Improvement of polyvinyl chloride nanofiltration membranes by incorporation of multiwalled carbon nanotubes modified with triethylenetetramine to use in treatment of dye wastewater

Journal of Environmental Management 242 (2019) 90–97

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Research article

Improvement of polyvinyl chloride nanofiltration membranes by incorporation of multiwalled carbon nanotubes modified with triethylenetetramine to use in treatment of dye wastewater

T

Vahid Vatanpour∗, Nasim Haghighat Department of Applied Chemistry, Faculty of Chemistry, Kharazmi University, 15719-14911, Tehran, Iran

A R T I C LE I N FO

A B S T R A C T

Keywords: Membrane Nanofiltration Modified carbon nanotubes Dye wastewater Polyvinyl chloride

Multiwalled carbon nanotubes modified with triethylenetetramine (TETA) as an organic nanofiller was used in fabrication of polyvinyl chloride (PVC) nanofiltration membranes. The membranes were prepared by the phase separation method and immersion precipitation technique. For this purpose, various percentages of the TETAMWCNTs were added to the casting solutions and the membrane films were formed and placed in a bath water. In order to identify the membranes and their properties, SEM images, contact angle and FTIR-ATR analyses were taken from the prepared nanocomposite membranes. The membranes performance in terms of water/protein/ dye permeability, protein rejection and Lanasol blue 3R dye rejection were investigated. Establishing hydrogen bond between the water molecules and the functional groups of MWCNTs enhanced the hydrophilicity of the fabricated membranes and caused an increase in permeability. The permeability in the membrane containing 0.25 wt% of TETA-MWCNTs reached its highest value, and adding more amounts reduced flux by blocking the membrane pores. There was also a significant decrease in the rate of membrane fouling for the hybrid membranes. Flux recovery ratio reached from 62.2% to 76.1%. Also, rejection of BSA and Lanasol blue 3R combination dye was increased for the modified membranes.

1. Introduction Recent advances in different industries, especially the water treatment industry, have been focused to increase the speed of the process and reducing energy consumption by respecting environmental laws. One of the most important processes in the water purification industry is the separation of various materials. In this regard, the use of membrane processes has been developed. The performance of membrane systems basically depends on the material used in the membrane structure (Cheng et al., 2018). The major limitation for membrane processes is fouling phenomenon. The membrane fouling is caused by deposition of colloidal particles and adsorption of natural organic matter (NOM) on to the membrane surface or into membrane pores; and has adversely affecting on flux decline for example in permeability and selectivity. It also reduces the useful life of the membrane (Guo et al., 2012). Many foulants are naturally hydrophobic and also, the polymeric membranes usually have less hydrophilicity (Rana and Matsurra, 2010). So, several techniques were used to improve hydrophilicity and decrease fouling phenomenon of the membranes such as grafting hydrophilic monomers on

∗

membrane surface (Hoseinpour et al., 2018) or using hydrophilic polymeric additives such as poly (ethylene glycol) (PEG) to main polymer of the membrane (Ma et al., 2011; Sun et al., 2018). Many studies are concentrated on efficacy of nanoparticle mixed matrix membranes (MMMs) on the membrane fouling decline (Li et al., 2007; Kim and Van der Bruggen, 2010). These membranes are a combination of solid particles which dispersed within the polymer matrix, are an effective solution for increasing the efficiency of polymer membranes. Zeolites, silica, metal oxide nanoparticles, clay, graphite, graphene oxide or carbon nanotubes are examples in this regard (Gopi et al., 2018; Merkel et al., 2002; Bottino et al., 2002; Farahani and Vatanpour, 2018; Nasrollahi et al., 2018; Cheng et al., 2017). Cellulose acetate (CA) (Saljoughi and Mohammadi, 2009), polysulfone (PSf) (Ghaemi et al., 2012), polyethersulfone (PES) (Zhao et al., 2013; Van der Bruggen, 2009), polyacrylonitrile (PAN) (Yang and Liu, 2003), polyvinyl chloride (PVC) (Davood Abadi Farahani et al., 2016; Fan et al., 2014), polyvinylidene fluoride (PVDF) (Zeng et al., 2016), and polyether imide (PEI) (Albrecht et al., 2001) are polymers that mainly use for membranes fabrication. Among these polymers, the PVC has received much attention because of favorable chemical, thermal

Corresponding author. E-mail addresses: [email protected], [email protected] (V. Vatanpour).

https://doi.org/10.1016/j.jenvman.2019.04.060 Received 12 September 2018; Received in revised form 1 April 2019; Accepted 16 April 2019 0301-4797/ © 2019 Elsevier Ltd. All rights reserved.

Journal of Environmental Management 242 (2019) 90–97

V. Vatanpour and N. Haghighat

List of symbols

PEG PVC Q R Rir Rr Rt SEM TETA V ω1 ω2 ε μ ΔP Δt

the membrane surface area [m2] particular concentration in feed [ppm] particular concentration in permeate [ppm] the mean density of water and isopropyl alcohol [0.892 g/ cm3] FRR flux recovery ratio FTIR Fourier transform infrared HA humic acid Jp the BSA solution flux [L/m2 h] jw,1 the pure water flux [L/m2 h] jw,2 the pure water flux after fouling [L/m2 h] L membrane thickness [m] MWCNTs Multiwalled carbon nanotubes NF Nanofiltration A CF CP dw

Polyethylene glycol Polyvinyl chloride the water flux [m3/s] rejection [–] irreversible fouling ratio [%] reversible fouling ratio [%] total fouling ratio [–] scanning electron microscope Triethylenetetramine the volume of permeated water [L] the weight of wet membrane [g] the weight of dry membrane [g] overall porosity [–] the viscosity of water [8.9 × 10−4 Pa s] the operation pressure [0.2 MPa] the permeation time [h]

between the carboxylated nanotubes and (5- isosyanoate iso-ethaulchloride). It has been found that adding the appropriate amount of MWCNTs leads to a decrease in the water content angle, an increase in the average size of the pores, and an improvement in permeability of the blended membranes (Qiu et al., 2009). Wu et al. created new ultrafiltration membranes using the MWCNTs in the body of BPPO (phenylene oxide). By doing so, they were able to increase the flux of membranes and to improve the chemical properties and membrane performance significantly (Wu et al., 2010). There are some reports that show presence of amine functional groups in membrane structure could increase membrane fouling resistance (Ma et al., 2016; Vatanpour et al., 2014, 2017). These amine groups could change surface charge of the membranes from negative to positive charge (Peydayesh et al., 2018). Among various amine based compounds, triethylenetetramine (TETA) by four active amine groups is a worthy nominee to react by MWCNTs (Wang et al., 2013). In addition, attachment of TETA to MWCNTs is a promising approach for overcoming two key difficulties of poor dispersibility and weak interfacial bonding of MWCNTs into matrix of polymer by improvement of the interfacial adhesion between the polymers and the functionalized MWCNTs (Li et al., 2008). In this study, our goal was to increase flux and reduce fouling of EPVC nanofiltration membranes. So, TETA attached MWCNTs were synthesized and used in membrane fabrication. The membrane's morphology was observed by SEM images. The pure water, protein and dye permeation, BSA protein and dye rejection of the membranes were examined. Fouling resistance of the prepared nanocomposite membranes was considered using BSA as a foulant. Using these procedures, the influence of TETA-MWCNTs as a modifier on the performance of microporous PVC membrane was investigated. To find out the industrial use of fabricated membranes we chose Lanazol blu-3R dye. This dye with azo-structure that has a color-bound agent dual nitrogen is one of the popular dyes in the industry textile and leather manufacturing. This dye and products from its degradation are toxic, carcinogenic and mutagenic. Hence, due to the toxicity and low degradability it is in the group of hazardous substances for the environment which must be refined before discharging.

and mechanical properties and also it's especially low cost compared with others. The PVC is well soluble in various solvents such as N,N dimethyl acetamide (DMAc) (Shu et al., 2011; Liu et al., 2012a, 2012b); tetrahydrofuran (THF) (An et al., 2003); dimethylformamide (DMF) (Bodzek and Konieczny, 1991); and N- Methylpyrrolidone (NMP) (Davood Abadi Farahani et al., 2016). In general, PVC polymer is produced in three commercial ways. Accordingly, PVC is the result of suspensions polymerization, emulsion polymerization and bulk or mass polymerization, respectively, which is called SPVC, EPVC and MPVC. In emulsion polymerization, surfactants are applied to disperse the vinyl chloride monomer in water. The hydrophobic nature of PVC increases the tendency for fouling in membranes fabricated from this polymer so it's a serious problem for using in water treatment. Therefore, blending modification is considered as an effectual approach to enhance the hydrophilicity of PVC membrane surface. Liu et al. used Pluronic F 127 as a polymer additive on the PVC membranes. Their research showed that this polymeric additive reduced the size and density of the pores and also reduced the flux of the membrane but improved the membranes antifouling properties (Liu et al., 2012b). Xu et al. examined ultrafiltration hollow fiber membranes fabricated by PVC and DMAc solvent. They also used PVP and PEG as polymeric pore former additives (Xu and Xu, 2002) and observed that by addition of PEG, mechanical properties and protein rejection were decreased; however, porosity and flux were increased. Peng and Sui produced PVC/polyvinyl butyral (PVB) blend membranes. The PVB was used as the second polymer component to enhance hydrophilicity of the PVC ultrafiltration membranes (Peng and Sui, 2006). Recently, in membrane separation processes, great attention has been paid to carbon nanotubes based mixed matrix membranes (MMMs). The results of mixing carbon nanotubes with polymer (CNTMMMs) provided hopeful results to overcome the limitations of common polymeric and inorganic membranes. The carbon nanotubes; both raw and functionalized are used in the process of making the membranes (Vatanpour and Safarpour, 2018). The efficiency of using CNTs in membranes depends on the solubility, dispersion and compatibility with the polymer body. This will be accomplished through functionalization of carbon nanotubes. Different types of functionalized carbon nanotubes have been developed to improve compatibility with the polymer. Choi et al. studies showed that polysulfone membranes mixed with multiwalled carbon nanotubes, functionalized with strong acid had better performance than the bare polysulfone membranes. Additionally, hydrophilicity of the mixed membranes improved (Choi et al., 2006). In another study, a polysulfone/carbon nanotube mixture was used to fabricate ultrafiltration membranes, and for suitable connection between the polymer and the nanotube, the MWCNTs were isolated by isocyanate and iso-phthaloyl chloride groups, which were reacted

2. Material and methods 2.1. Chemicals Raw MWCNTs were purchased from Plasm chem GmbH in Germany. TETA (C6H18N4), sulfuric acid, nitric acid, thionyl chloride (SOCl2), dimethyl formamide (DMF), ethanol, isopropyl alcohol, PEG additive (MW = 6000 g/mol), solvent of NMP with purity of 99.5%, bovine serum albumin (BSA) as a foulant, Lanasol blue 3R dye 91

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formation and fixation of the morphology. In this step, the membranes were removed from the coagulation bath and stored in a container for another 16 h in fresh distilled water until the morphology is completely stabilized. Since the most commercial membranes are dry, so the formed membranes were placed between two sheets of filter papers for 24 h at room temperature for drying. The thickness of the dried membranes was measured with a micrometer, and the membranes had a thickness of 80–100 μm. Fig. 1 presents the schematic illustration for the preparation of the TETA-MWCNT/PVC nanofiltration membranes.

(Reactive Blue 50) (Fig. S1) were supplied by Merck. The used dye is an acidic dye by molecular weight of 789.4 g/mol, soluble in water, containing negative charge in natural pH with isoelectric point of 3.9. EPVC from Arvand Petrochemical Co., Iran, was used as the polymerforming membrane. Distilled water was used as the non-solvent agent. 2.2. TETA-MWCNTs synthesis To synthesis the functionalized MWCNTs with TETA, first, in order to eliminate the possible impurities of this compound and to create acidic groups on the raw MWCNTs surfaces, the nanotubes were exposed in mixture sulfuric acid (98%) and nitric acid (68%) in ratio of 3:1 for 7 h at 40 °C in an ultrasonic bath. Then, 400 mg of acidified MWCNT was refluxed in DMF for 24 h at 70 °C in a mixture of 80 mL of SOCl2 and 4 mL of DMF. By evaporation, the SOCl2 residue was removed and the obtained black solid was dried at room temperature in a vacuum oven. The resulted solid was reacted with 120 mL of TETA for 96 h at 120 °C and washed with pure ethanol to remove excess TETA.

2.4. Characterization of the membranes 2.4.1. Membrane identification and morphology Functional groups created on the surface of MWCNTs and the presence of these modified MWCNTs on the surface of the prepared membranes were identified and validated using Fourier Transform Infrared Spectra (FTIR) PerkinElmer Spectrum RX1 using potassium bromide (KBr). Morphology of the MWCNTs powder and the TETAMWCNTs/PVC NF membranes was inspected under VEGA‖(TESCAN, Czech Republic) Scanning Electron Microscope (SEM). The membrane samples were prepared by fracturing them in liquid nitrogen. All samples were coated with gold. SEM images were taken under very high vacuum conditions, operating at 20 kV. The zeta potentials of the membranes were measured through a streaming potential method using an EKA Electro Kinetic Analyzer instrument (Anton Paar, Austria). The pH of the solution was adjusted by adding NaOH or HCl solution, and the measurement was performed at 25 °C.

2.3. Preparation of NF EPVC membranes TETA-MWCNTs blended EPVC nanofiltration membranes were synthesized by the phase separation method and immersion precipitation technique. To this aim, firstly, several specified amounts of TETAMWCNTs were carefully weighed and then added to pre-weighed solvent vessels. The solvent used in this study was NMP. The vessels containing magnets were first placed on a magnetic stirrer for 30 min and another 30 min in the ultrasonic bath. The carefully weighed EPVC and PEGs were then added to the resulting mixture to make 15 wt% PVC polymeric solutions. The weight percent of the MWCNTs is based on the weight of the polymer (0.0, 0.05, 0.1, 0.25.0.5 and 1.0 wt%). Table S1 represents the weight percent of the compounds in solution against the polymer. The solution was stirred at room temperature for one night to complete dissolving. The final solutions, depending on the amount of MWCNTs, were colored from colorless to dark. After dissolving, the casting solutions were placed in an ultrasonic bath for 30 min to remove soluble bubbles. Then, the dope solutions were casted on a glass plates, using the casting knife with a thickness of 170 μm. After that, the whole collection of glass and polymer films was rapidly entered to the water bathroom. The coagulation bath temperature was kept constant during the process and equal to 25 °C. After the coagulation process, the membranes separated from the glass surface after the

2.4.2. Hydrophilicity and surface wettability To measure the contact of angle and to evaluate the hydrophilicity of the membranesurface, the images of 3 μl deionized water droplets on the membrane surface were obtained, using OCA20, Dataphysics Instruments, Germany, which are taken at 25 °C with a relative humidity of 50%. To determine the average contact angle value, five measurements were done at different points on the membrane surface.

2.4.3. Porosity measurement The porosity of the membranes was obtained by applying the gravimetric method (Farahani and Vatanpour, 2018).

Fig. 1. Schematic illustration for the preparation of the TETA- MWCNT/PVC nanofiltration membranes. 92

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Table 1 Overall porosity of the prepared membranes.

ε=

Membrane name

Porosity (%)

Bare PVC 0.25% Raw CNT 0.05% TETA 0.1% TETA 0.25% TETA 0.5% TETA 1% TETA

2.0 ± ± 5.6 5.9 ± 2.2 ± 3.2 ± 4.1 ± 3.2 ±

74.9 73.1 81.8 82.6 83.8 80.2 79.5

ω1 − ω2 Α × l × dw

(1)

Where ω1 and ω2 (g) are the weights of the wet and dry membranes, l (m) is the membrane thickness, A (m2) is the membrane area, and dw is the mean density of water/isopropyl alcohol 50/50 vol% (0.892 g/ cm3). In this method, membrane samples were cut with 4 cm × 2 cm and their exact weight was determined. Then, the membranes were immersed in a 50/50 isopropyl alcohol-water solution for 24 h. After removing the samples from the solution, the membranes were weighed again. Their exact thickness was also calculated. The porosity of the membrane masses is evaluated using formula (1).

Fig. 2. Water contact angle and pure water flux of the TETA/MWCNTs/PVC membranes.

2.4.4. Water permeation experiments To evaluate the PWF and protein rejection performance, a dead-end filtration unit with a membrane effective area of 18.1 cm2 was used. The membranes were placed in distilled water for 1 h before the test started. After placing in the cells, the tank was filled with distilled water and the cells were filled with distilled water with air pressure. The pressure was applied 3 bar for 10 min to compact the membranes and the water leaked to a constant extent. The test was then carried out for 90 min at a pressure of 2 bar. The PWF was measured by formula (2).

JW , 1 =

V Α Δt

(2) 2

Where V (L) is the volume of permeated water, A (m ) is the membrane area, and Δt (h) is the permeation time. 2.4.5. Analysis of membrane fouling resistance In order to evaluate antifouling ability of the prepared membranes, a dead-end unit was also applied using BSA solution with a concentration of 250 mg/L and pH of 7.0 ± 0.1 as a foulant (using phosphate-buffered saline solution). BSA is a standard foulant, which its results will be comparable by other reported data. First, for 90 min, distilled water passed through the membrane at pressure of 2 bar and the flux was recorded. Then, the cells were refilled with BSA solution, and the membrane was placed under the same pressure for 90 min and the flux for BSA solution, Jp (L/m2 h), was obtained. To calculate the rejection of the BSA, Eq. (3) was used, where Cp (ppm) and CF (ppm) were the BSA concentration in permeate and feed solutions, respectively. Next, the membranes were removed from the cells and in order to washing stored at bath water for 20 min so that the sticking proteins were partially dissolved in water, and the membranes recovered. Afterwards, the membranes were re-inserted into the cells to measure second water flux. To evaluate the membrane fouling, using formula (4), the percentage of flux recovery ratio (FRR) was measured. The high percentage of FRR for modified membranes will indicate better antifouling properties.

CP ⎞ × 100 R (%) = ⎛1 − CF ⎠ ⎝

FRR (%) =

Jw, 2 × 100 Jw, 1

Fig. 3. (a). The results of BSA solution flux and BSA rejection test of the prepared mixed matrix membranes after 90 min, (b) BSA solution flux test for the prepared membranes.

fouling resistance including intrinsic resistance (Rm) induced by membrane pore size, chemistry and thickness, pore resistance (Rf) formed by the pore blocking, cake resistance (Rc) formed by the cake layer deposited on the surface of membrane, and also, total filtration resistance (Rt) were determined by following equations:

J =

ΔΡ μ ∑R

Rm =

(3)

(4)

To study the fouling process in NF membranes, the parameters of 93

ΔΡ μ Jw, 1

(5)

(6)

ΔΡ ⎞ Rf = ⎜⎛ ⎟ − Rm ⎝ μ JW , 2 ⎠

(7)

ΔΡ ⎞ Rc = ⎜⎛ ⎟ − Rm − Rf ⎝ μ Jp ⎠

(8)

Journal of Environmental Management 242 (2019) 90–97

V. Vatanpour and N. Haghighat

Fig. 4. Flux recovery ratio for the prepared membranes.

Rt = Rm + Rf + Rc

Fig. 6. The results of dye rejection by Lanasol blue 3R dye for the prepared membranes. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

(9)

Also, reversible fouling ratio (Rr) and irreversible fouling ratio (Rir) were calculated using Eqs. (10) and (11) (Zinadini et al., 2017).

Rr = ⎛ ⎝

Jw, 2 − Jp

⎜

⎞ × 100 ⎠

⎟

Jw, 1

(10)

Jw, 1 − Jw, 2 ⎞ Rir = ⎛ × 100 Jw, 1 ⎝ ⎠ ⎜

of the solutions and the sub-layer is responsible for protection and membrane strength (Wienk et al., 1996; Boom et al., 1992). As the images show, modified MWCNTs membranes have the same structure as the bare membrane so entering of nanotubes into a polymer solution does not alter the mechanism of membrane formation. The number of pores in hybrid membranes has been reduced by adding unmodified nanotubes. In fact, raw carbon nanotubes are impure and because of the van der Waals force between pipes, they tend to agglomeration and produce masses in a polymer solution, which blocks the surface of membranes made. Adding TETA-MWCNTs at low concentrations first converts the finger structure into macrovoid structure and adding more amounts of TETA-MWCNTs, converts the structure again into finger like but this time with more narrow and drawn pores. Also, the size of the upper pores first increases and then decreases in greater quantities. By comparing the surface SEM images (Fig. S4), it seems that the membranes containing TETA-MWCNTs have a sharper surface than the bare membrane. The uniform dispersion of MWCNTs on the surface of membrane is obvious in these pictures. Consequently, no possible aggregation could occur using nanotubes in the membrane structure. In addition, the surface of the fabricated membranes is relatively smooth and no cracks were recognized on the surface, which demonstrations that the membranes did not become brittle by the addition of the TETAMWCNTs, and have good stability.

⎟

(11)

3. Results and discussion 3.1. Characterization of TETA-MWCNTs The SEM images taken from the raw-MWCNTs and TETA-MWCNTs is shown in Fig. S2. It shows that the process of aminating has not destroyed the tube structure of the nanotubes and has maintained natural shape and size of the multiwalled nanotube. 3.2. Characterization of PVC nanocomposite membranes 3.2.1. The morphology of MWCNTs/EPVC membranes Fig. S3 shows the cross-section morphologies of membranes with different blend compositions. Since, the nanoparticles concentration was low and well spread, it was difficult to see them in the SEM images. All of the PVC membrane samples exhibit an asymmetric structure with a dense top layer and a porous sub-layer with a finger-like structure. The top dense layer has the responsibility for permeation and rejection

3.2.2. FTIR-ATR Fig. S5 (a) shows the FTIR spectra of MWCNTs and TETA-MWCNTs.

Fig. 5. Fouling parameters of the prepared membranes. 94

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Table 2 Researches done to improve PVC membranes in water treatment. Ref

Goal

Rejection (%)

Permeation (pressure) 2

Zhu et al. (2014)

Improvement of antifouling

75.2% (BSA)

Jhaveri et al. (2017)

Improvement of antifouling

100% (HA)

Yu et al. (2015a) Gholami et al. (2014) Yu et al. (2015b) Rabiee et al. (2015)

Improvement of antifouling Lead removal from water Resistance to bio-foulant Improvement of antifouling

90% (BSA) 35% Pb(II) 98.67% (E. coli) 97.5% (BSA)

Rabiee et al. (2014) Mishra and Mukhopadhyay (2017)

Improvement of antifouling Improvement in flux, rejection and antibacterial activity Hydrophilic moification of PVC membranes Improvement of antifouling

98% (BSA) 80% (BSA and HA) 100% (BSA) 97.66% (BSA)

Xu et al. (2015) This study

102.6 L/m h (0.1 Mpa) 152 L/m2 h (0.25 MPa) 31 L/m2 h (0.1 Mpa) 135 L/m2 h (0.1 Mpa) 49 L/m2 h (0.1 Mpa) 188.7 L/m2 h (0.2 Mpa) 222 L/m2 h (0.2 Mpa) 88 L/m2 h (0.2 Mpa) 54.3 (L/m2 h bar) 46 L/m2 h (0.2 Mpa)

Membrane module

Nanofiller

Polymer

UF

TETA

PVC

UF

GO-TiO2

PVC

UF NF Hollow fiber UF

SiO2 Fe3O4 Ag-n-TiO2 ZnO

PVC PVC PVC PVC

NF UF

TiO2 TiO2eMo.HNTs

PVC PVC

Hollow fiber NF

silica TETA-MWCNT

PVC PVC

(Zarrabi et al., 2016).

The raw MWCNTs FTIR spectrum does not show a characteristic absorption band, and only a widespread absorption band is located in the 3430 cm−1 area, which can be related to the high tendency of pure MWCNTs to absorb water and air humidity and stretching bond vibration of OeH groups (Manafi et al., 2017). In the FTIR spectrum of TETA-MWCNTs, the absorption bands at 869 cm−1 and 980 cm−1 are due to the bending vibration of the bond = CeH. The absorbance band of 1430 cm−1 could be related to the stretching vibration of the C]C bond in the carbon skeleton of MWCNTs. The absorbance bands at 1634 cm−1 (amide I), 1567 cm−1 (amide II), 1080 cm−1 (amide III) and 543 cm−1 (amide IV) are indicator of amide groups (Qiu et al., 2009) on the surface of MWCNTs, which approve the creation of TETA on the nanotube surface. Fig. S5 (b) shows the FTIR-ATR spectra of the bare and TETAMWCNTs/PVC membranes. The appearance of an absorption band at about 1600 cm−1 is because of the presence of amide (I) at the modified PVC NF membrane surface (Zhu et al., 2014). The reduction in the absorbance band at 1160 cm−1 is result of the presence of aliphatic CeN bond in the TETA-MWCNTs. The dual absorption band at 2910 cm−1 and 2964 cm−1 in the bare membrane, is related to the stretching vibration of aliphatic CeH and the increase in the intensity of this dual absorption band in the modified membrane is due to the stretching vibration of NeH in the groups of eCONH-, NH2, and eNH(Wang et al., 2006). The bands at 970 cm−1 and 1245 cm−1 indicate bending vibration outside and inside the bonding plate = CeH. The absorption band at 693 cm−1 refers to the stretching vibration of the CeCl bond in PVC. Two-branch absorption bands at the range of 750–850 cm−1 are related to the NH2 scissor motions. The absorption band in 1328 cm−1 is due to the stretching vibration of the CeH compound. And the band at 1710 cm−1 is due to the stretching vibration of the CeO bond.

3.3. Filtration properties 3.3.1. Permeation results Fig. 2 shows the results of PWF test for the prepared membranes and indicates that membranes containing raw MWCNT have less water flux than the bare membrane due to the agglomeration of raw carbon nanotubes and the obstruction of membrane pores. But all membranes mixed with modified carbon nanotubes showed more water flux than the bare membrane. The reason for this is due to the increase of porosity (Table 1) and the reduction of filtration resistance, and also improvement of membrane hydrophilicity (see Fig. 2, contact angle results). The flux increase continues until reaches it's maximum in the membrane with 0.25 wt% of modified carbon nanotubes. By increasing the amount of TETA-MWCNTs, flux decreases due to blockage of the membrane pores by these excess amounts and reducing porosity (Vatanpour et al., 2014). 3.3.2. Antifouling ability and rejection performance of the prepared membranes Resistance to fouling in the membranes is basically related to the hydrophilicity ability of the membranes. To evaluate antifouling performance of the fabricated NF membranes, the solution of BSA at a concentration of 250 mg/L was applied. The results are presented in Fig. 3a. It is observed that the NF membrane mixed with raw MWCNTs had the lowest flux and the lowest rejection for BSA and the membrane containing 0.25 wt% of modified MWCNTs had the highest flux. All the membranes rejected the BSA solution more than 95% (BSA retention is constant after 30 min filtration), but the membranes modified with TETA-MWCNTs showed better rejections. The rejections of BSA by 0.5 wt% of TETA-MWCNT was 97.7%. The slight increases in flux and rejections for BSA solution might be outcome of the enhancement in PVC membranes hydrophilicity and the antifouling ability. The flux charts in time for each of the seven membrane types is shown in Fig. 3b. As shown, 0.25 wt% presented best BSA solution flux during filtration. The best indicator for describing antifouling property is to use a flux recovery ratio. Fig. 4 shows the percentage of FRR for the prepared nanocomposite membranes. At this stage, the same membranes that were taken by PWF were used to filter the BSA protein. Then, the PWF was calculated again to determine the amount of fouling. All of the modified membranes showed a better percentage of FRR compared to the bare PVC membrane. The bare PVC membrane had a FRR of 62.2%, which increased to 76.1% by adding modified carbon nanotubes. This ability is due to the hydrophilicity improvement of the modified membranes. As described in section 2.4.5, fouling process in the fabricated hybrid membranes was studied using parameters Rm, Rf, Rc and Rt and its

3.2.3. Porosity and contact angle Table 1 shows the porosity of the prepared NF membranes. It was found that the porosity was increased with the addition of TETAMWCNTs. It can be interpreted that more gaps might be produced between the hydrophilic groups on TETA-MWCNTs and the hydrophobic PVC chains, which causes formation a more porous structure. Fig. 2 shows the contact angle of the prepared nanocomposite membranes. The contact angle decreases with addition of TETAMWCNTs and decreases regularly with increase in TETA-MWCNTs dosage. By addition of 1 wt% modified carbon nanotubes, the contact angle was dropped from 67.7° for the bare PVC to 51.6°. These improvements in hydrophilicity are due to the existence of polar groups like as free amines in the TETA-MWCNTs. Due to the polar nature of NeH bonds resulted from the difference in the electronegativity of H and N atoms, water molecules could absorb through hydrogen bonding interactions, leading to hydrophilicity improvement of the membranes 95

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results are shown in Fig. 5. As can be seen, a large part of resistance of PVC membranes is due to their intrinsic resistance (Rm), which has significantly reduced the flux of the bare and the hybrid membranes. Modification of PVC membranes by TETA-MWCNTs has been able to reduce this kind of resistance in the membranes. The results also show that, addition of TETA-MWCNTs has been a good way to reduce Rc, (formed by the cake layer deposited on the surface of membrane) and Rf in the NF membranes. These observations are the result of the hydrophilicity enhancement of the membranes by introducing an organic hydrophilic nanofiller (Xu and Xu, 2002). Fig. S6 shows the ratio of total fouling (Rt), reversible fouling ratio (Rr) and irreversible fouling (Rir) to water flux before using BSA solution and after removing it from the membrane surface after water washing. The results of this diagram are used to evaluate the prepared membranes antifouling properties. As the reversible fouling ratio increase and the irreversible fouling ratio decrease membranes show better performance against fouling. According this, as observed in Fig. S6, modified membranes show more reversible fouling ratio, as well as less irreversible fouling ratio. In fact, using of TETA-MWCNTs, has increased the hydrophilicity of prepared membranes so the adhesion of organic matter to the surface of the membrane and blocking it's pores has reduced. The highest amount of irreversible fouling observed for the bare membrane.

casting solution, including PEG 6k Da as a pore former and NMP as a polymeric solvent. The membranes were synthesized via the phase inversion method. The water flux increased from 61.9 L/m2h for the bare PVC membrane to 107.9 L/m2h for 0.25 wt% of TETA-MWCNT/PVC membrane. It is because of increase in the surface hydrophilicity of the synthetic membranes and more porous structure. By increasing the amount of modified carbon nanotubes, flux decreases due to blockage of the membrane pores by these excess amounts and reducing porosity. All of the modified membranes showed a better flux recovery percentage and better antifouling behavior in comparison with the pure PVC membrane. The results showed that addition of 0.25 wt% TETAMWCNT was optimum to achieve the best results for water flux, BSA solution flux and dye rejection.

3.3.3. Effect of dye separation performance Separation performance of the formed NF membranes was evaluated by solution of Lanasol blue 3R dye with concentration of 100 mg/ L and its natural pH in this concentration was 4.76. Rejection was done using dead-end unit during filtration period of 90 min under operating pressure of 2 bar and temperature of 25 °C. Fig. 6 shows the results of dye rejection. The results clearly show that the rejection capability was increased for the modified membranes which can be ascribed to enhancement of their hydrophilicity that prevents adsorption of dye molecules. Rejection for the membrane contains of 0.25 wt% of TETAMWCNTs was observed to be 86%. Table 2 shows some studies have been done to apply PVC membranes in water purification industry. Various types of organic and inorganic nanoparticles have been used to improve the PVC membranes with the aim of increasing the resistance against to various types of foulant and removal of heavy metals. As the studies show using TETAMWCNTs in PVC membranes by changing surface charge of the membranes from negative to positive charge has improved membrane fouling resistance. Therefore, we expected that by applying MWCNTs and attaching TETA to their surface, we would help better contribution of positive charge on the membrane surface, and we could create membranes with better resistance to fouling. The obtaining a higher percentage of rejection in the TETA-MWCNT/PVC membranes clearly confirms this aim has been achieved. Fig. S7 shows the zeta potential of bare EPVC and 0.5 wt% TETAMWCNTs embedded membranes at different pH. It could be seen that the zeta potential of the 0.5 wt% TETA-MWCNTs membrane is higher than that of the unfilled PVC membrane at all pH. The bare PVC membrane was negatively charged at pH 7.0, which is owing to the carboxyl group from vinisol, the additive used in PVC membrane preparation (Wu et al., 2018). The significant increase in zeta potential from −35.4 mV to −15.6 mV in pH = 7 after the nanotubes addition could be attributed to the amine groups of the TETA on the MWCNTs surface. This increase in positive charge could improve membrane antifouling properties.

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Acknowledgments The authors gratefully acknowledge the financial support of Kharazmi University (Grant Number: D/2063). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jenvman.2019.04.060.

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