- Email: [email protected]
Extraction of organic and inorganic compounds from aqueous solutions using hollow fibre liquidliquid contactor
Desalination 241 (2009) 337 341
Extraction of organic and inorganic compounds from aqueous solutions using hollow fibre liquid liquid contactor Serge Alexa*, Fabienne Biasottoa, German Arocab a
Centre of Chemical Process Studies of Quebec (CEPROCQ), 6220 Sherbrooke East, Montreal, Que., Canada, H1N 1C1 email: [email protected] b Escuela de Ingeniería Bioquímica, Universidad Catolica de Valparaíso, Valparaíso, Chile Received 27 August 2007; revised 22 October 2007; accepted 29 October 2007
Abstract Reactive solvent extraction is commonly used in industries to extract organic and inorganic products. In such a process, the solute present in an aqueous solution is extracted into an immiscible solvent containing a specific extracting agent. These experiments are usually carried out in conventional dispersion-based contacting devices such as colunm or mixer-settlers. Under this configuration, two problems arise: the extracting agent (usually a toxic or environmentally harmful compound) is dispersed into the medium and later in the environment and/or permanent emulsions are formed. To overcome these drawbacks, the introduction of a solid membrane between the two fluid phases using a hollow fibre liquid liquid contactor in order to keep a large surface of contact between the two media represents a good alternative. Two selected examples will be shortly presented in this article to illustrate the interest of this technology that uses the same membrane for the extraction of organic and inorganic compounds from aqueous solutions. Keywords: Extraction; Hollow fibres; Membranes; Contaminants; Heavy metals
1. Introduction Reactive solvent extraction has been widely used to efficiently remove toxic wastes from aqueous effluents. This technology works well and may be used to reduce either organic or *Corresponding author.
inorganic wastes. Usually, either a complexation reaction or an ionic exchange takes place between the aqueous and the organic phase. Whatever the extraction mechanism involved, many problems arise when this unit operation is carried out in column contactors or in mixer-settlers. The common problems encountered are water phase contamination and emulsion formation.
Presented at the Third Membrane Science and Technology Conference of Visegrad Countries (PERMEA), Siofok, Hungary, 2–6 September 2007. 0011-9164/09/$– See front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2007.10.103
338
S. Alex et al. / Desalination 241 (2009) 337 341
To overcome these disadvantages, the introduction of a solid membrane between the two fluids represents an alternative [15] unless complex biphasic reactors are used [6]. Contactor offers a higher efficiency than dispersive devices and they can treat fluids that do not present a significant density difference. Their scale-up is usually straightforward and this device totally prevents emulsion formations. Its principle is simple: first, the organic phase circulates inside the fibre and comes out to the outer side of the membrane wall. However, it could not be displaced further into the water phase (the outer phase) because a mild positive pressure is applied on the aqueous side. Typically, a 5 Psig (33.9 kPa) gradient fits most of the applications. The extraction occurs without emulsion formation as the organic phase is not dispersed but immobilised at the surface. Although not new, this technical approach discovered more than 20 years ago (i.e. see Ref. [1]) has never been fully exploited because commercial devices still do not exist. This is due to the fact that it was difficult to maintain a constant pressure gradient along the module, a key factor. In addition, the membrane choice is limited because most polymers exhibit poor chemical resistance towards solvents. By using a hollow fibre cartridge initially designed for liquid degassing, it is possible to overcome these two limitations. This will be illustrated by working out two examples. 2. Materials and methods 2.1. Materials All the liquidliquid extraction performed has been carried out on the unit built around the Liquid-Cel Extra-flow sold by Membrana (a division of Celgard Inc., Charlotte, NC, USA). It has been selected because this membrane device dedicated to solution degassing can be easily converted into a liquidliquid contactor. Larger modules are available from the same
Pump Pump
Liquid–liquid contactor
Organic phase Regeneration phase
Aqueous phase
Fig. 1. Schematics of the liquid liquid hollow fibre contactor.
company which are able to handle flow rates up to thousands of litres per hour. The schematics of the pilot process is pictured in Fig. 1. The membrane is made of polypropylene with 0.05 mm pore diameter and a porosity of 40% (surface area 1.4 m2). The two loops are alimented by two individual gear pumps [pump drives model 75211-10 (505000 rpm, 0.07 HP) equipped with precision pump heads model 74013-30 (0.91 mL/revolution) (Cole-Parmer, Montreal, Que., Canada)]. Protected manometers as well as stainless valves are intercalated. The pressure selection is done by restricting the flow, typically pressures are set at 10 Psig (67.9 kPa) and 5 Psig (33.9 kPa) for, respectively, the outer (shell side) and inner (lumen side) flows. All the connections and tubing are in stainless steel, but polypropylene is a better alternative for extractions performed in strongly acidic media. Membrane fouling is limited by using a washing procedure adapted from Scheler et al. [7] in order to improve its lifetime. 2.2. Chemicals All the chemicals used were obtained from SigmaAldrich (Montreal, Que., Canada) unless otherwise stated. Lactic Acid (LA) solutions are derived from the proper dilution of a 85% w/w stock solution (CAS number 50-21-5,
S. Alex et al. / Desalination 241 (2009) 337 341
CH3-CHOH-COOH, Mw 90 g/mol, pKa 3.76 (258C)). The pH of all these solutions is 2.1 and their concentrations are determined by Total Organic Carbon (TOC) measurements. The Cobalt (II) solution is made directly from solid dissolution of reagent grade CoCl2 ×6H2O salt (1% w/w into water). Cyanex 923 was kindly provided by Cytec Canada Inc. (Niagara Falls, Ont., Canada). Cyanex 923 (Cyanex hereafter) is a proprietary blend of Organophosphorous compounds made of Trioctylphosphine Oxide, Dioctylmonohexylphosphine Oxide, Dihexylmonooctylphosphine Oxide and Trihexylphosphine Oxide. Diethylhexylphosphoric Acid (DEHPA hereafter, formula C16H35PO4, Mw 322 g/mol, d 0.975 g/cm3, pKa 1.40 (258C), purity 95%) has been used without further purification. Cobalt concentrations have been measured using an UVVis. spectrophotometer (PharmaSpec1700, Shimadzu, Mississauga, Ont., Canada) set at 508 nm. /
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339
solutions have been put into contact with 1 L of organic phase (20% w/w Cyanex solution in Heptane). The organic phase should circulate inside the fibre otherwise smaller extraction yields are obtained. This is well documented [24]. During this operation, the 10 L of LA is recirculated at the speed of 1 L/min during 80 min. The organic phase is back extracted batch-wise (no regeneration) or simultaneously (with regeneration). In this latter case, 1 L of distilled water is added into the organic phase tank and stirred with a magnetic bar. From the plots displayed in Fig. 2, it is seen that for low LA loadings (1% w/w), there is only a slight difference between the LA quantities extracted between the two modes (without and with regeneration). The reasons are: at this percentage (1% w/w), the local concentration of the solute near the pore mouths is reduced and the regeneration of the organic phase using pure 60000
3. Results and discussion
3.1. Section I: LA extraction The extractions have been performed on 10 L of LA solution taken at various concentrations (1% w/w, 5% w/w and 10% w/w). These
Lactic Acid in mg/L
This section will introduce two examples of extractions performed with the hollow fibre contactor. The first one (Section I) will present the extraction of LA. This operation is of interest because its direct chemical synthesis is impossible. For this reason, it is mostly obtained directly from liquidliquid extraction from fermentation broth [8,9]. However, during this operation the extracting agent is often toxic to the bacteria, thus the use of a contactor may reduce this risk. The second part (Section II) will show data related to the liquidliquid extraction of Cobalt (II) from aqueous solutions that is difficult to perform as an emulsion is quickly formed.
50000 40000 30000 20000 10000 0 0
20
1 % w/w 5 % w/w 10 % w/w
40 60 Time in minutes
80
1 % w/w with regeneration 5 % w/w with regeneration 10 % w/w with regeneration
Fig. 2. Concentration of extracted LA with Cyanex 923 (20% w/w in Heptane) at various starting percentages with and without regeneration as a function of time (Experimental conditions : Flow rates 1 L/min (both sides), DP /5 Psig (Pinside /5 Psig, Poutside /10 Psig), Volumes of the respective circulating aqueous and organic phases 10 L:1 L).
S. Alex et al. / Desalination 241 (2009) 337 341
water is not efficient. However, water stripping bears the tremendous advantage to lead directly to pure LA solutions without undesired ionic species. Forty minutes are needed in order to complete the direct extraction for low LA content with no regeneration. For larger initial LA contents, the organic phase is quickly loaded. With solutions bearing a larger percentage of LA (10% w/w and 5% w/w), this happens earlier (roughly between 20 and 30 min) and reaches an asymptotic value after 7080 min. There is no more evolution; the extraction may go on only if the used distilled water is replaced by a new solution. This contactor is well designed for LA regulation because the organic phase never enters into contact with the fermentation medium. Also, this allows continuous extraction because the chemical and biochemical broth conditions are preserved. 3.2. Section II: metal salt extraction In this particular application, an ionic exchange takes place during the extraction [1012]. This is pictured by the equation: 2ðDEHPAÞHeptane þ Co2þ ðwaterÞ , ð2DEHPA CoÞHeptane þ 2Hþ ðwaterÞ As a consequence, the pH of the solution decreases when the extraction takes place. It has been already proved that Cobalt (II) is quantitatively and quickly extracted at pH 5.5 [12]. But at this pH, DEHPA is badly retained into the organic phase and tends to leak into the water phase. The membrane addition eliminates this drawback and avoids the loss of an expensive product. In addition, no permanent emulsions are formed while compared to the direct dispersion of the organic phase. The results obtained are reported in Fig. 3. This plot represents the extraction of Cobalt (II) ions starting with a 1% w/w Cobalt salt solution. Cobalt is extracted at 99% within
1.2 Absorbance at 508 nm × (x5) and Extraction yield
340
1 0.8 0.6 0.4 0.2 0 0
10
20 30 40 Time in minutes
50
60
Extraction yield Absorbance at 508 nm × (x5)
Fig. 3. Extraction of Cobalt (II) ions from a 1% w/w Cobalt Chloride solution with a 0.5 M DEHPA solution in Heptane (pH is controlled at 5.5 using an automatic neutralisation loop) (Experimental conditions : Flow rates 1 L/min (both sides), DP /5 Psig (Pinside /5 Psig, Poutside /10 Psig), Volumes of the respective aqueous and organic phases 1 L:1 L).
60 min. In this application, the kinetics of the extraction process is limited by the automatic pH adjustment device which unfortunately bears some inertia. The recovery of Cobalt is easily done using either a 2 M or a 6 M Chlorhydric Acid solutions. The back extraction yield is close to 99%. 4. Conclusions These two applications demonstrate that liquidliquid contactors are useful when one wants to limit close contact between the two phases. This technology should gain more popularity as most liquidliquid extractions are not industrially performed because the conventional liquidliquid extractors are limited by emulsion formations or other problems. This simple membrane process technology broadens the use of liquidliquid extraction. Theses devices are easy to built, to operate and are inexpensive. They rely on polypropylene
S. Alex et al. / Desalination 241 (2009) 337 341
membranes which are particularly resistant to chemicals. In addition, with the proper organic phase, it is possible to extract organic and inorganic compounds with the same module. It is surprising that this technique introduced in the late nineties remains so poorly used. Acknowledgements This work has been supported by the Ministere du Developpement Économique, de l’Innovation et de l’Exportation (MDEIE), Volet Initiative Internationale and by the PART and PSCCC programmes from Ministere de l’Éducation du Loisir et du Sport (MELS). The authors greatly acknowledge these two agencies for their major support. We also thank Cytec which kindly and freely provides us with Cyanex 923. References [1] D.O. Cooney and M.G. Poufos, Chem. Eng. Commun., 61 (1987) 159167.
341
[2] R. Prasad and K.K. Sirkar, J. Membr. Sci., 50 (1990) 153 175. [3] A. Gabelman and S.T. Hwang, J. Membr. Sci., 159 (1999) 61 106. [4] I.M. Coelhoso, M.M. Cardoso, R.M.C. Viegas and J.P.S.G. Crespo, Sep. Purific. Technol., 19 (2000) 183 197. [5] P.J. Harrington and G.W. Stevens, J. Membr. Sci., 192 (2001) 83 98. [6] S. Alex, Vect. Environ., 33 (2000) 47 51. [7] C. Scheler, M. Popovic, E. Iannotti, R. Mueller and R. Bajpai, Appl. Biochem. Biotechnol., 57/58 (1996) 29 38. [8] Y. Tong, M. Hirata, H. Takanashi, T. Hano, F. Kubota, M. Goto, F. Nakashio and M. Matsumoto, J. Membr. Sci., 143 (1998) 81 91. [9] Y. Tong, M. Hirata, H. Takanashi and T. Hano, J. Membr. Sci., 157 (1999) 189198. [10] S.H. Lin and R.S. Juang, J. Membr. Sci., 188 (2001) 251 262. [11] I. Van de Voorde, L. Pinoy and R.F. De Ketelaere, J. Membr. Sci., 234 (2004) 11 21. [12] S. Alex and A. Berthod, Can. J. Spectr. Anal. Sci., 44 (1999) 131 137.
Extraction of organic and inorganic compounds from aqueous solutions using hollow fibre liquid liquid contactor Serge Alexa*, Fabienne Biasottoa, German Arocab a
Centre of Chemical Process Studies of Quebec (CEPROCQ), 6220 Sherbrooke East, Montreal, Que., Canada, H1N 1C1 email: [email protected] b Escuela de Ingeniería Bioquímica, Universidad Catolica de Valparaíso, Valparaíso, Chile Received 27 August 2007; revised 22 October 2007; accepted 29 October 2007
Abstract Reactive solvent extraction is commonly used in industries to extract organic and inorganic products. In such a process, the solute present in an aqueous solution is extracted into an immiscible solvent containing a specific extracting agent. These experiments are usually carried out in conventional dispersion-based contacting devices such as colunm or mixer-settlers. Under this configuration, two problems arise: the extracting agent (usually a toxic or environmentally harmful compound) is dispersed into the medium and later in the environment and/or permanent emulsions are formed. To overcome these drawbacks, the introduction of a solid membrane between the two fluid phases using a hollow fibre liquid liquid contactor in order to keep a large surface of contact between the two media represents a good alternative. Two selected examples will be shortly presented in this article to illustrate the interest of this technology that uses the same membrane for the extraction of organic and inorganic compounds from aqueous solutions. Keywords: Extraction; Hollow fibres; Membranes; Contaminants; Heavy metals
1. Introduction Reactive solvent extraction has been widely used to efficiently remove toxic wastes from aqueous effluents. This technology works well and may be used to reduce either organic or *Corresponding author.
inorganic wastes. Usually, either a complexation reaction or an ionic exchange takes place between the aqueous and the organic phase. Whatever the extraction mechanism involved, many problems arise when this unit operation is carried out in column contactors or in mixer-settlers. The common problems encountered are water phase contamination and emulsion formation.
Presented at the Third Membrane Science and Technology Conference of Visegrad Countries (PERMEA), Siofok, Hungary, 2–6 September 2007. 0011-9164/09/$– See front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2007.10.103
338
S. Alex et al. / Desalination 241 (2009) 337 341
To overcome these disadvantages, the introduction of a solid membrane between the two fluids represents an alternative [15] unless complex biphasic reactors are used [6]. Contactor offers a higher efficiency than dispersive devices and they can treat fluids that do not present a significant density difference. Their scale-up is usually straightforward and this device totally prevents emulsion formations. Its principle is simple: first, the organic phase circulates inside the fibre and comes out to the outer side of the membrane wall. However, it could not be displaced further into the water phase (the outer phase) because a mild positive pressure is applied on the aqueous side. Typically, a 5 Psig (33.9 kPa) gradient fits most of the applications. The extraction occurs without emulsion formation as the organic phase is not dispersed but immobilised at the surface. Although not new, this technical approach discovered more than 20 years ago (i.e. see Ref. [1]) has never been fully exploited because commercial devices still do not exist. This is due to the fact that it was difficult to maintain a constant pressure gradient along the module, a key factor. In addition, the membrane choice is limited because most polymers exhibit poor chemical resistance towards solvents. By using a hollow fibre cartridge initially designed for liquid degassing, it is possible to overcome these two limitations. This will be illustrated by working out two examples. 2. Materials and methods 2.1. Materials All the liquidliquid extraction performed has been carried out on the unit built around the Liquid-Cel Extra-flow sold by Membrana (a division of Celgard Inc., Charlotte, NC, USA). It has been selected because this membrane device dedicated to solution degassing can be easily converted into a liquidliquid contactor. Larger modules are available from the same
Pump Pump
Liquid–liquid contactor
Organic phase Regeneration phase
Aqueous phase
Fig. 1. Schematics of the liquid liquid hollow fibre contactor.
company which are able to handle flow rates up to thousands of litres per hour. The schematics of the pilot process is pictured in Fig. 1. The membrane is made of polypropylene with 0.05 mm pore diameter and a porosity of 40% (surface area 1.4 m2). The two loops are alimented by two individual gear pumps [pump drives model 75211-10 (505000 rpm, 0.07 HP) equipped with precision pump heads model 74013-30 (0.91 mL/revolution) (Cole-Parmer, Montreal, Que., Canada)]. Protected manometers as well as stainless valves are intercalated. The pressure selection is done by restricting the flow, typically pressures are set at 10 Psig (67.9 kPa) and 5 Psig (33.9 kPa) for, respectively, the outer (shell side) and inner (lumen side) flows. All the connections and tubing are in stainless steel, but polypropylene is a better alternative for extractions performed in strongly acidic media. Membrane fouling is limited by using a washing procedure adapted from Scheler et al. [7] in order to improve its lifetime. 2.2. Chemicals All the chemicals used were obtained from SigmaAldrich (Montreal, Que., Canada) unless otherwise stated. Lactic Acid (LA) solutions are derived from the proper dilution of a 85% w/w stock solution (CAS number 50-21-5,
S. Alex et al. / Desalination 241 (2009) 337 341
CH3-CHOH-COOH, Mw 90 g/mol, pKa 3.76 (258C)). The pH of all these solutions is 2.1 and their concentrations are determined by Total Organic Carbon (TOC) measurements. The Cobalt (II) solution is made directly from solid dissolution of reagent grade CoCl2 ×6H2O salt (1% w/w into water). Cyanex 923 was kindly provided by Cytec Canada Inc. (Niagara Falls, Ont., Canada). Cyanex 923 (Cyanex hereafter) is a proprietary blend of Organophosphorous compounds made of Trioctylphosphine Oxide, Dioctylmonohexylphosphine Oxide, Dihexylmonooctylphosphine Oxide and Trihexylphosphine Oxide. Diethylhexylphosphoric Acid (DEHPA hereafter, formula C16H35PO4, Mw 322 g/mol, d 0.975 g/cm3, pKa 1.40 (258C), purity 95%) has been used without further purification. Cobalt concentrations have been measured using an UVVis. spectrophotometer (PharmaSpec1700, Shimadzu, Mississauga, Ont., Canada) set at 508 nm. /
/
/
/
/
/
339
solutions have been put into contact with 1 L of organic phase (20% w/w Cyanex solution in Heptane). The organic phase should circulate inside the fibre otherwise smaller extraction yields are obtained. This is well documented [24]. During this operation, the 10 L of LA is recirculated at the speed of 1 L/min during 80 min. The organic phase is back extracted batch-wise (no regeneration) or simultaneously (with regeneration). In this latter case, 1 L of distilled water is added into the organic phase tank and stirred with a magnetic bar. From the plots displayed in Fig. 2, it is seen that for low LA loadings (1% w/w), there is only a slight difference between the LA quantities extracted between the two modes (without and with regeneration). The reasons are: at this percentage (1% w/w), the local concentration of the solute near the pore mouths is reduced and the regeneration of the organic phase using pure 60000
3. Results and discussion
3.1. Section I: LA extraction The extractions have been performed on 10 L of LA solution taken at various concentrations (1% w/w, 5% w/w and 10% w/w). These
Lactic Acid in mg/L
This section will introduce two examples of extractions performed with the hollow fibre contactor. The first one (Section I) will present the extraction of LA. This operation is of interest because its direct chemical synthesis is impossible. For this reason, it is mostly obtained directly from liquidliquid extraction from fermentation broth [8,9]. However, during this operation the extracting agent is often toxic to the bacteria, thus the use of a contactor may reduce this risk. The second part (Section II) will show data related to the liquidliquid extraction of Cobalt (II) from aqueous solutions that is difficult to perform as an emulsion is quickly formed.
50000 40000 30000 20000 10000 0 0
20
1 % w/w 5 % w/w 10 % w/w
40 60 Time in minutes
80
1 % w/w with regeneration 5 % w/w with regeneration 10 % w/w with regeneration
Fig. 2. Concentration of extracted LA with Cyanex 923 (20% w/w in Heptane) at various starting percentages with and without regeneration as a function of time (Experimental conditions : Flow rates 1 L/min (both sides), DP /5 Psig (Pinside /5 Psig, Poutside /10 Psig), Volumes of the respective circulating aqueous and organic phases 10 L:1 L).
S. Alex et al. / Desalination 241 (2009) 337 341
water is not efficient. However, water stripping bears the tremendous advantage to lead directly to pure LA solutions without undesired ionic species. Forty minutes are needed in order to complete the direct extraction for low LA content with no regeneration. For larger initial LA contents, the organic phase is quickly loaded. With solutions bearing a larger percentage of LA (10% w/w and 5% w/w), this happens earlier (roughly between 20 and 30 min) and reaches an asymptotic value after 7080 min. There is no more evolution; the extraction may go on only if the used distilled water is replaced by a new solution. This contactor is well designed for LA regulation because the organic phase never enters into contact with the fermentation medium. Also, this allows continuous extraction because the chemical and biochemical broth conditions are preserved. 3.2. Section II: metal salt extraction In this particular application, an ionic exchange takes place during the extraction [1012]. This is pictured by the equation: 2ðDEHPAÞHeptane þ Co2þ ðwaterÞ , ð2DEHPA CoÞHeptane þ 2Hþ ðwaterÞ As a consequence, the pH of the solution decreases when the extraction takes place. It has been already proved that Cobalt (II) is quantitatively and quickly extracted at pH 5.5 [12]. But at this pH, DEHPA is badly retained into the organic phase and tends to leak into the water phase. The membrane addition eliminates this drawback and avoids the loss of an expensive product. In addition, no permanent emulsions are formed while compared to the direct dispersion of the organic phase. The results obtained are reported in Fig. 3. This plot represents the extraction of Cobalt (II) ions starting with a 1% w/w Cobalt salt solution. Cobalt is extracted at 99% within
1.2 Absorbance at 508 nm × (x5) and Extraction yield
340
1 0.8 0.6 0.4 0.2 0 0
10
20 30 40 Time in minutes
50
60
Extraction yield Absorbance at 508 nm × (x5)
Fig. 3. Extraction of Cobalt (II) ions from a 1% w/w Cobalt Chloride solution with a 0.5 M DEHPA solution in Heptane (pH is controlled at 5.5 using an automatic neutralisation loop) (Experimental conditions : Flow rates 1 L/min (both sides), DP /5 Psig (Pinside /5 Psig, Poutside /10 Psig), Volumes of the respective aqueous and organic phases 1 L:1 L).
60 min. In this application, the kinetics of the extraction process is limited by the automatic pH adjustment device which unfortunately bears some inertia. The recovery of Cobalt is easily done using either a 2 M or a 6 M Chlorhydric Acid solutions. The back extraction yield is close to 99%. 4. Conclusions These two applications demonstrate that liquidliquid contactors are useful when one wants to limit close contact between the two phases. This technology should gain more popularity as most liquidliquid extractions are not industrially performed because the conventional liquidliquid extractors are limited by emulsion formations or other problems. This simple membrane process technology broadens the use of liquidliquid extraction. Theses devices are easy to built, to operate and are inexpensive. They rely on polypropylene
S. Alex et al. / Desalination 241 (2009) 337 341
membranes which are particularly resistant to chemicals. In addition, with the proper organic phase, it is possible to extract organic and inorganic compounds with the same module. It is surprising that this technique introduced in the late nineties remains so poorly used. Acknowledgements This work has been supported by the Ministere du Developpement Économique, de l’Innovation et de l’Exportation (MDEIE), Volet Initiative Internationale and by the PART and PSCCC programmes from Ministere de l’Éducation du Loisir et du Sport (MELS). The authors greatly acknowledge these two agencies for their major support. We also thank Cytec which kindly and freely provides us with Cyanex 923. References [1] D.O. Cooney and M.G. Poufos, Chem. Eng. Commun., 61 (1987) 159167.
341
[2] R. Prasad and K.K. Sirkar, J. Membr. Sci., 50 (1990) 153 175. [3] A. Gabelman and S.T. Hwang, J. Membr. Sci., 159 (1999) 61 106. [4] I.M. Coelhoso, M.M. Cardoso, R.M.C. Viegas and J.P.S.G. Crespo, Sep. Purific. Technol., 19 (2000) 183 197. [5] P.J. Harrington and G.W. Stevens, J. Membr. Sci., 192 (2001) 83 98. [6] S. Alex, Vect. Environ., 33 (2000) 47 51. [7] C. Scheler, M. Popovic, E. Iannotti, R. Mueller and R. Bajpai, Appl. Biochem. Biotechnol., 57/58 (1996) 29 38. [8] Y. Tong, M. Hirata, H. Takanashi, T. Hano, F. Kubota, M. Goto, F. Nakashio and M. Matsumoto, J. Membr. Sci., 143 (1998) 81 91. [9] Y. Tong, M. Hirata, H. Takanashi and T. Hano, J. Membr. Sci., 157 (1999) 189198. [10] S.H. Lin and R.S. Juang, J. Membr. Sci., 188 (2001) 251 262. [11] I. Van de Voorde, L. Pinoy and R.F. De Ketelaere, J. Membr. Sci., 234 (2004) 11 21. [12] S. Alex and A. Berthod, Can. J. Spectr. Anal. Sci., 44 (1999) 131 137.