PVC based composite hollow fiber nanofiltration membranes: Effect of substrate on properties and performance

Author’s Accepted Manuscript Polyamide/PVC based composite hollow fiber nanofiltration membranes: Effect of substrate on properties and performance Xin Kong, Ming-Yong Zhou, Chun-Er Lin, Jun Wang, Bin Zhao, Xiu-Zhen Wei, Bao-Ku Zhu www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(16)30028-X http://dx.doi.org/10.1016/j.memsci.2016.01.028 MEMSCI14237

To appear in: Journal of Membrane Science Received date: 25 October 2015 Revised date: 12 January 2016 Accepted date: 14 January 2016 Cite this article as: Xin Kong, Ming-Yong Zhou, Chun-Er Lin, Jun Wang, Bin Zhao, Xiu-Zhen Wei and Bao-Ku Zhu, Polyamide/PVC based composite hollow fiber nanofiltration membranes: Effect of substrate on properties and p e r f o r m a n c e , Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2016.01.028 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.

Polyamide/PVC based composite hollow fiber nanofiltration membranes: Effect of substrate on properties and performance Xin Kong1, Ming-Yong Zhou1, Chun-Er Lin1, Jun Wang1, Bin Zhao2, Xiu-Zhen Wei3, Bao-Ku Zhu1,3 1

Joint Laboratory for Adsorption and Separation Materials, Key Laboratory of Macromolecule Synthesis and Functionalization (Ministry of Education), Engineering Research Center of Membrane and Water Treatment (Ministry of Education), Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China 2 College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China 3 Zhejiang Provincial Collaborative Innovation Center (ZJCIC) on Membrane and Water treatment Technology, College of Biological and Environmental Engineering, Zhejiang University of Technology, Hangzhou 310014, China

ABSTRACT Thin film composite nanofiltration membranes were successfully prepared via interfacial polymerization between piperazine and trimesoyl chloride on poly(vinyl chloride) hollow fiber substrates with different properties. Both the polyamide layer and underlying substrate were characterized with respect to chemical and morphological structures. The effects of substrate properties on the characteristics of the prepared nanofiltration membranes were investigated. It is found from the results that the differences in substrate properties, mainly the membrane pore size and porosity had significant contributions to the changes in the thickness and cross-linking degree of poly(piperazine-amide) layer during the interfacial polymerization process, leading to the varied performance of the prepared nanofiltration membranes in terms of MgSO4 rejection and permeate flux. The nanofiltration membrane prepared from the substrate with medium pore size and porosity exhibited a relatively higher permeate flux as well as a superior rejection for MgSO4, which showed typical rejection orders for different salts as well as good stabilities after backwash treatment and a long-term filtration process. These results indicate that the prepared poly(vinyl chloride) nanofiltration 

Corresponding author. Tel.: +86 57187953723. E-mail address: [email protected] (B.K. Zhu).

1

hollow fiber membranes could be applied to the desalination or water softening process.

Keywords: Nanofiltration; Hollow fiber membrane; Poly(vinyl chloride); Substrates; Pore size

1. Introduction Over the past decades, facing with the increasing shortage of water resources has driven humanity to search for alternative source of fresh and clean water worldwide. In view of this, desalination technique has attracted more attention due to its practical significance for providing a greater amount of clean water for industrial production and daily demand [1, 2]. Among the desalination techniques available, membrane separation is a green and feasible technology to meet the requirements of desalination. Thin film composite (TFC) membranes, generally prepared through interfacial polymerization, are considered the most efficient and successful membranes for various separation processes, particularly in the desalination and purification applications [3, 4]. These membranes are usually composed of an ultra-thin selective layer responsible for separation and a porous substrate providing mechanical support. The main advantage of TFC membranes is that the top selective layer and bottom porous substrate can be individually tailored and optimized to enhance selectivity and permeability at the same time as compared to other asymmetric membranes [5, 6]. Presently, most commercially available reverse osmosis (RO) and nanofiltration (NF) membranes suitable for desalination are TFC membranes [7]. A wide variety of studies have been conducted by researchers to develop high performance TFC RO or NF membranes. Nevertheless, it should be pointed out that most work have mainly focused on the top selective layer development. Substrate materials are paid much less attention during TFC membrane preparation. As substrate membranes can be prepared from various polymers, it is worthy to select a substrate that can be applied to different industrial areas with low cost of polymer material itself to save expenses [8-10]. Among

2

the polymers used, polysulfone (PSf) or polyethersulfone (PES) has been generally employed as substrate materials for TFC membrane preparation until the present. Although substrate membranes made of PSf or PES possess good thermal stability and relatively high hydrophilicity, their drawbacks like solvent sensitivity and relatively low mechanical strength are obvious [11-13]. It is very attractive to use polyvinylidene fluoride (PVDF) or polypropylene (PP) as substrate materials due to their high durability to chemical variation and good solvents resistance. However, as the PVDF or PP membrane surface is highly hydrophobic, special treatment on the membrane surface has to be conducted to improve its hydrophilicity before TFC membrane preparation using interfacial polymerization technique [13-16]. Polyacrylonitrile (PAN) material is more hydrophilic compared to other commercial polymers, but PAN has low mechanical strength and poor resistance to acid and alkali, leading to a limitation for its applications in many industrial separation processes [17, 18]. In addition to the aforementioned polymers, substrate membranes made of novel polymers like sulfonated poly(phthalazinone ether sulfone) (SPPES), sulfonated poly(ether ether ketone) (SPEEK) and sulfonated poly(phthalazinone ether sulfone ketone) (SPPESK) exhibit excellent thermal and chemical stabilities as well as better hydrophilicity compared to normal PSf or PES substrates [19-21]. Nevertheless, these novel polymers are really expensive and their synthetic processes are also extremely complex, resulting in a less possibility to be used as substrate materials for TFC membrane preparation. Polyvinyl chloride (PVC) is one of the most widely used raw materials in plastic production, and has become a common polymer for ultrafiltration and microfiltration membranes preparation due to its low cost, excellent mechanical strength and high resistance to acid and alkali as well as solvents [22, 23]. However, research concerning the use of PVC as a substrate material for TFC membrane preparation has only rarely been reported. Zhu et al. [24] first prepared a low operating pressure PVC NF membrane by interfacial polymerization between m-phenylenediamine and trimesoyl chloride, but the rejection rate of the membrane to MgSO4 was only 60% at 0.3 MPa, which is hard to meet the current requirements of desalination and water treatment. It should be noted that most available RO or NF modules are made from TFC flat

3

sheet membranes in the current market. Compared with flat sheet membranes, hollow fiber membranes can provide a higher packing density, a larger surface area to volume ratio and are easier to handle during module fabrication and operation [25]. Therefore, there has been a growing interest to develop TFC hollow fiber membranes for water treatment in recent years. As compared with RO, NF is considered to be better process in many applications because of its higher permeate flux and lower energy consumption especially when complete salt removal is not needed [26]. More importantly, with a less dense structure and a lower operating pressure compared to RO membranes, NF membranes are easier to achieve in hollow fiber modules. Based on these reasons, it is expected that a high performance PVC NF hollow fiber membrane could be obtained to expand the selection of substrate materials for the TFC membrane preparation. Apart from developing new substrate materials from various polymers, fundamental studies on the effects of substrate properties on the performance of TFC membranes should also be focused on. Currently, it still needs to be well understood what kind of substrate properties (e.g. pore size, porosity, surface roughness and hydrophilicity, etc.) could match perfectly with an interfacially polymerized selective layer. However, limited studies have been conducted on the interaction between substrates made of different properties and the performance of TFC membranes in recent years [8, 9, 27, 28]. Up to now, there is no literature available on the study of TFC NF membrane preparation over different PVC hollow fiber membranes. In this work, a series of TFC NF membranes were prepared through interfacial polymerization on commercial PVC hollow fiber substrates with different properties. The chemical and morphological structures of both polyamide selective layer and substrate

membrane

surface

were

characterized

using

attenuated

total

reflectance-Fourier transform infrared (ATR-FTIR) and field emission scanning electron microscopy (FESEM), respectively. Physical properties of each substrate membrane, such as the surface hydrophilicity, the porosity, the pore size and its distribution were measured to investigate the effects of substrate properties on the performance of the prepared TFC NF membranes in terms of salt rejection and permeate flux. In addition, the NF membrane rejection performance to different polyethylene glycols and salts as

4

well as the stabilities were also studied. 2. Experimental 2.1. Materials and reagents Three different PVC hollow fiber substrate membranes containing carboxyl as the hydrophilic content (carboxyl group in 0.3 wt%) were provided by Hainan Litree Purifying Technology Co., Haikou, China. The aqueous phase solution consisted of piperazine (PIP, Aladdin Reagent Co., China) as the reactive monomer and anhydrous sodium phosphate (Na3PO4, Sinopharm Chemical Reagent Co., China) as the acid acceptor. Trimesoyl chloride (TMC, purity > 99.0%) was purchased from Shanghai Kaisai Chemical Co., China, and used as the reactive monomer of the organic phase with n-hexane (Sinopharm Chemical Reagent Co., China) as the organic solvent. Magnesium sulfate (MgSO4), sodium sulfate (Na2SO4), magnesium chloride (MgCl2), sodium chloride (NaCl), sodium hydroxide (NaOH) and citric acid were purchased from Sinopharm Chemical Reagent Co., China. Polyethylene glycol (PEG) with molecular weights of 200, 400, 600, 1000, 2000 and 20,000 Da was purchased from Aladdin Reagent Co., China, while PEG with a molecular weight of 35,000 Da and polyethylene oxide (PEO) with higher molecular weights of 100,000 and 300,000 Da were both supplied by Sigma-Aldrich, USA. Deionized water (DI) (pH ≈ 6.5), which was treated with reverse osmosis membranes, was used in all experiments. All chemicals were analytic reagents and used as received. 2.2 Preparation of PVC based TFC NF hollow fiber membranes TFC NF hollow fiber membranes were prepared on three different PVC substrates denoted as PVC-UF1, PVC-UF2 and PVC-UF3 by interfacial polymerization at the same conditions. Piperazine (PIP) and trimesoyl chloride (TMC) were used as reactive monomers to form a polyamide layer on the inner surface of the membrane. To begin with, the aqueous solution of PIP (2.0 wt%) with 0.6 wt% Na3PO4 as the acid acceptor was extruded into the lumen side of hollow fiber membranes for 10 min, then excess aqueous solution was drained off the soaked surface and air-dried using an air

5

compressor. Subsequently, the organic phase solution with 0.5 wt% TMC in n-hexane was introduced in the same way into the surface saturated with PIP for 50 s, which resulted in the formation of a polyamide selective layer over the PVC membrane surface. After removing excess organic solution, the hollow fiber membranes were heated for approximately 10 min in an oven at 60 °C for further polymerization. Finally, the prepared NF hollow fiber membranes were stored in DI water before carrying out the NF experiments. The TFC NF membranes prepared on PVC-UF1, PVC-UF2 and PVC-UF3 hollow fiber membranes were then denoted as PVC-NF1, PVC-NF2 and PVC-NF3, respectively. 2.3 Membrane characterization 2.3.1 Chemical structures Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) was employed to analyze the near-surface region chemical composition changes of PVC hollow fiber membranes before and after interfacial polymerization. The ATR-FTIR spectra were recorded on a Nicolet 6700 FTIR (Thermo Fisher Nicolet, USA) with an Omni sampler over 32 scans. For ATR-FTIR analysis of the membrane sample, a Germanium crystal was used as the internal reflection element with an angle of incidence of 45° and a resolution of 2 cm−1. 2.3.2 Morphological structures Surface and cross-section morphologies of the prepared NF membranes and substrate membranes were observed under field emission scanning electron microscopy (FESEM, S-4800, Hitachi, Japan) after sputtering with gold. Cross-sections of the membranes were prepared by freeze-fracturing the samples in liquid nitrogen. 2.3.3 Surface hydrophilicity Surface hydrophilicity of the PVC substrate membranes was characterized by measuring the contact angle of water on the membrane surface. The water contact angle (WCA) of the membrane was measured using a contact angle meter (OCA20, Dataphysics, Germany) through a digital video image of the drop on the dried surface of

6

the hollow fiber at 25 °C. All the samples were tested for five different positions and the final results presented are an average of the measured values. 2.3.4 Porosity determination Membrane porosity was determined according to its dry-wet weight. Firstly, the PVC hollow fiber membranes were immersed in DI water for at least 24 h. Then the weights of each wet membrane were measured after wiping the excess water attached on both inner and outer membrane surfaces using a filter paper. After that, the wet membranes were dried in an electric blast drying oven for 10 h at 30 °C and then measured the dry membrane weights. The porosity of hollow fiber membrane can be calculated as [22, 29]: 𝜀 (%) =

𝑊𝑤 − 𝑊𝑑 × 100% (𝜋/4)(𝐷2 − 𝑑 2 )𝑙𝜌𝑤

(1)

where ε is the membrane porosity, Ww is the wet membrane weight (g), Wd is the dry membrane weight (g), ρw is the water density (1.0 g/cm3), l is the length of hollow fiber membrane (cm), D and d are the outer diameter and the inner diameter of hollow fiber membrane (cm), respectively. 2.3.5 Pore size and its distribution determination The solute transport method based on the correlation between solute sizes and solute rejection rates was used to determine the pore size and its distribution of the membrane [30]. Observed solute rejection rates were calculated by the relation: 𝑅 = (1 −

𝐶𝑝 ) × 100% 𝐶𝑓

(2)

where R is the percent solute rejection, and Cp and Cf (mg/L) are the concentrations of solute in the permeate and feed, respectively. In this study, feed solutions containing 200 ppm of different molecular weights of PEG (20,000 and 35,000 Da) and PEO (100,000 and 300,000 Da) were used to characterize the substrate membrane pore sizes and pore size distributions using the solute transport method. The Stokes diameter (nm) of the PEG or PEO used in the test at a given molecular weight can be calculated from the following equations [31]:

7

For PEG, 𝑟 = 33.46 × 10−12 × 𝑀0.557

(3)

For PEO, 𝑟 = 20.88 × 10−12 × 𝑀0.587

(4)

In this method, the solute rejection rate of the membrane is plotted as a function of the solute Stokes diameter based on the log-normal distribution. The geometric mean diameter of solute (µs) at R = 50% and the solute geometric standard deviation (σs) defined as the ratio of the Stokes diameter at R = 84.13% over that at R = 50% can be determined according to the obtained straight line equation. The mean effective pore size (µp) and the geometric standard deviation (σp) of the membrane can be considered to be the same as the µs and σs if the steric and hydrodynamic interactions between solute and membrane pores are disregarded. Therefore, based on the µp and σp, the pore size distribution of the membrane can be expressed by the probability density function as shown in the following equation [31-33]: 2

d𝑅(𝑟𝑝 ) (ln 𝑟𝑝 − ln 𝜇𝑝 ) 1 = exp [− ] 2 d𝑟𝑝 𝑟𝑝 ln 𝜎𝑝 √2π 2(ln 𝜎 )

(5)

𝑝

where 𝑟𝑝 is the membrane pore size. 2.4 Membrane performance tests Membrane performance tests were conducted through a laboratory-scale cross-flow filtration apparatus at a constant flow rate of 0.5 L/min. In the experiments, each membrane module consisted of 8 hollow fibers with an effective area of approximately 40 cm2. The feed solution was pumped into the lumen side of hollow fibers, while the permeate solution exited from the shell side. The pure water flux of the substrate membrane was determined by employing DI water as the feed. While the feed for the NF membrane performance tests was DI water with added solutes, such as MgSO4, Na2SO4, MgCl2, NaCl or PEG with different molecular weights. The concentrations of the salt and PEG solutions in the tests were 2000 ppm and 1000 ppm, respectively. The pure water flux or permeate flux was subsequently calculated according to the following equation: 𝐹=

𝑉 𝐴 ∙ ∆𝑡

(6)

8

where V is the volume of the water or solution permeated during the experiment (L), A represents the effective membrane area (m2), and Δt denotes the operation time (h). The other filtration characteristic of the membrane, solute rejection rate, was determined based on the equation (2) as previously mentioned. Both the permeate solution and rejection were recycled back to the feed tank in order to keep a constant concentration. The salt concentration in the feed and permeate was calculated according to the electrical conductivity of the corresponding salt solution using an electrical conductivity (DDS-11A, Hangzhou Dongxing Instrument Co., China), while the PEG concentration was measured by a total organic carbon analyzer (TOC-V CPN, Shimadzu, Japan). Measurements of the pH values were carried out using a pH-meter of PHS-3C (Shanghai Precision & Scientific Instruments Co., China). Adhesion strength between polyamide selective layer and substrate membrane was investigated through the backwashing experiment. Contrary to the process of normal performance tests, DI water used as the feed solution was pumped into the shell side, while the permeate solution exited from the lumen side of hollow fibers during the backwashing experiment. After backwash treatment for 30 min at a certain feed pressure, MgSO4 rejection rate and permeate flux of the TFC NF membrane at 0.4 MPa were measured again. During the experiments, the performance tests of the substrate membrane and the NF membrane were consistently carried out at the pressures of 0.1 MPa and 0.4 MPa, respectively. And the feed temperature was maintained at 25.0 °C. In order to assure the accuracy of the test results, at least three trials were carried out per performance test, and all of the results presented are an average of the measured values. 3. Results and discussion 3.1 Characterization of different PVC hollow fiber substrates The ATR-FTIR spectra of different substrate membranes are presented in Fig. 1. The absorption bands at 2927 cm-1, 1429 cm-1, 1328 cm-1 and 1249 cm-1 as well as 967 cm-1, were the typical characteristic peaks of substrates made of PVC, which correspond to the –CH stretching vibration, –CH2 deformation vibration, C–H bending vibration and

9

C–Cl stretching vibration, respectively [34]. Overall, all PVC hollow fiber substrates have the same chemical functional groups on the membrane surface.

Fig. 1. ATR-FTIR spectra of the PVC substrate membranes.

Fig. 2. FESEM images of the cross-section and surface morphologies of the PVC substrate

10

membranes (a) PVC-UF1, (b) PVC-UF2, (c) PVC-UF3; 1: cross section × 60, 2: cross section × 5K, 3: inner surface.

FESEM images of the cross-section and surface morphologies of each substrate membrane are provided in Fig. 2. Three substrate membranes possess a similar size and their inner and outer diameters are around 0.8 and 1.3 mm, respectively. The cross-section images show that a common asymmetric structure consisting of a dense top layer and a porous layer full of large finger-like structures could be clearly observed for all substrates. All the substrate membrane inner surfaces have observable pores and exhibit a relatively uniform pore size distribution. However, the pore sizes of each substrate membrane are significantly different, which could be determined by the solute transport method and would be further discussed in the following section. The property parameters of each substrate membrane, including pure water flux, water contact angle, porosity, pore size and its distribution are summarized in Table 1. The pure water fluxes of three substrate membranes, i.e. PVC-UF1, PVC-UF2 and PVC-UF3 reached 442, 280 and 130 L/m2 h at the operating pressure of 0.1 MPa, respectively. Generally speaking, surface hydrophilicity, porosity and pore size are considered as the main factors that determine the water flux of the membrane [32]. As shown in the table, the changes in the pure water flux for the membrane are noticeably, while the water contact angles of different substrate membranes are almost the same ranging from 74.6° to 77.3°. The results may be inconsistent with the fact that a relatively hydrophilic membrane probably produces a higher water flux. Consequently, the decrease in the pure water flux can be mainly attributed to a decrease in the membrane pore size, even though the substrate porosity increases in the sequence of PVC-UF3 > PVC-UF2 > PVC-UF1. A similar result was reported by Ghosh et al. [35], that is, pure water permeability data correlate moderately to strongly with the pore size, while weakly with the hydrophilicity. In addition, the substrate membranes exhibit pore sizes in the ranges of 13.0 and 25.8 nm. The pore size distribution curves of each substrate membrane based on the values of µp and σp are also presented in Fig. 3. It can be clearly seen from the cures that the PVC-UF3 membrane possesses a relatively

11

narrow pore size distribution, while the PVC-UF1 membrane has the widest pore size distribution among all the substrates. In the following sections, a systematic study will be conducted to what extent the changes in substrate properties would influence the polyamide selective layer formation and the performance of the prepared TFC NF membranes during the filtration process. Table 1 Pure water flux, water contact angle, porosity, µp and σp of the PVC substrate membranes. Substrate

Pure water flux (L/m2 h)

Water contact angle (degree)

Porosity (%)

µp (nm)

σp

PVC-UF1

442

77.3 ± 1.1

32.8

25.8

1.62

PVC-UF2

280

74.6 ± 1.2

45.7

17.9

1.99

PVC-UF3

130

75.8 ± 1.2

51.6

13.0

2.09

Fig. 3. Pore size distribution curves of the PVC substrate membranes.

3.2 Effect of substrate on the characteristics of the prepared TFC NF membranes 3.2.1 Chemical structures Fig. 4 shows the ATR-FTIR spectra of the TFC NF membranes prepared from different substrates. The spectra in the figure clearly indicated that the interfacial polymerization occurred over all PVC substrates as the acyl chloride peak at 1774 cm−1 is absent and the new peaks representing the amide groups at 1658 cm−1 (amide I, C＝O

12

stretching) and 1550 cm−1 (amide II, N–H bending) are present [36]. In addition, the extremely broad hydroxyl group (O–H) band found at 3400 cm−1 implies that carboxylic is introduced into the polyamide selective layer of the NF membrane. The appearance of carboxylic acid functional groups is attributed to the partial hydrolysis of the acyl chloride unit of TMC during interfacial polymerization [26]. To sum up, the ATR-FTIR spectra revealed that the polyamide selective layer could still be established on the membrane surface, even though there was a significant change in substrate properties, particularly the substrate pore sizes and their distributions.

Fig. 4. ATR-FTIR spectra of the TFC NF membranes prepared over different PVC substrates.

3.2.2 Morphological structures In order to further confirm the formation of polyamide selective layer which was interfacially polymerized over different PVC substrates, the inner surface and cross-section morphologies of the prepared TFC NF membranes were observed and their FESEM images are shown in Fig. 5. The FESEM images from Fig.5 and Fig.2 show large differences in the morphological structures of the TFC NF membranes and their corresponding substrates. The PVC substrate membrane presents a smooth surface with uniformly distributed pores, while the NF membrane surface is rough and loosely packed with small nodules, which is the typical structure of poly(piperazine-amide)

13

layer [27, 37]. It should be noted that little differences in surface morphology of the polyamide layer are found for the TFC NF membranes prepared from different substrates. On the other hand, the cross-section images show that the NF membrane is asymmetric and takes on a composite structure. A relatively dense selective layer with a thickness less than 0.2 µm firmly attached to the inner surface of the substrate membrane. The results indicate that the inner surface of the PVC hollow fiber substrate membrane has been successfully covered by a uniform dense functional layer after interfacial polymerization.

Fig. 5. FESEM images of the inner surface and cross-section morphologies of the prepared TFC NF membranes (a) PVC-NF1, (b) PVC-NF2, (c) PVC-NF3.

3.2.3 Performance of salt rejection and permeate flux The performance of the TFC NF membrane in terms of salt rejection and permeate flux using MgSO4 solution as the feed solution, is presented in Fig.6. It is apparent that the TFC NF membranes prepared over different PVC substrates exhibited obvious differences in the salt rejection rate and permeate flux. As the parameters of interfacial polymerization remain the same during the preparation of the TFC NF membranes, the PVC substrate properties including the pore size, the porosity and the surface hydrophilicity, are considered the major affecting factors in determining the performance of salt rejection and permeate flux for three NF membranes.

14

The permeate flux of the membrane decreases in the sequence of PVC-NF3 > PVC-NF2 > PVC-NF1, which coincides with the trend of changes in the pure water fluxes of the substrate membranes as mentioned in the previous section. The substrate membrane with bigger pores could produce a higher pure water flux, correspondingly, the NF membrane prepared over the substrate could also achieve a higher permeate flux. The comparable experimental result was also reported by Ghosh et al. [35], which can be explained by the following. According to the study by Singh et al. [28], the smaller substrate pores would limit the diffusion of PIP aqueous solution deep into the pores, which leads to a thicker polyamide layer formation, and thus, the permeate flux is lower accordingly. On the other hand, the bigger substrate pores would allow the PIP aqueous solution more easily to diffuse deep into the pores by forming an absorbed layer on the membrane surface, forming a thinner polyamide layer due to the deposition of cross-linking polyamide inside the pores. More importantly, bigger pores may not be entirely coated by the polyamide, resulting in a greater possibility of defects on the TFC surface which would significantly decrease the salt rejection rate of the membrane while increasing the permeate flux. However, the interpretation of Singh et al. only considered the effect of membrane pore sizes on forming polyamide layer during interfacial polymerization, other affecting factors such as the porosity and the surface hydrophilicity were completely ignored. It has been reported that interfacial polymerization reaction primarily occurs in the organic phase side near the interface between the two immiscible phases due to the low partition coefficient of the acyl chloride in the aqueous phase [38]. During interfacial polymerization, if the diamine monomer reacts with acyl chloride through amide linkage, a highly cross-linked polyamide structure would be formed, but the hydrolysis of acyl chloride units of TMC to carboxylic acid groups would result in a linear hydrophilic structure. Generally speaking, the cross-linking structure formed by the amide linkage increases salt rejection at the expense of water flux, while the linear structure formed by the hydrolysis reaction increases water flux due to its hydrophilic carboxylic acid group [39, 40]. The cross-linking degree of polyamide structure is affected by the monomer concentration as well as the concentration ratio of diamine

15

monomer and acyl chloride. If the hydrolyzed acyl chloride groups react with a higher concentration of diamine monomers, the amount of linear structure with pendant carboxylic acid groups will be reduced, and thus the polyamide structure is more cross-linked which contributes to the increase of salt rejection rate. In this work, as the concentrations of PIP and TMC in their respective solutions are fixed, the cross-linking degree of poly(piperazine-amide) is mainly dependent on the amount of PIP at the interface which diffuses from the substrate membrane surface. Based on the above argument, the porosity and the surface hydrophilicity of the substrate are considered to have significant influence on the formation of poly(piperazine-amide) structure, which will be further discussed in the following. On the one hand, the increased membrane porosity and surface hydrophilicity contribute to the preservation of PIP monomers in the membrane surface or pores of the substrate after soaking into the aqueous phase solution. Thus, more PIP monomers would erupt from the saturated pores of the substrate and then react with TMC to form a denser barrier layer, leading to an increase in the salt rejection rate and a decrease in the permeate flux. On the other hand, with an increase in the surface hydrophilicity, PIP monomer diffuses more slowly from the pores of the substrate to the interface due to the attractive interactions between PIP and the hydrophilic functional groups of the substrate. The slower the PIP is supplied, the more chance the formation of a linear structure

during

the

interfacial

polymerization

reaction.

As

a

result,

the

poly(piperazine-amide) layer is less cross-linked as the hydrophilicity of the substrate membrane increases, leading to a decrease in the salt rejection rate [41]. As all the substrate membranes possess similar surface hydrophilicity, the counteracting contributions of the above processes finally result in an increase in the salt rejection rate of the TFC NF membrane as shown in Fig.6. The highest salt rejection rate of the PVC-NF3 is most likely due to its relatively higher cross-linking degree of polyamide structure, which also creates additional resistance against water flux through the membrane as compared to other prepared TFC NF membranes. In view of this, it is fair to say that the effect of substrate porosity plays a more important role in the poly(piperazine-amide) formation than the membrane surface hydrophilicity.

16

Overall, the performance of the prepared TFC NF membranes including salt rejection and permeate flux are mainly determined by the structures of poly(piperazine-amide) formed over different PVC substrates, such as the thickness and cross-linking degree. The substrate properties greatly influence the interfacial polymerization process, especially the poly(piperazine-amide) formation. The PVC-NF2 membrane prepared over the substrate of PVC-UF2 was considered the optimum TFC NF membrane by taking into account both salt rejection rate and permeate flux. In order to fully understand the characteristics of the TFC NF membrane, the PVC-NF2 was employed to study the rejection performance of the prepared NF membranes to different PEGs and salts.

Fig. 6. MgSO4 rejection rate and permeate flux of the prepared TFC NF membranes.

3.3 Rejection performance of the PVC-NF2 membrane The rejection rates of the PVC-NF2 membrane to different PEGs with molecular weights of 200, 400, 600, 1000 and 2000 Da are presented in Fig. 7. It can be seen that the PEG rejection rate increased with the increase of the molecular weight of PEG. The value of molecular weight cut-off (MWCO) was determined by the molecular weight of the PEG molecules that are up to 90% rejected by the membrane. Therefore, the MWCO of the PVC-NF2 membrane was calculated to be around 310 Da, which is within the range of NF membranes.

17

Fig. 7. Rejection rates of the PVC-NF2 membrane to PEGs with different molecular weights.

The rejection performance of the PVC-NF2 membrane to MgSO4, Na2SO4, MgCl2 and NaCl were studied, and the rejection rate and permeate flux of different salts are presented in Fig. 8. As shown in the figure, the membrane possessed higher rejection to Na2SO4 than MgCl2, which suggested that the NF membrane surface is negatively charged according to the Donnan exclusion principle [42]. However, the order of MgCl2 and NaCl rejections is hardly explained by the Donnan exclusion principle. Therefore, the effect of steric hindrance must be considered. As shown in Table 2 [42, 43], Mg2+ ion has lager hydrated radius than Na+ ion while the diffusion coefficient of Mg2+ ion being smaller than that of Na+ ion, leading to the transportation across the membrane is more difficult for Mg2+ ion compared to Na+ ion. The better MgSO4 rejection comparing to Na2SO4 is also attributed to the larger hydrated radius of Mg2+ ion. It is worth noting that rejection rates of MgSO4 and NaCl at 0.4 MPa were 98.0% and 30.0%, respectively. In general, the prepared TFC NF hollow fiber membrane exhibits typical rejection characteristics of negatively charged NF membranes.

18

Fig. 8. Rejection rates and permeate fluxes of the PVC-NF2 membrane to different inorganic salts. Table 2 Bulk Diffusion coefficients and hydrated radii for different ions. Parameter

Na+

Mg2+

Cl−

SO42−

Hydrated diameters (10-10 m)

3.58

4.28

3.32

3.79

Diffusion coefficient (10-9 m2/s)

1.33

0.70

2.01

1.06

The performance comparison of the PVC-NF2 prepared in this work with several commercial NF membranes reported in the literature is shown in Table 3. All the commercial membranes listed in Table 3 are negatively charged TFC flat sheet membranes mostly prepared by interfacial polymerization on PSf or PES substrates in the current market [37, 44-46]. It is observed that although the PVC-NF2 membrane has a relatively moderate permeate flux, the adopted operating pressure is lower than all operating pressures applied in the listed membranes. Moreover, the PVC-NF2 membrane exhibits a superior rejection (98.0%) for MgSO4, which is comparable to most of the commercial NF membranes listed in the table. Overall, the performance of the PVC-NF2 membrane in terms of MgSO4 rejection and permeate flux indicate that the prepared TFC NF hollow fiber membranes are promising for the application of desalination or water softening.

19

Table 3 Performance comparison of various TFC NF membranes. Manufacturer

Membranes

Permeate flux (L/m2 MPa)

NF90

Rejection (%)

Operating pressure (Mpa)

Salt concentration (ppm)

Refs.

MgSO4

NaCl

64

> 97

85-95

0.5

2000

NF270

110

> 97

40-60

0.5

2000

GE osmonics

GE HL

69

97

33

0.76

1000

[45]

Koch

TFC-SR2

145

92

20

0.5

600

[46]

Nitto

NTR-7450

109

48

53

1.0

1000

[37]

This work

PVC-NF2

70

98.0

30

0.4

2000

Dow

[44]

3.4 Stabilities of the PVC-NF2 membrane Stability is very important for the practical application of the TFC NF membrane. Backwashing experiment was designed for evaluating the adhesion strength between selective layer and substrate membrane [14, 47]. Changes in the MgSO4 rejection rate and permeate flux of the PVC-NF2 membrane after backwash treatment are presented in Fig.9. It is obvious that the MgSO4 rejection rate decreased gradually while the permeate flux remained constant to some extent with an increase in the backwash pressure. Nevertheless, the rejection rate of the PVC-NF2 membrane to MgSO4 was still higher than 96.0%, even when the backwash pressure was up to 0.4 MPa. This result indicated that the adhesion between selective layer and substrate membrane is relatively stable for the prepared TFC NF membrane. The long-term stability of the prepared PVC-NF2 membrane was tested with 2000 ppm MgSO4 solution at an operation pressure of 0.4 MPa. The samples were collected every 24 h, and the MgSO4 rejection rate and permeate flux of the PVC-NF2 membrane during 30 days continuous filtration are shown in Fig. 10. From this figure, it can be seen that the permeate flux existed only slight fluctuation, and the membrane rejection rate for MgSO4 did not change significantly throughout the test process. The results suggested that the prepared TFC NF membrane showed a good long-term stability during the filtration process. Additionally, both salt rejection rate and permeate flux of

20

the PVC-NF2 membrane remained unchanged after continuous filtration for 24 h respectively using 0.5 wt% NaOH (pH ≈ 11) and 0.5 wt% citric acid (pH ≈ 2.5) as the feed solution, which indicated that the prepared TFC NF membrane could stay stable in the pH range from 2.5 to 11.

Fig. 9. Changes in the MgSO4 rejection rate and permeate flux of the PVC-NF2 membrane after backwash treatment.

Fig. 10. Changes in the MgSO4 rejection rate and permeate flux of the PVC-NF2 membrane during 30 days continuous filtration.

21

4. Conclusions A series of TFC NF membranes denoted as PVC-NF1, PVC-NF2 and PVC-NF3 were successfully prepared on PVC hollow fiber substrates via same interfacial polymerization conditions. The ATR-FTIR spectra and FESEM images revealed that the poly(piperazine-amide) selective layer could be formed over all PVC substrates regardless of their different properties. The prepared TFC NF membranes in terms of MgSO4

rejection

and

permeate

flux

confirmed

that

the

structures

of

poly(piperazine-amide) layer were obviously influenced by different substrate properties. The differences in the membrane pore size, porosity and surface hydrophilicity of different substrates had their respective contributions to the changes in the thickness and cross-linking degree of poly(piperazine-amide) layer during the interfacial polymerization process. It can be concluded that both the smaller membrane pores and larger substrate porosity led to an increase in the salt rejection rate of the TFC NF membrane at the expense of permeate flux, while the surface hydrophilicity might not be the main factor affecting the formation of poly(piperazine-amide) layer. After that, the PVC-NF2 membrane was employed to further study the rejection performance of the prepared TFC NF membranes. The PVC-NF2 membrane possessed a MWCO of around 310 Da and exhibited rejection rates of 98.0% and 30.0% to MgSO4 and NaCl at 0.4 MPa, which could be comparable to most of the commercial NF membranes reported in the literature. In addition, the stability tests showed that the prepared TFC NF hollow fiber membrane could keep stable after backwash treatment and a long-term filtration process within a feed pH range of 2.5-11, respectively.

Acknowledgments The authors gratefully acknowledge the financial support for this work provided by the National 863 Program of China (grant number 2012AA03A602), the National 973 Program of China (grant number 2009CB623402) and the Nature Science Foundation Committee of China (grant number 20974094).

22

References [1] P. Xu, T.Y. Cath, A.P. Robertson, M. Reinhard, J.O. Leckie, J.E. Drewes, Critical review of desalination concentrate management, treatment and beneficial use, Environ. Eng. Sci. 30 (2013) 502-514. [2] A.F. Ismail, M. Padaki, N. Hilal, T. Matsuura, W.J. Lau, Thin film composite membrane-Recent development and future potential, Desalination 356 (2015) 140-148. [3] R. Xu, J.H. Wang, M. Kanezashi, T. Yoshioka, T. Tsuru, Development of robust organosilica membranes for reverse osmosis, Langmuir 27 (2011) 13996-13999. [4] F. Fadhillah, S.M.J. Zaidi, Z. Khan, M.M. Khaled, F. Rahman, P.T. Hammond, Development of polyelectrolyte multilayer thin film composite membrane for water desalination application, Desalination 318 (2013) 19-24. [5] W. Fang, L. Shi, R. Wang, Interfacially polymerized composite nanofiltration hollow fiber membranes for low-pressure water softening, J. Membr. Sci. 430 (2013) 129-139. [6] Y. Cui, Z.K. Yao, K. Zheng, S.Y. Du, B.K. Zhu, L.P. Zhu, C.H. Du, Positively-charged nanofiltration membrane formed by quaternization and cross-linking of blend PVC/P(DMA-co-MMA) precursors, J. Membr. Sci. 492 (2015) 187-196. [7] S.C. Yu, Y.P. Zheng, Q. Zhou, S. Shuai, Z.H. Lu, C.J. Gao, Facile modification of polypropylene hollow fiber microfiltration membranes for nanofiltration, Desalination 298 (2012) 49-58. [8] W.J. Lau, A.F. Ismail, N. Misdan, M.A. Kassim, A recent progress in thin film composite membrane: A review, Desalination 287 (2012) 190-199. [9] N. Misdan, W.J. Lau, A.F. Ismail, T. Matsuura, D. Rana, Study on the thin film composite

poly(piperazine-amide)

nanofiltration

membrane:

Impacts

of

physicochemical properties of substrate on interfacial polymerization formation, Desalination 344 (2014) 198-205. [10] M. Fathizadeh, A. Aroujalian, A. Raisi, Effect of lag time in interfacial polymerization on polyamide composite membrane with different hydrophilic sub layers, Desalination 284 (2012) 32-41. [11] R.W. Baker, Membrane Technology and Applications, 2nd ed., John Wiley & Sons

23

Ltd., West Sussex, England, 2004. [12] R.J. Petersen, Composite Reverse-Osmosis and Nanofiltration Membranes, J. Membr. Sci. 83 (1993) 81-150. [13] E.S. Kim, Y.J. Kim, Q.S. Yu, B.L. Deng, Preparation and characterization of polyamide thin-film composite (TFC) membranes on plasma-modified polyvinylidene fluoride (PVDF), J. Membr. Sci. 344 (2009) 71-81. [14] H. Il Kim, S.S. Kim, Plasma treatment of polypropylene and polysulfone supports for thin film composite reverse osmosis membrane, J. Membr. Sci. 286 (2006) 193-201. [15] P.B. Kosaraju, K.K. Sirkar, Interfacially polymerized thin film composite membranes on microporous polypropylene supports for solvent-resistant nanofiltration, J. Membr. Sci. 321 (2008) 155-161. [16] A.R. Korikov, R. Kosaraju, K.K. Sirkar, Interfacially polymerized hydrophilic microporous thin film composite membranes on porous polypropylene hollow fibers and flat films, J. Membr. Sci. 279 (2006) 588-600. [17] M. Dalwani, N.E. Benes, G. Bargeman, D. Stamatialis, M. Wessling, Effect of pH on the performance of polyamide/polyacrylonitrile based thin film composite membranes, J. Membr. Sci. 372 (2011) 228-238. [18] C. Klaysom, S. Hermans, A. Gahlaut, S. Van Craenenbroeck, I.F.J. Vankelecom, Polyamide/Polyacrylonitrile (PA/PAN) thin film composite osmosis membranes: Film optimization, characterization and performance evaluation, J. Membr. Sci. 445 (2013) 25-33. [19] J.N. Han, D.L. Yang, S.H. Zhang, L.J. Wang, X.G. Jian, Preparation and performance of SPPES/PPES hollow fiber composite nanofiltration membrane with high temperature resistance, Desalination 350 (2014) 95-101. [20] T. He, M. Frank, M.H.V. Mulder, M. Wessling, Preparation and characterization of nanofiltration membranes by coating polyethersulfone hollow fibers with sulfonated poly(ether ether ketone) (SPEEK), J. Membr. Sci. 307 (2008) 62-72. [21] S.H. Zhang, X.G. Jian, Y. Dai, Preparation of sulfonated poly(phthalazinone ether sulfone ketone) composite nanofiltration membrane, J. Membr. Sci. 246 (2005) 121-126.

24

[22] X.C. Fan, Y.L. Su, X.T. Zhao, Y.F. Li, R.N. Zhang, J.J. Zhao, Z.Y. Jiang, J.N. Zhu, Y.Y. Ma, Y. Liu, Fabrication of polyvinyl chloride ultrafiltration membranes with stable antifouling property by exploring the pore formation and surface modification capabilities of polyvinyl formal, J. Membr. Sci. 464 (2014) 100-109. [23] W.D. Liu, Y.H. Zhang, L.F. Fang, B.K. Zhu, L.P. Zhu, Antifouling properties of poly(vinyl chloride) membranes modified by amphiphilic copolymers P(MMA-b-MAA), Chin. J. Polym. Sci. 30 (2012) 568-577. [24] J.H. Wang, Y.L. Wu, Y.H. Zhang, B.K. Zhu, Y.Y. Xu, Fabrication and performance of a low operating pressure nanofiltration poly(vinyl chloride) hollow fiber membrane, Chin. J. Polym. Sci. 32 (2014) 377-384. [25] S.P. Sun, T.A. Hatton, S.Y. Chan, T.S. Chung, Novel thin-film composite nanofiltration hollow fiber membranes with double repulsion for effective removal of emerging organic matters from water, J. Membr. Sci. 401 (2012) 152-162. [26] X. Wei, X. Kong, C. Sun, J. Chen, Characterization and application of a thin-film composite nanofiltration hollow fiber membrane for dye desalination and concentration, Chem. Eng. J. 223 (2013) 172-182. [27] N. Misdan, W.J. Lau, A.F. Ismail, T. Matsuura, Formation of thin film composite nanofiltration membrane: Effect of polysulfone substrate characteristics, Desalination 329 (2013) 9-18. [28] P.S. Singh, S.V. Joshi, J.J. Trivedi, C.V. Devmurari, A.P. Rao, P.K. Ghosh, Probing the structural variations of thin film composite RO membranes obtained by coating polyamide over polysulfone membranes of different pore dimensions, J. Membr. Sci. 278 (2006) 19-25. [29] H.L. Liu, C.F. Xiao, Q.L. Huang, X.Y. Hu, W. Shu, Preparation and interface structure study on dual-layer polyvinyl chloride matrix reinforced hollow fiber membranes, J. Membr. Sci. 472 (2014) 210-221. [30] N. Hilal, M. Al-Abri, H. Al-Hinai, Characterization and retention of UF membranes using PEG, HS and polyelectrolytes, Desalination 206 (2007) 568-578. [31] N. Widjojo, T.S. Chung, M. Weber, C. Maletzko, V. Warzelhan, The role of sulphonated polymer and macrovoid-free structure in the support layer for thin-film

25

composite (TFC) forward osmosis (FO) membranes, J. Membr. Sci. 383 (2011) 214-223. [32] J. Gao, S.P. Sun, W.P. Zhu, T.S. Chung, Polyethyleneimine (PEI) cross-linked P84 nanofiltration (NF) hollow fiber membranes for Pb2+ removal, J. Membr. Sci. 452 (2014) 300-310. [33] S.A. Al Malek, M.N. Abu Seman, D. Johnson, N. Hilal, Formation and characterization of polyethersulfone membranes using different concentrations of polyvinylpyrrolidone, Desalination 288 (2012) 31-39. [34] F. Liu, B.K. Zhu, Y.Y. Xu, Preparation and characterization of poly(vinyl chloride)-graft-acrylic acid membrane by electron beam, J. Appl. Polym. Sci. 105 (2007) 291-296. [35] A.K. Ghosh, E.M.V. Hoek, Impacts of support membrane structure and chemistry on polyamide-polysulfone interfacial composite membranes, J. Membr. Sci. 336 (2009) 140-148. [36] N.K. Saha, S.V. Joshi, Performance evaluation of thin film composite polyamide nanofiltration membrane with variation in monomer type, J. Membr. Sci. 342 (2009) 60-69. [37] W. Fang, L. Shi, R. Wang, Mixed polyamide-based composite nanofiltration hollow fiber membranes with improved low-pressure water softening capability, J. Membr. Sci. 468 (2014) 52-61. [38] V. Freger, Kinetics of film formation by interfacial polycondensation, Langmuir 21 (2005) 1884-1894. [39] A.P. Rao, S.V. Joshi, J.J. Trivedi, C.V. Devmurari, V.J. Shah, Structure-performance correlation of polyamide thin film composite membranes: effect of coating conditions on film formation, J. Membr. Sci. 211 (2003) 13-24. [40] R. Nadler, S. Srebnik, Molecular simulation of polyamide synthesis by interfacial polymerization, J. Membr. Sci. 315 (2008) 100-105. [41] S. Zhu, S. Zhao, Z. Wang, X. Tian, M. Shi, J. Wang, S. Wang, Improved performance of polyamide thin-film composite nanofiltration membrane by using polyetersulfone/polyaniline membrane as the substrate, J. Membr. Sci. 493 (2015)

26

263-274. [42] A.L. Ahmad, B.S. Ooi, A.W. Mohammad, J.P. Choudhury, Composite nanofiltration polyamide membrane: A study on the diamine ratio and its performance evaluation, Ind. Eng. Chem. Res. 43 (2004) 8074-8082. [43] B. Tansel, J. Sager, T. Rector, J. Garland, R.F. Strayer, L.F. Levine, M. Roberts, M. Hummerick, J. Bauer, Significance of hydrated radius and hydration shells on ionic permeability during nanofiltration in dead end and cross flow modes, Sep. Purif. Technol. 51 (2006) 40-47. [44] G. Bargeman, J.B. Westerink, C.F.H. Manuhutu, A. ten Kate, The effect of membrane characteristics on nanofiltration membrane performance during processing of practically saturated salt solutions, J. Membr. Sci. 485 (2015) 112-122. [45] N. Park, J. Cho, S. Hong, S. Lee, Ion transport characteristics in nanofiltration membranes: measurements and mechanisms, J. Water Supply: Res. Technol. 59 (2010) 179-190. [46] M.A. Zazouli, M. Ulbricht, S. Nasseri, H. Susanto, Effect of Hydrophilic and Hydrophobic Organic Matter on Amoxicillin and Cephalexin Residuals Rejection from Water by Nanofiltration, Iran. J. Environ. Health Sci. Eng. 7 (2010) 15-24. [47] M. Frank, G. Bargeman, A. Zwijnenburg, M. Wessling, Capillary hollow fiber nanofiltration membranes, Sep. Purif. Technol. 22-23 (2001) 499-506.

Figure captions Fig. 1. ATR-FTIR spectra of the PVC substrate membranes. Fig. 2. FESEM images of the cross-section and surface morphologies of the PVC substrate membranes (a) PVC-UF1, (b) PVC-UF2, (c) PVC-UF3; 1: cross section × 60, 2: cross section × 5K, 3: inner surface. Fig. 3. Pore size distribution curves of the PVC substrate membranes. Fig. 4. ATR-FTIR spectra of the TFC NF membranes prepared over different PVC substrates. Fig. 5. FESEM images of the inner surface and cross-section morphologies of the prepared TFC NF membranes (a) PVC-NF1, (b) PVC-NF2, (c) PVC-NF3.

27

Fig. 6. MgSO4 rejection rate and permeate flux of the prepared TFC NF membranes. Fig. 7. Rejection rates of the PVC-NF2 membrane to PEGs with different molecular weights. Fig. 8. Rejection rates and permeate fluxes of the PVC-NF2 membrane to different inorganic salts. Fig. 9. Changes in the MgSO4 rejection rate and permeate flux of the PVC-NF2 membrane after backwash treatment. Fig. 10. Changes in the MgSO4 rejection rate and permeate flux of the PVC-NF2 membrane during 30 days continuous filtration.

Highlights  TFC NF membrane was successfully prepared using PVC hollow fiber as new substrate.  Pore size and porosity of PVC substrates yield obvious influences on the formation of polyamide skin layer and the performance of PVC based TFC NF membranes.  The rejection rate and permeate flux of MgSO4 (2000 ppm, 0.4 MPa) reached 98.0% and 28 L/m2 h respectively for PVC-NF2 membrane.

28