Stirred cell ultrafiltration of aqueous micellar TX-100 solutions

Separation and Purification Technology 74 (2010) 21–27

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Stirred cell ultrafiltration of aqueous micellar TX-100 solutions M. Schwarze a,∗ , D.K. Le a , S. Wille b , A. Drews c , W. Arlt b , R. Schomäcker a a

Department of Chemistry, Berlin University of Technology, Straße des 17. Juni 135, 10623 Berlin, Germany Friedrich-Alexander-University Erlangen-Nuremberg, Chair of Separation Science and Technology, Egerlandstr. 3, 91058 Erlangen, Germany c HTW Berlin, School of Life Science Engineering, Wilhelminenhofstr. 75A, 12459 Berlin, Germany b

a r t i c l e

i n f o

Article history: Received 17 February 2010 Received in revised form 30 April 2010 Accepted 5 May 2010 Keywords: Micellar enhanced ultrafiltration (MEUF) Triton X-100 Gel concentration

a b s t r a c t Cellulose membranes with different molecular weight cut-offs (MWCO, 5–100 kDa) were used for the filtration of aqueous micellar TX-100 solutions. The experiments were performed in a stirred cell and surfactant concentration and pressure were varied. The gel-polarization model was employed and showed a gel concentration for TX-100 in the range of 150–300 g/L. Ultrafiltration was also carried out with dimethyl itaconate (DMI) and diethyl itaconate (DEI) as dissolved organic compounds and we found that the solute retention was directly associated to the hydrophobicity of the compounds. The flux was not affected as long as the solution appears homogeneous and the formation of a second phase with higher organic content was avoided. In order to use micellar enhanced ultrafiltration (MEUF) for the recovery of hydrophobic catalysts dissolved in aqueous micellar reaction media, a low MWCO membrane, low surfactant concentrations and hydrophilic reactants should be preferred. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Micellar enhanced ultrafiltration (MEUF) has nowadays become a well known technique in the purification of aqueous streams. Micelles formed in water at surfactant concentrations above the critical micelle concentration (cmc) are able to solubilize organic [1–4] or inorganic [5–8] material and can be separated from the aqueous solution by an ultrafiltration process using porous membranes. Related to the “Green Chemistry” discussion, it was suggested to use environmentally and user friendly solvents as reaction media that should be, e.g. non-toxic and non-flammable. It was found that surfactant based reaction media are a good alternative to organic solvents and different types of reactions like oxidations, reductions and coupling reactions can be transferred from conventional solvents to this new medium [9]. In addition to the green aspect, one big problem that usually occurs in homogeneously catalyzed reactions, the separation of product and catalyst, can be solved for micellar solutions by MEUF. In micellar systems, a hydrophobic catalyst is embedded into the surfactant micelles which can be rejected by an appropriate membrane [10,11]. In comparison to a conventional separation method, e.g. distillation, the usage of MEUF is less energy intensive and enables the recovery of the catalyst in its active state. For the design of a continuous process in which the catalyst is recycled and simultaneously separated from the product using MEUF, quantitative information about the filtration properties of aqueous micellar reaction solutions is

needed, e.g. retention values for the reactants and permeability of the membrane. Some papers already discussed the change of flux due to a specific interaction of the membrane with the surfactants [12–15]. Surfactants can deposit either on the membrane surface to build up a secondary layer or on the pore walls, reducing the mean pore diameter. We searched for an aqueous micellar reaction medium for the hydrogenation of itaconic acid and some of its esters and found that aqueous solutions of SDS and TX-100 are appropriate. In order to apply these systems to a continuous reaction and separation process (Fig. 1), the filtration step has to be analyzed. It is described in literature that membranes made of polyethersulfone are useful to reach high SDS retentions [10,16]. But polyethersulfone membranes have a higher tendency for fouling [16] than membranes made from regenerated cellulose. In this contribution the filtration of aqueous micellar TX-100 solutions with hydrophilic cellulose membranes at relatively high TX-100 concentrations is studied. The influence of pressure and surfactant concentration on flux and micelle retention is analyzed. Additionally, filtration experiments are performed with dissolved organic compounds. We chose dimethyl itaconate and diethyl itaconate as model substances to study the distribution behavior in such systems, because they are often used as substrates in homogeneously catalyzed hydrogenation reactions. 2. Experimental 2.1. Chemicals

∗ Corresponding author. E-mail address: [email protected] (M. Schwarze). 1383-5866/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2010.05.003

(p-tert-Octylphenoxy)Polyethoxyethanol (TX-100, cmc = 0.24 × 10−3 mol/L) was obtained from Sigma–Aldrich. Dimethyl itaconate

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M. Schwarze et al. / Separation and Purification Technology 74 (2010) 21–27

Fig. 1. Scheme for a continuous reaction and separation process using MEUF.

Fig. 2. Scheme of the ultrafiltration system: (1) pressure sensor, (2) stirred ultrafiltration cell, (3) magnetic stirrer, (4) balance, (5) permeate collection flask, (6) computer and (7) membrane.

(DMI, >97% purity) was received from Fluka. Diethyl itaconate (DEI, >98% purity) was obtained from TCI. 2-Propanol was obtained from Roth (≥99.5% purity). All chemicals were used without further purification. In all experiments, deionised water was used. 2.2. Membranes Hydrophilic membranes with different molecular weight cut-offs (MWCO, 5–100 kDa) made from regenerated cellulose (Microdyn-Nadir, Germany) were used for the experiments. Their pH-range is 1–11 and the maximum allowed temperature is 55 ◦ C. The pure water flux can be calculated from RM presented in Table 2. Before their use in the ultrafiltration experiments, the membranes were soaked in a 2-propanol/water (v/v = 1:1) solution for about 20 h to remove membrane production residuals. Afterwards, the membranes were soaked twice in deionised water for 30 min to remove the residual alcohol.

first, the surfactant solution was stirred for 45 min without pressure and then the desired trans-membrane-pressure (TMP) was set with nitrogen and monitored by a pressure sensor. In the experiments, the TMP was set to 0.1 MPa, 0.2 MPa and 0.3 MPa, respectively. Permeate was collected in a vessel which was placed on a balance. The balance was connected to a computer for automatic data recording. The ultrafiltration experiments were performed at room temperature (20 ± 2 ◦ C) with a constant stirrer speed of 600 min−1 . The concentrations of TX-100 in permeate and retentate were determined only at the end of the filtration experiment, i.e. cP is a mean value and cR a momentary one. The filtered volume was not always the same, i.e. both final retentate concentrations and deposition layers were different in different runs. However, earlier experiments [10] had shown that the micelle retention remains approximately the same for surfactant concentrations higher than 10 times the cmc, which was always the case in this work. Hence, the difference in filtered volume did not affect the evaluation of results. The mean micelle retention value RMicelle was calculated from Eq. (1). RMicelle =



1−

cP − cmc cR



× 100%

(1)

In filtration experiments with additional solutes, the volume of the feed solution was 200 mL or 400 mL and the TMP was set to 0.1 MPa. In order to study the solute distribution, permeate samples were collected at different times and analyzed. In all filtration experiments between successive runs the membrane was cleaned with water and the membrane permeability was checked to ensure that the clean water permeability remains essentially constant. 2.4. UV/vis-spectroscopy The concentration of TX-100 in the feed, permeate and retentate was determined using a UVIKON 943 spectrophotometer (Kontron Instruments, Germany). The samples were analyzed at 275 nm. An ultraviolet spectrum of an aqueous TX-100 solution is given by Smith et al. [17]. The calibration and the structural formula of TX100 are shown in Fig. 3.

2.3. Ultrafiltration procedure 2.5. High performance liquid chromatography (HPLC) A schematic diagram of the ultrafiltration setup is shown in Fig. 2. The ultrafiltration cell (GN 400, Berghoff, Germany), which was used in the experiments, has a maximum volume uptake of 400 mL, and the effective membrane area was 3.63 × 10−3 m2 . In filtration experiments without additional solutes, the pre-treated membrane was inserted into the cell and afterwards the cell was filled up with 400 mL of surfactant solution. The surfactant concentration was 20 g/L, 50 g/L and 80 g/L, respectively. At

The concentration of solute was determined by HPLC using a Dionex instrument (capillary column: Multospher 120 RP 18-5␮, ca. 250 mm × 4 mm; HPLC pump: P580A, 1.0 mL/min acetonitrile (ACN):water 70:30 (v/v); column thermostat: STH585, 25 ◦ C; UV/vis detector: 170S,  = 220 nm). Under these conditions the following retention times tR (Table 1) were obtained.

M. Schwarze et al. / Separation and Purification Technology 74 (2010) 21–27

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Table 1 HPLC retention times and structural formulas for investigated solutes. Solute

DMI

DEI

3.1

3.7

Structural formula

tR (min)

3.2. Stirred cell ultrafiltration of aqueous micellar TX-100 solutions 3.2.1. Determination of the initial flux From the mass–time curves, the initial slope (m/t)0 , for which the permeate volume was less than 5–10% of the feed volume, was used to calculate the initial flux J0 based on Eq. (3). J0 =

1 AM w

 m  t

(3) 0

In Eq. (3), AM is the membrane area, and w is the density of water. This range was chosen in order to neglect the change in the retentate concentration that normally influences the flux in a deadend ultrafiltration process. In cross-flow ultrafiltration, it was found that steady state fluxes are achieved within seconds [18], so the initial fluxes determined here should also be valid for steady state processes.

Fig. 3. Calibration curve for aqueous TX-100 solutions ( = 275 nm, dcuvette = 1 cm).

3. Results and discussion 3.1. Membrane resistance For all membranes, stirred-cell filtration of water was performed at 0.1 MPa, 0.2 MPa and 0.3 MPa and a linear relationship between water flux and pressure was obtained from which the membrane resistance RM (Table 2) was calculated based on Eq. (2).

RM

3.2.2. Influence of surfactant concentration and pressure Byhlin and Jönsson [19] already carried out fundamental research on the filtration of aqueous micellar TX-100 solutions with the same type of membranes in cross-flow mode but only in the low concentration range (cTX-100 ≤ 10 times cmc). When surfactants are applied for hydrogenation reactions with less polar substrates, much higher surfactant concentrations are needed in order to yield a macroscopic single phase system during the reaction and the separation, respectively. In our filtration experiments the surfactant concentration was varied in the range of 20–80 g/L (130–520 times cmc) and the TMP was varied between 0.1 MPa and 0.3 MPa. As a measure of ultrafiltration performance, the flux reduction FR (Table 3) was calculated from Eq. (4). FR =

P = w Jw

(2)

In Eq. (2), P is the trans-membrane-pressure, w is the viscosity of water, and Jw is the flux of water. It is shown that the membrane resistance decreases with increasing MWCO.



1−

J0 Jw



× 100%

(4)

In Eq. (4), J0 is the initial flux and Jw the pure water flux. For the C005, C010 and C030 membrane, FR increases with increasing surfactant concentration and increasing pressure mainly by an additional resistance due to concentration polarization. For the C100 membrane, FR only increases with increasing surfactant concentration.

Table 2 Membrane resistances and cut-off pore sizes for all investigated membranes. Membrane

C005

C010

C030

C100

RM (1012 m−1 ) d1 0 0 (nm)

22.7 ± 0.5 4.4

13.0 ± 0.4 5.1

1.34 ± 0.04 13.5

1.27 ± 0.06 28.8

d1 0 0 : cut-off pore size (determined according to [10]).

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M. Schwarze et al. / Separation and Purification Technology 74 (2010) 21–27

Table 3 FR for different MWCO, TMP and surfactant concentration. P (MPa)

cTX-100 (g/L)

FRC005 (%)

FRC010 (%)

FRC030 (%)

FRC100 (%)

0.1

20 50 80

10.8 25.0 39.5

15.8 47.5. 59.2

89.1 93.1 95.2

86.6 92.4 94.4

0.2

20 50 80

14.1 45.3 62.9

49.2 72.1 77.5

94.8 96.9 97.6

86.3 92.6 95.5

0.3

20 50 80

30.3 64.5 73.3

62.0 78.4 84.5

96.2 97.3 98.4

85.0 92.6 95.3

Fig. 4. Initial flux for different TMP and surfactant concentrations.

For the C100 membrane that has the biggest pore-size it is also possible that a part of the micelles is no longer rejected by the membrane surface but enters the membrane pores resulting in a partial pore blocking. In this case, the retention of the micelles should decrease. In Table 4 the mean micelle retention is given which for the membranes C005, C010 and C030 is higher than 95% for all tested combinations. Only for the C100 membrane, the mean micelle retention is significantly lower and the values are between 28% and 70%. If we compare the retention values obtained from stirred deadend ultrafiltration with the results obtained by Byhlin and Jönsson, from cross-flow ultrafiltration (cTX-100 = 10 times cmc), a small deviation is found. In our case, the retention values are up to 8% higher. A comparison of flux and retention is always a problem if the experimental conditions are not exactly the same. In this work, we used high surfactant concentrations which in the stirred dead-end cell can lead to a thicker surfactant film at the membrane surface so

high micelle retention would be favored. For a continuous reaction and separation process using MEUF for recovery of homogeneously dissolved catalysts it is clear that a high MWCO will lead to a high permeate flux and thereby more product will be separated but on the other hand, the catalyst loss will also be higher because of the lower micelle retention [10]. Therefore, the application of a membrane that enables high micelle retention is essential to avoid high catalyst losses and costs, respectively. From the tested membranes, C005 and C010 are the best choices with almost full micelle retention. For the process not only full catalyst retention is desired but also the influence of important parameters like surfactant concentration, TMP, temperature, etc., have to be known. In Fig. 4, for both, the C005 and the C010 membrane, the initial flux as a function of TMP for different surfactant concentrations is given. It is shown that only for the lowest surfactant concentration of 20 g/L the initial flux changes with pressure. In all other cases the flux is more or less pressure independent in the investigated range which indicates that the filtration is controlled by the surface layer at the membrane. In a process as shown in Fig. 1, reaction and separation can be performed in (a) sequential mode or (b) continuous mode. If method (a) is chosen, first the reaction proceeds and after achieving the desired conversion the reaction mixture is pumped into the ultrafiltration unit where retentate is recycled to the reactor and permeate is discharged. The filtration behavior is comparable with dead-end filtration in that the micelle concentration changes over time, and in this case the surfactant concentration is of big importance, if unnecessarily long dead-times in which no product is formed should be avoided. In Fig. 5 the initial flux and t1/2 (the time to filter half of the feed volume) are shown for different surfactant concentrations. With increasing surfactant concentration, the initial flux decreases and time to separate half of the feed volume increases. Because of the high surfactant retention, the surfactant concentration in the retentate strongly increases with filtration time, and permeate flux approaches zero. In method (b), reaction, dosing of new reactants and filtration happen at the same time. Therefore, the process is working in a full continuous mode and under steady state conditions surfactant concentration is constant (assuming quantitative micelle rejection).

Table 4 Mean micelle retention for different MWCO, TMP and surfactant concentration. P (MPa)

cTX-100 (g/L)

RMicelle,C005 (%)

RMicelle,C010 (%)

RMicelle,C030 (%)

RMicelle,C100 (%)

0.1

20 50 80

100.0 99.8 99.9

99.9 99.9 99.8

99.1 99.0 98.5

75.0 70.4 70.5

0.2

20 50 80

99.9 99.9 99.9

99.8 99.7 99.7

97.0 97.9 97.3

36.5 37.4 54.6

0.3

20 50 80

99.9 99.9 99.9

99.7 99.6 99.5

99.1 96.0 95.4

28.4 34.4 28.0

M. Schwarze et al. / Separation and Purification Technology 74 (2010) 21–27

Fig. 5. Initial flux and t1/2 for the ultrafiltration of aqueous TX-100 solutions (C010 membrane, P = 0.3 MPa, n = 600 min−1 ). (*) Calculated from the linearity of the first points and (**) time to filter a quarter of the feed solution.

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Fig. 6. Permeate concentration of DMI as a function of VCF for different initial DMI concentrations and initial surfactant concentration of 9 g/L.

3.2.3. Gel-polarization model In stirred-cell ultrafiltration experiments, very high TX-100 concentrations were used and the formation of a gel-layer at the membrane surface that acts as a second barrier in series with the membrane is most likely. The gel-polarization model (Eq. (5)) is used to obtain the gel-concentration and the calculated values are given in Table 5. In Eq. (5), J is the flux, k is the mass-transfercoefficient, cM is the concentration at the membrane surface, cG is the gel concentration, cP is the permeate concentration, and cF is the feed concentration. J = k · ln

c − c  cP →0 M P cF − cP

⇒ k · ln

c  G

cP

(5)

The mass-transfer-coefficient and gel concentration was calculated only for those conditions where micelle rejection is very high and TX-100 concentration in permeate can be neglected (cP  cF ): the C005, the C010 and the C030 membrane respectively. The changes in k and cG with pressure indicate that cG is only a pseudo gel concentration, because cG should be independent from operating conditions. But the changes from 0.2 MPa to 0.3 MPa are relative small, so that the determined cG value for 0.3 MPa seems to be valid and they are obvious higher than 52 g/L reported by Grieves et al. [20] using a polyelectrolyte membrane for the cross-flow ultrafiltration of aqueous TX-100 solutions. From Fig. 5 it is obvious that for the used experimental conditions a gel-concentration in this order of magnitude is adequate. The ultrafiltration of an aqueous TX-100 solution with a surfactant concentration of 140 g/L at 0.3 MPa using the C010 membrane leads to a flux reduction of about 93%. 3.3. Stirred cell ultrafiltration of aqueous micellar TX-100 solutions with dissolved organic compounds If aqueous micellar solutions are used as reaction media, the nature (hydrophilic/hydrophobic) of the solute is important and therefore we used two itaconates whose hydrophobic character increases in the following order: DMI < DEI as model solutes, in order to study solute retention and flux development. In Fig. 6, the permeate concentration is shown as a function of the volume concentration factor (VCF) for different initial solute concentrations. VCF is defined by Eq. (6). VCF =

VF VR

(6)

Fig. 7. Concentration of DMI and DEI in permeate as a function of VCF. The initial surfactant concentration is 9 g/L for DMI and 10 g/L for DEI.

In Eq. (6), VF is the feed volume and VR is the retentate volume that changes with time. From Fig. 6 it can be seen that the permeate concentration is approximately constant for VCF > 2 and does not change with time although the surfactant concentration strongly increases with VCF because of the high micelle retention. If we change the solute from DMI to the more hydrophobic DEI, the same variation is observed but the concentration of DEI in permeate is somewhat lower at similar conditions as shown in Fig. 7. Because of the more hydrophobic character of DEI, more solute is included into the micelle and in comparison to DMI, the solute retention is increased, which is shown in Fig. 8. The concentration of solute in the retentate was calculated from the mass balance (Eq. (7)). cR =

VF cF − VP cP VF − VP

(7)

For both solutes, the retention is permanently increasing with VCF for VCF > 2. From the results it is clear that if catalyst recovery and product isolation is carried out in a sequential mode, the flux decreases and in addition more solute will be retained with filtration time. From our point of view the best way to combine reac-

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Table 5 Gel-concentration cg obtained from the linear relationship J = k·ln cG + k·ln cF (r: regression coefficient, ln cG = ln cF for J = 0). Membrane

P (MPa)

k·ln cG (10−6 m3 m−2 s−1 )

k (10−6 m3 m−2 s−1 )

r

ln cG

cG (g/L)

C005

0.1 0.2 0.3

7.296 16.424 21.550

0.972 3.012 4.190

0.982 0.999 0.989

7.50 5.45 5.14

1808 233 171

C010

0.1 0.2 0.3

14.540 18.245 19.450

2.580 3.400 3.680

0.997 0.987 0.997

5.64 5.37 5.28

281 215 196

C030

0.1 0.2 0.3

18.010 17.770 18.440

3.280 3.220 3.330

0.999 0.995 0.989

5.49 5.52 5.54

242 250 255

Fig. 8. Solute retention for DMI and DEI as a function of VCF.

tion and separation by MEUF is to use a low amount of surfactant, hydrophilic reactants that will not accumulate in the micelles, and to use short filtration times. The knowledge of KMW, the partition coefficient between the micellar pseudo phase and the surrounding water phase, can be helpful to find an appropriate solute for these systems. In comparison to the common octanol–water partition coefficient POW the determination of KMW is more difficult because the separation of the water and micellar phase is not easy. Quantum chemical calculations like COSMO-RS (Conductor-like Screening Model for Real Solvents) can be used as a tool to obtain KMW . In Table 6, the predicted KMW values for the investigated itaconates are given and the trend for KMW agrees well with the found solute retentions. For all experiments we found that for DMI the flux of the aqueous micellar solution with solute is mainly the same as for the surfactant solution alone (Fig. 9). This observation agrees well with the results found by Doulia and Xiarchos [21] in the crossflow ultrafiltration of surfactant solutions with and without alachlor pesticide. For DEI, this behavior is only observed at high surfactant to solute ratio, and by decreasing this ratio less DEI is solubilized in the water phase. DEI therefore is able to interact with the membrane surface to form a hydrophobic layer or can enter the membrane pores and block them, both phenomena reduce the flux. Such behavior has

Table 6 KMW predicted by COSMO-RS. Solute

log KMW

DMI DEI

2.3 3.0

Fig. 9. Ultrafiltration of dissolved DMI and DEI with TX-100 solutions (C010 membrane, P = 0.1 MPa, n = 600 min−1 ).

also been reported for the ultrafiltration of an oil-in-water emulsion using a polysulfone membrane [22]. 4. Conclusions In order to find optimal conditions for a combined reaction and separation process using MEUF, ultrafiltration of aqueous TX-100 solution was studied in a stirred cell under variation of surfactant concentration and pressure. The highest micelle retention and the lowest flux reduction were obtained for the membranes with the smallest MWCO. In comparison to typical studies very high surfactant concentrations were chosen and in this case the flux was more dependent on the surfactant concentration than on the pressure. The gel polarization model was proven and a gel concentration in the range of 150–300 g/L was determined. This is much higher than the value of 52 g/L reported by Grieves et al. [20]. The ultrafiltration of aqueous TX-100 solutions with dissolved organic compounds, here DMI and DEI, shows that solute retention increases with increasing hydrophobic character and the flux is not affected as long as the solubilization capacity of the micelles is high enough. In a combined process, the separation of catalyst and product is aspired and therefore high solute retention is not wanted. We would propose (a) to use the C005 or the C010 membrane to achieve high catalyst recovery (agrees with [10]), (b) use low surfactant concentrations in order to achieve higher fluxes by decreasing surface layer formation and (c) use hydrophilic solutes as reactants that can be separated easily from the catalyst. COSMORS can be used as a tool to predict the solute KMW in order to find appropriate surfactants for given reactants.

M. Schwarze et al. / Separation and Purification Technology 74 (2010) 21–27

Acknowledgements This work is part of the Cluster of Excellence “Unifying Concepts in Catalysis” coordinated by the Technische Universität Berlin. Financial support by the Deutsche Forschungsgemeinschaft (DFG) within the framework of the German Initiative for Excellence is gratefully acknowledged (EXC 314). This work was also supported by the Deutsche Forschungsgemeinschaft (grant [SCHO 687/71]/[AR 236/32-1]). References [1] A. Deriszadeh, T.G. Harding, M.M. Husein, Improved MEUF removal of naphthenic acids from produced water, J. Membr. Sci. 326 (2009) 161–167. [2] N. Zaghbani, A. Hafiane, M. Dhahbi, Separation of methylene blue from aqueous solution by micellar enhanced ultrafiltration, Sep. Purif. Technol. 55 (2007) 117–124. [3] M.K. Purkait, S. DasGupta, S. De, Micellar enhanced ultrafiltration of phenolic derivatives from their mixtures, J. Colloid Interface Sci. 285 (2005) 395–402. [4] R.O. Dunn Jr., J.F. Scamehorn, S.D. Christian, Use of micellar-enhanced ultrafiltration to remove dissolved organics from aqueous streams, Sep. Sci. Technol. 20 (1985) 257–284. [5] Y.-Y. Fang, G.-M. Zeng, J.-H. Huang, J.-X. Liu, X.-M. Xu, K. Xu, Y.-H. Qu, Micellarenhanced ultrafiltration of cadmion ions with anionic-nonionic surfactants, J. Membr. Sci. 320 (2008) 514–519. [6] R. Juang, Y. Xu, C. Chen, Separation and removal of metal ions from delute solutions using micellar-enhanced ultrafiltration, J. Membr. Sci. 218 (2003) 257–267. [7] K. Baek, H. Lee, J. Yang, Micellar-enhanced ultrafiltration for simultaneous removal of ferricyanice and nitrate, Desalination 158 (2003) 157–166. [8] L. Gzara, M. Dhahbi, Removal of chromate anions by micellar-enhanced ultrafiltration using cationic surfactants, Desalination 137 (2001) 241–250.

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