Environ. Sci. Technol. 2000, 34, 5043-5050
Humic Acid Fouling during Ultrafiltration WEI YUAN AND ANDREW L. ZYDNEY* Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716
Recent studies have shown that natural organic matter (e.g., humic and fulvic acids) is a major foulant during ultrafiltration of surface water. The objective of this study was to develop a more complete understanding of the mechanisms governing humic acid fouling, including the effects of humic acid adsorption, concentration polarization, and aggregate deposition on the rate and extent of fouling. Data were obtained with Aldrich and Suwannee River humic acids using ultrafiltration membranes with a broad range of molecular weight cutoffs. Fouled membranes were also examined using streaming potential and contact angle measurements. The extent of flux decline was greatest for the largest molecular weight cutoff membranes due to the greater relative hydraulic resistance of the humic acid deposit formed on the surface of these membranes. This humic acid deposit reduced the apparent zeta potential and increased the membrane contact angle. Simple static adsorption and concentration polarization caused relatively little flux decline. Humic acid aggregates had a significant effect on fouling only for the larger molecular weight cutoff membranes. The rate and extent of humic acid fouling increased at low pH, high ionic strength, and in the presence of calcium due to changes in intermolecular electrostatic interactions. These results provide important insights into the mechanisms of humic acid fouling during ultrafiltration.
Introduction Membrane processes are increasingly used in drinking water treatment to meet more stringent water quality regulations (1-3). Microfiltration (MF) allows the removal of particles, turbidity, and microorganisms (bacteria, protozoans, algae), while ultrafiltration (UF) can also remove a variety of waterborne viruses and much of the dissolved organic matter. One of the critical issues in the successful development of these membrane processes is fouling, which arises from specific interactions between the membrane and various components present in the raw water. Several studies of UF and MF of surface, lake, and river waters have identified naturally occurring organic matter (NOM) as one of the major foulants during these membrane processes (4-7). Although the qualitative effects of natural organic matter on the performance of membrane systems are now reasonably well understood, there is still considerable disagreement over the actual mechanisms involved in the fouling process. For example, Laıˆne´ et al. (8) examined the flux decline during filtration of an untreated surface water through different molecular weight cutoff (MWCO) ultrafiltration membranes. Corresponding author phone: (302)831-2399; fax: (302)831-1048; e-mail:
[email protected]. 10.1021/es0012366 CCC: $19.00 Published on Web 10/21/2000
2000 American Chemical Society
The flux decline was greatest with the largest pore size membranes, with the hydrophilic membranes showing reduced levels of fouling compared to more hydrophobic surfaces. Most of the flux decline was found to be reversible, which the authors attributed to the formation of an organic layer on the membrane surface. Similar results were reported by Cho et al. (9) for the fouling of ultrafiltration membranes by a lake surface water. Fouling was negligible for very small pore (nanofiltration) membranes, with most of the flux decline during UF attributed to the reversible (weak) adsorption of natural organic matter to the membrane surface along with a small contribution from gel layer formation. Concentration polarization, the accumulation of a boundary layer of retained materials, was only important during filtration with the largest pore size (10 kD) polysulfone membrane. Crozes et al. (10) also observed that most of the fouling during river water filtration was reversible. A small amount of irreversible fouling was attributed to the adsorption of natural organic matter within the membrane pores. In sharp contrast to these results, Kulovaara et al. (11) reported that surface water fouling of a 50 kD polysulfone membrane was largely irreversible. The authors attributed this irreversible fouling to the formation of a strongly adherent deposit on the membrane surface. Maartens et al. (12) found that ultrafiltration of a naturally occurring brown water caused the operational flux to decrease to less than 65% of the original flux after 300 min of filtration, with most of the flux decline attributed to irreversible adsorption of humic substances. Scha¨fer et al. (13) also reported significant irreversible fouling during nanofiltration of surface waters containing natural organic matter, with the extent of fouling being greatest in the presence of high concentrations of calcium. To understand the results obtained during filtration of natural water sources, many investigators have examined the fouling characteristics of purified humic and fulvic acids, the two major classes of soluble natural organic matter found in aqueous systems. Nystro¨m et al. (14) examined the fouling behavior of a commercial humic acid solution obtained from Aldrich and found essentially no flux decline on either a polysulfone (50 kD nominal molecular weight cutoff) or zirconium oxide (10 kD) membrane. Humic acid retention was greater than 90% for both membranes. Kulovaara et al. (11) also found relatively little flux decline ( 0.99) allowing the apparent zeta potential to be evaluated directly from eq 2. Several studies have shown that eq 2 provides useful information on the charge characteristics of ultrafiltration membranes even though the Helmholtz-Smoluchowski equation neglects the effects of surface conductance and overlapping double layers (24-25). All results in this study are reported in terms of apparent zeta potential data as calculated from eq 2. Contact angle measurements were performed with a contact angle goniometer (Rame-Hart, CA) using the sessile drop technique. The membrane was mounted on a glass support, and a 20 µL drop of deionized water was then placed on the membrane surface using a microsyringe. The membrane (clean or fouled) was first dried at room temperature for 24 h. The surface of the drop was examined using an optical-electrical enlarging system, with the digitized image downloaded to a Dell microcomputer to evaluate the contact angle. Each measurement was completed within 2 min to minimize water evaporation effects. Contact angle measurements were performed in triplicate using separate pieces of membrane. Filtration Experiments. Humic acid filtration experiments were conducted in a 25 mm stirred cell (Model 8010, Amicon
Corp., Beverly, MA) connected to an air-pressurized 2 L solution reservoir. The stirred cell and reservoir were initially filled with filtered DI water. The water flux was measured as a function of time at a constant pressure (usually 69 kPa) until a quasi-steady flux was attained (usually within 20 min). The stirred cell was then emptied and refilled with a desired humic acid solution. The system was repressurized, and the stirring speed was set to 600 rpm using a Strobotac type 1531-AB strobe light (General Radio Co., Concord, MA). The filtrate flow rate was measured by timed collection with the filtrate mass determined using an analytical balance. Filtrate samples were collected periodically for subsequent concentration analysis. At the end of the filtration experiment, the stirred cell was emptied, the membrane was gently rinsed with DI water to remove any labile humic acids, and the stirred cell and reservoir were refilled with fresh DI water. The water flux was then measured as a function of applied pressure to determine the hydraulic resistance of the fouled membrane. All filtration experiments were conducted at room temperature (22 ( 2 °C). Concentration polarization effects were examined by measuring the flux of a humic acid solution as a function of the applied pressure from 21 to 139 kPa (3-20 psi). Data were obtained using an initially clean membrane, with the flux measured for a period of approximately 1 min at a given pressure to minimize fouling. The membrane was then cleaned with a 0.1 N NaOH solution for 10 min to restore the clean membrane resistance before measuring the flux at another pressure. Similar experiments were performed with the fouled membranes immediately after completing the constant pressure ultrafiltration. In this case, the data were obtained first with decreasing pressure (from 69 to 21 kPa) and then with increasing pressure (from 69 to 139 kPa). Each flux measurement was completed within 2 min, with the pressure rapidly reset (without cleaning the membrane) to evaluate the flux over the full pressure range.
FIGURE 1. Filtrate flux and rejection coefficients during filtration of a 2 mg/L Aldrich humic acid solution through different molecular weight cutoff membranes at pH 7 and 69 kPa (10 psi).
Results and Discussions Humic Acid Fouling during Ultrafiltration. Experimental data for the filtrate flux and overall humic acid rejection coefficient during the constant pressure (69 kPa) filtration of 2 mg/L solutions of the Aldrich humic acid through the different membranes are shown in Figure 1. The initial filtrate flux increased monotonically with increasing membrane pore size, ranging from 7.8 × 10-5 m/s for the 30 kD membrane to 1.3 × 10-3 m/s for the 0.16 µm membrane. The flux after 60 min of filtration had declined by more than 90% for the 0.16 µm membrane but only by 65% for the 300 kD membrane and 25% for the 30 kD membrane. At long times, the fluxes were nearly identical for the 30 kD, 50 kD, and 0.16 µm membranes (J ) 5.0-5.2 × 10-5 m/s), with these values about 50% smaller than those obtained with the 100 and 300 kD membranes. The lower panel in Figure 1 shows the results for the overall humic acid rejection coefficients
R)1-
Cfiltrate Cfeed
(3)
where the total humic acid concentration in the filtrate and feed solutions was evaluated spectrophotometrically. The 30 kD membrane was almost completely retentive to the Aldrich HA, with R > 0.96 throughout the filtration. The initial rejection coefficient for the 100 kD membrane was about 0.4, which is consistent with the presence of a significant amount of low molecular weight components in the Aldrich HA (16). The rejection coefficients for the 100 kD, 300 kD, and 0.16 µm membranes increased rapidly during the filtration, approaching values between 0.83 and 0.92 at long
FIGURE 2. Filtrate flux and rejection coefficients during filtration of a 2 mg/L Suwannee River humic acid solution through different molecular weight cutoff membranes at pH 7 and 69 kPa (10 psi). times. The 0.16 µm membrane had the smallest rejection coefficients over the first 50 min, but the rejection coefficients at longer times were somewhat greater than those obtained with the much smaller pore 100 kD and 300 kD membranes. Figure 2 shows the corresponding filtrate flux and rejection coefficient data for the Suwannee River HA. The Suwannee VOL. 34, NO. 23, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Effect of Humic Acid Adsorption and Filtration on the Filtrate Flux and Deposit Resistance for the Different Molecular Weight Cut-Off Membranes at pH 7 J / J0 membrane
initial flux, J0 (×105 m/s)
30 kD 100 kD 300 kD 0.16 µm 30 kD 100 kD 300 kD 0.16 µm
7.4 ( 0.2 21.5 ( 0.5 32.7 ( 0.7 128.4 ( 1.1 7.1 ( 0.3 20.3 ( 0.4 31.2 ( 0.5 127.8 ( 0.3
humic acid Aldrich
Suwannee River
DI water (after adsorption)
end of HA filtration
DI water (after filtration)
Rdeposit (×1011 m-1)
0.97 ( 0.01 0.95 ( 0.01 0.94 ( 0.01 0.93 ( 0.02 0.98 ( 0.01 0.96 ( 0.01 0.96 ( 0.01 0.95 ( 0.01
0.66 ( 0.03 0.55 ( 0.01 0.31 ( 0.02 0.05 ( 0.02 0.74 ( 0.02 0.49 ( 0.03 0.44 ( 0.02 0.34 ( 0.04
0.75 ( 0.02 0.50 ( 0.02 0.37 ( 0.01 0.10 ( 0.02 0.82 ( 0.02 0.54 ( 0.01 0.47 ( 0.02 0.38 ( 0.02
2.7 ( 0.4 2.8 ( 0.1 3.9 ( 0.1 5.1 ( 0.2 2.2 ( 0.1 2.8 ( 0.2 2.6 ( 0.1 1.2 ( 0.2
River HA has a higher percentage of functional groups (i.e., carboxylic acids), lower aromaticity, and smaller average molecular weight than the Aldrich HA (16, 21, 26). The filtrate flux data for the 30 kD membrane were similar for the two humic acids, with the flux declining by 26% for the Suwannee River HA compared to 34% for the Aldrich HA over the 2 h filtration. The flux decline for the other UF membranes was somewhat less pronounced for the Suwannee River HA. By far the greatest difference was seen with the 0.16 µm membrane, with the flux for the Suwannee River HA at the end of the 2 h filtration (J ) 4.2 × 10-4 m/s) being more than a factor of 8 larger than that obtained with the Aldrich HA. The data for the overall rejection coefficients also show very clear differences. The initial rejection coefficient for the Suwannee River HA through the 30 kD membrane was only R ) 0.70, consistent with the smaller average molecular weight of the Suwannee River HA (16). The humic acid rejection coefficients for the 30, 50, and 100 kD membranes increased throughout the filtration, with the final values (after 2 h of filtration) being greater than 0.95. Interestingly, the 100 kD membrane showed a greater rejection of the Suwannee River HA at long filtration times (R ) 0.96) than that seen previously for the Aldrich HA. Suwannee River HA rejection by the 0.16 µm membrane was negligible over the first 25 min of filtration, but R slowly increased to a value above 0.5 at long filtration times. These rejection coefficients remained well below those seen previously for the Aldrich HA. The surface properties of the fouled membranes also showed significant differences between the two humic acids. The apparent zeta potential of the clean 30 kD membrane at pH 7 was -16.0 ( 0.4 mV. This negative charge is due to the presence of a small number of negatively charged ionizable groups (e.g., carboxylic or phenolic acids) on the base polymer and the preferential adsorption of negative chloride ions from the solution (27). Humic acid filtration for 2 h caused a reduction in the apparent zeta potential to -10.6 ( 0.3 mV for the Aldrich HA and to -13.1 ( 0.2 mV for the Suwannee River HA. The reduction in the net negative charge on the membrane caused by HA filtration could be due to several factors. First, the charge density at the surface of the deposited humic acids may be smaller than that of the membrane pores. Similar behavior has been reported by Amy et al. (28, 29) after fouling with natural organic matter and by Jucker et al. (18) after humic acid adsorption. Second, the humic acid deposit will change the location of the plane of shear, which could lower the value of the apparent zeta potential (18, 30). The more negative apparent zeta potential seen after fouling with the Suwannee River HA is likely due to the slightly lower degree of fouling seen with the Suwannee River HA in combination with the presence of more negatively charged (e.g., carboxylic acid) groups in this humic acid. Similar behavior was also seen with the larger pore size membranes. For example, the apparent zeta potential of the 300 kD membrane increased from -21.8 mV to -12.1 mV 5046
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after filtration of the Aldrich HA compared to only -16.4 mV after filtration of the Suwannee River HA. Humic acid fouling also caused a significant increase in the membrane contact angle. The contact angle for the 30 kD membrane increased from 44 ( 4° to 74 ( 3° after filtration of the Aldrich HA compared to 53 ( 2° after filtration of the Suwannee River HA. The greater increase in membrane hydrophobicity after filtration of the Aldrich HA is consistent with the greater hydrophobicity of this soil-based humic acid. Contributions to Flux Decline. The flux decline observed during the humic acid filtration is due to the combined effects of humic acid adsorption on or within the membrane pores, humic acid deposition during filtration, and humic acid concentration polarization. To determine the relative importance of these effects, the flux of prefiltered DI water was evaluated for the clean membrane, for the same membrane after static adsorption in a 2 mg/L humic acid solution for 24 h to achieve equilibrium adsorption and then for the same membrane after a 2 h filtration (performed at 69 kPa using a 2 mg/L humic acid solution). All of the water flux data were taken at a pressure of 69 kPa, with the flux evaluated for a minimum of 10 min to ensure steady-state behavior. The results for the 30, 100, and 300 kD UF membranes and for the 0.16 µm MF membrane are summarized in Table 1. In each case, the adsorption and filtration experiments were performed using at least two (separate) membranes, with the results reported as the mean plus/minus the standard deviation. Humic acid adsorption caused a small (less than 7%) decline in the water flux, with the magnitude of this flux decline being somewhat greater for the Aldrich HA and the larger molecular weight cutoff membranes. The relatively small degree of humic acid adsorption is likely due to the electrostatic repulsion between the negatively charged humic acids and the negatively charged membranes at pH 7. The greater effect of adsorption in the larger pore size membranes is similar to that reported previously for protein adsorption (31) and is likely due to the greater accessibility of the membrane pores to the humic acid molecules. The water flux after the humic acid filtration was well below the clean membrane flux, reflecting the large additional hydraulic resistance to flow provided by the humic acid deposit. Scanning electron micrographs of the upper surface of the fouled membranes (given in ref 16 for the 0.16 µm membranes) showed the presence of a very distinct humic acid layer on the membrane surface. In contrast to the water flux data, humic acid adsorption caused a significant change in the apparent zeta potential. For example, the apparent zeta potential of the 30 kD membrane changed from -16.2 ( 0.2 mV to -12.0 ( 0.3 mV after adsorption of the Aldrich HA at pH 7, with only a small additional change to -10.6 ( 0.3 mV after humic acid filtration. Similarly, the apparent zeta potential of the 300 kD membrane was -12.8 mV after humic acid adsorption and -12.1 mV after humic acid filtration compared to -21.8 mV
for the clean membrane. Contact angle data for the preadsorbed 30 kD membranes showed only a slight increase from 44° for the clean membrane to 53°, with a much large increase to 74° occurring after the humic acid filtration. These results are completely consistent with the physical picture that humic acid adsorption occurs in a thin layer throughout the internal membrane pore structure, while humic acid deposition during fouling occurs primarily on the upper surface of the membrane. The thin layer of adsorbed humic acids within the membrane pores has a large effect on the apparent zeta potential by altering the exposed surface charge groups, while the humic acid deposit on the upper surface of the membrane has a much greater effect on the contact angle. The final column in Table 1 shows the hydraulic resistance of the humic acid deposit, which was evaluated as the difference between the resistance of the fouled membrane (determined from the water flux data measured after the filtration) and that of the humic acid preadsorbed membrane. Although the percentage flux decline was much greater for the larger pore size membranes, the hydraulic resistance of the humic acid deposits was fairly similar. The hydraulic resistance for the deposits formed by the Aldrich HA were greater than those for the Suwannee River HA, with the difference being as much as a factor of 4 for the 0.16 µm MF membrane. The lower resistance of the Suwannee River HA deposit is likely due to a smaller amount of humic acid deposition in combination with a more open packing within the deposit, with the latter effect consistent with the greater electrostatic repulsion between the more negatively charged Suwannee River humic acids. The middle column in Table 1 shows the flux measured with the humic acid solution at the end of the 2 h filtration, i.e., just before measuring the DI water flux after the filtration. The final filtrate flux with the humic acid solution was in each case somewhat less than the flux measured with the DI water, with this difference arising from humic acid concentration polarization (i.e. boundary layer effects) and/or the removal of some weakly bound humic acids from the membrane during the rinsing step. The relative contribution of concentration polarization to the total flux decline was estimated from the difference in flux evaluated with DI water (after the filtration) to that evaluated with the humic acid solution at the end of the filtration experiment. Concentration polarization accounted for about 35% of the flux decline during filtration of both the Aldrich and Suwannee River humic acids through the 30 kD membrane, but was the cause of less than 15% of the flux decline for the 100 kD membrane and less than 10% for the 300 kD and 0.16 µm membranes. The different contributions of concentration polarization, humic acid adsorption, and humic acid deposition on the overall flux decline for the different MWCO membranes help explain some of the apparent discrepancies in previous studies of humic acid fouling. The largely irreversible humic acid deposition dominates the fouling behavior of the higher molecular weight cutoff membranes. Concentration polarization effects, which are completely reversible, become important for the smaller pore size membranes due to the large reduction in the effective pressure driving force caused by humic acid retention. Concentration polarization effects will also become important for more highly concentrated humic acid solutions due to the concentration-dependence of the osmotic pressure. The effects of concentration polarization on the filtrate flux are examined in more detail in Figure 3. The circles show data for the water flux through the clean and fouled 300 kD membranes, while the squares show the flux obtained through these membranes with a 10 mg/L Aldrich HA solution. The water flux through the clean membrane varied approximately linearly with applied pressure, consistent with a constant membrane permeability. In contrast, the water
FIGURE 3. Filtrate flux as a function of transmembrane pressure for 10 mg/L solutions of the Aldrich humic acid through clean and fouled 300 kD membranes.
FIGURE 4. Humic acid rejection coefficients as a function of the filtrate flux during filtration of a 10 mg/L Aldrich humic acid solution through clean and fouled 300 kD membranes. flux through the fouled membranes shows a small curvature, corresponding to an increase in hydraulic resistance with increasing applied pressure, reflecting a slight compressibility of the deposit. The difference between the water and humic acid flux increases with increasing pressure but remains less than 30% even at the highest pressure of 139 kPa (20 psi). This behavior is completely consistent with predictions of the classical concentration polarization model. The humic acid concentration at the membrane surface increases with increasing pressure (i.e., increasing filtrate flux), causing a corresponding increase in the hydraulic resistance and/or osmotic pressure of the retained humic molecules. Figure 4 shows data for the humic acid rejection coefficients evaluated during the above experiments for several humic acid fractions from the Aldrich HA. The fractional rejection coefficients were evaluated using the ultrafiltration fractionation technique to determine the concentration of humic acids in each molecular weight range. The clean membrane shows minimal rejection (less than 10%) of humic acids with molecular weight between 30 and 50 kD at 69 kPa, but there is significant rejection of the larger molecular weight humic acids. The fouled membrane shows much greater humic acid rejection, with more than 70% rejection of even the 30-50 kD humic acid fraction at the lowest flux due to the sieving provided by the humic acid deposit on the membrane surface. The humic acid rejection coefficients for the clean and fouled membranes decrease with increasing filtrate flux due to the effects of concentration polarization. These polarization effects are more pronounced for the larger molecular weight fractions due to the smaller mass transfer coefficient and greater rejection of the large macromolecules. The nature of the humic acid deposit was further examined by subjecting the fouled membrane to a series of cleaning VOL. 34, NO. 23, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Hydraulic Permeability of the 30 and 300 KD Membranes Fresh, after Fouling (pH 7), after Physical Cleaning, and after Chemical Cleaning hydraulic permeability (×10-12 m) membrane
30 kD
300 kD
clean fouled physically cleaned chemically cleaned
1.05 0.77 0.96 0.97
4.72 1.66 2.30 3.44
steps. The membranes were first cleaned physically by carefully wiping the surface with several paper towels. Chemical cleaning was then done by soaking the same membranes in 0.1 N NaOH solution for 30 min. The membrane hydraulic permeability was evaluated both before and after each cleaning step using eq 1. The results are summarized in Table 2. Physical cleaning restored the permeability of the 30 kD membrane to more than 90% of the original value, indicating that the bulk of the fouling was localized on the upper surface of the 30 kD membrane. In contrast, physical cleaning of the 300 kD membrane only restored the permeability to about 49% of its original value. Chemical cleaning caused only a small further increase in the permeability, demonstrating that the remaining internal fouling on the larger pore size 300 kD membrane was largely irreversible even upon exposure to strong base. These results were confirmed using the streaming potential measurements. The apparent zeta potential value for the chemically cleaned 30 kD membrane was -15.4 mV, only slightly more than the -16.0 mV for the clean membrane. In contrast, the apparent zeta potential of the 300 kD membrane was -14.6 mV after cleaning compared to a value of -21.8 mV for the clean membrane. Effect of Prefiltration on Humic Acid Fouling. To understand the effects of different molecular weight components on humic acid fouling, a series of experiments were performed using humic acid solutions that had first been prefiltered through either a 100 kD or 0.16 µm membrane. The flux decline data for these prefiltered humic acid solutions through a 30 kD (top panel) and 300 kD (bottom panel) membrane are shown in Figure 5. Prefiltration significantly reduced the rate of flux decline through the 300 kD membrane, with the greatest effect seen when the prefiltration was performed using the 100 kD membrane. The flux data for the 0.16 µm-prefiltered Aldrich HA through the 300 kD membrane were very similar to those observed in Figure 2 with the standard (unfiltered) Suwanee River HA, suggesting that the primary difference in fouling behavior for these humic acids is due to the presence of some very large molecular weight species in the Aldrich HA. In contrast to the results seen with the 300 kD membrane, prefiltration of the Aldrich humic acid through a 0.16 µm membrane had no measurable affect on the filtrate flux through the 30 kD membrane. A small improvement in flux was seen when prefiltration was done using a 100 kD membrane, although the effect was much less pronounced than that seen with the 300 kD membrane. Thus, large humic acid aggregates/ particles have only a relatively small effect on fouling during ultrafiltration, in contrast to the very large effect seen previously with MF membranes (16). Effect of Solution Environment on Humic Acid Fouling. The effects of solution pH, salt concentration, and calcium on the flux decline during filtration of the Aldrich HA through a 30 kD membrane are examined in Figure 6. In each case, the pH or salt was adjusted immediately prior to the filtration experiment. The flux decline was much more pronounced at pH 3 and in the presence of 100 mM NaCl, with the flux at the end of the 2 h filtration varying from a high of 5.2 × 5048
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FIGURE 5. Filtrate flux during filtration of 2 mg/L Aldrich humic acid solutions (standard and prefiltered) through 30 kD (top panel) and 300 kD (bottom panel) membranes at pH 7 and 69 kPa (10 psi).
FIGURE 6. Filtrate flux and rejection coefficients during filtration of 2 mg/L solution of the Aldrich HA at different pH and salt concentrations through 30 kD membranes at 69 kPa (10 psi). 10-5 m/s in DI water at pH 7 to 2.6 × 10-5 m/s in the presence of 100 mM NaCl. Addition of the divalent calcium ion also caused a significant increase in the rate of flux decline. In contrast, the humic acid rejection coefficients were greatest in the DI water which was likely due to the strong electrostatic exclusion of the negatively charged humic acids from the
TABLE 3. Effect of Solution Environment on the Adsorption and Fouling for the Aldrich Humic Acid on 30 kD Membranes J/J0
solution conditions humic acid/ membrane Aldrich 30 kD
a
J0
(×10-5 m/s)
pH
I [NaCl] mM
7.4 ( 0.2 7.5 ( 0.1 7.2 7.4 ( 0.2 7.6 7.3 ( 0.1
7.0 3.0 7.0 7.0 7.0 7.0
a a 3 100 a a
I [CaCl2] mM
DI water (after adsorption)
end of HA filtration
DI water (after filtration)
Rdeposit (×1011 m-1)
a a a a 3 30
0.97 ( 0.01 0.73 ( 0.02 0.97 0.94 ( 0.01 0.93 0.81 ( 0.02
0.66 ( 0.03 0.41 ( 0.02 0.64 0.37 ( 0.02 0.45 0.28 ( 0.02
0.75 ( 0.02 0.47 ( 0.01 0.29 0.43 ( 0.02 0.50 0.34 ( 0.01
2.7 ( 0.4 7.0 ( 0.5 3.4 12.2 ( 0.6 8.5 16.0 ( 0.4
Indicates no additional salt added.
negatively charged pores. Increasing the solution ionic strength reduced the humic acid rejection by increasing the electrostatic shielding, while lowering the pH reduced the rejection by decreasing the net charge on the humic acids. The relative contributions to the flux decline in the different solution environments are examined in Table 3. The effects of humic acid adsorption were quite pronounced at pH 3, with the DI water flux (at pH 7) evaluated after exposure to the humic acid solution decreasing by more than 25% from the clean membrane flux. However, the filtrate flux through a clean 30 kD membrane and one which was preadsorbed overnight in a humic acid solution at pH 3 prior to use were nearly identical after a 2 h humic acid filtration at pH 3. Thus, the initial exposure to humic acids by static adsorption had minimal effect on the long-term flux behavior. In all cases, the large majority of the flux decline was caused by the hydraulic resistance of the humic acid deposit. The DI water flux after the filtration was only 5-10% larger than the flux measured with the humic acid solution, demonstrating that concentration polarization effects were small. The hydraulic resistance of the humic acid deposits formed at pH 3, in the presence of 100 mM NaCl, and in the presence of 10 mM CaCl2 were all much greater than the resistance of the deposit formed by filtration of a humic acid solution in DI water at pH 7 (Table 1). Data for the hydraulic permeability of a single fouled membrane with different salt solutions showed an increase in resistance at high salt, which was likely due to the tighter packing of the humic acids within the deposit associated with the greater electrostatic shielding. However, this effect was considerably smaller than the differences seen in Table 3, suggesting that there is also a greater mass of humic acid molecules/aggregates deposited during the filtration at high salt or at pH 3. Thus, the hydraulic resistance of the humic acid deposit was a fairly strong function of pH and salt concentration, even though it was largely unaffected by the membrane pore size. In contrast to the results in Table 3, the surface characteristics of the fouled membranes were only weakly affected by the solution conditions used during the humic acid filtration. For example, the contact angle of the fouled 30 kD membranes ranged from 74 ( 4° after filtration of the pH 7 humic acid solution in DI water to 80 ( 3° after filtration of the pH 3 solution and 66 ( 3° after filtration at pH 7 in the presence of 100 mM NaCl. The very similar values of the contact angle, despite the large differences in the resistances of the deposits, suggest that the upper surface of the deposits have surface characteristics which are largely independent of the solution conditions. The apparent zeta potential of the fouled 30 kD membranes, all evaluated using 10 mM NaCl at pH 7, ranged from -8.9 mV to -10.2 mV in the presence/absence of NaCl. The apparent zeta potential for the membrane fouled at pH 3 was somewhat less negative (-6.4 ( 0.4 mV), which may be due to the greater extent of humic acid adsorption within the membrane pores at this low pH. This is consistent with the large amount of humic acid adsorption seen under these conditions.
The ionic strength (0.1 M NaCl), pH, and calcium concentration (30 mM) examined in this study are much more extreme than those typically encountered in surface water treatment, although such conditions can be encountered in the treatment of certain groundwaters with high mineral content or significant saltwater intrusion. However, the use of these conditions highlights the effects of different solution conditions on the fouling behavior and the relative contributions of humic acid adsorption, deposition, and concentration polarization to the overall flux decline.
Acknowledgments The authors would like to acknowledge Filtron Technology Corp. for providing the polyethersulfone membranes used in this study and Paul Laibinis’ group at MIT for use of the contact angle goniometer.
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Received for review May 9, 2000. Revised manuscript received September 6, 2000. Accepted September 13, 2000. ES0012366
