Ind. Eng. Chem. Res. 1996, 35, 3687-3696
3687
Ultrafiltration of Surfactant and Aromatic/Surfactant Solutions Using Ceramic Membranes Frederic Gadelle,† William J. Koros, and Robert S. Schechter* Department of Chemical Engineering, The University of Texas, Austin, Texas 78712
Rejection and permeate flux taken together establish the efficiency of an ultrafiltration separation. The controllable factors that may influence the efficiency are systematically studied. These factors include transmembrane pressure, recirculation rate, membrane pore size, and solute and surfactant structure and concentration. Experiments carried out using both cationic and nonionic surfactants show that rejection decreases and permeate flux increases with membranes of increasing pore sizes. However, for the large pore size membrane (200 Å), it is also observed that rejection increases and permeate flux decreases as the filtration proceeds. These unexpected results suggest that micelles penetrate and accumulate into the larger pores, thereby reducing the effective membrane pore size. Depending on the molecular structure and concentration of the surfactant, rejection as high as 99.9% is achieved with a ceramic membrane having 65 Å pores. Permeate fluxes between 30 and 70% of pure water are observed. The addition of a solute tends to improve surfactant rejection and to decrease the permeate flux. Solute rejection increases with surfactant concentration and hydrophobicity. Solubilization isotherms determined here by ultrafiltration are shown to be in agreement with isotherms obtained with head space gas chromatography. Table 1. Properties of the Surfactants at 25 °C
Introduction Cost effective removal of toxic organic solutes from contaminated industrial waste waters is an increasingly important topic. The most widely used process is carbon-bed adsorption. This reliable technique can reduce pollutant concentrations to the parts per billion level. However, to regenerate activated carbon requires batch processing and is therefore costly. Alternatives such as distillation or solvent extraction are even less attractive. Ultrafiltration is generally limited to the removal of high molecular weight solutes. Another alternative, proposed by Leung (1979), is to combine surfactant solubilization and ultrafiltration. This new technique, called micellar-enhanced ultrafiltration, has been studied in some detail in the past decade by Scamehorn and co-workers (Dunn et al., 1985; Scamehorn and Harwell, 1988; Christian and Scamehorn, 1989; Scamehorn et al., 1989). In this process, surfactant at a concentration greater than its critical micelle concentration (cmc) is added to a polluted aqueous solution. The surfactant molecules then self-associate to form micelles which can solubilize the organic solute. The micellar solution can then be filtered using an ultrafiltration membrane to reject the aggregates containing the organic pollutant. The retentate is rich in solute and surfactant, with the solute molecules being primarily associated with the micelles. The permeate contains surfactant monomers at concentrations equal to or less than the cmc and solute at a reduced concentration. Theoretically, the solute concentration in the permeate is equal to the unbound or free solute concentration in the retentate. If the permeate concentrations are low enough, the permeate stream can be discarded; otherwise, further treatment is necessary. Such treatments can include other ultrafiltration stages and, at the end of the process, a more conventional approach such as reverse osmosis or carbon-bed adsorp* To whom correspondence should be addressed. † Present address: Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6038.
S0888-5885(95)00718-4 CCC: $12.00
surfactants
formula
cmc (wt %)
CO630 CO850 CO880 CO890 RC630 SDBzSo CPC P103 P123
C9PhEO9 C9PhEO20 C9PhEO30 C9PhEO40 C12PhEO10 C12PhSO3Na C12NC5H5Cl EO17PO60EO17 EO19PO69EO19
0.016 0.025 0.041 0.069 0.0017 0.055a 0.031a 0.07b-0.12c 0.003c-0.03b
a Data from van Os et al. (1993). b Data from Alexandridis et al. (1994). c Data from Wanka et al. (1994).
tion. Finally, the retentate must be treated to recover the surfactant. Successful removal of several pollutants using micellar-enhanced ultrafiltration has been reported in the literature. For example, Dunn and co-workers successfully filtered aqueous solutions containing 4-tert-butylphenol (TBP) and a cationic surfactant (cetylpyridinium chloride or CPC) (Dunn et al., 1985). They were able to recover about 99.7% of the TBP and CPC with a permeate flux only 30% less than that of pure water. Predictions, based on solubilization partition coefficients, have also been reported for the removal of aromatic solutes with CPC (Christian and Scamehorn, 1989). Scamehorn et al. obtained a rejection of 99.8% for divalent cations with high permeate flux using an anionic surfactant (sodium dodecyl sulfate or SDS) (Scamehorn et al., 1989). Nitrate ions have also been removed from water using tetradecyltrimethylammonium bromide (TTAB) as surfactant (Morel et al., 1991). The feasibility and efficiency of the separation rest on solubilization, rejection of the solute-surfactant aggregates, and solute and surfactant permeate concentrations. To facilitate study of the process, three separate phases may be considered: (1) solute solubilization in the micelles, (2) ultrafiltration of surfactant micelles, and (3) removal of solute and surfactant from aqueous streams. Regarding the first issue, Gadelle et al. identified several surfactants exhibiting attractively low cmcs (Table 1) and high solubilization capacities for © 1996 American Chemical Society
3688 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996
volatile organic compounds (VOCs) (Gadelle et al., 1995a,b; Gadelle, 1995). Such surfactants, therefore, appear suitable for micellar-enhanced ultrafiltration. The surfactants identified include cationic (e.g., CPC), nonionic (e.g., poly(oxyethylene alkyl phenyl ether) or CnPhEOm) and polymeric (poly(oxyethylene)-poly(oxypropylene) triblock copolymer or PEOmPPOnPEOm) surfactants. While CPC has already been reported to be a good candidate for micellar-enhanced ultrafiltration, only one study reported data for the filtration of polyoxyethylated alkyl phenols for the removal of phenol (Kandori and Schechter, 1990). Polymers and polymeric surfactants have recently been suggested for this application (Sasaki et al., 1989; Hurter and Hatton, 1992; Gadelle et al., 1995b), but, to date, filtration data have not been reported. The present research is concerned with the two remaining issues: filtration of pure surfactant solutions and removal of the solute using micellar-enhanced ultrafiltration. Little work is available on the filtration of surfactant solutions. This lack of interest is surprising since the viability of this separation process depends on the ability of the membrane to reject the surfactant micelles. Additionally, most of the available studies on surfactants and membrane separation were primarily designed to help characterize the surfactant-membrane interactions rather than to study rejection and permeate flux (Jo¨nsson and Jo¨nsson, 1991; Doulia et al., 1992; Mahdi and Sko¨ld, 1992; Yamagiwa et al., 1993). Studies by Markels and co-workers report the effect of membrane pore size on the surfactant intrinsic rejection and permeate flow rates. However, their work is limited to the cationic surfactant CPC (Markels et al., 1994; Markels et al., 1995a,b). Only one recent article reviewed the influence of several factors on permeate flux and surfactant rejection (Akay and Wakeman, 1993). The authors, Akay and Wakeman, studied the nature of the membrane, the surfactant type, the feed concentration, the cross-flow velocity (i.e., the recirculation flux), the transmembrane pressure, and the temperature. Yet, the influence of the membrane pore size and the effect of solutes on permeate flux and rejection of surfactant were not reported. Moreover, all the data reported were obtained with polymeric membranes (e.g., cellulose acetate, cellulose ether, polysulphone) prone to strong interactions with the surfactants. In the present research, inert ceramic membranes (γ-alumina) were used to minimize interaction between membrane and surfactants. Indeed, studies have shown that sorption of nonionic ethoxylated surfactants on alumina is negligible (Lawrence et al., 1987). Additionally, the positively charged alumina surface limits cationic surfactant adsorption. Therefore, the present study investigates several factors that affect the filtration efficiency (permeate flux and surfactant permeate concentrations) and consequently determine the economic viability of the process. The factors studied here are the pressure difference between the retentate and the permeate, the recirculation flux, the membrane pore size or molecular weight cutoff (MWCO), and the structure and concentration of the surfactant. The influence of a solute (benzene) on the rejection of solute-surfactant aggregates and permeate fluxes is also reported. Finally, a comparison between solubilization isotherms obtained using head space gas chromatography (HSGC) and the filtration system is presented.
Figure 1. Filtration system.
Experiment and Analysis Materials. Polyoxyethylated alkyl phenol (Igepal series, CnPhEOm) were provided by Rhoˆne-Poulenc (Cranbury, NJ). Sodium dodecylbenzenesulfonate (SDBzSo) was obtained from Witco Corp. (Houston, TX). Cetylpyridinium chloride (CPC or C16PyCl), a cationic surfactant, was purchased from Aldrich Chemical Co., Inc. (Milwaukee, WI). The block copolymers were provided by Rhoˆne-Poulenc and BASF Corp. (Parsippany, NJ). Table 1 summarizes the properties of the surfactants and block copolymers. The cmcs were determined experimentally for the nonionics from UV absorbance and obtained from the literature for the ionic and polymeric surfactants (van Os et al., 1993; Alexandridis et al., 1994; Wanka et al., 1994). Benzene was obtained from EM Science (ACS grade; Gibbstown, NJ). All of the chemicals were used as received. Distilled water used in the experiments was further purified and deionized to a resistivity of 18 MΩ‚cm by a Technic Central Systems Lab Five (Seattle, WA). Filtration Experiments. The filtration system is shown in Figure 1. It consists of a stirred stainless steel-Teflon tank or cell, a ceramic membrane in a stainless steel housing, and a Teflon diaphragm pump to assure retentate recirculation. To minimize surfactant and solute loss through sorption, all of the equipment in contact with the solution is either stainless steel or Teflon. Tubular ceramic membranes (Membralox) of 23 cm length with 0.76 cm inner diameter were purchased from U.S. Filter (Warrendale, PA). The membranes have pore diameter sizes of 65, 100, and 200 Å and surface areas of about 44.6-46.2 cm2. The separation layer is made of γ-alumina, while the support layer is R-alumina. During an experiment, an aqueous surfactant solution is first added to the holding tank. Combination of a vent port and the Teflon piston allows purging of all air and maintenance of an air-free environment. The cell is then sealed, the recirculation pump started, and nitrogen pressure applied to initiate flow through the membrane. Since the nitrogen pressure forces the piston down, the air-free environment is preserved during the experiment. Maintaining an air-free environment is a key issue when working with volatile solutes. Surfactant and solute concentrations in the permeate are determined using an on-line UV spectrophotometer (DU-40, Beckman Instrument, Inc.) with a flow-through cell (path length of 3 mm). A refractometer (R401, Waters) is used with the surfactants that are not UV absorbant (e.g., the polymeric surfactants). The permeate fluxes are monitored using a balance (Mettler
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AE163) and a stopwatch. It should be noted that the permeate fluxes reported in the following study are often normalized with respect to the pure solvent (i.e., water) permeate flux obtained for the same operating conditions (temperature, pressure, and recirculation flux). Before an experiment is conducted, the UV spectrophotometer is calibrated for the solutes with no surfactant. This procedure has already been detailed but is summarized below (Simpson, 1995). After the system is purged, the cell is capped to assure an air-free system and a solute increment is added into the cell via the syringe. Once equilibrium is reached (after approximately 2 h), the pump is turned on for 5 min and the retentate is analyzed with the on-line UV. The pump is then turned off, and a new solute increment is injected into the stirred cell. The procedure is repeated until the solution reaches near saturation conditions. Finally, a large amount of solute is injected, and the solution is stirred for several hours. The agitator is then turned off to allow phase separation to occur overnight. The UV absorbance obtained from this last step corresponds to the saturation concentration. This method enables the determination of the solute calibration curves (solute concentration as a function of the UV absorbance) and the solute aqueous saturation concentration. The solute permeate concentration was also measured to assess the loss by membrane sorption. When surfactant is not present, the solute permeate concentration equaled the solute retentate concentration, indicating that sorption is not sensible. Surfactant solutions of various concentrations were also prepared to calibrate the UV spectrophotometer and the refractometer. It should be noted that superposition of the solute and surfactant scans sometimes requires subtracting the surfactant UV absorbance from the solute UV readings to obtain the “true” solute absorbance. Data Analysis. With this experimental system, two modes of operation are available: (1) continuous filtration of the surfactant and solute-surfactant solutions (i.e., a transient mode) and (2) determination of steadystate conditions. The second mode of operation is achieved by restricting the permeate volume to a maximum of 10% of the initial volume. Retentate concentrations are then approximately equal to the initial concentrations. This mode yields flux and solute and surfactant rejection for given retentate conditions. This mode of operation also allows for the determination of one data point on the solute solubilization isotherm. The solute and surfactant concentrations in the permeate (Csol. and Csurf p p , respectively) are determined from the calibration curves. Csol. also corresponds to p the free solute concentration. Knowledge of the initial concentrations, Csol. and Csurf , and of the initial volume i i (Vi) and permeate volume recovered (Vp), combined with a mass balance, yield the retentate concentrations, Csol. and Csurf r r .
Cr )
ViCi - VpCp Vi - Vp
(1)
The solute and surfactant rejection (Rsol. and Rsurf) are defined as
(
Rsol. (%) ) 100 × 1 -
)
Csol. p Csol. r
(2)
(
Rsurf (%) ) 100 × 1 -
)
Csurf p Csurf r
(3)
A 100% rejection indicates that solute (or surfactant) molecules are not present in the permeate. The rejection values determined in the present study correspond to the observed rejection (based on the bulk retentate concentrations) rather than the intrinsic rejection (based on retentate concentrations at the membrane surface). It should be mentioned that Csurf is sometimes rer placed by (Csurf - cmc) in the denominator of eq 3 r (Kandori and Schechter, 1990). This correction was not accounted for in this study since permeate concentrations were often less than the cmc and since, for a practical approach, it is more interesting to present rejection levels based on initial concentrations. Finally, as already mentioned, Csurf is taken to be equal to r Csurf when operating under steady-state conditions. i Viscometry Experiments. The viscosity of the surfactant solutions is measured with a Ubbelohde viscometer immersed in a constant temperature bath (T ) 25 °C). Results and Discussion In the following, various factors influencing rejection and permeate flux are examined. First, the effect of operating conditions (i.e., pressure difference, recirculation flux, and membrane pore size) is determined. Then, the filtration of pure surfactants is reported as a function of the surfactant structure and concentration. The influence of a solute (benzene) is also discussed. Rejection of the solute and the nonionic surfactants is reported as functions of the solute concentration and surfactant structure and concentration. Finally, solubilization isotherms obtained using the filtration system and head space gas chromatography are compared. Influence of the Operating Conditions. Three operating parameters, studied in great detail, are the transmembrane pressure, the recirculation flux, and the membrane pore size. Temperature, another operating parameter, was kept constant at room temperature (24 °C). (a) Transmembrane Pressure. Experiments show that both permeate fluxes and permeate concentrations increase upon an increase in the transmembrane pressure. Data are not shown here for the sake of brevity. The increase in the permeate flux is not surprising, since the pressure difference between retentate and permeate is the driving force for the process. The higher permeate fluxes may also yield increased concentration polarization and, therefore, cause an increase in the permeate concentrations. These preliminary results suggest that a choice has to be made between high permeate flux and low rejection (or high permeate concentration), and low permeate flux and high rejection. In the remaining study, the nitrogen pressure was set at 40 psi. This pressure provides reasonable fluxes and high rejection. (b) Recirculation Flux. The influence of the retentate recirculation flux on permeate flux and rejection was also carefully studied. High permeate concentration and low permeate fluxes are observed at low recirculation rates. Most likely, concentration polarization contributes to these poor results. An increase in the recirculation flux yields lower permeate concentrations (therefore higher rejection) and higher permeate
3690 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996
Figure 2. Influence of the membrane pore size on the surfactant (CPC) rejection. Csurf ) 2.0 wt %. i
Figure 3. Influence of the membrane pore size on the permeate flux. Csurf ) 2.0 wt %. i
fluxes. These findings are not surprising since the high recirculation fluxes and the pulsations caused by the diaphragm pump decrease concentration polarization. In the following, a 400 cm3/min recirculation rate was chosen. (c) Membrane Pore Size. Two surfactant solutions, a cationic (CPC) and a nonionic (C12PhEO10), were filtered using membranes having 65, 100, and 200 Å diameter pores. Figure 2 presents the rejection of CPC as a function of the percentage of permeate volume recovered ()100Vp/Vi). Since the volumes recovered are a small fraction of the initial volume, the retentate surfactant concentrations remain relatively constant at the initial value of 2.0 wt %. Thus, the abscissa may be thought to correspond to time. Rejection by the 65 Å membrane is very high (Rcpc ) 99% and Ccpc p ) 0.6 × cmc, cmccpc ) 0.031 wt %) as may be expected since the hydrodynamic radius of the CPC micelles is about the same as the pore radius. When the pore size is increased to 100 Å, rejection decreases, ranging between 91 and 96% (Ccpc ) (5.7-3.0) × cmc), as anticipated. p Rejection is poor with the 200 Å membrane. For this membrane, Rcpc ranges from 47 to 75%, while the permeate concentrations are between 34 × cmc and 16 × cmc. A decrease in the rejection values with an increase in the membrane pore size is certainly to be expected. However, the unexpected feature of the results shown in Figure 2 is the consistent increase of rejection as a function of time seen for the 100 and 200 Å membranes. To help interpret these transient results, the permeate fluxes are presented in Figure 3. All of the fluxes decline with time, but the decrease for the membrane having the largest pores is dramatic, thereby suggesting a mechanism akin to fouling (Kulkarni et al., 1992). However, fouling is usually associated with adsorption onto the surface of the pore walls. Since
cationic (and also nonionic) surfactant adsorption is known to be negligible on alumina surface (Lawrence et al., 1987), the fouling hypothesis must be rejected. It is then proposed that deposition and accumulation of the surfactant aggregates within the pores are responsible for the phenomenon observed. This accumulation will continue until a steady state is established. Such an accumulation could reduce the effective pore diameters and thus increase rejection and decrease permeate flux. The relative permeate flux can also clarify the effect of the membrane pore size on filtration efficiency. The permeate fluxes relative to the pure water flux are calculated to be roughly 0.65, 0.53, and 0.09-0.11 for the 65, 100, and 200 Å membranes, respectively (the pure water permeate flow rates are 0.046, 0.058, and 1.136 cm3/(cm2‚min) at a pressure of 40 psi). The relative permeate flux decreases drastically with an increase in the membrane pore size, suggesting, again, that CPC micelles tend to readily obstruct flow through the large membrane pores. Furthermore, as already suggested with the data from Figures 2 and 3, deposition and accumulation of the micelles are much greater with the 200 Å than with the 100 or 65 Å membranes. As further evidence supporting this mechanism, we note that the same trends are observed with a nonionic surfactant solution and that, furthermore, the membranes were restored to their original state after filtering deionized water. This tendency for the micelles to accumulate in the larger pores of an ultrafiltration membrane even though adsorption is negligible does not seem to have been noticed before. This mechanism, if active, may imply that membranes having a wide distribution of pore sizes, some perhaps even several times the diameter of the micelles, will perform adequately after an initial start up period during which concentration of surfactant builds up within the larger pores. On the basis of these observations, 65 Å membranes were selected for further studies to avoid artifacts that might arise from accumulation of the micelles in the pores. Filtration of Pure Surfactant Solutions. The influence of the surfactant structure (i.e., size of the polar head, length of the hydrocarbon tail, and nature of the surfactant) has been studied for a cationic, two polymeric, and several nonionic surfactants. As already discussed, these surfactants have been shown to exhibit high solubilization capacities, and, therefore, appear to be good candidates for the removal of VOCs from aqueous streams using micellar-enhanced ultrafiltration. One experiment was carried out with an anionic surfactant: sodium dodecylbenzenesulfonate. In general, anionic surfactants should not be considered for micellar-enhanced ultrafiltration since they exhibit lower solubilization capacity than the corresponding cationics (Mukerjee and Cardinal, 1978; Rosen, 1989b; Gadelle, 1995). Furthermore, due to high Krafft temperature (when compared to cationics), they tend to precipitate at relatively low temperatures. (a) Influence of the Number of Ethylene Oxide Units in the Head Group. The influence of the number of ethylene oxide (EO) units on the efficiency of the filtration was studied with several polyoxyethylated nonyl phenols. Figures 4 and 5 show the surfactant rejection and permeate flux at several concentrations for nonionic surfactants having 9, 20, and 30 EOs. The surfactant having the lowest EO number exhibits the highest rejection (about 99.1% for C9PhEO9
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Figure 4. Influence of the number of EO units on the surfactant rejection.
Figure 5. Influence of the number of EO Units on the relative permeate flux.
Figure 6. Influence of the number of EO Units on the surfactant solution viscosity.
versus 98.7% for C9PhEO20 and C9PhEO30). C9PhEO9 is also the surfactant having the lowest cmc (0.016 wt %). Further calculations indicate that these rejection data correspond to permeate concentrations roughly equal to the cmc of the surfactants. These data suggest then that the cmc determines the rejection values. Permeate fluxes are reported in Figure 5. It appears that the fluxes for C9PhEO9 are significantly lower than for the two other surfactants. This result can further be analyzed upon examination of the viscosity data presented in Figure 6. One observes that the viscosity of the surfactant solutions decreases drastically when the EO number increases from 9 to 20 and then increases when the EO number increases again from 20 to 30, and to 40 for C9PhEO40 . Such a trend suggests a significant difference in the size and geometry of C9PhEO9 and C9PhEO20 micelles. The aggregation numbers, n, of the C9PhEOm surfactants subsequently verify this assumption. For these surfactants, n decreases drastically from about 350 to roughly 20 for EO numbers ranging from 9 to 50 (van Os et al., 1993). A
Figure 7. Influence of the surfactant nature on the rejection (P123 ) EO19PO69EO19).
closer inspection of experimental data reveals that for EO numbers less than 20, n is greater than the maximum aggregation number computed assuming spherical micelles (nmax ) 65) (Gadelle, 1995). Therefore, low EO content surfactants (i.e., EO number less than 20) can be assumed to form large nonspherical micelles, while higher EO content surfactant tend to form small spherical micelles. Bedo¨ et al. have previously suggested that a transition from oblate to spherical shape takes place at approximately 20 EOs (Bedo¨ et al., 1987). Thus, C9PhEO9 micelles are presumed to be large and oblate, while C9PhEO20, C9PhEO30, and C9PhEO40 aggregates are spherical, with the last having the smallest size and aggregation number. To conclude, a dramatic shape and size change (oblate to spherical shape) for nonylphenol micelles at an EO number of about 20 is thought to be responsible for the observed trends: the drop in viscosity followed by an increase when the EO number increases from 9 to 20 and from 20 to 40, and the lower permeate flux for C9PhEO9 when compared to C9PhEO20 and C9PhEO30. (b) Influence of the Length of the Hydrophobic Tail. The influence of the length of the hydrophobic chain was also determined by comparing the filtration of C9PhEO9 and C12PhEO10. The experiments show that the most hydrophobic surfactant exhibits the highest rejection. The rejection of C12PhEO10 is very high, around 99.9%, while the rejection of C9PhEO9 is somewhat lower, roughly 99.1%. Again, the surfactant having the lowest cmc (0.0017 wt % for C12PhEO10 versus 0.016 wt % for C9PhEO9) exhibits the highest rejection. As previously mentioned, these rejection data correspond to permeate concentrations near the cmc. The C9PhEO9 permeate fluxes are slightly lower than the C12PhEO10 fluxes. The slightly lower viscosity of C12PhEO10 aqueous solutions when compared to C9PhEO9 solutions may be responsible for the small permeate flux difference. (c) Influence of the Nature of the Surfactant. Finally, rejection of nonionic (C12PhEO10), polymeric (PEO19PPO69PEO19), and cationic (CPC) are compared in Figure 7. RC630 exhibits the best rejection (around 99.9%). It is also the surfactant with the lowest cmc (0.0017 wt %). CPC also shows high rejection (roughly 99.1%). Such rejection data correspond to permeate concentrations close to the cmc. On the other hand, the rejection of the polymeric surfactants is low, about 89%. These rejection levels correspond to permeate concentrations between 4 × cmc and 14.4 × cmc for feed concentrations ranging from 1 to 4 wt %. These filtration results indicate then that the polymer micelles leak through the membrane. Since these polymeric surfactants form very large micelles (hydrodynamic radii in
3692 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996
Figure 8. Influence of the surfactant nature on the relative permeate flux (P123 ) EO19PO69EO19).
Figure 9. Influence of the free solute concentration on surfactant (C12PhEO10) and solute (benzene) rejection. Csurf ) 1.0 wt %. i
the order of 100 Å have been reported (Wanka et al., 1990, 1994)), it is possible that these micelles are deformed (i.e., elongated) when subjected to stress and then penetrate inside the pores and flow through the membrane. Indeed, several decades ago, Tanford has suggested that macromolecular solutes can be elongated if exposed to high velocity gradients (Tanford, 1961). Such a deformation is now often reported during the transport of large flexible macromolecules (e.g., PEGs) in porous membranes (Long and Anderson, 1984; Davidson and Deen, 1988). Furthermore, unlike with conventional surfactant micelles, water molecules are believed to be located within the core of polymeric micelles (Linse and Malmsten, 1992; Hurter et al., 1993a,b; Gadelle et al., 1995b). The presence of water molecules in the micelle hydrophobic core would then increase the fluidity of the aggregates. Similar results, the same flux and a rejection of about 88-91%, were obtained with PEO17PPO60PEO17, another polymeric surfactant. An experiment with sodium dodecylbenzenesulfonate showed that the surfactant rejection is only 88.3% for Csurf ) 0.46 wt %, while the permeate i flux is about 62% that of pure water. The permeate fluxes for the nonionic, cationic and polymeric surfactants are reported in Figure 8. It appears that the flux for C12PhEO10 is slightly lower than that for CPC and PEO19PPO69PEO19. These results are again in agreement with viscosity measurements: the viscosities of the cationic and polymeric surfactant solutions are nearly identical and are also much less than that of the nonionic surfactant. (d) Influence of the Surfactant Concentration. The influence of the surfactant concentration is observed in Figures 4-8. For the nonionic and polymeric surfactants, rejection increases upon an increase in concentration and then reaches a plateau value at high surfactant concentrations. This effect is not surprising since an increase in the concentration increases the micelle concentration. On the other hand, fluxes are shown to be a decreasing function of the surfactant concentration. Permeate fluxes range from 0.75 to 0.25% of that of pure water for surfactant concentrations varying from 0.1 up to 6.0 wt %. Even though the high recirculation flux minimizes polarization, an increase in the surfactant concentration results in a greater viscosity and in an increased concentration polarization layer. The high concentration level may also facilitate surfactant deposition on/in the porous membrane and pore plugging (c.f., the discussion on the effect of the pore size). Also, the increase in the osmotic pressure difference across the membrane (related to the micelle concentrations in the retentate and permeate) reduces
the effective transmembrane pressure and, consequently, decreases the permeate fluxes. While the osmotic pressure difference (∆π) is negligible for the nonionic and polymeric surfactants for all of the range of concentrations considered here (∆π < 0.5 psi as shown by calculations and experiments (Attwood and Kayne, 1970)), it has been reported that this pressure difference can be significant at moderately high CPC concentrations (i.e., above 3 wt %) (Markels et al., 1995b) and may contribute to the slight decrease in rejection and permeate flux when the CPC initial concentration exceeds 3 wt %. Filtration of Benzene-Nonionic Surfactant Solutions. (a) Influence of the Solute. To study the effect of a solute, ultrafiltration experiments were conducted with C12PhEO10 and benzene. The data plotted in Figure 9 indicate that surfactant rejection increases with an increase in free solute concentration. The surfactant rejection ranges between 99.86 (in the absence of a solute) and 99.99% (for a free solute concentration around 675 mg/L). This increased rejection may be attributed to a change in the size and geometry of the aggregates upon solubilization. Indeed, the addition of an apolar solute into a micellar solution increases the size of the aggregates, not only via solubilization of the solute in the micelles but also through an increased aggregation number (i.e., the number of surfactant molecules in a micelle) (Rosen, 1989a; Nagarajan and Ruckenstein, 1991; Gadelle, 1995). The presence of the solubilizate also decreases the cmc, therefore decreasing the free monomer concentration (Rosen, 1989a,b; Nagarajan and Ruckenstein, 1991; Gadelle, 1995). Such phenomena can explain the trend observed in Figure 9. On the other hand, it appears that the solute rejection, Rbz, remains essentially constant around 48% as the free benzene concentration increases (note the two scales on the y-axis). Permeate fluxes were also monitored. The data in Figure 10 show that the permeate flux relative to pure water decreases from 40 to 26% upon an increase in the solute concentration. During the experiments, it was also observed that the addition of the solute increases the solution viscosity. This phenomenon coupled with the affect of the solute on the shape of the micelles could be responsible for the decline in the permeate flux. Figure 10 also illustrates the decrease in the surfactant permeate concentration that accompanies an increase in free benzene concentration. For the range of solute concentrations considered (0 to about 700 mg/L), the permeate concentration decreases from 0.9 × cmc to 0.15 × cmc. (b) Influence of the Concentration and Structure of the Surfactant. It was observed in Figures 4
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Figure 10. Influence of the free solute concentration on surfactant (C12PhEO10) permeate concentration (closed symbols) and permeate flux (open symbols). Csurf ) 1.0 wt %. i
Figure 11. Influence of the initial surfactant concentration on benzene rejection. Cbz i ) 440 mg/L.
and 7 that, with nonionic surfactants, surfactant rejection increases with an increase in surfactant concentration. Figure 11 shows that the solute rejection increases from 35 to about 70% for initial C12PhEO10 concentrations ranging from 0.5 to 3 wt %. An increase in the surfactant concentration results mostly in an increase in the micelle concentration, thereby increasing the amount of solute solubilized. Experimental results with the benzene-C9PhEO20 system showed that an increase in the C9PhEO20 initial concentration (from 1.5 to 3.0 wt %) increases benzene rejection from 45 to approximately 57.5%. Experiments with two other nonionic surfactants, C9PhEO9 and C9PhEO20, indicate that rejection of benzene with these surfactants is less than with C12PhEO10. Better results with C12PhEO10 stem from both a larger solubilization capacity (due to greater hydrophobicity) and a higher micelle rejection when compared to the other nonionic surfactants. The influence of the surfactant concentration on the permeate flux corresponds to that observed during the filtration of pure surfactant solutions: the permeate flux decreases with an increase in surfactant concentration. The data are not reported here. (c) Continuous Filtration. The continuous filtration of benzene-C12PhEO10 solutions was examined for varying surfactant concentrations to determine the feasibility and reliability of the separation process. It appears that for all of the surfactant concentrations considered, the solute permeate concentrations remain essentially constant as the feed is concentrated. This experimental result is not surprising since, as the filtration proceeds, the solute to surfactant ratio in the micellar phase remains constant. This observation is also consistent with results based on a simple mass balance around the membrane. This result also indicates that, to obtain solute permeate concentrations low
Figure 12. Benzene solubilization isotherm, Qr (%), at 25 °C. Comparison between filtration and head space gas chromatography (HSGC).
enough to permit discharge into the environment, high surfactant concentrations or more realistically multistage filtration processes are necessary. Experiments have shown, for example, that, during the filtration of the solute-micellar solution (Cbz ) 440 mg/L and i Csurf ) 0.48 wt %), the benzene permeate concentration i remains constant at 285 mg/L, while the retentate concentration continues to increase, reaching a value of 525 mg/L after about 38% of the initial volume is recovered as permeate. The surfactant concentrations follow the same pattern: the permeate concentration is nearly constant at the cmc, while the retentate surfactant concentration increases from about 280 × cmc to roughly 460 × cmc as the filtration proceeds. As already discussed, benzene permeate concentrations were observed to decrease upon an increase in the surfactant concentration. For instance, during the surf filtration of Cbz ) 3.0 wt %, the i ) 440 mg/L and Ci benzene permeate concentration reaches a constant value of about 130 mg/L. The permeate flux slightly decreases with the percentage of permeate volume recovered, thus suggesting again accumulation of micelles in the pores. In a previous paragraph, it was suggested that this accumulation may reduce permeate fluxes. Similar results were obtained during the filtration of the pure surfactant solutions (i.e., surfactant permeate concentrations nearly constant and reduced permeate fluxes as the filtration proceeds). The consistency of all the results for the filtration of organicsurfactant solutions suggests that micellar-enhanced ultrafiltration is a feasible and reliable process. Solubilization Isotherms. The filtration system may also be used to determine solubilization isotherms. These isotherms are simply obtained by plotting the solute uptake by the micelles, Qr (%), as a function of the permeate (or free) solute concentration. Qr (%) is obtained using the following equation:
Qr (%) ) 100 ×
(
Csol. - Csol. r p
)
Csurf - cmc r
) 100 ×
( ) Csol. mic
Csurf mic
(4)
The numerator corresponds to the concentration of solute solubilized in the micelles (g/L) and the denominator corresponds to the concentration of surfactant molecules in the micellar phase (g/L). Figure 12 compares the isotherm for benzene in C12PhEO10 obtained using the filtration apparatus with the isotherm obtained using an entirely independent method: head space gas chromatography (HSGC) (Gadelle, 1995). Good agreement is observed between the two isotherms.
3694 Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996
Figure 13. Benzene permeate and retentate concentration. Comparison between experiments and predictions. Csurf ) 1.0 wt i % and Cbz i ) 440 mg/L.
However, upon closer inspection of the graph, it appears that the solubilization isotherm from the filtration data is slightly higher than the one obtained with HSGC. Concentration polarization, resulting in a layer of higher surfactant concentration at the membrane surface, maybe responsible for the slight difference between the two isotherms. It should be noted that the discrepancy observed between these two isotherms cannot be attributed to solute loss through evaporation or sorption on the membrane. Indeed, as previously noted, no solute loss was detected during experiments with the benzene-water system. Additionally, Dunn et al. have also suggested that concentration polarization is responsible for a similar phenomena observed during the ultrafiltration of 4-tert-butylphenol with CPC as surfactant (Dunn et al., 1987). Solubilization isotherm data combined with results from the filtration of pure surfactant solutions can also be used to predict solute rejection and removal of organic pollutants using micellar-enhanced ultrafiltration. Figure 13 shows predictions for the continuous filtration of a benzene-C12PhEO10 solution. One can observe that prediction of the benzene permeate concentration is slightly higher than the actual observed concentration (275 versus 234 mg/L, respectively), indicating a solute rejection slightly lower than the rejection experimentally observed. Such a result would be consistent with the suggested concentration polarization phenomenon. Additionally, calculations show that, to obtain a permeate concentration of 234 mg/L, the surfactant concentration should be roughly 1.5 wt % instead of 1.0 wt %. Further calculations suggest that to obtain agreement between experimentally determined and predicted benzene permeate concentrations, the surfactant concentrations should be, on average, 45% greater than the initial concentrations. It is also observed in Figure 13 that both experimental and predicted benzene permeate concentrations are independent of the volume of permeate recovered. Such observation was reported in the previous paragraph. Figure 14 presents a comparison between benzene rejection obtained through experiments and obtained using predictions. As expected, predictions based on the HSGC solubilization isotherms slightly underestimate benzene rejection. However, these results suggest that the amount of solute removed from aqueous stream using micellar-enhanced ultrafiltration is essentially determined by the solubilization capacity of the surfactant micelles. Finally, predictions of solute removal using micellarenhanced ultrafiltration are reported in Table 2. These predictions are based on solubilization isotherms pre-
Figure 14. Benzene rejection. Comparison between experiments and predictions. Cbz i ) 440 mg/L. Table 2. Prediction of Solute Removal Using Micellar-Enhanced Ultrafiltration (Csurf ) 1.0 wt % and i Csol. ) Saturation/10; Permeate Volume Recovered, 85%) i solute removal, Rsol surfactant
benzene
toluene
chlorobz
p-xylene
C16PyCl C12PhEO10 EO17PO60EO17
89.8 77.2 52.9
95.5 93.6 76.7
95.1 84.3
86.4
sented in previous studies (Gadelle et al., 1995a,b; Gadelle, 1995) and the pure surfactant filtration results. These calculations assume that the initial solute concentration is one-tenth of the aqueous solubility, and the initial surfactant is 1.0 wt %. A permeate volume recovered of 85% was also assumed (i.e., 85% of the initial volume permeates through the membrane). Given the volume recovered, the maximum retentate surfactant concentration is still a reasonable 6.7 wt % ()1.0/ 0.15 wt %, assuming all the surfactant is rejected by the membrane). Predictions are presented for surfactants determined to exhibit high solubilization capacities for VOCs: a nonionic surfactant (C12PhEO10), a cationic surfactant (CPC), and a polymeric surfactant (EO17PO60EO17). It is calculated that the greatest removal is obtained for CPC and C12PhEO10. Removal of pollutants with the polymeric surfactants is relatively inefficient. Based on the filtration of the pure surfactant solutions, such results are not surprising. Indeed, while rejection of the pure surfactant micelles has been determined to be approximately 99.9% for the nonionic surfactant and 99.1% for the cationic surfactant, rejection is only about 90% for the polymer. Although the solute partitioning between polymeric micelle and the aqueous phase is large, micelle leakage is too significant to use a membrane-based separation process to remove the pollutants successfully. The table also indicate that the efficiency of the process increases with the hydrophobicity of the pollutants. Benzene, which has a fairly high aqueous solubility compared to the other solutes, exhibits a lesser degree of removal. On the other hand, toluene, chlorobenzene, and p-xylene appear to be good candidates for micellar-enhanced ultrafiltration. Finally, it should not be forgotten that the present predictions slightly underpredict the efficiency of the process. Conclusion The extensive aqueous surfactant filtration studies demonstrated the following: (1) Membrane pore size, recirculation flux, and transmembrane pressure all affect the filtration process. Most
Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3695
significantly, however, an increase in the membrane pore size (from 65 to 200 Å) tends to decrease both rejection and relative permeate flux. It is postulated that surfactant micelles are able to accumulate inside the pores of the 100 and 200 Å membranes and reduce the effective pore size. (2) Rejection of conventional surfactants is generally high. For hydrophobic surfactants such as CPC and C12PhEO10, rejection ranges from 99.1 to 99.9% using a 65 Å pore size membrane. Depending on the surfactant structure and concentration, the permeate fluxes range from 70 to 30% of that of pure water. Under equivalent testing conditions, polymeric surfactants exhibit a low rejection and high permeate fluxes. (3) Rejection was also shown to increase with surfactant concentration, while fluxes decrease with an increasing concentration. (4) Filtration of organic-micellar solutions indicates that the addition of a solute increases surfactant rejection and slightly decreases the permeate flux. Solute rejection increases with an increase in surfactant concentration. Continuous filtration shows that both surfactant and solute permeate concentrations remain essentially constant upon concentration of the feed (additionally, the surfactant permeate concentration remains near the cmc). (5) Comparisons between prediction based on solubilization isotherms obtained with HSGC and experimental filtration results show good agreement. The small discrepancy between the two isotherms could be explained by the formation of a gel layer at the membrane surface. These data also reveal that, for the surfactants that are retained by the membrane (e.g., C12PhEO10 or CPC), the amount of solute solubilized in the micelle mostly controls the efficiency of the process. Due to micelle leakage, removal of pollutants using polymeric surfactants is inefficient. (6) Contrary to benzene, hydrophobic solutes such as toluene, chlorobenzene, or p-xylene that exhibit large partition coefficients are good candidates for micellarenhanced filtration. Acknowledgment The authors thank the research assistant, Ying Sun, for her help during the filtration experiments. This research was funded in part by the State of Texas Energy Research in Applications Program Project No. 151 and the Separations Research Program, The University of Texas at Austin. Literature Cited Akay, G.; Wakeman, R. J. Ultrafiltration and Microfiltration of Surfactant Dispersions-An Evaluation of Published Research. Trans. Inst. Chem. Eng. 1993, 71, 411. Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Micellization of Poly(ethylene oxide)-Poly(propylene oxide)-Poly(ethylene oxide) Triblock Copolymers in Aqueous Solutions: Thermodynamics of Copolymer Association. Macromolecules 1994, 27, 2414. Attwood, D.; Kayne, S. B. Membrane Osmometry of Aqueous Micellar Solutions of Pure Nonionic Surfactants. J. Phys. Chem. 1970, 74, 3529. Bedo¨, Z.; Berecz, E.; Lakatos, I. Mass, Size and Shape of Micelles Formed in Aqueous Solutions of Ethoxylated Nonyl-Phenols. Colloid Polym. Sci. 1987, 265, 715. Christian, S. D.; Scamehorn, J. F. In Surfactant-Based Separation Processes; Scamehorn, J. F., Ed.; Dekker: New York, 1989; Vol. 33, p 3. Davidson, M. G.; Deen, W. M. Hydrodynamic Theory for the Hindered Transport of Flexible Macromolecules in Porous Membranes. J. Membr. Sci. 1988, 35, 167.
Doulia, D.; Gekas, V.; Tra¨gårdh, G. Interaction Behaviour in Ultrafiltration of Nonionic Surfactants. Part 1. Flux Behaviour. J. Membr. Sci. 1992, 69, 251. Dunn, R. O.; Scamehorn, J. F.; Christian, S. D. Use of MicellarEnhanced Ultrafiltration to Remove Dissolved Organics from Aqueous Streams. Sep. Sci. Technol. 1985, 20, 257. Dunn, R. O.; Scamehorn, J. F.; Christian, S. D. Concentration Polarization Effects in the Use of Micellar-Enhanced Ultrafiltration to Remove Dissolved Organic Pollutants from Wastewater. Sep. Sci. Technol. 1987, 22, 763. Gadelle, F. Solubilization of Aromatic Solutes in Surfactant Aggregates. Application to Micellar-Enhanced Ultrafiltration; The University of Texas: Austin, TX, 1995. Gadelle, F.; Koros, W. J.; Schechter, R. S. Solubilization Isotherms of Aromatic Solutes in Surfactant Aggregates. J. Colloid Interface Sci. 1995a, 170, 57. Gadelle, F.; Koros, W. J.; Schechter, R. S. Solubilization of Aromatic Solutes in Block Copolymers. Macromolecules 1995b, 28, 4883. Hurter, P. N.; Hatton, T. A. Solubilization of Polycyclic Aromatic Hydrocarbons by Poly(ethylene oxide-propylene oxide) Block Copolymer Micelles: Effect of Polymer Structure. Langmuir 1992, 8, 1291. Hurter, P. N.; Scheutjens, J. M. H. M.; Hatton, T. A. Molecular Modeling of Micelle Formation and Solubilization in Block Copolymer Micelles. 1. A Self-Consistent Mean-Field Lattice Theory. Macromolecules 1993a, 26, 5592. Hurter, P. N.; Scheutjens, J. M. H. M.; Hatton, T. A. Molecular Modeling of Micelle Formation and Solubilization in Block Copolymer Micelles. 2. Lattice Theory for Monomers with Internal Degrees of Freedom. Macromolecules 1993b, 26, 5030. 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. Kandori, K.; Schechter, R. S. Selection of Surfactants for MicellarEnhanced Ultrafiltration. Sep. Sci. Technol. 1990, 25, 83. Kulkarni, S. S.; Funk, E. W.; Li, N. N. In Membrane Handbook; Ho, W. S. W., Sirkar, K. K., Eds.; Van Nostrand Reinhold: New York, 1992; p 391. Lawrence, S. A.; Pilc, J. P.; Readman, J. R.; Sermon, P. A. Unexpectedly Low Extent of Adsorption of Nonionic Ethoxylated Surfactants on Alumina. J. Chem. Soc., Chem. Commun. 1987, 13, 1035. Leung, P. S. In Ultrafiltration Membranes and Applications; Cooper, A. R., Ed.; Plenum: New York, 1979; p 415. Linse, P.; Malmsten, M. Temperature-Dependent Micellization in Aqueous Block Copolymer Solutions. Macromolecules 1992, 25, 5434. Long, T. D.; Anderson, J. L. Flow-Dependent Rejection of Polystyrene from Microporous Membranes. J. Polym. Sci., Polym. Phys. Ed. 1984, 22, 1261. Mahdi, S. M.; Sko¨ld, R. O. Concentration and Temperature Effects on Aggregation, Clouding and Adsorption of a Polydisperse NonIonic Surfactant in Aqueous Solution. Colloids Surf. 1992a, 66, 203. Mahdi, S. M.; Sko¨ld, R. O. Reconcentration of a Polydisperse NonIonic Surfactant in Aqueous Solution by Adsorption Induced Ultrafiltration. Colloids Surf. 1992b, 68, 111. Markels, J. H.; Lynn, S.; Radke, C. J. Micellar-Enhanced Ultrafiltration in an Unstirred Batch Cell at Constant Flux. J. Membr. Sci. 1994, 86, 241. Markels, J. H.; Lynn, S.; Radke, C. J. Design of Micellar-Enhanced Ultrafilters. Ind. Eng. Chem. Res. 1995a, 34, 2436. Markels, J. H.; Lynn, S.; Radke, C. J. Design of Micellar-Enhanced Ultrafilters. AIChE J. 1995b, 41, 2058. Morel, G.; Graciaa, A.; Lachaise, J. J. Membr. Sci. 1991, 56, 1. Mukerjee, P.; Cardinal, J. R. Benzene Derivatives and Naphthalene Solubilized in Micelles. Polarity of Microenvironment, Location and Distribution in Micelles, and Correlation with Surface Activity in Hydrocarbon-Water Systems. J. Phys. Chem. 1978, 82, 1620. Nagarajan, R.; Ruckenstein, E. Theory of Surfactant Self-Assembly: A Predictive Molecular Thermodynamic Approach. Langmuir 1991, 7, 2934. Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley: New York, 1989a; p 108. Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley: New York, 1989b; p 170.
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Received for review December 1, 1995 Revised manuscript received May 28, 1996 Accepted May 28, 1996X IE9507181
X Abstract published in Advance ACS Abstracts, August 15, 1996.
