Desalination 184 (2005) 1–11
New composite hollow fiber membrane for nanofiltration S. Verı´ ssimoa, K.-V. Peinemannb*, J. Bordadoa a
Departamento de Engenharia Quı´mica, Torre Sul, Instituto Superior Te´cnico, Av. Rovisco Pais 1, 1049–001 Lisboa, Portugal Tel. þ49 4152 872492; Fax þ49 4152 872492; email:
[email protected] b GKSS Forschungszentrum Geesthacht, Max-Planck-Str., D-21502 Geesthacht, Germany Tel. þ49 4152 872420; Fax þ49 (0)52 872466; email;
[email protected] Received 28 February 2005; accepted 31 March 2005
Abstract A capillary nanofiltration module would give a better packing density than the tubular module and allows a less demanding pre-treatment and maintenance than the spiral-wound module. This module would be a solution for the treatment of low quality water sources by direct capillary nanofiltration. In this work, a new composite capillary membrane with high permeability was developed by interfacial polymerization of N, N0 -diaminopiperazine (DAP) and trimesoylchloride (TMC). The influence of the preparation conditions of the inner thin film on the separating performance of the hollow fibers was studied. The parameters with stronger influence were the concentration of N, N0 -diaminopiperazine and the residence times of trimesoylchloride and rinsing solution. By changing the preparation parameters, it was possible to obtain two different hollow fiber membranes. Their performance was evaluated with electrolyte and non-electrolyte solutions. One type gave water permeability in the range 12–22 l/m2/h/bar. The salt rejections for NaCl, MgSO4 and Na2SO4 were 10, 12 and 80%, respectively. The sugar rejections obtained were 12% for glucose and 21% for sucrose and lactose. The other type presented lower water permeability, around 6 l/m2/h/bar, and higher salt rejections for NaCl (14%), MgSO4 (60%) and Na2SO4 (87%). A stronger increase was observed on the sugar rejections which were 57% (glucose), 77% (sucrose) and 91% (lactose). Morphological studies of the hollow fiber surface were conducted by scanning electron microscopy (SEM) and Atomic Force Microscopy (AFM) and revealed an extreme flat film. It was also possible to conclude that the morphology more similar to the support hollow fiber lead to a more permeable membrane. Keywords: Nanofiltration; Composite membranes; Reverse osmosis; Hollow fiber; Polyamide; Interfacial polymerization; Capillary nanofiltration
*Corresponding author. Presented at the Conference on Desalination and the Environment, Santa Margherita, Italy, 22–26 May 2005. European Desalination Society. 0011-9164/05/$– See front matter Ó 2005 Elsevier B.V. All rights reserved doi:10.1016/j.desal.2005.03.069
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1. Introduction Nanofiltration (NF) membranes are often used in the pre-treatment of seawater for desalination, ground water treatment and removal of natural organic matter. Ground water is becoming scarce or expensive and lower quality sources, such as surface water and effluents of waste water treatment plants, become attractive for production of process water. Today, the main module geometries used are spiral-wound or tubular modules. The advantages of spiral modules are the large surface area of the membrane per m3 and the low cost of the module. On the other hand, they are extremely susceptible to floating matter, have high energy losses caused by friction in the spacers and it is not possible to reverse flush them [1]. Tubular modules do not require strong pre-treatment and back flushing is possible but they have low packing density and are expensive [2]. Efforts are recently being done to develop capillary modules to bridge the gap between the two module types [2,3]. A hollow fiber module would give a better packing density than the tubular module and allows a less demanding pretreatment and maintenance than the spiralwound module. This module would be a solution for the treatment of low quality water sources by direct capillary nanofiltration. It is also reported in the literature that an optimized hollow fiber nanofiltration module would give a 100% increase in performance compared to an optimized spiral-wound module [4]. Composite membranes offer higher water permeability and higher salt rejections when compared to asymmetric membranes. Previous work conducted with N,N0 -diaminopiperazine (DAP) revealed its excellent performance as a monomer for composite nanofiltration membranes. The reaction of DAP with trimesoylchloride (TMC) by interfacial polymerization, on a support membrane, leads to a
hydrophilic thin film with high water permeability and selective to electrolytes and low molecular weight molecules [5]. The reaction of amines and acid chlorides is known from the literature as being almost instantaneous. When preparing flatsheet membrane it is necessary to remove the DAP excess and drops from the surface of the support membrane with, for example, a rubber roller before putting the support membrane in contact with the solution of TMC. Otherwise, the film formed will not adhere to the support properly and pin-holes can appear caused by droplets of the amine solution. This problem is easily solved in the flatsheet geometry but the technique cannot be applied to hollow fibers with the thin film on the lumen side. Previous work from the authors has solved this problem by replacing the aqueous amine solution directly with pure organic solvent, which is then replaced by the organic acid solution. All three solutions, the aqueous amine solution, the intermediate organic solvent and the organic acid chloride solution should be in direct contact [6]. With this initial study, we intend to investigate the preparation of composite HF membranes for direct capillary nanofiltration. Composite hollow fiber membranes were then prepared and their performance evaluated by tests with electrolytes and sugar molecules. Morphological studies of the hollow fiber surface were conducted by SEM and AFM. The influence of the reaction conditions on the separating performance of the hollow fibers was also studied. 2. Experimental 2.1. Hollow fiber preparation Polyetherimide (PEI) hollow fiber membranes were used as support and were prepared according to the method described by Kneifel [7]. These fibers have an external
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Fig. 1. Constant AFM height image of the inner surface of the support hollow fiber.
diameter of 1.06 mm and inner diameter of 0.74 mm. Eight fibers were glued inside the module resulting in an total effective membrane area of 33.5 cm2. The fibers were wetted with 40 vol% methanol in water and left in deionized water overnight. The pure water permeability of the support fibers was 134 l/m2/h/bar. AFM image of the support hollow fiber is presented in Fig. 1. The support hollow fiber presents an asymmetric structure, with a very open finger structure except close to the internal surface which has a spongy structure, ideal for a thin film support. The composite hollow fibers were prepared by interfacial polymerization of DAP
(synthesized according to [5]) and TMC (Merck, recrystallized before use) leading to a thin polyamide film. The fibers were coated on the lumen side of the hollow fiber by the technique earlier described [6]. The preparation procedure involves the flow of reactant solutions and flushing liquid inside the fiber. The liquids were pumped inside the fibers with a peristaltic pump (Watson Marlow 520S/R) and left inside until they were displaced by the following liquid. The lumen side of the hollow fibers was wetted with the DAP solution, followed by the flushing with cyclohexane (Merck) and finally put in contact with the TMC solution in cyclohexane (c-hex). The module was then flushed with
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deionized water to remove the organic solution and kept in deionized water overnight. 2.2. Measurement of the RO performance The performance of the membranes was evaluated at 10 bar with 3 g/l solutions of NaCl (Merck), MgSO4 (Merck) and Na2SO4 (Merck) for the electrolyte solutes and with 0.2 wt% solutions of lactose (Merck), -d(þ)glucose (Merck) and sucrose (Merck) for the nonelectrolyte solutes. The tests were conducted in a laboratory-scale membrane Unit P-28 from CM-Celfa to which the hollow fiber modules were assembled. The water flux in this work was calculated by Eq. 1: Jw ¼ AðP P þ N Þ
ð1Þ
where Jw is the water flux, A the water permeability coefficient, Pis the appliedpressure, P the osmotic pressureofthepermeateandN theaverageosmotic pressure between the feed and retentate. The salt rejection of the membrane is defined as the following ratio: R¼
CF CP 100% CN
ð2Þ
where R is the salt rejection, CF the feed concentration, CP the permeate concentration and CN the average concentration between the feed and retentate. The electrolyte concentrations were obtained by electric conductance measurements using an Inolab conductivity meter from WTW and the nonelectrolyte concentrations by gel permeation chromatography (GPC). The equipments used for GPC were the pump K-501 Knauer and the detector Knauer Differential Refractometer. The conditions for chromatography analyses were (calibration with dextran mixture): Column: Prima lin 1 M, 8 300 mm i.d., particle size 10 mm
Eluent: H2O Flow rate: 1 ml/min Injection Volume: 100 ml Column temperature: 30 C Detector temperature: 30 C The AFM analysis was conducted with a NanoScope IIIa, Digital Instruments (scan size 100 mm2) in constant force mode, at room temperature. The very sharp AFM tip (NanoProbe ESPC-CONT, radius rtip15 nm, made from silicon) glides over the surface and tracks the profile line by line. Each line is a convolution between the tip and the local roughness profile of the sample. The images consist of 512 lines with 512 pixels per line. The flatten (factor 2) was used in order to eliminate the sample curvature and inclination by means of the original software (version 5.12b15). The SEM analysis were performed with a Leo 1550 VP Gemini from Zeiss.
3. Results and discussion An initial test was conducted and after the influence of liquid residence time inside the fibers and reactants concentration were studied. The membrane performance was evaluated mostly with Na2SO4 solutions. The two different hollow fiber modules developed were also tested with NaCl, MgSO4, lactose, -D(þ)-glucose and sucrose solutions. 3.1. Composite hollow fiber membrane and its performance Qiu [5] has reported in his work the preparation of a composite flatsheet membrane by reaction of N,N0 -diaminopiperazine and TMC. The mentioned conditions were adapted in order to prepare composite hollow fiber membranes based on the work presented earlier [6]. The preparation procedure involves the passage of reactant solutions and flushing liquid inside the fibers. The conditions for the initial procedure (P1) are presented in Table 1. The
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S. Verı´ssimo et al. / Desalination 184 (2005) 1–11 Table 1 Procedure P1 for the preparation of the hollow fibers Step number
Solution
Concentration (wt%)
Time (min)
1 2 3
DAP c-hexane TMC
2 100 0.13
1.5 1 2
membrane performance of P1 was then evaluated with NaCl, MgSO4 and Na2SO4 aqueous solutions Fig. 2). The water permeability of the three solutions was around 6.2 l/m2/h/bar. Concerning selectivities, it is possible to distinguish different rejections for the 3 salts used. The highest average rejection was for Na2SO4 with 87.3% followed by MgSO4 with 60.2%. The rejection to the monovalent salt, NaCl, was low, around 14.6%. The good performance obtained for separation of divalent salts in aqueous solutions was motivation to study and optimize the preparation conditions for the hollow fiber membranes. In order to reduce the amount of experimental work required, the influence of the preparation conditions on
the performance of the membrane was evaluated with Na2SO4 solutions only. 3.2. Influence of the preparation conditions on the hollow fibers performance 3.2.1. Residence time of the DAP solution: It was decided to investigate if the residence time of the DAP solution inside the fiber was an important factor to membrane performance. Hollow fiber modules were prepared where the DAP solution was inside the fibers between 0.5 and 3 min. All other preparation conditions were kept the same as before (Table 1). The results show that in the range tested, the residence time has no strong influence on the rejection or water permeability of the composite hollow fiber mod-
100 40
100
36
90
32
80
28
70
24
60
20
50
16
40
12
30
20
8
20
10
4
10
0
0
80
A (l/h/bar/m 2)
70 R (%)
60 50 40 30
0
1
2
3
4
5
6
7
8
9
10
A (l/h/m 2/bar) NaCl
MgSO4
Na2SO4
Fig. 2. Water permeability and rejection of NaCl, MgSO4 and Na2SO4 of the hollow fiber modules prepared by procedure P1. Preparation conditions presented in Table 1.
R (%)
90
0 0
1
2
3
4
t DAP (min) A
R Na2SO4
Fig. 3. Water permeability and rejection to Na2SO4 of the hollow fiber modules prepared with different times of DAP solution inside the fibers. Other preparation conditions indicated in Table 1.
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50
100
45
90 80
35
70
A (l/h/bar/m 2)
40
50
100
45
90
40
80
35
70
30
60
25
50
20
40
30
60
25
50
20
40
15
30
15
30
10
20
10
20
5
10
5
10
0
0
0
0
1
2
3
4
5
6
7
8
9
10
R (%)
A (l/h/bar/m 2)
higher concentrations, a plateau superior to 90% was reached. Concerning water permeabilities, a continuous decrease was observed as the concentrations increased, although stronger for concentrations inferior to 2.5 wt%. This behavior reflects the influence of the diamine monomer concentration, and consequently of the monomer ratio, on the formation of the selective film and on its thickness. For lower ratios, a thin, highly permeable film is obtained, at the expense of a lower selectivity. For higher ratios, the film selectivity is not further improved and the water permeability remains relatively constant or decreases slightly. 3.2.3. Residence time of the TMC solution: The thickness of the thin film depends on the reaction time and diffusion rate of the monomers through the interface. In general, the thickness increases with increasing contact time between the two phases. When the monomers are not able to diffuse through the interface any longer, the thickness stops growing and the performance of the membrane remains
11
C DAP (wt%) A
R Na2SO4
Fig. 4. Water permeability and rejection to Na2SO4 of the hollow fiber modules prepared with different concentrations of DAP solution. Other preparation conditions indicated in Table 1 except for the residence time of the DAP solution which was 0.5 min.
R (%)
ules (Fig. 3). Therefore the surface from the support fiber is completely wetted in less than half a minute. The average water permeability obtained was 8.1 l/m2/h/bar and the average rejection to Na2SO4 was 82.4%. 3.2.2. Concentration of the DAP solution: It is known from the literature [6,8], that the ratio of concentration of the monomers plays an important role in the preparation of a strong and coherent film. It was decided to study the influence of this parameter by changing the diamine concentration and keeping the one of acid chloride constant. DAP concentrations between 0.25 wt% and 10 wt% were tested. Preparation conditions differ from the ones in Table 1, the DAP contact time was 0.5 min. The results obtained show that both water permeability and rejection to Na2SO4 are strongly influenced by this parameter in the range tested (Fig. 4). Two distinct behaviors are visible. For concentrations below 2.5 wt%, the rejection was very sensitive to this factor and increased from 8% to around 90%. For
0 0
1
2
3
4
t T MC (min) A
R Na2SO4
Fig. 5. Water permeability and rejection to Na2SO4 of the hollow fiber modules prepared with different times of TMC solution inside the fibers. Residence time of the 2.5 wt% DAP solution was 0.5 min. Other preparation conditions indicated in Table 1.
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50
100
45
90
40
80
35
70
30
60
25
50
20
40
15
30
10
20
5
10
0
R (%)
A (l/h/bar/m 2)
constant. To investigate this effect, hollow fiber modules were prepared with different residence times of the TMC solution. The solution was kept inside the fiber for times between 0.5 and 3 min. The DAP solution (2.5 wt%) was for 0.5 min inside the fiber. All other preparation conditions were as in Table 1. In Fig. 5 is the performance of the samples represented. For times inferior to 2 min, the water permeability and the Na2SO4 rejection values are more dispersed than for times equal or superior to 2 min. The result indicates that the formation of a totally finalized film requires the permanence of TMC solution inside the fibers around 2 min. 3.2.4. Residence time of cyclohexane: The residence time of cyclohexane inside the fiber has most probably influence on the performance of the membranes due to the diffusion of the DAP monomer in the organic solvent which affects the monomer ratio. Hollow fiber modules were prepared with different residence time of cyclohexane. The solvent was kept inside the fiber for times between 0.5 and 4 min. Except for the DAP
concentration which stayed inside the fibers for 0.5 with a concentration of 2.5 wt%, the preparation conditions were as indicated in Table 1. The results obtained show that both water permeability and rejection to Na2SO4 are influenced by the studied parameter in the tested range (Fig. 6). When analyzing the water permeabilities, by increasing the residence time of cyclohexane inside the fibers from 0.5 to 2 min, the water permeability increased strongly (from an average 5 l/ m2/h/bar to an average 17 l/m2/h/bar). For longer times, the permeability remained relatively constant. The salt rejection remained relatively constant (around 80%) up to 2 min and decreased to around 60% after. A cyclohexane residence time of 2 min lead to a thin, highly permeable film that still keeps high rejection. 3.3. Procedure P2 In paragraph 3.1 the preparation conditions and performance of the procedure designated by P1 was presented. During the study of the influence of parameters mentioned in paragraph 3.2, it was observed that other performances can be obtained by careful selection of the preparation procedure. From this work, another procedure (P2) was established and is presented in Table 2. The performance of the hollow fiber modules prepared according to P2 is presented in Fig. 7.
0 0
1
2
3
4
t c-hex (min) A
R Na2SO4
Fig. 6. Water permeability and rejection to Na2SO4 of the hollow fiber modules prepared with different times of cyclohexane inside the fibers. Residence time of the 2.5 wt% DAP solution was 0.5 min. Other preparation conditions indicated in Table 1.
Table 2 Procedure P2 for the preparation of the hollow fibers Step number Solution Concentration Time (min) (wt%) 1 2 3
DAP 2.5 c-hexane 100 TMC 0.13
0.5 2 2
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Table 3 Crystal and hydratation radius of the ionic species used in this study
90 80 70
Ion
Crystal radius (A˚) [10]
Hydratation radius (A˚) [10]
Naþ Mg2þ Cl SO42
1.02 0.72 1.81 2.15
1.78 3.00 1.95 3.00
R (%)
60 50 40 30 20 10 0 0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30 A (l/h/m2/bar )
NaCl
MgSO4
Na2SO4
Fig. 7. Water permeability and rejection of NaCl, MgSO4 and Na2SO4 of the hollow fiber modules prepared by procedure P2. Preparation conditions presented in Table 2.
The water permeability of the NaCl and MgSO4 solutions varies between 7.3 l/m2/h/ bar and 22.0 l/m2/h/bar. Concerning rejections, both solutions give similar values around 11%. The water permeability of the Na2SO4 solutions was relatively constant around 15.1 l/m2/h/bar and the salt rejection was around 80.5%. When comparing the results from P1 and P2, a strong difference is visible concerning the rejection of MgSO4 and the water permeability. Procedure P2 gives higher average water permeability at the expense of a lower rejection of MgSO4. In accordance with the solution-diffusion model, the cations and anions first get dissolved at the membrane–water interface and then migrate through the active layer toward the product water in a manner that the negative charge of an anion is balanced by the positive charge of the accompanying cation, i.e. it is a case of coupled transport. The dielectric constant monotonically decreases from the bulk solution to the concentration polarization zone and then into the active membrane layer
[9]. Consequently, the hydrated radius of the individual ions shrinks along this path approaching more the value of its crystalline radius. In Table 3 the crystal and hydrated radius of the ionic species used in this study are represented. Although the hydrated radius of the Mg2þ cation is higher than the one of the Naþ, the crystal radius is smaller. So, when accompanied by the same anion, the magnesium salts will permeate more easily through the membrane. Comparing the results obtained for P1 and P2, P2 shows higher water permeability which is possibly due to larger membrane pores. This increase in pore size is then responsible for the reduction of the MgSO4 rejection in procedure P2. But the increase was low enough not to change significantly the Na2SO4 rejection. 3.4. Nonelectrolyte solutes Nanofiltration membranes are used also to separate small molecular weight organic molecules. The performance of the hollow fibers prepared by procedure P1 and P2 was Table 4 Rejection to lactose, -D(þ)-glucose and sucrose of the hollow fibers prepared by procedure P1 and P2 Procedure
R glucose (%)
R sucrose (%)
R lactose (%)
P1 P2
57 12
77 21
91 21
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tested with 0.2 wt% aqueous solutions of lactose, -D(þ)-glucose and sucrose. The rejection of the sugars is indicated in Table 4. Procedure P1 produced membranes that had 57% rejection to glucose, 77% to sucrose and 91% to lactose. On the other hand, procedure P2 resulted in 12% rejection to glucose, 21% to sucrose and lactose. As a consequence of the higher water permeabilities of P2, the retention of the low molecular weight organic molecules strongly decreased. 3.5. Morphological studies Morphological studies of the hollow fiber surface were conducted in order to better understand the differences in performance of the membranes prepared by procedure P1 and P2. The samples from P1 and P2 were analyzed by AFM and SEM. AFM surface analysis of the hollow fibers from procedure P1 and P2 were conducted with dry samples in contact mode (Fig. 8). The morphology of P2 is closer to the one of the support (Figs. 1 and 8), and therefore more open, leading to higher water permeability.
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The surface roughness was evaluated by determination of the average plane roughness (Ra), root mean square roughness (Rms), surface area (As) and maximum difference of peak and valley (Rmax). The definition of these quantities is indicated in [11,12]. The results are shown in Table 5. The sample from P1 presents higher roughness than the sample from P2. Also the maximum distance between peak and valley is higher for P1. If we compare the roughness of the two procedures with the support fiber (Table 5), the sample from procedure P1 is rougher than the support fiber and the sample from procedure P2 is less rough. Since the surface area of both composite hollow fibers is close to 100 mm2, they present a very flat surface, flatter than the supports surface. Unlike the AFM analysis, the SEM analysis did not reveal morphological differences between the two samples. 4. Conclusions An inner coated thin film composite hollow fiber was prepared on PEI hollow fiber with an internal diameter of 0.74 mm. The thin film was formed by interfacial polymerization of
Fig. 8. Constant force AFM height image of the surface of composite hollow fibers prepared by procedure P1 (a) and P2 (b).
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Table 5 AFM roughness analysis (image surface area ¼ 100 mm2) of the hollow fibers prepared by procedure P1 and P2 as well as of the support fiber Procedure Ra (nm) Rms (nm) Rmax (nm) As (mm2) P1 P2 Support
31.290 9.250 21.028
39.240 11.632 26.141
300.53 130.29 203.35
102.41 102.34 104.90
N,N0 -diaminopiperazine and trimesoylchloride. The preparation procedure involves the flow of reactant solutions and flushing liquid inside the fiber. The influence of residence times inside the fiber and reactants concentrations were studied. The parameters that showed stronger influence were the concentration of N,N0 -diaminopiperazine and the residence times of trimesoylchloride and flushing liquid. By selecting the adequate preparation conditions, two different hollow fiber membranes were prepared and their performance evaluated with electrolyte and nonelectrolyte solutions. One type gave water permeability in the range 12 to 22 l/m2/h/bar. The salt rejections for NaCl, MgSO4 and Na2SO4 were 10, 12 and 80%, respectively. The sugar rejections were 12% for glucose and 21% for sucrose and lactose. The other type presented lower water permeability, around 6 l/m2/h/bar, and higher salt rejections for NaCl (14%), MgSO4 (60%) and Na2SO4 (87%). A stronger increase was observed on the sugar rejections which were 57% (glucose), 77% (sucrose) and 91% (lactose). The sample with the highest water permeability showed lower surface roughness and a surface morphology more similar to the one from the support hollow fiber. Still, both samples have a very flat surface.
— feed concentration c-hex – cyclohexane — average concentration between the CN feed and retentate — permeate concentration CP DAP — N,N0 -diaminopiperazine GPC — gel permeation chromatography — water flux Jw NF — nanofiltration P — applied pressure P1 — procedure 1 P2 — procedure 2 PEI — polyetherimide R — salt rejection — average plane roughness Ra Rmax — maximum difference of peak and valley — root mean square roughness Rms SEM — scanning electron microscopy TMC — trimesoylchloride — average osmotic pressure between N the feed and retentate — osmotic pressure of the permeate P
CF
Acknowledgements We are grateful to K. Kratz, H. Kamusewitz and M. Keller at GKSS for performing the AFM experiments and for their help with the interpretation of the results. The authors greatly appreciate M. Schossig and M. Aderhold at GKSS for performing the SEM experiments and for their valuable comments. The authors appreciate as well R. Just and M. Eggers at GKSS for the preparation of the support hollow fiber and GPC measurements respectively. Gratefully acknowledged is also the financial support of the Portuguese Foundation for Science and Technology (Ph.D. scholarship SFRH/BD/6226/2001).
5. List of symbols and abbreviations A — permeability coefficient AFM — atomic force microscopy — surface area As
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