Ultrathin Polyamide Membranes Fabricated from Free-Standing

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Ultrathin polyamide membranes fabricated from the free-standing interfacial polymerization: Synthesis, modifications and post-treatment Yue Cui, Xiang Yang Liu, and Tai-Shung Chung Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04283 • Publication Date (Web): 21 Dec 2016 Downloaded from http://pubs.acs.org on December 24, 2016

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Industrial & Engineering Chemistry Research

Ultrathin polyamide membranes fabricated from the free-standing interfacial polymerization: Synthesis, modifications and post-treatment

Yue Cui1,2, Xiang-Yang Liu1,3, Tai-Shung Chung*2,4

1

2

Department of Chemistry

Department of Chemical and Biomolecular Engineering, 3

Department of Physics

National University of Singapore, Singapore 117542 4

Water Desalination & Reuse (WDR) Center, King Abdullah University of Science and Technology, 23955-6900 Saudi Arabia

*Corresponding author: Prof. Tai-Shung Chung E-mail address: [email protected] Tel.: +65 65166645 Fax: +65 67791936

1 ACS Paragon Plus Environment


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ABSTRACT The thin film composite (TFC) membrane synthesized via interfacial polymerization is the workhorse of the prevalent membrane technologies such as nano-filtration (NF), reverse osmosis (RO), forward osmosis (FO) and pressure retarded osmosis (PRO) membranes. The polyamide selective layer usually possesses a high selectivity and permeability, making it the heart of this membrane technology. To further improve and understand its formation, with entirely excluding the effect of substrate, an ultrathin membrane which consists of only the polyamide selective layer have been fabricated via free-standing interfacial polymerization between Mphenylenediamine (MPD) and trimesoylchloride (TMC) in this study. The influences of monomer concentration on polyamide layer formation is firstly examined. Different from previous studies which indicated the variation of MPD concentration might affect the polyamide layer formation even when in excess, the MPD concentration when in excess does not affect membrane properties significantly, while increasing the TMC concentration gradually densifies the polyamide layer and enhances its transport resistance. Adding lithium bromide (LiBr) and sodium dodecyl sulfate (SDS) in MPD solutions are found to facilitate the reaction between the two phases and result in a significant improvement in water permeability. However, a high amount of additives leads to an augmentation in transport resistance. The N, Ndimethylformamide (DMF) treatment on the polyamide membrane shows pronounced improvements on water flux under FO tests and water permeability under RO tests without compromising reverse salt flux and salt rejection because the dense polyamide core stays intact. This study may offer a different perspective on membrane formation and intrinsic properties of the polyamide selective layer and provide useful insights for the development of next-generation TFC membranes.


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Key words: polyamide membrane, free-standing interfacial polymerization, intrinsic properties, additives, solvent treatment



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INTRODUCTION The demands on clean water and energy are increasing globally because of rapid population growth, highly fluctuated oil price and extensive freshwater contamination1-4. Among many potential solutions, advances in membrane technology are one of the most direct, effective and feasible approaches to solve these two issues. Membranes are proven technologies that have been widely used, such as to recycle water and desalinate seawater

5-9

, to purify and produce clean

energy 10-12 and in gas separation 13, 14. Among various membranes, the thin film composite (TFC) membrane synthesized from interfacial polymerization displays the utmost importance because it is the dominant technology to fabricate reserve osmosis membranes for the current seawater desalination

15-17

. It is also widely used to design membranes for nanofiltration (NF)

forward osmosis (FO) production

26, 27

21-23

, pressure retarded osmosis (PRO)

16, 18-20

,

11, 24, 25

, biofuel and energy

. Therefore, the TFC membrane can be considered as the workhorse of the

current membrane technologies.

Generally, a TFC membrane is synthesized by interfacial polymerization between a diamine monomer dissolved in an aqueous phase and an acid chloride monomer dissolved in a non-polar organic phase

16, 28

. As a result, an ultra-thin cross-linked polyamide layer is formed at the

interface between the two immiscible phases. The formation of the polyamide layer proceeds as a succession of several different kinetic regimes, including insipient film formation, slowdown, and diffusion-limited growth. A loose polyamide layer firstly emerges at the incipient stage, followed by the densification of its core part. The film growth will abruptly decelerate if the permeability for both monomers become low, while it will undergo a smooth transition from the incipient stage to the diffusion-limited stage if the membrane is permeable


28, 29

. To improve the

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TFC membrane performance, many studies have been conducted to optimize and modify the reaction. For example, different monomers were utilized to adjust the membrane pore size and surface properties

30-34

. Various monomer concentrations and diamine/acid chloride ratios were

also explored 35-37. In addition, a variety of additives, such as surfactants 38, 39 and nanoparticles 16, 40

, were added into the monomer solutions to facilitate monomer diffusion from one phase to the

other phase in order to improve the formation and separation performance of the polyamide selective layer. Alternatively, solvent treatments were reported to be an effective method to leach out low molecular weight fragments from the polyamide layer and facilitate water transport 41-43. Besides, significant efforts have been made to understand the fundamentals of transport mechanisms across the polyamide selective layer 44-48.

Despite of tremendous research efforts, the intrinsic properties of the polyamide selective layer are still not fully understood. This is because most previous works were carried out directly on top of porous substrates, thus the influence of substrates could not be completely excluded. Therefore, in this study, an ultrathin polyamide membrane which consists of only the polyamide selective layer was fabricated via direct interfacial polymerization between the two monomers (i.e., M-phenylenediamine (MPD) and trimesoylchloride (TMC)). In addition, modifications such as addition of additives and solvent treatment were adopted to investigate their influences on membrane properties. Then the ultrathin polyamide membranes were tested under FO processes and RO processes in order to understand the transport mechanisms of water and salt across the membranes. Lastly, the physicochemical properties of polyamide membranes and their effects on membrane performance were systematically analyzed with the aid of advanced analytical tools. This study may not only reveal the basic properties of the polyamide membranes



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in terms of transport mechanism and separation performance, but also provide useful insights on the design of next-generation TFC membranes for clean water and energy applications.

MATERIALS AND METHODS Materials M-phenylenediamine (MPD, >99%), trimesoylchloride (TMC, >98%), sodium dodecyl sulfate (SDS, >99%) and lithium bromide (LiBr) were ordered from Sigma-Aldrich and used in the interfacial polymerization reaction. N, N-Dimethylformamide (DMF, >99.8%) acquired from Sigma-Aldrich was utilized to post-treat the polyamide membranes. Sodium chloride (Merck, Germany) was employed to characterize transport properties and determine membrane FO performance. A commercial nuclepore polycarbonate (PC) track-etched membrane (Whatman, UK) was utilized to support the polyamide membrane during the tests of transport properties while a non-woven fabric support was used in the FO tests. It should be noted that both the tracketched membrane and non-woven fabric mainly provide the mechanical support and are unlikely to impact the membrane performance due to their large openings. Deionized (DI) water was produced by a Milli-Q ultrapure water system (Millipore, USA). All chemicals were used as received.

Free-standing Interfacial Polymerization of the Polyamide Membranes The formation of the ultrathin polyamide membrane was achieved by a free-standing interfacial polymerization reaction between the MPD aqueous and TMC hexane solutions. No substrate was utilized during the membrane formation. As illustrated in Fig. 1(a), a 20 mL MPD aqueous solution containing additives (if any) was firstly poured into a crystalizing dish and allowed to


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stand still to stabilize the liquid surface. Subsequently, a 15 mL TMC hexane solution was added on the top surface of the MPD solution dropwise. The crystalizing dish was then covered with a lid. The growth of the thin film was clearly observed at the interface. After an overnight reaction, the crystalizing dish was drained and only a thin film remained in the crystalizing dish. It should be noted that since hexane evaporates quickly, the overnight duration only refers to the time taken for the complete evaporation of the hexane solution, which is more than 5 h. The resultant thin film was rinsed several times by ultrapure water to remove the excess monomers and preserved in the ultrapure water for further usage.

Addition of Additives and Post-treatment of the Polyamide Membranes SDS and LiBr were used as the additives for the interfacial polymerization reaction. Depending on weight ratio, various additive concentrations were added into the MPD aqueous solution following the aforementioned procedure. The resultant membranes were preserved in DI water overnight to remove the residual additives.

Since DMF has a similar Hildebrand solubility parameter to the polyamide membrane

41, 42

, it

was employed to post-treat the membrane. However, the treated membrane became so soft that defects might form during the post-treatment and handling, a 50 wt. % aqueous DMF solution was therefore used instead. For the post-treatment, the fabricated polyamide membranes were immersed into the aqueous DMF solution for 0.5 h before being rinsed thoroughly by DI water to remove any residues and dissolved portions of the membranes. Finally, they were kept in DI water for further usage.



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Water Reclamation through Forward Osmosis FO experiments were conducted on a lab-scale static FO cell 49-51, as illustrated in Fig. 1 (b). One piece of non-woven fabric was placed underneath the membrane such that the polyamide film can be perfectly transferred to the non-woven fabric when the non-woven was lifted up. The whole piece was then clamped between the two solution chambers and the polyamide membrane directly contacted with the salty solution. One of the chambers was then filled with 0.035 L ultrapure water as the feed, while the other with 0.035 L 1M NaCl as the draw solution. The opening of the chamber had a round shape with a diameter of 2 cm. Owing to the concentration gradient between the two solutions, water would spontaneously permeate across the membrane from the feed to the draw solution. An increase in volume would be observed in the draw solution chamber. On the other hand, NaCl would diffuse through the membrane and increase the NaCl concentration in the feed chamber. After stabilizing for 0.5 h, samples of 2 mL were taken out from both chambers and their NaCl concentrations were measured by a conductivity meter (Metrohm, Swiss). The entire test duration was 1.5 h with continuous stirring to minimize the concentration polarization (CP). The solution weights and volumes in both chambers were recorded before and after the test, and the feed conductivity was re-measured at the end of each experiment. The water flux (Jw, L·m-2·h-1, LMH) of the membrane was calculated based on Eq.1: ‫ܬ‬௪ =

∆௠ ଵ

(1)

∆௧ ஺೘

where Am is the effective area of the membrane, ∆m (g) is the average of the absolute weight loss in the feed solution and gain in the draw solution, and ∆t (h) is the test duration, which is 1.5 h. The reverse salt flux (Js, g·m-2·h-1, gMH) of the draw solution was calculated from the conductivity increment in the feed solution based on Eq.2: ‫ܬ‬௦ =

∆஼೟ ௏ ଵ

(2)

∆௧ ஺೘


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where ∆Ct (g/L) and V(L) are the changes of salt concentration and feed solution volume, respectively.

Characterizations of the Polyamide Membranes Water and salt transport properties of the polyamide membranes Water permeability, A (L·m-2·h-1·bar-1, LMH/bar), and salt rejection, R (%), of the polyamide membranes were determined under a RO process by testing the membranes under a transmembrane pressure, ∆P, of 5.0 bar in dead-end cells at room temperature, as described in our previous studies 43, 52. In the test, a PC track-etched membrane was utilized as the support for the polyamide membrane to withstand the pressure.

The pure water permeability (PWP) was calculated as per Eq.3: ‫ܬ‬௪ =

∆௠

ଵ

(3)

∆௧ ஺೘ ∆௉

where ∆m (g) is the absolute weight loss in the feed solution, ∆t (h) is the test duration, Am is the effective area of the testing cell, and ∆P (bar) is the applied trans-membrane pressure. In the salt rejection test, a 1000 ppm NaCl solution was used as the feed solution. The salt rejection R was calculated based on the given Eq. 4: ஼೛

ܴ = ൬1 − ஼ ൰ × 100%

(4)

೑

The concentrations of NaCl in the feed (Cf) and permeate (Cp) were determined by a conductivity meter.

Membrane morphology



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The membrane morphology was examined by field emission scanning electron microscopy (FESEM). First, the membrane was dried with a freeze-dryer and fractured in liquid nitrogen. After being coated with platinum by a Jeol JFC-1100E Ion Sputtering device, the membrane morphology was observed using a FESEM (JEOL JSM-6700). The membrane thickness was roughly estimated using the reference bar in the FESEM images.

RESULTS AND DISCUSSION Influence of monomer concentrations on the polyamide membranes Since the interfacial polymerization is initiated by a diamine monomer (i.e., MPD) in the aqueous phase reacting with an acid chloride monomer (i.e., TMC) in the organic phase at the interface, their concentrations may affect the polymerization reaction and the formation of the ultra-thin polyamide layer. Thus, it is crucial to investigate their influences on polyamide layer formation.

1. Influence of the MPD concentration During the interfacial polymerization, a highly asymmetric solubility of the monomers in the two phases is assumed

29

. The partitioning of TMC into the aqueous phase is negligible, whereas

MPD is soluble in both the organic and aqueous phases 29. Hence, only MPD would diffuse from the aqueous phase to the organic phase while the transport of TMC in the aqueous phase is negligible. As a result, we firstly investigate the role of MPD by varying its concentration from 1 to 4 wt. % with a constant TMC concentration of 0.1 wt. % (thereafter, % is used instead of wt. %). As illustrated in Fig. 2(a), the water flux in FO tests fluctuates within a narrow range of around 6 LMH when the MPD concentration increases from 1% to 4%. On the other hand, the


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reverse salt flux and the ratio of salt flux to water flux (i.e., Js/Jw) remain relatively constant at 0.3 gMH and 0.05, respectively. These results suggest that there is no significant difference in FO performance among these polyamide membranes when the MPD concentration varies from 1% to 4%. In addition, Table 1 summarizes the water and salt transport properties of the polyamide membranes under the RO mode as a function of MPD concentration. The water permeability slightly decreases from 1.25 LMH/bar to 0.9 LMH/bar when the MPD concentration increases from 1% to 4% while the salt rejection increases slightly from 90% to 92.7%. The aforementioned observations are contradictory to those previous findings when fabricating polyamide membranes on a substrate where the MPD concentration always affects the membrane performance even when it is in excess

37, 53

. This implies that some of the MPD might be

adsorbed by the substrate and the MPD diffusion from the substrate pores may affect the thin film formation. Thus, extra MPD is needed in the thin film formation when forming a polyamide layer on a substrate.

Similarly, no visible difference in top surface morphology is observed for these polyamide membranes synthesized from different MPD concentrations, as exhibited in Fig. 3. All membranes exhibit a similar ridge-and-valley top surface morphology. Previous studies have revealed that the ridge-and-valley morphology is likely resulted from rapid diamine diffusion from the substrate pores to the organic phase when a substrate is used 48, 54. As a result, a rougher surface morphology would be produced if the substrate pores are relatively large

48, 54

.

Nevertheless, the results of this study suggest that the presence of pores is not a necessary condition for the formation of the ridge-and-valley structure. The surface morphologies may be explained as follows. In the initial phase, only a limited amount of polyamide oligomers is



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formed at the interface and hence, the nascent film is not a completely continuous phase. Consequently, the MPD, together with any nascent polyamide oligomers, would keep diffusing from the interface into the TMC organic phase through the defects present in the loose incipient film until the formed polyamide layer is dense and thick enough to cease further diffusion. As a result, the ridge-and-valley morphology is observed on the top surface of the polyamide film.

On the other hand, the polyamide membranes have different bottom surfaces from those TFC membranes fabricated on a support layer

48

. The former has a relatively smooth bottom

morphology consisting of a few pits, while the latter is fully porous. This might be due to the fact that the polyamide membrane is synthesized via directly diffusion and convection of MPD to the organic phase, while the TFC membrane fabricated on a support is via limited transport channels for MPD diffusion from the substrate pores54. Consequently, the polyamide film has two parts of surface morphology; namely, a dense symmetric layer at the bottom (i.e., near the MPD side) and a ridge-and-valley morphology on the top (i.e., near the TMC side), as displayed in Fig. 3. The thickness of the bottom dense layer is around 100 nm, while the size of the ridge-and-valley structure varies. Comparing with the selective layer of TFC membranes fabricated on a substrate 47, 48

, the polyamide membrane is thicker. As a result, the latter has a relatively low water flux

and reverse salt flux than the former.

It is interesting to notice that the water flux and the reverse salt flux of the polyamide membrane under the FO mode and the PWP under the RO mode are all lower than the ones fabricated with substrates

21, 24

. This may be attributed to the different structure formed during the two different

fabrication processes. As observed from the membrane morphology, the bottom part of the


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polyamide membranes is a dense thin film, while the selective layers of the TFC membranes fabricated with substrates always possess more ridge-and-valley structure which are full of void structure 47, 48. Thus, the polyamide membrane has much higher transport resistance than the TFC membranes fabricated with substrates. The relatively low salt rejection (~90%) of the polyamide membrane under the RO mode is probably resulted from its dense structure. The transport resistance of NaCl is relatively high to begin with and the denser membrane will not increase its transport resistance significantly. On the other hand, the transport of water molecules may be more sensitive to the densification of the membrane, it may reduce significantly with an increase in membrane densification. As a result, the dilution effect is less significant and the rejection to NaCl appears to be relatively low. This suggests that an overly dense structure is actually not preferred for TFC membranes. The transport properties of the TFC membranes should be optimized in practical applications to reject the salt ions but maximize the passage of water molecules.

For the polyamide membranes, since the TMC concentration is 0.1% and MPD is in excess when its concentration varies from 1% to 4%, the TMC concentration is the limiting factor in the polymerization reaction

37

. Thus, although the diffusion of MPD to TMC might be accelerated

with an increase in MPD concentration, the reaction rate is not influenced. As a result, the resultant polyamide layer is not significantly affected by the MPD concentration. Because the RO and FO performance as well as the surface morphology are quite similar for these polyamide membranes fabricated from different MPD concentrations, it is reasonable to conclude that the variation in MPD concentration does not have significant influences on membrane formation.



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2. Influence of the TMC concentration As aforementioned, the TMC concentration is the limiting factor for the interfacial polymerization. Thus, its influence on membrane formation is investigated as follows: the MPD concentration is fixed at 2%, while the TMC concentration varies from 0.05% to 0.2%. As illustrated in Fig. 2(b), the FO water flux of the resultant TFC membrane drops from 12.5 LMH to around 6 LMH when the TMC concentration increases from 0.05% to 0.1%. The water flux remains at around 6 LMH when the TMC concentration is raised to 0.15%. A further increase in TMC concentration to 0.2%, the water flux slightly decreases to around 5 LMH. On the other hand, the reverse salt flux varies insignificantly; it decreases slightly from 0.11 gMH to 0.05 gMH with an increase in TMC concentration. Meanwhile, a high selectivity of water over salt is maintained with Js/Jw values constantly lower than 0.03.

As shown in Table 1, the water permeability of the polyamide membrane is as high as 2.46 LMH/bar when the TMC concentration is 0.05% while the salt rejection is slightly low (i.e., 87.8%). The PWP decreases to 0.95 LMH/bar when the TMC concentration increases to 0.1% and remains at 0.9 LMH/bar with a further increment in TMC concentration to 0.15%. This value further decreases to 0.52 LMH/bar when the TMC concentration reaches 0.2%. In contrast, the salt rejection of the polyamide membranes exhibits an opposite trend where it firstly increases with an increase in TMC concentration and eventually plateaus off at around 90%.

The effect of TMC concentration on membrane morphology is illustrated in Fig. 4. Basically, all membranes have a typical ridge-and-valley top surface and a smooth bottom surface regardless of TMC concentrations. However, when the TMC concentration increases from 0.05% to 0.2%,


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the number of ridge-and-valley increases and the top surface becomes rougher. The top surface is only partially covered by the ridge-and-valley structure when the TMC concentration is relatively low at 0.05%, while it is almost fully covered by the ridge-and-valley structure when the TMC concentration reaches 0.2%. In contrast, the bottom surfaces and cross-sectional morphology do not change significantly as the TMC concentration increases. All membranes have a similar dense layer thickness with an increase in TMC concentration, whereas the number of the ridge-and-valley structure has increased significantly.

The influence of the TMC concentration may be explained as follows. Given that the TMC concentration is the limiting factor for the reaction, an increase in TMC concentration should accelerate the thin film formation. At a low TMC concentration (i.e., 0.05%), the rate of thin film formation might be relatively slow and there might be difficult to form a uniform dense polyamide film 29,

55

. Therefore, the resultant membrane is less tight and has less transport

resistance. As a result, it has a relatively high water flux and reverse salt flux. On the other hand, when the TMC concentration increases, the formation of the dense layer is more rapid and complete since a greater amount of MPD is able to migrate to the organic phase and react with TMC

25

. The resultant membrane possesses more ridge-and-valley morphology but with a tight

dense-layer structure. Consequently, its water flux and reverse salt flux decreases with a higher concentration of TMC. Similarly, its pure water permeability declines while the salt rejection is improved.

In conclusion, the TMC concentration has greater effects than the MPD concentration on membrane formation and performance. Since the polyamide membrane synthesized from 2%



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MPD and 0.1% TMC appears to be more desirable in terms of water flux and selectivity, subsequent investigations are carried out on this formulation.

Influence of additives on polyamide membranes Additives such as LiBr and SDS have been reported to promote the MPD diffusion to the TMC organic phase 24, 56 and greatly enhance the reaction. In both cases, reactions are no longer limited to the interface between the aqueous MPD and organic TMC phases. Hence, it is crucial to investigate the influence of additives on the formation of free-standing polyamide membranes and their properties.

1. The addition of LiBr LiBr was firstly added in the MPD solution owing to its ability to facilitate the interaction between the two phases

57

. Fig. 5 (a) shows the FO performance of the resultant membrane as a

function of LiBr concentration. The water flux firstly increases with the LiBr addition and reaches a peak value of 9.5 LMH at 1% LiBr. However, when the LiBr concentration is further increased to 2%, the water flux decreases to 7.4 LMH. On the other hand, the reverse salt flux slightly reduces and reaches a plateau at about 0.1 gMH when the LiBr amount is further increased. The selectivity of water over salt also remains high with a Js/Jw value lower than 0.01 for all membranes under this study with various LiBr concentrations.

Fig. 6 illustrates the effects of LiBr on membrane morphology. As mentioned in the previous section, the top surface of the pristine membrane consists of a ridge-and-valley morphology and a smooth and continuous bottom surface, while its cross-section has a dense continuous


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morphology with isolated ridge-and-valley structures on top. With an increase in LiBr concentration (i.e., from 0.1% to 0.5%), the number of ridge-and-valley structure increases and the top surface becomes rougher. However, the surface morphology gradually evolves into a “flake-like” morphology with a further increase in LiBr concentration to 1%. Eventually, the flakes on the surface grow and merge with each other to form another dense layer at 2% LiBr. On the other hand, the bottom surface morphology gradually becomes more crumpled with more pits and wrinkles. When the LiBr concentration is further increased to 2%, holes can even be observed on the bottom surface. Synchronously, a significant increase in the amount of ridgeand-valley structure can be discerned in the cross-section. It appears that a high LiBr concentration of around 1% initiates the merging of these ridge-and-valley structures and forms another dense layer on their top.

Therefore, LiBr may have the following influences. First, LiBr promotes the MPD diffusion to the organic phase and facilitates the formation of the polyamide layer. Second, the addition of LiBr will probably enable the TMC diffusion from the organic phase to the aqueous phase because LiBr is able to form a hydrophilic complex with the carbonyl (C=O) group of TMC molecules 57. These two factors would result in a larger reaction zone between MPD and TMC. As a result, there is a morphology change on the bottom surface morphology because the interface between MPD and TMC solutions become obscure. Meanwhile, the complexation between LiBr and TMC may promote the hydrolysis of TMC and result in a more hydrophilic and permeable polyamide layer with a higher water flux

57

. However, when the LiBr

concentration exceeds 1%, the reaction zone is extensively enlarged. The flakes gradually



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emerge and form multiple layers that decline the water flux because of higher transport resistance.

2. The addition of SDS Fig. 5(b) illustrates the FO performance of the resultant membranes synthesized from different SDS concentrations. The water flux shows a convex trend as the SDS concentration increases. When a small amount of SDS is added into the MPD solution (0.1%), the water flux soars to 11.5 LMH, which is more than twice of the value of the pristine membrane, but it starts to decline with a further increment in SDS concentration, reaching 6.5 LMH at 2% SDS. The reverse salt flux, however, does not vary much with the SDS addition. Consequently, the enhancement in selectivity of water over salt is procured with a value of Js/Jw constantly lower than 0.01. Clearly, the SDS is able to significantly enhance the membrane’s FO performance.

The influence of SDS on transport properties under the RO mode is then investigated, as illustrated in Table 2. The pristine polyamide membrane has a PWP slightly below 1 LMH and a salt rejection marginally lower than 90%. However, with the addition of a small amount of SDS (0.1%), the PWP soars to 1.7 LMH/bar, which is almost twice of that obtained from the pristine TFC membrane. The enhancement in PWP exhibits a similarity with the water flux tested under the FO mode: it gradually drops with a rise in SDS concentration. On the contrary, the salt rejection keeps escalating with increasing SDS concentration. The salt rejection improves from 89.8% to 94.57% with 0.1% SDS addition, and it rises further to 97% when 2% SDS is added into the MPD solution.


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The membrane morphology as a function of SDS concentration is exhibited in Fig. 7. Compared with the pristine polyamide membrane, the morphology of the polyamide membranes has been significantly changed with the SDS addition into the MPD solution. Unlike the pristine one, the polyamide membranes possess highly rough top surfaces full of multi-layer ridge-and-valley structure even with a small amount of SDS (i.e., 0.1%). Meanwhile, the cross-section of the multi-layer ridge-and-valley structure is fully porous with multi-layers of voids and a greater thickness. In addition, the thickness of the polyamide membrane has been largely increased with the addition of SDS. Interestingly, a highly porous bottom surface is also observed. This morphology does not change significantly with a further increment in SDS concentration. It always has a rough top surface, multi-layer porous structure and fully porous bottom surface.

The addition of SDS in monomer solutions has been reported to influence both the diffusion of monomers between the aqueous and organic phases as well as the extent of interfacial polymerization

38, 58-60

. Owing to its amphiphilicity, the presence of SDS in the MPD solution

facilitates the MPD diffusion to the organic phase and remarkably enhances the polyamide formation. Since MPD keeps diffusing into the organic phase through the nascent film, it results in multi-interfaces. Meanwhile, the rapid MPD diffusion gradually shifts the reaction interfaces further to the organic phase. As a consequence, the bottom surface becomes discontinuous with large pores on it. Although MPD continues to diffuse to the organic phase, less MPD is able to diffuse through the nascent film, leading to less reaction sites and larger voids inside the ridgeand-valley structure

47, 48

. Therefore, compared with the pristine TFC membrane, the water flux

doubles without increasing the reverse salt flux under FO tests when the SDS concentration is low (0.1%). Similarly, the water permeability under RO tests also increases with an improved



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salt rejection because of the reduced transport resistance and discontinuity in the bottom surface 24

. However, the thin dense core would gradually shape inside the initially formed loose film

during the incipient film formation and eventually form the sandwich structure 29. Thus, a further increment in SDS concentration may possibly result in a denser or thicker core portion. Consequently, the water flux and water permeability would start to decline, whereas the salt rejection will increase.

In conclusion, the addition of an optimal amount of SDS into the MPD solution is able to promote the interaction between aqueous and organic phases and significantly improve the membrane transport properties. High concentrations of SDS, however, would lead to excessive transport resistance. Meanwhile, although the FO and RO performance have been improved, a thicker polyamide membrane may induce additional transport resistance. Thus, the reaction duration should be carefully optimized in order to obtain better performance. Since the addition of SDS into the MPD solution gives superior membrane performance compared to LiBr, SDS is utilized as the additive in the subsequent study.

Influence of the DMF treatment on polyamide membranes Solvent treatment, which is an effective method to improve TFC membrane properties

41-43

, is

also applied in this study. The polyamide membranes fabricated using different SDS concentrations in the MPD solution, are treated with a 50% DMF aqueous solution for 30 min. After being rinsed thoroughly to remove any residues, their FO performance is determined and displayed in Fig. 8. Compared with Fig. 5(b), a significant increment in water flux is observed after the solvent treatment. For example, water flux increases from 5.4 LMH to 9.8 LMH for the


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pristine membrane without any additives; it is an increment of 81.5%. Similarly, water flux increases over 40% for the membranes fabricated from the MPD solution containing SDS. Surprisingly, the polyamide membrane fabricated from the MPD solution containing 2% SDS almost doubles its water flux after the DMF treatment. Notably, the reverse salt flux does not pronouncedly increase after the DMF treatment. As a result, the selectivity of water over salt of the membrane remains at a Js/Jw value lower than 0.01. This suggests that the DMF treatment reduces the transport resistance for water while maintaining the selectivity.

To further delve into the effects of DMF on the membrane properties, the water and salt transport properties under the RO mode are evaluated, as listed in Table 2. Compared to membranes without the DMF treatment, a pronounced increment in water permeability is observed after the DMF treatment. For example, the PWP of the pristine membrane rises from 0.95 LMH/bar to 1.22 LMH/bar after undergoing the DMF treatment, which corresponds to an increment of 28%. For the membrane fabricated with 0.1% SDS, the PWP after the DMF treatment jumps from 1.74 to 2.31 LMH/bar. Moreover, it is noticeable that more than 24% increment in water permeability is obtained after the DMF treatment on both of TFC membranes fabricated with 1 and 2% SDS. These results indicate that the water transport resistance across the membranes is reduced after the DMF treatment. On the other hand, the salt rejections of all four tested membranes are still higher than 93%. Compared with the membranes without the DMF treatment, no significant loss in salt rejection is observed, suggesting that the transport resistance for salt is maintained.

The surface morphologies before and after the DMF treatments are displayed in Fig. 7 and Fig.S1. There are no observable differences before and after the treatment. This indicates that the



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comprehensive structures of the TFC membranes are preserved even after the DMF treatment.

Since water transport properties are improved while the salt rejection is maintained, the effect of the DMF treatment on polyamide membranes may be explained by the sandwich-structured polyamide layer. It is believed that the polyamide layer possesses an ultrathin dense core (i.e., the actual selective barrier) sandwiched between two looser polymer structures

29, 55

. Owing to

the similarity between in solubility parameter between DMF and MPD-TMC polyamide (i.e., 24.8 MPa1/2 vs. 23 MPa1/2), the loose parts, inclusive of low molecular weight molecules and loosely crosslinked polyamide fragments, would be dissolved and washed away by the DMF treatment. However, the integrity of the core part would not be compromised by the DMF treatment due to its high density and molecular weight. As a consequence, the polyamide membrane treated with DMF not only exhibits an enhanced water flux without suffering from an escalated reverse salt flux in FO tests, but also possesses a higher the water permeability without sacrificing the salt rejection. Thus, it can be concluded that the DMF treatment is a promising method to improve the performance of TFC membranes possessing a polyamide selective layer.

CONCLUSION In this study, we have investigated the intrinsic properties of polyamide membranes which consist of only the polyamide selective layer fabricated via free-standing interfacial polymerization. The influences of monomer concentration, additives and DMF solvent treatment on the polyamide membranes were explored. The following conclusions can be drawn: 1. Differently from previous studies, the TMC concentration has greater effects than the MPD concentration on membrane formation and performance when MPD is in excess.


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Owing to the excess concentration of MPD compared to TMC, the increment in MPD concentration does not demonstrate significant influences on membrane properties. 2. As the limiting factor of the interfacial polymerization, the TFC membrane evolves from a less uniform and loose film with low water and salt transport resistance to a denser film when increasing the TMC concentration from 0.05% to 0.2%. As a result, the water flux gradually decreases and the membrane surface becomes rougher with more ridge-andvalley structures when increasing the TMC concentration. 3. Additives added into the MPD solution, including LiBr and SDS are able to promote the interaction between aqueous and organic phases and thus largely improve the membrane transport properties. However, overly high concentrations of additives will lead to excessive transport resistance. Thus, a low concentration of additive is preferred to modify the TFC membranes. 4. The DMF treatment on the TFC membrane will remove the loose portions of the polyamide layer but leave the core part untouched. Thus, the transport resistance is pronouncedly reduced while the selectivity is maintained. In the FO performance, the water flux could be doubled with the DMF treatment. The increment of water permeability can be as high as 74.5%, without compromising the salt rejection.

SUPPORTING INFORMATION Figure S1. The typical surface morphologies of the polyamide membranes post-treated with DMF aqueous solution

ACKNOWLEDGEMENT



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This research is supported by the National Research Foundation- Prime Minister's office, Republic of Singapore under its Competitive Research Program entitled “Advanced FO Membranes and Membrane Systems for Wastewater Treatment, Water Reuse and Seawater Desalination” (Grant numbers: R-279-000-336-281 & R-279-000-339-281).


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FIGURES Fig. 1. Schematic diagram of (a) the fabrication process of the polyamide membrane and (b) the static FO permeation cell Fig.2 FO performance as a function of (a) MPD concentration and (b) TMC concentration Fig. 3. Typical surface morphologies of the polyamide membranes fabricated from different MPD concentrations Fig. 4. Typical surface morphologies of the polyamide membranes fabricated from different TMC concentrations Fig.5 The effects of additives (a) LiBr and (b) SDS on FO performance Fig.6. Typical surface morphologies of the polyamide membranes fabricated from MPD containing different LiBr concentrations Fig. 7. Typical surface morphologies of the polyamide membranes fabricated from MPD containing different SDS concentrations Fig.8 The effect of DMF treatment on FO performance TABLES Table 1. Transport properties of the polyamide membranes with different monomer concentrations Table 2. Transport properties of the polyamide membranes with different treatments



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Table of Content



1 2 3 4 5 6 7 8 9 (a) 10 11 12 13 14 15 MPD Solution 16 17 18 19 20 21 Certain duration 22 23 Drain 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

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(b) Add the TMC solution on top

Mechanic stirrer Membrane

TMC in hexane MPD solution

Draw solution

Feed solution

Immersion in water

A polyamide membrane in water

Figure 1. Schematic diagram of (a) the fabrication process of the polyamide membrane and (b) the static FO permeation cell

ACS Paragon Plus Environment

(a)

0 1

2

3

4

0.10

0.05

0.00

Water flux (LMH) Reverse salt flux (gMH) Js/Jw

10

1

5

0.20

0.15

Js/Jw

1

15

0.15

2

(b)

Reverse salt flux (gMH)


20

0.20

Js/Jw

Water flux (LMH)

2


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Water flux (LMH)

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0.10

0.05

0

0 0.05

MPD concentration (%)

0.10 0.15 TMC concentration (%)

0.20

Figure 2. FO performance as a function of (a) MPD concentration and (b) TMC concentration


0.00


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

1%MPD

2%MPD

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4%MPD

Top X 15K

Bottom X 15K

Crosssection X 30K

Figure 3. Typical surface morphologies ofParagon the polyamide membranes fabricated from different ACS Plus Environment MPD concentrations

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1 2 3 4 5 6 Top 7X 15K 8 9 10 11 12 13 14 15 16 17 Bottom 18 19X 15K 20 21 22 23 24 25 26 27 28 29 Cross-section 30 31 X 30K 32 33 34 35 36 37 38 39 40 41 42


0.05% TMC

0.1% TMC

0.15% TMC

0.2% TMC

Figure 4. Typical surface morphologies of the polyamide membranes fabricated from different ACS Paragon Plus Environment TMC concentrations


Water flux (LMH)

1.0 1.5 LiBr concentration (%)

2.0

0.10

0.05

0.00

(b)

2


15

10

1

5

0.20

0.15

Js/Jw

0.5

0.15

Js/Jw

0

0.20



1

0 0.1

20

2

Water flux (LMH)

(a)


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0.10

0.05

0

0 0 0.1

0.5

1.0

1.5

SDS concentration (%)

Figure 5. The effects of additives (a) LiBr and (b) SDS on FO performance


2.0

0.00

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1 2 3 4 5 Top 6 X 15K 7 8 9 10 11 12 13 14 15 16 17 Bottom 18 X 15K 19 20 21 22 23 24 25 26 Cross27 28 section 29 X 30K 30 31 32 33 34 35 36 37 38 39 40 41 42


0 LiBr

0.1% LiBr

0.5% LiBr

1% LiBr

2% LiBr

Figure 6. Typical surface morphologies of the polyamide membranes fabricated from MPD containing different LiBr concentrations ACS Paragon Plus Environment

1 2 3 4 5 6 Top 7X 15K 8 9 10 11 12 13 14 15 16 17 Bottom 18 19X 15K 20 21 22 23 24 25 26 27 28 29 Cross-section 30 31 X 30K 32 33 34 35 36 37 38 39 40 41 42


No SDS

0.1 % SDS

1% SDS

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2% SDS

Figure 7. Typical surface morphologies of the polyamide membranes fabricated from MPD ACS Paragon Plus Environment containing different SDS concentrations

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Water flux (LMH) 20

2


0.20

Js/Jw

10

1

5

0.15

Js/Jw

Water flux (LMH)

15


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0.10

0.05

0

0 0 0.1

0.5

1.0

1.5

2.0

SDS concentration (%)

Figure 8. The effect of DMF treatment on FO performance ACS Paragon Plus Environment

0.00


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Table 1. Transport properties of the polyamide membranes with different monomer concentrations

MPD concentration

Concentration

PWP (LMH/bar)

Salt Rejection (%)

1%

1.25±0.05

90.42±0.2

2%

0.95±0.02

89.80±0.10

4%

0.90±0.09

92.66±0.69

0.05 %

2.46± 0.28

87.8±0.48

0.1%

0.95±0.02

89.80±0.10

0.15%

0.90±0.05

92.3±0.44

0.2%

0.52±0.01

90.42±0.2

TMC concentration


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Table 2. Transport properties of the polyamide membranes with different treatments W/O DMF treatment Concentration

With DMF treatment

PWP (LMH/bar)

Salt Rejection (%)

PWP (LMH/bar)

Salt Rejection (%)

0%

0.95±0.02

89.80±0.10

1.22 ±0.02

93.81±0.02

0.1 %

1.74±0.02

94.57±0.79

2.31±0.03

96.08±0.09

1%

1.46±0.04

96.46±0.07

1.81±0.16

96.67±0.28

2%

1.02±0.09

97.18±0.32

1.78±0.04

95.26±0.08

SDS concentration


Ultrathin Polyamide Membranes Fabricated from Free-Standing

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