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Unraveling flux behavior of superhydrophilic loose nanofiltration membranes during textile wastewater treatment
Author’s Accepted Manuscript Unravelling flux behavior of superhydrophilic loose nanofiltration membranes during textile wastewater treatment Jiuyang Lin, Chuyang Y. Tang, Wenyuan Ye, ShiPeng Sun, Shadi H. Hamdan, Alexander Volodin, Chris Van Haesendonck, Arcadio Sotto, Patricia Luis, Bart Van der Bruggen
PII: DOI: Reference:
www.elsevier.com/locate/memsci
S0376-7388(15)30046-6 http://dx.doi.org/10.1016/j.memsci.2015.07.018 MEMSCI13837
To appear in: Journal of Membrane Science Received date: 9 June 2015 Revised date: 9 July 2015 Accepted date: 10 July 2015 Cite this article as: Jiuyang Lin, Chuyang Y. Tang, Wenyuan Ye, Shi-Peng Sun, Shadi H. Hamdan, Alexander Volodin, Chris Van Haesendonck, Arcadio Sotto, Patricia Luis and Bart Van der Bruggen, Unravelling flux behavior of superhydrophilic loose nanofiltration membranes during textile wastewater t r e a t m e n t , Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2015.07.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Unravelling
flux
behavior
of
superhydrophilic
loose
nanofiltration membranes during textile wastewater treatment Jiuyang Lina, Chuyang Y. Tangb, Wenyuan Yea*, Shi-Peng Sunc, Shadi H. Hamdana, Alexander Volodind, Chris Van Haesendonckd, Arcadio Sottoe, Patricia Luisf, Bart Van der Bruggena* * Corresponding author: [email protected] (W. Ye); [email protected] (B. Van der Bruggen) a
Department of Chemical Engineering, KU Leuven, Willem de Croylaan 46, B-3001
Heverlee, Belgium b
Department of Civil Engineering, The University of Hong Kong, Pokfulam
HW619B, Hong Kong c
National Engineering Research Center for Special Separation Membrane, College of
Chemical Engineering, Nanjing Tech University, 210009 Nanjing, China d
Laboratory of Solid-State Physics and Magnetism, Department of Physics and
Astronomy, K.U. Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium e
Department of Chemical and Environmental Technology, Rey Juan Carlos
University, 28933 Móstoles, Madrid, Spain f
Materials & Process Engineering (iMMC-IMAP), Université catholique de Louvain,
Place Sainte Barbe 2, 1348 Louvain-la-Neuve, Belgium
1
Abstract: Loose nanofiltration (NF) membranes can be used to treat textile wastewater effectively, providing an attracted avenue for resource recovery (i.e., dye purification and salt reuse) at the premise of the integration of high dye retention and salt permeation. However, the issue of membrane fouling has to be adequately addressed in view of its practical application. In this study, superhydrophilic loose NF membranes (Sepro NF 6 and NF 2A, Ultura) were applied for textile wastewater treatment. Synthetic solutions containing dyes and NaCl were used as the feed stream of a NF unit. It was found that two factors, namely cake-enhanced concentration polarization and the formation of a dye cake layer, dramatically deteriorated the flux of NF membranes with a synergic effect. In viewpoint of realistic application, diafiltration of a binary dye/salt mixture indicates that cake-enhanced concentration polarization plays a dominant role for the low membrane flux. As the diafiltration continued, cake-enhanced concentration polarization was alleviated with a decreasing concentration of salt in the feed. At the subsequent post-concentration procedure, the formation of a dye cake layer slightly compromised the membrane flux, but the negative impact of cake-enhanced concentration polarization was negligible due to the small quantity of salt remained in the feed.
Keywords: Superhydrophilic loose NF membranes; Flux decline; Dye/salt binary mixture; Cake-enhanced concentration polarization; Fouling and cleaning
2
Graphical abstract:
1. Introduction Sustainable wastewater treatment calls for a new paradigm shift to emphasize on resource recovery from wastewater in addition to contaminants removal [1, 2]. The development of advanced technology and practices potentially enables cost-effective resource recovery, including water, nutrients, bioplastics and energy from the industrial wastewater [1]. For example, in textile wastewater, a high amount of organic matters (i.e., dyestuffs) can be applied for energy production or recovered for further dyeing [3-6]. Furthermore, a high salinity (~6.0% NaCl or ~5.6% Na2SO4) in the textile wastewater endows its potential reuse as a draw solution for forward osmosis/pressure retard osmosis or an agent for base/acid generation by bipolar membrane electrodialysis, expediting the close of water and material loop [7, 8]. However, direct treatment and reuse of textile wastewater can cause the severe fouling of forward osmosis membrane or ion exchange membrane due to dye adsorption, thus requiring the effective fractionation of dye and salt in textile wastewater [9, 10]. Nanofiltration (NF)
membranes, with a molecular weight cutoff (MWCO) of 3
200-1000 Da and pore size of ~0.5 to 2.0 nm, exhibit one of the most competitive separation and purification processes in textile wastewater treatment [11]. The unique separation mechanism of NF membranes, which involves size exclusion and electrostatic repulsion, allows for its wide applications in textile wastewater treatment, endowing high selectivity of dye molecules as well as partial permeation for inorganic salt solutes [7]. The performance of NF membranes can be severely limited by membrane fouling and concentration polarization [12-18]. Generally, in textile wastewater treatment, membrane fouling caused by cake layer formation and pore blocking plays a dominant role for the permeation flux decline [19-21]. To optimize the operational conditions of NF membranes for dye/salt separation, Koyuncu et al. intensively investigated the operation factors involving membrane flux decline in textile wastewater, indicating that the increasing cross-flow velocity can be a pronounced factor to improve the membrane flux by alleviating the membrane fouling and concentration polarization [22]. Furthermore, the adsorption and accumulation of dyes in the membrane pore structure, mainly resulting from hydrophobic interaction or electrostatic attraction, gives rise to the high risk of irreversible fouling, which in turn induces a dramatic flux decline [23]. Inevitably, this requires an increase in frequency of chemical cleaning, which may shorten the membrane lifespan. Apart from dye adsorption and accumulation, Van der Bruggen et al. demonstrated that the high salinity in the textile wastewater has a negative impact on the membrane flux, related to the high osmotic pressure caused by high salt rejection of commercially available 4
dense NF membranes [8]. Thus, advanced NF membranes with an enhanced hydrophilicity and high salt transmission (i.e., low salt rejection) are required in the treatment of textile wastewater for flux enhancement. The employment of loose NF membrane can be established as an effective strategy for the fractionation of dye/salt mixture [24-28]. Loose NF membranes with relatively low salt rejection are preferred to reduce the trans-membrane osmotic pressure difference and thus the overall energy consumption. Lin et al. applied loose negative Sepro NF membrane with a MWCO of ~850 Da for the fractionation of direct dye/salt binary mixture, which allows for ~98% of NaCl pass through the membrane, with complete retention of direct dye [7]. However, the negative charge of membrane surface still prevents the free permeation of divalent salts in the textile wastewater, such as Na2SO4, due to the strong electrostatic repulsion. A loose NF membrane with positive charge which was fabricated by Yu et al. through incorporating poly (ionic liquid) brushes modified silica spheres, promotes the free passage of both Na2SO4 and NaCl [29]. Expectedly, through the process intensification with electrodialysis or bipolar membrane electrodialysis, a sustainable water recovery and salt reuse can be achieved from textile wastewater [30]. In order to further explore realistic applications of these loose NF membranes for the dye and salt fractionation, their flux behavior as well as the mechanisms for membrane flux decline should be investigated. Specifically, the use of superhydrophilic loose NF membranes was proposed to avoid membrane fouling; nevertheless, fouling remains an unknown factor, which needs confirmation and comparison. 5
In this work, the flux of the superhydrophilic loose NF membranes (Sepro NF 6 and NF 2A, Ultura) in a single-component (i.e., NaCl or dyes) solution was studied at variable pressures and concentrations. Furthermore, the flux decline of these NF membranes was systematically investigated for simulated textile wastewater (i.e., dye/salt mixture). In addition, interactions between salt, dyes and the membrane surface were related to flux decline by characterizing the membrane surface after fouling. The combination of diafiltration and post-concentration for textile wastewater was performed to understand the membrane flux behavior in the overall treatment process, in view of assessing an industrial application.
2. Materials and methods 2.1 Chemicals and NF membranes Direct dyes, i.e., direct red 80 (Pure, Sigma-Aldrich, Belgium), direct red 23 (Dye content: >30%, Sigma-Aldrich, Belgium), and Congo red (Pure, Avocado Research Chemicals Ltd., Belgium), were used at different concentrations, i.e., 50, 200, 500, 1000, and 2000 ppm. The molecular structure of the direct dyes is shown in Fig. 1. NaCl with analytical grade was supplied by Sigma-Aldrich (Belgium). All the chemicals were used as received. Ultrapure water (18.2 MΩ·cm) was used throughout all the experiments.
6
Fig. 1 Molecular structure of the dyes used in the experiments
Two thin film composite NF membranes with different salt rejections were kindly supplied by Ultura (USA). The properties of the tested NF membranes are shown in Table 1.
Table 1 Properties of NF membranes used in this study [31] Membrane
Sepro NF 2A
Sepro NF 6
Composition of top layer
Semi-aromatic polyamide
Semi-aromatic polyamide
Contact angle (°)
21.7 ± 1.4
14.3 ± 0.9
Permeability (L·m−2·h−1·bar−1) at 25 °C
10.1
16.7
Molecular weight cutoff (Da)
529
847
7
Salt rejection (NaCl, %)
21.2
7.3
pH for isoelectric point
3.78
5.06
Process pH limitation
3.0-10.0
3.0-10.0
2.2 Permeation experiments 2.2.1 Steady-state nanofiltration in cross-flow mode The filtration properties of Sepro NF membranes were evaluated through a lab-scale cross-flow permeation setup described elsewhere [7]. Before the membrane filtration, membrane samples with a surface area of 2.29 × 10-3 m2 were pre-compacted at 10 bar for 2 hours in all the experiments to achieve a stable flux. Firstly, the NF process was performed by a feed of single-component solution (i.e., NaCl or dye) to evaluate the effect of solute concentration and operation pressure on the membrane flux. The salt concentration was set up to 40 g·L-1; the dye concentration was 0.05, 0.2, 0.5, 1.0 and 2.0 g·L-1 (pH~7.6). Then the flux of NF membranes at 6 bar in dye/NaCl binary solutions with different salt concentrations (i.e., 5, 10, 20, 30, and 40 g·L-1) was further investigated to reveal the effect of salt addition.
2.2.2 Antifouling test After the filtration of dye solutions, the fouled NF membranes were cleaned by pure water or HCl solution (pH~3.0) in the cross-flow setup until the dye was totally removed from the membranes. The pure water flux of cleaned NF membranes was 8
measured at 6 bar; the pure water flux was recorded to determinate the flux recovery ratio (η):
%
J w,c J w,0
100
(1) where Jw,0 is the pure water flux of original NF membranes, and Jw,c is the pure water flux of NF membranes after cleanning.
2.2.3 Diafiltration and post-concentration In order to simulate the application of loose NF membranes in industry, diafiltration of dye/salt mixture and its post-concentration was performed. In diafiltration, a 350 mL direct red 80/NaCl mixed solution (1.0 g·L-1 dye and 20.0 g·L-1 NaCl) was applied as the feed solution. Pure water was continuously added to the feed solution to keep the feed at a constant volume. The NF permeate was sampled in a fixed time interval for flux measurement. A total of 1400 mL pure water was added for diafiltration. Subsequent post-concentration of dye solution, following diafiltration, was carried out. 625 mL diluted direct red 80 solution (originated from solution after diafiltration with proper dilution) with a small amount of salt (0.5 g·L-1 dye and 0.3 g·L-1 NaCl) was employed as feed solution for continuous concentration with a factor of 4.0. All the experiments were conducted at 6 bar and 25 ± 1 °C.
9
2.2.4 Membrane flux calculation NF membranes are subject to fouling and concentration polarization. Taking these phenomena into consideration, the permeate flux (Jw) can be described by a resistance-in-series model, given in Equation 2 [32]:
(2) where ∆P denotes the transmembrane pressure, Δπ represents the osmotic pressure difference between bulk and permeate solution, µ is the solvent viscosity, Rm is the intrinsic membrane resistance, Rf is related to the hydraulic resistance caused by fouling, such as cake layer formation and pore blocking, and Rcp is the resistance originating from the concentration polarization of salts. For a multi-component solution, the osmotic pressure difference (Δπ) is calculated by superimposing the contribution from each individual component [33]:
(3) For the dilute solution, the osmotic pressure can be estimated by the Van’t Hoff equation: (4) where Ci is the molarity of solute, Rg is the universal gas constant, and T is the thermodynamic (absolute) temperature. In Equation 2, the intrisnic membrane resistance (Rm) was calculated from the flux of pure water, the value of Rf was estimated from independent filtration tests using individual dye solution, and the resistance caused by the concentration polarization of 10
NaCl (Rcp) was determined by the independent filtration of pure NaCl solutions.
2.3 Characterization of fouled membranes In order to investigate the alteration of surface chemical functionality of the fouled membranes, Fourier transform infrared spectroscopy (FTIR) measurements were performed through an ATR-FTIR spectrometer (Bruker, Germany), which is equipped with a platinum diamond for single reflection. The FTIR spectra of the dried membrane samples were taken over the range of 400-4000 cm-1 at a resolution of 1.43 cm-1. Scanning electron microscopy (SEM) measurements were conducted to visualize the surface morphology of fouled membranes before and after cleaning. The SEM images of the membrane samples were taken at a voltage of 10 kV with a Philips Scanning Electron Microscope XL30 FEG. Prior to the measurements, the membrane samples were dried in a vacuum chamber, and then sputtered by gold nanoparticles. Atomic force microscopy (AFM) measurements of the dried membrane samples were carried out using an Agilent 5500 AFM with scan areas of 1×1 μm2 at ambient condition. AFM images were obtained in tapping mode using NCSTR probes from NanoAndMore GmbH.
3. Results and discussion 3.1 Membrane filtration of single salt solutions Loose NF membranes have potential for fractionation of dye/salt mixtures with a high 11
salt permeation [7, 27, 34]. However, a further investigation of the flux behavior is essential because of the complexity of the dye/salt system, with enhanced interaction among salt, dye, and membrane surface. As a reference, the flux behavior was studied in single-component solutions. Fig. 2 shows the flux of Sepro NF membranes in the NaCl solutions at variable salt concentrations and operation pressures. (A) 200
(B) 200 Mea.: Cal.:
180
2 g/L 2 g/L
5 g/L 5 g/L
10 g/L 10 g/L
20 g/L 20 g/L
40 g/L 40 g/L
Pure water
160
140
140
Mea.: Cal.:
2 g/L 2 g/L
5 g/L 5 g/L
10 g/L 10 g/L
20 g/L 20 g/L
40 g/L 40 g/L
Pure water
Flux, Lm h
-1
160 -1
120
-2
-2
Flux, Lm h
180
100 80
120 100 80
60
60
40
40
20
20
0
0 0
1
2
3
4
5
6
7
Operation pressure, bar
8
9
10
0
1
2
3
4
5
6
7
Operation pressure, bar
8
9
10
Fig. 2 Flux of Sepro NF membranes in NaCl solution filtration. (A): Sepro NF 6; (B): Sepro NF 2A (Dot: experimental data; Solid line: theoretical flux calculated without taking concentration polarization into account)
As illustrated in Fig. 2 and Supplementary Fig. S1, the flux of Sepro NF membranes in the NaCl solutions was positively related with the operation pressures. However, the flux of NF membranes decreased with the salt concentration at any fixed pressure. For instance, at 10 bar, the flux of Sepro NF 6 and NF 2A dropped by 22.8% and 53.3% in the whole range of salt concentration, respectively. This is due to the following factors: (1) Firstly, the viscosity of salt solution. As shown in Supplementary Fig. S2, the 12
viscosity of NaCl solutions follows the Jone-Dole equation, and increases with the salt concentration [35]. Indeed, the solution viscosity increased by 6.38% when NaCl concentration increased from 0 to 40 g·L-1, which will negatively impact on membrane permeate flux. (2) Secondly, concentration polarization. In Fig. 2, theoretical flux values have been calculated taking into consideration of the change in solution viscosity as well as osmotic pressure difference across the membrane (see Supplementary Fig. S3). However, the calculation did not take account of concentration polarization effect. The difference between the theoretical and experimental results thus revealed the role of concentration polarization. Generally, the concentration polarization resistance can be negligible at low salt concentrations. As shown in Fig. 3, Sepro NF 6 had an obvious effect of concentration polarization at salt concentrations ranging from 20 to 40 g·L-1, whereas Sepro NF 2A experienced a larger impact of concentration polarization in NaCl solutions from 5 to 40 g·L-1. Furthermore, the additional hydraulic resistance caused by concentration polarization was significantly greater for Sepro NF 2A. The greater concentration polarization effect for Sepro NF 2A can be explained by its relatively higher salt rejection compared to that of Sepro NF 6 (Table 1). Once again, this reveals the advantage of highly salt-permeable membranes. The low rejection nature of such membranes allows solutes to easily pass through them, which reduces the osmotic pressure difference across the membrane and the concentration 13
polarization of salts. Rm=2.3908E13 m
-1
4 bar 6 bar 8 bar 10 bar
0.3
(B)
Rm=4.1762E13 m
-1
2.5
13
13
CP resistance, Rcp (X10
0.4
0.2
0.1
3.0
-1
CP resistance, Rcp (X10 m )
(A)
-1
m )
0.5
Low conceontration polarization
0.0
2.0
4 bar 6 bar 8 bar 10 bar
1.5 1.0 0.5
Low conceontration polarization
0.0
0.1
1
0.1
10
1
10
Concentration of NaCl (g/L)
Concentration of NaCl (g/L)
Fig. 3 Concentration polarization resistance (Rcp) of loose NF membranes in different NaCl solutions. (A): Sepro NF 6; (B): Sepro NF 2A
3.2 Membrane filtration in pure dye solutions Fig. 4 shows the flux of Sepro NF membranes in the dye solutions as a function of dye concentration and pressure. As shown in Fig. 4, the flux of loose NF membranes in dye solutions was substantially lower than the pure water flux, particularly at high pressure. This can be attributed to membrane fouling. More severe membrane fouling tends to occur at higher dye concentrations. Compared to Sepro NF 2A, Sepro NF 6 had greater flux drop which can be attributed to its higher pure water permeability [36].
14
0.05 g/L 1.0 g/L
(B)
0.2 g/L 2.0 g/L
-2
100 80 60 40 20
Congo red
80 60 40 20
Direct red 23
140 0
-1
120
-2
100
Flux, Lm h
-2 -1
0.2 g/L 2.0 g/L
60 40 20
Congo red
-1 -2
100
Flux, Lm h
Flux, Lm h
-2 -1
120
Flux, Lm h
0.05 g/L 1.0 g/L
0 80
140 0
80 60 40 20 0
Water 0.5 g/L
80
-1
Water 0.5 g/L
120
Flux, Lm h
Flux, Lm h
-2 -1
(A) 140
Direct red 80 0
1
2
3
4
5
6
7
60 40 20
Direct red 23
0 80 60 40 20
Direct red 80 0
8
0
1
2
3
4
5
6
7
8
Applied pressure, bar
Applied pressure, bar
Fig. 4 Membrane flux of pure dye solutions for NF membrane at viable operation pressures and dye concentrations. (A): Sepro NF 6; (B): Sepro NF 2A The permeability of Sepro NF membranes in the dye solutions showed no obvious decline at a low pressure (linear relationship between the flux and pressure). However, as the operation pressure increased, the membrane flux deviated from the linear relationship, possibly due to the severe compaction of the dye cake layer which increased the fouling resistance (Rf). In some severe fouling cases (2 g·L-1 direct red 23 for both NF membranes), increasing pressure above the threshold value did not result in enhanced permeability. In the current study, the threshold pressure of Sepro NF membranes in dye filtration lied at 6 bar. Similar type of limiting flux behavior has been previously reported for surfactants [37] and macromolecules [38]. Indeed, a limiting flux value of ~ 40 L·m-2·h-1 was observed for both Sepro NF 6 and NF 2A at 6 bar. This type of membrane independence suggests that the threshold flux value was likely governed by foulant-foulant interaction instead of foulant-membrane interaction 15
[38]. In order to obtain an explanation for the mechanisms of membrane fouling, the fouling resistance of Sepro NF membranes for these three dyes was calculated, as shown in Fig. 5. In general, Sepro NF 2A appeared to have a stronger antifouling property than Sepro NF 6, as indicated by its relatively lower hydraulic resistance caused by membrane fouling (Rf). At low dye concentrations, Rf for Sepro NF 2A was relatively small compared to its intrinsic membrane resistance. In contrast, Sepro NF 6 had higher values of Rf despite of its more hydrophilic membrane surface (Table 1). A plausible explanation may be its relatively large pore size allows dye molecules to block its membrane pores more easily, leading to greater permeability drop. The presence of larger nodules on the surface of Sepro NF 6 can also possibly facilitate the deposit of dye molecules on the “valley” regions, resulting in a higher hydraulic resistance of Sepro NF 6 [31]. Furthermore, Sepro NF 6 is less negatively charged compared to Sepro NF 2A, which may allow the negatively charged dye molecules (due to their -SO3- groups) to approach Sepro NF 6 more easily. At very high dye concentrations (e.g., 2.0 g·L-1 Congo red or direct red 23), similar Rf values were observed. In the latter case, fouling was more likely dominated by cake layer formation (e.g., see SEM images of Sepro NF membranes fouled by 2000 ppm direct red 23 solution in Fig. 6), where foulant-foulant interaction prevails over foulant-membrane interaction [39]. Specifically, Sepro NF 2A had a higher calculated hydraulic resistance in the direct red 23 solution, compared to Sepro NF 6. This is mainly due to the complexity of the direct red 23 solution (~70% impurity, including 16
5.8% Cl-, 12.6% SO42-, 5.2% HCO3- and other intermediate during dye synthesis), diminishing the flux of Sepro NF 2A through a higher salt retention. 0.05 g/L 0.2 g/L 0.5 g/L 1.0 g/L 2.0 g/L
(A) 1.4 1.2
1.0
0.8
0.8
0.6
0.6
0.4
0.4
Direct red 23
0.0 5
-1
Rm=2.444E13 m
-1
13
4
0.2
Congo red
Rf, 10 m
Rf, 10 m
-1
0.0 5
3 2 1
4
Congo red
Rm=4.0078E13 m
-1
Direct red 23
3 2 1
0.9 0
0.9 0
0.6
0.6
0.3
0.05 g/L 0.2 g/L 0.5 g/L 1.0 g/L 2.0 g/L
1.2
1.0
0.2
13
(B) 1.4
Direct red 80
0.3
Direct red 80
0.0
0.0 2
3
4
5
6
7
8
2
Applied pressure, bar
3
4
5
6
7
8
Applied pressure, bar
Fig. 5 Hydraulic resistance of Sepro NF membranes caused by fouling (Rf) in different dye solutions. (A): Sepro NF 6; (B): Sepro NF 2A
Fig. 6 SEM images of Sepro NF membranes fouled by 2000 ppm direct red 23. (A): Sepro NF 6; (B): Sepro NF 2A In order to clarify the mechanism of the flux decline for Sepro NF membranes in dye solutions, digital pictures of NF membranes cleaned by rinsing with water and the 17
FTIR measurements are shown in Figs. 7 and 8, respectively. As shown in Fig. 7, the fouled Sepro NF 6 was less effectively cleaned after water rinsing, with a small amount of dye deposited on the membrane, indicating that the reversible membrane fouling may control the flux behavior. By contrast, the surface of Sepro NF 2A appeared to be better cleaned, once again confirming its better antifouling ability. Furthermore, the FTIR spectra in Fig. 8 indicate no obvious alteration in the observed functional groups, from which it can be concluded the dyes had no chemical reaction with the surface of Sepro NF membranes. This is in direct contrast to some earlier studies claiming that chemical interactions (e.g., covalent bonds) between dye and the surface of polyamide membrane occurs [40].
A
B
Fouled by direct red 23
Fouled by direct red 23
Fig. 7 Digital pictures of fouled NF membranes after water rinsing. (A): Sepro NF 6; (B): Sepro NF 2A Fouled by direct red 23 Fouled by direct red 80 Fouled by congo red Original
Absorpiton 1800
Fouled by direct red 23 Fouled by direct red 80 Fouled by congo red Original
(B)
Absorpiton
(A)
1600
1400
1200
Wavenumber (cm-1)
1000
800
1800
1600
1400
1200
1000
800
Wavenumber (cm-1)
Fig. 8 FTIR spectra for fouled NF membranes. (A): Sepro NF 6; (B): Sepro NF 2A 18
3.3 Membrane filtration in dye/salt binary mixtures Real textile waste streams have a higher complexity than the single-component solutions described above. Hence, it is crucial to investigate the membrane flux in these waste streams. Fig. 9 shows the flux of Sepro NF membranes in dye/salt binary mixtures. 110
(A)
Direct red 80 (mea.) Congo red (mea.) Direct red 23(mea.) Direct red 80(cal.) Congo red(cal.) Direct red 23 (cal.)
100 80
Direct red 80 (mea.) Congo red (mea.) Direct red 23 (mea.) Direct red 80 (cal.) Congo red (cal.) Direct red 23 (cal.)
40
-2
70
(B)
50
Flux, Lm h
-2
Flux, Lm h
-1
90
60
-1
120
60 50 40
30 20
30 20
10
10 0
0 0
10
20
30
Concentration of NaCl, gL
40
0
-1
10
20
30
Concentration of NaCl, gL
-1
40
Fig. 9 Flux of Sepro NF membranes in the dye/salt binary mixtures at 6 bar. (A): Sepro NF 6; (B): Sepro NF 2A As shown in Fig. 9, the flux of membrane in the dye/salt mixture decreases with the salt concentration. This is due to the increase in osmotic pressure difference between the feed and permeate side of NF membrane (see Supplementary Fig. S4), lowering the driving force. This is consistent with previous studies [7, 8, 22, 41]. It is worth noting that in the filtration of direct red 23 or Congo red/NaCl mixture, Sepro NF membranes had an extremely low flux (< 5.0 L·m-2·h-1) at high salt concentrations. As shown in Supplementary Fig. S4, the osmotic pressure difference levels off at higher salt concentrations, indicating that the driving force almost keeps constant. However, the measured membrane flux in dye/NaCl solutions with different salt concentrations is much lower than the calculated theoretical flux. This implies 19
that Sepro NF membranes may have different fouling mechanisms in dye/salt binary mixture, which are responsible for the greatly enhanced flux decline compared to the theoretical predictions. One plausible mechanism is 'cake-enhanced concentration polarization', originally proposed by Hoek and Elimelech [42]. The formation of an unstirred cake layer of foulant may promote concentration polarization in the porous cake layer and lead to an increase of the osmotic pressure difference and a significant flux decline [42-47]. Furthermore, the increased salt concentration (thus higher ionic strength) weakens the electrostatic repulsive force between foulant-foulant and foulant-membrane [12]. As shown in Fig. 10, precipitation of dye occurs with the increasing salt concentration, since the presence of background electrolytes can effectively shield the electrostatic double layer interaction [12] as well as increase the hydrophobicity of dyes [48]. This implies a reduced stability of dye molecules which promotes more severe membrane fouling.
Fig. 10 Digital pictures for the dye/NaCl mixtures with different salt concentrations. (A): direct red 80; (B): Congo red; (C): direct red 23
3.4 Membrane fouling and cleaning In the dye/salt mixtures, loose NF membranes are subjected to a severe fouling, hindering the application of loose NF membranes by a drastic hindrance of the flux. 20
Therefore, effective membrane cleaning strategies are required for industrial applications [49]. Fig. 11 shows flux recovery after cleaning the fouled membranes by water and acid flushing. 110
Flux recovery (%)
105
After water flushing (Sepro NF6) After water flushing (Sepro NF2A) After HCl cleaning (Sepro NF6) After HCl cleaning (Sepro NF2A)
100 95 90 85 80 Direct red 80
Direct red 23
Congo red
Dye
Fig. 11 Flux recovery of fouled NF membranes after water rinsing or HCl cleaning As shown in Fig. 11, Sepro NF membranes have a high flux recovery (>95.6%) after rinsing with pure water. This is due to their superhydrophilicity which reduces the affinity of dye molecules on the membrane surface as well as no chemical interactions between dye molecules and membrane surface [31]. Compared to Sepro NF 2A, Sepro NF 6 had a lower flux recovery ratio, even though it had a more hydrophilic surface. This is ascribed to the fact that a trace amount of dye foulant deposited on the surface of Sepro NF 6, which can be confirmed by SEM images in Fig. 12. After rinsing with water, Sepro NF 2A exhibits a smooth surface with a ~99% flux recovery. AFM measurement also shows that the membrane surface can be cleaned with water, indicating the physical cleaning can be an acceptable approach for flux recovery of Sepro NF membranes, especially for Sepro NF 2A.
21
(A1)
(B1)
Dye foulant
Impurities
(A2)
(B2)
(A3)
(B3)
Fig. 12 SEM and 3D AFM images of cleaned NF membrane surfaces by water rinsing. (A): Sepro NF 6; (B): Sepro NF 2A
Additionally, cleaning membranes with HCl gave rise to a nearly complete recovery (>99.4%) of the water flux for both Sepro NF membranes. Fig. 13 shows that both Sepro NF membranes had a clean surface, only with a small amount of crystalline impurity deposited on the membrane surface. Therefore, Sepro NF membranes had an 22
excellent resistance to fouling. HCl cleaning offered a more effective route for the complete flux recovery of Sepro NF membranes, compared to rinsing with water.
(A1)
(B1) Impurities
(A2)
(B2)
(A3)
(B3)
Fig. 13 SEM and 3D AFM images of cleaned NF membrane surface by HCl cleaning. (A): Sepro NF 6; (B): Sepro NF 2A
3.5 Membrane flux in diafiltration and psot-concentration process 23
Fig. 14 shows the flux of Sepro NF membranes for direct red 80/NaCl binary mixture in diafiltration and post-concentration. Mea. (Diafiltration) Mea. (Concentration)
Flux, Lm-2h-1
100
3 times 1 time 2 times
90
(B) 100
Cal. (Diafiltration) Cal. (Concentration)
Cake layer
CECP
70 1 time 2 times
60
Postconcentration
Diafiltration
3 times
4 times
40 Postconcentration
Diafiltration
20
50
Cake layer
50 CECP
30
60
Cal. (Diafiltration) Cal. (Concentration)
80
4 times
80 70
Mea. (Diafiltration) Mea. (Concentration)
90
Flux, Lm-2h-1
(A) 110
10 0
100
200
300
400
500
600
0
200
400
600
800
1000
Operation time, min
Operation time, min
Fig. 14 Flux of Sepro NF membranes in diafiltration and post-concentration. (A): Sepro NF 6; (B): Sepro NF 2A (CECP:
cake-enhanced concentration polarization;
Mea.: measured; Cal.: calculated)
Fig. 14 indicates that the flux of both Sepro NF membranes had a similar tendency. In the initial stage of diafiltration, the large gap between the measured and calculated flux of Sepro NF membranes resulted from the cake-enhanced concentration polarization. As the diafiltration proceeded, the decrease of salt content mitigated the cake-enhanced concentration polarization phenomenon, enhancing the driving force for water transfer through the NF membranes. Furthermore, the reduction of the salt concentration also alleviated the formation of a cake layer, since the salt effect on the dye aggregation was limited, as demonstrated in Fig. 10. As the concentration of salt in the feed was reduced to a fixed small content, the cake-enhanced concentration polarization phenomenon can be ignored. Fig. 15 schematically shows the evolution 24
of cake-enhanced concentration polarization and cake layer formation in diafiltration to demonstrate the flux enhancement. During the diafiltration of the direct red 80/NaCl mixture, the flux of Sepro NF 6 and NF 2A membrane increased from 59.6 to 86.3 L·m-2·h-1 and from 34.3 to 53.9 L·m-2·h-1, respectively.
Fig. 15 Schematic diagram for the evolution of cake-enhanced concentration polarization phenomenon and cake layer formation in diafiltration
Following the diafiltration, the post-concentration procedure was performed. The flux of Sepro NF 6 and NF 2A in this stage slightly decreased from 86.6 to 81.9 L·m-2·h-1 and from 56.2 to 52.4 L·m-2·h-1, after a concentration factor of 4.0, respectively. Due to the high salt permeation of Sepro NF membranes, the concentration of NaCl in the feed had a slight increase (see Supplementary Fig. S5), and no obvious adverse effect on promoting cake-enhanced concentration polarization occurred. Therefore, as schematically demonstrated in Fig. 16, the mild difference between the calculated and measured flux of Sepro NF membranes was due to the evolution of dye cake layer in post-concentration with different concentration factors.
25
Fig. 16 Schematic diagram for the evolution of cake layer formation in post-concentration with different concentration factors
4. Conclusions In this study, two superhydrophilic loose NF membranes (Sepro NF 6 and NF 2A membrane, Ultura) were applied for dye/NaCl fractionation with a full investigation on flux behavior. It was demonstrated that in the treatment of textile wastewater with a high salinity, the loose NF membranes can generally suffer the concentration polarization and the formation of dye cake layer. Particularly, the pore blocking and dye deposition can be partially involved in the flux decline of loose NF membranes with a lager pore size and nodules. The enhancing hydrophilicity of surface for Sepro NF membranes can reduce the physical or chemical adsorption of dye. Furthermore, in the highly loaded textile wastewater, a specific fouling mechanism, denoted as ‘cake-enhanced concentration polarization’, can significantly deteriorate the membrane flux through increasing osmotic pressure as well as exacerbating the formation of dye cake layer. Application of diafiltration for a dye/salt binary mixture indicates that cake-enhanced concentration polarization played a dominant role for the low flux at the initial step. 26
As the diafiltration proceeds, cake-enhanced concentration polarization was alleviated, enhancing the membrane flux. During the subsequent post-concentration, the evolution of dye cake layer slightly compromised the flux of superhydrophilic loose NF membranes, ignoring the negative impact of cake-enhanced concentration polarization. Therefore, the investigation of flux behavior for Sepro NF membranes in textile wastewater gives the convincing evidence that loose NF membranes can be an attracted option for textile wastewater treatment, apart from the term of high dye rejection. However, new strategies should be developed in future to mitigate the cake-enhanced concentration polarization for flux enhancement.
Acknowledgments J. Lin and W. Ye would like to acknowledge the support provided by China Scholarship Council of the Ministry of Education, China. Michèle Vanroelen from CIT, KU Leuven, is acknowledged for performing Ion Chromatography measurements. Ultura (USA) is greatly thanked for supplying the NF membrane samples.
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Desalination, 359 (2015) 176-185. [27] J. Zhu, M. Tian, Y. Zhang, H. Zhang, J. Liu, Fabrication of a novel “loose” nanofiltration membrane by facile blending with Chitosan–Montmorillonite nanosheets for dyes purification, Chem. Eng. J., 265 (2015) 184-193. [28] J. Zhu, N. Guo, Y. Zhang, L. Yu, J. Liu, Preparation and characterization of negatively charged PES nanofiltration membrane by blending with halloysite nanotubes grafted with poly (sodium 4-styrenesulfonate) via surface-initiated ATRP, J. Membr. Sci., 465 (2014) 91-99. [29] L. Yu, Y. Zhang, Y. Wang, H. Zhang, J. Liu, High flux, positively charged loose nanofiltration membrane by blending with poly (ionic liquid) brushes grafted silica spheres, J. Hazard. Mater., 287 (2015) 373-383. [30] J. Lin, W. Ye, J. Huang, R.B. Castillo, M.-C. Baltaru, B. Greydanus, S. Balta, J. Shen, M. Vlad, A. Sotto, P. Luis, B. Van der Bruggen, Toward resource recovery from textile wastewater: dye extraction, water and base/acid regeneration using a hybrid NF-BMED process, Submitted for publication. [31] J. Lin, C.Y. Tang, C. Huang, Y. Tang, W. Ye, J. Li, J. Shen, R. Van der Broeck, J. Van Impe, A. Volodin, C. Van Haesendonck, A. Sotto, P. Luis, B. Van der Bruggen, A comprehensive physico-chemical characterization of superhydrophilic nanofiltration membranes, Submitted for publication. [32] J. Luo, L. Ding, Y. Wan, M.Y. Jaffrin, Threshold flux for shear-enhanced nanofiltration: experimental observation in dairy wastewater treatment, J. Membr. Sci., 409 (2012) 276-284. 31
[33] K. Pastagia, S. Chakraborty, S. DasGupta, J. Basu, S. De, Prediction of permeate flux and concentration of two-component dye mixture in batch nanofiltration, J. Membr. Sci., 218 (2003) 195-210. [34] F. Liu, B.-r. Ma, D. Zhou, L.-J. Zhu, Y.-Y. Fu, L.-x. Xue, Positively charged loose nanofiltration membrane grafted by diallyl dimethyl ammonium chloride (DADMAC) via UV for salt and dye removal, React. Funct. Polym., 86 (2015) 191-198. [35] H.-L. Zhang, S.-J. Han, Viscosity and density of water+ sodium chloride+ potassium chloride solutions at 298.15 K, J. Chem. Eng. Data, 41 (1996) 516-520. [36] C.Y. Tang, Y.-N. Kwon, J.O. Leckie, Fouling of reverse osmosis and nanofiltration membranes by humic acid—effects of solution composition and hydrodynamic conditions, J. Membr. Sci., 290 (2007) 86-94. [37] C.Y. Tang, Q.S. Fu, A. Robertson, C.S. Criddle, J.O. Leckie, Use of reverse osmosis membranes to remove perfluorooctane sulfonate (PFOS) from semiconductor wastewater, Environ. Sci. Technol., 40 (2006) 7343-7349. [38] C.Y. Tang, J.O. Leckie, Membrane independent limiting flux for RO and NF membranes fouled by humic acid, Environ. Sci. Technol., 41 (2007) 4767-4773. [39] C.Y. Tang, Y.-N. Kwon, J.O. Leckie, Probing the nano-and micro-scales of reverse osmosis membranes—a comprehensive characterization of physiochemical properties of uncoated and coated membranes by XPS, TEM, ATR-FTIR, and streaming potential measurements, J. Membr. Sci., 287 (2007) 146-156. [40] A. Akbari, J. Remigy, P. Aptel, Treatment of textile dye effluent using a polyamide-based nanofiltration membrane, Chem. Eng. Process., 41 (2002) 601-609. 32
[41] R. Jiraratananon, A. Sungpet, P. Luangsowan, Performance evaluation of nanofiltration membranes for treatment of effluents containing reactive dye and salt, Desalination, 130 (2000) 177-183. [42] E.M. Hoek, M. Elimelech, Cake-enhanced concentration polarization: a new fouling mechanism for salt-rejecting membranes, Environ. Sci. Technol., 37 (2003) 5581-5588. [43] T. Mahlangu, E. Hoek, B. Mamba, A. Verliefde, Influence of organic, colloidal and combined fouling on NF rejection of NaCl and carbamazepine: Role of solute-foulant-membrane interactions and cake-enhanced concentration polarisation, J. Membr. Sci., 471 (2014) 35-46. [44] G. Fimbres Weihs, D. Wiley, CFD analysis of tracer response technique under cake-enhanced osmotic pressure, J. Membr. Sci., 449 (2014) 38-49. [45] T. Chong, F. Wong, A. Fane, Implications of critical flux and cake enhanced osmotic pressure (CEOP) on colloidal fouling in reverse osmosis: experimental observations, J. Membr. Sci., 314 (2008) 101-111. [46] D. Vogel, A. Simon, A.A. Alturki, B. Bilitewski, W.E. Price, L.D. Nghiem, Effects of fouling and scaling on the retention of trace organic contaminants by a nanofiltration membrane: the role of cake-enhanced concentration polarisation, Sep. Purif. Technol., 73 (2010) 256-263. [47] H.Y. Ng, M. Elimelech, Influence of colloidal fouling on rejection of trace organic contaminants by reverse osmosis, J. Membr. Sci., 244 (2004) 215-226. [48] K. Yeung, S. Shang, The influence of metal ions on the aggregation and 33
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Highlights - A loose nanofiltration (NF) membrane was employed for dye/salt separation - CECP mechanism substantially deteriorated the flux of loose NF membrane in dye/salt mixture - The loose NF membrane had an excellent antifouling against dye due to its superhydrophilicity - Water rinsing can be an effective method to recover the flux of loose NF membrane
34
PII: DOI: Reference:
www.elsevier.com/locate/memsci
S0376-7388(15)30046-6 http://dx.doi.org/10.1016/j.memsci.2015.07.018 MEMSCI13837
To appear in: Journal of Membrane Science Received date: 9 June 2015 Revised date: 9 July 2015 Accepted date: 10 July 2015 Cite this article as: Jiuyang Lin, Chuyang Y. Tang, Wenyuan Ye, Shi-Peng Sun, Shadi H. Hamdan, Alexander Volodin, Chris Van Haesendonck, Arcadio Sotto, Patricia Luis and Bart Van der Bruggen, Unravelling flux behavior of superhydrophilic loose nanofiltration membranes during textile wastewater t r e a t m e n t , Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2015.07.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Unravelling
flux
behavior
of
superhydrophilic
loose
nanofiltration membranes during textile wastewater treatment Jiuyang Lina, Chuyang Y. Tangb, Wenyuan Yea*, Shi-Peng Sunc, Shadi H. Hamdana, Alexander Volodind, Chris Van Haesendonckd, Arcadio Sottoe, Patricia Luisf, Bart Van der Bruggena* * Corresponding author: [email protected] (W. Ye); [email protected] (B. Van der Bruggen) a
Department of Chemical Engineering, KU Leuven, Willem de Croylaan 46, B-3001
Heverlee, Belgium b
Department of Civil Engineering, The University of Hong Kong, Pokfulam
HW619B, Hong Kong c
National Engineering Research Center for Special Separation Membrane, College of
Chemical Engineering, Nanjing Tech University, 210009 Nanjing, China d
Laboratory of Solid-State Physics and Magnetism, Department of Physics and
Astronomy, K.U. Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium e
Department of Chemical and Environmental Technology, Rey Juan Carlos
University, 28933 Móstoles, Madrid, Spain f
Materials & Process Engineering (iMMC-IMAP), Université catholique de Louvain,
Place Sainte Barbe 2, 1348 Louvain-la-Neuve, Belgium
1
Abstract: Loose nanofiltration (NF) membranes can be used to treat textile wastewater effectively, providing an attracted avenue for resource recovery (i.e., dye purification and salt reuse) at the premise of the integration of high dye retention and salt permeation. However, the issue of membrane fouling has to be adequately addressed in view of its practical application. In this study, superhydrophilic loose NF membranes (Sepro NF 6 and NF 2A, Ultura) were applied for textile wastewater treatment. Synthetic solutions containing dyes and NaCl were used as the feed stream of a NF unit. It was found that two factors, namely cake-enhanced concentration polarization and the formation of a dye cake layer, dramatically deteriorated the flux of NF membranes with a synergic effect. In viewpoint of realistic application, diafiltration of a binary dye/salt mixture indicates that cake-enhanced concentration polarization plays a dominant role for the low membrane flux. As the diafiltration continued, cake-enhanced concentration polarization was alleviated with a decreasing concentration of salt in the feed. At the subsequent post-concentration procedure, the formation of a dye cake layer slightly compromised the membrane flux, but the negative impact of cake-enhanced concentration polarization was negligible due to the small quantity of salt remained in the feed.
Keywords: Superhydrophilic loose NF membranes; Flux decline; Dye/salt binary mixture; Cake-enhanced concentration polarization; Fouling and cleaning
2
Graphical abstract:
1. Introduction Sustainable wastewater treatment calls for a new paradigm shift to emphasize on resource recovery from wastewater in addition to contaminants removal [1, 2]. The development of advanced technology and practices potentially enables cost-effective resource recovery, including water, nutrients, bioplastics and energy from the industrial wastewater [1]. For example, in textile wastewater, a high amount of organic matters (i.e., dyestuffs) can be applied for energy production or recovered for further dyeing [3-6]. Furthermore, a high salinity (~6.0% NaCl or ~5.6% Na2SO4) in the textile wastewater endows its potential reuse as a draw solution for forward osmosis/pressure retard osmosis or an agent for base/acid generation by bipolar membrane electrodialysis, expediting the close of water and material loop [7, 8]. However, direct treatment and reuse of textile wastewater can cause the severe fouling of forward osmosis membrane or ion exchange membrane due to dye adsorption, thus requiring the effective fractionation of dye and salt in textile wastewater [9, 10]. Nanofiltration (NF)
membranes, with a molecular weight cutoff (MWCO) of 3
200-1000 Da and pore size of ~0.5 to 2.0 nm, exhibit one of the most competitive separation and purification processes in textile wastewater treatment [11]. The unique separation mechanism of NF membranes, which involves size exclusion and electrostatic repulsion, allows for its wide applications in textile wastewater treatment, endowing high selectivity of dye molecules as well as partial permeation for inorganic salt solutes [7]. The performance of NF membranes can be severely limited by membrane fouling and concentration polarization [12-18]. Generally, in textile wastewater treatment, membrane fouling caused by cake layer formation and pore blocking plays a dominant role for the permeation flux decline [19-21]. To optimize the operational conditions of NF membranes for dye/salt separation, Koyuncu et al. intensively investigated the operation factors involving membrane flux decline in textile wastewater, indicating that the increasing cross-flow velocity can be a pronounced factor to improve the membrane flux by alleviating the membrane fouling and concentration polarization [22]. Furthermore, the adsorption and accumulation of dyes in the membrane pore structure, mainly resulting from hydrophobic interaction or electrostatic attraction, gives rise to the high risk of irreversible fouling, which in turn induces a dramatic flux decline [23]. Inevitably, this requires an increase in frequency of chemical cleaning, which may shorten the membrane lifespan. Apart from dye adsorption and accumulation, Van der Bruggen et al. demonstrated that the high salinity in the textile wastewater has a negative impact on the membrane flux, related to the high osmotic pressure caused by high salt rejection of commercially available 4
dense NF membranes [8]. Thus, advanced NF membranes with an enhanced hydrophilicity and high salt transmission (i.e., low salt rejection) are required in the treatment of textile wastewater for flux enhancement. The employment of loose NF membrane can be established as an effective strategy for the fractionation of dye/salt mixture [24-28]. Loose NF membranes with relatively low salt rejection are preferred to reduce the trans-membrane osmotic pressure difference and thus the overall energy consumption. Lin et al. applied loose negative Sepro NF membrane with a MWCO of ~850 Da for the fractionation of direct dye/salt binary mixture, which allows for ~98% of NaCl pass through the membrane, with complete retention of direct dye [7]. However, the negative charge of membrane surface still prevents the free permeation of divalent salts in the textile wastewater, such as Na2SO4, due to the strong electrostatic repulsion. A loose NF membrane with positive charge which was fabricated by Yu et al. through incorporating poly (ionic liquid) brushes modified silica spheres, promotes the free passage of both Na2SO4 and NaCl [29]. Expectedly, through the process intensification with electrodialysis or bipolar membrane electrodialysis, a sustainable water recovery and salt reuse can be achieved from textile wastewater [30]. In order to further explore realistic applications of these loose NF membranes for the dye and salt fractionation, their flux behavior as well as the mechanisms for membrane flux decline should be investigated. Specifically, the use of superhydrophilic loose NF membranes was proposed to avoid membrane fouling; nevertheless, fouling remains an unknown factor, which needs confirmation and comparison. 5
In this work, the flux of the superhydrophilic loose NF membranes (Sepro NF 6 and NF 2A, Ultura) in a single-component (i.e., NaCl or dyes) solution was studied at variable pressures and concentrations. Furthermore, the flux decline of these NF membranes was systematically investigated for simulated textile wastewater (i.e., dye/salt mixture). In addition, interactions between salt, dyes and the membrane surface were related to flux decline by characterizing the membrane surface after fouling. The combination of diafiltration and post-concentration for textile wastewater was performed to understand the membrane flux behavior in the overall treatment process, in view of assessing an industrial application.
2. Materials and methods 2.1 Chemicals and NF membranes Direct dyes, i.e., direct red 80 (Pure, Sigma-Aldrich, Belgium), direct red 23 (Dye content: >30%, Sigma-Aldrich, Belgium), and Congo red (Pure, Avocado Research Chemicals Ltd., Belgium), were used at different concentrations, i.e., 50, 200, 500, 1000, and 2000 ppm. The molecular structure of the direct dyes is shown in Fig. 1. NaCl with analytical grade was supplied by Sigma-Aldrich (Belgium). All the chemicals were used as received. Ultrapure water (18.2 MΩ·cm) was used throughout all the experiments.
6
Fig. 1 Molecular structure of the dyes used in the experiments
Two thin film composite NF membranes with different salt rejections were kindly supplied by Ultura (USA). The properties of the tested NF membranes are shown in Table 1.
Table 1 Properties of NF membranes used in this study [31] Membrane
Sepro NF 2A
Sepro NF 6
Composition of top layer
Semi-aromatic polyamide
Semi-aromatic polyamide
Contact angle (°)
21.7 ± 1.4
14.3 ± 0.9
Permeability (L·m−2·h−1·bar−1) at 25 °C
10.1
16.7
Molecular weight cutoff (Da)
529
847
7
Salt rejection (NaCl, %)
21.2
7.3
pH for isoelectric point
3.78
5.06
Process pH limitation
3.0-10.0
3.0-10.0
2.2 Permeation experiments 2.2.1 Steady-state nanofiltration in cross-flow mode The filtration properties of Sepro NF membranes were evaluated through a lab-scale cross-flow permeation setup described elsewhere [7]. Before the membrane filtration, membrane samples with a surface area of 2.29 × 10-3 m2 were pre-compacted at 10 bar for 2 hours in all the experiments to achieve a stable flux. Firstly, the NF process was performed by a feed of single-component solution (i.e., NaCl or dye) to evaluate the effect of solute concentration and operation pressure on the membrane flux. The salt concentration was set up to 40 g·L-1; the dye concentration was 0.05, 0.2, 0.5, 1.0 and 2.0 g·L-1 (pH~7.6). Then the flux of NF membranes at 6 bar in dye/NaCl binary solutions with different salt concentrations (i.e., 5, 10, 20, 30, and 40 g·L-1) was further investigated to reveal the effect of salt addition.
2.2.2 Antifouling test After the filtration of dye solutions, the fouled NF membranes were cleaned by pure water or HCl solution (pH~3.0) in the cross-flow setup until the dye was totally removed from the membranes. The pure water flux of cleaned NF membranes was 8
measured at 6 bar; the pure water flux was recorded to determinate the flux recovery ratio (η):
%
J w,c J w,0
100
(1) where Jw,0 is the pure water flux of original NF membranes, and Jw,c is the pure water flux of NF membranes after cleanning.
2.2.3 Diafiltration and post-concentration In order to simulate the application of loose NF membranes in industry, diafiltration of dye/salt mixture and its post-concentration was performed. In diafiltration, a 350 mL direct red 80/NaCl mixed solution (1.0 g·L-1 dye and 20.0 g·L-1 NaCl) was applied as the feed solution. Pure water was continuously added to the feed solution to keep the feed at a constant volume. The NF permeate was sampled in a fixed time interval for flux measurement. A total of 1400 mL pure water was added for diafiltration. Subsequent post-concentration of dye solution, following diafiltration, was carried out. 625 mL diluted direct red 80 solution (originated from solution after diafiltration with proper dilution) with a small amount of salt (0.5 g·L-1 dye and 0.3 g·L-1 NaCl) was employed as feed solution for continuous concentration with a factor of 4.0. All the experiments were conducted at 6 bar and 25 ± 1 °C.
9
2.2.4 Membrane flux calculation NF membranes are subject to fouling and concentration polarization. Taking these phenomena into consideration, the permeate flux (Jw) can be described by a resistance-in-series model, given in Equation 2 [32]:
(2) where ∆P denotes the transmembrane pressure, Δπ represents the osmotic pressure difference between bulk and permeate solution, µ is the solvent viscosity, Rm is the intrinsic membrane resistance, Rf is related to the hydraulic resistance caused by fouling, such as cake layer formation and pore blocking, and Rcp is the resistance originating from the concentration polarization of salts. For a multi-component solution, the osmotic pressure difference (Δπ) is calculated by superimposing the contribution from each individual component [33]:
(3) For the dilute solution, the osmotic pressure can be estimated by the Van’t Hoff equation: (4) where Ci is the molarity of solute, Rg is the universal gas constant, and T is the thermodynamic (absolute) temperature. In Equation 2, the intrisnic membrane resistance (Rm) was calculated from the flux of pure water, the value of Rf was estimated from independent filtration tests using individual dye solution, and the resistance caused by the concentration polarization of 10
NaCl (Rcp) was determined by the independent filtration of pure NaCl solutions.
2.3 Characterization of fouled membranes In order to investigate the alteration of surface chemical functionality of the fouled membranes, Fourier transform infrared spectroscopy (FTIR) measurements were performed through an ATR-FTIR spectrometer (Bruker, Germany), which is equipped with a platinum diamond for single reflection. The FTIR spectra of the dried membrane samples were taken over the range of 400-4000 cm-1 at a resolution of 1.43 cm-1. Scanning electron microscopy (SEM) measurements were conducted to visualize the surface morphology of fouled membranes before and after cleaning. The SEM images of the membrane samples were taken at a voltage of 10 kV with a Philips Scanning Electron Microscope XL30 FEG. Prior to the measurements, the membrane samples were dried in a vacuum chamber, and then sputtered by gold nanoparticles. Atomic force microscopy (AFM) measurements of the dried membrane samples were carried out using an Agilent 5500 AFM with scan areas of 1×1 μm2 at ambient condition. AFM images were obtained in tapping mode using NCSTR probes from NanoAndMore GmbH.
3. Results and discussion 3.1 Membrane filtration of single salt solutions Loose NF membranes have potential for fractionation of dye/salt mixtures with a high 11
salt permeation [7, 27, 34]. However, a further investigation of the flux behavior is essential because of the complexity of the dye/salt system, with enhanced interaction among salt, dye, and membrane surface. As a reference, the flux behavior was studied in single-component solutions. Fig. 2 shows the flux of Sepro NF membranes in the NaCl solutions at variable salt concentrations and operation pressures. (A) 200
(B) 200 Mea.: Cal.:
180
2 g/L 2 g/L
5 g/L 5 g/L
10 g/L 10 g/L
20 g/L 20 g/L
40 g/L 40 g/L
Pure water
160
140
140
Mea.: Cal.:
2 g/L 2 g/L
5 g/L 5 g/L
10 g/L 10 g/L
20 g/L 20 g/L
40 g/L 40 g/L
Pure water
Flux, Lm h
-1
160 -1
120
-2
-2
Flux, Lm h
180
100 80
120 100 80
60
60
40
40
20
20
0
0 0
1
2
3
4
5
6
7
Operation pressure, bar
8
9
10
0
1
2
3
4
5
6
7
Operation pressure, bar
8
9
10
Fig. 2 Flux of Sepro NF membranes in NaCl solution filtration. (A): Sepro NF 6; (B): Sepro NF 2A (Dot: experimental data; Solid line: theoretical flux calculated without taking concentration polarization into account)
As illustrated in Fig. 2 and Supplementary Fig. S1, the flux of Sepro NF membranes in the NaCl solutions was positively related with the operation pressures. However, the flux of NF membranes decreased with the salt concentration at any fixed pressure. For instance, at 10 bar, the flux of Sepro NF 6 and NF 2A dropped by 22.8% and 53.3% in the whole range of salt concentration, respectively. This is due to the following factors: (1) Firstly, the viscosity of salt solution. As shown in Supplementary Fig. S2, the 12
viscosity of NaCl solutions follows the Jone-Dole equation, and increases with the salt concentration [35]. Indeed, the solution viscosity increased by 6.38% when NaCl concentration increased from 0 to 40 g·L-1, which will negatively impact on membrane permeate flux. (2) Secondly, concentration polarization. In Fig. 2, theoretical flux values have been calculated taking into consideration of the change in solution viscosity as well as osmotic pressure difference across the membrane (see Supplementary Fig. S3). However, the calculation did not take account of concentration polarization effect. The difference between the theoretical and experimental results thus revealed the role of concentration polarization. Generally, the concentration polarization resistance can be negligible at low salt concentrations. As shown in Fig. 3, Sepro NF 6 had an obvious effect of concentration polarization at salt concentrations ranging from 20 to 40 g·L-1, whereas Sepro NF 2A experienced a larger impact of concentration polarization in NaCl solutions from 5 to 40 g·L-1. Furthermore, the additional hydraulic resistance caused by concentration polarization was significantly greater for Sepro NF 2A. The greater concentration polarization effect for Sepro NF 2A can be explained by its relatively higher salt rejection compared to that of Sepro NF 6 (Table 1). Once again, this reveals the advantage of highly salt-permeable membranes. The low rejection nature of such membranes allows solutes to easily pass through them, which reduces the osmotic pressure difference across the membrane and the concentration 13
polarization of salts. Rm=2.3908E13 m
-1
4 bar 6 bar 8 bar 10 bar
0.3
(B)
Rm=4.1762E13 m
-1
2.5
13
13
CP resistance, Rcp (X10
0.4
0.2
0.1
3.0
-1
CP resistance, Rcp (X10 m )
(A)
-1
m )
0.5
Low conceontration polarization
0.0
2.0
4 bar 6 bar 8 bar 10 bar
1.5 1.0 0.5
Low conceontration polarization
0.0
0.1
1
0.1
10
1
10
Concentration of NaCl (g/L)
Concentration of NaCl (g/L)
Fig. 3 Concentration polarization resistance (Rcp) of loose NF membranes in different NaCl solutions. (A): Sepro NF 6; (B): Sepro NF 2A
3.2 Membrane filtration in pure dye solutions Fig. 4 shows the flux of Sepro NF membranes in the dye solutions as a function of dye concentration and pressure. As shown in Fig. 4, the flux of loose NF membranes in dye solutions was substantially lower than the pure water flux, particularly at high pressure. This can be attributed to membrane fouling. More severe membrane fouling tends to occur at higher dye concentrations. Compared to Sepro NF 2A, Sepro NF 6 had greater flux drop which can be attributed to its higher pure water permeability [36].
14
0.05 g/L 1.0 g/L
(B)
0.2 g/L 2.0 g/L
-2
100 80 60 40 20
Congo red
80 60 40 20
Direct red 23
140 0
-1
120
-2
100
Flux, Lm h
-2 -1
0.2 g/L 2.0 g/L
60 40 20
Congo red
-1 -2
100
Flux, Lm h
Flux, Lm h
-2 -1
120
Flux, Lm h
0.05 g/L 1.0 g/L
0 80
140 0
80 60 40 20 0
Water 0.5 g/L
80
-1
Water 0.5 g/L
120
Flux, Lm h
Flux, Lm h
-2 -1
(A) 140
Direct red 80 0
1
2
3
4
5
6
7
60 40 20
Direct red 23
0 80 60 40 20
Direct red 80 0
8
0
1
2
3
4
5
6
7
8
Applied pressure, bar
Applied pressure, bar
Fig. 4 Membrane flux of pure dye solutions for NF membrane at viable operation pressures and dye concentrations. (A): Sepro NF 6; (B): Sepro NF 2A The permeability of Sepro NF membranes in the dye solutions showed no obvious decline at a low pressure (linear relationship between the flux and pressure). However, as the operation pressure increased, the membrane flux deviated from the linear relationship, possibly due to the severe compaction of the dye cake layer which increased the fouling resistance (Rf). In some severe fouling cases (2 g·L-1 direct red 23 for both NF membranes), increasing pressure above the threshold value did not result in enhanced permeability. In the current study, the threshold pressure of Sepro NF membranes in dye filtration lied at 6 bar. Similar type of limiting flux behavior has been previously reported for surfactants [37] and macromolecules [38]. Indeed, a limiting flux value of ~ 40 L·m-2·h-1 was observed for both Sepro NF 6 and NF 2A at 6 bar. This type of membrane independence suggests that the threshold flux value was likely governed by foulant-foulant interaction instead of foulant-membrane interaction 15
[38]. In order to obtain an explanation for the mechanisms of membrane fouling, the fouling resistance of Sepro NF membranes for these three dyes was calculated, as shown in Fig. 5. In general, Sepro NF 2A appeared to have a stronger antifouling property than Sepro NF 6, as indicated by its relatively lower hydraulic resistance caused by membrane fouling (Rf). At low dye concentrations, Rf for Sepro NF 2A was relatively small compared to its intrinsic membrane resistance. In contrast, Sepro NF 6 had higher values of Rf despite of its more hydrophilic membrane surface (Table 1). A plausible explanation may be its relatively large pore size allows dye molecules to block its membrane pores more easily, leading to greater permeability drop. The presence of larger nodules on the surface of Sepro NF 6 can also possibly facilitate the deposit of dye molecules on the “valley” regions, resulting in a higher hydraulic resistance of Sepro NF 6 [31]. Furthermore, Sepro NF 6 is less negatively charged compared to Sepro NF 2A, which may allow the negatively charged dye molecules (due to their -SO3- groups) to approach Sepro NF 6 more easily. At very high dye concentrations (e.g., 2.0 g·L-1 Congo red or direct red 23), similar Rf values were observed. In the latter case, fouling was more likely dominated by cake layer formation (e.g., see SEM images of Sepro NF membranes fouled by 2000 ppm direct red 23 solution in Fig. 6), where foulant-foulant interaction prevails over foulant-membrane interaction [39]. Specifically, Sepro NF 2A had a higher calculated hydraulic resistance in the direct red 23 solution, compared to Sepro NF 6. This is mainly due to the complexity of the direct red 23 solution (~70% impurity, including 16
5.8% Cl-, 12.6% SO42-, 5.2% HCO3- and other intermediate during dye synthesis), diminishing the flux of Sepro NF 2A through a higher salt retention. 0.05 g/L 0.2 g/L 0.5 g/L 1.0 g/L 2.0 g/L
(A) 1.4 1.2
1.0
0.8
0.8
0.6
0.6
0.4
0.4
Direct red 23
0.0 5
-1
Rm=2.444E13 m
-1
13
4
0.2
Congo red
Rf, 10 m
Rf, 10 m
-1
0.0 5
3 2 1
4
Congo red
Rm=4.0078E13 m
-1
Direct red 23
3 2 1
0.9 0
0.9 0
0.6
0.6
0.3
0.05 g/L 0.2 g/L 0.5 g/L 1.0 g/L 2.0 g/L
1.2
1.0
0.2
13
(B) 1.4
Direct red 80
0.3
Direct red 80
0.0
0.0 2
3
4
5
6
7
8
2
Applied pressure, bar
3
4
5
6
7
8
Applied pressure, bar
Fig. 5 Hydraulic resistance of Sepro NF membranes caused by fouling (Rf) in different dye solutions. (A): Sepro NF 6; (B): Sepro NF 2A
Fig. 6 SEM images of Sepro NF membranes fouled by 2000 ppm direct red 23. (A): Sepro NF 6; (B): Sepro NF 2A In order to clarify the mechanism of the flux decline for Sepro NF membranes in dye solutions, digital pictures of NF membranes cleaned by rinsing with water and the 17
FTIR measurements are shown in Figs. 7 and 8, respectively. As shown in Fig. 7, the fouled Sepro NF 6 was less effectively cleaned after water rinsing, with a small amount of dye deposited on the membrane, indicating that the reversible membrane fouling may control the flux behavior. By contrast, the surface of Sepro NF 2A appeared to be better cleaned, once again confirming its better antifouling ability. Furthermore, the FTIR spectra in Fig. 8 indicate no obvious alteration in the observed functional groups, from which it can be concluded the dyes had no chemical reaction with the surface of Sepro NF membranes. This is in direct contrast to some earlier studies claiming that chemical interactions (e.g., covalent bonds) between dye and the surface of polyamide membrane occurs [40].
A
B
Fouled by direct red 23
Fouled by direct red 23
Fig. 7 Digital pictures of fouled NF membranes after water rinsing. (A): Sepro NF 6; (B): Sepro NF 2A Fouled by direct red 23 Fouled by direct red 80 Fouled by congo red Original
Absorpiton 1800
Fouled by direct red 23 Fouled by direct red 80 Fouled by congo red Original
(B)
Absorpiton
(A)
1600
1400
1200
Wavenumber (cm-1)
1000
800
1800
1600
1400
1200
1000
800
Wavenumber (cm-1)
Fig. 8 FTIR spectra for fouled NF membranes. (A): Sepro NF 6; (B): Sepro NF 2A 18
3.3 Membrane filtration in dye/salt binary mixtures Real textile waste streams have a higher complexity than the single-component solutions described above. Hence, it is crucial to investigate the membrane flux in these waste streams. Fig. 9 shows the flux of Sepro NF membranes in dye/salt binary mixtures. 110
(A)
Direct red 80 (mea.) Congo red (mea.) Direct red 23(mea.) Direct red 80(cal.) Congo red(cal.) Direct red 23 (cal.)
100 80
Direct red 80 (mea.) Congo red (mea.) Direct red 23 (mea.) Direct red 80 (cal.) Congo red (cal.) Direct red 23 (cal.)
40
-2
70
(B)
50
Flux, Lm h
-2
Flux, Lm h
-1
90
60
-1
120
60 50 40
30 20
30 20
10
10 0
0 0
10
20
30
Concentration of NaCl, gL
40
0
-1
10
20
30
Concentration of NaCl, gL
-1
40
Fig. 9 Flux of Sepro NF membranes in the dye/salt binary mixtures at 6 bar. (A): Sepro NF 6; (B): Sepro NF 2A As shown in Fig. 9, the flux of membrane in the dye/salt mixture decreases with the salt concentration. This is due to the increase in osmotic pressure difference between the feed and permeate side of NF membrane (see Supplementary Fig. S4), lowering the driving force. This is consistent with previous studies [7, 8, 22, 41]. It is worth noting that in the filtration of direct red 23 or Congo red/NaCl mixture, Sepro NF membranes had an extremely low flux (< 5.0 L·m-2·h-1) at high salt concentrations. As shown in Supplementary Fig. S4, the osmotic pressure difference levels off at higher salt concentrations, indicating that the driving force almost keeps constant. However, the measured membrane flux in dye/NaCl solutions with different salt concentrations is much lower than the calculated theoretical flux. This implies 19
that Sepro NF membranes may have different fouling mechanisms in dye/salt binary mixture, which are responsible for the greatly enhanced flux decline compared to the theoretical predictions. One plausible mechanism is 'cake-enhanced concentration polarization', originally proposed by Hoek and Elimelech [42]. The formation of an unstirred cake layer of foulant may promote concentration polarization in the porous cake layer and lead to an increase of the osmotic pressure difference and a significant flux decline [42-47]. Furthermore, the increased salt concentration (thus higher ionic strength) weakens the electrostatic repulsive force between foulant-foulant and foulant-membrane [12]. As shown in Fig. 10, precipitation of dye occurs with the increasing salt concentration, since the presence of background electrolytes can effectively shield the electrostatic double layer interaction [12] as well as increase the hydrophobicity of dyes [48]. This implies a reduced stability of dye molecules which promotes more severe membrane fouling.
Fig. 10 Digital pictures for the dye/NaCl mixtures with different salt concentrations. (A): direct red 80; (B): Congo red; (C): direct red 23
3.4 Membrane fouling and cleaning In the dye/salt mixtures, loose NF membranes are subjected to a severe fouling, hindering the application of loose NF membranes by a drastic hindrance of the flux. 20
Therefore, effective membrane cleaning strategies are required for industrial applications [49]. Fig. 11 shows flux recovery after cleaning the fouled membranes by water and acid flushing. 110
Flux recovery (%)
105
After water flushing (Sepro NF6) After water flushing (Sepro NF2A) After HCl cleaning (Sepro NF6) After HCl cleaning (Sepro NF2A)
100 95 90 85 80 Direct red 80
Direct red 23
Congo red
Dye
Fig. 11 Flux recovery of fouled NF membranes after water rinsing or HCl cleaning As shown in Fig. 11, Sepro NF membranes have a high flux recovery (>95.6%) after rinsing with pure water. This is due to their superhydrophilicity which reduces the affinity of dye molecules on the membrane surface as well as no chemical interactions between dye molecules and membrane surface [31]. Compared to Sepro NF 2A, Sepro NF 6 had a lower flux recovery ratio, even though it had a more hydrophilic surface. This is ascribed to the fact that a trace amount of dye foulant deposited on the surface of Sepro NF 6, which can be confirmed by SEM images in Fig. 12. After rinsing with water, Sepro NF 2A exhibits a smooth surface with a ~99% flux recovery. AFM measurement also shows that the membrane surface can be cleaned with water, indicating the physical cleaning can be an acceptable approach for flux recovery of Sepro NF membranes, especially for Sepro NF 2A.
21
(A1)
(B1)
Dye foulant
Impurities
(A2)
(B2)
(A3)
(B3)
Fig. 12 SEM and 3D AFM images of cleaned NF membrane surfaces by water rinsing. (A): Sepro NF 6; (B): Sepro NF 2A
Additionally, cleaning membranes with HCl gave rise to a nearly complete recovery (>99.4%) of the water flux for both Sepro NF membranes. Fig. 13 shows that both Sepro NF membranes had a clean surface, only with a small amount of crystalline impurity deposited on the membrane surface. Therefore, Sepro NF membranes had an 22
excellent resistance to fouling. HCl cleaning offered a more effective route for the complete flux recovery of Sepro NF membranes, compared to rinsing with water.
(A1)
(B1) Impurities
(A2)
(B2)
(A3)
(B3)
Fig. 13 SEM and 3D AFM images of cleaned NF membrane surface by HCl cleaning. (A): Sepro NF 6; (B): Sepro NF 2A
3.5 Membrane flux in diafiltration and psot-concentration process 23
Fig. 14 shows the flux of Sepro NF membranes for direct red 80/NaCl binary mixture in diafiltration and post-concentration. Mea. (Diafiltration) Mea. (Concentration)
Flux, Lm-2h-1
100
3 times 1 time 2 times
90
(B) 100
Cal. (Diafiltration) Cal. (Concentration)
Cake layer
CECP
70 1 time 2 times
60
Postconcentration
Diafiltration
3 times
4 times
40 Postconcentration
Diafiltration
20
50
Cake layer
50 CECP
30
60
Cal. (Diafiltration) Cal. (Concentration)
80
4 times
80 70
Mea. (Diafiltration) Mea. (Concentration)
90
Flux, Lm-2h-1
(A) 110
10 0
100
200
300
400
500
600
0
200
400
600
800
1000
Operation time, min
Operation time, min
Fig. 14 Flux of Sepro NF membranes in diafiltration and post-concentration. (A): Sepro NF 6; (B): Sepro NF 2A (CECP:
cake-enhanced concentration polarization;
Mea.: measured; Cal.: calculated)
Fig. 14 indicates that the flux of both Sepro NF membranes had a similar tendency. In the initial stage of diafiltration, the large gap between the measured and calculated flux of Sepro NF membranes resulted from the cake-enhanced concentration polarization. As the diafiltration proceeded, the decrease of salt content mitigated the cake-enhanced concentration polarization phenomenon, enhancing the driving force for water transfer through the NF membranes. Furthermore, the reduction of the salt concentration also alleviated the formation of a cake layer, since the salt effect on the dye aggregation was limited, as demonstrated in Fig. 10. As the concentration of salt in the feed was reduced to a fixed small content, the cake-enhanced concentration polarization phenomenon can be ignored. Fig. 15 schematically shows the evolution 24
of cake-enhanced concentration polarization and cake layer formation in diafiltration to demonstrate the flux enhancement. During the diafiltration of the direct red 80/NaCl mixture, the flux of Sepro NF 6 and NF 2A membrane increased from 59.6 to 86.3 L·m-2·h-1 and from 34.3 to 53.9 L·m-2·h-1, respectively.
Fig. 15 Schematic diagram for the evolution of cake-enhanced concentration polarization phenomenon and cake layer formation in diafiltration
Following the diafiltration, the post-concentration procedure was performed. The flux of Sepro NF 6 and NF 2A in this stage slightly decreased from 86.6 to 81.9 L·m-2·h-1 and from 56.2 to 52.4 L·m-2·h-1, after a concentration factor of 4.0, respectively. Due to the high salt permeation of Sepro NF membranes, the concentration of NaCl in the feed had a slight increase (see Supplementary Fig. S5), and no obvious adverse effect on promoting cake-enhanced concentration polarization occurred. Therefore, as schematically demonstrated in Fig. 16, the mild difference between the calculated and measured flux of Sepro NF membranes was due to the evolution of dye cake layer in post-concentration with different concentration factors.
25
Fig. 16 Schematic diagram for the evolution of cake layer formation in post-concentration with different concentration factors
4. Conclusions In this study, two superhydrophilic loose NF membranes (Sepro NF 6 and NF 2A membrane, Ultura) were applied for dye/NaCl fractionation with a full investigation on flux behavior. It was demonstrated that in the treatment of textile wastewater with a high salinity, the loose NF membranes can generally suffer the concentration polarization and the formation of dye cake layer. Particularly, the pore blocking and dye deposition can be partially involved in the flux decline of loose NF membranes with a lager pore size and nodules. The enhancing hydrophilicity of surface for Sepro NF membranes can reduce the physical or chemical adsorption of dye. Furthermore, in the highly loaded textile wastewater, a specific fouling mechanism, denoted as ‘cake-enhanced concentration polarization’, can significantly deteriorate the membrane flux through increasing osmotic pressure as well as exacerbating the formation of dye cake layer. Application of diafiltration for a dye/salt binary mixture indicates that cake-enhanced concentration polarization played a dominant role for the low flux at the initial step. 26
As the diafiltration proceeds, cake-enhanced concentration polarization was alleviated, enhancing the membrane flux. During the subsequent post-concentration, the evolution of dye cake layer slightly compromised the flux of superhydrophilic loose NF membranes, ignoring the negative impact of cake-enhanced concentration polarization. Therefore, the investigation of flux behavior for Sepro NF membranes in textile wastewater gives the convincing evidence that loose NF membranes can be an attracted option for textile wastewater treatment, apart from the term of high dye rejection. However, new strategies should be developed in future to mitigate the cake-enhanced concentration polarization for flux enhancement.
Acknowledgments J. Lin and W. Ye would like to acknowledge the support provided by China Scholarship Council of the Ministry of Education, China. Michèle Vanroelen from CIT, KU Leuven, is acknowledged for performing Ion Chromatography measurements. Ultura (USA) is greatly thanked for supplying the NF membrane samples.
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Highlights - A loose nanofiltration (NF) membrane was employed for dye/salt separation - CECP mechanism substantially deteriorated the flux of loose NF membrane in dye/salt mixture - The loose NF membrane had an excellent antifouling against dye due to its superhydrophilicity - Water rinsing can be an effective method to recover the flux of loose NF membrane
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