Incorporation of layered double hydroxide nanofillers in polyamide nanofiltration membrane for high performance of salts rejections

Journal of the Taiwan Institute of Chemical Engineers 97 (2019) 1–11

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Incorporation of layered double hydroxide nanofillers in polyamide nanofiltration membrane for high performance of salts rejections Muhammad Hanis Tajuddin a,b, Norhaniza Yusof a,b,∗, Norfadhilatuladha Abdullah a,b, Muhammad Nidzhom Zainol Abidin a,b, Wan Norharyati Wan Salleh a,b, Ahmad Fauzi Ismail a,b, Takeshi Matsuura c, Nur Hanis Hayati Hairom d, Nurasyikin Misdan d a

Advanced Membrane Technology Research Centre (AMTEC), Universiti Teknologi Malaysia, 81310 Skudai, Johor Bahru, Malaysia School of Chemical and Energy Engineering (SCEE), Faculty of Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor Bahru, Malaysia Department of Chemical and Biological Engineering, Industrial Membrane Research Institute, University of Ottawa, 161 Louis Pasteur St., Ottawa, ON K1N 6N5 Canada d Faculty of Engineering Technology, Universiti Tun Hussein Onn Malaysia, 86400 Parit Raja, Johor, Malaysia b c

a r t i c l e

i n f o

Article history: Received 27 June 2018 Revised 23 December 2018 Accepted 19 January 2019 Available online 5 February 2019 Keywords: Layered double hydroxide Polyamide Thin film composite membrane Nanofiltration

a b s t r a c t In this work, thin film composite (TFC) membrane embedded with self-synthesized layered double hydroxide (LDH) nanofillers was prepared via interfacial polymerization technique for nanofiltration (NF) process. Prior to filtration experiments, the morphologies and physicochemical properties of the prepared LDH nanofillers and TFC membranes were characterized using TEM, XRD, FESEM, FTIR, AFM, zeta potential analyzer and contact angle goniometer. The results revealed that the self-synthesized LDH nanofillers possessed layered structured materials with typical hexagonal plate-like shape. Meanwhile, it was found that the membrane contact angle decreased remarkably from 53° to 33.96° upon addition of 0.1 wt% LDH nanofillers in the active polyamide layer. TFN membrane of 0.1 wt% LDH loading possessed the highest water flux of 54.62 Lm−2 h−1 at 7 bar. The highest rejections of Na2 SO4 (97.3%), MgSO4 (95.5%), MgCl2 (95.2%) and NaCl (63.7%) were also achieved by the TFN embedded with 0.1 wt% LDH in polyamide layer. In addition to the excellent water flux and rejection, incorporation of LDH also enhanced the fouling resistance, proven by the improvement of pure water flux recovery by 52% after BSA filtration. The findings indicated the potential of the newly developed TFN membrane incorporated with LDH nanofillers for efficient NF membrane in water softening and pre-desalination process. © 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction Nanofiltration (NF) is a membrane separation process where its performance and pore size lie in between ultrafiltration and reverse osmosis (RO). In comparison to RO process, despite the lower operating pressure, NF exhibits higher flux and higher rejection of multivalent salts at lower operation cost [1–3]. Generally, the membranes used in NF are thin film composite (TFC) membranes manufactured by interfacial polymerization where the active polyamide (PA) layer is formed via the reaction between the monomers dissolved in aqueous and organic phase. Despite many superior properties, TFC membrane often suffers from structural deterioration as a result of chemical exposure and fouling.

∗ Corresponding author at: School of Chemical and Energy Engineering (SCEE), Faculty of Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor Bahru, Malaysia. E-mail address: [email protected] (N. Yusof).

Consequently, the membrane performance is aggravated over time [4]. Due to that, thin film nanocomposite (TFN) membranes have been introduced as a new way to enhance the membrane properties by impregnating inorganic materials, such as zeolites [5–7], TiO2 [8], CNTs [9,10] and cerium carbonate [11], in the active PA layer. The incorporation of these nanomaterials has been proven to improve surface hydrophilicity, surface roughness, water permeability and anti-fouling resistance [12,13]. However, one of the primary drawbacks encountered by TFN membrane fabrication is the agglomeration of nanomaterials in PA layer. Limited dispersion mainly occurs due to nanoparticles agglomeration and uneven distribution in polymer matrix. In recent years, layered materials such as layered double hydroxide (LDH) have received remarkable interest in water/wastewater treatment [14,15]. With its ability to intercalate molecules and ions into the interlayer regions and flexibility in ratio composition, LDH is prominently used in various applications as photocatalysts, adsorbents and additives in membrane [16]. LDH

https://doi.org/10.1016/j.jtice.2019.01.021 1876-1070/© 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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M.H. Tajuddin, N. Yusof and N. Abdullah et al. / Journal of the Taiwan Institute of Chemical Engineers 97 (2019) 1–11

is known to have a layered structure that consists of metal ions (either di-or trivalent) surrounded by hydroxyl ions. Anions such as Cl− and NO3− are intercalated between the layers classified as one member of the clay minerals family [17]. LDH is hydrophilic in nature due to the presence of OH− group at its surface and its positive charge is primarily attributed to the isomorphous substitution of clay layer [18]. Despite these interesting features, the incorporation of LDH as nanofillers in TFN membranes for NF process has not been extensively studied. Herein, this work aims to fabricate TFN membranes by incorporating LDH as the additive into the active PA layer of TFN membranes and to study its loading effects on membrane water permeability and separation performances. Moreover, LDH nanofillers were synthesized by the new method proposed by Meng et al., in which shorter reaction time and high crystallinity were achieved [19]. The fabricated membranes were characterized in terms of their morphological structures, surface roughness, membrane hydrophilicity, membrane surface charges and chemical functional groups. The TFN membranes were also evaluated by their capability to reject various inorganic salts.

Table 1 Composition of the monomer solution in aqueous and organic phase with respective LDH loading. Membrane

PIP (w/v%)

TMC (w/v%)

LDH loading in TMC solution (wt%)

TFC TFN TFN TFN TFN

2.0 2.0 2.0 2.0 2.0

0.1 0.1 0.1 0.1 0.1

0 0.05 0.1 0.15 0.2

0.05 0.1 0.15 0.2

TMC solution was poured onto the substrate and kept there for 1 min, which resulted in the formation of ultra-thin PA by in-situ interfacial polymerization. Subsequently, the membrane was cured in an oven at 60 °C for 5 min, followed by rinsing and storage in DI water prior to usage. The resulting membranes were designated as TFC, TFN 0.05, TFN 0.1, TFN 0.15 and TFN 0.2, respectively, according to the composition given in Table 1. The pristine TFC membrane without LDH nanofillers was developed as a control in this experiment.

2. Materials and methods 2.1. Materials For synthesis of LDH nanoparticles, aluminum nitrate nonahydrate Al(NO3 )3 ·9H2 O and copper (II) nitrate trihydrate Cu(NO3 )2 ·3H2 O powder, and sodium hydroxide (NaOH) were purchased from Sigma Aldrich and Merck, respectively. For the preparation of TFN membranes, commercial US020 PSf membranes with molecular weight cut-off of 20 kDa were obtained from Rising Sun Membrane to be used as the substrate. Piperazine (PIP), trimesoyl chloride (TMC) and n-hexane (purity > 99.5%) were purchased from Merck and used, respectively as the aqueous monomer, the organic monomer and the organic solvent. For membrane performance testing, Na2 SO4 , MgSO4 , MgCl2 and NaCl, supplied by Sigma Aldrich were used as the electrolyte solutes. Bovine serum albumin (BSA), 67 kDa, used as a model foulant was supplied by Sigma Aldrich. 2.2. Synthesis of LDH LDH was synthesized by the co-precipitation method, following the procedure of Meng et al. [19]. Firstly, Cu(OH)2 was precipitated by adding 100 mL of 0.2 mol L−1 NaOH into 100 mL of 0.1 mol L−1 Cu(NO3 )2 solution. The freshly prepared Cu(OH)2 precipitate was then added to 100 mL of 0.05 mol L−1 Al(NO3 )3 solution and vigorously stirred for 60 min. The pH was adjusted to 12 by adding 1 M NaOH solution to allow the reaction between two hydroxides. The precipitates obtained were aged for 6 h and separated by centrifugation and washed with deionized water several times to purify the LDH. Finally, the obtained LDH powder was dried at 60 °C for 24 h and stored until further use. 2.3. Preparation of thin film nanocomposite membrane The compositions of aqueous and organic phase with respective LDH loading are summarized in Table 1. Firstly, LDH powder was added to 30 mL of 0.1 w/v% TMC in n-hexane and dispersed via ultrasonication for 1 h. Meanwhile, 2 w/v% PIP solution was prepared by adding PIP to de-ionized (DI) water. The PSf commercial substrate membrane was clamped in between a viton rubber sheet and a glass plate with a square window and 30 mL of 2 w/v% PIP aqueous solution was poured on top of the PSf substrate and kept there for 2 min. Droplets of PIP solution were removed from the surface by using a rubber roller and tissue. Next, 30 mL of 0.1 w/v%

2.4. LDH nanofillers characterizations The crystalline structure of LDH nanofillers was studied by Xray diffraction (XRD) (D50 0 0, Siemens). Using CuKα radiation at a ˚ the diffraction patterns were recorded wavelength (λ) of 1.54 A, within the range of 2θ = 10˚−80˚. The structure of the LDH particles was also studied by transmission electron microscopy (TEM) (HT 7700, Hitachi). The LDH fillers were loaded onto the sample holder and the images of Cu-Al LDH were captured by Orius SC 10 0 0A camera. Zeta potential of LDH was determined by Zetasizer 30 0 0HSA (Malvern Instruments) at pH 7, using water as a dispersant.

2.5. Thin film composite/nanocomposite membranes characterizations Field-emission scanning electron microscopy (FE-SEM) (Model: Hitachi SU8020) was used to investigate the morphologies of the membranes. Different FESEM micrographs with various magnifications (5.0k and 20.0k) were obtained from the surface of the TFC and TFN membranes. The functional groups in the TFC and TFN membranes were identified using ATR-FTIR spectrometer (Model: IRTRACE100, Shimadzu). Each sample has been scanned for a total of 600 times at a wavelength range of 500 cm−1 to 4000 cm−1 with a scanning resolution of 0.25 cm−1 . The roughness of the membrane surface was analyzed using atomic force microscopy (AFM) (Model: DFM SPA-300 HV) in a dynamic force mode coupled with 20 μm scanner and SI-DF 40 (spring constant = 42 N m−1 ) cantilever. The top surface of the membranes was observed within the scan size of 2 μm × 2 μm to obtain the mean surface roughness, Ra. The contact angle of the membranes was obtained by the sessile drop method using a goniometer (Kruss Gambult, Germany). A 0.50 μL of water droplet coming out from a motor-driven syringe was placed onto the top surface of the membrane sample and the contact angle was measured. Reading was taken at least 15 times at different locations of the membrane and the average value was reported. The surface charge of the TFN membranes was determined by zeta potential measurement using an electrokinetic analyzer (Model: Malvern Zetasizer Nano Zsp). The membrane samples were soaked in RO water for no less than 12 h to complete the hydration. Two membrane samples of 20 mm × 10 mm were placed on the sample holder with double-sided tape and the measurement was taken.

M.H. Tajuddin, N. Yusof and N. Abdullah et al. / Journal of the Taiwan Institute of Chemical Engineers 97 (2019) 1–11

2.6. Nanofiltration performances The pure water flux and salt rejection of prepared TFC and TFN membranes were assessed using a cross-flow filtration system (Model: SterlitechTM Stirred Cell). The effective surface area of the membrane was 20.6 cm2 . Prior to the pure water flux measurement, all membrane samples were compacted at a pressure of 8 bar for about 30 min. Then, the pressure was reduced to 7 bar and the pure water flux was measured. After that, salt rejection performance was determined using 10 0 0 ppm of different inorganic salts, including Na2 SO4 , MgSO4 , MgCl2 and NaCl. The permeate flux (J) was given by:

J=

V t ×A

(1)

where V is the permeate volume (L), A is the membrane area (m2 ) and t (t) is the time required to collect the permeate volume V. The salt concentrations in the feed and permeate were measured by bench conductivity meter (Model: Jenway 4520). The salt rejection was calculated by the equation:



R (% ) =

1−

Cp Cf



× 100

(2)

where Cp is the permeate salt concentration (ppm) and Cf is the feed salt concentration (ppm). 2.7. Fouling study Fouling experiments were carried out using 500 ppm bovine serum albumin (BSA) as the model foulant. The operating pressure was 7 bar and the feed pH was 6.8 for all tests. In addition, the normalized flux (Jf /J0 ) was used to evaluate the fouling resistance of TFC NF membranes, where Jt and J0 are the fluxes at the operational period of t and 0, respectively. For pure water flux recovery, each membrane was cleaned by immersing the membrane in a beaker of deionized water, shaken for 15 min to remove BSA residual. 3. Results and discussion 3.1. Characteristics of LDH nanoparticles The morphological structure of synthesized LDH nanoparticles was characterized using TEM analysis as can be seen from Fig. 1(a) and (b), the LDH nanofillers possessed crystalline structure and typical hexagonal shape, which is in agreement with the previously reported study [20]. This structure was resulted from crystallographic habits of LDH during the synthesis process. In addition, the average lateral dimension of synthesized LDH is about 31.62 nm. The crystallinity of LDH was further studied via XRD. Based on the XRD spectra shown in Fig. 1(c), the peaks at 2θ = 11.5° and 2θ = 22.0° can be assigned to the typical wellcrystallized hydrotalcite-like LDH materials [21]. The basal spacing of LDH was 0.85 nm which is equal to the sum of the ionic size of NO3 − (0.37 nm) and the interlayer distance of brucite like structure materials (0.48 nm) [22,23]. However, the diffractions at 2θ = 30, 35, 38 were caused by the oxidation of Cu2+ into Cu3+ during the preparation of LDH nanofillers [24]. Fig. 2(a) depicts the zeta potential behavior of LDH nanofillers in water at neutral condition (pH = 7.0). From the figure, it can be observed that LDH nanofillers have a positive zeta potential of about 26.2 ± 4.41 mV. This result was mainly attributed to the excess positive charge due to the substitution of divalent ions Cu2+ with trivalent ions Al3+ [25]. In other cases, the interlayer anions such as NO3− and OH− neutralized these positive charges [21,22]. However, it was presumed that the positive charge of LDH

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nanofillers was caused by the weak electrostatic interactions between anions and the particle surface [26]. Consequently, these anions easily detached from the surface and LDH nanofillers maintained their positive charge [26]. The chemical functional group of LDH nanofillers in Fig. 2(b) were examined by FTIR analyses. The peak at 3407 cm−1 can be attributed to the stretching –OH functional groups that attached to the metals ion [27]. The small node that appears at 1628 cm−1 were assigned to the interlayer of water in LDH structures [28]. Band at 1383 cm−1 corresponded to interlayer anion (NO3 − ). Furthermore, the frequency at low wavenumber can be ascribed to Cu/Al–OH and Al–OH translation modes [29]. 3.2. Effects of LDH on surface morphology of membranes The surface structures of pristine TFC and TFN membranes were characterized using FE-SEM analysis. The top surface images of TFC and TFN 0.1 membranes are presented in Fig. 3(a) and (b) respectively. It can be observed that the pristine TFC and TFN 0.1 membranes exhibited the typical “ridge and valley” like structure with nodules and firm globules formed by the reaction of PIP and TMC [30]. However, the rate of formation of nodules and firm globules decreased by the incorporation of LDH in the PA layer, indicated by the smaller nodule size of TFN 0.1 (as can be clearly seen from Fig. 3(b) at high magnification). This observation was most probably related to the strong adsorption of water molecules to LDH. When the aqueous solution of PIP interacts with the organic solution containing TMC and LDH, water molecules are driven from the aqueous to organic phase due to the presence of LDH and polycondensation takes place in a water-rich environment [15]. In addition, at higher magnification (20.0k) of FESEM image shown in Fig. 3(b), the presence of LDH was observed on top of the PA layer. The incorporation of LDH can be further supported by EDS analysis as shown in Fig. 4(a) and (b). The EDS mapping of TFN membrane showed the presence of Cu and Al elements, indicating the successful addition of LDH nanofillers into the top surface layer. The chemical functional groups of prepared TFC and TFN membranes were detected by ATR-FTIR analysis. Fig. 4(c) shows the IR spectra of the PA layer of composite membranes prepared at various LDH loadings. The results indicated that PA layer was successfully formed on top of the composite membranes, owing to the presence of strong band at 1633 cm−1 , which corresponded to the C=O band of an amide group [31]. A peak appeared at 1456 cm−1 can be also detected which was attributed to the C–N stretching of the amide group. In addition, a broad band appeared at around 3427 cm−1 could be assigned to the presence of –COOH groups of PA layer due to partial hydrolysis of the acyl chloride of TMC [32]. However, the peaks which corresponded to the LDH nanoparticles were not clearly been detected in the IR spectra. This is probably due to a low content of LDH nanoparticles that had been added into the PIP solution during the fabrication of TFN membranes. Besides, the LDH peaks could be overlapped with the peaks which assigned to other functional groups such as amide, imide and hydroxyls group [33,34]. Fig. 5 illustrates the AFM topographic images of pristine TFC membrane and TFN membranes. As can be seen from the figures, the incorporation of LDH nanofillers resulted in smoother surfaces, indicated by the following sequence of the mean roughness, Ra: TFC (27.8 nm) > TFN 0.20 (11.72 nm) > TFN 0.1 (11.11 nm) > TFN 0.15 (10.97 nm) > TFN 0.05 (8.98 nm). The increase in the LDH loading did not necessarily lead to a further decrease in surface roughness [35]. The decrease in surface roughness was related to the formation of smaller nodules as mentioned earlier in Section 3.2. The smoother membrane surface, with the lack of ridge and valley structure, is supposed to provide better antifouling properties [36].

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Fig. 1. TEM images of LDH nanofillers at different magnification (a) 80.0k and (b) 600.0k and (c) XRD patterns of LDH nanofillers.

Fig. 2. (a) Zeta potential analysis of LDH nanofillers (b) FTIR spectrum of LDH nanofillers.

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Fig. 3. Surface morphological structures of pristine TFC and LDH-filled TFN membranes at different magnifications (left 5.0k and right 20.0k) (a) TFC and (b) TFN 0.1.

The contact angle of TFC and TFN membranes was measured and the results are shown in Fig. 6. The contact angle decreased from 54.56° (TFC membrane) to 35.29° (TFN 0.05). The contact angle kept decreasing progressively as the LDH loading increased. The enhancement of membrane hydrophilicity by incorporation of LDH was mainly attributed to the hydrophilic property of LDH itself [19]. It was presumed that some of the nanofillers were exposed directly on the membrane surface, particularly at higher LDH loadings. Moreover, the membrane with higher hydrophilicity can attract more water molecules onto its surface and facilitate the transport of water across the membrane [37]. In addition, better fouling resistance could also be achieved as hydrophilic membrane could reduce the potential of attachment of hydrophobic foulants due to the formation of the water layer on the membrane surface. Table 2 shows the surface zeta potential of TFC and TFN membranes. TFC membrane exhibited more negative zeta potential (−29.0 mV) than all TFN membranes (−16.8, −15.9, −14.3 and −14.2 mV). This result was comparable to the positive zeta potential of LDH (26.2 mV), due to the replacement of divalent ions (Cu2+ ) with trivalent ions (Al3+ ) in their layered structured. This phenomenon occurred when Al3+ rapidly reacted with Cu(OH)2 and substitute Cu2+ in the octahedral sheet leading to the transformation from copper hydroxide to LDH phase [19]. It is also worth to mention that LDH nanofillers were present on the outer surface

Table 2 Surface zeta potential of pristine TFC and TFN membranes. Membrane

Surface zeta potential (mV)

TFC TFN0.05 TFN0.1 TFN0.15 TFN0.2

−29.0 −16.8 −15.9 −14.3 −14.2

of the TFN membrane, as shown in Fig. 3, hence inducing more positive charge to the membrane surface [14].

3.3. NF separation performances 3.3.1. Pure water flux and salts rejection Fig. 7(a) shows the pure water flux of TFC and TFN membranes. TFN membranes displayed significantly higher pure water flux than TFC membrane with a maximum increment of 141%, as recorded by TFN 0.1. The increase in the pure water flux was attributed by the presence of extra interlayer pathways by LDH, which facilitated water transport across the membrane. In addition, membrane

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Fig. 4. EDS mapping of (a) pristine TFC (b) TFN 0.1 membranes (c) ATR-FTIR spectra of composite membranes prepared at various LDH loadings.

hydrophilicity has also contributed to the pure water flux increment. However, a further increase in LDH loading has resulted in the decrease of pure water flux due to the formation of stacked layers or agglomeration of LDH that reduced the pathways for water transport [14]. As illustrated in Fig. 7(b), the solute rejection became higher as the molecular weight is increased. In this study, the molecular weight cut-off (MWCO) of TFC and TFN membranes was determined using different types of polyethylene glycol (PEG) molecules with different molecular weights (20 0, 40 0, 60 0 and 10 0 0 Da, respectively). MWCO was obtained according to the molecular weight where the rejection of PEG is 90% referring to previously reported studies [38,39]. The results of PEG rejection are presented in Fig. 7(b), whereas the results of MWCO and effective pore

Table 3 MWCOs and effective pore size of NF membranes. Membrane

Molecular weight (g mol−1 )

rp (nm)a

TFC TFN0.05 TFN0.1 TFN0.15 TFN0.2

420 350 209 198 180

0.533 0.493 0.394 0.386 0.373

a

Effective diameter = rp = 0.0397MWCO0.43 [13].

diameter are tabulated in Table 3. The results revealed that at higher LDH loading, the pore size of the membranes was slowly decreasing. Moreover, the membrane pore diameter was

M.H. Tajuddin, N. Yusof and N. Abdullah et al. / Journal of the Taiwan Institute of Chemical Engineers 97 (2019) 1–11

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Fig. 5. Membrane surface roughness.

Table 4 Ion radius, hydrated radius and diffusion coefficients of ions. Ions +

Na Mg2+ Cl− SO4 2−

Ion radius (nm) [44]

Hydrated radius (nm) [44]

Diffusion coefficients (10−9 m2 s−1 ) [43]

0.10 0.07 0.18 0.29

0.36 0.43 0.33 0.38

1.33 0.71 2.03 1.07

systematically compared with the hydrated ionic radius. Based on the data presented in Table 4, it can be confirmed that the hydrated ionic radii of the ions were close to the membrane pore size. These findings proved that the separation mechanism of the ions was not only based on surface charge, but rather a combination of surface charge and size exclusion [40,41].

Fig. 7(c) represents a rejection of different types of inorganic salts (MgSO4 , MgCl2 , Na2 SO4 and NaCl) by TFC and TFN membranes. From the figure, the descending order of salt rejection is Na2 SO4 > MgSO4 ≈ MgCl2 > NaCl for both TFC and TFN membranes. NF mechanisms are highly influenced on the membrane pore size and Donnan exclusion effects [41,42]. Comparing

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Membrane hydrophilicity 60.00

54.56

Contact angle (°)

50.00

40.00

35.29

33.96

32.33 28.84

30.00

20.00

10.00

0.00 TFC

TFN0.05

TFN0.1

TFN0.15

TFN0.2

Membranes

Fig. 6. Membrane hydrophilicity.

between TFC and TFN membranes, the rejection of the latter was higher than the former. As all membranes possessed negative surface charge, higher rejections of divalent anions rather than low-valent anions can be seen due to the strong electrostatic repulsion of high-valent anions [43]. Thus, higher rejection was obtained for Na2 SO4 and MgSO4 than those of MgCl2 and NaCl. The increase in MgCl2 rejection was probably caused by the weak electrostatic attraction of Mg2+ to the membrane surface, contributed by the less negative charged of TFN membranes compared to TFC membrane. Table 4 shows the ion radius, hydrated radius and diffusion coefficient of each salt. The low diffusion coefficient of SO4 2− ions has made the divalent ions more difficult to pass through the membrane, proving that the higher rejection of SO4 2− ions compared to Cl− ions was slightly contributed by size exclusion [43,44]. The effect of size exclusion, in addition to electrostatic repulsive force on salt rejection became more notable as the membrane pore size approaches the hydrated radius of certain ions (Cl− , Na+ ). Furthermore, the lower rejection of NaCl compared to MgCl2 was also attributed to the higher diffusivity of Na+ ions than that of Mg2+ ions [43]. In that case, the steric hindrance mechanism was irrelevant due to the small size of NaCl ions and therefore causing NaCl rejection to drop [45,46]. Since the TFN membranes surface was less negatively charged, the higher salt rejection of the TFN membranes is difficult to be explained by the Donnan effect alone. Considering the average size of the water and the ions, the effect of size exclusion could be seen as the space of the pathway was reduced by the incorporation of LDH. In that case, the trade-off rule between the flux and rejection has been broken, which constitutes the most desirable effect of LDH incorporation. TFN0.1 membrane exhibited the best separation performance of inorganic salts (MgSO4 , MgCl2 , Na2 SO4 and NaCl). The TFN0.1 membrane showed higher rejection towards Na2 SO4 (97.34%), MgSO4 (95.51%), MgCl2 (95.21%) and NaCl (63.2%) compared to TFC membrane (Na2 SO4 = 96.70%,

MgSO4 = 91.23%, MgCl2 = 81.11% and NaCl = 46.81%) with considerably high pure water flux (54.62 L m−2 h−1 ).

3.4. Fouling study The fouling effect of TFC and TFN membranes was studied using commercial BSA as a model protein foulant. The normalized water flux versus time for TFC and TFN 0.1 membranes is shown in Fig. 8(a). The initial water flux of TFC and TFN0.1 membranes were recorded at 24.28 L m−2 h−1 and 53.78 L m−2 h−1 , respectively. During the first hour, the normalized water flux of both TFC and TFN 0.1 dropped to 80%, which indicates the fast deposition of protein foulant due to the strong van der Waals force between BSA and membrane surface. The electrostatic force was repulsive since both BSA (with an isoelectric point of 5.8) and the membranes (with negative zeta potential) were negatively charged at pH 6.8. After 3 h of BSA filtration, the normalized water flux of TFC and TFN 0.1 membranes was 39% and 50%, respectively. Therefore, TFN 0.1 membrane was less fouled. Considering the following effects reported in this work: (1) increase in surface hydrophilicity [14,47] (2) increase in negative surface charge and (3) decrease in surface roughness, the fouling resistance of the membrane after the incorporation of LDH was enhanced. The experimental data hence indicated that the second effect was not as strong as the sum of the first and the third effect. The fouling study of TFC and TFN membranes was further continued by measuring the pure water flux recovery after the BSA filtration. Fig. 8(b) displays the pure water flux of TFC and TFN 0.1 measured before and after the filtration of the BSA solution. TFC showed a significant reduction (by 63%) of pure water flux after the BSA filtration test, whereas TFN only displayed 48% pure water flux reduction. The unrecoverable flux reduction is usually considered as the permanent flux reduction. Therefore, the incorporation of LDH also mitigated the permanent flux reduction.

M.H. Tajuddin, N. Yusof and N. Abdullah et al. / Journal of the Taiwan Institute of Chemical Engineers 97 (2019) 1–11

Fig. 7. (a) Pure water flux (b) different molecular weight of PEG rejection (c) salts rejection of TFC and TFN membranes.

Fig. 8. (a) BSA filtration of pristine TFC and TFN0.1 membranes (b) comparison of pure water flux before and after BSA filtration for TFC and TFN0.1 membranes.

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4. Conclusion From the experimental results, the following conclusions can be drawn. (1) In-situ polymerization of piperazine and TMC could be successfully carried out in the presence of synthesized LDH in the organic phase. (2) SEM, XRD, FTIR and EDS all confirmed the successful incorporation of LDH in the top active layer of TFN membranes. (3) By incorporation of LDH, zeta potential became less negative, membrane surface became more hydrophilic and smoother. (4) By incorporation of LDH, both flux and salt rejection increased. In particular, the best performing TFN 0.1 displayed MgSO4 rejection of 96% with pure water flux of 54.62 L m−2 h−1 at 7 bar and 25 °C. (5) By incorporation of LDH, the fouling resistance and pure water flux recovery of the membrane after BSA filtration were enhanced. In conclusion, it is believed that the incorporation of LDH nanofillers in thin film nanocomposite membrane can serve as an excellent alternative to produce high-performance nanofiltration membranes for water treatment process such as water softening and pre-desalination process. Acknowledgment The author would like to acknowledge the financial support from the Ministry of Education Malaysia and Universiti Teknologi Malaysia under Fundamental Research Grant Scheme (R.J130 0 0 0.7846.4F929), GUP grant (Q.J130 0 0 0.2546.16H29), PRGS-ICC (R.J130 0 0 0.7746.4J329) and Higher Institution Centre of Excellence (HiCOE) grant (R.J090301.7846.4J180 and R.J090301.7846.4J179). References [1] Pourjafar S, Rahimpour A, Jahanshahi M. Synthesis and characterization of PVA/PES thin film composite nanofiltration membrane modified with TiO2 nanoparticles for better performance and surface properties. J Ind Eng Chem 2012;18:1398–405. [2] Liu M, Zheng Y, Shuai S, Zhou Q, Yu S, Gao C. Thin-film composite membrane formed by interfacial polymerization of polyvinylamine (PVAm) and trimesoyl chloride (TMC) for nanofiltration. Desalination 2012;288:98–107. [3] Rajaeian B, Rahimpour A, Tade MO, Liu S. Fabrication and characterization of polyamide thin film nanocomposite (TFN) nanofiltration membrane impregnated with TiO2 nanoparticles. Desalination 2013;313:176–88. [4] Yan F, Chen H, Lü Y, Lü Z, Yu S, Liu M, et al. Improving the water permeability and antifouling property of thin-film composite polyamide nanofiltration membrane by modifying the active layer with triethanolamine. J Memb Sci 2016;513:108–16. [5] Fathizadeh M, Aroujalian A, Raisi A. Effect of added NaX nano-zeolite into polyamide as a top thin layer of membrane on water flux and salt rejection in a reverse osmosis process. J Memb Sci 2011;375:88–95. [6] Ma N, Wei J, Liao R, Tang CY. Zeolite-polyamide thin film nanocomposite membranes: towards enhanced performance for forward osmosis. J Memb Sci 2012;405–406:149–57. [7] Dong LX, Huang XC, Wang Z, Yang Z, Wang XM, Tang CY. A thin-film nanocomposite nanofiltration membrane prepared on a support with in situ embedded zeolite nanoparticles. Sep Purif Technol 2016;166:230–9. [8] Bet-moushoul E, Mansourpanah Y, Farhadi K, Tabatabaei M. TiO2 nanocomposite based polymeric membranes: a review on performance improvement for various applications in chemical engineering processes. Chem Eng J 2015;283:29–46. [9] Yin J, Deng B. Polymer-matrix nanocomposite membranes for water treatment. J Memb Sci 2015;479:256–75. [10] Mahdavi MR, Delnavaz M, Vatanpour V. Fabrication and water desalination performance of piperazine–polyamide nanocomposite nanofiltration membranes embedded with raw and oxidized MWCNTs. J Taiwan Inst Chem Eng 2017;75:189–98. [11] Gu Z, Cui S, Liu S, An Q, Qin Z, Guo H. Superhydrophilic nanofiltration membrane with antifouling property through in-situ mineralization of Ce2 (CO3 )3 nanoparticles. J Taiwan Inst Chem Eng 2018;88:70–7.

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