Surface energy of experimental and commercial nanofiltration membranes: effects of wetting and natural organic matter fouling

Journal of Membrane Science 175 (2000) 61–73

Surface energy of experimental and commercial nanofiltration membranes: effects of wetting and natural organic matter fouling Anna R. Roudman∗,1 , Prof. Francis A. DiGiano Department of Environmental Sciences and Engineering, University of North Carolina, Chapel Hill, NC 27599-7400, USA Received 5 November 1999; received in revised form 17 March 2000; accepted 22 March 2000

Abstract Contact angle measurements (captive bubble technique) were used to determine the surface energy of three experimental thin-film composite nanofiltration membranes and a commercial nanofiltration membrane (Hydranautics NTR 7450). The two experimental membranes of practical interest were thin film composites (diblock copolymer on a polysulfone support layer). The two blocks were poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) and poly(1,1-dihydroperfluorooctyl methacrylate) (PFOMA). The concept was to devise a membrane material that takes advantage of the low adhesion of PFOMA to prevent fouling and the hydrophilic nature of PDMAEMA to produce high water permeation rates. Hydranautics NTR 7450 is a sulfonated polysulfone membrane that purportedly lessens fouling because the surface is more hydrophilic. The change in surface energy upon wetting, permeation of water containing natural organic matter (NOM) and chemical cleaning was of interest. Wetting caused reorganization of the experimental block copolymer surface to move more of PDMAEMA block to the membrane–water interface. After permeation of ultrapure water, however, the surface became more hydrophilic. After permeation of NOM containing water, the surface of both experimental and commercial membranes reached about the same surface energy, indicative of adsorption of NOM. The contact angle measurements were used to calculate a negative change in surface free energy for all but the PFOMA membrane; hence, with this exception, the deposition of NOM into a layer adjacent to the membrane surface was spontaneous. Scanning electron micrographs and atomic force micrographs showed that rigorous chemical cleaning failed to remove the NOM. Although the new polymeric materials were not more resistant to NOM fouling than commercial membranes, the surface energy calculations may help in the search for more successful polymers. Systematic study of charge, molecular size and specific functional groups of NOM on membrane fouling warrants further research to understand why similar fouling occurred on very different polymeric materials. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Nanofiltration; Contact angle; Membrane; Geometric mean equation; Surface property; Natural organic matter; Poly(1,1-dihydroperfluorooctyl methacrylate) and poly(2-dimethylaminoethyl methacrylate) diblock copolymer

1. Introduction

∗ Corresponding author. Fax: +919-966-7911. E-mail address: fran [email protected] (A.R. Roudman) 1 Post-doctoral Researcher.

Nanofiltration (NF) of drinking water may offer an economically competitive process to remove natural organic matter (NOM), the precursors to disinfection by-products (DBPs) formed upon chlorination. These DBPs are under increasing government regulation

0376-7388/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 0 ) 0 0 4 0 9 - 9

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because of their implication as carcinogens. While NOM removals greater than 80% have been shown in pilot plant studies of NF, fouling of the membrane surface is a serious problem that leads to unacceptable flux decline for economic operation. Discussions of the nature of NOM fouling in recent literature point to the complexity of interaction between NOM and the membrane surface [1–5]. The role of membrane surface chemistry in NOM fouling is not clearly understood mainly because the chemistry of NOM is still largely unknown [3,6]. NOM is a mixture of polymeric, charged macromolecules. As such its size, configuration and charge are largely dependent on pH and ionic strength; at the pH of natural waters, NOM tends to be negatively charged. In contrast to NOM, the chemistry of proteins and polymers that have been most studied as foulants is relatively simple. The general conclusion from such studies is that hydrophilic surfaces are less foulant than hydrophobic surfaces. For example, based on free energy of adhesion calculations from contact angle measurements, cellulose acetate and polyacrylonitrile were found less foulant than polyethersulfone and polyvinylidenedifluoride when human serum albumin and polyethylene glycol were the fouling agents [7]. However, these conclusions cannot be extended to NOM necessarily owing to the very different chemistry of NOM. A study of NOM fouling showed interactions with both a hydrophilic and hydrophobic membrane by either polar or non-polar interactions [5]. Operational definitions of hydrophilic and hydrophobic fractions have been

used to sub-divide NOM for studies of fouling [1,2]. It appears that both fractions may be responsible for fouling a relatively hydrophilic, sulfonated polysulfone NF membrane surface. The purpose of this paper, is to investigate the change in membrane surface properties upon wetting and exposure to NOM for several different materials that vary greatly in their polymer composition. A unique aspect of the study was the control of block copolymer chemistry to produce experimental thin-film composite (TFC) membranes for comparison to a commercial membrane. In our research, we have been exploring the development of new membrane materials that may be potentially less fouling [8]. The initial synthesis efforts involved block copolymers. The repeating pattern of block copolymers leads to microphase segregation in which a co-continuous morphology exists in the form of ‘channels’ of one polymer. The concept is schematically represented in Fig. 1. The fluorocarbon segment provides a minimally adhesive veneer that is characteristics of dihydroperfluoro compounds [9]. This veneer could reduce membrane fouling and/or improve membrane cleaning efficiency. The hydrophilic segments selfassemble to form hydrophilic channels or ‘highways’ for high rates of water permeation. We quantified the surface energy and membrane– water interfacial energy under the following conditions: before wetting; after wetting; after permeation of ultrapure water; after permeation of NOM-containing water; and after chemical cleaning of the surface. The approach is based on the captive bubble technique

Fig. 1. Conceptualization of the anti-foulant and permeation functions (hydrophobic and hydrophilic and segments, respectively) of diblock copolymer in proposed thin-film composite membrane.

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to measure contact angles [10]. The captive bubble was either a drop of octane or an air bubble at the membrane–water interface. This allows for estimating the fully hydrated polymer (membrane)–water interfacial energy as well as the polar and dispersive components of polymer–vapor surface energy. The ultimate goal is, to determine the membrane surface chemistry that would provide the most resistance to NOM fouling.

2. Experimental 2.1. Membranes The experimental TFC membranes consisted of a thin layer (about 1 ␮m) of block copolymers that was solvent (Freon) cast on a polysulfone support layer. The diblock composition was poly(1,1-dihydroperfluorooctyl methacrylate) (PFOMA)-poly (2-dimethylaminoethyl methacrylate) (PDMAEMA) as depicted in Fig. 2. Synthetic pathways are provided by Betts et al. [11]. PDMAEMA is very hydrophilic while PFOMA is very hydrophobic. In fact, PFOA, which differs from PFOMA only in the absence of a methyl group, has a surface tension of about 11 dyn/cm; this is about one-half that of teflon [12]. The glass transition temperatures (Tg ) of PDMAEMA and PFOMA are 25 and 50◦ C, respectively. The resulting diblock exhibited two Tg values intermediate between these values. When cast into a thin-film for a membrane application, a block copolymer with Tg > ambient temperature will be brittle or glassy as opposed to soft or rubbery. High Tg polymers will be more resistant to compaction upon pressurization but may also have lower water permeability; thus, a compromise is reached in performance. Two different compositions of PDMAEMA and PFOMA were synthesized and made into TFC mem-

Fig. 2. Structure of the experimental diblock copolymer.

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branes. The approximate molecular weights of the PDMAEMA and PFOMA in these diblock formulations were 29,000 and 82,000 g/mol, respectively, for the 29% PDMAEMA sample and 29,000 and 40,000 g/mol, respectively, for the 40% PDMAEMA sample. Companion bench-scale studies of cross-flow filtration showed that the experimental membranes produced a flux of 15–30 L/h/m2 compared to 10 L/ h/m2 for a Hydranautics NTR 7450 membrane [8]. A TFC membrane consisting of PFOMA on a polysulfone support layer was also included to provide a frame of reference for the behavior of the block copolymer TFC membranes in which PFOMA served as the hydrophobic block. Because of its very hydrophobic surface, the PFOMA membrane did not permeate water at the pressures of practical interest (i.e. <6 atm). Thus, only surface adsorptive fouling by NOM was possible. The fouling results from experimental membranes were compared to the NTR thin-film composite membrane (Hydranautics, San Diego, CA) which has a hydrophilic skin layer of sulfonated polysulfone and a support layer of polysulfone. Results of NOM fouling of this membrane have been reported [13,14]. Comparisons were also made with the polysulfone support layer alone. 2.2. NOM source water The NOM source water was collected from the P.O. Hoffer water treatment plant (WTP) in Fayetteville, NC in May 1997. This WTP draws surface water from the Cape Fear River and employs alum coagulation, sedimentation and filtration. Settled rather than filtered water was taken to avoid chemical complications arising from the addition of chlorine at the WTP. The samples were subsequently filtered through nominal 5 ␮m spun-polypropylene cartridge filter (Model P5, Ametek Inc.) before use in experiments. NOM is typically characterized by measuring the total organic carbon (TOC) concentration and UV absorbance (measured in a 5 cm cell path length) at 254 nm. The former is a surrogate for the total mass of NOM and the latter for the aromatic structural features that relate to hydrophobicity. The TOC of settledfiltered water was 2.2 mg/l and the UV254 absorbance was 0.25 for a 5 cm cell path length. Expressing UV254 absorbance per m of cell path length, the

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UV254 –TOC ratio is 2.3 l/mg/m, which suggests a relatively low hydrophobic content that is typical of natural waters after chemical coagulation-sedimentation and filtration [15]. 2.3. Exposure of membranes to NOM and subsequent cleaning A 1-l sample of settled-filtered water from the P.O. Hoffer WTP was passed through the following membranes in a dead-end, 5 cm diameter filtration cell (Amicon, Bedford, MA): (PDMAEMA-PFOMA (29 wt.% PDMAEMA); PDMAEMA-PFOMA (40 wt.% PDMAEMA); Hydranautics NTR-7450; and the polysulfone support). The applied pressure was about 3 atm. Because the PFOMA TFC membrane did not permeate water, this membrane sample (also 5 cm in diameter) was immersed in 1 l of the water sample and stirred for 48 h. The procedure for membrane cleaning were adapted from larger bench-scale, cross-flow testing of 135 cm2 samples of flat sheets [14]. The cleaning solution was MC-3 (Zenon Water System, Burlington, Ontario), an inorganic caustic detergent that was prepared by dissolving 0.34 g of MC-3 into ultrapure water (Dracor Water Purification System — Chapel Hill tap water through activated carbon filter and a mixed ion exchange resin) and adjusting the pH to 10.3 with NaOH. The steps in cleaning were: (1) expose the membrane to 20 ml of MC-3 cleaning solution for 1 h using a magnetic mixer for stirring; (2) rinse the membrane with ultrapure water for 0.5 h; and (3) repeat steps (1) and (2). 2.4. Polyacrylic acid as a reference foulant The P.O. Hoffer WTP sample, while providing a realistic test of a NOM-containing foulant solution,

also contains other potential foulants (e.g. residual colloidal aluminum hydroxide from chemical coagulation). Moreover, the characteristics of NOM are highly site dependent [6]. Thus, a specific, well-defined reference foulant was also included. Polyacrylic acid (PAA) was selected because it is readily available and has some of the general characteristics of NOM. In particular, PAA has a negative charge as is true of NOM in the pH range of natural waters and its molecular weight was 100,000 (Aldrich Chemicals) which is on the high end of the range typically reported for NOM [6]. The PAA was dissolved in ultrapure water to produce a PAA concentration of 1000 mg/l. Although this concentration is about two orders of magnitude higher than NOM, comparisons of fouling among different membrane materials was still possible. 2.5. Contact angle measurements The VCA 2500 video camera (AST Products, Billerica, MA) together with the captive bubble assembly was used to measure contact angles in a three-phase system consisting of water, membrane surface and bubble of either air or liquid n-octane. A schematic representation is given in Fig. 3 wherein the measured contact angle and the components of surface tension are shown. The advantage of the captive bubble technique over the sessile drop technique is complete hydration of the surface and thus, the surface energy of interest between water and membrane should not change with time of measurement [16]. The glass cell of the VGA captive bubble assembly was filled with ultapure water. A 1 cm2 sample of the test membrane was then attached to a poly(methyl methacrylate) cover with double sided tape (Macbond IB1190, MACtac, Stow, OH). A specially designed and shaped syringe needle (AST Products, Billerica, MA) in the form of a ‘J’ dispenses the liquid (n-octane

Fig. 3. Contact angles and surface energies corresponding to the captive bubble technique with n-octane and air bubbles.

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from Fisher Scientific, Fair Lawn, NJ) or gas (air) bubble from beneath the sample. A video screen provides an image of the captive bubble from which five points are selected on the bubble curvature. The supporting computer software (MS-DOS system AST Products, Billerica, MA) uses these data to calculate the contact angles (Fig. 3) between n-octane and the membrane, θ o , and the between air and membrane, θ a , to the nearest degree. The complementary angle would be measured by the sessile drop technique. Contact angles were measured for 10–20 air or n-octane bubbles. An average contact angle was calculated, including only those points for which deviation from the average value was less than the standard deviation. In general, the contact angles in the air/water/membrane system had a lower standard deviation those in the n-octane/water/membrane system. 2.6. Sequence of membrane testing Contact angle measurements were made for several different membrane conditions. Two reference conditions were included. These are referred to as the ‘dry’ and ‘wetted’ conditions. A dry membrane is before exposure to any solutions whereas the wetted membrane is after exposure for 20 min to pure ethanol followed by water. This treatment is recommended by membrane manufacturers to open the pores of the polysulfone support material. The membrane was next placed in the dead-end test cell for filtration of ultrapure water (UPW), defined as water produced by passing local tap water through a 1 ␮m prefilter, an activated carbon bed, two mixed-bed ion exchangers in series and a macro reticular resin (Dracor Water System, Durham, NC). Following UPW filtration, the NOM containing solution from the P.O. Hoffer WTP was filtered and finally, the membrane was cleaned as described above. A separate membrane sample was tested by under the following conditions: dry; wetted; and after exposure to 1000 mg/l of PAA. 2.7. Calculation of surface and interfacial energy from contact angles Contact angles from air and n-octane captive bubble experiments can be used to resolve the polar and dispersive interaction components of surface energy [16,17]. The components of interfacial energy are

65

shown in Fig. 3. The appendix shows the derivation of the following equations used to calculate the components of surface energy (in Appendix A, these equations appear as (A.8), (A.9) and (A.10)): 

2 q p γwv (1 − cos θa ) − 2 50.8γpv

d γpv =

d 4γwv  q 2 p = 2.3307 5.0930(1 − cos θa ) − γpv

p

γpv =

(γwv − γov − γwv cos θo )2 d 4γpv

= 12.7(1 − cos θo )2 γpw

(1)

q q d − 14.2548 γ p = 72.6+γpv −9.3381 γpv pv

(2) (3)

where γ is the interfacial energy, θ the contact angle shown in Fig. 3, the subscripts pv, w and a refer to the phase of interest (polymer, i.e., membrane surface; vapor of polymer; water and air, respectively) and the superscripts d and p refer to the dispersive and polar components of interfacial energy, respectively. 2.8. Morphological examination Both scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to inspect the membrane surface after fouling and cleaning. SEMs were determined with a Cambridge Stereoscan, Type S200, Leica configured with an accelerating voltage of 20 kV. Specimens were fixed (2 days, 2.5% glutaraldehyde in water), dehydrated (5–10 min sequentially in 35, 55, 75 and 95% ethanol), and dried at the critical point (31.3◦ C, 72.9 atm, with several changes of liquid carbon dioxide to completely replace ethanol). Dried samples were cut, mounted with double-sided tape on an aluminum stubs and sputter coated (Hummer X sputter coater, Anatech, Springfild, VA) with a thin layer of approximately 10–20 nm of a conductive metal (60% gold/40% palladium alloy). AFMs were made with an Explorer with ECU electronics (TopoMetrix, Santa Clara, CA). Samples were dried and stored in pure nitrogen atmosphere and mounted with double-sided tape on stubs. The dry method of contact was used for AFM and the silicon

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tip was scanned above the surface (high amplitude, resonance-amplitude detection). Frictional forces were reduced by oscillating the silicon cantilever at high frequency and amplitude.

3. Results and discussion The contact angles measured for each membrane under each different condition are listed in Table 1. In the dry condition, the contact angle for the Hydranautics NTR 7450 surface is much greater than for the three experimental membranes. The surface of Hydranautics NTR 7450 is thus relatively hydrophilic compared to these other materials. After exposure to ethanol and water, the contact angle changed very little for the Hydranautics NTR 7450 membrane, polysulfone, and the PFOMA membrane. However, the contact angle increased significantly for both the two block copolymer membranes with the increase being greater for the higher percentage content of the hydrophilic block. Phase reorganization may explain these results. The block-copolymer membranes were cast from Freon-113 because it is a ‘good’ solvent for hydrophobic block and thus, after evaporation of the solvent we should expect to find the microphase separated hydrophilic blocks mostly in the bulk and hydrophobic blocks on the surface. However, after exposure to ethanol which is a ‘good’ solvent

for hydrophilic block, we would expect surface reorganization whereby the hydrophilic chains move to the surface thus increasing the contact angle. Ethanol would not be expected to affect contact angle for the PFOMA homopolymer and this was indeed the case. Given that the PFOMA membrane is completely hydrophobic, filtration of UPW was impossible and thus, no data are given for these entries in Table 1. Contact angle changed for all the other membranes after filtration of UPW, perhaps because of pressurization of the membrane surface. In most cases, the membrane became more hydrophilic. The change in contact angles after exposure to the NOM-containing solution and then after cleaning are the most significant for this research. Foremost, these show that the contact angle for Hydranautics NTR 7450 membrane and polysulfone was little changed by NOM exposure but there was a large increase for the two block copolymer membranes. In fact, after NOM exposure the Hydranautics NTR 7450, polysulfone, and two block copolymer membranes all had about the same contact angle. As important, cleaning did not decrease the contact angle of the two block copolymer membranes to their original state. These conclusions are the same for the results of both the air bubble/water and n-octane bubble/water measurements. Static contact of the PFOMA membrane with the NOM-containing solution also increased the contact angle although not as much as for the two block

Table 1 Contact angle (degrees) using captive bubble technique Membrane

Angle (Fig. 3)

Dry

Wetted

After UPW filtration

After exposure to NOM-containing solution

After cleaning of membrane surface

After exposure to PAA-containing solution

Hydranautics NTR 7450 7450

θa θo

144±4 114±8

152±6 112±8

158±3 138±6

144±5 139±8

153±5 136±8

154±1 107±8

40% PDMAEMA

θa θo

85±4 61±8

103±4 96±8

112±4 80±9

155±4 149±8

158±2 157±5

157±8 154±4

29% PDMAEMA

θa θo

74±4 62±8

92±4 71±8

119±8 55±8

156±4 153±8

164±2 151±5

162±7 148±5

PFOMAa

θa θo

70±4 10±3

70±4 16±7

105±3 22±6

112±2 48±8

124±8 25±8

Polysulfone

θa θo

128±4 69±8

129±8 68±8

160±5 134±8

163±6 150±7

152±4 138±4

a

158±1 146±4

Static contact with NOM solution because PFOMA membrane was impermeable to water.

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copolymer membranes. Hence, the PFOMA surface, while being extremely hydrophobic, is still affected to some extent by adsorption of NOM. The accumulation of NOM on the Hydranautics NTR 7450 and the two experimental block copolymer membranes was also observed in cross-flow, flat sheet bench-scale experiments with the NOM-containing solutions from the P.O. Hoffer WTP [8,14]. In those experiments, SEMs and AFMs showed that a foulant layer had accumulated on each membrane surface after 2–3 days of continuous operation. Typical SEMs and AFMs (before and after cleaning) are given in Figs. 4 and 5. SEMs and AFMs of membranes before exposure of foulant are not included here because these showed a uniformly flat surface [18]. However, for reference purposes, the largest surface deformations shown by AFM were on the order of 100 nm and the average was about 13 nm. Flux declined during cross flow filtration by about 30%. Although flux increased upon cleaning with the same rigorous procedure as used in the dead-end cell tests reported here, both SEMs and AFMs showed that a foulant layer remained. The mass of NOM recovered by cleaning was measured and, depending on the membrane, was from 30 to 70% of that lost from the feed solution during the filtration; this provides further evidence for strong adsorption of NOM to the surface. The last column in Table 1 shows that the contact angle after exposure of each membrane to a 1000 mg/l PAA solution was about the same as after exposure to NOM. PAA has very high density of carboxylic groups that could form an ionic bond with the amine group to give (–C2 H5 (CH3 )2 NH+ COO− –) or hydrogen bond with (–C=O) of the PDMAEMA; only the latter is possible for the PFOMA homopolymer. However, the number of carboxylic groups in PAA most likely exceeds the stoichiometric amount needed to react with the block copolymer structure. The remaining free carboxylic groups could thus control the hydrophilic nature of the surface. The polar and dispersive components of SFE and the IFE (between membrane surface and water) as calculated by the procedure described in Section 2.7 are listed in Table 2. The polar component for the ‘dry’ PFOMA surface is extremely low. This should be expected because the PFOMA contains very few polar and dipole groups. The polar component for the two block copolymer formulations was somewhat higher

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than for PFOMA. However, the surface is still dominated by the hydrophobic block (PFOMA). The IFEs for all three membranes was very high. The interaction between the polymer surface and water is measured by γ pw . A high γ pw corresponds to weak interaction that would be true for a hydrophobic surface in water. In contrast, a low γ pw corresponds to strong interaction that would be true for a hydrophilic surface in water. The results in Table 2 show, therefore, that the Hydranautics NTR 7450 membrane surface is hydrophilic whereas the PFOMA membrane surface is hydrophobic. Upon wetting of the diblock copolymer membranes, Table 2 shows that the polar component of SFE increased by a factor of 1.5–3 whereas the IFE decreased. Both results suggest movement of the hydrophilic block to the membrane surface. The dispersive component of SFE increased significantly for the 29% PDMAEMA but decreased for the 40% PDMAEMA block copolymer. Opposite effects are not surprising because dispersion forces do not relate closely to the structure of the polymer. In contrast to the experimental membranes, wetting had little effect on SFE or IFE for the Hydranautics 7450 and polysulfone membranes. Table 2 also indicates that UPW filtration decreased the polar component of SFE and increased the IFE of the experimental membranes. One explanation is pressure-induced reorganization, possibly forcing PDMAEMA deeper into the membrane structure. Filtration of the NOM-containing solution caused the SFE of all membranes to reach about the same value. Thus, NOM associated with each of these surfaces. The IFEs were very low probably because NOM, although soluble in water, formed a transitional layer between the membrane and the bulk water. In effect, the NOM coated surface is the same regardless of the chemical characteristics of the starting membrane. Although the PFOMA homopolymer membrane sorbed NOM, the SFE increased whereas it decreased for all other membranes. This result may be explained by selective sorption of more hydrophobic fractions of NOM. With the exception of the Hydranautics 7450 membrane, all of the results discussed for NOM were similarly noted for PAA. Once NOM is sorbed, the components of SFE and IFE listed in Table 2 did not change upon rigorous

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Fig. 4. SEM of 42%wt PDMAEMA-58%wt PFOMA thin film composite membrane after bench-scale filtration of settled-cartridge filtered water from the P.O. Hoffer Water Treatment Plant (Fayetteville, N.C. for 90 hours: (a) before cleaning and (b) after cleaning with inorganic caustic detergent (Zenon MC-3) and UPW rinsing.

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Fig. 5. AFM of Hydranautics NTR 7450 thin-film composite membrane (sulfonated polysulfone) after bench-scale filtration of settled-cartridge filtered water from the P.O. Hoffer Water Treatment Plant (Fayetteville, N.C. for 90 hours (14): (a) before cleaning and (b) after cleaning with inorganic caustic detergent (Zenon MC-3) and UPW rinsing.

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Table 2 SFE Components and IFE (dyn/cm) determined by contact angle measurements and geometric mean equation Membrane

SFE or IFE Dry

Wetted After filtration of UPW

After filtration of NOM solution

After cleaning

After filtration of PAA solution

p

25.1 41.1 66.2 7.5

24.0 51.3 75.3 11.2

38.6 30.3 68.9 1.5

39.1 20.4 59.5 0.8

37.5 28.6 66.2 1.5

21.2 59.8 81.0 15.7

Hydranautics NTR 7450 γpv d γpv γ pv γ pw 40% PDMAEMA

γpv d γpv γ pv γ pw

p

3.4 18.4 21.8 28.1

15.5 12.4 27.9 11.5

8.7 38.3 47.0 19.8

43.8 22.3 66.1 0.3

46.8 20.6 67.4 0.1

45.8 21.2 67.0 0.1

29% PDMAEMA

γpv d γpv γ pv γ pw

p

3.6 7.5 11.1 31.1

5.8 19.2 24.9 22.4

2.3 85.1 87.4 52.2

45.4 21.1 66.5 0.2

44.6 25.5 70.1 0.3

43.4 26.2 69.5 0.5

PFOMAa

γpv d γpv ␥pv ␥pw

p

0.003 0.02 25.3 24.1 25.3 24.1 50.2 48.9

0.07 88.2 88.3 69.5

1.4 80.0 80.4 53.2

0.1 86.1 86.2 67.4

Polysulfone

γpv d γpv ␥pv ␥pw

p

5.2 82.3 87.5 42.8

36.5 34.4 70.8 2.6

44.2 25.6 69.8 0.4

38.6 26.6 65.2 1.1

a

5.0 85.9 90.8 45.1

42.5 25.3 67.8 0.5

Static contact with NOM solution because PFOMA membrane was impermeable to water.

chemical cleaning of the membrane. It appears that NOM is very strongly sorbed to the surface. The spontaneity of the NOM fouling process can be assessed by recognizing that the components of SFE listed in Table 2 are Gibbs free energies. For any component of SFE, a negative difference between the final and initial state of the membrane would indicate the spontaneous process direction. The SFE and IFE values in Table 2 of the dry membrane were subtracted from those after NOM filtration to calculate the change caused by NOM exposure. These changes p are presented in Table 2. The 1γpv values are posid and tive for all the membrane surfaces whereas 1γpv 1γ pv values are negative only in case of polysulfone and the Hydranautics NTR 7450 membrane surfaces. However, the most important of these changes, is the negative value of 1γ pw which was measured for all but the PFOMA membrane surface. The implication is that NOM sorption was a spontaneous and inevitable process for all but the PFOMA membrane surface although the tendency is less for the Hydranautics NTR 7540 than for polysulfone surface. Bouchard et al. [5] noted the same trends after exposure of a cellulose

acetate membrane to a calcium amended NOM solution; they proposed that calcium reduced electrostatic repulsion between NOM colloids and membrane surface to permit adsorption.

4. Conclusions Although this research failed to produce a new membrane surface that was more resistant to NOM fouling than commercial membranes, information was obtained that may guide selection of more successful polymers. Changes in SFE and IFE of five different membrane materials after wetting, filtration of UPW, filtration of a NOM- and PAA-containing solutions, and chemical cleaning were quantified by contact angles measurements using the captive bubble technique. Wetting of the experimental, block-copolymer membranes with ethanol and water increased the hydrophilicity of the surface possibly because the hydrophilic PDMAEMA polymer was free to move to the surface. Based on the IFE and SFE results, the order of decreasing hydrophilicity for dry as well as

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wetted membrane condition was: Hydranautics NTR 7450 (sulfonated polysulfone)>40% PDMAEMA-bPFOMA >29% PDMAEMA-b-PFOMA >polysulfone >PFOMA. UPW filtration changed this order; causing the Hydranautics NTR 7450 and polysulfone membranes to become extremely hydrophilic in comparison with the two block copolymer membranes and the impermeable, hydrophobic PFOMA membrane; pressurization is thus implicated as responsible. Adsorption of NOM increased the hydrophilic nature of all the membrane surfaces although the experimental block-copolymer membranes were more noticeably affected than the Hydranautics NTR 7450. The IFE decreased to nearly 0 dyn/cm after filtration of NOM solution for all but the PFOMA membrane, consistent with formation of a hydrated NOM layer. Based on the negative direction of change in SFE, deposition of NOM into a layer adjacent to the membrane surface was a spontaneous process for all but the PFOMA membrane. Rigorous chemical cleaning and rinsing with UPW failed to remove the NOM as evidenced by scanning electron micrographs and atomic force micrographs. The interaction of PAA with each membrane surface produced similar changes in contact angle as after NOM deposition suggesting that mostly acid–base interaction and hydrogen bond formation between membranes and NOM or PAA. The fact that NOM produced a similar change in the surface energies of several chemically different membrane materials suggests multiple pathways of chemical interactions. Despite extensive research cited in the literature, the influence of charge, molecular size and specific functional groups of NOM on membrane fouling remains poorly understood. Systematic study of these factors warrants further research. CAM and subsequent calculation of SFE components may be helpful in screening of membranes to determine which are most resistant to fouling. This would reduce the testing program required for more time intensive and complex, membrane filtration experiments.

γ po γ wo γ iv γivd p γiv θa θo

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interfacial energy between polymer and octane interfacial energy between water and octane surface free energy between any phase (i) and its vapor (v) dispersive component of surface free energy polar component of surface free energy contact angle between air and polymer contact angle between octane and polymer

Acknowledgements This research was supported by the National Water Research Institute (Fountain Valley, CA) and the American Water Works Association Research Foundation (Denver, CO). The assistance of Dr. Benny Freeman, Michelle Arnold and Kazu Nagai of the Department of Chemical Engineering, North Carolina State University and Dr. Joseph DeSimone and Terri Johnson, Department of Chemistry, University of North Carolina is greatly appreciated. Thanks also Dr. Richard Superfine of the Department of Physics and Astronomy, University of North Carolina for use of the atomic force microscope.

Appendix A. Derivation of surface energy relationships Beginning with Young’s equation for the air-watermembrane system shown in Fig. 3 we have: γpw = γpa − γwa cos θa

(A.1)

where γ is the interfacial energy and the subscripts represent the phases of interest (p: polymer, i.e. membrane surface; w: water and a: air). The knowns are γ wa (72.6 erg/cm2 at 20◦ C) and θ a but both γ pw and γ pa are unknown and so the difference (γ pa −γ pw ) is reported as the adhesion tension. The contact angle from the n-octane captive bubble provides an additional Young’s equation relationship from which to calculate ␥pw :

5. Nomenclature

γpw = γpo − γwo cos θo

γ pw γ pa γ wa

The surface free energy of interaction between any phase (i) and its vapor (v) is represented by γ iv and it follows that

interfacial energy between polymer and water interfacial energy between polymer and air interfacial energy between water and air

(A.2)

72

A.R. Roudman, F.A. DiGiano / Journal of Membrane Science 175 (2000) 61–73 p

γiv = γivd + γiv

(A.3)

where the superscripts refer to the dispersive (d) and polar (p) components of surface free energy. The interfacial free energy (IFE) between two phases (1 and 2) can be estimated from the polar and dispersive components of surface free energy (SFE). Two approximations have been used for polymers: Harmonic mean equation γ12 ≈ γ1v + γ2v −

d γd 4γ1v 2v d + γd γ1v 2v

p

−

p

4γ1v γ2v p

p

γ1v + γ2v

Geometric mean equation q q d γd − 2 γp γp γ12 ≈ γ1v + γ2v − 2 γ1v 1v 2v 2v

p

(A.4)

p

γwa = γwv = 72.6 erg/cm2 p

γwv = 50.8 erg/cm2 p

γov = 0 erg/cm2 d d γov = γov = γwv = 21.8 erg/cm2

(γwv − γov − γwv cos θo )2 d 4γwv (A.9)

p

(A.5)

(A.7)

Eq. (A.6) and (A.7) are solved with Eq. (A.1) and (A.2) where γ pa and γ wa in Eq. (A.1) are the same as γ pv and γ wv , respectively, in Eq. (A.6) and (A.7). The d ) of SFE is obtained from dispersive component (γpv Eq. (A.1), (A.6) and (A.7) and the polar component p (γpv ) from Eq. (A.2), (A.6) and (A.7). The following components of SFEs all at 20◦ C are known [10,20,21]: d = γav = γav = 0 γav

γpv =

= 12.7(1 − cos θo )2

Either Eq. (A.4) or (A.5) can be solved together with Young’s equation and experimental measurements of contact angle. The geometric equation does not account for polar interaction at the interface because of hydrogen bonding [10,19,20]. However, the harmonic mean equation together with contact angle measurements leads in some cases to calculation of negative IFEs, a result that is difficult to explain without assuming thermodynamic instability [5]. For this reason, the geometric mean equation was used. The polymer–water and polymer–octane interfaces are given by Eq. (A.5) as: q q d γd − 2 γp γp (A.6) γpw = γpv + γwv − 2 γpv wv wv wv q q d γd − 2 γp γp γpo = γpv + γov − 2 γpv pv ov ov

The results of solving the two sets of equations are:  2 q p γwv (1 − cos θa ) − 2 50.8γpv d = γpv d 4γwv  q 2 p = 2.3307 5.0930(1 − cos θa ) − γpv (A.8)

d is calculated After calculating γpv from Eq. (A.9), γpv from Eq. (A.8). It follows from Eq. (A.3) that γ pv is then known. Finally, returning γ pv and other known values to Eq. (A.6) gives: q d γpw = 72.6 + γpv − 9.3381 γpv q p −14.2548 γpv (A.10)

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