Selective adsorption of natural organic foulants by polysulfone colloids: Effect on ultrafiltration fouling

Journal of Membrane Science 281 (2006) 472–479

Selective adsorption of natural organic foulants by polysulfone colloids: Effect on ultrafiltration fouling Liang Chee Koh, Won-Young Ahn, Mark M. Clark ∗ Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, 205 North Mathews Avenue, 3207 Newmark CE Lab, MC, Urbana, IL 61801, USA Received 1 December 2005; received in revised form 11 April 2006; accepted 17 April 2006 Available online 29 April 2006

Abstract The problem of membrane fouling by natural organic matter (NOM) is commonly experienced in membrane filtration of natural waters for drinking water treatment. This study continues exploration of a novel pretreatment to reduce membrane fouling using a colloidal polysulfone (PSf) adsorbent that removes compounds that would normally adsorb on membranes [M.M. Clark, W.Y. Ahn, X. Li, N. Sternisha, R.L. Riley, Formation of polysulfone colloids for adsorption of natural organic foulants, Langmuir 21 (2005) 727–7213.]. Pretreatment of high-fouling lake water samples yields continued decrease in adsorptive fouling of 20-kDa molecular weight cut-off ultrafilters as the colloid concentration is increased incrementally from 5 to 100 mg/L; it is also shown that kinetics of adsorption of the critical NOM foulant by PSf colloids is quite fast in the water treatment context, occurring within minutes of contact with the water samples. Supporting previous work, measurements of dissolved organic carbon and apparent molecular weight distribution reveal that only a small fraction of the NOM is removed by PSf colloids, hence, only a small fraction of NOM causes fouling. High-pressure size-exclusion chromatography suggests that the foulant has an apparent molecular weight ranging from 20 to 200 kDa. In addition, study of the foulants adsorbed on the colloids using Fourier-transform infrared spectrometry and energy-dispersive spectroscopy indicates the presence of trace amounts of silicon, indicating a role of silicon in NOM adsorption. © 2006 Elsevier B.V. All rights reserved. Keywords: Adsorption; Membrane fouling; Natural organic matter; Polysulfone; Pretreatment; Ultrafiltration

1. Introduction In membrane filtration of natural waters, fouling of membranes by natural organic matter (NOM) is frequently encountered. Fouling restricts membrane performance and represents the major limitation to the extended use of membranes in drinking water treatment [2–4]. NOM represents a broad, structurally complex matrix of compounds formed in both soil and aquatic environments by biological degradation. NOM is typically quantified by dissolved organic carbon (DOC), UV absorption, apparent molecular weight (AMW) distribution, and functional group composition. Organic fouling is typically thought of as the loss of membrane permeability due to adsorption of NOM on the surface of the membrane and/or within its pores. Adsorption of NOM on

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membrane surfaces has been established in a number of previous studies. In static adsorption experiments using ultrafiltration (UF) membranes and lake water, Laˆın´e et al. [5] found permeability losses as high as 50%, simply by dipping membranes into samples of lake water. The more hydrophobic ultrafilters, like polysulfone (PSf), underwent a greater permeability loss than hydrophilic ultrafilters. Laˆın´e et al. [5] attributed this to preferential adsorption of NOM by the hydrophobic ultrafilters. Jucker and Clark [6] studied the kinetics of adsorption of humic and fulvic acids on various UF membranes during static adsorption; more hydrophobic membranes had a stronger interaction with the organic matter. They also found that inorganic constituents, viz. calcium, and low pH, played an important role in increasing adsorption of organics on the membrane. Crozes et al. [7] filtered synthetic solutions containing tannic acid (700 Da) and dextran (10 kDa) through 50- and 100-kDa molecular weight cut-off (MWCO) UF membranes. The permeate concentrations of both tannic acid and dextran decreased after filtration — presumably by a desorption mechanism since the steric rejection

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mechanism was minimized due to the use of organic compounds with molecular weights smaller than the filter MWCO. The authors also ascertained that hydrophobic ultrafilters were more susceptible to fouling by adsorption. More recently, Koh et al. [8,9] sent a large volume of particle-free lake water through a 0.2-␮m polypropylene microfiltration (MF) membrane, and collected the permeate in individual batches. Initial batches of MF permeate caused little flux decline when filtered through 20-kDa polyethersulfone (PES) UF membranes, but subsequent batches exhibited an increase in membrane fouling. The lower fouling caused by the initial batches was due to removal of foulants by adsorption onto the microfilter. However, as the microfilter approached its adsorption capacity, gradual breakthrough of NOM resulted in fouling of the ultrafilter by the later batches. Some studies have indicated the majority of adsorptive fouling during UF is actually attributed to a minor fraction of the NOM matrix. Howe [10] examined sequential filtration through two 20-kDa PES UF membranes in series using prefiltered (particle-free) natural waters, and measured the amount of DOC retained by each ultrafilter. The first ultrafilter retained less than 5% of DOC and almost all the DOC passed through the second ultrafilter; significant flux decline was observed for the first ultrafilter, while the second ultrafilter exhibited almost no flux decline. Makdissy et al. [11] filtered solutions containing dissolved NOM isolated from two surface waters through 100-kDa PES UF membranes. The isolation procedure consisted of passing the bulk NOM through a 1-␮m filter, followed by retention in a 3.5-kDa dialysis bag and subsequent filtration through a 0.45-␮m filter. Flux reduction was observed, but like the study of Howe [10], NOM removed by the ultrafilter was barely discernible. Several researchers have claimed that membrane fouling is most affected by the larger molecular weight NOM while others have identified the smaller molecular weight NOM as the key in membrane fouling. Yuan and Zydney [12] prefiltered humic acid solutions using PES membranes with various MWCO and filtered the permeate through a hydrophobic MF membrane. Permeate from membranes with successively lower MWCO (lowest at 100 kDa) caused progressively less fouling of the microfilter. Fan et al. [13] segregated NOM concentrated from natural waters into different size fractions and sent the diluted solutions through 0.22-␮m hydrophobic MF membranes. The higher molecular weight fraction (greater than 30 kDa) was found to be responsible for majority of the flux decline. Likewise, Howe and Clark [14] fractionated prefiltered natural water through regenerated cellulose membranes and analyzed the flux decline through 0.2␮m polypropylene membranes. The authors reported that the critical membrane foulants were in a molecular weight range from 3 to 100 kDa. A different approach was made by Carroll et al. [3] who filtered reconstituted NOM fractions through MF membranes and noted the greatest flux reduction with the neutral hydrophilic fraction. Characterization by size-exclusion chromatography revealed that the neutral hydrophilic fraction had the smallest molecular weight distribution. A number of pretreatment methods have been recommended to limit adsorptive fouling by NOM. These include adding metalbased coagulants or powdered activated carbon (PAC) upstream

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of membrane filtration process to remove foulants. LahoussineTurcaud et al. [15] investigated coagulation of river water with polyaluminum and ferric chloride using UF membranes operated in cross-flow mode. Coagulation marginally improved membrane flux and retarded reversible fouling but was ineffective in decreasing the degree of irreversible fouling. Carroll et al. [3] studied the flux decline through MF membranes using natural water that was pretreated with an alum dose for optimum DOC and UV254 removal. While coagulation was able to eliminate selective fractions of NOM and reduce fouling, the flux decline was still appreciable. The use of PAC to minimize adsorptive fouling has been far less promising. Zhang et al. [16] examined fouling of UF membranes using natural water with different concentrations of PAC. A decline in membrane performance was noted even though NOM was adsorbed onto the PAC. A similar observation was reported in another study by Mozia et al. [17] using PAC and a model solution containing humic acid and phenol. The authors attributed the decline in flux to the attractive forces among the PAC particles, humic acid molecules and membrane surface. It was recently proposed that the inability of activated carbon to significantly reduce fouling is due to its pore size being more tuned to removal of the smaller molecular weight NOM [1]. This study illustrates an innovative approach to reduce membrane fouling wherein a new colloidal adsorbent developed from PSf, a common hydrophobic membrane material, is used in the pretreatment of natural waters before membrane filtration. The PSf colloids preferentially remove foulants that would normally adsorb onto the membrane and cause fouling. Since the chemical composition of the foulants has not yet been established, it is also not yet possible to measure the foulant concentrations analytically; therefore, the kinetics of adsorption of foulants by PSf colloids was assessed in terms of UF flux decline during filtration of PSf-pretreated natural waters. DOC concentration, AMW distribution, energy-dispersive spectroscopy (EDS), and attenuated total reflection (ATR) Fourier-transform infrared (FTIR) spectrometry were used to characterize NOM in solution and on the colloids. 2. Materials and methods 2.1. Membrane filtration Surface waters from Lake Decatur, Illinois, and Lake Michigan were used in this study. Available or measured water quality data are provided in Table 1. To remove gross particulate matter and provide some stability, the lake waters were prefiltered through 0.7-␮m glass fiber filters (AP40, Millipore Co., Bedford, MA) followed by 0.45-␮m nylon filters (GE Osmonics Inc., Minnetonka, MN), and stored at 4 ◦ C. (In the remainder of this paper, the water prepared in this way is called “prefiltered” water or lake water.) Before use, the microfilters were rinsed by filtering 1000 mL of reagent-grade water (resistivity ≈18 M
cm, DOC < 0.1 mg/L) to restrict leaching of organic carbon into the lake waters during prefiltration. Initial dissolved organic matter losses to the washed glass fibers and nylon filters by adsorption were addressed through discarding the first 400 and 100 mL of

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Table 1 Raw water quality Parameter

Lake Decatur

Lake Michigan

pH Turbidity (NTU) Alkalinity (mg/L as CaCO3 ) DOC (mg/L) Calcium (mg/L) Magnesium (mg/L) Sodium (mg/L) Silicon (mg/L) Sulfur (mg/L)

8.3 33 140 3.81 48.8 18.0 4.9 5.0 5.8

8.1a <1 105.5a 1.84 36.1 12.2 7.2 0.6 8.2

Sampling of Lake Decatur water occurred in June 2004, at the Decatur South Water Treatment Facility; sampling of Lake Michigan water occurred in August 2004, from the inlet of the Kenosha (Wisconsin) Membrane Drinking Water Treatment Plant. a Source: Kenosha Water Utility laboratory report, July 2004.

prefiltered water, respectively. A new set of glass fiber and nylon filters was used with every 1500 mL of lake water. PSf colloids were prepared from a solution of 2 g of polysulfone (Udel P-3500, Amoco Performance Products, Atlanta, GA) in 56.1 g of 1-methyl-2-pyrrolidinone (Sigma–Aldrich, St. Louis, MO) and 41.9 g of propionic acid (Aldrich, Milwaukee, WI). The detailed steps in preparation of PSf colloids are described in Clark and Riley [18] and Clark et al. [1]. A scanning electron microscope (SEM) image of the fresh PSf colloids is shown in Fig. 9a. Some physical characteristics of PSf colloids are summarized in Table 2. (A physical–chemical model for colloid formation is provided in [1].) Membrane filtration was carried out using 44.5-mm diameter flat-sheet membranes in a stirred batch filtration cell (Amicon Model 8050, Millipore Co., Billerica, MA) at a temperature of 22 ± 1 ◦ C and constant pressure of 15 psi (103 kPa). Filtration experiments were performed using nitrogen gas as the pressure source. Membrane flux was determined by recording the weight of permeate with an electronic top-loading balance (Model PB3002-S, Mettler-Toledo Inc., Columbus, OH) at 60-s intervals using data acquisition software (Winwedge Standard, TAL Technologies Inc., Philadelphia, PA). The flux data were corrected to standard temperature and pressure conditions of 25 ◦ C and 15.0 psi (103 kPa), respectively, using the following equation adapted from the American Society for Testing and Materials [19]:
 Ps Js = JM × 1.024Ts −TM PM where J is the flux (L m−2 h−1 ), P is the transmembrane pressure, T the temperature, and subscripts S and M refer to stanTable 2 Characteristics of polysulfone adsorbent [1] Parameter

Measurement

Average primary particle size Average aggregate size Total surface area Average ‘pore’ diameter

50 nm 25 ␮m 105 m2 /g 25 nm

dard and measured conditions, respectively. During flux tests, a higher initial pressure of 35 psi (241 kPa) was applied for 5 min to ensure that the membranes were fully wet. Following that, the clean water flux of each membrane was measured by filtering reagent-grade water (typically 100 mL) until a constant flux was achieved. Membranes are likely to have small variations in initial permeability (clean water flux). In order to compare the flux decline of different membranes, the flux results were recorded as J/J0 where J is the measured sample flux, and J0 is the flux through a clean membrane. To determine the benefit of pretreatment with PSf colloids, the colloids were contacted with the prefiltered lake water for fixed times prior to filtration. The fouling potential was assessed with a fresh 20-kDa MWCO PES UF membrane (Sepa, GE Osmonics Inc., Minnetonka, MN). Filtration was performed at a constant stirrer speed of ‘2’ on the Amicon mixer (Amicon Model MT 2, Millipore Co., Billerica, MA). Depending on the objective of the experiment, about 100–250 mL of sample was used for each filtration. 2.2. Physical and chemical analyses Measurements of pH, turbidity, and alkalinity were carried out according to procedures in Standard Methods [29]. DOC was determined with a total organic carbon analyzer (Phoenix 8000, Teledyne Tekmar, Mason, OH) using persulfate-ultraviolet oxidation method [29]. The analysis for inorganic elements was conducted at the Illinois State Water Survey (Champaign, IL) using an inductively coupled plasma vacuum spectrometer (Thermo Jarrell Ash 61E ICP vacuum spectrometer with a fixed channel polychromator, Thermo Electron Corporation, San Jose, CA), following USEPA Method 200.7, Revision 4.4, 1994. The AMW distribution was measured by high-pressure sizeexclusion chromatography (HPSEC) using a high-pressure liquid chromatograph (HPLC) (VP Series, Shimazu Scientific Instruments, Columbia, MD) with UV–vis detector (SPDM10Avp, Shimazu Scientific Instruments, Columbia, MD) and a silica gel column (Protein-Pak 125, Waters Co., Milford, MA). The eluent was prepared with 0.1N NaCl buffered with 2 mM monobasic and dibasic potassium phosphate. The eluent was filtered through a 0.2-␮m filter (MCE, Fisher Scientific, Pittsburgh, PA) and degassed overnight. UV absorbance readings were collected for wavelengths of 224 nm at intervals of 0.24 s. Calibration was carried out using polystyrene sulfonates (Polysciences Inc., Warrington, PA) with molecular weights of 4.6, 8, 18 and 35 kDa in combination with acetone. The functional groups associated with PSf colloids and membrane foulants were analyzed using an infrared spectrometer (Nexus 670 FT-IR, Thermo Electron Corporation, Madison, WI) fitted with praying mantis drifts accessories (DRP-SAP, Harrick Scientific Corporation, Ossining, NY). The spectrometer was mounted with a deuterated triglycine sulfate–potassium bromide (DTGS–KBr) detector and spectra were collected using FT-IR software (OMNIC, Thermo Electron Corporation, Madison, WI). About 400 mg of ground KBr was used for collecting the background spectrum. For the collection of sample spectrum, 10 mg of dried colloid sample was mixed with 400 mg of KBr. A total of 500 scans were collected for each sample and added

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at 4 cm−1 resolution. Interpretation of ATR-FT-IR spectra were based on studies by Cho et al. [2], Howe et al. [20], Kimura et al. [4], Koh et al. [8,9], Mayo et al. [21], Ouatmane et al. [22], and Zhu [23]. PSf colloids with and without foulants were imaged using an environmental scanning electron microscope (Philips Model XL30, ESEM-FEG, FEI Company, Hillsboro, OR). EDS was also performed for elemental analysis of PSf colloids that were contacted with prefiltered lake water. The sample preparation involved coating the PSf colloids with a 4-nm thick layer of gold–palladium using a sputter coater (Desk II TSC, Denton Vacuum LLC, Moorestown, NJ). EDS imaging was carried out on an area that was magnified 500 times, with an accelerating voltage of 15 kV and spot size of ‘4’. Several analyses required dried samples of PSf colloids that were contacted with prefiltered lake water. To obtain these samples, PSf colloids were contacted with prefiltered lake water and the solution sent through a 0.45-␮m nylon filter (GE Osmonics Inc., Minnetonka, MN). The PSf colloids that were retained on the nylon filter were placed in sterilized Petri dishes and stored in a 4 ◦ C dark room. The samples were allowed to dry by transferring them to a dessicator 72 h before analysis.

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The kinetics of adsorption of foulants by PSf colloids was evaluated by measuring the flux decline during filtration of Lake Decatur water pretreated with PSf colloids. Batches of prefiltered water (100 mL) were contacted with the same concentration of PSf colloids for different amounts of time. Each experiment resulted in a filtration time of 30–40 min, depending on the concentration of PSf colloids. The flux results are shown in Fig. 1. After filtering 100 mL of prefiltered water through a 20-kDa PES membrane, raw water flux (without PSf colloids pretreatment) declined to 63% of initial permeability. With 50 mg/L of PSf colloids added just before filtration (<1 min contact time), the flux stabilized at 90% of its initial value. To examine if a longer contact time would improve flux, the same

concentration of PSf colloids was allowed to mix with prefiltered water on a laboratory shaker for about 25 h prior to filtration. But, it appears that the additional contact time did not enhance membrane performance. The experiment was repeated with a lower concentration (5 mg/L) of PSf colloids. The flux decline exhibited trends similar to higher concentration case. The comparable flux decline curves despite the difference in contact time suggest that the adsorption kinetics of foulants by PSf colloids is very fast. Rapid adsorptive fouling of hydrophobic UF membranes has been reported by Lahoussine-Turcaud et al. [15] and Crozes et al. [7]. Fig. 2 shows a series of flux decline curves of prefiltered Lake Decatur water pretreated with various concentrations of PSf colloids. Different concentrations of PSf colloids ranging from 5 to 100 mg/L were added to the prefiltered water just before filtration (<1 min contact time). After filtering 200 mL of prefiltered water, the raw water flux declined to 56% of initial permeability. When the prefiltered water was equilibrated with 5 mg/L of PSf colloids, a barely significant improvement (∼5%) in flux was observed. However, higher concentrations significantly reduced the extent of flux decline. This can be seen from the flux curve for 100 mg/L PSf colloids, in which the flux stabilized at 83% of initial permeability. In general, it was found that increasing the concentration of PSf colloids during pretreatment improves membrane performance. As the concentration increased, foulants were more effectively removed by adsorption onto the PSf colloids, resulting in less fouling. The contribution of PSf colloids to membrane filtration resistance is presented in Fig. 3. In this experiment, 50 mg/L of PSf colloids were mixed with prefiltered Lake Decatur water and the solution sent through a 0.45-␮m nylon filter. The PSf colloids were retained on the nylon filter, while the permeate was collected and fed through a 20-kDa PES membrane. The flux for the permeate from the nylon filter stabilized at 83% of its initial permeability after 200 mL of filtration. In contrast, the flux for prefiltered water containing 50 mg/L of PSf colloids dropped to 78% of its initial value. Therefore, the colloids themselves contributed to a small decline in flux, probably due to an increased colloid cake resistance. In a separate experiment where a 50-mg/L colloid solution in reagent-grade water

Fig. 1. Flux decline for prefiltered Lake Decatur water pretreated with PSf colloids with different contact time through 20-kDa PES membranes.

Fig. 2. Flux decline for prefiltered Lake Decatur water pretreated with different concentrations of PSf colloids through 20-kDa PES membranes.

3. Results and discussion 3.1. Performance of polysulfone colloids

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Fig. 3. Flux decline for prefiltered Lake Decatur water pretreated with 50 mg/L of PSf colloids through 20-kDa PES membranes with and without prefiltration through a 0.45-␮m nylon filter.

was filtered through a 20-kDa PES membrane, the steady state flux declined to about 95% of the initial value, thus confirming the small hydraulic resistance attributable to the colloids themselves. 3.2. Characterization of membrane foulants The DOC concentration measured for prefiltered Lake Decatur water (feed) was 3.81 mg/L, while permeate DOC (without PSf colloids pretreatment) from the 20-kDa PES membrane was 3.11 mg/L. This implied that 82% of DOC went through the membrane. This level of DOC rejection is typical for ultrafiltration of this lake water [5,10]. As illustrated earlier in Fig. 1, when 200-mL of this prefiltered water was fed through a 20-kDa PES membrane, the flux declined by a relatively large amount (44% of initial permeability). Interpreted together with this flux data, the DOC results demonstrate that the majority of the dissolved NOM present in this natural water does not contribute to fouling. This observation is consistent with findings of Howe [10], Koh et al. [8,9] and Clark et al. [1] showing that about 85–90% of dissolved NOM in several river and lake waters did not cause membrane fouling. DOC measurements for permeate from 20-kDa PES membrane corresponding to various concentrations of PSf colloids pretreatment are given in Fig. 4. The permeate DOC values were found to be similar regardless of pretreatment concentrations. This shows that a hardly detectable quantity of DOC was associated with the PSf colloids. Intriguingly, the flux data shown in Fig. 2 revealed a significant improvement in flux with 100 mg/L of PSf colloids as compared to 5 mg/L of PSf colloids. Bearing in mind that the DOC adsorbed by different concentrations of PSf colloids was barely noticeable (even though there was a substantial difference in flux), it is reasonable to suggest that only a small fraction of DOC is the primary cause of membrane fouling in these water samples. It is important to note that the permeate DOC with PSf colloid pretreatment appears to be higher than without any pretreatment. The increase in DOC could be due to trace amounts of unaggregated PSf colloids getting though the pores of the 20-kDa PES membrane. To confirm this hypothesis,

Fig. 4. DOC of permeate from 20-kDa PES membranes for prefiltered Lake Decatur water pretreated with different concentrations of PSf colloids.

reagent-grade water and a 50 mg/L colloid solution (in reagentgrade water) were filtered through 20-kDa PES membranes and the DOC of both permeates were measured. It was found that the permeate DOC from the 50 mg/L colloid solution (in reagentgrade water) was slightly (but consistently) higher (<0.1 mg/L) than the DOC of the reagent-grade water permeate. The AMW distribution of NOM in prefiltered Lake Decatur Water is presented in Fig. 5. The chromatogram illustrates a primary peak corresponding to a molecular weight of 400 Da and a very small secondary peak of much larger molecular weight centered at 60 kDa. This is consistent with Laˆın´e et al. [5] and Howe [10] who measured the AMW distribution of Lake Decatur water and also found two main molecular weight peaks. In addition, Fig. 5 also reveals that the majority of the NOM (with molecular weight of 400 Da) remained in the permeate after filtration through a 20-kDa PES membrane, which is consistent with the relatively large membrane MWCO. Interpreted together with flux data of prefiltered water (Fig. 2), the HPSEC results supported the earlier conclusion that the majority of the NOM does not cause membrane fouling. Fig. 6 shows that the most significant difference in NOM removal for Lake Decatur water after filtration through the 20kDa PES membrane was in the larger molecular weight range,

Fig. 5. HPSEC (AMW between 100 Da and 1000 kDa) for prefiltered Lake Decatur water and permeate from 20-kDa PES membrane.

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Fig. 6. HPSEC (AMW between 1000 Da and 1000 kDa) for prefiltered Lake Decatur water (feed), permeate from 20 kDa PES membrane, permeate from 20 kDa PES membrane for feed pretreated with 50 mg/L of PSf colloids and permeate from 0.45-␮m nylon filter for feed pretreated with 50 mg/L of PSf colloids.

between 20 and 200 kDa. When water pretreated with 50 mg/L of colloids was filtered through a 20-kDa PES membrane, the AMW distribution of the permeate also followed a similar trend (Fig. 6). In contrast, substantial difference in flux (Fig. 2) was observed for prefiltered water with and without 50 mg/L of PSf colloids pretreatment. Relating the HPSEC results to flux data, it appears that this larger AMW fraction of NOM was removed by the PSf colloids, resulting in an improvement in membrane flux (see also Kwon et al., 2005). Therefore, it seems likely that this portion of the NOM is the primary foulant. To confirm that this larger AMW fraction of NOM was indeed adsorbed on the PSf colloids and not the PES membrane, the experiment was repeated using a 0.45-␮m nylon filter. AMW measurement of the permeate from the nylon filter for prefiltered water pretreated with the same concentration of PSf colloids was made. The result (Fig. 6) indicates that the AMW distribution of permeate from the nylon filter was almost identical to the permeate from PES membrane. This confirms that the foulants were adsorbed by the PSf colloids, which would have otherwise passed through the nylon filter. The finding from this experiment agrees with Howe and Clark [14] who determined that the size of the critical UF foulants was primarily between 3 and 100 kDa (∼3–20 nm). The 25-nm pores of the PSf colloids seem well tuned to adsorption of these foulants [1]. Fig. 7 shows EDS spectra for “fouled” PSf colloids, taken at two different locations on the colloid deposit. The spectra were taken after contacting 60 mg of PSf colloids with 4000 mL of Lake Michigan water. The “fouled” PSf colloids were observed to have peaks corresponding to carbon (C), oxygen (O) and sul-

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Fig. 8. ATR-FT-IR spectra for “fresh” colloids and “fouled” colloids (contacted with prefiltered Lake Michigan water).

fur (S), which were expected based on the chemical structure of PSf. Besides the PSf material, the marked carbon (C) peak was also due to trace amounts of organic carbon deposited on the PSf colloids. The gold (Au) and palladium (Pd) peaks were due to the coating deposited during sample preparation. However, the spectrum in Fig. 7b, corresponding to a different location, suggests that trace amounts of silicon (Si) were also adsorbed on or associated with the PSf colloids. Many studies have suggested that inorganic ions like iron, calcium, aluminum and silicon contribute to membrane fouling in natural waters, although it is not yet clear if these interactions are a result of metal–ion complexation, double-layer compression, or a metal–ion “bridge” between organic matter and the membrane surface [24,20,25,8,9]. In natural waters, silicon exists in the form of silicon dioxide-related compounds like silica or silicates. Agglomerated silica and silicates are typically present in lakes as colloids, with sizes ranging ˚ and 1 ␮m [26]. If the silicon in Lake Michigan between 10 A water is present in colloidal form, EDS analysis is likely to pick up spatial inhomogeneities in silicon distribution. Fig. 8 shows the ATR-FT-IR spectra for “fresh” and “fouled” PSf colloids. Similar to EDS analysis, the spectrum for “fouled” colloids was taken after contacting 60 mg of PSf colloids with 4000 mL of prefiltered Lake Michigan water. The PSf colloids encompass a range of functional groups that give rise to a complex spectrum with several overlapping adsorption bands. The difference in intensity between the spectrum for “fouled” and “fresh” colloids appears to be relatively weak which suggests that only a small amount of foulants accumulated on the PSf colloids. This is consistent with the observations drawn from the measurements of DOC and AMW distribution.

Fig. 7. EDS of “fouled” PSf colloids (contacted with prefiltered Lake Michigan water).

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Fig. 9. Scanning electron microscope images of (a) “fresh” PSf colloids and (b) “fouled” PSf colloids (contacted with prefiltered Lake Decatur water).

The complex spectrum of PSf colloids unfortunately interferes with the comparatively weak spectrum of the foulants and therefore makes it difficult to identify specific foulant functional groups. Nonetheless, it was observed that the strong peaks of aromatic double-bonded carbon (around 1500 and 1600 cm−1 ), C–O bond of ethers (1100–1250 cm−1 ) and aromatic sulfone (550 cm−1 ) exhibited by the “fresh” colloids were lowered in absorbance intensity when fouled with Lake Michigan water. This suggests that these peaks were masked with foulants. The marked increase in absorbance intensity observed with “fouled” colloids in the region near 1050 cm−1 indicates the presence of C–O bonds associated with polysaccharide-like substances or Si–O bonds of silicates. The small peak at 3691 cm−1 supports the presence of silicates, since very few materials absorb in this region [20]. This interpretation seems consistent with the above EDS analysis. Another region of increased adsorption occurs at 1652 and 1546 cm−1 , peaks that can be assigned to amide groups, which could be associated with proteins — another common constituent of natural waters. Unfortunately, the remaining peaks could not be clearly identified. 3.3. Scanning electron microscopy imaging of polysulfone colloids Fig. 9 shows SEM images of “fresh” and “fouled” PSf colloids. The SEM image for “fouled” colloids was taken after contacting PSf colloids with 800 mL of prefiltered Lake Decatur water. The SEM image in Fig. 9a reveals that the PSf primary particles are fairly spherical and uniform, with an average size of about 50 nm [27,1]. Visual inspection of “fouled” colloids, however, revealed a yellowish-brown deposit. Under SEM, the “fouled” colloids appeared to be coated with substantially different material (Fig. 9b).

treated with increasing concentrations of PSf colloids exhibited correspondingly less fouling when filtered through 20-kDa PES membranes. Measurements of DOC and AMW distribution showed that only a small fraction of NOM was removed by adsorption on PSf colloids. Although numerous studies have named NOM as a major foulant during filtration of natural waters, this work emphasizes that only a small subset of NOM is responsible for the majority of membrane fouling. Evidence presented by HPSEC further suggests that this portion of NOM has an AMW ranging from 20 to 200 kDa. It is postulated that the PSf colloids may be well tuned to the adsorption of foulants in this size range. Characterization of the foulants by EDS and ATR-FT-IR indicated the presence of trace amounts of silicon (Lake Michigan water). The relatively small difference in absorbance intensity between the ATR-FT-IR spectrum for “fresh” and “fouled” PSf colloids suggests that only a small amount of foulants was adsorbed on the PSf colloids. The ATR-FT-IR spectra analyses also revealed that the organic foulants contain amide functional groups. NOM adsorption kinetics cannot currently be examined quantitatively do to the fact that the adsorbing species have not been determined. However, data presented here do clearly demonstrate accumulation of foulant on the polysulfone adsorbent, and that this effect can be detected to occur rapidly via the secondary effect — membrane fouling. (In fact, the adsorption time scale is presumably less than the resolution of the combined adsorption-fouling experiments, ca. 5–10 min.) Although it is not possible to make any quantitative conclusions on the adsorption kinetics, the speed at which the secondary effect is detected is certainly consistent with diffusional transport limitation of sorbates within mesoporous materials [28].

Acknowledgements 4. Conclusions This study looked into the performance of a novel NOM adsorbent, mesoporous aggregates of polysulfone nanoparticles. Experiments investigated the kinetics of adsorption of foulants by PSf aggregates, and demonstrated that adsorption took place within minutes of contact with the PSf colloids. Lake water pre-

This work was supported by the Illinois Board of Higher Education, the National Water Research Institute, and the University of Illinois WaterCAMPUS — a National Science Foundation Science and Technology Center. The student (L.C. Koh) was funded by the Public Utilities Board (PUB) of Singapore and the comments and views detailed in this research may not reflect

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