Thin-film nanocomposite forward osmosis membrane for water desalination: synthesis, characterization and performance improvement

A_m

effective membrane area (m²)

A_p

permeability of pure water (l/m³ · h. bar)

B_s

solute permeability (g/m³ h¹. bar)

C_t

salt concentration of the water feed (mg/l)

i

Van't Hoff Constant (dimensionless)

Ε

membrane porosity (%)

Ρ

solution density (kg/m³)

π

osmotic pressure (atm)

J_s

reverse solute flux (g/m² h.)

J_v

pure water flux based on FO (l/m² · h¹)

V

permeate volume (m³)

L

membrane thickness (m)

W_w

weight of wet membrane (g)

W_d

weight of dry membrane (g)

M

molarity of the aqueous solution (mole/l)

AL

active layer

CVM

compact video microscopy

EDX

energy-dispersive X-ray

ICP

internal concentration polarization

FO

forward osmosis

IP

interfacial polymerization

DS

draw solution

FS

feed solution

PA

polyamide

PSF

polysulfone

PVP

polyvinylpyrrolidone

RO

reverse osmosis

FO

forward osmosis

UF

ultrafiltration

MF

microfiltration

The desalination process is critical in addressing worldwide water scarcity issues (Charcosset 2009; Razmjou et al. 2013; Ahmed et al. 2021). While current desalination technologies like thermal distillation and membrane technologies have greatly contributed to the success of sea/brackish water desalination, their high energy consumption remains a source of concern for many, especially when the majority of the energy consumed comes from non-renewable sources (Charcosset 2009).

Forward osmosis (FO) desalination utilizing membrane technology has piqued the interest of membrane scientists and manufacturers in recent years as a possible desalination technique to incorporate with reverse osmosis (RO) in the future (Cath et al. 2006). The difference in osmotic pressure between the two solutions, feed and draw, generates a critical performance index of FO due to the flux flow, and so there is no need for an external driving force. Furthermore, when compared to the RO process, which is a pressure-driven process, FO provides great contamination rejection and lower membrane fouling (Cornelissen et al. 2008). The power consumption at FO is significantly reduced as a result of this working paradigm. Another advantage of using low (Emadzadeh et al. 2014) operating pressure in the FO process is that it lowers fouling, which reduces the cleaning procedure's periodic time and, as a result, enhances the membrane's life (Lee et al. 2010; Zhao et al. 2012; Emadzadeh et al. 2014).

Despite the significant advantages that the FO membrane gives in the desalination process, scientists are still working on ways to improve the characteristics of thin-film composite (TFC-FO) membranes, particularly in the upper active skin layer (Ma et al. 2012) The attention has shifted to a new idea known as ‘thin-film nanocomposite (TFNC),’ which displays a breakthrough in the way super-hydrophilic nanoparticles (NPs) are synthesized and integrated into the polyamide (PA) thin layer (Hoek et al. 2020). Several researchers used this new generation of thin films to insert hydrophilic nanomaterials into PA thin layers using NPs dispersed in iso-par-TMC monomer solution (Jeong et al. 2007; Lind et al. 2009, 2010; Fathizadeh et al. 2011).

Inorganic NPs have been reported as potential fillers to improve the properties of microporous ultrafiltration (UF) membranes in terms of water permeability, fouling propensity, mechanical, and thermal properties in recent years, provided the number of NPs added was not in excess amount. The titanium dioxide (TiO₂) nanoparticle is the most extensively employed nanomaterial in the creation of nanocomposite membranes among the NPs examined (Emadzadeh et al. 2014; Ismail et al. 2021).

Incorporating hydrophilic NPs, such as zeolite NPs, has been demonstrated in the previous study to be beneficial (Zhao et al. 2012). Also, silica dioxide (SiO₂) can be used (Li et al. 2009; Yasukawa et al. 2015). Carbon nanotube (CNT) is considered one of the most important NPs (Chen et al. 2017). Halloysite nanotube (HNTs) (Tian et al. 2014) and graphene oxide (GO) (Baghbanzadeh et al. 2016; Sirinupong et al. 2018; Song et al. 2021) could enhance not just the hydrophilicity of the back support layer or substrate (Lai et al. 2016), but also its morphology, which is linked to the structural (S) parameter (Yasukawa et al. 2015). TFC-FO membranes with hydrophilic polymers in the support layers increased water flux while reducing internal concentration polarization (ICP) during the FO procedure (Xing et al. 2021; Han et al. 2012; Bui & McCutcheon 2013; Huang et al. 2013; Huang & McCutcheon 2014; Puguan et al. 2014; Tian et al. 2014; Ong et al. 2015; Yasukawa et al. 2015; Obaid et al. 2016; Chen et al. 2017). Incorporating Zeolite nanocomposite into a PSF-based membrane, for example, lowered the water contact angle and enhanced water flux by 55–64%, according to the dose added (Lind et al. 2009).

In all of these studies, nanoparticle agglomerations were inescapable, severely limiting the TFN membrane's ability to attain its full potential. Chemical modification of nanoparticle surfaces has lately attracted interest as a means of circumventing the drawbacks of nanoparticle aggregation. This research tries to achieve this by using a method based on the interaction (or lack there of) between NPs and the resulting PA matrix.

In mixed-matrix membranes, TiO₂ is one of the most often used NPs (Han et al. 2012). Due to their tendency to breakdown natural organic materials, generate more hydrophilic structures, chemical stability, minimal toxicity, and commercial availability, they have recently been used in membrane surface and structure changes (Yang et al. 2007; Emadzadeh et al. 2014; Mollahosseini & Rahimpour 2014). The key properties of TiO₂ NPs explored for nanocomposite membrane development are their exceptionally small particle size (less than 21 nm) and super-hydrophilic surface (Kim et al. 2003; Emadzadeh et al. 2014). The usage of TFC membrane as a FO membrane for separation is due to its good separation capabilities with a smaller skin layer covering and wide pH resistance. In recent studies, wastewater treatment has received a lot of attention (Jia et al. 2021).

TFN membranes were synthesized in this study by integrating PSF, PVP, and TiO₂ with the solvent NMP. Interfacial polymerization (IP) was used to produce the membranes. These membranes had different structures, e.g., integral asymmetric membrane (Wei et al. 2021). The findings of this study are important because they show that, rather than changing the properties of the PA layer of TFC membranes, the substrate properties can also be changed to produce appropriate FO membranes. Membrane porosities were calculated (Kim et al. 2003).

The major goals of the current work are to fabricate a flat sheet TFNC-FO membrane and its performance which was enhanced by adding TiO₂ NPs to the substrate membrane and to look at the impacts of TiO₂ NPs loading on the properties of the PSF substrate, as well as how changes in substrate properties affect the TFC membrane performance in the FO process.

The research findings are significant in showing that modifying the properties of the PSF substrate rather than changing the characteristics of the PA layer can improve the FO membrane.

Transparent Polysulfone (PSF) (Pellets) was used as the main polymer which is provided by Sigma–Aldrich, with an average molecular weight of ∼35,000 with a linear formula of [C₆H₄–4–C(CH₃)₂C₆H₄–4–OC₆H₄–4–SO₂C₆H₄–4–O]_n. The solvent of PSF (Pellets) was acquired from Sigma–Aldrich Company; N-methyl pyrrolidone with the synonym N-methyl-2-pyrrolidone, 1-methyl-2-pyrrolidone; NMP, C₅H₉NO (more than 99.5%), with an average molecular weight of ∼99.13. The substrate membrane was made from polyvinylpyrrolidone (PVP, K30, Sigma–Aldrich). PVP is a pore-forming ingredient that can also enhance the viscosity of the casting solution. It has been claimed that adding PVP to a membrane structure can cause macro voids to form, resulting in increased water flux. 1,3-M-phenylenediamine with the chemical formula 1,3-(NH₂) ₂C₆H₄; MPD (more than 99%), n-hexane (more than 99%) and 1,3,5-benzenetricarbonyl trichloride, C₆H₃(COCl)₃, TMC (more than 98%), with an average molecular weight of ∼265.48 were the monomers utilized for the preparation of PA selective layer for TFC membranes, and they were obtained from Sigma–Aldrich Company. Sigma–Aldrich synthesized titanium dioxide NPs (TiO₂ NPs) with a particle size of less than 21 nm. TiO₂ NPs were utilized as an additive to modify the characteristics of the substrate. Table 1 presents the physicochemical properties of TiO₂ NPs. Pure sodium chloride, NaCl (more than 99.5%), was used for producing salt (draw) solution at different concentrations for FO tests.

The method of TFNC-FO membrane synthesis is described in the following.

The main procedures for the preparation of the TFC-FO membrane are shown in Figure 1. The dope solution compositions used to form PSF substrate with and without NPs incorporation are presented in Figure 2. To prepare the dope solution, an appropriate amount of TiO₂ NPs was first added to NMP and introduced to ultra-sonication for 60 min to minimize the TiO₂ NPs agglomeration. PVP was then added to the mixture that was then introduced to a vigorous stirring by the hot plate magnetic stirrer for 15 min at 80 °C. It was later taken by including PSF into the mixture under vigorous blending for about 120–180 min until all PSF pellets were dissolved. The prepared homogeneous solution was left at room temperature for about 2 h to expel the air bubbles trapped within the solution. The dope solution was cast on a glass plate. The casting step was carried out by the automatic film applicator to form a wet membrane thickness of 200 μm. This step was immediately followed by immersion into a coagulation water bath at room temperature for phase inversion to take place. After membrane formation, the formed membranes were transferred to a second water bath (distilled water) and further soaked for at least 24 h to remove solvent/PVP residual. After that, the membranes were stored in deionized water until using it.

A rejection (PA) layer sits on top of a very porous support layer in a conventional TFC membrane, the PSF substrate. This active rejection layer of the TFC membrane was formed by IP. IP is an irreversible process, a rapid reaction that has been used to create a thin rejection layer for TFC membranes that can be employed in FO preparation. It is carried out as follows: first, on the top surface of the PSF substrate, 50 ml of a 2% (w/v) aqueous MPD solution were decanted, which stayed horizontally for 2 min to ensure the breakthrough of MPD aqueous solution into the pores of the substrate (Table 2). The remaining MPD was removed using a smooth rubber roller after the excess MPD solution was drained. Then, 50 ml of the organic solution was poured onto the top surface of the substrate by the slope to remove any residue from the MPD, which was applied horizontally for 1 min to ensure the completion of the IP process. Then, the excess of the TMC solution was drained off and dried in the oven for 10 min at 60 °C. Finally, the TFC membrane was saved in deionized water until use. The polymerization that occurs at the interface depends on the reactivity and concentration of the reacting monomers, the number of reactive groups on each monomer, and the stability of the solvent interface. All these influence the properties of the produced polymer.

Scanning electron microscopy (SEM) was used to observe the morphology of the cross-section, top and bottom surface of the TFC and TFNC. The membrane samples were covered with gold to produce electrical conductivity. The top surfaces of membranes were scanned with a JSM-IT200 scanning electron microscope (SEM) to show the pores of membranes.

Fourier Transform Infrared (FT-IR) analysis was performed on the prepared membrane. It was used to distinguish the functional groups of the membrane. In the spectral region, 3,500–500 cm⁻¹ with a resolution of 5 cm⁻¹, a smart type of beam splitter was equipped with the FT-IR spectrophotometer.

The knowledge of the thermal properties of materials is very important in determining their thermal stability. Detection of chemical and physical changes, characterization, and application possibilities TGA are thus used to test the polymer thermal stability at high temperatures. The weight of the sample changes with the temperature, and this is used to analyze the thermal stability. The TGA test is carried out with a Thermogravimetric Analyzer (TGA-50, SHIMADZU) from 0 to 600 °C at a rate of 5°/min (minute variations in mass can be observed).

EDX (energy-dispersive X-ray) spectroscopy is an analytical technique for determining the compositions of various elements in a sample (Hilal et al. 2017).

Membrane porosity (ɛ) was determined by measuring the dry mass (m_dry) and wet mass (m_wet) of membrane samples according to the following equation (Li et al. 2009).

The hydrophilic characteristic and moist ability of the membrane surface are determined by measuring the contact angle. A compact video microscope (CVM) was used to calculate the contact angle (Abdallah et al. 2018).

The experimental setup for this membrane performance test was carried out as follows: A laboratory-scale FO circulated system was applied to evaluate the overall efficiency of FO membranes (Graphical abstract). The cross-flow FO cell was provided with two pumps for both feed and draw solutions. Experiments were performed at 25 °C by keeping the pressure at 6 psi using the feed and drawing solution concentrate valves (Ezugbe et al. 2021). The TFC membranes were tested in both operating modes for asymmetric membrane (FO) regarding the membrane orientation. The active layer of a pressure-retarded osmosis (PRO) system faces the draw solution, whereas the active layer of an FO system faces the feed solution. To accurately determine the water flux, weight changes of the draw solution were calculated using a digital weight balance that was placed under the tank of the draw solution. In addition, the conductivity of the feed solution was estimated by a bench conductivity meter (AD 310, Adwa). Then, the conductivity was converted into a concentration by employing a calibration curve to determine the reverse salt flux. In the FO experiment, NaCl aqueous solution was used as a feed solution with different concentrations (10–120 mM) and suggested drawing solutions with different concentrations (0.5–2.5 M) NaCl.

The most important measurements to evaluate the membrane performance are:

• [1] The FO membrane flux J_v (l/m² · h)

• [2] Reverse salt flux (J_s, gm⁻² h⁻¹)

The characterization of the synthetic FO membranes TFC and TFNC were carried out using:

• SEM,

• FT-IR,

• TGA,

• Membrane porosity test,

• Membrane contact angle.

In this work, an SEM was used to capture the top, bottom, and cross-sections of the membranes. Membranes were coated with gold for 2 min using a current (15 mA) sputter coater. Results of SEM for the series of nanocomposite PSF substrates' top and bottom surfaces as well as their cross-sectional form are presented in Figure 3(a). In a comparison of the top and bottom morphologies for the four types of PSF substrates (A–E), it was shown that the pores in the bottom surface are clearer than the top surface. This reflects that more nanomaterials were accumulated on the bottom part of the substrate during the phase inversion stage (Sirinupong et al. 2018).

The addition of TiO₂ NPs in the main structure of the membrane leads to an increase in membrane porosity as the number of pores increases in the TFNC as shown in the bottom surface of the membrane in Figure 3(d) compared to the bottom surface of the TFC membrane as shown in Figure 3(a). Also, Figure 3(e) indicates the image of the bottom surface of the TFNC membrane, where the NPs are agglomerated due to increasing TiO₂ percentage to 1.5%. This suggests that adding more than 1.0% of TiO₂ results in clear aggregation on the surface of the substrate, minimizing the contact area of hydroxyl groups occurred by TiO₂ NPs and likely deteriorating the substrate structural integrity (Emadzadeh et al. 2014).

From the cross-section images, it can be observed that the sponge-like holes and voids extended from the top to the bottom of the substrate surface. Figure 4(b) and 4(d) illustrate long finger-like voids compared to the pure PSF substrate Figure 4(a), because of the inclusion of TiO₂ NPs in the dope solution, water passage from the water coagulation bath to the cast polymer layer is greatly facilitated, resulting in the formation of long finger-like pores that improves overall porosity and facilitates water flux (Emadzadeh et al. 2014).

EDX element analysis of the upper and bottom surfaces of the substrate without adding TiO₂ NPs and in the presence of different doses of the NPs is shown in Table 3. Due to the chemical structure of TiO₂, which contains (O) elements in the organic structure, the surface of the substrate TiO₂ displays substantially more oxygen (O) elements than the surface of the substrate control. The substrate TiO₂ investigates the increase in (Ti) element on the bottom surface as a result of more nanomaterials accumulating on the bottom surface of the substrate during the phase inversion process, resulting in a decrease in internal concentration polarization of the TFC membrane during the FO process.

Equation (1) was used to calculate the porosity of the substrate membrane (%). The overall porosity percentage of the prepared TFC/TFNC membranes is shown in Figure 5. The total porosity of the membrane rose from 65% in the TFC membrane to 77% in the TFNC membrane, this means that the use of TiO₂ NPs increased the membrane porosity that leads to a decrease in the porosity of the membrane's internal structures during the FO processes that increase the water permeability. The excessively porous support can also make constructing an excellent active selective layer with the required mass transport capabilities more challenging. The sponge-like structure with small pores surrounded by dense walls, on the other hand, may make it easier to construct an integrated thin active layer and may provide superior mechanical stability to the finger-like characteristic; however, it increases mass transfer resistance.

The addition of hydrophilic nanomaterials to the substrate layer of a TFC-FO membrane can result in increased porosity, greater hydrophilicity, and decreased tortuosity, all of which work together to reduce ICP. The water contact angle (WCA: θ) is the common measurement to estimate the membrane surface hydrophilicity, and the findings are shown in Figure 6. The hydrophilic membranes have a WCA of 0° < θ < 90°, while hydrophobic membranes have a WCA of 90° < θ < 180°. In the case of the prepared TFC/TFNC membranes, there was a decrease in contact angle with increasing the TiO₂ percentage. The contact angle reduces from 66.4° to 52.2° as the TiO₂ percentage increased from 0 to 1.5 wt% due to the increase of membrane hydrophilicity. Increasing hydrophilicity leads to improvement in membrane performance.

One of the most widely used analytical techniques for materials analysis is infrared spectroscopy. The main concept is summarized as follows: when infrared light passes through a sample, some of the light is absorbed by the sample's chemical bonds, creating molecular vibrations. The absorbed IR radiation's energies are determined by the bond strengths and atoms, and the resulting spectrum shows molecular absorbance. Figure 7 shows the FT-IR spectrum of the prepared flat sheets membranes for substrate (control) and TFC membrane. TFC spectra indicate that the absorption peak of wave number equals 1,154.03 cm⁻¹ which shows the presence of symmetric O = S = O stretching and indicated in the 1,151–1,155 cm⁻¹ range. The absorption peak at 1,294.71 cm⁻¹ indicates the presence of asymmetric O = S = O stretching and is indicated in the 1,294–1,295 cm⁻¹ range. The absorption peak at 1,240.96 cm⁻¹ indicates the presence of asymmetric C − O − C stretching and is indicated in the 1,240–1,258 cm⁻¹ range. The absorption peak at 1,490.47 cm⁻¹ indicates the presence of CH₃ − C − CH₃ stretching and indicated in the 1,486–1,490 cm⁻¹ range and 1,490 cm⁻¹ (C = C aromatic ring stretching) correspond to the functional groups of substrates made of the PSF polymer. Regarding the PA layer, the characteristic peak at 1,652.00 cm⁻¹ indicates the presence of C = O, where the carbonyl functional group spectra are in the range from 1,600 to 1,800 cm⁻¹ wave number (C = O amide stretching) and 1,583.30 cm⁻¹ (C − N amide stretching). So, it can be concluded that there are PVP, PSF, TMC, and MPD in the produced membrane and this is considered for both TFC and TFNC membranes. As can be seen, the peaks of virgin PSF and PSF substrate with TiO₂ NPs have no obvious change. FT-IR spectroscopy is mostly used for a qualitative study of various materials, but it may also be used for quantitative analysis utilizing modern software that calculates the area of various peaks in the IR spectrum (Stuart 2004).

TFC-FO and TFNC-FO, as well as their components, have thermal stability. The weight change that occurs while a sample is heated at a constant pace was assessed using TGA, which was employed as an analytical technique to determine the fraction of volatile components. Figure 8 presents TGA curves for the four prepared membranes; the first one for the TFC-FO membrane, (b), (d), and (e) of the figure represent TFNC-FO membranes (0.5, 1, 1.5 TiO₂). From the obtained results, all the prepared membrane samples have relatively good thermal stability. The weight losses were evaluated, and no significant values occurred until 100 °C. From 100 to 500 °C is the step of removing residual solvent from the membrane matrix. Solvent removal in the TFC-FO case is more than in the TFNC-FO cases. Figure 8 indicated that the weight loss (degradation) in the TFC occurred in two steps at midpoint 100 °C by 53.52% and at midpoint 584 °C by 24.52%. A total overall weight loss of 38%, in the case of TFNC-0.5% TiO₂, at midpoint 100 °C, and 21.15%, at midpoint 521 °C. In general, similar results were obtained at the degradation step (100–500 °C) in TFNC 1, 1.5; there was no significant difference in TFNC with different NPs doses. It is observed that the weight loss of the TFN-0.5% is less than the weight loss in the TFC membrane, which improves the thermal stability in the nanocomposite membrane. This improvement may be attributed to the interaction of coordinate bonds, covalent bonds, Van Der Waals forces, or hydrogen bonds between TiO₂ NPs and polymeric chains.

The difference in osmotic pressures between the draw and feed solutions is thought to provide the driving factor for the FO process, hence draw solutions are crucial. Because the FO membrane has an asymmetric structure, water flows have been determined at both membrane orientations using 10 mM NaCl as the feed solution and 1.5 M NaCl as the draw solution. To improve the efficiency of the FO process, the optimum draw solutions must have specific qualities. To begin, it must produce a larger osmotic pressure than the feed solution, which can be accomplished by raising the concentration of the feed solution. Second, there must be a low reverse solute flux as a result. Third, following the FO process, the diluted draw solution must be able to collect with minimal energy and expense. Finally, to decrease ICP, the orientation of the membrane has a critical factor. The two main membrane orientations are (1) active layer facing draw solution (AL-DS) and (2) active layer facing feed solution (AL-FS) (Ahmed et al. 2021). When the active layer of the membrane is facing the feed solution, it is in FO mode; when the active layer is facing the draw solution, it is in PRO mode. As presented in Figure 9, the water flux was 1.7–2.0 times higher in the PRO mode as compared to the FO mode due to internal concentration polarization being more severe in the FO mode. But placing the support layer against the feed solution would result in higher fouling and scaling (Razmjou et al. 2013; Yasukawa et al. 2015).

The reverse solute flux of the membranes tested in PRO and FO modes is shown in Figure 10. When comparing the PRO and FO modes, it was discovered that the PRO mode had a greater reverse draw solute flux. Due to the strong reverse draw solute flux in the PRO mode, which minimizes the osmotic driving force, all experiments in this study were conducted in the FO mode. As a result, the membrane efficiency is reduced. Results agreed with Ismail et al. (2021); Ismail et al. found a significant increase in solute flux in TFNC in the AL-DS mode and in the AL-FW mode compared to control TFC membrane in the AL-DS mode and the AL-FW mode.

The effects of TiO₂ dose on membrane water flux and reverse solute flux were assessed using a cross-flow FO experimental laboratory setup. The studies were carried out with a feed solution of 10 mM NaCl and various draw solution concentrations. (0.5 M NaCl (29,500–24.74 bar), 1 M NaCl (59,000–49.48 bar), 1.5 M NaCl (88,500–74.24 bar), 2 M NaCl (118,000–98.98 bar), and 2.5 M NaCl (147,500–123.7 bar)). The results indicate that TFN membranes prepared from PSF–TiO₂ nanocomposite substrates (0.5–1 wt%) provide much higher water flux regardless of changing the draw solution concentrations compared to the control TFC membrane. This significant improvement is due to the improved structural properties of PSF–TiO₂ substrates which minimize the transport resistance against water permeation. Nevertheless, a further increase in TiO₂ NPs loading to 1.5 wt% showed lower water flux due to agglomeration of TiO₂ on the substrate surface. These results agreed with previous work (Emadzadeh et al. 2014), which proved that the TFN membranes prepared from PSF–TiO₂ nanocomposite substrates showed much higher water flux for both orientations regardless of draw solution concentration as compared to the blank (control) TFC membrane. These findings are similar in the case of using zeolite NPs. The water flux of the fabricated FO membranes increased sequentially with the increase in the content of zeolite NPs and increase in the PSF sheet. (Ismail et al. 2021).

Figure 11(a) and 11(b) shows the effect of TiO₂ doses on membrane water flux and reverse solute flux, the results indicating that the TFNC-0.5 (0.5 wt%TiO₂) provides the higher water flux by 120% from the TFC (control) (from 12.2 to 24 l/m² h). Likewise, it can be seen that there is an insignificant increase in solute flux in the TFN-0.5 membrane compared to the control TFC membrane (from 2.4 to 4.3 g/m² h), using draw solution concentrations (1.5 M NaCl) vs. 600 mg/l feed water.

Increased TiO₂ NPs loading to more than 0.5 wt%, on the other hand, had a negative influence on the separation performance, resulting in a lower degree of PA cross-linking, reduced membrane efficiency, and increased reverse solute flow due to increased membrane porosity (Emadzadeh et al. 2014). Previous work compared the separation properties of similar membranes which were made in this study to commercial cellulose triacetate (CTA) membranes. In terms of water permeability, they discovered that the PSF substrate with TiO₂ added had a significant impact on TFC membrane performance. The water permeability of all TFN membranes made from a PSF–TiO₂ nanocomposite substrate was significantly higher than that of commercial CTA membranes (Emadzadeh et al. 2014; Ismail et al. 2021).

The FO membrane flux was characterized by different draw solution concentrations. As can be seen from Figure 12, increasing the draw solution concentration from 0.5 to 2.5 M resulted in a flux increase from 7.14 to 17 l/m² h of TFC (control) and from 16.2 to 31 l/m² h of TFNC-0.5. As a result, as the concentration of the draw solution increased, the FO membrane flux increased. This was predicted because a higher concentration causes a stronger osmotic driving force in FO, which leads to a higher flow (Yasukawa et al. 2015).

Table 4 shows the results of both water permeability and solute permeability for the tested TFNC membrane prepared from the PSF substrate made of different TiO₂ loadings. The water permeability of our synthesized TFNC-0.5 membrane is A = 0.355 l/m² h bar; this value is 53.5% higher than the TFC control membranes. This result is due to the high porosity of TFNC-0.5 which is 70% as a result of adding TiO₂. On the contrary, TFC (control) has the lower permeate flux due to its low porosity which is 65%.

Also, Table 4 shows the effect of TiO₂ loading on solute permeability, which indicates that the TFNC-0.5 provides a lower solute permeability of 0.25 l/m² h compared to the TFC (control) which had a higher solute permeability of 0.31 l/m² h.

In this current work, fabrication of a TFC-FO membrane was investigated and the effect on the performance of the membrane by adding TiO₂ NPs into the PSF matrix was studied. Based on the findings, we can conclude that:

• The support layer of the TFC-FO membrane has a significant impact on its performance. To achieve high salt retention, the active layer of an ideal FO membrane must be exceedingly thin and thick.

• TiO₂ NPs were incorporated into a PSF matrix, and both surface porosity and hydrophilicity of PSF-based substrates were enhanced. We can say that the modified surface hydrophilicity and porous structure formed are the most important factors leading to higher water flux compared with the TFC membrane/control.

• The increase in the concentration from 0.5 to 1 wt% of TiO₂ leads to an increase in both the porosity and hydrophilicity of the nanocomposite substrate and consequently, an increase in the water flux. To obtain high flux and minimum fouling, the hydrophilicity must be high. According to the current research, using innovative nanomaterials, substrates, and layer-by-layer assumptions in the fabrication of FO membranes significantly improve water flux.

• TFNC-FO membrane has slightly reverse salt rejection compared with the TFC-FO membrane.

• Under the same conditions, the fabricated TFNC-FO membrane indicated a much higher FO water flux showing a slight increase in the reverse solute flux.

• Water flux TFNC-FO membrane under long filtration time was also recorded to be much higher, basically due to the enhancing properties of the substrate upon addition of nanoparticle of TiO₂.

• FO member water flux increases with the increase of draw solution concentration. As a result, a higher concentration produces a higher osmotic driving force.

• The problem of internal concentration polarization is a problem because of its impact to decrease the effective osmotic pressure through the membrane's active layer.

• The performance of an FO membrane is determined by a complex mix of external test conditions, including the osmotic strength of feed and draw solutions, the orientation of the FO membrane, designing an experiment (run-in times, measuring times, equilibration times, etc.).

A.H.K. conceptualized the study, refined the research idea, and edited and revised the study. H.Z.A. wrote the manuscript, drew the figures, wrote the results, and helped in discussion. S.N. wrote, edited, and revised the manuscript. M.G.E. wrote the manuscript, drew the figures, edited, and revised the manuscript.

The authors declare that they have no conflict of interest.

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All relevant data are available from an online repository or repositories. https://www.doi.org/10.2166/wqrj.2022.034.