Chlorine-resistant sulfochlorinated and sulfonated polysulfone for reverse osmosis membranes by coating method

Journal of Colloid and Interface Science 541 (2019) 434–443

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Regular Article

Chlorine-resistant sulfochlorinated and sulfonated polysulfone for reverse osmosis membranes by coating method Yali Zhao a,b, Lei Dai a, Qifeng Zhang a,⇑, Shengyang Zhou a,b, Suobo Zhang a,b,c,⇑⇑ a

Key Laboratory of Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China University of Science and Technology of China, Hefei 230026, China c Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), China b

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 26 December 2018 Revised 22 January 2019 Accepted 24 January 2019 Available online 25 January 2019 Keywords: RO membrane Chlorine resistance Sulfochlorinated polysulfone Polysulfone-sulfamide

a b s t r a c t In this work, sulfochlorinated polysulfone (SC-PSf), a functional polysulfone with chlorine resistance, was synthesized through metalation sulfochlorination of polysulfone (PSf). For endowing the required hydrophilicity of SC-PSf, the sulfonyl chloride groups on SC-PSf were partially hydrolyzed to sulfonic groups to produce sulfonated SC-PSf (SC-S-PSf). Thin film composite (TFC) membranes for reverse osmosis application were fabricated by coating solution of SC-S-PSf on porous PSf substrate and then crosslinked by piperazine (PIP) to form polysulfone-sulfamide (PSSA) skin layer. In order to enhance the spreadability of polymer solution on PSf supporting layer, tetrabutylammonium chloride (TBAC), a common surfactant, was added into the coating solution. A flawless membrane was acquired only by a single coating step and at dilute SC-S-PSf solution when TBAC was added into polymer solution. Through optimizing coating conditions, the NaCl rejection and water flux of PSSA membrane reached 96.9% and 17.8 L/m2h under brackish desalination conditions. Moreover, the PSSA membrane exhibited the long-term stability against chlorine during the operation condition of 2000 ppm NaOCl for 10 days. The salt rejection of PSSA only decreased by 1%. In contrast, the salt rejection of polyamide membrane decreased by 8% under the same condition. Ó 2019 Elsevier Inc. All rights reserved.

Abbreviations: A, effective membrane area (m2); ATR-IR, attenuated total reflectance infrared; Cp, concentration of the permeation (mg mL
1); Cf, concentration of the feed (mg mL
1); EM, 2-methoxyethanol; J, water flux (L/m2 h); L, liter; Mc-1, the membrane prepared by one coating and cross-linking step; Mc-2, the membrane prepared by two coating and cross-linking steps; PA, polyamide; PIP, piperazine; R, salt rejection (%); RO, reverse osmosis; SEM, scanning electron microscope; t, time (h); TBAC, tetrabutylammonium chloride; TFC, thin film composite; V, volume of permeate water (L); XPS, X-ray photoelectron spectroscopy. ⇑ Corresponding author. ⇑⇑ Corresponding author at: Key Laboratory of Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China. E-mail addresses: [email protected] (Q. Zhang), [email protected] (S. Zhang). https://doi.org/10.1016/j.jcis.2019.01.104 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

Y. Zhao et al. / Journal of Colloid and Interface Science 541 (2019) 434–443

1. Introduction Polyamide (PA) reverse osmosis (RO) and nanofiltration (NF) membranes fabricated by interfacial polymerization have been widely used in desalination of water all over the world thanks to their high separation efficient, wide pH range and compaction resistance [1–4]. However, PA membrane is highly sensitive towards chlorine [5]. Chlorine is a common bactericide added into the feed solution to mitigate the biofouling of membranes in water treatment process [6]. The poor chlorine resistance of PA membranes can be ascribed to the electron effect of ANHCO- group. The amidic nitrogen is willing to share its lone-pair electron with chlorine atom, which is electron deficiency. This results in a reversible NACl reaction on polyamide chain [7]. The further destruction of polyamide is an irreversible Orton rearrangement, producing chlorinated ring [8]. Based on the mechanism of PA chlorinated degradation, many strategies have been proposed for improving the chlorine resistance of PA membranes, such as using novel monomer [9,10], surface modification with sacrificial layer [11,12], and the modification of amide bond [13]. However, these methods only mitigate the rate of chlorinated degradation of PA membrane rather than eradicating the problem. Adopting non-polyamide based material with high chlorine resistance to prepare desalination membrane is an available method [14–16]. The chain structure of PSf and polyethersulfone (PES) materials possesses outstanding chlorine resistance compared with polyamide [17,18]. The performance of membranes fabricated by PSf and PES presents slight decline when undergoing chlorine-resistance test [18,19]. Therefore, they are candidates to fabricate chlorine-resistant desalination membrane. However, one drawback of PSf and PES to be used for desalination membrane is their hydrophobicity. Introduction of hydrophilic groups such as ACOOH [20], ANH2 [21], and ASO3H [22] into polymer chain is an efficient method to enhance the hydrophilicity of PSf and PES. Thereinto, the synthesis of sulfonated polymer is the simplest and most practicable routing. In addition, the hydrophilicity of sulfonated polymer is outstanding. Hence, sulfonated polymer is usually utilized as membrane materials for desalination. Nowadays, nanofiltration (NF) membrane prepared by sulfonated polymer has been applied widely and sulfonated polyethersulfone NF membrane has been commercialized [23]. However, it is an aporia to achieve the NaCl rejection benchmark for RO operation using sulfonated polymer. Sulfonated degree of polymer plays a ‘‘trade-off” relationship with the salt rejection of membrane, and plays a positive correlation with the water flux of membrane. These are because that highly sulfonated degree of polymer usually induces high swelling of the membranes in water. In order to obtain high salt rejection of prepared membrane, using polymer with low sulfonated degree and crosslinking treatment are two selectable methods. Park et al. [18] used a series of sulfonated poly(arylene ether sulfone) copolymers (BPS) synthesized from 3,3’-disulfonato-4,4’-dichloro diphenyl sulfone and 4,4’-dichlorodiphenyl sulfone to prepare desalination membrane. The results indicated that the salt rejection of fabricated membranes was inverse proportionality with the percentage of sulfone groups on synthesized polymer. A continuously chlorine-resistant test was ran to compare the chlorine resistance of membrane prepared by sulfonated poly(arylene ether sulfone) copolymers and PA membrane. The results showed that the salt rejection of sulfonated poly(arylene ether sulfone) membrane did not change under 10,000 ppmhours of continuous exposure to chlorine. However, the salt rejection of PA membrane declined by more than 20% under the same condition. Lee et al. [24] utilized poly(arylene ether sulfone) of low sulfonated degree to prepare TFC RO membranes by brush coating method. Through adjusting the coating conditions, the NaCl rejection of membranes

435

could reach to 97%. To the polymer of high sulfonated degree, crosslinking is an available method to restrict polymer swelling in water. Paul et al. [25] used BPS with highly sulfonated degree (BPS-50) to prepare RO membrane and used tetraglycidyl bis(paminophenyl) methane (Araldite MY721 epoxy resin) to react with phenoxide endgroups for crosslinking the polymer chain. Finally, the NaCl rejection of obtained membrane could reach to 97.2% through controlling crosslinking conditions. By contrast, the NaCl rejection of non-crosslinked membrane was only 73.4%. Kim et al. [26] used thermal curing method to crosslink the membranes prepared from sulfonated poly(arylene ether) with ethynyl end groups. The salt rejection of membrane was also improved. Two types of techniques including the non-solvent induced phase inversion (NIPS) [23] and surface coating [18] can be used to fabricate RO membranes via sulfonated polymers. The membrane prepared by coating method is TFC membrane, whose separation layer is usually thinner than that of prepared by NIPS. Hence, coating is thought to be a prefer technology for developing RO membrane from sulfonated polymers. However, since the PSf support layer should not be soluble in the polymer solvents, the selection of suitable solvents is a challenge for coating method. Formic acid (FA) [27], an organic acid, is routinely used as the solvent for coating technology. Nevertheless, FA is toxic and corrosive. Thus, it is meaningful to try new solvent for preparing sulfonated polymers based RO membranes. In this study, sulfochlorinated polysulfone (SC-PSf) was synthesized and was used as the barrier layer material to fabricate TFC RO membrane via coating method. The sulfonyl chloride groups on SCPSF were partially hydrolyzed to sulfonic groups for enhancing the hydrophilicity of SC-PSF. The residual sulfonyl chloride groups reacted with piperazine (PIP) for crosslinking the barrier layer to restrict membrane swelling in water. 2-Methoxyethanol (EM), a solvent of lower corrosivity compared with formic acid, was utilized as the solvent of coating solution [28]. In addition, tetrabutylammonium chloride (TBAC), a surfactant, was added into the coating solution for improving the wettability of the coating solution to the supporting layer. Through optimizing coating conditions, a TFC RO membrane with suitable separation performance and excellent chlorine resistance was developed. 2. Experimental 2.1. Materials Polysulfone (PSf, UdelÒ P-3500, Solvay Comp) was dried at 120 °C under vacuum for 12 h. Sulfuryl chloride, N-butyllithium (2.5 M solution in hexane), piperazine (PIP, Reagent Plus), and tetrabutylammonium chloride (TBAC) were purchased from sigma-aldrich. Tetrahydrofuran (THF) was dried by calcium hydride for 24 h, and then was distilled and stored into SCHLENK under nitrogen. N-hexane, 2-methoxyethanol (EM), dimethyl formamide (DMF), hydrochloric acid (HCl), glycerol and NaOCl were obtained from Sinopharm Chemical Reagent Co., Ltd. 2.2. The synthesis of sulfochlorinated polysulfone (SC-PSf) The preparation of SC-PSf referred to a previous literature as shown in (Fig. 1) [29]. Typically, PSf (6 g, 13.6 mmol) and 400 ml of THF were added into a four-necked round bottom flask, which was assembled by mechanical stirrer, argon inlet and thermometer. Subsequently, the PSf solution was cooled to
70 °C utilizing dry ice and acetone, and 2.1 equivalents (11.5 ml) of nbutyllithium was added into the flask at
70 °C. The solution was stirred for 1 h at
70 °C before 2.2 equivalents of sulfuryl chloride was added into the flask. Finally, 100 ml of deionized water

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Fig. 1. Synthesis of SC-PSf. Fig. 3. Quantitative analysis of the amount of - SO2Cl group on one structural unit.

was added into the solution for quenching active substances. The SC-PSf precipitate was obtained via pouring the polymer solution into i-propanol. The SC-PSf was washed by i-propanol for several times, and then was dried at 50 °C for 24 h under vacuum. 2.3. The synthesis of sulfochlorinated and sulfonated polysulfone (SCS-PSf) Fig. 2 exhibits the hydrolysis of -SOCl2 SC-PSf (4 g) and 60 ml of THF were added into a flask, and the mixture was stirred by magnetic stirring until the solution was clear. Subsequently, 10 ml of deionized water was added into the solution at100 °C. The reaction hold for 2 h at100 °C. Thereafter, the solution was cooled to room temperature and was poured into i-propanol. The precipitate was collected and washed by i-propanol and deionized water for several times. Finally, the polymer was freeze-dried. The hydrolysis degree of SC-PSf could be adjusted by controlling the hydrolysed time. The obtained partially hydrolyzed polymer was labeled as SC(x)-S(y)-PSf, where x and y were the average numbers of sulfonyl chloride and sulphonic acid groups per repeating unit of prepared polymer, respectively. In order to calculate the amount of surplus ASO2Cl groups, the THF solution of methylamine was utilized to react with SC-S-PSf (see Fig. 3). Concretely, 0.5 g of SC-S-PSf was dissolved into 10 ml of DMF, and then excessive methylamine was added into the solution. This reaction was stirred at room temperature for 12 h. Subsequently, the polymer solution was slowly poured into 2 mol/L of HCl aqueous solution. Through a acidification of 24 h, the polymer was acquired by filtration, and then dialyzed by dialysis bag (MD34, MWCO 500 D) until the conductivity of dialyzate was lower than 10 ls. Finally, the polymer was dried at 80 °C under vacuum for 12 h. 2.4. The preparation of membrane Thin film composite (TFC) was prepared by coating method as shown in Fig. 4. The PSf supporting layer was treated by 25% (v/ v) glycerol aqueous solution before using. Then, SC-S-PSf and tetrabutylammonium chloride (TBAC) with various weight concentrations were dissolved in EM to form clear solution. The solution was filtrated by a 0.22 mm PTFE syringe filter before using. In a

O

O

ClO2S O S O SO2Cl

H2O n

O

O S O SO2Cl x

Fig. 2. Synthesis of SC-S-PSf.

2.5. Characterization 2.5.1. NMR spectroscopy of prepared polymer A Bruker Avance 500 MHz spectrometer was used to tested the 1 H NMR spectra of polymer. 2.5.2. Chemical constitution characterization The chemical constitution of polymer and membrane was characterized by attenuated total reflectance infrared (ATR-IR) that uses a Bio-Rad Digilab Division FTS-80 spectrometer and an Irtran crystal at an incidence angle of 45°. X-ray photoelectron spectroscopy (XPS) with a Thermo ESCALAB 280 system using Mg/Ka as the radiation source was utilized to characterize the element composition of membrane surface. 2.5.3. Water uptake and swelling A homogeneous membrane was used to characterize the water uptake and swelling of SC-S-PSf. The preparation detail of homogeneous membrane was as following: a 5 wt% DMF solution of SC-SPSf was cast onto a clean glass plate, and then DMF was slowly evaporated at 80 °C for 12 h. For producing crosslinked membrane, the dried membrane was immersed into PIP solution for 30mins. The prepared membranes were tailored to a 60 mm * 40 mm rectangle and then was placed into deionized water for 36 h at 25 °C. Then, fetch the membranes, wipe the water droplets on membranes, weight the membranes, and measure the length of the membranes. The wet membranes were dried under vacuum at 90 °C for 12 h and the weight and length of dried membranes were measured. Water uptake and swelling were calculated by Eqs. (1) and (2), respectively.

Water uptake ðWU; %Þ ¼

100°C

SwellingðS; %Þ ¼

y HO3S

O

typical coating process, the solution was poured onto PSf supporting layer and then evenly shook for 2mins to ensure the uniformity of prepared membrane. Subsequently, the coated membrane was placed into an oven and dried at 60 °C for 4 min. This noncrosslinked membrane was recorded as Muc. After that, a PIP solution in n-hexane (the content of PIP was set at 5% of SC-S-PSf weight in solution) was poured onto membrane surface and kept for 3mins to react with ASO2Cl group for preparing crosslinked membrane. Next, the membrane was dried in an oven at 60 °C for 5mins. This membrane crosslinked once was recorded as Mc1. Repeating above operations twice, a denser membrane which was recorded as Mc-2 could be obtained.

n

Wwet
Wdry  100 Wdry

Lwet
Ldry  100 Ldry

ð1Þ

ð2Þ

2.5.4. Membrane morphology and thickness A scanning electron microscope (SEM, XL 30 ESEMFEG, FEI Company) was used to characterize the morphology and thickness of membranes. The cross-section sample was prepared through peel-

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Fig. 4. The preparation process of membrane.

ing off the sample from non-woven fabric under the condition of liquid nitrogen freezing. All samples were sprayed with gold before SEM characterized.

3. Results and discussions

2.6. Membranes performance test

SC-PSf was synthesized by metalation routing. The chemical stability of polymer prepared by this method was higher than that of polymer prepared by concentrated sulfuric acid or chlorosulfonic acid due to the difference of substitution site [31]. The degree of sulfochlorination of SC-PSf (the number of ASO2Cl group per repeating unit), as calculated by 1H NMR (Fig. S1), was 1.64. Then, the SC-PSf reacted with deionized water for different time to produce a series of sulfonated SC-PSf. The obtained polymer was recorded as SC-S-PSf. Table 1 summarizes the solubility of SC-SPSf prepared by different hydrolyzed time in EM, where the solubility increases with the extension of hydrolyzed time. However, the polymer hydrolyzed for 3 h could dissolve in water with 0.25% (g/g). Considering the coating concentration of SC-S-PSf in EM (>0.3 wt%) and polymer swelling ratio in water, SC-S-PSf hydrolyzed for 2 h was selected as membrane materials for coating. For quantifying the residual sulfonyl chloride groups of SC-SPSf, NH2CH3 was used to react with sulfonyl chloride groups. The degree of sulfochlorination of SC-S-PSf, as obtained by 1H NMR (Fig. S2), was 0.6. The average numbers of sulfonyl chloride and sulphonic acid groups per repeating unit of SC-S-PSf which was hydrolyzed for 2 h were 0.6 and 1.04, respectively. The polymer was recorded as SC(0.6)-S(1.04)-PSf.

2.6.1. Seperation performance of membrane The salt rejection and water flux of membrane were tested by a cross-flow configuration. The operation pressure was 1.55 MPa, and the feed solution was 2000 ppm NaCl solution. The valid membrane area of each cell was 15.79 cm2. The membranes sustained a pure water pressure of 1.55 MPa for 4 h before testing. The water flux was calculated by Eq. (3), and the salt rejection was calculated by Eq. (4).

J¼

V At

ð3Þ

where v(L) is the volume of permeated solution, A(m2) is the tested membrane area, and t(h) is the time.

R¼

Cf
Cp Cf

ð4Þ

where Cp (mg/L) is the concentration of permeated solution, and Cf (mg/L) is the concentration of feed solution. These two concentrations were represented by the conductance of solution, which was measured by a conductance meter (DDS-11A, China). Each membrane chose six parallel samples, and the final result was the average value. 2.6.2. Chlorine tolerance of membrane A 2000 ppm of chlorine aqueous solution was prepared using NaOCl and deionized water. The pH of solution was adjusted to 6.0 using potassium acid phthalate. The experiment of chlorine tolerance was implemented by static soaking method at 25 °C. Specifically, the membrane coupons were inserted into a beaker which contained active chlorine aqueous solution and was packaged by aluminum foil to avoid the decrease of active chlorine. When reaching stated ageing time, the coupon was fetched and washed by deionized water for 30 mins before testing the seperation performance. The total exposure of sample to active chlorine was presented by ppm h. Besides, the chlorine tolerance of a PA TFC membrane which was synthesized according to our previous work [30] was also tested as comparation.

3.1. Polymer synthesis and characterization

3.2. The stability of SC(0.6)-S(1.04)-PSf in 2-methoxyethanol (EM) 2-Methoxyethanol (EM), which was low corrosivity, was selected to be the solvent. Because there was a hydroxyl group on EM, the AOH may react with - SO2Cl. Therefore, the stability of SC(0.6)-S(1.04)-PSf in EM was tested. The specific operation Table 1 The solubility of SC-S-PSf prepared by different hydrolysis time in EM. Hydrolyzed time (h)

Solubility (% g/g)

0 1 2 3

0.1 0.3 2 3.5

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was depicted as follows. Firstly, 1 wt% solution of SC(0.6)-S(1.04)PSf in EM was heated at 60 °C for 1 h. Secondly, the solution was cast onto a glass plate, and dried at 60 °C for 20 min. Finally, the produced film was washed with ethyl alcohol for several times and dired at atmosphere for 24 h. Fig. 5 exhibits the 1H NMR of SC(0.6)-S(1.04)-PSf undergoing stability test. According to the reference, EM possesses three kinds of protons characteristic peaks, which are ACH3 (3.30 ppm, s), ACH2 (3.39 ppm, t) and ACH2 (3.55 ppm, t) [32], respectively. After stability test, the protons characteristic peaks of EM do not appear in range of 3.0–4.0 ppm as shown in Fig. 5. The result demonstrates - SO2Cl groups can not react with EM solution at testing time and SC(0.6)-S(1.04)PSf is stable in EM. 3.3. Preparation of TFC reverse osmosis membrane The membrane was fabricated via coating polymer solution on PSf porous supporting layer. The supporting layer was treated referencing E. McGrath means [24], where glycerol was used to keep an open pore structure on supporting layer. EM was the solvent of polymer solution and the curing temperature was selected at 60 °C.

Fig. 5. 1H NMR of SC(0.6)-S(1.04)-PSf undergoing stability test.

Other preparation conditions of TFC membrane such as additive, solution concentration, and crosslinking conditions were discussed in the next sections.

3.3.1. The effect of tetrabutylammonium chloride (TBAC) on seperation preformance of prepared membranes Usually, a single coating step is insufficient for producing a nondefective separation layer on PSf supporting layer[24]. The possible reason is that PSf supporting layer is poor hydrophlilicity. Nevertheless, the sulfonated polymer solution has good hydrophilicity. Therefore, the polymer solution hardly spreads out on PSf surface with defectless status. In order to solve this problem, a surfactant, tetrabutylammonium chloride (TBAC), was added into polymer solution for decreasing the surface tension of coating solution. The spreadability improvement of coating solution was in favour of preparation for defect-free layer. A single coating step and 0.5 wt% solution of SC(0.6)-S(1.04)-PSf were implemented to prepare membranes. As shown in Fig. 6, the surface of the membrane which was prepared use 0.5 wt% TBAC as additive was smooth and flawless. In contrast, there were many pores on the surface of membrane whose coating solution did not add TBAC, and the pores were similar with the pores on supporting layer. This verified the above conjecture that PSf supporting layer had poor affinity with the coating solution without adding TBAC, and the addition of TBAC decreased the coating solution surface tension to enhance this affinity. In addition, the thickness of membrane prepared by adding TBAC was thinner than that of membrane prepared by not adding TBAC, which also was attributed to the excellent spreadability of the coating solution adding TBAC. Fig. 7(a) exhibits the influence of TBAC on the seperation performance of membranes. The NaCl rejection of membranes improved from 23% to 80% with adding TBAC. The membrane prepared with TBAC as additive possessed agreeable rejection with only a single coating step. Subsequently, the influence of TBAC concentration was disscussed. As shown in Fig. 7(b), the NaCl rejection enhanced from 50% to 80% with TBAC concentration increased from 0.2 wt% to 0.5 wt%. However, the seperation performance of membranes obviously changed slowly when the concentration of TBAC exceeded 0.35 wt%. This might be due to the mild change of the surface tension of coating solution when the concentration of TBAC exceeded 0.35 wt%. Therefore, 0.35 wt% was selected as the adding concentration of TBAC.

Fig. 6. SEM images of surfaces and cross-section of prepared membrane: (a),(d) the PSf supporting membrane; (b),(e) the membrane prepared by not adding TBAC; (c),(f) the membrane prepared by using 0.5 wt% TBAC as additive.

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439

Fig. 7. (a) The influence of TBAC to membrane seperation performance (testing with 2000 ppm NaCl), (b) The effect of TBAC concentration on the seperation performance of membranes (0.5 wt% polymer solution as coating solution).

3.3.2. The effects of crosslinking and coating solution concentration PIP was selected as the crosslinker. The number of ASO2Cl groups per repeating unit was 0.6. Hence, the PIP concentration was chose as 5% of SC(0.6)-S(1.04)-PSf weight concentration. At this concentration ratio, the mole ratio of -NH- on PIP to SO2Cl groups on polymer was 1.2, which guaranteed the sufficient crosslinking between SO2Cl and ANH- groups as shown in Fig. 4. Fig. 8(a) shows the separation performance of non-crosslinked Muc and crosslinked Mc-1. The NaCl rejection of membranes improved from 80% to 90% after crosslinking, which manifested crosslinking made a denser chain structure. The effect of coating solution concentration on the seperation performance of Mc-1 was depicted in Fig. 8(b). For Mc-1, NaCl rejection varied from 87% to 95%, while the water flux declined from 19.3 L/m2h to 15 L/m2h as the coating solution concentration increased from 0.3 wt% to 0.75 wt%. Higher concentration solution coated onto PSf supporting layer would form thicker seperation layer. Hence, the rejection improved and water flux decreased. As shown in Fig. 8(b), even if high polymer concentration (0.75 wt%) was used, a single coating step could not satisfy the rejection benchmark of RO membrane. A method of two coating steps was used to further enhance the NaCl rejection of membranes. The crosslinking procedure trod on the heels of coating steps. Compared with the NaCl rejection of Mc-1 in Fig. 8(b), the rejection of Mc-2 presents an obvious improve-

ment as shown in Fig. 9. This illustrates two coating steps is effective. To the membrane prepared by 0.3 wt% polymer solution, the rejection of Mc-2 was 96.9%, while the rejection of Mc-1 was 87%. The rejection enhanced dramatically. The water flux changed

Fig. 9. The effect of solution concentration on seperation performance of Mc-2 which was prepared by two coating and crosslinking steps.

Fig. 8. (a) the seperation performance of uncrosslinked Muc and crosslinked Mc-1 (0.5 wt% polymer solution as coating solution), (b) effect of coating solution concentration on the seperation performance of Mc-1 (Mc-1 represents the membrane with a single coating and crosslinking step).

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Fig. 10. SEM images of cross-section of Mc-2:(a) Mc-2

Table 2 Water flux and salt rejection of Mc-2

0.3/0.3,

Water flux (Lm

Mc-2 0.3/0.3 SPAES-X-1.0 BPS-32 BW30-4040

1.15 0.62 0.54 3.4

Mc-2

0.5/0.5,(c)

Mc-2

0.75/0.75.

literature reported membranes which were fabricated by coating method, and commercial PA membrane.


2
1

Membranes

0.3/0.3,(b)

h

bar
1)

NaCl Rejection (%)

Operating pressure (bar)

Salt concentration (ppm)

Ref

96.9 96.8 96.5 99.5

15.5 20 27.6 15.5

2000 2000 2000 2000

this work [27] [24] Dow Filmtech

for RO membrane preparation considering the water flux and NaCl rejection. The prepared membrane was recorded as Mc-2 0.3/0.3. 3.3.3. The comparison of seperation performance of Mc-2 0.3/0.3 with that of other membranes Table 2 summarizes the water flux and NaCl rejection of Mc-2 0.3/0.3 and other referenced membranes. Compared with SPAES-X1.0 and BPS-32 which were also prepared by coating method, Mc2 0.3/0.3 had a water flux 185% and 213% higher than SPAES-X-1.0 and BPS-32, while its NaCl rejection preceded that of SPAES-X1.0 and BPS-32. The result may be because the thickness of Mc-2 0.3/0.3 was lower than that of SPAES-X-1.0 and BPS-32 (250 nm vs 700 nm vs 679 nm) [24,27] and a crosslinking procedure was

Fig. 11. ATR-IR spectrum of PSF, Muc and Mc-2.

slightly, which varied from 19.3 L/m2h of Mc-1 to 17.8 L/m2h of Mc2. Unfortunately, when the coating solution was 0.75 wt%, the method of two coating steps would result in an obvious decrease of water flux which changed from 15 L/m2h of Mc-1 to 10 L/m2h of Mc-2. Meanwhile, the salt rejection enhanced blandly compared to the membrane prepared by 0.3 wt% polymer solution. Fig. 10 shows the cross-section images of Mc-2 0.3/0.3, Mc-2 0.5/0.5, Mc-2 0.75/0.75, which were prepared with 0.3 wt%, 0.5 wt% and 0.75 wt% coating solution by two coating steps, respectively. The thickness of Mc-2 sharply increased from 220 to 250 nm to 1.2 lm as the increasing of solution concentration. The thick separation layer of Mc-2 0.75/0.75 caused an obvious decline of water flux. Finally, 0.3 wt% of coating solution and two coating steps were selected

Fig. 12. The water uptake and swelling of uncrosslinked and crosslinked homogeneous membrane.

Table 3 XPS analysis of membrane up surface composition. Samples

PSF Muc Mc-2

0.3/0.3

Surface elemental composition (mol %) C

O

S

Cl

N

80.55 74.35 74.8

15.31 19.45 19.18

4.14 4.95 4.56

– 1.25 0.45

– – 1.01

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implemented. Nevertheless, the gap of the seperation performance between Mc-2 0.3/0.3 and commercial PA membrane is obvious as shown in Table 2. 3.4. Characterization of membranes The chemical constitutions of PSf, Muc and Mc-2 were characterized by ATR-IR and XPS. As shown in Fig. 11, for Muc, new peaks at 1388 cm
1 and 3470 cm
1 appeared compared with the spectrum of PSf, and they corresponded to the characteristic bands of ASO2Cl [33]and hydroxy of ASO3H, respectively. In addition, the Cl element was detected on Muc surface and the content of S element

on Muc surface increased compared to PSf as summerized in Table 3. These results directly suggested that ASO2Cl and ASO3H groups were introduced onto PSf. After crosslinking, the characterization band of ASO2AN- at 925 cm
1 [34] appeared, and N element was detected on Mc-2, which manifested the occurence of crosslinking reaction. There still was a little amount of Cl element existing on Mc-2 surface, that might be because ASO2Cl groups uncompletely reacted with PIP in short time. 5 wt% DMF solution of SC(0.6)-S(1.04)-PSf and casting method were used to prepared homogeneous membrane (HM) for testing water uptake and swelling ratios of non-crosslinked and crosslinked membrane(HMuc, HMc). As shown in Fig. 12, the swelling

Fig. 13. The chlorine resistant of Mc-2

Fig. 14. (a),(b) ATR-IR spectrum of PA and Mc-2 chlorine exposure for 10 days.

0.3/0.3

0.3/0.3

and PA membrane.

before and after chlorine exposure for 10 days; (c),(d) the surface morphology of PA membrane and Mc-2

0.3/0.3

after

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ratios of membranes decreased from 9.7% to 5.2% after crosslinking. Crosslinking could astrict the movement of polymer chain to suppress the swelling of membrane in water [35]. Therefore, the salt rejection of membrane improved after crosslinking. Meanwhile, dense chain structure decreased water uptake of polymer. Hence, the water flux of crosslinked membrane decreased.

rejection of Mc-2 0.3/0.3 only decreased by 1%, while that of PA membrane decreased by 8%. Overall, a RO membrane with chlorine resistance was successfully produced based on the functional polysulfone which was not sensitive to chlorine. The present study may open a new avenue for the development of novel desalination membranes with chlorine resistance.

3.5. Chlorine resistant of membranes

Acknowledgments

The prepared membranes were immersed into 2000 ppm NaOCl aqueous solution at pH 6 to investigate the chlorine-tolerance property. Fig. 13 depicts the water flux and rejection of Mc-2 0.3/0.3 and PA membranes as a function of chlorine exposure time. The NaCl rejection of Mc-2 0.3/0.3 decreased by 1% after 4.8 * 105 ppm h (10 days) of chlorine exposure, while the NaCl rejection of PA membrane declined by 8%. The water flux of PA membrane deteriorated severely, which exhibited a decline of 70%. Interestingly, the water flux of Mc-2 0.3/0.3 displayed an increase of 20% compared with that of the membrane not undergoing chlorine-resistant test. Fig. 14(a),(b) shows the ATR-IR spectrum of PA and Mc-2 0.3/0.3 before and after chlorine-resistant test. The bands of 1608 cm
1 and 1545 cm
1 were assigned to the characterization band of ANH- in ACONH- in Fig. 14(a). When the membranes underwent chlorine exposure for 10 days, the ACONHcharacteristic bands of PA membrane disappeared. Amide bond was entirely destoried. In contrast, the ATR-IR spectrum of Mc-2 0.3/0.3 did not have an obvious change and the characteristic band of ASO2N- at 925 cm
1 still existed in Fig. 14(b). However, the peak intensity of hydroxyl of ASO3H at 3470 cm
1 enhanced after chlorine-resistant test. In the 3.4 section, a few ASO2Cl groups existing on Mc-2 0.3/0.3 had been proved by ATR-IR and XPS. The ASO2Cl groups could be hydrolyzed to ASO3H under the condition of chlorine-resistant test. Therefore, the peak intensity of hydroxyl of -SO3H increased, and then the water flux enhanced and rejection decreased slightly (1%) of Mc-2 0.3/0.3 after chlorine-resistant test for10 days. The surface morphology of PA membrane and Mc-2 0.3/0.3 was characterized after chlorine exposure for 10 days. As shown in Fig. 14(c),(d), the surface of PA membrane was destoried seriously and defects appeared on membrane surface. The surface of Mc-2 0.3/0.3 still held smooth and dense. Based on the above results, Mc-2 0.3/0.3 had better chlorine resistance compared with PA membrane.

The authors gratefully acknowledge the National Basic Research Program of China (no. 2015CB655302), the National Science Foundation of China (no. 51473163) and the Development of Scientific and Technological Project of the Jilin Province (no. 20140203004GX, 20160519006JH).

4. Conclusions Here, the chlorine-resistant sulfochlorinated and sulfonated polysulfone (SC-S-PSf) was successfully synthesized and used to prepare TFC RO membrane by coating on PSf supporting layer. Piperazine as the crosslinker reacted with sulfonyl chloride groups for crosslinking the coating layer. In the preparation process, tetrabutylammonium chloride (TBAC), a common surfactant, was added into polymer solution to refrain from forming defects on separation layer. The NaCl rejection of the membrane fabricated by adding TBAC increased by 3.5 times in comparison to that of the bare membrane (80% vs 23%). Through adjusting coating conditions, 2-methoxyethanol as the solvent, 0.3 wt% of SC-S-PSf, 0.35 wt% of TBAC, two coating steps, and two crosslinking steps by PIP were chose for fabricating membrane Mc-2 0.3/0.3. The NaCl rejection and water flux of Mc-2 0.3/0.3 were 96.9% and 17.8 L/m2h (2000 ppm NaCl feed at 1.55 MPa operation pressure). Compared with other membranes prepared by coating method, Mc-2 0.3/0.3 had a water flux 200% higher than them [24,27]. When immersed Mc-2 0.3/0.3 and PA membranes into 2000 ppm and pH = 6 NaOCl aqueous solution for 10 days, the salt

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