Iron oxide (FeO) nanoparticles embedded thin-film nanocomposite nanofiltration (NF) membrane for water treatment

Accepted Manuscript Iron oxide (FeO) nanoparticles embedded thin-film nanocomposite nanofiltration (NF) membrane for water treatment Sonia R. Lakhotia, Mausumi Mukhopadhyay, Premlata Kumari PII: DOI: Reference:

S1383-5866(18)30779-2 https://doi.org/10.1016/j.seppur.2018.09.034 SEPPUR 14932

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

7 March 2018 3 August 2018 10 September 2018

Please cite this article as: S.R. Lakhotia, M. Mukhopadhyay, P. Kumari, Iron oxide (FeO) nanoparticles embedded thin-film nanocomposite nanofiltration (NF) membrane for water treatment, Separation and Purification Technology (2018), doi: https://doi.org/10.1016/j.seppur.2018.09.034

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Iron oxide (FeO) nanoparticles embedded thin-film nanocomposite nanofiltration (NF) membrane for water treatment Sonia R. Lakhotiaab, Mausumi Mukhopadhyayb*, Premlata Kumaria

a

Applied Chemistry Department, Sardar Vallabhbhai National Institute of Technology,

Surat-395007, Gujarat, India. b

Department of Chemical Engineering, Sardar Vallabhbhai National Institute of Technology,

Surat-395007, Gujarat, India. *

Corresponding author. Tel.:. 0261-2251645, Fax: +91 261 2227334

E-mail

address:

[email protected];

[email protected]

(M.

Mukhopadhyay)

Abstract This research work investigated the usage of pre-seeding interfacial polymerization method using iron oxide (FeO) nanoparticles on the polyethersulfone (PES) support membrane for water treatment. The two different concentrations (low (0.05 wt%) and high (2 wt%) of FeO nanoparticles were used as the membrane surface modification parameter. Surface modified membranes were characterized by several characterizations methods. Membrane surface charge was calculated qualitatively by using Grahame equation with the help of contact angle measurements. Synthesized membranes showed 90% rejection with polyethylene glycol (PEG) with M.W. 1500Da, which was fitted in the range of nanofiltration. The membrane performance towards salt rejection, flux (at different pressure) and anti-fouling activity was evaluated using a various salt solution like CaCl2, MgCl2, C6H5Na3O7, Na2SO4, NaCl with an initial concentration of 2000 mg/L, saltwater (collected from Suvali beach, Surat, India). With the increase in the concentration of FeO nanoparticles, hydrophilic nature (84.7 to 49.6°), surface charge (-6.27 to 14.21 mC/m2) and flux (27.46 to 36.85 L/m2h) of the nanocomposite membranes were effectively enhanced, and also showed the high salt rejections (>90%). Anti-fouling activity of 1

the membrane with saltwater was analyzed by flux recovery ratio that showed FeO nanoparticles embedded membrane notably reduced the fouling. Keywords: FeO nanoparticle; Thin-film nanocomposite membrane; Nanofiltration; Antifouling membrane; Salt rejection;

1. Introduction One of the main challenges to sustain the society is to secure suitable water resources of needed quality for many designated uses. To address this challenge, membrane separation technique is expected to play a vital role in drinking water treatment, brackish and seawater desalination due to several advantages like low cost, low-process temperature, no by-product and high separation efficiency [1-4]. After the significant commercial success of reverse osmosis (RO) and ultrafiltration (UF), membranes with separation characteristics in between these two technologies are foreseen to have a promising market. Such membranes are referred to as nanofiltration (NF) membranes. It is an essential technique in food, chemical, and pharmaceutical industries [5]. Typically, NF membranes involve the separation of salts (monovalent and divalent), and/or organic solutes. NF technique is one of the largely used filtration technique especially in desalination and many industrial applications for the production of clean and safe water. Although the NF membranes have gone through the several improvements, there are still challenges for further development of the NF membranes such as trade-off relationship between permeability and selectivity, fouling and scaling are the current 2

major constraints of NF membranes. In current years, there have been several kinds of work carried out on enhancing the membrane performances [6]. Improvement in the flux and separation efficiency of the membrane has been the aim of these investigations [7]. Usually, the scheme that increases the water flux decreases the membrane performance and vice versa [8]. Therefore, it is more advantageous to use such a type of methods that can increase both flux and separation efficiency of the membrane, simultaneously [9]. Nanoparticle embedded membrane is the solution where nanoparticles contribute their inherent properties to the embedded membrane and enhance the membrane performance [6,9]. The aim of embedding of nanoparticles to the membrane preparation is to enhance the flux, and the separation efficiency compared to raw membranes [8]. Various types of nanoparticles are reported for the preparation of polymer composite [10] membranes to get benefits of their properties. Titanium oxide (TiO2) [11-14], zinc oxide (ZnO) [15] and silver (Ag) [16-19] nanoparticles have an antibacterial property; silica oxide (SiO2) nanoparticles [20-22] have a nature of electrical conductivity; carbon nanotubes (CNTs) such as single-walled CNTs (SWCNTs), multiwalled CNTs (MWCNTs) [23-25] and graphene oxide (GO) [26] have new pathways for water transport, iron (Fe) has catalytic properties, and iron oxide (FeO) nanoparticles give a magnetic property to the membrane [27-32]. Among all these nanoparticles, FeO nanoparticle is an effective material for water/wastewater treatment because of its large surface area, ease availability, cheaper, non-toxic hydrophilic nature with magnetic property and considered as no/low toxicity [33,34]. Due to these known properties of FeO, it is also used for many applications like cleaning of contaminated land, catalysis, sensors and environment remediation [28]. Literature found that FeO nanoparticles are highly effective sorbents for bacteria (E. coli and Stapyloccus aureus), viruses and heavy metals like As (V) and As (III) from water/wastewater [35]. Huang et al. 2006 and his co-author are evaluated the performance of magnetized and non-magnetized ultrafiltration membranes using pig blood solution. They proved that the non-magnetized membrane has a lower recovery of blood protein, flux, and relative flux compared to the equivalent magnetized membrane [29]. Its hydrophilic and magnetic nature is the important factor for flux and high-protein recovery. FeO blended polyethersulfone (PES) NF membrane [36] is fabricated to observe the copper removal ability from wastewater and its reusability checked using ethylene diamine tetra acetic acid (EDTA) as eluting agent. They found that membrane prepared with 0.1 wt% FeO nanoparticles shows the highest copper removal of about 3

92% and best performance observed after several times treatment with EDTA [36]. FeO nanoparticles are also important in polyvinylidene fluoride blended microfiltration membrane system for the degradation of trichloroethylene from water and real groundwater samples [37]. FeO nanoparticles embedded polysulfone (PS) membrane is prepared for dehydration process of ethanol/water mixtures [38]. The addition of FeO nanoparticles is effective for improving the physicochemical properties of membrane and mitigates the membrane fouling due to the formation of bio-films. Blending nanoparticles to the polymeric matrix for membrane preparation is the most common method but now-a-days, membrane surface coating with nanoparticles has been widely used to modify membrane functionality [7]. Homayoonfal and his co-author are focused on a comparative study of the effectiveness of blending and coating methods for membrane separations. They pointed out that nanoparticle coated membrane has improved separation processes (in terms of flux, hydrophilicity, and rejection) [8]. But, nanoparticles coated layer is less sustainable compared to blended nanoparticles in the polymeric matrix [6,8]. Therefore, they have put some modification to change the membrane surface using self-sealing interfacial polymerization technique to synthesize surface modified thin-film nanocomposite (TFN) NF membrane [7]. For the preparation of this type of surface modified membrane, PS and PES, a low pressure driven membrane are commonly used. Kim et al. 2013 evaluates membrane filtration systems for the separation of organic fraction from the oil and sands affected water [39]. Experiments were accomplished using membrane that consisted of low pressure driven membrane (LPM), high pressure driven membrane (HPM) and nanoparticles embedded high pressure-driven membrane (NHPM). To modify membrane physicochemical properties, membrane (NHPM) was prepared with MWCNTs material. The results showed that carbon material embedded surface that increasing the hydrophilicity, rejection of hydrophobic contaminants, permeate flux, and significantly reducing membrane surface fouling. Researchers are embedded several types of nanoparticles such as zeolite-A, TiO2, SiO2, and silver nanoparticles into thin-film NF membrane, which resulted in high flux with constant salt rejection, increased surface hydrophilicity and anti-bio-fouling properties. This type of surface modified membrane helps to create a stronger chemical bond between two highly reactive monomers (phenylenediamine and trimesoyl chloride) as compared to the nanoparticles blended membrane and in turn make sustainable surface layer. Using this method many 4

nanoparticles are coated on the porous membrane support such as TiO2 [14,40,41], SiO2 [10,20,42,43] Ag [44-46], zeolite [47-49] and CNTs [7,25] etc. They observed that surface modified nanocomposite membrane is a promising solution to the challenges like low flux, rejection, and resistance towards the fouling. It gives a self-cleaning membrane with TiO2, fouling-resistant with Ag, compaction-resistant with SiO2 and highly permeable membrane with zeolite and CNTs nanoparticles. By considering the advantages of FeO nanoparticles, in this work, FeO nanoparticles embedded thin-film coated nanocomposite membranes are prepared by a pre-seeding interfacial polymerization method. The surface charges characteristics, flux recovery and fouling study with FeO nanoparticles embedded membrane have not been studied so far. The prepared membrane is analyzed by the molecular weight cut-off (MWCO), contact angle, scanning electron microscopy (SEM), Fourier transform infrared (FTIR) attached with attenuated total reflection (ATR) spectroscopy and scanning probe microscopy (SPM). The effects from the presence of FeO nanoparticles are examined for the separation performance using a cross-flow filtration setup using a different salt solution like CaCl2, MgCl2, C6H5Na3O7, Na2SO4, NaCl, saltwater (collected from Suvali beach, Surat, India) for rejection performance and anti-fouling study. 2. Materials and Methods 2.1 Materials Polyethersulfone (PES) porous support (75 kDa) was purchased from Permionics Ltd., Baroda, India. FeO nanoparticles (Fe3O4, ~50 nm), m-phenylenediamine (MPD, Mw= 108.14), trimesoyl chloride (TMC, Mw= 265.48) were obtained from Sigma-Aldrich (St. Louis, MO, USA). The magnetic properties of iron oxide nanoparticles such as saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc) values are represented in the Supplementary file (Table S1). For the proper attachment of FeO nanoparticles sodium lauryl sulfate (SLS), hexane and ethanol were purchased from Finar Chemicals Ltd., Ahemdabad, India. Polyethylene glycol (PEG, Fisher Scientific, Mumbai, India) with different molecular weights were used for molecular weight cut-off (MWCO) study to analyze the type of membrane (ultrafiltration or nanofiltration). To evaluate the prepared membrane's rejection performance, different salts like sodium chloride (NaCl), sodium sulfate (Na2SO4), trisodium citrate (C6H5Na3O7), calcium chloride (CaCl2) and magnesium chloride (MgCl2) were purchased from 5

Finar Chemicals Ltd., Ahemdabad, India. Flux recovery experiment was done for anti-fouling study using saltwater collected from Suvali beach, Surat, India. The composition of saltwater was shown in the Supplementary file (Table S2). 2.2. Membrane preparation PES supported membrane with 5.0 cm diameter was placed in an aqueous solution of 0.15 wt% SLS and 2 wt% MPD for 3 min. After that excess solution of SLS and MPD was removed from the surface using a roller. A particular concentration (0, 0.05, and 0.2 wt%) of FeO nanoparticles were properly dispersed in 5 wt% ethanol solutions, separately, and subjected to sonication for 5 min. Then, 0.05 wt% organic phase (hexane in ethanol) containing TMC was mixed to this solution and sonicated for 5 min again to get proper dispersed pre-assembled FeO nanoparticles. Afterwards, a particular amount (0.4 ml) of this sonicated solution was quickly spread on the MPD coated layer. Spreading was done quickly because of the positive spreading coefficient of hexane solvent. After the completion of the reaction, membranes were dried in an open air at room temperature and then rinsed with distilled water. Four membranes were used in the experiments: low pressure driven membrane (LPDM) PES porous support, high pressure driven membrane (HPDM), high pressure driven membrane with 0.05 wt% FeO nanoparticles (NHPDM1), and high pressure driven membrane with 0.2 wt% FeO (NHPDM2). 2.3. Characterizations To observe surface chemistry and roughness of the membrane, FTIR-ATR (IRAffinity1S, Shimadzu) and scanning probe microscopy (SPM, Bruker, USA) analyses were used. To observe prepared membrane nature, a sessile drop (1 μL drop of pure water) method containing contact angle analysis (Data-Physics OCA15, GmbH, Germany) was used. The contact angle of buffered and unbuffered solutions at different pH was also used to calculate the surface charge (using fractional ionization (α), effective acid dissociation constant (pKa) and surface at a concentration of acidic groups [COO-]o) of membrane qualitatively by using the Grahame equation [28]. MWCO study was used to observe the type of the membrane using 1000 mg/L solution of PEG with different molecular weights (600, 1000, 1500 Da). This solution was passed through cross-flow filtration system (Fig. S1) and permeates were collected after reaching the steady-state condition. The concentration of PEG in feed and permeate were determined using UV spectrophotometer (DR 6000, HACH, USA) at a wavelength of 535 nm against a 6

reagent blank (barium chloride, iodine, and potassium iodide). Membrane morphologies (surface and cross-section) were analyzed under a FEG-SEM instrument by JSM-7600F (JEOL, Japan). Before imaging, membranes were freeze-dried, cracked in liquid nitrogen, and then covered with gold particles. Thickness of the membranes was measured by using thickness gauge meter. At least, three measurements were taken at different positions of the membrane to determine the average thickness of polyamide membrane. Thermal behavior of the membranes was also determined using a thermo gravimetric analyzer (TGA, Perkin Elmer) over a temperature range of 25–800 °C under oxygen atmosphere. To observe the bonding effect between FeO nanoparticles and membrane, mechanical strength was measured by mechanical testing machine (Tensometer, Kudale PC-2000) with the specific dimension of membrane 10 cm length and 1.5 cm width and estimated at 20 mm/min tensile rate. 2.4. Membrane performance (anti-fouling and salt rejection) Anti-fouling or re-usability performance of the FeO embedded membrane was studied by performing flux recovery experiment using saltwater collected from the Suvali beach according to the following steps: (i) pure water flux (PWF), (ii) filtration of saltwater solution was performed and flux is measured, (iii) washing in pure water and (iv) again pure water flux was measured. A laboratory-made cross-flow membrane filtration setup was used for all the experiments, in which four stainless steel membrane modules attached in series. The membrane modules were possessed space to put circular flat-sheet membrane piece having 4.9 cm diameter. A reciprocating diaphragm pump (model no: m ROY B-13) was used to pass the feed solution through the membrane. The valve located at the end of the module was used to pressurize the feed solution and control the feed pressure. All membranes were compressed at 200 psi pressure until the steady-state flow was achieved. Flux (J), flux recovery ratio (FRR) and flux reduction (FR) of the membrane were calculated using equation (1), (2) and (3) as shown below.

7

Where, J: the flux (L/m2h), A: the membrane area (m2), Vp: the permeate volume (ml) and t: treatment time (h). Jw1: the average pure water flux during the first stage, Jf: the feed flux (saltwater) solution and Jw2: the average of pure water flux during the third stage. The salt rejection performance of the membranes was observed by filtering NaCl, Na2SO4, C6H5Na3O7, CaCl2 and MgCl2 salt solution at an initial concentration of 2000 mg/L. The conductivity of a solution (feed and permeate) was measured by a conductivity meter connected with the system, and a calibration curve was used to relate the solution conductivity to salt concentration. The salt rejection, R, was calculated from the equation (4) as shown below:

Where Cp: the conductivity of salt in permeates and Cf: the conductivity of salt in feed solutions.

3. Results and discussion 3.1. Incorporation of FeO nanoparticles in membrane Membrane surface modification is one of the main approaches in the fouling mitigation way. An appropriate method for modification of membrane surface is as essential as a selection of particular nanoparticles. Basically, two methods are used for incorporation of nanoparticles in the membrane surface modification shown in the Fig. 1 viz. (i) conventional method and (ii) preseeding interfacial polymerization (IP) method [7,47]. (i) The conventional method is of again two types depending on, whether the nanoparticles are added in TMC or MPD solution. When the nanoparticles are added in TMC solution, an IP zone is formed below the nanoparticles as shown in Fig. 1(i)(a). Since the nanoparticles are on the upper most layers on the surface, it gets agglomerate and also has a possibility of leaching. If nanoparticles are added to the MPD solution, IP zone is occurred above the nanoparticles followed by the formation of polyamide layer shown in Fig. 1(i)(b). Since the polyamide layer is on the top surface now, fouling will be more [9]. Here FeO nanoparticles used to prepare surface modified nanocomposite membrane, but one of the main constrains of using FeO nanoparticles is its fast aggregation during membrane preparation. This aggregation behavior of FeO nanoparticles leads to decrease its 8

hydrophilic nature; block surface pores and increase in fouling of the membrane. (ii) To overcome this aggregation problem of FeO nanoparticles, an intermediate pre-seeding step, in which hexane solution having a low concentration of TMC, nanoparticles, and ethanol is introduced between MPD and TMC layer to assist the proper dispersion of FeO nanoparticles on the membrane surface. This form an IP zone at the same layer on the membrane where the nanoparticles are present (neither above nor below the nanoparticles) shown in Fig. 1(ii).

Fig. 1. (i) Conventional method can be done in two ways: (a) by dispersing NPs in TMC solution (b) by dispersing NPs in MPD solution and (ii) pre-seeding interfacial polymerization method. The organic materials (hexane in ethanol) that are used to pre-assemble the FeO nanoparticles serve here as seeds. The pre-assembled FeO nanoparticles have thus impregnated with the MPD layer to form a thin, stable and defect-free layer of FeO nanoparticles. This prepared support is then kept in an open air at room temperature to evaporate the extra solution. Afterwards, a solution containing 0.1 wt% TMC and hexane is immediately poured into this prepared support and kept for 30-40 s for polymerization to take place. Here, the polyamide layer is formed with the embedded FeO nanoparticles (Fig. 1(ii)). Since a good bonding between the nanoparticles and polyamide layer exists, problems like agglomeration [50] and leaching of the nanoparticles, fouling, etc. are rectified. 9

3.2. Effect of FeO nanoparticles on the membrane surface properties Characterization is an important step for any modified/prepared membrane to observe its surface chemistry, roughness, nature (hydrophilic or hydrophobic), charges and morphology. The presence of FeO nanoparticles effects, hydrophilicity, pore size, surface charge density, roughness, and surface uniformity, of the studied membrane surfaces, as discussed below.

3.2.1. Contact angle The effect of embedding FeO nanoparticles on the surface of the membrane is analyzed using contact angle (left and right side angle) measurements. The nature (hydrophilic or hydrophobic) of LPDM, HPDM, NHPDM1, and NHPDM2 membranes has been compared to the experiment and after the saltwater experiment, and the results are shown in Table 1. An initial contact angle of LPDM is 84.7˚. This contact angle decreases up to 49.6˚ (NHPDM2) with increasing the concentration of FeO nanoparticles. Before experiment, LPDM shows 84.7˚ angle with only 0.2˚ angle differences (between left and right) but after an experiment with saltwater it shows 98.1˚ angle with 0.7 differences which show that membrane will get unevenly fouled surface due to the attachment of natural foulants (hydrophobic substance) of saltwater. While FeO embedded membrane (NHPDM2) shows 49.6˚ angle before the experiment and after experiment it shows 51.3 with only 0.3˚ angle differences (between left and right) due to its improvement in the hydrophilicity. The contact angle of the membrane decreases, with increases in FeO nanoparticles concentration, where improvement of membrane surface hydrophilicity and increased ionization is observed. As a précis, an improvement in the hydrophilicity comes out to be an acceptable approach to improve self-cleaning ability to overcome membrane fouling problem. This shows that natural foulants present in the saltwater not attached to the membrane surface due to its hydrophilic and smoother surface of the membrane which is confirmed by the roughness analysis of the membrane. Table 1 The contact angle measurements of membranes. Membranes

Before experiment

After experiment

Left (˚)

Right (˚)

Left (˚)

Right (˚)

LPDM

84.7

84.9

98.1

94.8

HPDM

76.1

76.3

86.3

87.8 10

NHPDM1

60.9

60.1

65.4

65.9

NHPDM2

49.6

49.5

51.3

51.6

3.2.2. AFM analysis The effect of embedding FeO nanoparticles surface roughness of the membrane and membrane morphology is examined by SPM analysis and the results resented in Fig. 2.

Fig. 2. 3D SPM morphology of membranes: (a) LPDM, (b) HPDM, (c) NHPDM1, and (d) NHPDM2. As can be observed from the Fig. 2, LPDM (Fig. 2a) and HPDM (Fig. 2b) shows a surface with the high roughness. An addition of FeO nanoparticles with the thin polyamide layer it enhances the surface smoothness. The increase in the smoothness of membrane is due to preseeded FeO nanoparticles bind with the polyamide layer all through the polymerization process. The embedded FeO nanoparticles are fascinated in the polyamide layer which helps to improve the surface roughness of NHPDM1 (5.67 nm, Fig. 2c) and NHPDM2 (2.67 nm, Fig. 2d) decreases as compared to LPDM (16.6 nm, Fig. 2a) and HPDM (9.07 nm, Fig. 2b). Membrane

11

surface with low roughness has possessed a stronger anti-fouling activity due to its smoother surface and hydrophilic nature.

3.2.3. FTIR-ATR Polymerization reaction occurs by the self-sealing polycondensation reaction between two highly reactive monomers MPD and TMC. For surface chemical vibrations of the prepared membrane, FTIR-ATR spectroscopy analysis is used, and the spectra are presented in Fig. 3.

Fig. 3. FTIR-ATR spectroscopy: (a) NHPDM1, (b) HPDM, and (c) LPDM. The peaks at 1486 cm-1 are assigned to aromatic C–C stretching, 1298 cm-1 to the doublet from the asymmetric O=S=O stretching of a sulfone group, 1237 cm-1 to the asymmetric C–O–C 12

stretching of an ether group and 1151 cm-1 to the symmetric O=S=O stretching of sulfone group of LPDM. A thin layer is coated on the LPDM by the pre-seeding interfacial polymerization method; several new peaks are also appeared (Fig. 3b). Moreover, the observable peaks at 1662 cm−1 and 1577 cm−1 are related to the C=O stretching vibrations of amide I and N―H deformation vibration of amide II, respectively. The peak is assigned at 1482 cm−1 of amide II, N-H deformation and C-N stretching vibration –CO-NH– group originated from the prearranging interfacial polymerization reaction. Amide group of polyamide layer is a strong electron donor which attached to the oxygen atom of the nanoparticle. At 557 cm−1 a peak is observed in FeO nanoparticles embedded polyamide membrane spectra (Fig. 3a), which belongs to Fe-O group of FeO nanoparticles. The polycondensation reaction is occurring during the pre-seeding interfacial polymerization reaction. In this reaction, amide group (-NH2) of polyamide layer act as a strong electron donor [51] may attached to the oxygen atoms of Fe3O4 to form an unstable complex which absorbs a proton (–OH groups) from the aqueous solution of MPD monomer and rearranged to R-NH-Fe3O4–OH in order to achieve more steady state. This reaction could be promoted after coupling with another amine group to formed the bond between polyamide layer and FeO nanoparticles be the most vital for the formation of a track-free uniform polyamide layer with smoother and hydrophilic surface on the neat PES support membrane to beat the fouling problem of the membrane [52]. 3.2.4. Surface charge For surface charge characteristics, contact angle measurements of aqueous electrolyte's solution at different pH (4, 7, and 9 pH) values are used. This solution has reduced the control of functional groups on the pH of the drop itself, and allows the hydroxide ion to ionize the surface acids directly with increasing pH, showed contact angles decreased. Table 2 represents the surface concentration of acidic groups [COO-]o, fractional ionization (α), effective acid dissociation constant (pKa), and their values are supported to calculate the surface charges (σ) of the membrane surface. The surface charge across the HPDM membrane is -7.58 mC/m2 (calculated from contact angle measurements) at 7 pH. This value is nearer to the values of other researchers, where the surface charge across the HPDM membrane is -7.2 mC/m2 (calculated from streaming potential 13

measurements) at 7 pH [53]. With increasing the amount of FeO nanoparticles (up to 0.2 wt %), the surface charge of the membrane is improved up to be -14.21 mC/m2 (NHPDM2). Negatively charged surface repelled the foulants (negatively charged substances) from the surface and clean the membrane that increased membrane’s durability, fouling-resistant, and selectivity in a separation process, imparted by embedding FeO nanoparticles. The pKa values increases (around 5 to 9 across the entire pH range shown in Table 2) with increasing ionization, because more −COO− groups appeared on the surface, and remaining −COOH groups are weaker acids with higher pKa. Table 2 Fractional ionization (α), effective acid dissociation constant (pKa), surface concentration of acidic groups [COO-]o and surface charges (σ). Membranes pH Contact angle (θ)

LPDM

4

HPDM

NHPDM1

NHPDM2

84.7

Fractional

Effective acid Surface

ionization (α)

dissociation

concentration charges (σ)

constant (pKa)

[COO-]o

mC/m2

5.598

3.61×1016

-0.44

16

0

Surface

7

75.7

0.4

7.172

3.12×10

-6.27

9

70.3

0.7

8.641

2.86×1016

-9.95

4

73.4

0.2

4.621

5.99×1017

-2.33

7

66.9

0.6

6.909

6.8×1017

-7.58

9

54.2

1.2

-

5.54×1017

-13.44

4

61.2

0.1

4.778

17

-2.97

17

7.8×10

7

53.1

0.6

6.887

6.9×10

-10.29

9

46.4

0.9

8.128

6.1×1017

-14.97

4

51.4

0.1

4.838

1.6×1018

-3.72

7

42.8

0.6

6.889

1.4×1018

-14.21

9

39.9

0.7

8.639

1.3×1018

-16.69

3.2.5. SEM analysis The surface and cross-sectional morphologies of the prepared membrane are analyzed by SEM analysis shown in Fig. 4.

14

Fig. 4b shows the seed-like covered surface morphology of NHPDM2 membrane by embedding FeO nanoparticles compared with peeling type structure (Fig. 4a) visible on the surface of HPDM membrane (for high magnification images of (a) HPDM (b) NHPDM2 see Fig. S2) with an average polyamide membrane thickness 150 nm to 210 nm. Thickness of polyamide membrane is very frequently reported to be 100-500 nm [9]. It also shows some collective nanoparticles due to the covering of thin layer on the surface of nanoparticles (Fig. 4d). This thin covering on the membrane surface provides uniform and stable attachment of FeO nanoparticles, which is confirmed in the cross-sectional view. This cross-sectional image shows a finger-like structure with thin layer covering (Fig. 4c of HPDM and Fig. 4d of NHPDM2) with its crosssectional EDS analysis (Fig. 4e of HPDM and Fig. 4f of NHPDM2) to observe elemental composition (for the content of FeO nanoparticles in the polyamide polymer matrix see Fig. S3). The content of FeO nanoparticles in the polyamide polymer matrix can be observed using EDS analysis.

15

Fig. 4. SEM surface images: (a) HPDM (b) NHPDM2, cross-sectional images: (c) HPDM (d) NHPDM2, and EDX analysis: (e) HPDM (f) NHPDM2. 3.2.6. Mechanical and thermal stability The mechanical strength of membranes (tensile strength and elongation at break point) was measured by Tensometer machine and presented in the Table 3 to observe the bonding effect between FeO nanoparticles and membrane. Table 3. Tensile strength and elongation at break for the membranes

16

Tensile strength (MPa)

Elongation at break (%)

LPDM

5.0

0.04

HPDM

13.2

4.31

NHPDM1

29.2

9.33

NHPDM2

49.8

18.71

Membranes

The strength of the LPDM membrane is 5.0 MPa. This is closer to the results of other researchers, where the value is 5.3 MPa. It is observed that with addition of FeO nanoparticles that the tensile strength and elongation at break point increases from 29.2 to 49.8 Mpa due to the formation of strong bonds between FeO nanoparticles and membrane which was attributed to the dispersion of FeO nanoparticles in polymer matrix. Thermal stability of the membranes was also determined by TGA analysis and presented in the Fig. S5. From the TGA graph (Fig. S4), it is observed that with the incorporation of FeO nanoparticles, the degradation temperature of the membrane increases, as specified by the change of the TG curves. The introduction of the FeO nanoparticles improved the thermal stability of the membranes, and which is expected for the organic-inorganic nanocomposite membranes. Literature attributed the fact that the nanoparticles improve mass transport barrier properties to both the oxidizing and the volatile compounds generated during the degradation [7]. The XRD analysis of FeO nanoparticles embedded nanocomposite membrane (NHPDM2) is also done and incorporated in the supplementary file of the modified manuscript as Fig. S6. 3.3. Effect of the presence of FeO nanoparticles on membrane performance The membranes performance is studied for flux, MWCO, salt rejection, and flux recovery using saltwater.

3.3.1. Flux study Permeate flux of membrane was determined by pure water. Fig. 5 shows the permeate flux vs. applied trans-membrane pressure of LPDM, HDPM, NHPDM1, and NHPDM2. The flux for each membrane showed a linear increase with an increase in pressure. The initial pure water flux of HPDM (17.56 L/m2h) was lower than the initial pure water flux of NHPDM1 and 17

NHPDM2 (19.56 L/m2h and 23.88 L/m2h) at 10.34 bar. The results indicate that the presence of FeO nanoparticles in the membrane (i.e., NHPDM1 and NHPDM2) enhanced the pure water permeate flux compared to membranes without FeO nanoparticles (i.e., HDPM). As FeO nanoparticles increase the membrane surface hydrophilicity, water uptake is expected to increase and can also reduce the membrane compaction to sustain a more stable flux. 40

LPDM HPDM NHPDM1 NHPDM2

30

2

F lu x (L /m h )

35

25

20

15 0

4

8

12

16

20

24

P re ssu re (b a r)

Fig. 5. Pure water flux of membranes based on the different trans-membrane pressure.

Similarly, as can be observed in Fig. 5, with increase in FeO nanoparticles concentration, enhancement in membrane flux is observed. The membrane hydrophilicity is increases shown in Table 1 with increase in the concentration of FeO nanoparticles in the thin polyamide layer which helps to improve the flux. On the contrary, protract the proper concentration of nanoparticles; particles cannot accumulate in the polyamide layer increases the pure water flux and membrane surface hydrophilicity. But after optimal concentration of nanoparticles, as a polyamide layer containing FeO nanoparticles forms, aggregation of nanoparticles that shrink pores and the flux decreases (confirmed through the SEM analysis of 0.3 wt% FeO embedded membrane and represented as Fig. S5). 3.3.2. MWCO 18

To observe pore size of the membrane, MWCO is a starting point to describe the retention capabilities of the membrane. It is defined as the lowest molecular weights at which around 90% of solute with a known molecular weight is retained by the membrane. Rejection performance of PEG with different molecular weights (600, 1000, 1500 Da) is calculated and presented in Table 4. Table 4 MWCO study of the membranes. Membranes

Rejection (%) PEG (600 Da) PEG (1000 Da) PEG (1500 Da)

LPDM

37.49

54.98

71.36

HPDM

49.04

64.32

89.13

NHPDM1

53.16

72.44

90.21

NHPDM2

58.71

78.46

91.87

Based on the pore size, membranes for the different application can be categorized as either nanofiltration or ultrafiltration membranes. Prepared membranes (HPDM, NHPDM1, and NHPDM2) show around 90% rejections with PEG 1500 Da, which is the range of nanofiltration membrane. 3.3.3. Salt rejection performance Table 5 represents salt rejection performance on the basis of the conductivity of salt solution using different salts (NaCl, Na2SO4, C6H5Na3O7, MgCl2, and CaCl2).

Table 5. Salt rejection (%) and flux (salt solution) study of the membranes. Membranes

Salt rejection (R %) MgCl2

CaCl2

C6H5Na3O7

Na2SO4

NaCl

LPDM

69.66

67.66

63.92

53.21

36.44

HPDM

80.66

79.06

76.32

72.22

66.23 19

NHPDM1

97.56

96.35

96.05

94.22

82.12

NHPDM2

98.92

98.77

97.88

97.16

92.14

For nanofiltration membranes modified by FeO nanoparticles, the main effects on the rejection behavior are observed: the increase of hydrophilicity and surface negativity improving the productivity. As a result, a salt rejection of HPDM has been enhanced when a minor amount of FeO nanoparticles was added into polyamide membrane structure. FeO nanoparticles embedded polyamide membrane is prohibited salt rejection by positively charged inner layer (due to the presence of unreacted amines) of the thin layer for multivalent cations (CaCl2, MgCl2, etc.) and by the negatively charged outer layer (due to the presence of unreacted acid chloride groups) for multivalent anions (e.g. C6H5Na3O7 and Na2SO4). Therefore membrane shows more adsorption capacity for divalent cations (Mg2+ and Ca2+) than the monovalent cations (Na+) due to the positively charged inner layer and thus follows the sequence of rejection CaCl2>MgCl2>NaCl. This table also represents the selectivity of rejection, in which membrane showed more rejection with trivalent ions (C6H5Na3O7) as compared to divalent (Na2SO4) and monovalent (NaCl) ions. Because, number of cations increases, the size of the molecules increases and it forms the massive molecule which does not pass through the membrane. Table 4 also shows the high removal of low-valence cations and high-valence anions (Na2SO4) than high-valence cations and low-valence anions (NaCl), due to the size sieving processes and negatively charged surface. Due to the size of the ions, SO4-2 ions (3 Å) face strong resistance than Cl- ions (1.95 Å) and the negatively charged surface shows strong repulsive force against the divalent ions.

3.3.4. Anti-fouling study Anti-fouling or re-usability study of prepared membranes is analyzed by performing flux recovery experiment using seawater (saltwater) shown in Fig. 6 and Table 6.

20

Fig. 6. (a) Flux-recovery of HPDM and NHPDM2 Membrane. Surface morphology of (b) HPDM (c) NHPDM2 after flux recovery study of saltwater

Anti-fouling performance is studied by taking flux-recovery experiment using cross-flow membrane filtration system shown in the Fig. 6. In this graph (Fig. 6a), first and third steps (0– 120 min and 240–360 min) are of pure water permeate flux and the middle (second) one is of saltwater (120–240 min). In the first cycle, the flux of the FeO nanoparticles embedded membrane is greater than that of the HPDM. The abrupt decline of flux is noticed (before starting the second step) due to the increase in osmotic pressure of the saltwater. During the second cycle of the experiment, FeO embedded membrane (NHPDM2) got less fouled than the HPDM due to its hydrophilic (Table 1) negatively charged smooth surface (Table 2). Because foulants present in the saltwater have been described as a negatively charged substance that 21

would be attached to the positively charged substances. After 30 min rinse of pure water, the third cycle of the experiment is the recovery of flux, and NHPDM2 showed higher flux recovery due to its self-cleaning behavior and lower surface fouling. Anti-fouling study of the membrane is analyzed by SEM analysis and shown in the Fig. 6b and Fig. 6c to observe the fouled surface of the membrane after flux recovery study. SEM analysis (images of both the membranes are taken at the same magnifications at X 20, 000) shows that the HPDM (Fig. 6b) is completely fouled, demonstrating that the HPDM has no antifouling activity compared to FeO nanoparticles embedded NHPDM2 (Fig. 6c) membrane. Table 6 represents FR and FRR of the membranes. It shows that the FRR of NHPDM2 is higher (81.16%) as compared to LPDM (50.16%) and HPDM (64.26%). Here, high flux recovery of NHPDM2 can be credited to its higher hydrophilicity (Table 1) and high surface negativity (Table 2). Table 6. Fouling resistance of the membranes. Membranes

Fouling resistant (%) FRR

FR

LPDM

50.16

45.72

HPDM

64.26

37.42

NHPDM1

69.38

32.92

NHPDM2

81.16

29.26

An improvement in the hydrophilicity and surface negativity comes out to be an acceptable approach to improve self-cleaning ability to overcome membrane fouling problem [54]. Natural foulants (hydrophobic substance) present in the saltwater have been described as negatively charged substance that is preferred to attach to the positively charged substances. The charge of the natural foulants present in the saltwater (-7.68 mV) is measured using zeta potential measurement. Conclusions FeO nanoparticles embedded thin-film coated membranes are prepared by pre-seeding interfacial polymerization method to overcome FeO nanoparticles agglomeration problem and studied the ability of anti-fouling, and separation performance. FeO nanoparticles embedded 22

membrane performances are evaluated using saltwater (seawater) collected from the Suvali beach, Surat, India for anti-fouling study, and found that FeO embedded membrane surface increased the removal of hydrophobic contaminants, and remarkably reduced the fouling. Therefore, it has a wide range of applications in the water treatment.

Acknowledgment Authors express sincere gratitude to the Sophisticated Analytical Instrument Facility (SAIF), Materials Science or Materials Engineering (MEMS) Indian Institute of Technology Bombay (IIT-B) for providing FEG-SEM, SPM characterization facilities.

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29

Graphical abstract

30

Highlights



Preparation of FeO nanoparticles embedded thin-film nanocomposite NF membrane.



Ethanol was used as a co-solvent, to obtain well dispersed FeO nanoparticles in hexane solution.



Thin film gives the best performance at 0.2 wt% of FeO, compared to a polyamide coated membrane.



The prepared membranes have demonstrated superior fouling-resistance and separation performances.

31