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Hypochlorite degradation of crosslinked polyamide membranes
Journal of Membrane Science 282 (2006) 456–464
Hypochlorite degradation of crosslinked polyamide membranes II. Changes in hydrogen bonding behavior and performance Young-Nam Kwon ∗ , James O. Leckie Department of Civil and Environmental Engineering, Stanford University, Terman B-13, Stanford, CA 94305-4020, United States Received 28 February 2006; received in revised form 29 May 2006; accepted 3 June 2006 Available online 10 June 2006
Abstract In this work the effect of chlorination on the change of hydrogen bonding behavior and performance of crosslinked aromatic polyamide membrane has been systematically investigated. Chlorination replaced hydrogen with chlorine on the amide group of the membrane polymer and caused the loss of hydrogen bonding sites, confirmed by the systematic shift of the amide I band (C O stretching vibration) to the higher wavenumbers and the disappearance of the amide II band (N–H bending vibration) by FT-IR spectrum analysis. Performance experiments demonstrated that the change of hydrogen bonding behavior due to the chlorination caused the change of flexibility of polymer chains followed by the change of initial flux and subsequent flux change with filtration time. © 2006 Elsevier B.V. All rights reserved. Keywords: Degradation; Chlorine; Crosslinked polyamide; Hydrogen bonding; Performance
1. Introduction Chlorination and chemical cleaning are used effectively to decrease membrane fouling and rejuvenate membranes in use. The water/wastewater treatment plants such as the NEWater Factory in Singapore dose continuously sodium hypochlorite into feed water (which sometimes contains ammonia) to form chloramines and to maintain 2.0–2.5 ppm total residual chlorine [1,2]. However, these chemicals are aggressive toward most commercial polymer-based membranes, eventually resulting in impaired performance. Several studies have been performed to investigate the cause of performance change of linear polyamide membranes caused by chlorination. The reason for the performance change has been identified as likely due to specific structural changes within the polymers caused by the chemical exposure. The performance decline of linear polyamide membrane seems to be from the loss of structural integrity of constituent polymers. The chlorine substituents on the amide nitrogen or aromatic rings may cause physical deformation or chemical cleavage at amide linkages in the linear polymer chain [3]. Polymer deformation of the linear polyamide was proposed by Glater ∗
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0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.06.004
and Zachariah [4], who suggested that hydrogen bonding shifts from inter to intramolecular within a linear polymer chain and that this shift causes chain deformation followed by alteration in gross polymer properties. Avlonitis et al. [5] proposed that the decreased intermolecular bonding of the linear polyamide has an effect on the structural transition from a crystalline to amorphous state where chemical attack occurs preferentially. Singh [6] showed the loss in viscosity of linear polyamide due to chain cleavage by chlorine. The transition of integral structure to dispersed swollen polymer structure, due to the decreased intermolecular bonding or molecular weight, seemed to explain the performance decline of linear polyamide membranes. Unlike the linear polyamides, the cause of performance change of crosslinked polyamide membranes due to chlorination had not been systematically discussed. Koo et al. [7] proposed that flux decline was associated with the hydrophobic character introduced by added chlorine and that flux increase was from chain cleavage. Glater et al. [8,9] proposed that flux decline was due to membrane tightening resulting from chlorine addition and that flux increase was caused by bond cleavage. Soice et al. [10,11] proposed the major cause of performance decline caused by chlorination is not due to polymer chain cleavage but physical separation of the polyamide skin layer from the polysulfone support layer. However, these explanations could not explain the different performance behavior of the
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crosslinked polyamide membranes chlorinated at various pH conditions. The purpose of this work was to systematically investigate the effect of membrane exposure to hypochlorite oxidant (disinfecting agent) on the changes of hydrogen bonding behavior and performance of thin film composite crosslinked polyamide membranes and to gain insight into the causes of performance change of crosslinked membranes.
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of a polyamide skin top layer of the membrane. A minimum of 200 scans at a resolution of 1.0 cm−1 were signal-averaged. The membranes were placed on the ATR crystal and pressed onto the surface with a plate press. The instrument was covered and continuously purged with dry air to prevent interference of atmospheric moisture with the spectra. 2.3. Filtration experiments
2.2. ATR-FT-IR characterization
2.3.1. Filtration system The system in Fig. 1 was set up to provide the same operating conditions for four identical, stainless steel RO test cells (Sepa CF II, GE Osmonics Inc.) arranged in parallel. This system contained four independent sets of pressure channels with a recirculation mode. Each pressure vessel houses 140 cm2 of effective surface area of RO membrane with nominal dimensions of 19.1 cm × 14.0 cm. The feed solution for these cells was contained in a 100 L stainless steel cylindrical tank and was mechanically stirred by a mixer. The temperature of the feed solution was held constant (25 ◦ C) during the experiment by a circulating water bath (RTE-111, Neslab). The solution was pumped out of the reservoir and pressurized by a Hydracell pump (Wanner Engineering), capable of delivering 11.3 l/min (LPM) at a maximum pressure of 1000 psi (69.0 bar). Through careful adjustment of the valves and a back pressure regulator on a bypass line, the crossflow velocity and feed pressure was finely controlled.
Attenuated total reflection-Fourier transform infrared (ATRFT-IR) spectra were recorded on a Nicolet Nexus 470 spectrometer. As an internal reflection element, a flat plate Ge crystal at an incident angle of 45◦ was used to get a higher contribution
2.3.2. Protocols for the performance measurements The filtration test protocol can be divided into six steps: presoaking, compaction, conditioning, chlorination, compaction, and conditioning.
2. Experimental materials and methods 2.1. Polyamide membrane We have used a commercially available Hydranautics ©LFC1 membrane (Oceanside, CA) as a representative of polyamide membranes. The membrane is a thin film composite crosslinked aromatic polyamide membrane which is produced by interfacial polymerization of 1,3-phenylenediamine and 1,3,5benzentricarbonyl chloride, having amide bonds (–CONH–) and crosslinked/non-crosslinked portions of the structure. The LFC1 membrane is reported to have a chlorine tolerance of 1000 ppm h [12,13]. Each sample was thoroughly rinsed with flowing deionized (DI) water for 6 h, sonicated in a Milli-Q water bath for 30 min, and then dried at room temperature.
Fig. 1. Schematic of the reverse osmosis filtration system.
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Fig. 2. Protocol for the performance measurements. (1) Operating conditions: pressure = ∼220 ± 10 psi (15.2 ± 0.7 bar), temperature = 25 ± 1 ◦ C, flowrate = 1 ± 0.2 LPM. (2) Operating conditions (specifically at the point of performance measurement): pressure = 225 ± 1 psi (15.5 ± 0.1 bar), temperature = 25 ± 1 ◦ C, flowrate = 1 ± 0.02 LPM, salt = 2000 ppm NaCl solution. (3) Chlorination conditions: chlorine concentration = 100, 500, 1000, 2000 ppm, exposure time = 1 h, pH 4 and 9.
The protocol developed for performance measurements of chlorinated membranes is summarized in Fig. 2. The membranes were first presoaked in deionized water for 24 h. The membranes were then compacted near 220 psi (15.2 bar) until the permeate flux stabilized. Concentrated NaCl solution was added to the feed tank to maintain 2000 ppm concentration. The feed solution was then fed to the RO test cells from the feed tank to condition the membranes at the same operating condition as the compaction stage. After compacting and conditioning the virgin membranes, the performance of the membranes was characterized in terms of permeate flux and salt rejection. Permeate flux was measured volumetrically (100 ml vol. flaks and stopwatches) and salt rejection was determined by measurement of feed and permeate conductivity (Ultrameter
4P, Myron L Company). After performance measurements, the membranes were taken out of the test cells, and thoroughly rinsed with DI water. The membranes were then exposed to 100, 500, 1000, 2000 ppm chlorine solutions for 1 h at pH 4 and 9. The total exposure of polyamide membrane samples to chlorine was expressed as ppm h. The chlorine exposure was performed in Pyrex glass bottles covered with PTFE (polytetrafluoroethylene) caps, and the contents were mixed on a shaker. The chlorinated membranes were thoroughly rinsed with DI water and re-loaded in the GE Osmonics test cells. The membranes were compacted again near 220 psi (15.2 bar), and conditioned with 2000 ppm NaCl. Samples used to determine the permeate flux and salt rejection of the chlorinated membranes were taken at various time intervals. Whenever performance was measured,
Fig. 3. ATR-FT-IR absorption spectra of polysulfone and virgin LFC1 membrane. Polysulfone spectra modified from FDM Electronic Handbook [23].
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the operating conditions were finely readjusted to the desired values (225 psi (15.5 bar) and 1 l/min). Three measurements of flux and six measurements of rejection were averaged for each representative data point for performance measurements. 3. Results and discussion 3.1. Hydrogen bonding behavior 3.1.1. Virgin LFC1 membrane Fig. 3 presents FT-IR spectra of a polysulfone and a virgin LFC1 polyamide membrane. The polysulfone is composed of benzene rings, CH3 , ether and sulfone groups, and thus its FT-IR spectrum exhibited peaks corresponding to the functional groups. Aromatic in-plane ring bend stretching vibrations occurred in the region of 1625–1430 cm−1 . A C–H symmetric deformation vibration of C(CH3 )2 was observed as two weak-medium bands (almost equal intensity) in the range of 1385–1365 cm−1 [14]. Aryl-O-aryl group had a characteristic strong band near 1250 cm−1 associated with the C–O–C asymmetric stretching vibration [15,16]. The asymmetric SO2 stretching vibration occurred in the 1350–1280 cm−1 region and the band due to the symmetric stretching vibration in the 1180–1145 cm−1 region. The band near 830 cm−1 resulted from the in-phase out-of-plane hydrogen deformation for parasubstituted phenyl groups [15]. Since the penetration depth of the IR beam, below wavenumber 2400 cm−1 in the Germanium ATR crystal, is more than
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the thickness (∼300 nm) of the polyamide skin layer of the LFC1 membrane, the FT-IR spectrum of LFC1 membrane in Fig. 3 includes bands of both the polyamide skin top layer and polysulfone sublayer of the membrane. From comparison of two spectra in the Figure, it appears that the peaks at 1663, 1609, 1541, 1444 cm−1 are unique to the polyamide skin top layer. The peaks at 1663 and 1541 cm−1 are the amide I and II bands, respectively. The 1663 cm−1 peak, which is usually identified as an amide I mode in a secondary amide group, may consist of contributions from the C O stretching (largest contribution), the C–N stretching, and the C–C–N deformation vibration [17]. The amide II band (1541 cm−1 ) is due to a motion combining both N–H in-plane bending and the N–C stretching vibrations of the group –CO–NH– in its trans form. The amide II bands are mainly due to the N–H bending motion [14]. 3.1.2. Effect of chlorine concentration The infrared spectra of LFC1 membranes, recorded as a function of increasing chlorine concentration at both pH 4 and 9, are displayed in Fig. 4. After exposure to chlorine, the frequencies of the peaks at 1663, 1609, 1541, and 1416 cm−1 shifted, and the intensities of the peaks at 1609, 1587, 1541, 1488, 1444, and 1416 cm−1 changed systematically. It is interesting to note that the extent of peak shift and the decrease of peak intensity was greater for the membrane exposed to 20 ppm h chlorine at pH 4 than for the membrane exposed to
Fig. 4. ATR-FT-IR absorption spectra for virgin and degraded LFC1 membranes. (Left) LFC1 membranes degraded at pH 9 and various chlorine concentrations. (Right) LFC1 membranes degraded at pH 4 and various chlorine concentrations.
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II band disappeared due to the decreased number of hydrogen bonding sites caused by the substitution of chlorine for hydrogen on the amide group, and due to too much shift and merging with other peaks. The decreasing intensity of the 1541 cm−1 peak at pH 9 can also be explained by means of the decreasing number of N–H bonds due to hydrogen replacement by chlorine. The peak intensities at 1609 cm−1 , also, systematically decreased with increasing concentration of chlorine in the soaking solution (Fig. 4). Since the peak may be assigned as a C C ring stretching vibration motion [21], the decline of the peak intensities shows the aromatic rings are monotonically chlorinated after chlorination.
Fig. 5. Peak shift of amide I and II bands after chlorination of LFC1 membrane at pH 4 and 9.
2000 ppm h chlorine at pH 9. This indicates that the 20 ppm h at pH 4 environment was more reactive. Fig. 5 shows peak shifts of the amide I (1663 cm−1 ) and amide II (1541 cm−1 ) bands of the LFC1 membrane. As the chlorine concentration in a soaking bath increased in a stepwise fashion, the amide I peak after chlorination shifted systematically to higher frequency. The systematic shift of the amide I peak to the higher frequency is thought to result from breakage of hydrogen bonds between the C O and N–H groups. When the C O groups form hydrogen bonds with N–H groups in the virgin membrane, the hydrogen bonding tends to decrease the doublebond character of the C O moiety, shifting the adsorption band to lower frequency [14]. Breaking hydrogen bonds due to the chlorination shifted the amide I band back to its original high frequency. The chlorination of the LFC1 membrane substitutes Cl for H in the amide bond of the polymer since the amide nitrogen is vulnerable to chlorine attack [4,18–20]. The substitution decreases the average hydrogen bond strength among the polymer chains. Therefore, the systematic shift to higher frequency of the amide I band with increasing concentration of chlorine can be explained by a decrease in the average hydrogen bond strength following the transformation of H-bonded carbonyl state to free carbonyl. The amide II band (1541 cm−1 ) is mainly due to the N–H bending motion. With increasing chlorine exposure at pH 9, the peak shifted monotonically to lower frequency and peak intensity decreased. When the N–H groups form hydrogen bonds with C O or other N–H groups in the virgin membrane, the bending vibration of the N–H bond is restricted due to the hydrogen bonds, therefore, shifting the absorption band to higher frequency [14]. After chlorination, hydrogen bonding was weakened, allowing the N–H bending peak to shift to lower wavenumber due to the stretched hydrogen bond length (decreased interaction of the bonding) resulting from the conversion of neighbor N–H groups to N–Cl groups. At pH 4, the amide
3.1.3. Effect of pH Figs. 6 and 7 show FT-IR spectra and the peak shift of the amide I/II band of the LFC1 membranes exposed to various pH values at 100 ppm h chlorine. Peak shifts at amide I/II peaks and the intensity change of the amide II peak due to the change of pH showed the same trend as those observed for the change of chlorine concentration in the soaking solution (Figs. 4 and 5). This was due to the fractional increase of the HOCl species, the more aggressive form of chlorine, and lower negative zeta potential of the membrane with decreasing pH.
Fig. 6. ATR-FT-IR absorption spectra of LFC1 membranes degraded at various pH and 100 ppm h Cl.
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Fig. 7. Peak shift of amide I and II bands of the LFC1 membrane degraded at various pH and 100 ppm h chlorine solution.
3.2. Performance 3.2.1. Chlorination of LFC1 membrane at pH 4 Table 1 shows the permeate flux and salt rejection of the virgin LFC1 membrane, as reported by the manufacturer (Hydranautics ©) and measured in our lab. Fig. 8 shows the normalized flux and rejection of LFC1 membranes which were chlorinated under conditions of 100, 500, 1000, 2000 ppm h chlorine and pH 4. Normalized flux1 (or rejection1) was defined as the flux (or rejection) of chlorinated membranes divided by the flux (or rejection) of their virgin membranes under comparable conditions. The flux of LFC1 membranes decreased after chlorination at pH 4. Flux decreases of 37%, 39%, 40%, and 49% occurred at 100, 500, 1000, 2000 ppm h chlorinated membranes (Table 2) when measured at 71 h total filtration time (equivalent to the 6 h DI water filtration and following 12 h filtration with 2000 ppm NaCl solution after chlorination. Please refer to Fig. 2). The first measurement of performance after chlorination (at 71 h total filtration time) in Fig. 8 showed exposure of the membrane lowered the permeate flux in proportion to the degree of chlorine exposure. According to the X-ray photoelectron spectroscopy (XPS) investigation in our preceding paper [22], chlorine in the soaking bath reacted with the LFC1 membrane and was bound Table 1 Performance data of the membranes reported by the manufacturers and measured at our lab
LFC1
Reported by manufacturersa
Measuredb
Flux (l/m2 h)
Rejection (%)
Flux (l/m2 h)
Rejection (%)
46.64
99.5
45.76 ± 4.65
98.2 ± 0.2
Operating condition. a 1500 ppm NaCl, 225 PSI, 25 ◦ C, 15% recovery. b 2000 ppm NaCl, 225 PSI, 25 ◦ C, 1 l/min (LPM).
Fig. 8. Normalized flux1 and rejection1 of LFC1 membranes degraded at 100, 500, 1000, 2000 ppm h Cl and pH 4. The performance was measured at 71, 83, 92 h total filtration time (12, 24, 33 h filtration with 2000 ppm NaCl solution following 6 h DI water filtration after chlorination). Normalized flux1 (or rejection1) was defined as the flux (or rejection) of chlorinated membranes divided by the flux (or rejection) of their virgin membranes under compatible conditions.
to the membrane surface up to 2.6% atomic percent at pH 4. The replacement of hydrogen with chorine on the amide group (N–H) of the membrane caused the loss of most of the hydrogen bonding sites, confirmed by the systematic shift of the amide I band (C O stretching vibration) to a higher wavenumber and the disappearance of the amide II band (N–H bending vibration) in the FT-IR spectrum analysis of the membrane. More hydrogen bonds were broken in highly chlorinated membranes (Figs. 4 and 5). The breakage of most hydrogen bonds supporting the structure of the polyamide layer along with crosslinking could provide a large increase of rotational freedom and flexibility of the polymer chains due to (i) the removal of H-bonds anchoring a polymer chain to another Table 2 Normalized performance1 of the LFC1 membranes chlorinated at pH 4
Virgin LFC1 100 ppm h Cl 500 ppm h Cl 1000 ppm h Cl 2000 ppm h Cl
Normalized flux1
Normalized rejection1
1.00 0.63 0.61 0.60 0.51
1.00 1.01 1.00 0.98 0.98
Measured at 71 h total filtration time (6 h deionized water filtration and following 12 h filtration with 2000 ppm NaCl solution).
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polymer chain and (ii) a lowered energy barrier of cis–trans rotation around the C and N of the chlorinated amide bond [19]. By the application of high pressure (225 psi (15.5 bar)) during the filtration test, the polymer chains having large rotational freedom and flexibility could collapse or compact. The collapse or compaction of the polymer chains might block the passage of water molecules through the polymeric membrane, resulting in the decline of flux after chlorination at pH 4. The rejection of the LFC1 membrane, shown at the bottom of Fig. 8, increased a little after chlorination and then decreased with increasing concentration of chlorine in the soaking bath. The change of rejection due to chlorination was not significant compared with the change of flux. Similar to membrane flux, membrane rejection, also, increased with filtration time. The increase in rejection with filtration time seemed to be due to increased permeate flux. It is interesting to note that the flux and rejection of the membranes changed with increasing filtration time. The flux of the membranes chlorinated at higher concentrations of chlorine increased more quickly than that of membranes degraded at lower chlorine concentrations. Fig. 8 presents the flux as a function of the degree of chlorine exposure to the membrane. When the flux was expressed as a function of total filtration time, it was more clearly observed that the flux systematically increased with increasing filtration time (Fig. 9). Normalized flux1 shown at the top in Fig. 9 was derived by taking the ratio of the flux of chlorinated membranes to the steady-state flux of their virgin membranes under similar conditions. The normalized flux1, then, was normalized again based on the first normalized flux measurement (at 71 h total filtration time for Fig. 9) after chlorination, which was designated the normalized flux2 at the bottom of Fig. 9. The flux of the membrane degraded at 100 ppm h chlorine at pH 4 remained almost the same after 6 hr filtration of DI water and 33 h filtration of 2000 ppm NaCl solution (Fig. 9 and Table 3). On the other hand, membranes chlorinated at 500 and 1000 ppm h showed a slight increase of flux and the 2000 ppm h chlorined membrane showed the largest increase of flux after the same filtration time. This might be caused by increased rotational freedom or flexibility of polymer chains due to chlorination. Even if the polymer chains were collapsed by pressure (225 psi (15.5 bar)) applied in the filtration process, the flexibility of the polymer chains, provided by chlorination, still remained the same as the chlorinated but unpressurized membranes. Highly chlorinated membranes had more rotational freeTable 3 Normalized Flux2 of the LFC1 membranes chlorinated at pH 4 Normalized flux2
100 ppm h Cl 500 ppm h Cl 1000 ppm h Cl 2000 ppm h Cl
71 h (6 + 12)
83 h (6 + 24)
92 h (6 + 33)
1.00 1.00 1.00 1.00
1.00 1.07 1.12 1.27
0.99 1.08 1.16 1.32
Measured at 71, 83, and 92 h total filtration time (6 h deionized water filtration and following 12, 24, and 33 h filtration with 2000 ppm NaCl solution).
Fig. 9. Normalized flux2 of LFC1 membranes degraded at 100, 500, 1000, 2000 ppm h Cl and pH 4. The performance was measured at 71, 83, 92 h total filtration time (12, 24, 33 h filtration with 2000 ppm NaCl solution following 6 h DI water filtration after chlorination). Normalized flux2 (or rejection2) was defined as the normalized flux1 (or rejection1) of the membrane divided by the first normalized flux (or rejection) measurement (at 71 h total filtration time for this figure).
dom of the polymer chains. The continuous passage of water molecules at 225 psi (15.5 bar) pressure through the polymeric membrane might enable the polymer chains to rearrange themselves to find the most thermodynamically stable structures, which can allow more passage of water molecules and hence flux recovery. Therefore, highly chlorinated membranes, which have the most flexibility of the polymer chains, could most quickly be collapsed by the initial application of pressure, but most easily find stable positions for the polymer chains. 3.2.2. Chlorination of LFC1 membrane at pH 9 Fig. 10 shows that the permeate flux data of the LFC1 membrane chlorinated at pH 9 were higher than the flux of its virgin membrane. This was opposite experimental results from the membranes chlorinated at pH 4. As shown by the initial flux of chlorinated membranes measured at 70 h total filtration time, the permeate flux rapidly increased for those membranes exposed to low chlorine concentration (100 ppm h Cl), and then the flux gradually decreased with increasing concentration of chlorine in the soaking bath (Table 4). After chlorination with 100 ppm h chlorine at pH 9, small amounts of hydrogen bonds on the surface of LFC1 membranes were broken. The rupture of hydrogen bonds increased the rotational freedom of the polymer chains. But, unlike pH 4
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Fig. 10. Normalized flux1 and rejection1 of LFC1 membranes degraded at 100, 500, 1000, 2000 ppm h Cl and pH 9. The performance was measured at 70, 82, 94 h total filtration time (12, 24, 36 h filtration with 2000 ppm NaCl solution following 6 h DI water filtration after chlorination).
conditions, the polymer frames, supported by hydrogen bonds and crosslinking between the polymer chains, still remained the same as the virgin membrane since the number of broken hydrogen bonds was not sufficient to exceed the membrane’s compressive yield points which would result in the mass compaction or collapse of polymer chains. The increased rotational flexibility of the chains with an intact polymer structure might release the restriction of water passage through the polymeric membrane, resulting in the increase of initial flux at the soaking condition of 100 ppm h chlorine and pH 9. With increasing concentration of chlorine in a soaking bath, more chlorine was bound to the membrane surface and there could be local collapse or compaction in the membrane. This appears to cause the subsequent flux decline (measured at 70 h total filtration time) for 500, 1000, 2000 ppm h chlorination at pH 9. Table 4 Normalized performance1 of the LFC1 membranes chlorinated at pH 9
Virgin LFC1 100 ppm h Cl 500 ppm h Cl 1000 ppm h Cl 2000 ppm h Cl
Normalized flux1
Normalized rejection1
1.00 1.12 1.11 1.07 1.02
1.00 1.00 1.00 0.99 0.98
Measured at 70 h total filtration time (6 h deionized water filtration and following 12 h filtration with 2000 ppm NaCl solution).
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Fig. 11. Normalized flux2 of LFC1 membranes degraded at 100, 500, 1000, 2000 ppm h Cl and pH 9. The performance was measured at 70, 82, 94 h total filtration time (12, 24, 36 h filtration with 2000 ppm NaCl solution following 6 h DI water filtration after chlorination).
The normalized flux of LFC1 membranes chlorinated at pH 9 changed with increasing filtration time similar to the membranes degraded at pH 4. As shown in Fig. 11, the flux of 1000 and 2000 ppm h chlorinated membranes increased with filtration time. On the other hand, the flux of 500 ppm h chlorinated membranes remained almost constant, and the flux of 100 ppm h chlorinated membrane slightly decreased with filtration time. The data for 500 ppm h chlorine experiment showed no change at pH 9. The flux increase with increasing filtration time of membranes chlorinated at 1000 and 2000 ppm h chlorine and pH 9 can be explained by the same reasoning as the flux increase at pH 4 (highly chlorinated membranes can be easily collapsed by the initial application of pressure, but the membranes can easily find stable positions for the polymer chains). The slight flux decrease with increasing filtration time of 100 ppm h chlorinated membrane was likely attributed to the distortion of polymer chains due to the local collapse of polymer structures neighboring the chlorinating amide bond resulting from continuous application of high pressure (225 psi). The flux change with filtration time was determined by the balance between rearrangement of flexible polymer chains and distortion of the polymer chains. 4. Conclusion A systematic investigation of the relationship between chlorine exposure of polyamide LFC1 membranes and changes in hydrogen bonding behavior, and performance of the membranes has been presented. FT-IR spectra showed the amide I band
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(C O stretching peak at 1663 cm−1 ) of polyamide membranes shifted systematically to a higher wavenumber and the peak intensity of amide II band (N–H bending at 1541 cm−1 ) monotonically decreased with increasing concentration of chlorine and decreasing pH of the soaking bath solution. These systematic changes of the peaks are recognized as resulting from breakage of hydrogen bonds between C O and N–H groups of the membranes. The FT-IR experimental results demonstrated that the chlorination changed the hydrogen bonding behavior among the crosslinked polyamide chains. Chlorination of polyamide membranes showed (i) initial flux change, and then (ii) following systematic flux shifts with filtration time. The initial flux change and flux change with filtration time depended on pH and concentration of chlorine in the soaking bath. The initial flux of the membrane decreased at low pH and increased for slightly chlorinated membranes at high pH. The flux shift with filtration time was grater for highly chlorinated membranes. The highly chlorinated membranes, which have the most flexibility of the polymer chains, could most quickly be collapsed by the initial application of pressure, but most easily find stable positions for the polymer chains. These flux changes caused by chlorination were attributed to the change of increased rotational freedom or the flexibility of polymer chains due to changes in hydrogen bonding behavior caused by chlorine attack. Systematic investigation of chlorine exposure to the polyamide membranes on the change of performance (water flux and salt rejection) demonstrated that the change of hydrogen bonding behavior due to the chlorination caused the change of rotational freedom or the flexibility of polymer chains followed by the change from initial performance and subsequent flux changes with filtration time. Acknowledgements This work was funded by the Clean Water Programme (CWP) with Nanyang Technological University (NTU) and STC WaterCAMPWS of the National Science Foundation under agreement #CTS-0120978. Membranes used for this study were generously donated by FilmTec ©. References [1] C.R. Bartels, M. Wilf, K. Andes, J. Iong, Design considerations for wastewater treatment by reverse osmosis, Water Sci. Technol. 51 (2005) 473–482. [2] M. Wilf, S. Alt, Application of low fouling RO membrane elements for reclamation of municipal wastewater, Desalination 132 (2000) 11–19.
[3] J. Glater, S.K. Hong, M. Elimelech, Search for a chlorine-resistant reverse osmosis membrane, Desalination 95 (1994) 325–345. [4] J. Glater, M.R. Zachariah, Mechanistic study of halogen interaction with polyamide reverse-osmosis membranes, ACS Symp. Ser. (1985) 345– 358. [5] S. Avlonitis, W.T. Hanbury, T. Hodgkiess, Chlorine degradation of aromatic polyamides, Desalination 85 (1992) 321–334. [6] R. Singh, Polyamide polymer solution behavior under chlorination conditions, J. Membr. Sci. 88 (1994) 285–287. [7] J.-Y. Koo, R.J. Petersen, J.E. Cadotte, ESCA characterization of chlorinedamaged polyamide reverse osmosis membrane, Polym. Prepr. 27 (1986) 391–392. [8] J. Glater, J.W. McCutchan, S.B. McCray, M.R. Zachariah, Effect of halogens on the performance and durability of reverse-osmosis membranes, ACS Symp. Ser. (1981) 171–190. [9] J. Glater, M.R. Zachariah, S.B. McCray, J.W. McCutchan, Reverse osmosis membrane sensitivity to ozone and halogen disinfectants, Desalination 48 (1983) 1–16. [10] N.P. Soice, A.R. Greenberg, W.B. Krantz, A.D. Norman, Studies of oxidative degradation in polyamide RO membrane barrier layers using pendant drop mechanical analysis, J. Membr. Sci. 243 (2004) 345– 355. [11] N.P. Soice, A.C. Maladono, D.Y. Takigawa, A.D. Norman, W.B. Krantz, A.R. Greenberg, Oxidative degradation of polyamide reverse osmosis membranes: Studies of molecular model compounds and selected membranes, J. Appl. Polym. Sci. 90 (2003) 1173–1184. [12] W. Bates, Reducing the fouling rate of surface and waste water RO systems, IWC-98-08, 1998. [13] W. Bates, R. Cuozzo, Integrated membrane systems, Technical Paper, Hydranautics, 2000. [14] G. Socrates, Infrared Characteristic Group Frequencies, Wiley/ Interscience, 1994. [15] Sadtler, The Infrared Spectra Atlas of Monomers and Polymers, Sadtler Research Laboratories, 1980. [16] Y.Q. Song, J. Sheng, M. Wei, X.B. Yuan, Surface modification of polysulfone membranes by low-temperature plasma-graft poly(ethylene glycol) onto polysulfone membranes, J. Appl. Polym. Sci. 78 (2000) 979– 985. [17] D.J. Skrovanek, S.E. Howe, P.C. Painter, M.M. Coleman, Hydrogenbonding in polymers—infrared temperature studies of an amorphous polyamide, Macromolecules 18 (1985) 1676–1683. [18] C. Ingold, Structure and Mechanism in Organic Chemistry, Cornell University Press, 1953. [19] J.S. Jensen, Y.F. Lam, G.R. Helz, Role of amide nitrogen in water chlorination: Proton NMR evidence, Environ. Sci. Technol. 33 (1999) 3568–3573. [20] J. Zabicky, The Chemistry of Amides, Interscience, 1970. [21] S. Wu, G. Zheng, H. Lian, J. Xing, L. Shen, Chlorination and oxidation of aromatic polyamides. I. Synthesis and characterization of some aromatic polyamides, J. Appl. Polym. Sci. 61 (1996) 415–420. [22] Y.-N. Kwon, J.O. Leckie, Hypochlorite degradation of crosslinked polyamide membranes. I. Changes in chemical/morphological properties, J. Membr. Sci. 283 (2006) 21–26. [23] FDM Electronic Handbook, Fiveash Data Management, Inc., Madison, WI, USA, 2006.
Hypochlorite degradation of crosslinked polyamide membranes II. Changes in hydrogen bonding behavior and performance Young-Nam Kwon ∗ , James O. Leckie Department of Civil and Environmental Engineering, Stanford University, Terman B-13, Stanford, CA 94305-4020, United States Received 28 February 2006; received in revised form 29 May 2006; accepted 3 June 2006 Available online 10 June 2006
Abstract In this work the effect of chlorination on the change of hydrogen bonding behavior and performance of crosslinked aromatic polyamide membrane has been systematically investigated. Chlorination replaced hydrogen with chlorine on the amide group of the membrane polymer and caused the loss of hydrogen bonding sites, confirmed by the systematic shift of the amide I band (C O stretching vibration) to the higher wavenumbers and the disappearance of the amide II band (N–H bending vibration) by FT-IR spectrum analysis. Performance experiments demonstrated that the change of hydrogen bonding behavior due to the chlorination caused the change of flexibility of polymer chains followed by the change of initial flux and subsequent flux change with filtration time. © 2006 Elsevier B.V. All rights reserved. Keywords: Degradation; Chlorine; Crosslinked polyamide; Hydrogen bonding; Performance
1. Introduction Chlorination and chemical cleaning are used effectively to decrease membrane fouling and rejuvenate membranes in use. The water/wastewater treatment plants such as the NEWater Factory in Singapore dose continuously sodium hypochlorite into feed water (which sometimes contains ammonia) to form chloramines and to maintain 2.0–2.5 ppm total residual chlorine [1,2]. However, these chemicals are aggressive toward most commercial polymer-based membranes, eventually resulting in impaired performance. Several studies have been performed to investigate the cause of performance change of linear polyamide membranes caused by chlorination. The reason for the performance change has been identified as likely due to specific structural changes within the polymers caused by the chemical exposure. The performance decline of linear polyamide membrane seems to be from the loss of structural integrity of constituent polymers. The chlorine substituents on the amide nitrogen or aromatic rings may cause physical deformation or chemical cleavage at amide linkages in the linear polymer chain [3]. Polymer deformation of the linear polyamide was proposed by Glater ∗
Corresponding author. Tel.: +1 650 723 0315; fax: +1 650 725 3162. E-mail address: [email protected] (Y.-N. Kwon). URL: http://www.ce.stanford.edu.
0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.06.004
and Zachariah [4], who suggested that hydrogen bonding shifts from inter to intramolecular within a linear polymer chain and that this shift causes chain deformation followed by alteration in gross polymer properties. Avlonitis et al. [5] proposed that the decreased intermolecular bonding of the linear polyamide has an effect on the structural transition from a crystalline to amorphous state where chemical attack occurs preferentially. Singh [6] showed the loss in viscosity of linear polyamide due to chain cleavage by chlorine. The transition of integral structure to dispersed swollen polymer structure, due to the decreased intermolecular bonding or molecular weight, seemed to explain the performance decline of linear polyamide membranes. Unlike the linear polyamides, the cause of performance change of crosslinked polyamide membranes due to chlorination had not been systematically discussed. Koo et al. [7] proposed that flux decline was associated with the hydrophobic character introduced by added chlorine and that flux increase was from chain cleavage. Glater et al. [8,9] proposed that flux decline was due to membrane tightening resulting from chlorine addition and that flux increase was caused by bond cleavage. Soice et al. [10,11] proposed the major cause of performance decline caused by chlorination is not due to polymer chain cleavage but physical separation of the polyamide skin layer from the polysulfone support layer. However, these explanations could not explain the different performance behavior of the
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crosslinked polyamide membranes chlorinated at various pH conditions. The purpose of this work was to systematically investigate the effect of membrane exposure to hypochlorite oxidant (disinfecting agent) on the changes of hydrogen bonding behavior and performance of thin film composite crosslinked polyamide membranes and to gain insight into the causes of performance change of crosslinked membranes.
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of a polyamide skin top layer of the membrane. A minimum of 200 scans at a resolution of 1.0 cm−1 were signal-averaged. The membranes were placed on the ATR crystal and pressed onto the surface with a plate press. The instrument was covered and continuously purged with dry air to prevent interference of atmospheric moisture with the spectra. 2.3. Filtration experiments
2.2. ATR-FT-IR characterization
2.3.1. Filtration system The system in Fig. 1 was set up to provide the same operating conditions for four identical, stainless steel RO test cells (Sepa CF II, GE Osmonics Inc.) arranged in parallel. This system contained four independent sets of pressure channels with a recirculation mode. Each pressure vessel houses 140 cm2 of effective surface area of RO membrane with nominal dimensions of 19.1 cm × 14.0 cm. The feed solution for these cells was contained in a 100 L stainless steel cylindrical tank and was mechanically stirred by a mixer. The temperature of the feed solution was held constant (25 ◦ C) during the experiment by a circulating water bath (RTE-111, Neslab). The solution was pumped out of the reservoir and pressurized by a Hydracell pump (Wanner Engineering), capable of delivering 11.3 l/min (LPM) at a maximum pressure of 1000 psi (69.0 bar). Through careful adjustment of the valves and a back pressure regulator on a bypass line, the crossflow velocity and feed pressure was finely controlled.
Attenuated total reflection-Fourier transform infrared (ATRFT-IR) spectra were recorded on a Nicolet Nexus 470 spectrometer. As an internal reflection element, a flat plate Ge crystal at an incident angle of 45◦ was used to get a higher contribution
2.3.2. Protocols for the performance measurements The filtration test protocol can be divided into six steps: presoaking, compaction, conditioning, chlorination, compaction, and conditioning.
2. Experimental materials and methods 2.1. Polyamide membrane We have used a commercially available Hydranautics ©LFC1 membrane (Oceanside, CA) as a representative of polyamide membranes. The membrane is a thin film composite crosslinked aromatic polyamide membrane which is produced by interfacial polymerization of 1,3-phenylenediamine and 1,3,5benzentricarbonyl chloride, having amide bonds (–CONH–) and crosslinked/non-crosslinked portions of the structure. The LFC1 membrane is reported to have a chlorine tolerance of 1000 ppm h [12,13]. Each sample was thoroughly rinsed with flowing deionized (DI) water for 6 h, sonicated in a Milli-Q water bath for 30 min, and then dried at room temperature.
Fig. 1. Schematic of the reverse osmosis filtration system.
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Fig. 2. Protocol for the performance measurements. (1) Operating conditions: pressure = ∼220 ± 10 psi (15.2 ± 0.7 bar), temperature = 25 ± 1 ◦ C, flowrate = 1 ± 0.2 LPM. (2) Operating conditions (specifically at the point of performance measurement): pressure = 225 ± 1 psi (15.5 ± 0.1 bar), temperature = 25 ± 1 ◦ C, flowrate = 1 ± 0.02 LPM, salt = 2000 ppm NaCl solution. (3) Chlorination conditions: chlorine concentration = 100, 500, 1000, 2000 ppm, exposure time = 1 h, pH 4 and 9.
The protocol developed for performance measurements of chlorinated membranes is summarized in Fig. 2. The membranes were first presoaked in deionized water for 24 h. The membranes were then compacted near 220 psi (15.2 bar) until the permeate flux stabilized. Concentrated NaCl solution was added to the feed tank to maintain 2000 ppm concentration. The feed solution was then fed to the RO test cells from the feed tank to condition the membranes at the same operating condition as the compaction stage. After compacting and conditioning the virgin membranes, the performance of the membranes was characterized in terms of permeate flux and salt rejection. Permeate flux was measured volumetrically (100 ml vol. flaks and stopwatches) and salt rejection was determined by measurement of feed and permeate conductivity (Ultrameter
4P, Myron L Company). After performance measurements, the membranes were taken out of the test cells, and thoroughly rinsed with DI water. The membranes were then exposed to 100, 500, 1000, 2000 ppm chlorine solutions for 1 h at pH 4 and 9. The total exposure of polyamide membrane samples to chlorine was expressed as ppm h. The chlorine exposure was performed in Pyrex glass bottles covered with PTFE (polytetrafluoroethylene) caps, and the contents were mixed on a shaker. The chlorinated membranes were thoroughly rinsed with DI water and re-loaded in the GE Osmonics test cells. The membranes were compacted again near 220 psi (15.2 bar), and conditioned with 2000 ppm NaCl. Samples used to determine the permeate flux and salt rejection of the chlorinated membranes were taken at various time intervals. Whenever performance was measured,
Fig. 3. ATR-FT-IR absorption spectra of polysulfone and virgin LFC1 membrane. Polysulfone spectra modified from FDM Electronic Handbook [23].
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the operating conditions were finely readjusted to the desired values (225 psi (15.5 bar) and 1 l/min). Three measurements of flux and six measurements of rejection were averaged for each representative data point for performance measurements. 3. Results and discussion 3.1. Hydrogen bonding behavior 3.1.1. Virgin LFC1 membrane Fig. 3 presents FT-IR spectra of a polysulfone and a virgin LFC1 polyamide membrane. The polysulfone is composed of benzene rings, CH3 , ether and sulfone groups, and thus its FT-IR spectrum exhibited peaks corresponding to the functional groups. Aromatic in-plane ring bend stretching vibrations occurred in the region of 1625–1430 cm−1 . A C–H symmetric deformation vibration of C(CH3 )2 was observed as two weak-medium bands (almost equal intensity) in the range of 1385–1365 cm−1 [14]. Aryl-O-aryl group had a characteristic strong band near 1250 cm−1 associated with the C–O–C asymmetric stretching vibration [15,16]. The asymmetric SO2 stretching vibration occurred in the 1350–1280 cm−1 region and the band due to the symmetric stretching vibration in the 1180–1145 cm−1 region. The band near 830 cm−1 resulted from the in-phase out-of-plane hydrogen deformation for parasubstituted phenyl groups [15]. Since the penetration depth of the IR beam, below wavenumber 2400 cm−1 in the Germanium ATR crystal, is more than
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the thickness (∼300 nm) of the polyamide skin layer of the LFC1 membrane, the FT-IR spectrum of LFC1 membrane in Fig. 3 includes bands of both the polyamide skin top layer and polysulfone sublayer of the membrane. From comparison of two spectra in the Figure, it appears that the peaks at 1663, 1609, 1541, 1444 cm−1 are unique to the polyamide skin top layer. The peaks at 1663 and 1541 cm−1 are the amide I and II bands, respectively. The 1663 cm−1 peak, which is usually identified as an amide I mode in a secondary amide group, may consist of contributions from the C O stretching (largest contribution), the C–N stretching, and the C–C–N deformation vibration [17]. The amide II band (1541 cm−1 ) is due to a motion combining both N–H in-plane bending and the N–C stretching vibrations of the group –CO–NH– in its trans form. The amide II bands are mainly due to the N–H bending motion [14]. 3.1.2. Effect of chlorine concentration The infrared spectra of LFC1 membranes, recorded as a function of increasing chlorine concentration at both pH 4 and 9, are displayed in Fig. 4. After exposure to chlorine, the frequencies of the peaks at 1663, 1609, 1541, and 1416 cm−1 shifted, and the intensities of the peaks at 1609, 1587, 1541, 1488, 1444, and 1416 cm−1 changed systematically. It is interesting to note that the extent of peak shift and the decrease of peak intensity was greater for the membrane exposed to 20 ppm h chlorine at pH 4 than for the membrane exposed to
Fig. 4. ATR-FT-IR absorption spectra for virgin and degraded LFC1 membranes. (Left) LFC1 membranes degraded at pH 9 and various chlorine concentrations. (Right) LFC1 membranes degraded at pH 4 and various chlorine concentrations.
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II band disappeared due to the decreased number of hydrogen bonding sites caused by the substitution of chlorine for hydrogen on the amide group, and due to too much shift and merging with other peaks. The decreasing intensity of the 1541 cm−1 peak at pH 9 can also be explained by means of the decreasing number of N–H bonds due to hydrogen replacement by chlorine. The peak intensities at 1609 cm−1 , also, systematically decreased with increasing concentration of chlorine in the soaking solution (Fig. 4). Since the peak may be assigned as a C C ring stretching vibration motion [21], the decline of the peak intensities shows the aromatic rings are monotonically chlorinated after chlorination.
Fig. 5. Peak shift of amide I and II bands after chlorination of LFC1 membrane at pH 4 and 9.
2000 ppm h chlorine at pH 9. This indicates that the 20 ppm h at pH 4 environment was more reactive. Fig. 5 shows peak shifts of the amide I (1663 cm−1 ) and amide II (1541 cm−1 ) bands of the LFC1 membrane. As the chlorine concentration in a soaking bath increased in a stepwise fashion, the amide I peak after chlorination shifted systematically to higher frequency. The systematic shift of the amide I peak to the higher frequency is thought to result from breakage of hydrogen bonds between the C O and N–H groups. When the C O groups form hydrogen bonds with N–H groups in the virgin membrane, the hydrogen bonding tends to decrease the doublebond character of the C O moiety, shifting the adsorption band to lower frequency [14]. Breaking hydrogen bonds due to the chlorination shifted the amide I band back to its original high frequency. The chlorination of the LFC1 membrane substitutes Cl for H in the amide bond of the polymer since the amide nitrogen is vulnerable to chlorine attack [4,18–20]. The substitution decreases the average hydrogen bond strength among the polymer chains. Therefore, the systematic shift to higher frequency of the amide I band with increasing concentration of chlorine can be explained by a decrease in the average hydrogen bond strength following the transformation of H-bonded carbonyl state to free carbonyl. The amide II band (1541 cm−1 ) is mainly due to the N–H bending motion. With increasing chlorine exposure at pH 9, the peak shifted monotonically to lower frequency and peak intensity decreased. When the N–H groups form hydrogen bonds with C O or other N–H groups in the virgin membrane, the bending vibration of the N–H bond is restricted due to the hydrogen bonds, therefore, shifting the absorption band to higher frequency [14]. After chlorination, hydrogen bonding was weakened, allowing the N–H bending peak to shift to lower wavenumber due to the stretched hydrogen bond length (decreased interaction of the bonding) resulting from the conversion of neighbor N–H groups to N–Cl groups. At pH 4, the amide
3.1.3. Effect of pH Figs. 6 and 7 show FT-IR spectra and the peak shift of the amide I/II band of the LFC1 membranes exposed to various pH values at 100 ppm h chlorine. Peak shifts at amide I/II peaks and the intensity change of the amide II peak due to the change of pH showed the same trend as those observed for the change of chlorine concentration in the soaking solution (Figs. 4 and 5). This was due to the fractional increase of the HOCl species, the more aggressive form of chlorine, and lower negative zeta potential of the membrane with decreasing pH.
Fig. 6. ATR-FT-IR absorption spectra of LFC1 membranes degraded at various pH and 100 ppm h Cl.
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Fig. 7. Peak shift of amide I and II bands of the LFC1 membrane degraded at various pH and 100 ppm h chlorine solution.
3.2. Performance 3.2.1. Chlorination of LFC1 membrane at pH 4 Table 1 shows the permeate flux and salt rejection of the virgin LFC1 membrane, as reported by the manufacturer (Hydranautics ©) and measured in our lab. Fig. 8 shows the normalized flux and rejection of LFC1 membranes which were chlorinated under conditions of 100, 500, 1000, 2000 ppm h chlorine and pH 4. Normalized flux1 (or rejection1) was defined as the flux (or rejection) of chlorinated membranes divided by the flux (or rejection) of their virgin membranes under comparable conditions. The flux of LFC1 membranes decreased after chlorination at pH 4. Flux decreases of 37%, 39%, 40%, and 49% occurred at 100, 500, 1000, 2000 ppm h chlorinated membranes (Table 2) when measured at 71 h total filtration time (equivalent to the 6 h DI water filtration and following 12 h filtration with 2000 ppm NaCl solution after chlorination. Please refer to Fig. 2). The first measurement of performance after chlorination (at 71 h total filtration time) in Fig. 8 showed exposure of the membrane lowered the permeate flux in proportion to the degree of chlorine exposure. According to the X-ray photoelectron spectroscopy (XPS) investigation in our preceding paper [22], chlorine in the soaking bath reacted with the LFC1 membrane and was bound Table 1 Performance data of the membranes reported by the manufacturers and measured at our lab
LFC1
Reported by manufacturersa
Measuredb
Flux (l/m2 h)
Rejection (%)
Flux (l/m2 h)
Rejection (%)
46.64
99.5
45.76 ± 4.65
98.2 ± 0.2
Operating condition. a 1500 ppm NaCl, 225 PSI, 25 ◦ C, 15% recovery. b 2000 ppm NaCl, 225 PSI, 25 ◦ C, 1 l/min (LPM).
Fig. 8. Normalized flux1 and rejection1 of LFC1 membranes degraded at 100, 500, 1000, 2000 ppm h Cl and pH 4. The performance was measured at 71, 83, 92 h total filtration time (12, 24, 33 h filtration with 2000 ppm NaCl solution following 6 h DI water filtration after chlorination). Normalized flux1 (or rejection1) was defined as the flux (or rejection) of chlorinated membranes divided by the flux (or rejection) of their virgin membranes under compatible conditions.
to the membrane surface up to 2.6% atomic percent at pH 4. The replacement of hydrogen with chorine on the amide group (N–H) of the membrane caused the loss of most of the hydrogen bonding sites, confirmed by the systematic shift of the amide I band (C O stretching vibration) to a higher wavenumber and the disappearance of the amide II band (N–H bending vibration) in the FT-IR spectrum analysis of the membrane. More hydrogen bonds were broken in highly chlorinated membranes (Figs. 4 and 5). The breakage of most hydrogen bonds supporting the structure of the polyamide layer along with crosslinking could provide a large increase of rotational freedom and flexibility of the polymer chains due to (i) the removal of H-bonds anchoring a polymer chain to another Table 2 Normalized performance1 of the LFC1 membranes chlorinated at pH 4
Virgin LFC1 100 ppm h Cl 500 ppm h Cl 1000 ppm h Cl 2000 ppm h Cl
Normalized flux1
Normalized rejection1
1.00 0.63 0.61 0.60 0.51
1.00 1.01 1.00 0.98 0.98
Measured at 71 h total filtration time (6 h deionized water filtration and following 12 h filtration with 2000 ppm NaCl solution).
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polymer chain and (ii) a lowered energy barrier of cis–trans rotation around the C and N of the chlorinated amide bond [19]. By the application of high pressure (225 psi (15.5 bar)) during the filtration test, the polymer chains having large rotational freedom and flexibility could collapse or compact. The collapse or compaction of the polymer chains might block the passage of water molecules through the polymeric membrane, resulting in the decline of flux after chlorination at pH 4. The rejection of the LFC1 membrane, shown at the bottom of Fig. 8, increased a little after chlorination and then decreased with increasing concentration of chlorine in the soaking bath. The change of rejection due to chlorination was not significant compared with the change of flux. Similar to membrane flux, membrane rejection, also, increased with filtration time. The increase in rejection with filtration time seemed to be due to increased permeate flux. It is interesting to note that the flux and rejection of the membranes changed with increasing filtration time. The flux of the membranes chlorinated at higher concentrations of chlorine increased more quickly than that of membranes degraded at lower chlorine concentrations. Fig. 8 presents the flux as a function of the degree of chlorine exposure to the membrane. When the flux was expressed as a function of total filtration time, it was more clearly observed that the flux systematically increased with increasing filtration time (Fig. 9). Normalized flux1 shown at the top in Fig. 9 was derived by taking the ratio of the flux of chlorinated membranes to the steady-state flux of their virgin membranes under similar conditions. The normalized flux1, then, was normalized again based on the first normalized flux measurement (at 71 h total filtration time for Fig. 9) after chlorination, which was designated the normalized flux2 at the bottom of Fig. 9. The flux of the membrane degraded at 100 ppm h chlorine at pH 4 remained almost the same after 6 hr filtration of DI water and 33 h filtration of 2000 ppm NaCl solution (Fig. 9 and Table 3). On the other hand, membranes chlorinated at 500 and 1000 ppm h showed a slight increase of flux and the 2000 ppm h chlorined membrane showed the largest increase of flux after the same filtration time. This might be caused by increased rotational freedom or flexibility of polymer chains due to chlorination. Even if the polymer chains were collapsed by pressure (225 psi (15.5 bar)) applied in the filtration process, the flexibility of the polymer chains, provided by chlorination, still remained the same as the chlorinated but unpressurized membranes. Highly chlorinated membranes had more rotational freeTable 3 Normalized Flux2 of the LFC1 membranes chlorinated at pH 4 Normalized flux2
100 ppm h Cl 500 ppm h Cl 1000 ppm h Cl 2000 ppm h Cl
71 h (6 + 12)
83 h (6 + 24)
92 h (6 + 33)
1.00 1.00 1.00 1.00
1.00 1.07 1.12 1.27
0.99 1.08 1.16 1.32
Measured at 71, 83, and 92 h total filtration time (6 h deionized water filtration and following 12, 24, and 33 h filtration with 2000 ppm NaCl solution).
Fig. 9. Normalized flux2 of LFC1 membranes degraded at 100, 500, 1000, 2000 ppm h Cl and pH 4. The performance was measured at 71, 83, 92 h total filtration time (12, 24, 33 h filtration with 2000 ppm NaCl solution following 6 h DI water filtration after chlorination). Normalized flux2 (or rejection2) was defined as the normalized flux1 (or rejection1) of the membrane divided by the first normalized flux (or rejection) measurement (at 71 h total filtration time for this figure).
dom of the polymer chains. The continuous passage of water molecules at 225 psi (15.5 bar) pressure through the polymeric membrane might enable the polymer chains to rearrange themselves to find the most thermodynamically stable structures, which can allow more passage of water molecules and hence flux recovery. Therefore, highly chlorinated membranes, which have the most flexibility of the polymer chains, could most quickly be collapsed by the initial application of pressure, but most easily find stable positions for the polymer chains. 3.2.2. Chlorination of LFC1 membrane at pH 9 Fig. 10 shows that the permeate flux data of the LFC1 membrane chlorinated at pH 9 were higher than the flux of its virgin membrane. This was opposite experimental results from the membranes chlorinated at pH 4. As shown by the initial flux of chlorinated membranes measured at 70 h total filtration time, the permeate flux rapidly increased for those membranes exposed to low chlorine concentration (100 ppm h Cl), and then the flux gradually decreased with increasing concentration of chlorine in the soaking bath (Table 4). After chlorination with 100 ppm h chlorine at pH 9, small amounts of hydrogen bonds on the surface of LFC1 membranes were broken. The rupture of hydrogen bonds increased the rotational freedom of the polymer chains. But, unlike pH 4
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Fig. 10. Normalized flux1 and rejection1 of LFC1 membranes degraded at 100, 500, 1000, 2000 ppm h Cl and pH 9. The performance was measured at 70, 82, 94 h total filtration time (12, 24, 36 h filtration with 2000 ppm NaCl solution following 6 h DI water filtration after chlorination).
conditions, the polymer frames, supported by hydrogen bonds and crosslinking between the polymer chains, still remained the same as the virgin membrane since the number of broken hydrogen bonds was not sufficient to exceed the membrane’s compressive yield points which would result in the mass compaction or collapse of polymer chains. The increased rotational flexibility of the chains with an intact polymer structure might release the restriction of water passage through the polymeric membrane, resulting in the increase of initial flux at the soaking condition of 100 ppm h chlorine and pH 9. With increasing concentration of chlorine in a soaking bath, more chlorine was bound to the membrane surface and there could be local collapse or compaction in the membrane. This appears to cause the subsequent flux decline (measured at 70 h total filtration time) for 500, 1000, 2000 ppm h chlorination at pH 9. Table 4 Normalized performance1 of the LFC1 membranes chlorinated at pH 9
Virgin LFC1 100 ppm h Cl 500 ppm h Cl 1000 ppm h Cl 2000 ppm h Cl
Normalized flux1
Normalized rejection1
1.00 1.12 1.11 1.07 1.02
1.00 1.00 1.00 0.99 0.98
Measured at 70 h total filtration time (6 h deionized water filtration and following 12 h filtration with 2000 ppm NaCl solution).
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Fig. 11. Normalized flux2 of LFC1 membranes degraded at 100, 500, 1000, 2000 ppm h Cl and pH 9. The performance was measured at 70, 82, 94 h total filtration time (12, 24, 36 h filtration with 2000 ppm NaCl solution following 6 h DI water filtration after chlorination).
The normalized flux of LFC1 membranes chlorinated at pH 9 changed with increasing filtration time similar to the membranes degraded at pH 4. As shown in Fig. 11, the flux of 1000 and 2000 ppm h chlorinated membranes increased with filtration time. On the other hand, the flux of 500 ppm h chlorinated membranes remained almost constant, and the flux of 100 ppm h chlorinated membrane slightly decreased with filtration time. The data for 500 ppm h chlorine experiment showed no change at pH 9. The flux increase with increasing filtration time of membranes chlorinated at 1000 and 2000 ppm h chlorine and pH 9 can be explained by the same reasoning as the flux increase at pH 4 (highly chlorinated membranes can be easily collapsed by the initial application of pressure, but the membranes can easily find stable positions for the polymer chains). The slight flux decrease with increasing filtration time of 100 ppm h chlorinated membrane was likely attributed to the distortion of polymer chains due to the local collapse of polymer structures neighboring the chlorinating amide bond resulting from continuous application of high pressure (225 psi). The flux change with filtration time was determined by the balance between rearrangement of flexible polymer chains and distortion of the polymer chains. 4. Conclusion A systematic investigation of the relationship between chlorine exposure of polyamide LFC1 membranes and changes in hydrogen bonding behavior, and performance of the membranes has been presented. FT-IR spectra showed the amide I band
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(C O stretching peak at 1663 cm−1 ) of polyamide membranes shifted systematically to a higher wavenumber and the peak intensity of amide II band (N–H bending at 1541 cm−1 ) monotonically decreased with increasing concentration of chlorine and decreasing pH of the soaking bath solution. These systematic changes of the peaks are recognized as resulting from breakage of hydrogen bonds between C O and N–H groups of the membranes. The FT-IR experimental results demonstrated that the chlorination changed the hydrogen bonding behavior among the crosslinked polyamide chains. Chlorination of polyamide membranes showed (i) initial flux change, and then (ii) following systematic flux shifts with filtration time. The initial flux change and flux change with filtration time depended on pH and concentration of chlorine in the soaking bath. The initial flux of the membrane decreased at low pH and increased for slightly chlorinated membranes at high pH. The flux shift with filtration time was grater for highly chlorinated membranes. The highly chlorinated membranes, which have the most flexibility of the polymer chains, could most quickly be collapsed by the initial application of pressure, but most easily find stable positions for the polymer chains. These flux changes caused by chlorination were attributed to the change of increased rotational freedom or the flexibility of polymer chains due to changes in hydrogen bonding behavior caused by chlorine attack. Systematic investigation of chlorine exposure to the polyamide membranes on the change of performance (water flux and salt rejection) demonstrated that the change of hydrogen bonding behavior due to the chlorination caused the change of rotational freedom or the flexibility of polymer chains followed by the change from initial performance and subsequent flux changes with filtration time. Acknowledgements This work was funded by the Clean Water Programme (CWP) with Nanyang Technological University (NTU) and STC WaterCAMPWS of the National Science Foundation under agreement #CTS-0120978. Membranes used for this study were generously donated by FilmTec ©. References [1] C.R. Bartels, M. Wilf, K. Andes, J. Iong, Design considerations for wastewater treatment by reverse osmosis, Water Sci. Technol. 51 (2005) 473–482. [2] M. Wilf, S. Alt, Application of low fouling RO membrane elements for reclamation of municipal wastewater, Desalination 132 (2000) 11–19.
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