Relating Nanofiltration Membrane Performance to Membrane Charge

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Environ. Sci. Technol. 2000, 34, 3710-3716

Relating Nanofiltration Membrane Performance to Membrane Charge (Electrokinetic) Characteristics A M Y E . C H I L D R E S S * ,† A N D MENACHEM ELIMELECH‡ Department of Civil Engineering, Mail Stop 258, University of Nevada, Reno, Nevada 89557-0152, and Department of Chemical Engineering, Environmental Engineering Program, P.O. Box 208286, Yale University, New Haven, Connecticut 06520-8286

The performance (i.e., water flux and solute rejection) of a thin-film composite aromatic polyamide nanofiltration membrane and its relation to membrane surface charge (electrokinetic) characteristics were investigated. Membrane performance and streaming potential measurements were carried out as a function of pH for several solution chemistries, including an indifferent electrolyte, humic acid, and anionic and cationic surfactants. Performance results for the membrane were interpreted by relating the water flux and salt/ion rejection to the membrane charge characteristics. In the case of the indifferent electrolyte (NaCl), water flux and salt passage were maximal at the membrane pore isoelectric point (pH 5) primarily due to decreased electrostatic repulsion and increased pore volume (size) in the cross-linked polymer network. Ion rejection is directly related to the membrane pore charge and is attributed to co-ion electrostatic repulsion (exclusion). At low pH, negative rejection of protons was observed, demonstrating the classical behavior of a more mobile co-ion in a mixture of electrolytes (NaCl and HCl). Suwannee River humic acid was found to have very little effect on the shortterm performance of the membrane, despite its significant influence on membrane ζ-potential. Sodium dodecyl sulfate, on the other hand, had significant effects on the water flux and salt rejection. Association of the surfactant molecules (i.e., hemimicelle formation) at the membranesolution interface was analyzed in terms of membrane charge characteristics. It is proposed that the adsorbed surfactant molecules in the form of hemimicelles or a bilayer provide an additional filtration layer that results in reduced water flux and increased salt rejection.

Introduction Nanofiltration (NF) membranes are used in a wide range of drinking water, wastewater, and industrial applications (14). Separation by nanofiltration membranes occurs primarily due to size exclusion and electrostatic interactions (1, 5-8). For uncharged molecules, sieving or size exclusion is most responsible for separation; for ionic species, both sieving and electrostatic interactions are responsible for separation * Corresponding author phone: (775)784-6942; fax: (775)784-1390; e-mail: [email protected]. † University of Nevada. ‡ Yale University. 3710

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(7-9). For all applications, membrane surface and pore charge characteristics play a significant role in the transport of water and solute molecules through the membrane. Additionally, the interaction of colloids and charged macromolecules with the membrane and subsequent fouling of the membrane are dependent on the surface and pore charge properties (10). Previous experimental studies on nanofiltration membranes have investigated to some extent the role of surface and pore charge characteristics on the performance of nanofiltration membranes [e.g., Nystrom et al. (1), Bowen et al. (11), Braghetta et al. (12), Hong and Elimelech (13), Pontalier et al. (14), Blank et al. (15), Peeters et al. (8)]. However, these investigations did not systematically study membrane performance as a function of solution pH. Studying water flux and solute rejection as a function of pH is mandatory because as pH changes so do several of the system characteristics. The charge and resulting ζ-potential of the membrane depend on the pH of the system because membrane functional groups protonate and deprotonate over the pH range. The charge of the membrane is significant to membrane performance because charge affects the electrostatic repulsion between the ions or charged molecules and the membrane surface. Additionally, because of dissociation of membrane functional groups, the pH of the system may affect the “openness” of the membrane. This affects the size exclusion mechanism of removal for nanofiltration membranes. Last, pH may affect the characteristics of the molecules in the test solution. For example, at low pH, humic functional groups will protonate, and at high pH, they will deprotonate. This, in turn, will play a role in the interaction between the humic molecules and the membrane. A thorough understanding of the interrelation between membrane performance and membrane chemical characteristics is of paramount importance in membrane research. The objective of this paper is to systematically investigate the performance (i.e., water flux and solute rejection) of a thin-film composite aromatic polyamide nanofiltration membrane under a wide range of solution chemistries and to relate the performance of the membrane to its chemical and charge characteristics. Both the performance and the charge properties are evaluated at the various solution chemistries as a function of pH. A mechanistic explanation for the water flux and salt/ion rejection behaviors at the various solution chemistries is provided.

Materials and Methods Representative NF Membrane. The representative NF membrane used in this investigation was the NF-55 membrane (FilmTec Corp., Minneapolis, MN). It is a fully aromatic thinfilm composite membrane. According to the manufacturer, it has an operational pH range of 3-9 and a maximum operating temperature of 35 °C. The recommended operating pressure is approximately 70 psi (483 kPa). The membrane was received as a wet flat sheet and was stored in deionized water (NanoPure II, Dubuque, IA) at 5 °C. Solution Chemistries. Certified ACS grade sodium chloride (Fisher Scientific, Pittsburgh, PA) was used for the background inorganic electrolyte solution. Certified grade hydrochloric acid and sodium hydroxide (Fisher Scientific, Pittsburgh, PA) were used for pH adjustments. Suwannee River humic acid standard (SRHA) was received in a freeze-dried form from the International Humic Substances Society (Golden, CO). The humic acid solution was made by dissolving the freeze-dried powder in deionized 10.1021/es0008620 CCC: $19.00

 2000 American Chemical Society Published on Web 07/26/2000

water and adjusting the pH to 8.2. The molecular weight of the SRHA is between 3000 and 5000 (16), and the major functional groups are carboxylic (4.1 mequiv/g) and phenolic (2.1 mequiv/g) (17). Certified grade sodium dodecyl sulfate (SDS) (Fisher Scientific, Pittsburgh, PA) and dodecyl trimethylammonium bromide (DTAB) (Aldrich Chemicals, Milwaukee, WI) were used as model anionic and cationic surfactants, respectively. The surfactant concentration used in the experiments was 1 mM. This concentration is below the corresponding critical micelle concentrations of approximately 3.2 and 7.9 mM (in the presence of 0.01 M NaCl) for SDS and DTAB, respectively (18). Streaming Potential Measurements. Streaming potential measurements were performed with the BI-EKA (Brookhaven Instruments Corp., Holtsville, NY). This instrument utilizes silver/silver chloride electrodes to measure the streaming potential that is induced when an electrolyte solution flows across a stationary, charged membrane. ζ-potential is calculated from the measured streaming potential using the Helmholtz-Smoluchowski equation (19, 20) with the Fairbrother and Mastin substitution (21). A detailed description of the instrument, measurement procedure, and ζ-potential calculation can be found elsewhere (22, 23). The streaming potential of the NF-55 membrane was evaluated over the pH range of 3-9 for each solution chemistry. The results shown for each pH value are an average of six measurements taken at that pH. Measurements were performed at room temperature (approximately 23 °C). Membrane Performance Measurements. The performance (water flux and solute rejection) of the NF-55 membrane under the various solution chemistries was evaluated using a closed-loop bench-scale membrane test unit. In this unit, the test solution is pumped from a solution reservoir, through two membrane cells (in parallel), and back to the reservoir. The pump (Hydra-Cell, Wanner Engineering, Inc., Minneapolis, MN) is capable of providing pressures up to 1000 psig (6900 kPa) and flow rates up to 4.2 L/min. The solution reservoir sits on a magnetic stirrer and houses a stainless steel cooling coil that is connected to a refrigerated recirculating chiller (Fisher Scientific, Pittsburgh, PA). The temperature of the system is monitored with a thermometer and is kept constant at 20 °C. The membrane cells have an active area of 3.2 by 8.2 cm and operate in cross-flow mode; both the permeate and concentrate are recycled. The operating pressure [either 60 or 80 psi (414 or 552 kPa) depending on the solution chemistry] is controlled by a backpressure regulator and monitored with a pressure gauge. The flow rate across each membrane cell (1.9 L/min corresponding to a cross-flow velocity of 33 cm/s) is also monitored during the entire run. The membranes for each run are taken from storage and rinsed in flow-through mode with 15 L of deionized water. They are then placed into the membrane cells and given 45 h to stabilize with 0.01 M NaCl. For the humic acid and surfactant runs, the humic acid or surfactant is added immediately after the stabilization period. Prior to making any measurements, there are two 30-min equilibration periods during which the water flux is monitored. If the water flux is constant over these equilibration periods, the membrane performance measurements are started. Performance is first evaluated at ambient pH (pH ∼5.7). The solution is then adjusted to pH 9 by the addition of NaOH. From pH 9, the pH is incrementally dropped to pH 3 by additions of HCl, and performance is evaluated at each pH increment after a 10-min equilibration period. After pH 3, the pH is raised back to pH 5.7, and the performance is evaluated there to ensure that fouling has not occurred and that the initial water flux and salt rejection are maintained.

FIGURE 1. Comparison of ζ-potential of NF-55 membrane in the presence of an indifferent electrolyte (NaCl), Suwannee River humic acid (SRHA), and sodium dodecyl sulfate (SDS). The SRHA and SDS experiments were carried out with a background electrolyte of 0.01 M NaCl. Sample Analyses. Solution pH was monitored with a pH meter (Accumet model 15, Fisher Scientific, Pittsburgh, PA), and total ion rejection was measured with a conductance meter (YSI Co., Inc., Yellow Springs, OH). Sodium and chloride ion selective electrodes (ISE models 86-11BM and 96-127BN, respectively, Orion, Boston, MA) were used to determine individual ion rejection. For each electrode, a calibration curve was developed using three serial dilutions. The three standards (0.1, 1, and 10 mM) were selected to bracket the concentrations of the feed and permeate solutions. The appropriate amount of ionic strength adjuster (Orion Research Inc., Boston, MA) was added to each sample prior to measurement.

Results and Discussion Membrane ζ-potential in the Presence of Indifferent Electrolyte. In the presence of an indifferent electrolyte (0.01 M NaCl), the NF-55 membrane has a slightly positive ζ-potential at the lowest pH (pH 3), passes through an isoelectric point at approximately pH 3.2, and is negatively charged above pH 3.2 (Figure 1). ζ-potential curves of this shape are characteristic of amphoteric surfaces, or surfaces with both acidic and basic functional groups. Because fully aromatic thin-film composite membranes are made by the interfacial polymerization reaction of 1,3benzenediamine with trimesoyl chloride, carboxyl and amine functional groups would be expected on the membrane surface (22-26). The positive surface charge below the isoelectric point would result from the protonation of the amine functional groups (tNH2 f tNH3+), and the negative charge above the isoelectric point would result from deprotonation of the carboxyl groups (tCOOH f tCOO-). A detailed description of the mechanisms controlling the surface charge of various thin-film composite RO and NF membranes was presented in an earlier publication (23). Membrane ζ-potential in the Presence of Humic Acid. The most obvious effect of SRHA on the ζ-potential of the NF-55 membrane (Figure 1) is that it causes the membrane to be more negatively charged over the entire pH range. This indicates that the humic macromolecules readily adsorb to the membrane surface and that the negatively charged VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Schematic of adsorption of sodium dodecyl sulfate (SDS) molecules onto the membrane surface.

FIGURE 2. Comparison of ζ-potential of NF-55 membrane in the presence of sodium dodecyl sulfate (SDS) at two pH conditions. The experiments were carried out with a background electrolyte of 0.01 M NaCl. functional groups of the humics dominate the membrane surface charge. In the presence of SRHA, the NF-55 membrane displays no isoelectric point over the pH range investigated. At low pH, where the membrane and SRHA are oppositely charged, adsorption of SRHA is favorable because of both electrostatic and hydrophobic interactions. At higher pH values, where the SRHA and the membrane are similarly charged, adsorption is likely dominated by hydrophobic interactions. Discussion on the effect of SRHA on the ζ-potential of other NF (and RO) membranes can be found elsewhere (23, 27). Membrane ζ-potential in the Presence of Surfactants. The effect of SDS on the ζ-potential of the NF-55 membrane is somewhat similar to the effect of SRHA in that the negatively charged sulfate functional groups of the surfactant molecules cause the membrane to become more negatively charged (Figure 1). The adsorption characteristics of surfactants are governed by the molecular structure of the surfactant molecules (e.g., type of polar head, structure and length of hydrocarbon chain) and the characteristics of the membrane surface (e.g., charge, hydrophobicity) (28-30). As the solution pH changes, the dominant mechanism of adsorption also changes. At high pH, hydrophobic interactions between the SDS molecules and the negatively charged membrane surface result in significant SDS adsorption and a more negative membrane ζ-potential. At low pH, electrostatic attraction and possible surfactant association (i.e., hemimicelle formation) at the membrane surface have an even greater effect on ζ-potential. This is best demonstrated in Figure 2 where the higher concentrations of SDS have a much more dramatic effect on ζ-potential at pH 3 as compared to pH 8. Hemimicelle formation was initially proposed by Gaudin and Fuerstenau (31). Hemimicelles result from the surfactant ions associating with each other to remove their hydrocarbon chains from the bulk water and, hence, to reduce the free energy of the system (32). Hemimicelles, or two-dimensional surfactant aggregates, form at the solid-solution interface when the hemimicelle concentration (HMC) of the system has been exceeded. The HMC is the point at which there is a sharp increase in adsorption which reflects the transition from individual surfactant ion adsorption to surfactant association at the surface (32-34). In a study of SDS 3712

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FIGURE 4. Comparison of ζ-potential of NF-55 membrane in the presence of dodecyl trimethylammonium bromide (DTAB) at two pH conditions. The experiments were carried out with a background electrolyte of 0.01 M NaCl. adsorption onto alumina, the HMC was found to occur at approximately 0.06 mM (33). The differences in SDS adsorption at low pH and high pH (Figure 2) are shown schematically in Figure 3. At pH 3, the membrane initially has a slight positive charge (Figure 1), and adsorption occurs due to electrostatic attraction between the negatively charged polar head of the surfactant ions and the charged membrane surface (Figure 3). As their concentration increases, the surfactant ions begin associating with each other and form surfactant aggregates (or hemimicelles). From Figure 2, the HMC appears to occur around 0.01 mM SDSsat this concentration there is a dramatic change in the slope of the ζ-potential curve representing transition from individual ion adsorption to surfactant association. At pH 8, the membrane initially has a negative charge (Figure 1), and adsorption occurs due to hydrophobic interactions between the surfactant tail and the membrane surface (Figure 3). As the surfactant concentration increases, the membrane becomes slightly more negative (Figure 2) due to a larger number of adsorbed surfactant molecules (Figure 3). Figures 4 and 5 show the adsorption of DTAB, a cationic surfactant, at two pH values. At pH 3, when the membrane is initially positively charged (Figure 1), adsorption is due to hydrophobic interactions (Figure 5). Adsorption increases

FIGURE 5. Schematic of adsorption of dodecyl trimethylammonium bromide (DTAB) molecules onto the membrane surface. with increasing DTAB concentration, and the membrane becomes more positively charged. At pH 8, the membrane is initially negatively charged (Figure 1), and adsorption occurs due to electrostatic attraction (Figure 5). Hemimicelle formation (Figure 5) may start to occur at the highest concentration shown in Figure 4 (0.1 mM DTAB). Membrane Performance in the Presence of Electrolyte Solution. The NF-55 is a “loose” nanofiltration membrane. It is one of several NF membranes that is believed to have the same polymeric structure as the FT-30 membrane (FilmTec Corp., Minneapolis, MN) but has been post-treated with phosphoric acid and tannic acid to open up the pores (26). Other membranes in this category include the NF-40, NF-70, and NF-90 (FilmTec Corp., Minneapolis, MN). Nystrom et al. (1) performed pore streaming potential measurements on the NF-40 and found the pores to have a ζ-potential curve similar to the ζ-potential curve of the NF55 membrane surface (measured in the current investigation) with two exceptions. First, the magnitude of the positive and negative pore ζ-potential is less than it is for the NF-55 membrane surface. Second, and more important, the isoelectric point of the NF-40 membrane pores is approximately at pH 5. By comparing the NF-40 membrane pore isoelectric point [from Nystrom et al. (1)] with observations of water flux and electrolyte rejection for the NF-55 membrane in the current investigation (Figure 6), it appears that the charge of the membrane pores may have a significant effect on membrane flux and rejection. Because pore ζ-potential measurements were not conducted in the current investigation, this could not be verified. However, it would be expected that pore streaming potential measurements are as important, if not more important, than surface streaming potential measurements in controlling water flux and salt rejection because the NF-55 is a loose, porous NF membrane. (a) Water Flux. The flux of the NF-55 membrane (Figure 6a) is relatively constant over the entire pH range with the exception of a slight peak at pH 5 where it is expected that the NF-55 membrane pores are uncharged. The peak in flux (and corresponding dip in rejection shown in Figure 6b) may be caused by several mechanisms including (i) increased pore size due to conformational changes of the cross-linked membrane polymer structure, (ii) increased apparent water permeability due to decreased electroviscous effect, and (iii) increased net driving pressure due to decreased osmotic pressure at the membrane surface. The potential effect of each of these mechanisms on the NF-55 membrane is discussed below. The lack of electric surface charge in the pores may cause the membrane to be “looser” than when the pores are charged. In a study of UF membranes with carboxylic acid groups (35), evidence was presented that carboxylic acid segmental conformation is responsible for the pH-sensitive molecular sieve effect of copolymer membranes. The pore

FIGURE 6. Flux (a) and conductivity rejection (b) of NF-55 membrane as a function of pH. Experiments were carried out with 0.01 M NaCl at a pressure of 414 kPa, a cross-flow velocity of 33 cm/s, and a temperature of 20 °C. size of the membrane was found to be significantly reduced at higher pH values because the charged tCOO- groups adopt an extended chain conformation due to electrostatic repulsion between them. This expanded conformation reduces the pore size (or pore volume) of the membrane and thereby causes decreased flux and increased salt rejection. The pore size of the membrane changes with pH because the cross-linked polymer network will either shrink or expand (36). Extending this explanation to the NF-55 membrane, which has both carboxyl and amine functional groups, the pore size of the membrane would be reduced at both high and low pH. At high pH, the carboxyl groups would be deprotonated (tCOO-), and at low pH, the amino groups would be protonated (tNH3+). In both cases, the electrostatic repulsion between the charged groups would cause a reduction in pore size. However, at the pore isoelectric point, the pore size would not be reduced, and therefore, water flux would be at a maximum and salt rejection would be at a minimum. The experimental results in Figure 6 confirm this proposed mechanism. From pH 3 to pH 5, where the pores are gradually expanding as they are becoming less positively charged, the water flux increases. At pH 5, the water flux peaks, and salt rejection reaches a local minimum. From pH 5 to pH 8, where the pores are gradually shrinking as they are becoming more negatively charged, the water flux declines. Above pH 8, the ζ-potential of the pores begins to level off and so does the water flux. The electroviscous effect is a physical phenomenon that occurs when an electrolyte solution is pressed through a narrow capillary or pore with charged surfaces (37). This effect refers to the back-flow of counterions and water in the VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 8. Total molar rejection (combined rejection of [H+], [Na+], and [Cl-]) of NF-55 membrane as a function of pH. Experiments were carried out with 0.01 M NaCl at a pressure of 414 kPa, a cross-flow velocity of 33 cm/s, and a temperature of 20 °C.

TABLE 1. Ion Characteristics

FIGURE 7. Individual ion rejection (a) and proton rejection (b) of NF-55 membrane as a function of pH. Experiments were carried out with 0.01 M NaCl at a pressure of 414 kPa, a cross-flow velocity of 33 cm/s, and a temperature of 20 °C. double layer adjacent to the capillary pore surface due to a streaming potential that develops between the capillary ends. The electroviscous effect is least pronounced at the pore surface point of zero charge (or isoelectric point) where double-layer effects are negligible. At low pore surface charge, the permeating solution appears to exhibit a reduced viscosity when its flow rate is compared with the flow at high pore surface charge. Accordingly, flux would be at a maximum when the capillary is uncharged, or in other words, at the membrane pore isoelectric point. This phenomenon has been shown previously for ceramic (38-41) and polymeric membranes (40) and may contribute to the flux maximum shown in Figure 6a for the NF-55 membrane. Changes in osmotic pressure at the membrane surface may also be used to explain the flux curve in Figure 6a. As the pH increases from pH 5 to pH 9, the salt rejection increases (Figure 6b). Increased salt rejection, coupled with concentration polarization, results in increased osmotic pressure. Because the operating pressure of the system was kept constant, the increased osmotic pressure results in a decreased net driving pressure which, in turn, leads to decreased water flux. (b) Salt, Individual Ion, and Total Molar Rejection. The salt rejection (Figure 6b) increases as pH increases with the exception of the dip at pH 5. Rejection of loose nanofiltration membranes is due to both size exclusion and co-ion electrostatic repulsion (or charge exclusion). Generally, when dealing with loose NF membranes, size exclusion is more important in understanding the rejection of uncharged solute molecules, and electrostatic repulsion is more important for the rejection of ionic species. 3714

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ion

ionic mobility (10-8 m2 V-1 s-1)

hydrated radius (nm)

molar conductance (10-3 m2 mol-1 Ω-1)

H+ OHNa+ Cl-

36.3 20.5 5.19 7.91

0.28 0.30 0.36 0.33

35.0 19.8 5.01 7.63

In Figure 6b, the decrease in salt rejection from pH 9 to pH 5 is attributed to decreasing co-ion charge exclusion (1, 3, 26). At pH 9, where the membrane pore is more negatively charged, the chloride ion (the co-ion) experiences electrostatic repulsion from the membrane pore and will be rejected by the membrane (Figure 7a). Because electroneutrality of the permeate solution must be maintained, the Na+ will also be rejected (Figure 7a). (At pH 9, the proton and hydroxide ion concentrations, 10-9 M and 10-5 M, respectively, are negligible.) As the pH decreases, so does the electrostatic repulsion and therefore ion rejection. At pH 5, the rejection reaches a local minimum, as the lack of charge leads not only to no electrostatic repulsion but also to increased pore size and salt passage. From pH 4 to pH 3, where the membrane pores are positively charged, the sodium ion (the co-ion) will be rejected by the membrane, and because of electroneutrality, the chloride ion will also be rejected (Figure 7a). However, from pH 4 to pH 3, salt rejection (Figure 6b) decreases dramatically because of extensive proton (H+) passage through the membrane (Figure 7b), as discussed in the next subsection. When the graph of conductivity rejection as a function of pH (Figure 6b) is compared to a graph of total molar rejection (the total number of moles of all ions that are rejected) as a function of pH (Figure 8), one major difference is apparent. In the plot of salt rejection (Figure 6b), the minimum at pH 5 is shown to be a local minimum. On the other hand, in the plot of total molar rejection (Figure 8), the minimum at pH 5 is shown to be a global minimum. The high molar conductance (35 × 10-3 m2/mol-Ω) (Table 1) of the protons in the permeate contribute much more significantly to the permeate conductivity than that of sodium ions (5.0 × 10-3 m2/mol-Ω) or chloride ions (7.6 × 10-3 m2/mol-Ω). The plot of total molar rejection (Figure 8) takes into account only the concentration of ions, not their molar conductance and, therefore, shows the rejection to improve as the pH decreases from 5 to 3. (d) Negative Rejection of Protons. Negative rejection of protons is shown in Figure 7b. When the pH of the feed

FIGURE 9. Effect of Suwannee River humic acid (SRHA) on the water flux (a) and conductivity rejection (b) of NF-55 membrane as a function of pH. Experiments were carried out at a pressure of 414 kPa, a cross-flow velocity of 33 cm/s, a temperature of 20 °C, and with a background electrolyte of 0.01 M NaCl. solution is 3.0, the pH of the permeate solution is 2.7; this indicates that negative rejection occurs due to significant proton passage through the membrane. Negative rejection of protons occurs when the concentration of protons in the permeate (10-2.7 M) is greater than the concentration of protons in the feed (10-3 M); it does not imply that protons are created, just that the small volume of permeate has proportionately more protons than the much larger volume of feed. Negative rejection of protons at pH ∼3 demonstrates the classical behavior of a more mobile co-ion in a mixture of electrolytes. The electrolytes are HCl and NaCl; the coions at pH 3, where the membrane pores are positively charged, are H+ and Na+. Because protons have a higher ionic mobility and are smaller than sodium ions (Table 1), proton rejection is made worse by the presence of the less permeable sodium ions (8). Furthermore, at pH 3, the molar ratio of protons to sodium ions is 0.1. When the ratio of the more permeable ion to the less permeable ion is low, negative rejection of the more permeable ion is possible (8). Note that due to the higher passage of protons, more chloride ions will transport through the membrane to maintain electroneutrality in the permeate as shown in Figure 7a. Numerous investigators have encountered negative rejection, both experimentally [e.g., Hoffer and Kedem (42), Lonsdale et al. (43), Akred et al. (44), Vonk and Smit (45), Tsuru et al. (46)] and theoretically [e.g., Hoffer and Kedem (42), Dresner (47), Lonsdale et al. (43), Akred et al. (44), Tsuru et al. (46), Nielsen and Jonsson (48), Hagmeyer and Gimbel (3)]. Membrane Performance in the Presence of Humic Acid. The water flux and total ion rejection of the NF-55 membrane

FIGURE 10. Effect of sodium dodecyl sulfate (SDS) on the water flux (a) and conductivity rejection (b) of NF-55 membrane as a function of pH. Experiments in the presence of SDS were carried out at a pressure of 552 kPa, a cross-flow velocity of 33 cm/s, a temperature of 20 °C, and with a background electrolyte of 0.01 M NaCl. Experiments in the absence of SDS were carried out at a pressure of 414 kPa, a cross-flow velocity of 33 cm/s, a temperature of 20 °C, and with a background electrolyte of 0.01 M NaCl. does not change significantly in the presence of SRHA (Figure 9). Therefore, the presence of humics is not believed to significantly affect the short-term performance of the membrane. In a recent study of the effect of humic fouling layers on membrane performance (49), salt rejection was found to improve in the presence of organic fouling layers. In the current investigation, flux was found to decrease just slightly in the presence of SRHA. Several steps were taken to ensure that the observed effects of SRHA on the membrane performance characteristics were due to membrane surface modification rather than membrane fouling. After adding the SRHA to the solution reservoir, the humic acid was given a short time to equilibrate with the membrane surface. After the equilibration, a measurement of flux was taken to serve as the baseline flux. Then, after 30 min, a second measurement of flux was taken and checked to make sure that flux had not decreased over the 30-min period. Finally, after another 30-min equilibration period and another flux measurement to ensure that the flux still had not changed, the actual salt rejection and water flux measurements were taken. The second way of verifying that the effects of the humic on the membrane was only due to short-term surface modification was to compare performance characteristics initially (at ambient pH) with performance characteristics at the end of the experiments (when the pH is returned to approximately ambient pH) to make sure the membrane had not fouled through the duration of the VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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experiment. In this manner, it is assured that any changes in performance at different pH values are due to surface modification of the membrane at that pH. Membrane Performance in the Presence of Surfactants. Unlike the SRHA, the SDS had significant effects on the flux and rejection of the NF-55 membrane (Figure 10). Advective transport of the SDS may result in a secondary filtration layer of SDS molecules on the membrane surface and cause decreased flux and increased rejection of the membrane. Additionally, the anionic surfactant alters the charge and hydrophobicity of the membrane. Because of significant flux loss through the membrane, the operating pressure of the NF-55 membrane was immediately increased from 60 to 80 psi (414-552 kPa) upon addition of SDS to the system. This contributes to the increased flux at high pH (Figure 10a). The effect of the anionic surfactant is most apparent at low pH because at low pH the SDS molecules will associate and form hemimicelles as discussed earlier. When the negatively charged headgroups of the surfactant molecules are aligned toward the membrane surface, the hydrocarbon chains of the surfactant will be dangling in the bulk solution. Because this is not a thermodynamically favorable formation for the hydrocarbon chains, the hydrocarbon chains of the surfactant ions on the surface will associate with the hydrocarbon chains of the surfactant ions in the bulk solution to form hemimicelles or a bilayer of SDS molecules (32). The result is substantially decreased water flux and increased salt rejection (Figure 10). This situation would not be as likely to occur at high pH because the membrane is negatively charged, and the hydrocarbon chains will be attracted to the membrane surface leading to adsorption of individual molecules (23). Therefore, only a minimal effect of the surfactant ions is observed at high pH (Figure 10).

Acknowledgments This research was supported in part by the U.S. Bureau of Reclamation, Water Treatment and Engineering Research Group. The findings reported in this paper do not necessarily reflect the views of this agency, and no official endorsement should be inferred. We would also like to acknowledge Shivaji Deshmukh for his contribution to the streaming potential and fouling experiments.

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Received for review January 3, 2000. Revised manuscript received June 7, 2000. Accepted June 12, 2000. ES0008620