Chemosphere 136 (2015) 204–210

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Alginate fouling reduction of functionalized carbon nanotube blended cellulose acetate membrane in forward osmosis Hyeon-gyu Choi a, Moon Son a, SangHyeon Yoon a, Evrim Celik b, Seoktae Kang c, Hosik Park d, Chul Hwi Park e, Heechul Choi a,⇑ a

School of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, Republic of Korea Suleyman Demirel University, Faculty of Engineering, Department of Environmental Engineering, 32260 Isparta, Turkey Department of Civil Engineering, Kyung Hee University, Gyeonggi 446-701, Republic of Korea d Center for Membranes, Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT), Daejeon 305-600, Republic of Korea e School of Environmental Engineering, The University of Seoul, Seoul 130-743, Republic of Korea b c

h i g h l i g h t s  The fCNT-CA membrane showed enhanced performance than the bare CA membrane in FO.  The fCNT-CA membrane became more hydrophilic and negatively charged.  Dominant alginate foulant–membrane interaction force was firstly investigated.  The fCNT-CA membrane was more repulsive against alginate foulant via AFM analysis.

a r t i c l e

i n f o

Article history: Received 12 December 2014 Received in revised form 7 April 2015 Accepted 1 May 2015

Keywords: Forward osmosis Nano-enhanced membrane Alginate fouling Electrostatic repulsion Carbon nanotube

a b s t r a c t Functionalized multi-walled carbon nanotube blended cellulose acetate (fCNT-CA) membranes were synthesized for forward osmosis (FO) through phase inversion. The membranes were characterized through SEM, FTIR, and water contact angle measurement. AFM was utilized to investigate alginate fouling mechanism on the membrane. It reveals that the fCNT contributes to advance alginate fouling resistance in FO (57% less normalized water flux decline for 1% fCNT-CA membrane was observed than that for bare CA membrane), due to enhanced electrostatic repulsion between the membrane and the alginate foulant. Furthermore, it was found that the fCNT-CA membranes became more hydrophilic due to carboxylic groups in functionalized carbon nanotube, resulting in approximately 50% higher water-permeated flux than bare CA membrane. This study presents not only the fabrication of fCNT-CA membrane and its application to FO, but also the quantification of the beneficial role of fCNT with respect to alginate fouling in FO. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction As water scarcity and energy exhaustion become severe, a paradigm of water treatment aims at obtaining clean water with less energy. Membrane process is replacing conventional process in drinking water due to its excellent retention of pollutants (Kim and Lee, 2011). However, fouling is a major issue in membrane process operation, as it causes severe water flux decline and a shortened lifespan of the membrane. Hence, adequate cleaning is necessary, but this requires large energy inputs (Lee et al., 2010). Recently, new types of membranes have been introduced to overcome this issue. The convergence of nanotechnology and membrane science has been actively attempted to functionalize ⇑ Corresponding authors. E-mail address: [email protected] (H. Choi). http://dx.doi.org/10.1016/j.chemosphere.2015.05.003 0045-6535/Ó 2015 Elsevier Ltd. All rights reserved.

membranes with discrete nano-material named ‘‘nano-enhanced membranes (NEMs)’’, which improves membrane performances (Pendergast and Hoek, 2011). NEMs have been developed with nano-sized zeolite, titania, alumina, iron oxide, silver, and carbon nanotube, resulting in enhanced membrane performance with regard to water flux, rejection, fouling resistance, and microorganism inactivation (Kwak et al., 2001; Yan et al., 2005; Taurozzi et al., 2008; Lind et al., 2010; Celik et al., 2011b; Park and Choi, 2011). Reverse osmosis (RO) has been widely utilized in both industrial applications and academic researches, due to its high rejection rate of target material or pollutant. In desalination, large quantities of fresh water can be obtained from seawater via RO with lower energy costs rather than via distillation. However, RO also results in severe membrane fouling due to the high hydraulic pressures required. Biofouling is particularly problematic, with biofilm formation on the membrane surface by variable bacteria, which is

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difficult to control. High-molecular weight compounds are secreted by bacteria in biofilm, named extracellular polymeric substance (EPS). EPS is mostly composed of polysaccharides and proteins. It constitutes 50–90% of the total organic matter in biofilm (Donlan and Costerton, 2002). Recently, forward osmosis (FO) has been studied not only for the direct production of freshwater from seawater but also for utilizing as pretreatment prior to water treatment and desalination. It utilizes osmotic pressure gradients alone, having great potential due to its advantages – no hydraulic pressure required, low operating cost, and low fouling tendency. Draw solute development, process optimization, and FO membrane fabrication are considered as core research areas for the real application of FO (Cath et al., 2006; Zhao et al., 2012). Various natural and artificial draw solutes have been studied, including NH4CO3, intracellular proteins, magnetic draw solutes, fertilizer, and nanoparticles (McCutcheon et al., 2006; Adham et al., 2007; Ling et al., 2010; Ling and Chung, 2011; Phuntsho et al., 2011). A draw solute for FO should have high osmotic pressure, be easy to separate, and cause no chemical or physical damage to membranes (Zhao et al., 2012). The reverse solute diffusion phenomenon has been reported as a critical factor for consideration in the draw solute for FO (Cath et al., 2006; Phillip et al., 2010). Monovalent ions, such as sodium chloride, have the potential to penetrate nonporous membranes. The diffusivity of draw solute strongly affects thicker fouling layer formation, due to cake enhanced osmotic pressure (CEOP) caused by concentration polarization (Hoek and Elimelech, 2003). Past studies have attempted to investigate membrane fouling reversibility in FO. As no hydraulic pressure is required in FO, accumulated fouling layers are less compacted onto membrane surfaces. Therefore, no chemical cleaning is necessary and physical cleaning is an effective means of restoring water flux to initial levels. Commercial RO membranes have been utilized for FO research before a cellulose triacetate (CTA) based polymeric membrane was synthesized by HTI Inc. (Herron, 2006). It triggered FO membrane fabrication research. Cellulose-based asymmetric membranes (Su et al., 2010) and thin-film composite membranes (Yip et al., 2010) have been widely utilized. The critical aspects of membrane performance are water flux, ion rejection, and fouling resistance. Several studies of NEM for FO application have been reported, using functionalized carbon nanotube (Amini et al., 2013; Dumée et al., 2013; Goh et al., 2013; El Badawi et al., 2014), vertically aligned carbon nanotube (Baek et al., 2014), zeolite (Emadzadeh et al., 2014), and silver nanoparticle (Liu et al., 2013). In the case of CNT, enhanced water flux and hydrophilicity were mostly reported. Organic foulant has been applied to investigate the fouling tendency of the CNT-based NEM for FO. In this study, a functionalized CNT blended polymeric composite membrane was fabricated and characterized, and its performance was evaluated with a lab-scale FO system. A cellulose acetate was chosen as a membrane polymer due to its good chlorine tolerance and anti-biofouling property (Lee et al., 2011). Alginate was chosen as a hydrophilic organic EPS foulant surrogate in biofilm, and alginate fouling reduction on the NEM was evaluated in a lab-scale FO system. Interaction force measurement between the alginate foulant and the membrane surface was conducted by atomic force microscope (AFM) for the first investigation of the major mechanism of alginate fouling on the synthesized NEM. 2. Materials and methods 2.1. Chemicals Cellulose acetate (CA, average M.W. 30,000), nitric acid (HNO3, 70%), sulfuric acid (H2SO4, 98%), 1,4-dioxane, acetone, methanol,

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lactic acid, sodium chloride (NaCl), and sodium alginate were purchased from Sigma–Aldrich. Multi-walled carbon nanotubes (MWCNTs) were purchased from Hanhwa Nanotech., and calcium sulfate (CaSO4) was purchased from Junsei. All of the aqueous solutions for the experiments were prepared using deionized (DI) water from a water purification system (Synergy, Millipore, USA) which has a resistivity of 18.2 mO cm. A commercial asymmetric cellulose triacetate (HTI FO) membrane was provided by Hydration Technology Innovations, Albany, OR. 2.2. Carbon nanotube functionalization and membrane fabrication MWCNTs were functionalized in a mixture of nitric and sulfuric acids, in order to increase dispersion in organic solvent (Liu et al., 1998). The MWCNTs were refluxed in a 1:3 (v/v) HNO3 and H2SO4 mixture at 100 °C. Following this, the MWCNTs were washed with DI water until the solution pH reached 7.0, and then dried overnight at room temperature. The dried MWCNTs were ultrasonicated in a 1:3 (v/v) HNO3 and H2SO4 mixture at 70 °C for nine hours. Next, the MWCNTs were again washed with DI water until the pH of the solution reached 7.0, and the functionalized MWCNTs were left to dry in a vacuum oven. When dried, the functionalized MWCNTs blended cellulose acetate (fCNT-CA) membranes were synthesized through a phase inversion method. The functionalized MWCNTs were ultrasonicated in 1,4-dioxane for homogeneous dispersion. After dispersion, cellulose acetate and acetone were added to the MWCNT solution. Methanol and lactic acid were subsequently also added and carefully mixed into the MWCNT and polymer solution. The chemical composition of MWCNTs and polymer solution is shown in Table S2. The polymer solution was ultrasonicated and placed at room temperature to remove air bubbles and then casted with a casting knife onto glass plates with a 280-lm casting thickness. The casted polymer film was then allowed to evaporate for one minute and then dipped in DI water for coagulation, and the synthesized membranes were stored in DI water. 2.3. Membrane characterization The morphologies of the functionalized MWCNTs and fCNT-CA membranes were observed by SEM (S-4700, Hitachi, Japan). Their functional groups were analyzed using FTIR spectrometry (Nicolet iS10, Thermo Scientific, USA). The hydrophilicity of the membrane surface was evaluated through the dynamic sessile drop method, using a contact angle goniometer (Model 100, Rame-Hart, USA). A minimum of seven contact angles were averaged to ensure reliable values. 2.4. Lab-scale forward osmosis test A laboratory-scale FO system was operated using the synthesized fCNT-CA membrane and the HTI FO membrane, in order to evaluate water-permeated flux and reverse solute flux. To determine the water-permeated flux, 3 M NaCl aqueous solution was used as a draw solution and DI water was used as a feed solution. To determine the reverse solute flux, 1 M NaCl solution was used as a draw solution and DI water was used as a feed solution. A crossflow membrane test unit was used, in which the temperature of draw and feed solutions was controlled at 20 ± 1 °C with a water-cooled chiller (DH-003BH, Daeho Auto Chiller, Korea). The solutions in the test unit were circulated with a peristaltic pump (BP-60601, Won Corp., Korea) with a 1000 cm3/min crossflow rate and an effective membrane area of 20 cm2. The water-permeated flux was measured using a calculation in which the permeated volume rate was divided by the effective membrane area

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Jw ¼

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V F0  V F Am t

where Jw is the measured water-permeated flux, VF0 is the initial volume of feed solution, VF is the final volume of feed solution after operation, Am is the effective membrane area, and t is time. To determine the reverse solute flux, the final concentration of feed solution was measured with electrical conductivity using a dissolved oxygen/conductivity instrument (YSI-85, YSI Incorporated, USA) after operation

JS ¼

cF V F Am t

where JS is the measured reverse solute flux, cF is the final NaCl concentration of feed solution after operation, VF is the final volume of feed solution after operation, Am is the effective membrane area, and t is time. 2.5. Alginate fouling test Alginate was added to the feed solution and the lab-scale system was operated for a fouling test. The concentration of the alginate solution was 200 mg/L and 1000 mg/L, with 0.5 mM and 1 mM of calcium cation added, respectively. In 1000 mg/L alginate fouling, Ca2+ concentration was increased to 1 mM to maximize bridge effect as alginate foulant for severe fouling. 3 M NaCl aqueous solution was used as a draw solution and DI water was used as a feed solution. Normalized water flux was calculated based on water flux decline. Additionally, the interfacial force between the alginate foulant and the membrane surface was measured using a carboxylate-modified latex (CML) particle attached to the tipless cantilever of an AFM instrument (Innova INSC-090, Bruker, USA). The CML particle was mechanically attached onto the end of the tipless cantilever, as described (Asatekin et al., 2007). The interfacial force was measured at the tip speed of 1 lm/s and with a piezo displacement of 1 lm in the various aquatic chemistries: DI water and 1.0 mM of Ca2+ at pH 6.0 ± 0.2. Interaction force measurements were taken at four different locations on the membrane surfaces, with 20 force curves at each location. 3. Results and discussion

between the acetyl groups of CA and the observed groups of the functionalized MWCNTs(Li et al., 2010), as shown in Fig. S3. The contact angles decreased as the MWCNTs were blended, indicating that the membrane hydrophilicity increased. The hydrophilic functional groups of the MWCNTs, confirmed in FTIR spectra, contributed to the enhancement of the membranes’ hydrophilicity (Choi et al., 2006; Brady-Estévez et al., 2008). 3.2. Lab-scale forward osmosis test The 0.5% and 1% fCNT-CA membranes with contact angles of less than 65° were selected for the lab-scale FO test. Fig. 3 shows the water permeate and reverse solute fluxes of the synthesized membranes and HTI FO membrane in the lab-scale FO process. The water permeated flux of the 0.5% and 1% fCNT-CA membranes was higher than that of the bare membranes (bare CA: 9.58 LMH, 0.5% fCNT-CA: 10.8 LMH, 1% fCNT-CA: 14.11 LMH). Hydrophilicity of the membrane surface and internal concentration polarization (ICP) are critical to determine water flux. ICP is governed by membrane structural characteristics and draw solute diffusivity (Cath et al., 2006; Choi et al., 2006). In this study, membrane thickness has the greatest contribution in determining the ICP phenomena. By blending the fCNT, the membrane became hydrophilic and thin, resulting in enhanced water permeated flux. Other minor hypotheses could be considerable in the water flux enhancement of the fCNT-CA membrane. Firstly, the ionized carboxyl group of COO was able to enhance negative surface charge, contributing to water flux increase (Zhao et al., 2013). Secondly, the fCNTs were randomly dispersed in the membrane matrix due to applied ultrasonication in membrane fabrication; some of the fCNT might be aligned vertically. It was found that MWCNTs provide hydrophobic inner channels that dramatically accelerate water transport (Hummer et al., 2001), thus it is possible that vertically aligned fCNTs could enhance water flux. With regard to reverse salt flux, the fCNTs allowed only little more reverse diffusion of salt ions. It might be assumed that the decreased membrane thickness with the fCNT and the microvoids between the fCNT and CA in the dense layer are causes. Finally, the 1% fCNT-CA membrane was selected for the alginate fouling resistance test, since this showed the highest enhanced water-permeated flux without sacrificing significant reverse salt flux. The fCNT-CA membranes showed higher water permeated flux than HTI FO, while bare CA membrane had the lowest water permeated flux.

3.1. Membrane characterization 3.3. Alginate fouling test Fig. 1 shows the surface and cross-section morphologies of the fCNT-CA membranes. All of the synthesized membranes had asymmetric structures with dense top layers and porous sub-layers. Their thicknesses were measured by the SEM. In the presence of the fCNT, thinner membranes were synthesized than the CA bare membrane and the fCNT-CA membranes had similar thicknesses. It has been reported that the blended CNT increases the viscosity of the polymer solution for phase inversion (Han and Nam, 2002; Amirilargani et al., 2010). Here, the casting of the polymer solution, caused it to be compacted due to the increased viscosity, resulting in the fabrication of a thin membrane. Table S3 shows the increase in polymer solution viscosity induced by the blended fCNTs. The functional groups of the membranes were analyzed by FTIR spectroscopy, and the water contact angles of the membranes were measured by the sessile bubble method (Fig. 2). It was observed that the fCNT-CA membranes exhibited five peaks in FTIR spectra, representing the presence of the following functional groups: 3480 cm1 (AOH), 1740 cm1 (C@O), 1640 cm1 (>C@O), 1430 cm1 (ACOO), and 1360 cm1 (ACOOH) (Choi et al., 2006; Yuen et al., 2008). As such, we suggest that the fCNT-CA membranes may be formed via hydrogen bonding interactions

For the alginate fouling resistance test, the 200 mg/L and 1000 mg/L alginate foulants were added to the feed solution in the presence of Ca2+ (0.5 mM and 1 mM, respectively), and the normalized water flux decline was measured (Fig. 4). The same types of membranes had similar initial water fluxes (bare CA: 13.21 LMH, 1% fCNT-CA: 20.27 LMH for different foulant concentrations) due to the membrane surface characteristics. As the membranes were operated, the normalized fluxes decreased at the initial stage due to alginate fouling layer formation, and, then the normalized fluxes stabilized (stabilized flux in 200 mg/L alginate foulant, 0.31 of the bare CA and 0.49 of the 1% fCNT-CA; in 1000 mg/L alginate foulant, 0.15 of the bare CA and 0.35 of the 1% fCNT-CA) as shown in Table S4. The fCNT-CA membrane showed 37% less flux decline in the 200 mg/L alginate fouling and 57% less flux decline in the 1000 mg/L alginate fouling than the bare CA membrane. Also, less alginate fouling layer accumulated on the 1% fCNT-CA membrane. An interaction force between foulant and membrane surface is one of the critical factors governing membrane fouling phenomena (Mi and Elimelech, 2008). In this study, the alginate has 3.2 log Kow and 1.5–3.5 pKa values, which demonstrates that it is

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1430 1360

3480

1740 1640

Fig. 1. Sem images of surface (a–d) and cross-sectional (e–i) structures of the fCNT-CA membranes (a, e: Bare CA, b, f: fCNT 0.1%, c, g: fCNT 0.5%, d, h, i: fCNT 1%). The fCNT is shown in the 1% fCNT-CA membrane (i).

4000

Contact angle (°)

Transmittance (%)

75

Bare CA fCNT 0.1% fCNT 0.5% fCNT 1%

3500

70 65 60 55 50

Bare CA 3000

2500

2000

1500

1000

Wavelength (cm-1)

0.1%

0.5%

1.0%

f-CNT Content

Fig. 2. Ftir spectra (left) and water contact angles (right) of the fCNT-CA membranes. Five peaks of the fCNT-CA membranes, representing the presence of the following functional groups in FTIR spectra: 3480 cm1 (AOH), 1740 cm1 (C@O), 1640 cm1 (>C@O), 1430 cm1 (ACOO), and 1360 cm1 (ACOOH).

hydrophilic and negatively charged in neutral pH. Fig. 2 shows that the fCNT-CA membranes became more hydrophilic as the fCNTs were blended. With regard to considerations of hydrophobic interaction, the fCNT-CA membrane might induce severe alginate fouling, caused by enhanced hydrophobic direct interaction between the alginate and the membrane surface. Simultaneously, hydrophilic membrane could reduce the direct interaction by adsorbing a thin layer of water molecules on the membrane surface (Tiraferri et al., 2012). Regarding electrostatic interaction, the acid-treated MWCNTs enhanced the membranes surface negativity (Esumi et al., 1996; Fogden et al., 2008; Liao et al., 2008; Celik et al., 2011a). Further, the fCNT-CA membranes possess new peaks of COO, supporting an enhanced negative surface charge of the membrane surface. To investigate the interaction force between the alginate foulant and the membrane surface, the interaction force distribution was measured by AFM. Carboxylate-modified latex (CML) particle was utilized as an alginate surrogate. Fig. 5 shows the adhesive interaction force distribution between the CML particle and the

membrane surface in DI water and in a 1 mM Ca2+ solution. This figure consists of three regions: a negative region as adhesive force, zero as neutral, and positive as repulsive. In DI water, the average values of the forces were measured at 0.11 nN for the bare CA membrane and 0.38 nN for the 1% fCNT-CA membrane. Both membranes have similar proportions of adhesive force frequencies (fCNT-CA membrane: 8%, bare CA membrane: 9%). The fCNT-CA membrane has approximately twice the level of repulsive force proportion of the bare CA membrane (fCNT-CA: 50%, bare CA: 24%). This demonstrates that the fCNT-CA membrane repulses alginate foulant more strongly than does the bare CA membrane due to surface charge. In the presence of Ca2+, the force distributions were shifted to the adhesive region, which means that the two membranes are more adhesive with alginate foulant. This indicates that calcium cation induces severe alginate fouling due to the bridge effect of calcium cation and alginate (Mi and Elimelech, 2008). The average forces were 1.17 nN for the bare CA membrane and 0.81 nN for the 1% fCNT-CA membrane. Based on adhesive force distribution frequency, the bare CA membrane is more adhesive

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Water Permeate Flux Reverse Solute Flux

(a)

15

5

5

Bare CA

fCNT 0.5%

fCNT 1%

0 -5

-4

-3

-5

0

1

2

-4

-3

-2

-1

0

1

2

1

2

1

2

Adhesion Force (nN)

Fig. 3. Water permeate flux and reverse solute flux of the membranes. Water permeate flux (LMH) measured in FO mode (draw solution: 3 M NaCl, feed solution: DI water, temperature: 20 ± 1 °C, crossflow: 1000 ccm). Reverse solute flux (mol/ m2 h, molMH) measured in FO mode (draw solution: 1 M NaCl, feed solution: DI water, temperature: 20 ± 1 °C, crossflow: 1000 ccm).

(b)

0.6 0.4 0.2 0.0

-3

-2

-1

0

fCNT 1% (m=-0.81 nN)

20

-4

-3

-2

-1

0

Fig. 5. Interfacial force distribution between the membrane surfaces and carboxylate-modified latex (CML) particle of AFM cantilever; (a) in DI water, (b) in 1 mM Ca2+ solution.

acids in order to increase the dispersion in organic solvent. FTIR analysis of the fCNT-CA membranes showed that there might be hydrogen bonding interactions between the carboxylic groups of fCNTs and the acetyl groups of CA polymer. (2) The membrane characterization and lab-scale FO testing confirmed that the fCNT content of the synthesized membrane increased the membrane hydrophilicity and water permeated flux. Although bare CA membrane showed less water flux than HTI FO membrane, the fCNT-CA membrane showed better water flux without significant sacrifice of reverse solute diffusion.

(b)

no alginate 200mg/L alginate 1000 mg/L aiglnate

fCNT 1% 1.0

Normalized Flux

0.8

-4

Adhesion Force (nN)

(1) The fCNT-CA membrane was fabricated via a phase inversion method. Prior to membrane synthesis, MWCNTs were functionalized with a strong acid mixture of nitric and sulfuric no alginate 200mg/L alginate 1000 mg/L aiglnate

-5 40

-5

The fabrication and alginate anti-fouling behavior of fCNT-CA membrane for forward osmosis was firstly investigated, leading to several conclusions. They include the following:

Bare CA

0

0

4. Conclusions

1.0

Bare CA (m=-1.17 nN)

40 20

Frequency (%)

with CML particles than the 1% fCNT-CA membrane (negative force distribution proportion for bare CA: 72%, fCNT-CA: 62%). Furthermore, the 1% fCNT-CA membrane had a proportion of repulsive force that was four times greater than that of the bare CA membrane (bare CA: 4%, fCNT-CA: 18%). This indicates that the fCNT-CA membrane is much more repulsive against alginate foulant in Ca2+ presence than the bare CA membrane due to enhanced negative charge of the membrane surface. Hence, it is concluded that electrostatic force is dominant in alginate fouling, and alginate fouling is reduced in the fCNT-CA membrane. It has been reported, moreover, that the alginate fouling tendency is extremely reversible in the FO mode (Mi and Elimelech, 2010). Thus, this fCNT-CA membrane is expected to show very high flux recovery after membrane cleaning and also to require reduced energy for the cleaning as compared to bare CA membrane because it is more repulsive against alginate foulant.

Normalized Flux

-1

0

HTI FO

Membrane

(a)

-2

fCNT 1% (m=0.38 nN)

80 40

0

0

Bare CA (m=0.11 nN) 80 40

Frequency (%)

10

10

Reverse Solute Flux (molMH)

Water Permeate Flux (LMH)

15

0.8 0.6 0.4 0.2 0.0

0

100

200

Time (min)

300

400

0

100

200

300

400

Time (min)

Fig. 4. Water flux decline comparison of the fCNT-CA membranes on alginate fouling; (a) Bare CA, (b) fCNT 1%; Feed solution: DI water (no alginate), 200 mg/L alginate & 0.5 mM Ca2+ solution (200 mg/L alginate), and 1000 mg/L alginate & 1 mM Ca2+ solution (1 g/L alginate) respectively, draw solution: 3 M NaCl solution, crossflow rate: 1000 ccm, temperature: 20 ± 1 °C.

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(3) In alginate fouling, the fCNT-CA membrane displayed 57% less flux decline than bare CA membrane. A comparative AFM force distribution analysis of the alginate surrogate and the membrane surface further confirmed that the fCNT-CA membrane is more repulsive against the alginate than the bare CA membrane. This indicates that the electrostatic repulsive force is dominant in alginate fouling of the fCNT-CA membrane. (4) This synthesized fCNT-CA membrane showed membrane performance enhancement compared to the CA bare membrane. Since the same fabrication was utilized for this NEM, fCNT could be easily applied to commercial polymeric FO membrane. This demonstrates that the fCNT-NEM can contribute to a greater applicability of the FO process via performance enhancement. Under the consideration of fouling reduction, this membrane could be applied to pretreatment processes prior to wastewater treatment and desalination.

Acknowledgments This research was supported by the R&D Program of the Society of the National Research Foundation (NRF) and funded by the Ministry of Science, ICT & Future Planning (Nos. NRF-2014M3C8A4030498, 2012R1A2A2A03046711, and 2014K000274) and the Basic Research Projects in High-tech Industrial Technology funded by GIST. The authors acknowledge Miji Kim in ‘‘Environmental Geochemistry Laboratory’’ of GIST for her contributions in ion concentration measurement. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere. 2015.05.003. References Adham, S., Oppenheimer, J., Liu, L., Kumar, M., 2007. Dewatering Reverse Osmosis Concentrate from Water Reuse Applications Using Forward Osmosis. Water Reuse Association. Amini, M., Jahanshahi, M., Rahimpour, A., 2013. Synthesis of novel thin film nanocomposite (TFN) forward osmosis membranes using functionalized multiwalled carbon nanotubes. J. Membr. Sci. 435, 233–241. Amirilargani, M., Sadrzadeh, M., Mohammadi, T., 2010. Synthesis and characterization of polyethersulfone membranes. J. Polym. Res. 17, 363–377. Asatekin, A., Kang, S., Elimelech, M., Mayes, A.M., 2007. Anti-fouling ultrafiltration membranes containing polyacrylonitrile-graft-poly(ethylene oxide) comb copolymer additives. J. Membr. Sci. 298, 136–146. Baek, Y., Kim, C., Seo, D.K., Kim, T., Lee, J.S., Kim, Y.H., Ahn, K.H., Bae, S.S., Lee, S.C., Lim, J., Lee, K., Yoon, J., 2014. High performance and antifouling vertically aligned carbon nanotube membrane for water purification. J. Membr. Sci. 460, 171–177. Brady-Estévez, A.S., Kang, S., Elimelech, M., 2008. A single-walled-carbon-nanotube filter for removal of viral and bacterial pathogens. Small 4, 481–484. Cath, T.Y., Childress, A.E., Elimelech, M., 2006. Forward osmosis: principles, applications, and recent developments. J. Membr. Sci. 281, 70–87. Celik, E., Liu, L., Choi, H., 2011a. Protein fouling behavior of carbon nanotube/ polyethersulfone composite membranes during water filtration. Water Res. 45, 5287–5294. Celik, E., Park, H., Choi, H., Choi, H., 2011b. Carbon nanotube blended polyethersulfone membranes for fouling control in water treatment. Water Res. 45, 274–282. Choi, J.-H., Jegal, J., Kim, W.-N., 2006. Fabrication and characterization of multi-walled carbon nanotubes/polymer blend membranes. J. Membr. Sci. 284, 406–415. Donlan, R.M., Costerton, J.W., 2002. Biofilms: survival mechanisms of clinically relevant microorganisms. Clin. Microbiol. Rev. 15, 167–193. Dumée, L., Lee, J., Sears, K., Tardy, B., Duke, M., Gray, S., 2013. Fabrication of thin film composite poly(amide)-carbon-nanotube supported membranes for enhanced performance in osmotically driven desalination systems. J. Membr. Sci. 427, 422–430. El Badawi, N., Ramadan, A.R., Esawi, A.M.K., El-Morsi, M., 2014. Novel carbon nanotube–cellulose acetate nanocomposite membranes for water filtration applications. Desalination 344, 79–85.

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