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Polysulfone composite nanofiltration membrane crosslinked with epichlorohydrin
Desalination 233 (2008) 147–156
Preparation and characterization of N,O-carboxymethyl chitosan/Polysulfone composite nanofiltration membrane crosslinked with epichlorohydrin Jing Miaoa,b, Guohua Chenc, Congjie Gaoc, Shengxiong Donga* a
College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350002, China Tel. þ86 (591) 83852701; email: [email protected] b School of Environmental Science and Engineering, Ocean University of China, Qingdao 266003, China c School of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266003, China Received 16 July 2007; accepted revised 21 September 2007
Abstract N,O-carboxymethyl chitosan (NOCC) composite nanofiltration membranes crosslinked by ECH were developed using a method of coating and crosslinking, where an epichlorohydrin/ethanol 96.7% (0.067 M KOH) solution was used as the crosslinking agent. The structure and the morphology of the resulting membrane were characterized by attenuated total reflection infrared spectroscopy (ATR-IR) and environmental scanning electron microscopy (ESEM). The effects of preparation conditions on the rejection performance of the resulting composite membrane were also investigated. At 20 C and 0.40 MPa the rejections of the resulting membrane to Na2SO4 and NaCl solutions (1000 mg L1) were 90.4% and 27.4%, respectively, and the permeate fluxes were 7.9, and 10.8 kg m2 h1, respectively. The rejections of this kind of NOCC/PSF composite NF membrane to the inorganic electrolyte solutions decreased in the order of Na2SO4, NaCl, MgSO4, and MgCl2. Keywords: N,O-carboxymethyl chitosan (NOCC); Composite nanofiltration membrane; Structure and morphology; Rejection performance
1. Introduction During the past decades, the thin-film composite (TFC) membranes have become the main type of nanofiltration (NF) membranes [1]. *Corresponding author.
Chitosan is a cationic polysaccharide obtained by alkaline N-deacetylation of chitin. Due to the advantages of abundance, hydrophilicity, antibacterial, and environmental benignancy, chitosan and its derivatives have been attracting more and more attention to the further utilization
Presented at the Fourth Conference of Aseanian Membrane Society (AMS 4), 16–18 August 2007, Taipei, Taiwan. 0011-9164/08/$– See front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2007.09.037
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in membrane materials. Various kinds of composite NF membranes, which employed chitosan and its derivatives as the materials of the active layers, have been fabricated through the methods of surface crosslinking [2–4], blending [5], ultraviolet irradiation [6], etc. N,O-carboxymethyl chitosan (NOCC) is the product of the chitosan carboxylation having carboxymethyl substituents on some of both the amino and primary hydroxyl sites of the glucosamine units [7]. NOCC is hydrophilic and an excellent candidate for membrane material. However, within our knowledge, there has been no reported literature on NOCC composite NF membrane surface crosslinked by epichlorohydrine (ECH). In this paper a novel kind of NOCC composite NF membranes having a polysulfone (PSF) ultrafiltration membrane as the substrate were developed through the method of coating and crosslinking, where ECH was used to be the crosslinking agent. The structure, the morphology, and the rejection performance of the resulting NOCC/PSF composite NF membrane were investigated, respectively. The objective of this study was to develop a novel kind of NOCCbased composite NF membrane and investigate systematically the effects of preparation variables on the NF performances. The information would be useful for the further study on this kind of membrane materials.
NOCC aqueous solution was coated onto the PSF UF membrane, and then cured in an oven at 60 C for 1 h. The cured membranes were crosslinked in an ECH/ethanol 96.7% (0.067 M KOH) solution at 50 C for a certain period of time. After crosslinking, the resulting membrane was washed with deionized water extensively and kept in pure water to prevent crystal formation. Scheme 1 shows the cross-linking of NOCC with ECH.
2. Experimental
2.4. Characterization of the resulting composite NF membrane
2.1. Materials and methods Chitosan (MW 5.4105 Da, degree of deacetylation (d.d) 91%) was purchased from Haihui Bioengineering Co., Qingdao (China). The ultrafiltration membrane evaluation apparatus and the PSF UF membrane (MWCO ¼ 10,000 Da) that had a pure water flux of 251 kg m2 h1 at 20 C and 0.10 MPa were provided by the Development Center of
Water Treatment Technology, State Oceanic Administration, Hangzhou (China). All other reagents and solvents were of analytical grade and used without further purification. The conductivity was determined using Model DDS-11A conductivity meter (Shanghai Lida Instrument Co., China). Total organic carbon (TOC) was determined using TOC-VCPN Analyzer (Shimadzu Scientific Instruments, Inc., Japan). 2.2. Synthesis of NOCC NOCC was synthesized as per the procedure described in the literatures [8,9]. The carboxymethylation substitutive degree was determined by potentiometric titration [10] and its value was 0.96. 2.3. Preparation of NOCC/PSF composite NF membranes
The sample for characterization was prepared under the following conditions: NOCC concentration 1.7 wt.%, curing time 1 h at 60 C, ECH concentration 3.0 wt.%, and crosslinking time 3 h at 50 C. ATR-IR characterization of the membrane surface was made with an ATR accessory of Nicolet Avatar360 IR Spectrometer. ZnSe crystal was used in the ATR accessory. Membrane samples were mounted flush to the ATR crystal.
J. Miao et al. / Desalination 233 (2008) 147–156 CH2OH
CH2OH
CH2OCH2COOR O
O
O
O
OH
CH2OH O
O
OH
OH NH2
NH2
NH2
O
O
OCH2COOR
NHCH2COOR
NHAc
149
NHAc
NH2
OCH2COOR
NHCH 2COOR OH
OH O
O
O
O
OH O
O
O CH2OH
O CH2OCH2COOR
CH2OH
CH2OH
X, KOH
CH2OH
CH2OH
CH2OCH2COOR O
O
O
O
O
OH O
CH2OH
OCH2COOR
NHCH2COOR
NH
R O
O
O
O CH2OH
R
O
O
O
CH2OH
O
O
OH
CH 2OCH2COOR
CH2CHCH2Cl
X:
NHAc
NHCH2COOR
OCH2COOR
O
O
NH
NH
NH
O
O
R
R
NHAc
O
O CH2OH
CH 2CHCH2
, R:
OH
Scheme 1. Schematic representation of the cross-linking of NOCC with ECH.
Environmental scanning electromicroscopy was performed with an environmental electronic scanning microscope (Philips XL-30 ESEM) operating at 15.0 kV. The sample for crosssectional viewing was prepared as described in the literature [11].
2.5. Permeation experiment Na2SO4, NaCl, MgSO4, and MgCl2 were employed as the inorganic electrolytes. The concentrations of the single inorganic electrolytes were determined conductometrically. The
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permeation tests were carried out using a UF membrane evaluation apparatus at a temperature, pressure, feed flow, and concentration of inorganic electrolyte solution of 20 C, 0.40 MPa, 40 L/h, and 1000 mg L1, respectively. The permeation flux (F) of the membrane was determined by weighing the permeate penetrated through the membrane during a certain period of time. Rejection (R) was calculated with the following equation: R¼1
Cp Cf
where Cp and Cf are the concentrations of the permeate and the feed solution, respectively.
2.6. Molecular weight cut-off (MWCO) MWCO of the resulting membrane for characterization was measured using 500 mg L1 aqueous solutions of polyethylene glycols (PEG) (MW 200–1000 Da), respectively. The concentrations of the test solutes in the feed and the permeate were determined using Shimadzu TOC-VCPN analyzer.
2.7. Streaming potential measurement A streaming potential is the potential difference at zero current caused by the convective flow of charge due to a pressure gradient through a charged membrane. Therefore, surface charge characteristics of a membrane can be determined by the streaming potential, and the slope of a EP plot could reflect the membrane charge properties reliably. The streaming potential of NOCC/PSF composite NF membrane was measured at 20 C and different pressures ranging from 0 to 0.35 MPa in 0.01 M KCl solution by the method described in the literatures [12–14] using a digital multimeter (Model MY60, Hangzhou Huayi Electric Industrial Co. Ltd., China).
3. Results and discussion 3.1. Membrane characterization 3.1.1. ATR-IR spectra of NOCC/PSF composite NF membrane Fig. 1 shows the ATR-IR spectra of a virgin PSF UF membrane with just the NOCC coating (PSF-NOCC), and with the crosslinked NOCC active layer (PSF-NOCC-ECH). As for PSF-NOCC-ECH, there is a distinct band at 1586 cm1 attributed to the bending vibration of aliphatic secondary amine, which was synthesized by the reaction between NH2 and ECH. The band at 1150 cm1 became relatively intense and sharper, corresponding to the formation of the ether bonds synthesized by the etherization between ECH and OH. The sharper band at 1103 cm1, corresponding to the stretching vibration of the chain aliphatic alcohol, confirmed the introduction of the hydroxyl groups into the straight chains.
3.1.2. SEM image of NOCC/PSF composite NF membrane Fig. 2 shows the cross-sectional morphology of NOCC/PSF composite NF membrane with the magnification of 10,000. As can be seen clearly from the figure, there is a thin, dense active layer coating on the base membrane, which
Fig. 1. ATR-IR spectra of NOCC/PSF composite NF membrane.
J. Miao et al. / Desalination 233 (2008) 147–156
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The active layer of NOCC/PSF composite membrane contains carboxymethyl groups. Additionally, the active layer of this amphoteric NF membrane might acquire a negative surface charge distribution by adsorbing anions from the electrolyte solution [15].
3.1.4. MWCO of NOCC/PSF composite NF membrane Fig. 2. Cross-sectional morphology of NOCC/PSF composite NF membrane.
reveals the asymmetric and composite structure of this membrane. This figure also indicated relatively good compatibility between the NOCC active layer and PSF UF membrane.
3.2. Preparing conditions on the rejection performance
3.1.3. Streaming potential of the NOCC/PSF composite NF membrane
P (MPa) 0
E (mV)
–0.5
0.1
0.2
3.2.1. Effect of NOCC concentration A series of NOCC/PSF composite membranes were prepared from NOCC solutions of different concentrations in the range of 1.0–2.0 wt.%
100 90 80 R (%)
Fig. 3 illustrates the curve about streaming potential (E) against operating pressure (P) using 0.01 M KCl as the electrolyte solution for the NOCC/PSF composite NF membrane. All streaming potential were below zero and decreased linearly with the increase of the operating pressure. The slope of EP plot is equal to 6.00 mV MPa1, suggesting this kind of composite NF membrane is negatively charged.
0
Fig. 4 shows the rejections of the resulting NF membrane to polyethylene glycol (PEG) with different molecular weights. As shown in the figure, the observed rejections to PEG increased with the growth of their MWs under the operating pressure of 0.40 MPa. The MWCO of the resulting NF membrane is 760 Da (corresponding to a rejection of 90%).
0.3
0.4
70 60 R 50
–1
40
–1.5
30
–2 –2.5
Fig. 3. Streaming potential (E)-operating pressure (P) figure of NOCC/PSF composite NF membrane.
200
400
600
800
1000
Molecular weight of PEG (Da)
Fig. 4. Rejections of NOCC/PSF composite NF membrane to PEG with different molecular weights.
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20
R F
90
14 84 12 82
F (kg m–2 h–1)
R (%)
16 86
10
80
8 1.0
1.2
1.4
1.6
1.8
2.0
Dried at 20 C for 12 h, or cured in an oven at 50 C for 1 h, or cured in an oven at 60 C for 1 h, NOCC/PSF membranes prepared from 1.7 wt.% NOCC solution were crosslinked with 3.0 wt.% ECH/EtOH 96.7% (0.067 M KOH) at 50 C for 3 h. Table 1 illustrates the effect of the curing temperature on the rejection performance of the resulting membranes to 1000 mg L1 Na2SO4 solution. As can be seen clearly from Table 1, the permeate flux decreased and the rejection increased with the increase of the curing temperature. The elevated temperature would promote the shrinkage of the base membrane and the active layer simultaneously. Hence, both the pore sizes of the base membrane and the active layer were reduced simultaneously, which resulted in a decrease in the permeate flux and an increase in the rejection.
18
88
78
3.2.2. Effect of the curing temperature
22
6
NOCC concentration (wt.%)
Fig. 5. Effect of NOCC concentration on the rejection performance of NOCC/PSF composite membranes.
following the same preparation technique as mentioned in Sect. 2.4. Fig. 5 shows the effect of NOCC concentration on the rejection performance of the resulting composite membranes to 1000 mg L1 Na2SO4 solution. The Na2SO4 rejection increased, and the permeate flux decreased with the increase of the NOCC concentration until it was 1.7 wt.%, which was likely to be resulted from the following reasons. The amount of amino and hydroxyl groups was increased with the increase of NOCC concentration, which led to the increase of the amount of the crosslinking points, the increase of the crosslinking density, and the decrease of the dimension of the crosslinking network. The rejection to Na2SO4 solution began to decrease as NOCC concentration was ca. 1.7 wt.%, which might be due to the blemished active layer resulted from the partial precipitation of NOCC during the curing period and the decrease of the crosslinking degree of the membrane surface. The resulting membrane prepared from NOCC solution concentration of 1.7 wt.% has high rejection (90.4%). Therefore, a NOCC solution concentration of 1.7 wt.% was selected while the effects of the other preparation conditions on the rejection performance were investigated.
3.2.3. Effect of ECH concentration Crosslinked with ECH solutions at different concentrations in the range of 1.0–5.0 wt.%, a series of NOCC/PSF composite membranes were prepared from 1.7 wt.% NOCC solutions following the same preparation technique as mentioned in Sect. 2.4. Fig. 6 shows the effect of ECH concentration on the rejection performance of the resulting membranes to 1000 mg L1 Na2SO4 solution. As known from Fig. 6, ECH concentration had a large effect on the permeate flux of the composite membranes but influenced Table 1 Effect of the curing temperature on the rejection performance of NOCC/PS composite membranes Curing temperature ( C)
F (kg m2 h1)
R (%)
20 50 60
16.0 9.6 7.9
85.7 87.6 90.4
J. Miao et al. / Desalination 233 (2008) 147–156
R (%)
86
35
90
30
88
25
84
20
82
22 R F
20 18 16
86 14 84 12
15
82
10
80
5
78
10
80
8
78
1
2
3
4
5
ECH concentration (wt.%)
Fig. 6. Effect of the crosslinking solution concentration on the rejection performance of NOCC/PSF composite membranes.
the rejection little. The permeate fluxes were 36.9–7.9 kg m2 h1 and the rejections were 78.5–90.4%, respectively, as ECH concentration ranged from 1.0 to 3.0 wt.%, which may be due to the sieving effect. The increase of the crosslinking solution concentration would result in the reduction in pore size and water absorption, and the increase in hydrophobicity and pore tortuosity, which would cause the decrease of the permeate flux and the increase of the rejection to inorganic electrolyte. The similar trend was also observed in the literature [2,16]. However, the rejection began to decrease, as ECH concentration was higher than 3.0 wt.%, which could not be explained plausibly yet.
3.2.4. Effect of the cross-linking time A series of NOCC/PSF composite membranes coated with 1.7 wt.% NOCC solutions were cured in an oven at 60 C for 1 h, and then crosslinked with 3.0 wt.% ECH solutions at 50 C for a period of time ranging from 1 to 5 h. Fig. 7 shows the effect of the crosslinking time on the rejection performance of NOCC/PSF composite membranes to 1000 mg L1 Na2SO4 solution. As known from the figure, the crosslinking time
F (kg m–2 h–1)
88
92
R (%)
R F
90
40
F (kg m–2 h–1)
92
153
6
1
2 3 4 Cross-linking time (h)
5
Fig. 7. Effect of the crosslinking time on the rejection performance of NOCC/PSF composite membranes.
also had a more remarkable effect on the permeate flux than on the rejection while it was 3 h. The permeate flux decreased from 20.6 to 7.9 kg m2 h1 by 62% and the rejection increased from 79.8% to 90.4% by only 13% as the crosslinking time increased from 1 to 3 h. The trends of the decrease of the permeate flux and the increase of the rejection as the crosslinking time was 3 h could be also explained by the increase of the crosslinking degree, the pore contraction, and the increase in tortuosity. However, the rejection began to decrease after ca. 3 h, for which there is no plausible explanation yet. 3.2.5. Effect of low-molecular-weight organic additives in the casting solution A low-molecular-weight organic substance could be employed as an additive to control the pore size of the active layer, and then adjust the rejection properties of the resulting membrane. NOCC/PSF composite membranes coated with 1.7 wt.% NOCC solution in the presence of four such kinds of low-molecular-weight organic additives were fabricated using the same preparation technique described in Sect. 2.4, where all the organic additives were each employed at 5.0 wt.% concentration. Table 2 illustrates the
J. Miao et al. / Desalination 233 (2008) 147–156 180
Table 2 Effect of the low-molecular-weight organic additives on the rejection performance of NOCC/PSF composite membranes
In the absence of organic additive Glycerol 1,4-Butanediol PEG200 PEG400
F (kg m2 h1)
R (%)
150 80
R (%)
Low-molecular-weight organic additives
90
120
R F
70
90
60
7.9
90.4
41.9 25.1 14.5 15.2
81.1 77.9 86.3 69.9
60
50
40
F (kg m–2 h–1)
154
30
0
5
10
15
20
25
30
35
40
0
PEG200 concentration (wt.%)
Fig. 8. Effect of PEG200 concentration on the rejection performance of NOCC/PSF composite membranes.
rejection performance of the resulting composite membranes to 1000 mg L1 Na2SO4 solution. The rejections decreased in the order of PEG200, glycerol, 1, 4-butanediol, and PEG400, while the permeate flux increased in the order of PEG200, PEG400, 1, 4-butanediol, and glycerol. Hence, PEG200 is an optimal choice for NOCC/PSF composite membranes. The permeate flux increased by 83% and the rejection decreased little by only 4.5% while the casting solution was added into 5.0 wt.% PEG200. To investigate the effects further, a series of NOCC/PSF composite membranes were prepared from 1.7 wt.% NOCC solutions, which were added into PEG200 at the weight ratios ranging from 5.0 to 40.0 wt.%. 3.2.6. Effect of PEG200 concentration To investigate the effect of PEG200 concentration on the rejection performance, a series of NOCC/PSF composite membranes were prepared from 1.7 wt.% NOCC solutions, which were added into PEG200 at the weight ratios in the range of 5.0–40.0 wt.%. Fig. 8 shows the effect of PEG200 concentration on the rejection performance to 1000 mg L1 Na2SO4 solution. As can be seen clearly from Fig. 8, the permeate flux increased and the rejection decreased with increasing PEG200 concentration, and it can be concluded that the resulting composite
membranes showed better rejection performance as PEG200 concentration was in the range of 5.0–30.0 wt.%. With the addition of 30.0 wt.% PEG200, the permeate flux increased by approximately four times to 30.2 kg m2 h1 and the rejection decreased by only 10–81.7%. When the concentration of PEG200 was more than 30.0 wt.%, the permeate flux increased sharply and the rejection decreased with the increase of the PEG200 concentration. Although PEG 200 is a pore-forming agent, the addition of a certain amount of PEG200 would decrease the interfacial tension and increase the swelling of the base membrane, thereby the casting solution would be coated more uniformly. However, when the concentration of PEG200 was more than 30.0 wt.%, the molecules integrated with each other via hydrogen bonds and the congeries formed with the increase of the PEG200 concentration, which resulted in the increase in the pore size of the active layer.
3.3. Rejection to different inorganic electrolytes by NOCC/PSF composite NF membrane NOCC/PSF composite NF membranes, coded as Mem-1 and Mem-2, were prepared by the method described in 2.4 from a 1.7 wt.% NOCC
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solution and a 1.7 wt.% NOCC solution in the presence of 10.0 wt.% PEG200, respectively. The rejection performances to different inorganic electrolyte solutions (1000 mg L1) by the resulting NOCC/PSF composite NF membranes are shown in Table 3. The order of the rejections is RNa2 SO4 > RNaCl > RMgSO4 > RMgCl2 . This sequence shows the predictable Donnan characteristic of salt rejection of a negatively charged nanofiltration membranes [1,17]. The rejection to the inorganic electrolytes containing identical anion such as Cl decreased in the order of Naþ and Mg2þ, and the rejection to the inorganic electrolytes containing identical cation such as Naþ decreased in the order of SO2 4 and Cl , which could be explained by Donnan exclusion effect. The active layer of NOCC/PSF composite membrane contains carboxymethyl groups, which will have a stronger repulsion to SO2 4 than Cl. Additionally, the active layer of NOCC/PSF composite NF membrane could acquire a negative surface charge distribution by the adsorption of anions from the electrolyte inorganic solution [17]. In this study MgSO4 separations were lower than NaCl separations. Lower salt rejections for divalent cations have been reported in the literatures [17–19]. This might result from an affinity between Mg2þ and the anions on the membrane surface, which would decrease the effective surface charge of the membranes and thus reduce the rejection. Table 3 Rejections to different inorganic electrolytes by NOCC/ PSF composite NF membranes Feed flow
Mem-1
Mem-2
F (kg m2 h1) R (%) F (kg m2 h1) R (%) Na2SO4 7.9 NaCl 10.8 MgSO4 9.1 8.2 MgCl2
90.4 27.4 24.4 8.9
20.9 43.2 38.6 41.1
83.2 18.8 18.0 5.6
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4. Conclusion Novel amphoteric composite NF membranes were prepared through a method of coating and crosslinking using NOCC, PSF UF membranes, and ECH as the active layer material, the base membranes, and the crosslinking agent, respectively. The results suggest that NOCC/PSF composite NF membranes with excellent rejection performance could be prepared under the following conditions: NOCC concentration 1.7 wt.%, curing time 1 h at 60 C, ECH/EtOH 96.7% (0.067 M KOH) concentration 3.0 wt.%, and crosslinking time 3 h at 50 C. The MWCO of the resulting NF membrane was 760 Da. At 20 C and 0.40 MPa the rejections to Na2SO4 and NaCl solutions (1000 mg L1) were 90.4% and 27.4%, respectively, and the permeate fluxes were 7.9 and 10.8 kg m2 h1, respectively. The rejection of this kind of composite NF membrane to inorganic electrolyte solutions decreased in the order of Na2SO4, NaCl, MgSO4, and MgCl2. It can be concluded from the results that the amphoteric NOCC/PSF composite membrane showed similar rejection performance to negatively charged composite NF membranes. Additionally, the curve for the streaming potential also illustrates the negatively charged characteristics of the composite membrane. The resultant NOCC/PSF composite NF membrane with excellent rejection performance was characterized with ATR-IR and ESEM, respectively. The ATR-IR spectra revealed the crosslinking of hydroxyl groups and amino groups with ECH. The rejection properties of the developed composite membranes could be adjusted by changing the concentration of the casting solution of the active layer, the concentration of the crosslinking agent, the crosslinking time, the curing temperature, the addition of the low-molecular-weight organic additive, etc. It was found that PEG200 is a kind of excellent low-molecule-weight organic
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additive for NOCC/PSF composite NF membranes, and the rejection characteristic of the resultant membrane would be improved when the casting solution was added into PEG200 in the range of 5.0–30.0 wt.%.
[9]
[10]
Acknowledgement The authors are grateful to the financial support from the Major State Basic Research Development Program of China (973 Program, No. 2003CB615706).
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Preparation and characterization of N,O-carboxymethyl chitosan/Polysulfone composite nanofiltration membrane crosslinked with epichlorohydrin Jing Miaoa,b, Guohua Chenc, Congjie Gaoc, Shengxiong Donga* a
College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350002, China Tel. þ86 (591) 83852701; email: [email protected] b School of Environmental Science and Engineering, Ocean University of China, Qingdao 266003, China c School of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266003, China Received 16 July 2007; accepted revised 21 September 2007
Abstract N,O-carboxymethyl chitosan (NOCC) composite nanofiltration membranes crosslinked by ECH were developed using a method of coating and crosslinking, where an epichlorohydrin/ethanol 96.7% (0.067 M KOH) solution was used as the crosslinking agent. The structure and the morphology of the resulting membrane were characterized by attenuated total reflection infrared spectroscopy (ATR-IR) and environmental scanning electron microscopy (ESEM). The effects of preparation conditions on the rejection performance of the resulting composite membrane were also investigated. At 20 C and 0.40 MPa the rejections of the resulting membrane to Na2SO4 and NaCl solutions (1000 mg L1) were 90.4% and 27.4%, respectively, and the permeate fluxes were 7.9, and 10.8 kg m2 h1, respectively. The rejections of this kind of NOCC/PSF composite NF membrane to the inorganic electrolyte solutions decreased in the order of Na2SO4, NaCl, MgSO4, and MgCl2. Keywords: N,O-carboxymethyl chitosan (NOCC); Composite nanofiltration membrane; Structure and morphology; Rejection performance
1. Introduction During the past decades, the thin-film composite (TFC) membranes have become the main type of nanofiltration (NF) membranes [1]. *Corresponding author.
Chitosan is a cationic polysaccharide obtained by alkaline N-deacetylation of chitin. Due to the advantages of abundance, hydrophilicity, antibacterial, and environmental benignancy, chitosan and its derivatives have been attracting more and more attention to the further utilization
Presented at the Fourth Conference of Aseanian Membrane Society (AMS 4), 16–18 August 2007, Taipei, Taiwan. 0011-9164/08/$– See front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2007.09.037
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in membrane materials. Various kinds of composite NF membranes, which employed chitosan and its derivatives as the materials of the active layers, have been fabricated through the methods of surface crosslinking [2–4], blending [5], ultraviolet irradiation [6], etc. N,O-carboxymethyl chitosan (NOCC) is the product of the chitosan carboxylation having carboxymethyl substituents on some of both the amino and primary hydroxyl sites of the glucosamine units [7]. NOCC is hydrophilic and an excellent candidate for membrane material. However, within our knowledge, there has been no reported literature on NOCC composite NF membrane surface crosslinked by epichlorohydrine (ECH). In this paper a novel kind of NOCC composite NF membranes having a polysulfone (PSF) ultrafiltration membrane as the substrate were developed through the method of coating and crosslinking, where ECH was used to be the crosslinking agent. The structure, the morphology, and the rejection performance of the resulting NOCC/PSF composite NF membrane were investigated, respectively. The objective of this study was to develop a novel kind of NOCCbased composite NF membrane and investigate systematically the effects of preparation variables on the NF performances. The information would be useful for the further study on this kind of membrane materials.
NOCC aqueous solution was coated onto the PSF UF membrane, and then cured in an oven at 60 C for 1 h. The cured membranes were crosslinked in an ECH/ethanol 96.7% (0.067 M KOH) solution at 50 C for a certain period of time. After crosslinking, the resulting membrane was washed with deionized water extensively and kept in pure water to prevent crystal formation. Scheme 1 shows the cross-linking of NOCC with ECH.
2. Experimental
2.4. Characterization of the resulting composite NF membrane
2.1. Materials and methods Chitosan (MW 5.4105 Da, degree of deacetylation (d.d) 91%) was purchased from Haihui Bioengineering Co., Qingdao (China). The ultrafiltration membrane evaluation apparatus and the PSF UF membrane (MWCO ¼ 10,000 Da) that had a pure water flux of 251 kg m2 h1 at 20 C and 0.10 MPa were provided by the Development Center of
Water Treatment Technology, State Oceanic Administration, Hangzhou (China). All other reagents and solvents were of analytical grade and used without further purification. The conductivity was determined using Model DDS-11A conductivity meter (Shanghai Lida Instrument Co., China). Total organic carbon (TOC) was determined using TOC-VCPN Analyzer (Shimadzu Scientific Instruments, Inc., Japan). 2.2. Synthesis of NOCC NOCC was synthesized as per the procedure described in the literatures [8,9]. The carboxymethylation substitutive degree was determined by potentiometric titration [10] and its value was 0.96. 2.3. Preparation of NOCC/PSF composite NF membranes
The sample for characterization was prepared under the following conditions: NOCC concentration 1.7 wt.%, curing time 1 h at 60 C, ECH concentration 3.0 wt.%, and crosslinking time 3 h at 50 C. ATR-IR characterization of the membrane surface was made with an ATR accessory of Nicolet Avatar360 IR Spectrometer. ZnSe crystal was used in the ATR accessory. Membrane samples were mounted flush to the ATR crystal.
J. Miao et al. / Desalination 233 (2008) 147–156 CH2OH
CH2OH
CH2OCH2COOR O
O
O
O
OH
CH2OH O
O
OH
OH NH2
NH2
NH2
O
O
OCH2COOR
NHCH2COOR
NHAc
149
NHAc
NH2
OCH2COOR
NHCH 2COOR OH
OH O
O
O
O
OH O
O
O CH2OH
O CH2OCH2COOR
CH2OH
CH2OH
X, KOH
CH2OH
CH2OH
CH2OCH2COOR O
O
O
O
O
OH O
CH2OH
OCH2COOR
NHCH2COOR
NH
R O
O
O
O CH2OH
R
O
O
O
CH2OH
O
O
OH
CH 2OCH2COOR
CH2CHCH2Cl
X:
NHAc
NHCH2COOR
OCH2COOR
O
O
NH
NH
NH
O
O
R
R
NHAc
O
O CH2OH
CH 2CHCH2
, R:
OH
Scheme 1. Schematic representation of the cross-linking of NOCC with ECH.
Environmental scanning electromicroscopy was performed with an environmental electronic scanning microscope (Philips XL-30 ESEM) operating at 15.0 kV. The sample for crosssectional viewing was prepared as described in the literature [11].
2.5. Permeation experiment Na2SO4, NaCl, MgSO4, and MgCl2 were employed as the inorganic electrolytes. The concentrations of the single inorganic electrolytes were determined conductometrically. The
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permeation tests were carried out using a UF membrane evaluation apparatus at a temperature, pressure, feed flow, and concentration of inorganic electrolyte solution of 20 C, 0.40 MPa, 40 L/h, and 1000 mg L1, respectively. The permeation flux (F) of the membrane was determined by weighing the permeate penetrated through the membrane during a certain period of time. Rejection (R) was calculated with the following equation: R¼1
Cp Cf
where Cp and Cf are the concentrations of the permeate and the feed solution, respectively.
2.6. Molecular weight cut-off (MWCO) MWCO of the resulting membrane for characterization was measured using 500 mg L1 aqueous solutions of polyethylene glycols (PEG) (MW 200–1000 Da), respectively. The concentrations of the test solutes in the feed and the permeate were determined using Shimadzu TOC-VCPN analyzer.
2.7. Streaming potential measurement A streaming potential is the potential difference at zero current caused by the convective flow of charge due to a pressure gradient through a charged membrane. Therefore, surface charge characteristics of a membrane can be determined by the streaming potential, and the slope of a EP plot could reflect the membrane charge properties reliably. The streaming potential of NOCC/PSF composite NF membrane was measured at 20 C and different pressures ranging from 0 to 0.35 MPa in 0.01 M KCl solution by the method described in the literatures [12–14] using a digital multimeter (Model MY60, Hangzhou Huayi Electric Industrial Co. Ltd., China).
3. Results and discussion 3.1. Membrane characterization 3.1.1. ATR-IR spectra of NOCC/PSF composite NF membrane Fig. 1 shows the ATR-IR spectra of a virgin PSF UF membrane with just the NOCC coating (PSF-NOCC), and with the crosslinked NOCC active layer (PSF-NOCC-ECH). As for PSF-NOCC-ECH, there is a distinct band at 1586 cm1 attributed to the bending vibration of aliphatic secondary amine, which was synthesized by the reaction between NH2 and ECH. The band at 1150 cm1 became relatively intense and sharper, corresponding to the formation of the ether bonds synthesized by the etherization between ECH and OH. The sharper band at 1103 cm1, corresponding to the stretching vibration of the chain aliphatic alcohol, confirmed the introduction of the hydroxyl groups into the straight chains.
3.1.2. SEM image of NOCC/PSF composite NF membrane Fig. 2 shows the cross-sectional morphology of NOCC/PSF composite NF membrane with the magnification of 10,000. As can be seen clearly from the figure, there is a thin, dense active layer coating on the base membrane, which
Fig. 1. ATR-IR spectra of NOCC/PSF composite NF membrane.
J. Miao et al. / Desalination 233 (2008) 147–156
151
The active layer of NOCC/PSF composite membrane contains carboxymethyl groups. Additionally, the active layer of this amphoteric NF membrane might acquire a negative surface charge distribution by adsorbing anions from the electrolyte solution [15].
3.1.4. MWCO of NOCC/PSF composite NF membrane Fig. 2. Cross-sectional morphology of NOCC/PSF composite NF membrane.
reveals the asymmetric and composite structure of this membrane. This figure also indicated relatively good compatibility between the NOCC active layer and PSF UF membrane.
3.2. Preparing conditions on the rejection performance
3.1.3. Streaming potential of the NOCC/PSF composite NF membrane
P (MPa) 0
E (mV)
–0.5
0.1
0.2
3.2.1. Effect of NOCC concentration A series of NOCC/PSF composite membranes were prepared from NOCC solutions of different concentrations in the range of 1.0–2.0 wt.%
100 90 80 R (%)
Fig. 3 illustrates the curve about streaming potential (E) against operating pressure (P) using 0.01 M KCl as the electrolyte solution for the NOCC/PSF composite NF membrane. All streaming potential were below zero and decreased linearly with the increase of the operating pressure. The slope of EP plot is equal to 6.00 mV MPa1, suggesting this kind of composite NF membrane is negatively charged.
0
Fig. 4 shows the rejections of the resulting NF membrane to polyethylene glycol (PEG) with different molecular weights. As shown in the figure, the observed rejections to PEG increased with the growth of their MWs under the operating pressure of 0.40 MPa. The MWCO of the resulting NF membrane is 760 Da (corresponding to a rejection of 90%).
0.3
0.4
70 60 R 50
–1
40
–1.5
30
–2 –2.5
Fig. 3. Streaming potential (E)-operating pressure (P) figure of NOCC/PSF composite NF membrane.
200
400
600
800
1000
Molecular weight of PEG (Da)
Fig. 4. Rejections of NOCC/PSF composite NF membrane to PEG with different molecular weights.
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J. Miao et al. / Desalination 233 (2008) 147–156 92
20
R F
90
14 84 12 82
F (kg m–2 h–1)
R (%)
16 86
10
80
8 1.0
1.2
1.4
1.6
1.8
2.0
Dried at 20 C for 12 h, or cured in an oven at 50 C for 1 h, or cured in an oven at 60 C for 1 h, NOCC/PSF membranes prepared from 1.7 wt.% NOCC solution were crosslinked with 3.0 wt.% ECH/EtOH 96.7% (0.067 M KOH) at 50 C for 3 h. Table 1 illustrates the effect of the curing temperature on the rejection performance of the resulting membranes to 1000 mg L1 Na2SO4 solution. As can be seen clearly from Table 1, the permeate flux decreased and the rejection increased with the increase of the curing temperature. The elevated temperature would promote the shrinkage of the base membrane and the active layer simultaneously. Hence, both the pore sizes of the base membrane and the active layer were reduced simultaneously, which resulted in a decrease in the permeate flux and an increase in the rejection.
18
88
78
3.2.2. Effect of the curing temperature
22
6
NOCC concentration (wt.%)
Fig. 5. Effect of NOCC concentration on the rejection performance of NOCC/PSF composite membranes.
following the same preparation technique as mentioned in Sect. 2.4. Fig. 5 shows the effect of NOCC concentration on the rejection performance of the resulting composite membranes to 1000 mg L1 Na2SO4 solution. The Na2SO4 rejection increased, and the permeate flux decreased with the increase of the NOCC concentration until it was 1.7 wt.%, which was likely to be resulted from the following reasons. The amount of amino and hydroxyl groups was increased with the increase of NOCC concentration, which led to the increase of the amount of the crosslinking points, the increase of the crosslinking density, and the decrease of the dimension of the crosslinking network. The rejection to Na2SO4 solution began to decrease as NOCC concentration was ca. 1.7 wt.%, which might be due to the blemished active layer resulted from the partial precipitation of NOCC during the curing period and the decrease of the crosslinking degree of the membrane surface. The resulting membrane prepared from NOCC solution concentration of 1.7 wt.% has high rejection (90.4%). Therefore, a NOCC solution concentration of 1.7 wt.% was selected while the effects of the other preparation conditions on the rejection performance were investigated.
3.2.3. Effect of ECH concentration Crosslinked with ECH solutions at different concentrations in the range of 1.0–5.0 wt.%, a series of NOCC/PSF composite membranes were prepared from 1.7 wt.% NOCC solutions following the same preparation technique as mentioned in Sect. 2.4. Fig. 6 shows the effect of ECH concentration on the rejection performance of the resulting membranes to 1000 mg L1 Na2SO4 solution. As known from Fig. 6, ECH concentration had a large effect on the permeate flux of the composite membranes but influenced Table 1 Effect of the curing temperature on the rejection performance of NOCC/PS composite membranes Curing temperature ( C)
F (kg m2 h1)
R (%)
20 50 60
16.0 9.6 7.9
85.7 87.6 90.4
J. Miao et al. / Desalination 233 (2008) 147–156
R (%)
86
35
90
30
88
25
84
20
82
22 R F
20 18 16
86 14 84 12
15
82
10
80
5
78
10
80
8
78
1
2
3
4
5
ECH concentration (wt.%)
Fig. 6. Effect of the crosslinking solution concentration on the rejection performance of NOCC/PSF composite membranes.
the rejection little. The permeate fluxes were 36.9–7.9 kg m2 h1 and the rejections were 78.5–90.4%, respectively, as ECH concentration ranged from 1.0 to 3.0 wt.%, which may be due to the sieving effect. The increase of the crosslinking solution concentration would result in the reduction in pore size and water absorption, and the increase in hydrophobicity and pore tortuosity, which would cause the decrease of the permeate flux and the increase of the rejection to inorganic electrolyte. The similar trend was also observed in the literature [2,16]. However, the rejection began to decrease, as ECH concentration was higher than 3.0 wt.%, which could not be explained plausibly yet.
3.2.4. Effect of the cross-linking time A series of NOCC/PSF composite membranes coated with 1.7 wt.% NOCC solutions were cured in an oven at 60 C for 1 h, and then crosslinked with 3.0 wt.% ECH solutions at 50 C for a period of time ranging from 1 to 5 h. Fig. 7 shows the effect of the crosslinking time on the rejection performance of NOCC/PSF composite membranes to 1000 mg L1 Na2SO4 solution. As known from the figure, the crosslinking time
F (kg m–2 h–1)
88
92
R (%)
R F
90
40
F (kg m–2 h–1)
92
153
6
1
2 3 4 Cross-linking time (h)
5
Fig. 7. Effect of the crosslinking time on the rejection performance of NOCC/PSF composite membranes.
also had a more remarkable effect on the permeate flux than on the rejection while it was 3 h. The permeate flux decreased from 20.6 to 7.9 kg m2 h1 by 62% and the rejection increased from 79.8% to 90.4% by only 13% as the crosslinking time increased from 1 to 3 h. The trends of the decrease of the permeate flux and the increase of the rejection as the crosslinking time was 3 h could be also explained by the increase of the crosslinking degree, the pore contraction, and the increase in tortuosity. However, the rejection began to decrease after ca. 3 h, for which there is no plausible explanation yet. 3.2.5. Effect of low-molecular-weight organic additives in the casting solution A low-molecular-weight organic substance could be employed as an additive to control the pore size of the active layer, and then adjust the rejection properties of the resulting membrane. NOCC/PSF composite membranes coated with 1.7 wt.% NOCC solution in the presence of four such kinds of low-molecular-weight organic additives were fabricated using the same preparation technique described in Sect. 2.4, where all the organic additives were each employed at 5.0 wt.% concentration. Table 2 illustrates the
J. Miao et al. / Desalination 233 (2008) 147–156 180
Table 2 Effect of the low-molecular-weight organic additives on the rejection performance of NOCC/PSF composite membranes
In the absence of organic additive Glycerol 1,4-Butanediol PEG200 PEG400
F (kg m2 h1)
R (%)
150 80
R (%)
Low-molecular-weight organic additives
90
120
R F
70
90
60
7.9
90.4
41.9 25.1 14.5 15.2
81.1 77.9 86.3 69.9
60
50
40
F (kg m–2 h–1)
154
30
0
5
10
15
20
25
30
35
40
0
PEG200 concentration (wt.%)
Fig. 8. Effect of PEG200 concentration on the rejection performance of NOCC/PSF composite membranes.
rejection performance of the resulting composite membranes to 1000 mg L1 Na2SO4 solution. The rejections decreased in the order of PEG200, glycerol, 1, 4-butanediol, and PEG400, while the permeate flux increased in the order of PEG200, PEG400, 1, 4-butanediol, and glycerol. Hence, PEG200 is an optimal choice for NOCC/PSF composite membranes. The permeate flux increased by 83% and the rejection decreased little by only 4.5% while the casting solution was added into 5.0 wt.% PEG200. To investigate the effects further, a series of NOCC/PSF composite membranes were prepared from 1.7 wt.% NOCC solutions, which were added into PEG200 at the weight ratios ranging from 5.0 to 40.0 wt.%. 3.2.6. Effect of PEG200 concentration To investigate the effect of PEG200 concentration on the rejection performance, a series of NOCC/PSF composite membranes were prepared from 1.7 wt.% NOCC solutions, which were added into PEG200 at the weight ratios in the range of 5.0–40.0 wt.%. Fig. 8 shows the effect of PEG200 concentration on the rejection performance to 1000 mg L1 Na2SO4 solution. As can be seen clearly from Fig. 8, the permeate flux increased and the rejection decreased with increasing PEG200 concentration, and it can be concluded that the resulting composite
membranes showed better rejection performance as PEG200 concentration was in the range of 5.0–30.0 wt.%. With the addition of 30.0 wt.% PEG200, the permeate flux increased by approximately four times to 30.2 kg m2 h1 and the rejection decreased by only 10–81.7%. When the concentration of PEG200 was more than 30.0 wt.%, the permeate flux increased sharply and the rejection decreased with the increase of the PEG200 concentration. Although PEG 200 is a pore-forming agent, the addition of a certain amount of PEG200 would decrease the interfacial tension and increase the swelling of the base membrane, thereby the casting solution would be coated more uniformly. However, when the concentration of PEG200 was more than 30.0 wt.%, the molecules integrated with each other via hydrogen bonds and the congeries formed with the increase of the PEG200 concentration, which resulted in the increase in the pore size of the active layer.
3.3. Rejection to different inorganic electrolytes by NOCC/PSF composite NF membrane NOCC/PSF composite NF membranes, coded as Mem-1 and Mem-2, were prepared by the method described in 2.4 from a 1.7 wt.% NOCC
J. Miao et al. / Desalination 233 (2008) 147–156
solution and a 1.7 wt.% NOCC solution in the presence of 10.0 wt.% PEG200, respectively. The rejection performances to different inorganic electrolyte solutions (1000 mg L1) by the resulting NOCC/PSF composite NF membranes are shown in Table 3. The order of the rejections is RNa2 SO4 > RNaCl > RMgSO4 > RMgCl2 . This sequence shows the predictable Donnan characteristic of salt rejection of a negatively charged nanofiltration membranes [1,17]. The rejection to the inorganic electrolytes containing identical anion such as Cl decreased in the order of Naþ and Mg2þ, and the rejection to the inorganic electrolytes containing identical cation such as Naþ decreased in the order of SO2 4 and Cl , which could be explained by Donnan exclusion effect. The active layer of NOCC/PSF composite membrane contains carboxymethyl groups, which will have a stronger repulsion to SO2 4 than Cl. Additionally, the active layer of NOCC/PSF composite NF membrane could acquire a negative surface charge distribution by the adsorption of anions from the electrolyte inorganic solution [17]. In this study MgSO4 separations were lower than NaCl separations. Lower salt rejections for divalent cations have been reported in the literatures [17–19]. This might result from an affinity between Mg2þ and the anions on the membrane surface, which would decrease the effective surface charge of the membranes and thus reduce the rejection. Table 3 Rejections to different inorganic electrolytes by NOCC/ PSF composite NF membranes Feed flow
Mem-1
Mem-2
F (kg m2 h1) R (%) F (kg m2 h1) R (%) Na2SO4 7.9 NaCl 10.8 MgSO4 9.1 8.2 MgCl2
90.4 27.4 24.4 8.9
20.9 43.2 38.6 41.1
83.2 18.8 18.0 5.6
155
4. Conclusion Novel amphoteric composite NF membranes were prepared through a method of coating and crosslinking using NOCC, PSF UF membranes, and ECH as the active layer material, the base membranes, and the crosslinking agent, respectively. The results suggest that NOCC/PSF composite NF membranes with excellent rejection performance could be prepared under the following conditions: NOCC concentration 1.7 wt.%, curing time 1 h at 60 C, ECH/EtOH 96.7% (0.067 M KOH) concentration 3.0 wt.%, and crosslinking time 3 h at 50 C. The MWCO of the resulting NF membrane was 760 Da. At 20 C and 0.40 MPa the rejections to Na2SO4 and NaCl solutions (1000 mg L1) were 90.4% and 27.4%, respectively, and the permeate fluxes were 7.9 and 10.8 kg m2 h1, respectively. The rejection of this kind of composite NF membrane to inorganic electrolyte solutions decreased in the order of Na2SO4, NaCl, MgSO4, and MgCl2. It can be concluded from the results that the amphoteric NOCC/PSF composite membrane showed similar rejection performance to negatively charged composite NF membranes. Additionally, the curve for the streaming potential also illustrates the negatively charged characteristics of the composite membrane. The resultant NOCC/PSF composite NF membrane with excellent rejection performance was characterized with ATR-IR and ESEM, respectively. The ATR-IR spectra revealed the crosslinking of hydroxyl groups and amino groups with ECH. The rejection properties of the developed composite membranes could be adjusted by changing the concentration of the casting solution of the active layer, the concentration of the crosslinking agent, the crosslinking time, the curing temperature, the addition of the low-molecular-weight organic additive, etc. It was found that PEG200 is a kind of excellent low-molecule-weight organic
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additive for NOCC/PSF composite NF membranes, and the rejection characteristic of the resultant membrane would be improved when the casting solution was added into PEG200 in the range of 5.0–30.0 wt.%.
[9]
[10]
Acknowledgement The authors are grateful to the financial support from the Major State Basic Research Development Program of China (973 Program, No. 2003CB615706).
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