stirred cell, a cake of latex particles is formed on the membrane surface. This can be achieved by a simple calculation as follows. The number of latex particles in a commercial 10% sample is 1.7 10(11), the latex diameter is 1.0 mm and the projected area of each particle is 0.817 mm2. Membrane surface is 15 cm2 or 15 108 mm2. If it is assumed that the particles reaching the membrane are distributed regularly on the membrane surface, in total 1.835 109 particles are needed to form a monolayer cake. This means that the ltration of 100 ml of a 10 ppm (10 mg l 1) suspension provides a monolayer cake of latex particles on the membrane surface. However this is oversimpli ed and only provides a rough estimation of the minimum volume of suspension needed for cake formation. A picture of the membrane surface taken with an electron microscope
The step by step technique5,7 was used to nd the critical ux. In this technique ux is increased stepwise and remains constant for a short time (around half an hour). Transmembrane pressure is measured independently. The highest ux with constant transmembrane pressure is the critical ux. The critical ux for 10 ppm of 1 mm latex suspension using GVHP membrane with a cross ow velocity of 0.5 m s 1 was 160 l m 2 h 1. For 50 nm Au sol with the same conditions the critical ux was 140 l m 2 h 1. In the rst set of experiments, the latex suspension was ltered for one hour with the ux higher than the critical value (275 l m 2 h 1). After pre ltration of latex suspension, Au sol was ltered for two hours without changing the ux, i.e. the ux was higher than the critical value. Figure 2 shows the variation of ux and transmembrane pressure. The transmembrane pressure is increased indicating that fouling occurs due to the formation of a cake layer. This is clearly seen in the SEM micrograph Figure 3. The
Figure 1. Equipment set-up for pumped permeate tests. 1 Pump, 3 Valve, 4 Pressure element (transducer), 5 6 Permeate, 7 Balance, 8 Computer.
Figure 2. Flux and DP pro les of Au sol using GVHP membrane after ltration of latex (10 mg l 1, 1000 ml, 1 hour, 0.5 m s 1) for uxes higher than Au sol critical ux (2 hours).
Feed tank, 2 Cross ow cell,
Trans IChemE, Vol 79, Part B, November 2001
Scanning Electron Microscopy of Membranes Surfaces of the used membrane specimens were thinly coated with 2–3 nm of chromium (Cr) using a Dynavac Xenosput 2000 and imaged with a Hitachi S-900 Field Emission Scanning Electron Microscope (FESEM) at 2 kV. RESULTS AND DISCUSSION
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Figure 3. SEM micrograph of GVHP membrane surface after ltration of latex (10 mg l 1, 1000ml, 1 hour, 0.5 m s 1) for uxes higher than Au sol critical ux (2 hours).
Figure 5. SEM micrograph of GVHP membrane surface after ltration of latex (10 mg l 1, 1000ml, 1 hour, 0.5 m s 1) for uxes higher than Au sol critical ux (5 hours).
Figure 4. Flux and DP pro les of Au sol using GVHP membrane after ltration of latex (10 mg l 1, 1000 ml, 1 hour, 0.5 m s 1) for uxes higher than Au sol critical ux (5 hours).
Figure 6. Flux and DP pro les of Au sol using GVHP membrane after ltration of latex (10 mg l 1, 1000ml, 1 hour, 0.5 m s 1) for uxes lower than Au sol critical ux (2 hours).
rejection of Au sol was complete in this set of experiments. In the same conditions but with a longer ltration time ( ltration of Au sol for ve hours) (Figure 4) a thicker cake is formed (Figure 5). The rejection was 100%, i.e. all particles were retained by the membrane. In the next set of experiments, after pre ltration of latex suspension with a ux higher than the critical value, i.e. 275 l m 2 h 1, Au sol was ltered for two hours with a ux lower than the critical ux (75 l m 2 h 1). Figure 6 shows that the transmembrane pressure is constant for ltration of Au sol. This means that a cake layer is not formed on the membrane surface during the course of ltration. The SEM micrograph (Figure 7) shows no formation of a cake on the membrane surface. The rejection was complete in this set of experiments.
If Au sol is ltered for a longer time ( ve hours), the transmembrane pressure is again constant (Figure 8) indicating that fouling does not occur during the ltration time. The SEM micrograph Figure 9 shows some particles on the membrane surface, which are not a cake. Rejection was also 100%. The rejection of small (50 nm) gold colloids using micro lter GVHP membrane with a nominal pore size of 0.22 mm is complete in the dead-end mode (stirred cell). The result was not changed with pre ltration of latex suspension. However, if Au sol is mixed with 1 mm latex particles, rejection is reduced (Figure 10). The reasons for this phenomenon have been given elsewhere11,16. The complete rejection of small gold colloids with micro ltration membrane has been attributed to the formation of a cake layer without the presence of latex particles. This selfTrans IChemE, Vol 79, Part B, November 2001
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Figure 7. SEM micrograph of GVHP membrane surface after ltration of latex (10 mg l 1, 1000 ml, 1 hour, 0.5 m s 1) for uxes lower than Au sol critical ux (2 hours).
Figure 9. SEM micrograph of GVHP membrane surface after ltration of latex (10 mg l 1, 1000 ml, 1 hour, 0.5 m s 1) for uxes lower than Au sol critical ux (5 hours).
rejecting cake layer may form on the membrane surface or even on the top of the pre ltered latex particles. If Au sol is mixed with large latex particles the self-rejecting cake layer is not formed and the new cake layer of mixed particles is not able to retain the colloidal gold particles. Electron microscopy con rms this conclusion. For details refer to the original papers11,16. The rejection of small particles in pumping permeate micro ltration is now discussed. The results obtained in the rst two sets of experiments, i.e. ltration of gold sol with a ux higher than the critical value which results in formation of a cake layer, were anticipated. In this process, similar to the dead-end mode, a self-rejecting cake layer is formed on the membrane surface and acts as a secondary barrier to retain the particles attempting to pass through the membrane. However the complete rejection of Au sol in the last two sets of experiments, i.e. ltration of gold sol with a
ux lower than the critical value, requires more attention. In this process, cake is not formed so the complete rejection cannot be explained by the self-rejecting cake layer or secondary barrier. To explain the complete rejection requires more detailed understanding of the pumping process regime. As explained elsewhere17, the particles move towards the membrane due to diffusion and convection, settle on the membrane surface and form a cake. However, back-transport of particles away from the membrane may also occur. In the pumping permeate regime the back-transport is signi cant. The critical ux occurs when convective transport balances back-transport. The main mechanisms for back-transport are lateral migration and diffusivity. Potentially stable uxes are where convection is less than back-transport. This means that the particles are moved away from the membrane instead of going towards it. If the particles do not move
Figure 8. Flux and DP pro les of Au sol using GVHP membrane after ltration of latex (10 mg l 1, 1000 ml, 1 hour, 0.5 m s 1) for uxes lower than Au sol critical ux (5 hours).
Figure 10. Rejection of Au sol in different conditions.
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towards the membrane, they do not reach it and cannot pass through it. This means complete rejection. However a small proportion of the particles reach the membrane and pass through it. Another important mechanism in micro ltration is adsorption of particles on the membrane matrix or in the membrane pores. This means that the particles which enter the membrane may stay in the membrane and do not appear in the permeate. The dilute nature of the gold sol suspension in this study leads to the complete removal of gold sol particles due to either back-transport or adsorption in the pores.
5. 6.
7. 8.
CONCLUSIONS
9.
The rejection of small colloidal gold particles with micro ltration membrane in the pumping permeate regime was complete. The experiments carried out both below and above the critical ux showed the same results, i.e. complete rejection. Above the critical ux a cake layer of Au sol particles is formed on the membrane surface and rejects the other particles. This secondary barrier acts similarly if it is formed on the membrane surface or on the top of pre ltered latex beads. While working below the critical ux, a cake is not formed and therefore complete rejection may not be attributed to the self-rejecting cake layer. Below the critical ux a cake is not formed because the particles move away from the membrane due to back-transport. When most of the particles do not move towards the membrane, they may not reach the membrane to pass through. However the small proportion of the particles that go through the membrane adsorb on the membrane matrix and do not appear in the permeate.
10.
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ADDRESS Correspondence concerning this paper should be addressed to Dr S. S. Madaeni, Chemical Engineering Department, Razi University, Daneshgah Road, Tagh Bostan 67149, Kermanshah, Iran. E-mail: [email protected] The manuscript was received 19 April 2001 and accepted for publication after revision 22 October 2001.
Trans IChemE, Vol 79, Part B, November 2001