Reactable substrate participating interfacial polymerization for thin film composite membranes with enhanced salt rejection performance

Desalination 436 (2018) 1–7

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Reactable substrate participating interfacial polymerization for thin film composite membranes with enhanced salt rejection performance

T

Zhikan Yaoa, Hao Guoa, Zhe Yanga, Chuner Linb, Baoku Zhub, Yingchao Dongc, ⁎ Chuyang Y. Tanga, a

Department of Civil Engineering, The University of Hong Kong, Pokfulam, Hong Kong, China Key Laboratory of Macromolecule Synthesis and Functionalization (Ministry of Education, MOE), ERC of Membrane and Water Treatment (MOE), Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, China c Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Dalian, China b

G RA P H I C A L AB S T R A C T

A R T I C L E I N F O

A B S T R A C T

Keywords: Thin film composite Interfacial polymerization Reactable substrate Poly vinyl chloride Polyamide

In this study, we investigate the use of reactable substrates for interfacial polymerization of polyamide membranes. Two reactable group (hydroxyl group) contained substrates were prepared. Using m-phenylenediamine (MPD) and trimesoyl chloride (TMC) as monomers, thin film composite (TFC) membrane formed thereon displayed varying film morphology, chemical composition and separation performance. The substrate participating TFC membrane showed a thicker rejection layer with greater crosslinking density. The latter was caused by the formation of ester bonds in addition to amide bonds. This polyamide/polyester composite rejection layer had greatly enhanced NaCl and Na2SO4 rejection compared to the control membrane prepared on inert substrate. We further proposed a conceptual model that highlighted the important role of amphiphilic copolymers as (1) MPD enrichment loaders, (2) reactive monomers and (3) surfactants in the interfacial polymerization process.

1. Introduction Membrane based technologies, such as reverse osmosis (RO) and nanofiltration (NF), play an increasingly important role in addressing

⁎

the grand challenges of water shortage [1–4]. Most of the modern RO and NF membranes used for desalination and water reclamation are thin film composite (TFC) membranes prepared by interfacial polymerization [5–11]. In a typical interfacial polymerization process, a

Corresponding author. E-mail address: [email protected] (C.Y. Tang).

https://doi.org/10.1016/j.desal.2018.01.039 Received 23 September 2017; Received in revised form 21 December 2017; Accepted 30 January 2018 0011-9164/ © 2018 Elsevier B.V. All rights reserved.

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TFC membrane is obtained by forming an ultra-thin selective polyamide layer on a porous substrate through a condensation polymerization [12,13]. Water phase composition [14–18], organic phase composition [18–21], morphology and physicochemical property of substrate [22–26] and polymerization conditions [27,28] are important factors influencing the performance of TFC membrane. As the sites for the interfacial polymerization, the substrate not only provides the appropriate mechanical strength to the formed polyamide rejection layer, but also has direct impacts on the formation of the rejection layer. The structure and surface morphology of the substrate plays a critical role on the formation of the polyamide rejection layer [10,23,24]. For example, TFC membrane formed on polysulfone substrates with small pore sizes had better salt rejection [10]. In addition, the membrane separation properties also depend strongly on the surface wetting properties of the substrate. Several studies have reported enhanced salt rejection for TFC membranes formed on hydrophilic substrates due to the formation of thicker rejection layers [7,23,25]. However, the use of hydrophilic substrates often lead to inadequate adhesion between the polyamide layer and the substrate [29]. In this work, we designed polyvinyl chloride (PVC) substrates containing reactable hydroxyl groups that can participate in the interfacial polymerization process. Specifically, the reactable groups were introduced by blending amphiphilic copolymers of methyl methacrylate and 2-hydroxyethyl methacrylate (P(MMA-co-HEMA)) into the PVC substrates. The separation properties of the TFC membranes prepared on these reactable substrates were systematically investigated. The findings in this study provide new insights on the role of substrate on the formation of polyamide rejection layer and thus the overall performance of TFC membranes.

Table 2 The compositions of casting solution for membrane substrates.

Table 1 Molecular weight and HEMA content for the synthesized copolymers.

Copolymer-1 Copolymer-2

36.1 54.9

38,200 37,200

68,700 58,800

1.80 1.58

a b

LiCl (wt%)

DMAc (wt%)

M0 M1 M2

12 14 14

– 1.5 –

– – 1.5

3 1 1

1 0.5 0.5

84 83 83

Membrane surface and cross-section (fractured in liquid nitrogen) morphologies were imaged by a field-emission scanning electron microscope (FE-SEM, S-4800, Hitachi) after sputter-coated with a thin layer of gold (SCD 005, BAL-TEC). SEM images were analyzed to calculate the surface pore size of the substrate using image analysis software (Image-Pro Plus, Media Cybernetics, Inc.). Atomic force microscopy (AFM, Multimode 8, Bruker) was applied to exam the surface roughness of membrane, in which Ra (average roughness) and roughness ratio were analyzed by software Nanoscope Analysis (Bruker) with 5 μm × 5 μm scanning range. The water contact angle (WCA) of the substrates and TFC membranes were measured with an optical instrument (Attension Theta, Biolin Scientific) equipped with video capture for predicting hydrophilicity. X-ray photoelectron spectroscopy (XPS, PHI 5000C ESCA system, PHI Co.) was used to measure the composition of a membrane surface. Al Ká radiation (1486.6 eV) was adopted as Xray source and run at a power of 250 W (14.0 kV, 93.9 eV) with an electron take off angle of 30° relative to the sample plane. The data were analyzed using the AugerScan 3.2 software provided by RBD Enterprises, Inc. SurPASS electrokinetic analyzer (SurPASS™ 3, Anton Paar) equipped with an adjustable gap cell was utilized to evaluate the surface charge properties (zeta potentials) and isoelectric point (IEP) of the membranes, following a standard operation using 1 mmol/L KCl aqueous solution as the testing solution [30].

The reactable substrates M1 and M2 were prepared by the traditional non-solvent induced phase separation (NIPS) method by blending copolymer-1 and copolymer-2 into the PVC dope solution, respectively

PDIb

PEG400 (wt%)

2.4. Membrane characterization

2.2. Preparation of membrane substrates

Mwb

Copolymer-2 (wt%)

MPD and TMC were used as monomers to form rejection layer on the reactable substrate by interfacial polymerization following a reported procedure [15]. In summary, an aqueous solution of 1% (w/v) MPD with 0.6% (w/v) Na3PO4 as the acid acceptor was poured onto a substrate surface and allowed to contact the surface for 1 min. After draining off the excess MPD solution, the residual droplets of MPD solution were removed from the substrate by rolling a rubber roller across the surface. Afterwards, the organic phase solution with 0.05% (w/v) TMC in n-hexane was poured onto the substrate surface for 30s. After removing excess organic solution, the membrane was rinsed using n-hexane to wash away residual reagents. The resulting membrane was immersed in deionized water at 50 °C for 10 min for further polymerization. Finally, the prepared membrane was stored in deionized water before characterization. The membrane prepared on M0, M1 and M2 were then denoted as TFC-0, TFC-1 and TFC-2, respectively.

Two amphiphilic copolymers, P(MMA-co-HEMA), were prepared different ratios of methyl methacrylate and 2-hydroxyethyl methacrylate. Their molecular structures and the synthesis procedures are presented in Supplementary Information S1. Table 1 shows the key properties of the two copolymers, including HEMA content, molecular weight, and polydispersity index (PDI). Unless specified otherwise, all other chemicals were used without further purification. PVC (Mn ~ 47,000), N,N-dimethylacetamide (DMAc, 99%), n-hexane (for HPLC, ≥95%), lithium chloride (LiCl, anhydrous, 99%), m-phenylenediamine (MPD, 99%) and trimesoyl chloride (TMC, 1,3,5-benzenetricarbonyl trichloride, 98%) were purchased from Sigma-Aldrich. Polyethylene glycol 400 (PEG400), trisodium phosphate dodecahydrate (Na3PO4·12H2O), sodium chloride (NaCl) and sodium sulfate (Na2SO4, anhydrous) were all analytical reagents and obtained from Dieckmann company. Deionized water used throughout this work was supplied from a Milli-Q system (Millipore).

Mnb

Copolymer-1 (wt%)

2.3. Interfacial polymerization

2.1. Materials

HEMA content (wt%)a

PVC (wt%)

(Table 2). In a typical casting, a dope solution was spread into a thick film by an automatic film applicator (Elcometer 4340, Elcometer, adjusted at a gate height of 150 μm). The nascent film was coagulated in a deionized water bath at room temperature (about 25 °C) and the formed substrate was stored in deionized water before further use. A control substrate of M0 was also casted without the addition of any reactable copolymers.

2. Materials and methods

ID

ID

2.5. Filtration performance tests A laboratory-scale cross-flow membrane filtration setup was applied to evaluate the pure water flux and salt rejection of the prepared

Calculated from 1HNMR measurement. Calculated from GPC result.

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Information S3.1), the surface hydroxyl group content increased in the order of M0 (0%) < M1 (2.37%) < M2 (5.54%). These experimental eOH contents for M1 and M2 were significantly higher than the respective theoretical values (0.59% and 0.88%) calculated assuming a homogenous distribution of the amphiphilic copolymers (Supplementary Information S3.1). These results imply the surface migration and enrichment of the hydrophilic eOH groups during the membrane formation [30,33–35]. Corresponding to the increasing eOH content, M1 and M2 were significantly more hydrophilic compared to M0 (Table 3). M1 and M2 were less negatively charged compared to M0 (Supplementary Information S3.2). In the current study, PVC lacks charged functional groups and its negative surface charge can be attributed to the adsorption of potential determining ions (e.g., Cl−) onto its relative hydrophobic surface [36,37]. The reduced surface charge of M1 and M2 can thus be explained by their more hydrophilic surfaces. Fig. 1 presents the SEM micrographs of the prepared substrates. The PVC substrate (M0) and the copolymer blended substrates (M1 and M2) had similar finger-like pore structure. Addition of amphiphilic copolymers resulted in an increase in surface pore size, surface porosity and surface roughness (Table 3). Similar with the previous studies using amphiphilic copolymer as additives [38,39], these structural changes may also be attributed to the compatibilization of the casting solution and coagulation bath at the nascent substrate surface. The pure water flux of M0, M1 and M2 were listed in Table 3. With the largest surface pore size and highest surface porosity, the most hydrophilic substrate (M2) showed the highest pure water flux.

membranes under a constant pressure at around 25 °C (Supplementary Information S2). For each test, a membrane with an effective area of 42 cm2 was placed in a cross-flow cell (CF042, Sterlitech). The crossflow velocity was about 22.4 cm/s in the tests. Each substrate was precompacted using deionized water at 150 kPa for 1 h to achieve a stable water flux and tested at 100 kPa. The TFC membranes were pre-compacted at 350 kPa for 2 h and tested at 300 kPa. The pure water flux was determined by measuring the volume of the permeate water collected over a specified time interval according to the following equation (Eq. (1)):

JV =

ΔV Δt × α

(1) −2

−1

where Jv (L m h ) is the water flux, ΔV (L) is the volume of permeate water collected over Δt (h), a (m2) is the effective membrane area. The pure water permeability coefficient (A) was calculated from Eq. (2) using deionized water as the feed solution:

A=

JV Δp

(2) −2

−1

where A (m/s Pa) is the water permeability coefficient, Jv (L m h ) is the water flux and Δp (Pa) is the hydraulic pressure difference across the membrane. Salt rejection was measured using a 5 mmol/L of Na2SO4 or a 7.5 mmol/L of NaCl solution as the feed. Membrane rejection R was calculated by Eq. (3):

R=

Cf − Cp Cf

× 100%

(3)

3.2. TFC membrane characterization

where Cp (mol/L) and Cf (mol/L), determined by conductivity meter (Ultrameter II, Myron L company), are the concentrations of the permeate and feed, respectively. The salt permeability coefficient (B), which was an intrinsic measure of a membrane's ability to retain salt, was determined by Eq. (4) based on the classical solution-diffusion theory [31]:

B=

(1 − R) × A × (ΔP − Δπ ) R

SEM surface micrographs (Fig. 2 (a)–(c)) show that all the three TFC membranes had ridge-and-valley surface structures. These features are characteristic to polyamide membrane formed through the interfacial polymerization of MPD and TMC [23,41]. Fig. 2(d–f) reveal that the rejection layers of TFC-1 (204.6 ± 18.8 nm) and TFC-2 (226.6 ± 30.9 nm) were marginally thicker compared to that of TFC-0 (185.3 ± 32.8 nm), which is consistent with the greater MPD adsorption by the polar hydroxyl and ester groups contained substrate surface [23,25]. Surface chemical compositions were analyzed on the basis of XPS measurements (Table 4 and Supplementary Information S4.1). All the membranes had similar amide group (O]CeN) content. On the other hand, the carbonyl group contents increased significantly in the order of TFC-0 < TFC-1 < TFC-2. Generally, the carbonyl peak can be attributed to ester and/or carboxyl groups. In the current study, the increased carbonyl content of M1 and M2 can be attributed to the formation of ester groups due to the greater presence of the reactable eOH groups on their surfaces (Table 3). Indeed, zeta potential results (Fig. 3) show that TFC-1 and TFC-2 were less negatively charged compared to TFC-0, indicating reduced carboxyl contents likely due to the competition between esterification and hydrolysis of TMC. The significantly reduced surface charge of TFC-2 also implies a greater crosslinking degree of the rejection layer [42]. As a rule of thumb, polyamides (WCA ~ 70°) [23] are much more hydrophilic compared to polyesters (WCA ~ 90°) [43,44]. The WCA results (Supplementary Information S4.2) that TFC-0 (71.4°) was more hydrophilic compared to TFC-1 (82.5°) and TFC-2 (85.0°) was consistent with the formation of ester groups.

(4)

where B (m/s) is the salt permeability coefficient, Δπ (Pa) is the osmotic pressure difference across the membrane. 3. Results and discussion 3.1. Substrate characterization In this study, two amphiphilic copolymers with similar molecular weight but different HEMA content were synthesized (Supporting Information S1). According to the 1HNMR and GPC results (Table 1), the hydroxyl group (eOH) content of copolymer-1 and copolymer-2 were about 2.78 mmol/g and 4.22 mmol/g, respectively. These amphiphilic copolymers were blended with PVC, a commonly used UF membrane material [32], to prepare substrates containing reactable eOH groups. Based on XPS analysis (Table 3 and Supplementary Table 3 The compositions and performance of substrates. ID

M0

M1

M2

Surface pore diameter (nm) Surface roughness Ra (nm) Roughness ratio (−) Measured WCA (°) Corrected WCA (°)a Surface OH content (%Atm) IEP (−) Flux (L/m2h)

13.7 ± 5.2 8.4 1.03 85.9 ± 1.6 86.0 ± 1.5 – 3.86 ± 0.02 248.4 ± 49.0

13.8 ± 6.3 15.7 1.07 75.1 ± 1.8 75.5 ± 1.7 2.37 3.84 ± 0.10 220.9 ± 20.7

20.7 ± 6.5 20.4 1.09 66.4 ± 2.1 67.1 ± 2.0 5.54 3.85 ± 0.03 321.7 ± 21.8

a

3.3. TFC membrane performance Fig. 4 shows the flux and salt rejection performance of the TFC membranes. Compared with the TFC membrane based on inert substrate (TFC-0), the TFC membranes based on reactable substrates (TFC1 and TFC-2) showed enhanced salt rejection but decreased water flux, which may be attributed to the marginally thickened rejection layer and increased crosslinking degree of the rejection layer (Fig. 2).

Corrected WCA was calculated based on the Wenzel's equation [40].

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Fig. 1. Surface SEM images of substrates: (a) M0, (b) M1, (c) M2; and cross-section SEM images: (d) M0, (e) M1, (f) M2.

Fig. 2. Surface SEM images of TFC membranes: (a) TFC-0, (b) TFC-1, (c) TFC-2; and cross-section SEM images: (d) TFC-0, (e) TFC-1, (f) TFC-2.

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coefficient (BNa2SO4) increased according to TFC-0 < TFC-1 < TFC-2 (Table 5). The BNaCl/A ratio and BNa2SO4/A ratio (Table 5), which are important selectivity parameters for TFC membranes [45,46], greatly reduced in order of TFC-0 > TFC-1 > TFC-2, indicating the significantly enhanced salt rejection due to the participating of the substrates. In conventional approach, the separation properties of a TFC membrane can be tuned by varying monomer concentrations [27]. To provide better understanding of the role of reactable substrates, we prepared an additional control membrane TFC-3 using inert substrate M0. The monomer concentrations for preparing TFC-3 (2% (w/v) MPD and 0.1% (w/v) TMC) were chosen in such a way that TFC-3 had similar water permeability with TFC-1 and TFC-2. Nevertheless, the salt rejections (RNaCl and RNa2SO4) of TFC-3 were much lower compared to the respective values of TFC-1 and TFC-2 (Fig. 4 and Table 5). This result reveals the feasibility of enhancing salt rejection without necessarily scarifying water permeability with the use of reactable substrates, thanks to the enhancement in crosslinking (Section 3.2).

Table 4 The surface compositions of TFC membranes. ID

TFC-0 TFC-1 TFC-2

C 1s (%)

74.03 72.42 71.94

N 1s (%)

O 1s (%)

eNH3+ (401.7 eV)

O]CeN (400.0 eV)

O]CeN (531.2 eV)

O]CeO (532.6 eV)

0.36 0.17 0.10

10.60 10.55 10.16

10.76 10.79 10.17

4.25 6.07 7.63

3.4. Conceptual model for the role of reactable substrate in interfacial polymerization We propose a conceptual model to explain the results obtained in this study and the role of hydroxyl groups contained reactable substrate in the interfacial polymerization (Fig. 5). During the MPD immersion step, greater amount of MPD is loaded on the reactable substrate since the MPD aqueous solution would easily enter into the pores of the more hydrophilic substrates (M1 and M2). In addition, due to presence of polar groups (hydroxyl groups and ester groups) contained in the amphiphilc copolymer chains on the surface of M1 and M2, MPD molecules would be more easily captured by and enrich around the amphiphilc copolymers by the interaction between MPD and polar groups through hydrogen bonding [9,23]. With high MPD loaded amount, a thicker polyamide layer would be formed on the reactable substrates. Meanwhile, the esterification between acyl chloride groups and hydroxyl groups in reactable substrate leads to a denser and thicker polyamide/polyester rejection layer with greatly improved rejection. Furthermore, the amphiphilic copolymers on the reactable substrate may behave as nonionic surfactants [34,39]. A prior study [16] showed that the addition of a nonionic surfactant (Triton X-100) improved the migration of the amine monomer and thereby enhanced the formation of a denser rejection. The obtained results in our study showed similar trends to the previous research on TFC membrane prepared using non-ionic surfactants as aqueous phase additives [16]. In summary, the amphiphilic copolymers in the substrates played the role as (1) MPD enrichment loaders, (2) reactive monomers in aqueous phase and (3) surfactants in reaction process.

Fig. 3. Zeta potential curves of TFC membranes.

Fig. 4. The flux, NaCl rejection and Na2SO4 rejection of the TFC membranes (TFC-3 was prepared on M0 with CMPD = 2% and CTMC = 0.1%).

4. Conclusions

Table 5 Separation properties of the TFC membranes. ID

Water permeability A (×10−11 m/ s Pa)

NaCl permeability BNaCl (×10−7 m/s)

Na2SO4 permeability BNa2SO4 (×10−7 m/s)

BNaCl/ A (kPa)

BNa2SO4/ A (kPa)

TFC-0 TFC-1 TFC-2 TFC-3a

0.30 0.17 0.14 0.17

2.64 0.44 0.21 1.14

1.13 0.25 0.06 0.40

84.9 26.5 14.7 67.5

36.3 13.9 4.6 23.3

a

± ± ± ±

0.07 0.05 0.03 0.01

± ± ± ±

1.01 0.17 0.06 0.08

± ± ± ±

0.47 0.14 0.01 0.11

Amphiphilic copolymers contained reactable substrates were designed and prepared for the investigation on the interfacial polymerization. Based on the reactable substrate, a dense and thick polyamide/polyester rejection layer was formed. The resulting TFC-2 membrane showed greatly enhanced salt rejection. Our study reveals that amphiphilic copolymers in reactable substrate played important role in the interfacial polymerization process, as (1) MPD enrichment loaders, (2) reactive monomers and (3) surfactants. These findings have important implications for the TFC membrane preparations by interfacial polymerization and for the design of high performance RO, NF, forward osmosis (FO) or pressure retarded osmosis (PRO) membranes.

TFC-3 was prepared on M0 with CMPD = 2% and CTMC = 0.1%.

Acknowledgments

Corresponding to the results of flux and salt rejection, the water permeability (A) decreased according to TFC-0 > TFC-1 > TFC-2, the NaCl permeability coefficient (BNaCl) and the Na2SO4 permeability

The study is supported by the General Research Fund (Project 17207514) and the Strategic Research Theme (Clean Energy) at the 5

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Fig. 5. Conceptual model illustrating the role of reactable substrate during interfacial polymerization.

University of Hong Kong. We also appreciated the partial support from the 111 Talent Program, the Programme of Introducing Talents of Discipline to Universities (B13012) and the Haitian Scholar Program from Dalian University of Technology. We acknowledge the help on amphiphilic copolymer synthesis by Dr. Patrick-Henry Toy and Mr. Zhiqi Lao.

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