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Nanofiltration of Nonionic Surfactants: Effect of the Molecular Weight Cutoff and Contact Angle on Flux Behavior Geert Cornelis,*,† Katleen Boussu,† Bart Van der Bruggen,† Ilse Devreese,‡ and Carlo Vandecasteele† Laboratory of Applied Physical Chemistry and Environmental Technology, Department of Chemical Engineering, K.U. Leuven, W. De Croylaan 46, B-3001 Leuven, Belgium, and Centexbel-Gent, Technologie Park 7, B-9052 Zwijnaarde, Belgium
Nonionic surfactants are widely used in industry, and large amounts of wastewater containing nonionic surfactants are produced each year. Nanofiltration (NF) is a possible option to purify these waters, reducing the overall water consumption and enhancing biological purification. However, the flux behavior of NF during purification of wastewaters containing nonionic surfactants is not well understood. NF tests were performed with both synthetic solutions and real wastewaters containing nonionic surfactants from carpet rinsing. When a membrane with a relatively high molecular weight cutoff (MWCO) was chosen, flux decreased to a level lower than that with most ultrafiltration membranes. When a low MWCO was chosen, flux either increased above pure water flux when a relatively hydrophobic membrane was chosen or decreased when a relatively hydrophilic membrane was chosen. NF thus seems feasible to reduce water usage in industrial processes involving nonionic surfactants when a proper membrane is selected. It appeared that flux is controlled by three mechanisms: first, the narrowing of membrane pores through adsorption of monomers when the MWCO is comparable or larger than the monomer size, causing flux decline; second, an improved wettability of the membrane surface through adsorption of monomers on hydrophobic groups, causing flux to increase above pure water flux; third, a decreased wettability through adsorption of monomers on hydrophilic groups, causing flux decline. The nonionic surfactant concentration, MWCO, and membrane’s hydrophilicity determine which mechanism is dominant. Introduction Surfactants are used in a number of crucial industrial processes, e.g., in the textile industry or metal processing industry, and can be found in the produced wastewaters. These waters can be treated to a certain extent in a biological wastewater treatment plant, but some surfactants are barely biodegradable1 and reduce the plant’s performance.2 Furthermore, these effluents cannot meet the high quality requirements of most industrial processes for reuse. In the production of polyamide carpet, for example, large amounts of high-quality groundwater are used to remove spin finish from the carpet prior to dyeing. Many spin finishes are comprised of several nonionic surfactants and have to be removed from the carpet because they hamper the dyeing process. The resulting wastewater stream can be reused partially, after which it is discharged in the biological treatment plant. As a consequence, problems with sludge settling have been reported. Because groundwater reserves can be limited, there is a need to manage surfactant-containing wastewater in a more sustainable way without compromising process water quality. Online pressure-driven membrane purification of surfactant-containing wastewaters is a valid option in this respect. First of all, it can easily be installed online in an industrial process and can be adapted to treat individual wastewater streams so that a very high * To whom correspondence should be addressed. E-mail:
[email protected]. † K.U. Leuven. ‡ Centexbel-Gent.
quality is reached for the permeate. Furthermore, only a limited buffer capacity is needed because the purification is online and the elevated temperature of the wastewater from many industrial processes is quite beneficial for the membrane filtration unit’s performance. To reach a sufficiently low permeate concentration of surfactants in the permeate, a nanofiltration (NF) membrane is necessary.3 Retentions of surfactants in ultrafiltration (UF) are always too low for reuse of the permeate because only micelles are retained and membrane fouling can be a serious problem.4,5 Nonionic surfactants are known to adsorb strongly to pores of UF membranes, thus causing flux decline, especially for relatively hydrophobic membranes.5 Much better results were obtained when more hydrophilic UF membranes were used. When a NF membrane with a low molecular weight cutoff (MWCO) was applied to purify bottlewashing solutions, high fluxes and retentions were obtained because monomers can no longer penetrate the pores.6 Some other authors6-9 used NF to treat surfactant-containing solutions, but although nonionic surfactants generally have the most dramatic effect on flux in membrane filtration, their effect on flux in NF remains largely unclear. In two isolated cases, a flux increase above pure water flux was even observed when treating nonionic surfactant-containing solutions.7,9 It appears that the MWCO and hydrophilicity or surface wettability are very important membrane characteristics to explain flux behavior. In this investigation, the mechanisms leading to the observed flux behavior are investigated by using various membranes, with
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cutoffs both larger and smaller than the surfactant monomer and very good to poor wetting characteristics. In this way, membrane selection for the purification of wastewaters containing nonionic surfactants can be made more purposeful. Methods and Materials Experimental Setup. All filtration experiments were carried out in a laboratory-scale NF unit (Amafilter) using flat-sheet membranes with an effective surface area of 59 cm2. The flow channel was rectangular with a hydraulic diameter of 4.2 mm. The total channel length was 293 mm. The transmembrane pressure was 8 bar, and the rectangular flow was 600 L/h. A high rectangular flow was used to minimize concentration polarization. In all experiments, the temperature was kept at 50 °C because this is the temperature at which the textile wastewaters are actually produced. Membrane Characterization. The used membranes were NTR7450 (Nitto-Denko), N30F (Nadir), NF270 (Dow/Filmtech), UTC-20 (Toray), and Desal 5 DL (GE Osmonics). The MWCO of these membranes was determined by filtering solutions containing poly(ethylene glycol) molecules of different molecular weights ranging from 150 to 3000 Da. The temperature was 20 °C, the transmembrane pressure was 8 bar, and the rectangular flow was 600 L/h. Using gel permeation chromatography (Shodex OH Pak SB-802-5 HQ), retention as a function of the molecular weight was determined. The column size was 8 × 300 mm. The exclusion limit value was 8.5 × 103. The injection volume was 20 µL. From these data, the MWCO was calculated using the adapted log-normal model as proposed by Van der Bruggen et al.10 The contact angle of wet membranes with pure water was determined by a drop-shape analysis system (Kru¨ss, DSA Mk2). The contact angle is an indication of the membrane’s hydrophilicity. Pure water flux was determined at the experimental conditions used in all experiments. Filtration of Synthetic Solutions. To prepare synthetic surfactant solutions, a spin finish, Fasavin CA 73 (Zschimmer & Schwarz, Lahnstein, Germany), was used. This spin finish is comprised of several nonionic surfactants and minor amounts of anionic surfactants. The molecular size of the monomers ranges from 742 up to 1072 Da. The hydrogen carbon chain length ranges from 9 to 18 carbon atoms, while the size of the hydrophilic ethoxy headgroup varies between 10 and 16. The critical micelle concentration (cmc) was determined by measuring the surface tension as a function of the Fasavin CA 73 concentration using a drop-shape analysis system (Kru¨ss, DSA Mk2). Solutions with increasing surfactant concentrations show decreasing surface tension. Because only monomers contribute to the surface tension, the decline in the surface tension as a function of the surfactant concentration stops once the cmc is reached. From that point on, the monomer concentration is constant, while the number of micelles increases. Several experiments were carried out with a fixed surfactant concentration (200 ppm Fasavin CA 73) and different NF membranes. In this way, not only the MWCO but also the hydrophilicity was varied. Fluxes were measured during the 180-min filtration time, and fluxes relative to pure water fluxes were calculated. On the other hand, increasing surfactant concentrations were filtered with a single membrane (Desal 5 DL) to
Table 1. Characteristics of the NF Membranes Used membrane
MWCO (Da)
pure water flux [L/(m2 h)]
contact angle (deg)
N30F NTR7450 NF270 Desal DL UTC-20
578 312 136 255 163
89 78 220 90 80
87.5 69.9 26.0 54.4 34.8
investigate the effect of varying concentration. In this case, fluxes were recorded until steady-state flux was reached. Static Adsorption Tests. Increasing concentrations (5-100 ppm) of Fasavin CA 73 were allowed to adsorb to Desal 5 DL membranes for 24 h. The membrane’s surface area was 19.63 cm2, and the solution volume was 50 mL. The solutions were continuously stirred. After 24 h, the adsorption was compared to a blank and the adsorbed amount was calculated. Filtration of Real Wastewaters. Rinsing waters were obtained from Nelca NV (Lendelede, Belgium; samples 1-3) and Desso NV (Dendermonde, Belgium, sample 4). Rinsing is applied prior to dyeing in order to remove the spin finish from the polyamide carpet. Normally, this water is reused several times per day before it is discharged in the biological treatment plant. The Nelca wastewaters were therefore taken at different times of the day in order to produce wastewaters of varying spin finish concentration. These waters contained spin finish but also an antifoam, polyamide fibers, and minor amounts of mineral oils and dust. The Desso wastewater also contained about 1 g/L of another unknown nonionic surfactant. To remove suspended solids, paper filters (Whatman No. 5) were used. This had no effect on the surfactant concentration in the wastewaters. Samples were stored at 4 °C. To avoid biodegradation, filtration took place within a maximum of 3 days after sampling. Chemical Analysis. Retention of nonionic surfactants was determined by a two-phase titration with natrium tetrakis(4-fluorophenyl)borate.11 To 10 mL of an unknown surfactant solution was added 5 mL of a 6 M KOH solution, a few drops of Victoria blue B (0.04% in ethanol), and 5 mL of 1,2-dichloroethane. This mixture was titrated with a 5 × 10-5 M solution of natrium tetrakis(4-fluorophenyl)borate with vigorous mixing until the color changed from red to blue in the organic phase. A calibration curve was set up using Fasavin CA 73 solutions. Results Membrane Characterization. Table 1 shows the main properties of the NF membranes used in this experiment. Membranes were purchased from Nadir (N30F), Nitto-Denko (NTR7450), Dow/Filmtech (NF270), Osmonics (Desal DL), and Toray (UTC-20). The MWCOs of the used membranes vary widely. Because the monomers are between 742 and 1072 Da in size, some membranes have MWCOs slightly smaller than the monomer size while others have MWCOs much smaller than the monomer size. The membranes with the largest cutoff, NTR7450 and N30F, are relatively hydrophobic. On the other hand, the membranes with the lowest cutoff, UTC-20 and Desal 5 DL, are relatively hydrophilic. Because hydrophilic membranes generally have high fluxes and membranes with high cutoffs do too, all of these membranes have comparable pure water
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Figure 1. Results of the static adsorption tests of solutions of increasing Fasavin CA 73 concentration to Desal 5 DL membranes. Fasavin CA 73 concentration to Desal 5 DL membranes.
Figure 3. Steady-state fluxes during filtration of solutions with increasing concentrations of Fasavin CA 73 with Desal 5 DL.
Figure 2. Surface tension of solutions containing Fasavin CA 73 as a function of the surfactant concentration at 50 °C.
Figure 4. Nonionic surfactant retentions during filtration of solutions with increasing concentrations of Fasavin CA 73 with Desal 5 DL (closed symbols). The retentions of experiments with wastewaters are also included (open symbols).
Table 2. Nonionic Surfactant Concentrations and Retentions in Filtration Experiment with Real Wastewaters with Desal 5 DL sample feed (ppm) permeate (ppm) retention (%)
1
2
3
4
1030 27 97
280 15 95
38 3 93
1828 30 98
fluxes. NF270, a membrane with an intermediate cutoff, is the most hydrophilic, resulting in the highest pure water flux. Static Adsorption Tests. Figure 1 shows that increasing amounts of surfactants result in more adsorption in static conditions. A linear correlation was observed. Preliminary testing (results not shown) showed that equilibrium was attained within the time span of the experiments. No concentrations of Fasavin CA 73 higher than 100 ppm could be used because the difference with the blank was smaller than the standard deviation of repeated measurements. Filtration of Increasing Concentrations of Surfactant with Desal 5 DL. A typical behavior of surfactant-containing solutions is shown in Figure 2. A single cmc was not observed but rather a set of different cmc’s between 100 and 1000 ppm. This is a confirmation of the fact that Fasavin CA 73 is not a single compound but a mixture of several nonionic surfactants. To only study the effect of monomers on NF, 200 ppm was chosen as a suitable concentration to be used in experiments with synthetic solutions. This concentration is still high enough to be representative for real wastewaters (Table 2) and is lower than the cmc’s of the surfactants in CA 73 (Figure 2). Fluxes for NF of Fasavin CA 73 solutions ranging from 0.001 to 10 000 ppm using Desal 5 DL are given
Figure 5. Flux relative to pure water flux as a function of time for five NF membranes during the filtration of a 200 ppm Fasavin CA 73 solution.
in Figure 3. At concentrations above 0.1 ppm, steadystate flux was reached within 60 min. At lower concentrations, it took up to 270 min to reach steady state. At low concentration, flux increases up to 1.4 times pure water flux. At concentrations above 0.1 ppm, flux dropped to a near constant value of 1.1 times pure water flux. Retention of nonionic surfactants increases as a function of the concentration (Figure 4). When the cmc is reached (100 ppm), the slope increases, but as concentrations increase further, the slope decreases. Filtration of Surfactant Solutions with Membranes with Varying MWCO and Contact Angle. Figure 5 shows the behavior of flux relative to pure water flux as a function of time for five NF membranes when filtering a 200 ppm Fasavin CA 73 solution. When the membrane’s MWCO is slightly smaller than the monomer’s molecular size (N30F and NTR7450), a dramatic flux decrease is observed. N30F, the membrane with the largest MWCO (Table 1), showed a slightly more pronounced flux decline than NTR7450
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Figure 6. Flux relative to pure water flux as a function of time of nonionic surfactants for five NF membranes after replacement of the 200 ppm Fasavin CA 73 feed by deionized water after 180 min. The MWCO of the membranes is indicated.
Figure 8. Flux relative to pure water flux as a function of time during filtration of sample 4. The water flux measured after finishing filtration of the wastewater is indicated.
when filtering sample 4 (Figure 8). Even after 900 min of filtration time, no steady-state flux was attained. Retentions of the wastewaters are given in Table 2 and were included in Figure 4 (open symbols). The retentions of the wastewaters followed the general trend in Figure 4, except for sample 3. Discussion
Figure 7. Flux relative to pure water flux as a function of time during filtration of rinsing waters from the textile industry (samples 1-3). The results from 200 ppm Fasavin CA 73 are included for comparison. Table 3. Relative and Absolute Fluxes and Retentions of Five NF Membranes When Filtering a 200 ppm Fasavin CA 73 Solution
membrane
pure water flux [L/(m2h)]
flux after 180 min [L/(m2h)]
relative flux after 180 min (%)
retention (%)
N30F NTR7450 NF270 Desal DL UTC-20
89.0 77.6 226.2 95.7 82.8
1.4 4.4 180.2 106.2 138.4
1.5 5.7 79.7 110.9 167.1
76.9 63.5 91.4 93.7 97.3
(Table 3). When the membrane’s MWCO is much smaller than the monomer’s molecular size (UTC-20, Desal 5 DL, NF270), a flux increase or a slight flux decrease is observed. The increase in flux is more pronounced as the MWCO increases and the contact angle decreases. Table 3 shows the observed rejections for the five membranes. Retention increases as the MWCO and contact angle decrease except for N30F. After the filtration experiments, the feed surfactant solution was replaced by deionized water for three membranes and fluxes were again measured (Figure 6). Fluxes improved for all three membranes, in particular for UTC-20, the membrane with the lowest MWCO. For NF270, flux was restored to the original pure water level, and for NTR7450, fluxes stayed low. Filtration of Wastewaters with Desal 5 DL. Figure 7 shows flux as a function of time during filtration of samples 1-3 with Desal 5 DL. Although the nonionic surfactant load differs significantly, no difference in steady-state flux can be observed. Just as in Figure 3, flux always attained a constant value of ca. 1.1 times pure water flux after 60 min. Quite in contrast to Figures 3, 5, and 7, a declining flux was observed
Based on the results given above, it is postulated that flux during NF of nonionic surfactants is controlled by three mechanisms: (1) adsorption of surfactants in pores, leading to flux decrease; (2) adsorption of surfactants to hydrophobic groups on the membrane surface, leading to flux increase; (3) adsorption of surfactants to hydrophilic groups on the membrane surface, leading to flux decrease. Adsorption of Surfactants in Pores. A comparison of relative fluxes allows one to distinguish between the effects of the surfactants on flux evolution and intrinsic differences in pure water flux. Flux decline can be caused by reversible and irreversible mechanisms.12 Adsorption of surfactant monomers inside membrane pores is generally not reversible.13 When the surfactantcontaining feed solution was replaced by deionized water, flux could not be restored to the original water flux when using NTR7450 or N30F (Figure 6). Although these membrane have a MWCO smaller than the monomer size, surfactants still could penetrate the pores, as indicated by the relatively low retention. Because nonionic surfactants consist of long chains that can be folded to a substantial degree, they can penetrate and adsorb in pores with a smaller MWCO than their molecular size.5 Adsorbed surfactants may thus have caused narrowing of the pores accompanied by flux decline (Figure 9 a). Because the NTR7450 and N30F membranes are relatively hydrophobic and their pores are already quite small compared to UF membranes, the resulting flux decline will be quite dramatic with these NF membranes: UF membranes normally show a flux decline up to 60% of pure water flux,4,5 whereas NTR7450 showed a decline down to 6% and N30F down to 1.5% (Table 3). Because adsorbed monomers can still be transported through the pores,13,14 this narrowing would not lead to an increase in surfactant retention. Retentions were indeed not higher than 80%. When membranes with lower MWCOs were used (NF270, UTC-20, and Desal 5 DL), surfactants much less penetrated the pores, as indicated by high retentions (Table 3). Because the used membranes were also
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Figure 9. Overview of the proposed mechanisms influencing flux during NF of solutions containing nonionic surfactants.
more hydrophilic than N30F and NTR7450, narrowing of the pores is not likely because adsorption of nonionic surfactants in relatively hydrophilic membranes is weak and barely leads to flux decline.5,6 Adsorption of Surfactants to the Membrane Surface, Leading to Flux Increase. Water flux is also related to membrane characteristics such as hydrophilicity or wettability.15 Hydrophilic membranes usually show good wettability and hence high fluxes. In this study, an increase above pure water flux was observed for the membranes UTC-20 and Desal 5 DL (Figures 3, 5, and 7). It is hypothesized that an improved wetting of the membrane surface caused this increase after adsorption of monomers on hydrophobic groups. Nonionic surfactants interact with hydrophobic groups on membrane surfaces predominantly through London and van der Waals interactions.16 This means that the hydrophobic groups will adsorb the hydrophobic tails while the hydrophilic heads are directed to the bulk solution (Figure 9c). Mietton-Peuchot et al.17 found that when such an adsorption occurs in the pores of microfiltration membranes, flux is increased. Some authors have, in fact, observed a similar flux increase above water flux when filtering wastewaters containing nonionic surfactants with NF.7,9 The more hydrophobic a membrane is, the stronger the hydrophobic tails that are attracted.5 This is the case in Figure 5, where flux increase is most pronounced for UTC-20, which has a relatively high contact angle. For NF270, flux increase did not occur because this membrane is highly hydrophilic, which means that there are much less hydrophobic groups on the membrane surface. Hydrophobic interactions and improved wettability were thus much less pronounced. Adsorption of Surfactants to the Membrane Surface, Leading to Flux Decrease. When the surfactant solution feed was replaced by pure water, flux improved for all three membranes tested (Figure 6). Hence, before replacement of the feed, a reversible fluxdecreasing mechanism must have been active, which was relieved upon replacement of the feed solution by pure water. Concentration polarization can be ruled out as such a mechanism here because of the high rectangular flow (600 L/h). It is, therefore, hypothesized that a reversible adsorption to the membrane surface oc-
curred, which lowered wettability. This is clearly demonstrated with NF270. In what preceded, it was stated that, in the case of NF270, neither adsorption in the pores due to low MWCO nor improved wettability due to low contact angle occurred. Flux evolution must have been influenced by a third mechanism. It is, therefore, hypothesized that flux decrease was caused by a decreased wettability of the membrane surface due to adsorption of surface monomers by hydrophilic groups on the membrane surface. Because surfactants in Fasavin CA 73 are relatively hydrophilic because of relatively large ethoxy headgroups and the surface of NF270 is too, adsorption of headgroups to hydrophilic groups is possible.18 The hydrophobic tails will be extended into the bulk solution, thereby decreasing the membrane surface wettability and hence also flux (Figure 9d). Adsorption of nonionic surfactants to hydrophilic membranes is quite weak because not much gain in free energy is obtained through adsorption of a hydrophilic compound out of water on a hydrophilic surface.19 Consequently, the adsorption and resulting flux decline will be reversible. Influence of the Surfactant Concentration. The flux behavior as a function of increasing surfactant concentration in Figure 3 can be partially explained according to these two mechanisms. Because hydrophobic interactions are much stronger than hydrophilic ones, monomers will adsorb to hydrophobic groups already at low concentrations (0.1 ppm), hydrophilic interactions will take place, decreasing surface wettability. The resulting flux will be dependent on which mechanism is dominant. It is unclear why flux stays stabile at 110% because Figure 1 indicates that adsorption of Fasavin CA 73 increases between 5 and 100 ppm. It is possible that, from a certain concentration on, more adsorption of monomers causes just as much increase in wettability through hydrophobic adsorption as it causes decrease in wettability through hydrophilic adsorption. Once the cmc is reached, the monomer concentration attains a constant value, so logically an adsorption equilibrium should be attained because micelles do not adsorb to surfaces.18
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Filtration of Wastewaters. The same observations as those for synthetic solutions were made for real wastewaters. Just as in Figure 3, flux was independent of the surfactant concentration in Figure 7. When NF was used for sample 4, a slow flux decline over the time span of the experiment was observed (Figure 8). This decline cannot be attributed to spin finish components because no flux decline was observed in Figure 3, even at high concentrations. At high concentration, anionic surfactant monomers can associate at the membrane surface to form bilayers, also called hemimicelles. Childess and Elimelech20 proposed that, during NF of ionic surfactants, hemimicelle formation can significantly lower flux. Possibly monomers of the unknown nonionic surfactant associated to form hemimicelles on the membrane surface. Nonionic surfactants differ from many other surfactants in that quite small changes in the concentration and molecular structure can have large effects on its adsorption behavior. Some nonionic surfactants can indeed form bilayers on relatively hydrophilic surfaces above their cmc.18 In any case, this decline was reversible because flux climbed to 119% of pure water flux upon replacement of the wastewater feed by pure water. In this way, NF proves to be a valuable purification technology because its performance is independent of the wastewater surfactant concentration. Figure 4 and Table 2 show that in the concentration range relevant for the treated wastewaters retentions are 90% or higher, so a NF unit seems feasible for water recycling.
Conclusion During NF, adsorption of nonionic surfactants can cause both flux decrease and flux increase, depending on the MWCO and surface hydrophilicity. Figure 9 gives an overview of the mechanisms possible. When the MWCO is slightly lower or comparable to the monomer size, monomers can penetrate the pores. When the membrane has a high surface density of hydrophobic groups, or in other words when it is relatively hydrophobic, strong adsorption of monomers occurs and the pore radius is reduced, leading to flux decline (Figure 9a). This occurred in the case of N30F and NTR7450. When the membrane has a low surface density of hydrophobic groups but a high density of hydrophilic groups, adsorption of monomers is much less strong, leading to a limited flux decline. This behavior was observed using relatively hydrophilic UF membranes.5 When, however, the MWCO is much lower than the monomer size, the monomers cannot penetrate the pores and flux will only be influenced by changes to the membrane’s surface. On hydrophobic groups, strong irreversible adsorption of hydrophobic tails occurs, which improves surface wettability, whereas on hydrophilic groups, the hydrophilic heads are adsorbed and wettability is decreased. Depending on the relative density of hydrophobic or hydrophilic groups, or in other words the hydrophilicity, the net effect is flux increase or flux decrease (Figure 9c,d). Carpet-rinsing waters containing nonionic surfactants were also tested, and the same observations as those for synthetic solutions were made. Fluxes and retentions stayed high over a broad concentration range. A membrane filtration unit thus appears to be feasible for water recycling. When a suitable membrane is to be
selected, the MWCO should be chosen to be much lower than the monomer size. Hydrophilic membranes show high water fluxes, but this flux can be decreased by adsorption of monomers, a more hydrophobic membrane can offer an improvement because its flux is increased because of adsorption of monomers. The high pure water flux and the limited decrease made in this case NF270 the most suitable membrane. Acknowledgment We thank IWT for making a valuable financial contribution to the project from which this publication emerged. We also kindly thank Prof. Delcour for his assistance with gel permeation chromatography. Literature Cited (1) Kitis, M.; Adams, C. D.; Daigger, G. T. The effects of Fenton’s reagent pretreatment on the biodegradability of nonionic surfactants. Water Res. 1999, 33, 2561. (2) Metcalf & Eddy, Inc. Wastewater Engineering: Treatment, Disposal and Reuse; McGraw-Hill: New York, 1991. (3) Goers, B.; Wozny, G. Flexible design and operation of a twostep UF/NF-system for product recovery from rinsing waters in batch production. Water Sci. Technol. 2000, 41, 93. (4) Van der Bruggen, B.; Vandecasteele, C.; Cornelis, G.; Van Baelen, D.; Devreese, I. Fouling of nanofiltration and ultrafiltration membranes applied for wastewater regeneration in the textile industry. Proceedings on fouling and critical flux, Lappeenranta, Finland, June 16-18, 2004. (5) Jo¨nsson, A.-S.; Jo¨nsson, B. The influence of nonionic and ionic surfactants on hydrophobic and hydrophilic ultrafiltration membranes. J. Membr. Sci. 1991, 56, 49. (6) Ro¨gener, F.; Willems, M.; Mavrov, V.; Chmiel, H. The influence of cleaning additives on rejection and permeability in nanofiltration and ultrafiltration of bottle washing solutions. Sep. Purif. Technol. 2002, 28, 207. (7) Wendler, B.; Goers, B.; Wozny, G. Nanofiltration of solutions containing surfactantssprediction of flux decline and modelling of mass transfer. Desalination 2002, 147, 217. (8) Goers, B.; Mey, J.; Wozny, G. Optimised product and water recovery from batch-production rinsing waters. Waste Manage. 2000, 20, 651. (9) Rozzi, A.; Antonelli, M.; Arcari, M. Membrane treatment of secondary textile effluents for direct reuse. Water Sci. Technol. 1999, 40, 409. (10) Van der Bruggen, B.; Schaep, J.; Wilms, D.; Vandecasteele, C. A comparison of models to describe the maximal retention of organic molecules in nanofiltration. Sep. Sci. Technol. 2000, 35, 169. (11) Tsubouchi, M.; Yamasaki, N.; Yanagisawa, K. Two-phase titration of poly(oxyethylene) Nonionic surfactants with tetrakis(4-fluorophenyl)borate. Anal. Chem. 1985, 57, 783. (12) Mulder, M. Basic Principles of Membrane Technology; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1991. (13) Bakx, A.; Timmerman, A. M. D. E.; Frens, G. Shear stimulated adsorption of surfactants from micellar solutions. Colloids Surf. A 2001, 183-185, 149. (14) Bakx, A.; Timmerman, A. M. D. E.; Frens, G. The flow of concentrated surfactant solutions through narrow capillaries. Colloid Polym. Sci. 2000, 278, 418. (15) Roudman, A. R.; DiGiano, F. A. Surface tension of experimental and commercial nanofiltration membranes: effects of wetting and natural organic matter fouling. J. Membr. Sci. 2000, 175, 61. (16) Doulia, D.; Tra¨gårdh, G.; Gekas, V. Interaction behaviour in ultrafiltration of nonionic surfactants. Part II. Static adsorption below CMC. J. Membr. Sci. 1997, 123, 133.
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(17) Mietton-Peuchot, M.; Ranisio, O.; Peuchot, C. Study of behaviour of membranes in the presence of anionic or nonionic surfactants. Filtr. Sep. 1997, 38, 883. (18) Paria, S.; Khilar, K. C. A review on experimental studies of surfactant adsorption at the hydrophilic solid-water interface. Adv. Colloid Interface Sci. 2004, 110, 75. (19) Fane, A. G.; Fell, C. J. D.; Kim, K. J. The effect of surfactant pre-treatment on the ultrafiltration of proteins. Desalination 1985, 53, 37.
(20) Childess, A. E.; Elimelech, M. Relating nanofiltration membrane performance to membrane charge (electrokinetic) characteristics. Environ. Sci. Technol. 2000, 34, 3710.
Received for review February 1, 2005 Revised manuscript received April 29, 2005 Accepted May 2, 2005 IE0501226
