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Frontiers in Membrane Science and Technology ·

Volume 5 - 2026 | https://doi.org/10.3389/frmst.2026.1894848

Ultralong hydroxyapatite nanowire-incorporated PVDF mixed-matrix membranes for enhanced forward osmosis performance

  • Mohamed Gamal Gomaa1
  • El-Sayed Ali Abdel-Aal1
  • Francesca Russo2
  • Francesco Galiano2
  • Alberto Figoli2
  • Hamdy Maamoun Abdel-Ghafar1*corresponding author
  1. 1Central Metallurgical Research and Development Institute (CMRDI), Helwan, Cairo, Egypt
  2. 2Institute on Membrane Technology (CNR-ITM), Rende, Italy
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Abstract

A simple and novel treatment strategy was developed to fabricate poly (vinylidene fluoride) (PVDF)-based forward osmosis mixed matrix membranes (MMM) with enhanced desalination performance. The proposed approach focuses on improving the surface properties of PVDF membranes through incorporation of ultralong hydroxyapatite (UHA) nanowires as hydrophilic inorganic additives. PVDF and UHA possess excellent mechanical strength, thermal stability, and chemical resistance, making them promising materials for membrane fabrication. However, the intrinsic hydrophobicity of PVDF limits water permeability and flux, reducing membrane efficiency and increasing operational costs. Therefore, UHA nanowires were incorporated into the PVDF matrix to enhance membrane hydrophilicity and separation performance. The fabricated membranes were comprehensively characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) to investigate their structural, chemical, and morphological properties. Membrane performance was evaluated through forward osmosis desalination experiments by measuring water flux and reverse salt flux. The pristine PVDF membrane exhibited a water flux of 2.1 LMH, while the modified PVDF/UHA membrane achieved 3.6 LMH, corresponding to approximately a 70% enhancement in water permeability. In addition, the modified membranes demonstrated improved antifouling behavior and operational stability. The obtained results indicate that incorporation of UHA nanowires effectively improves membrane hydrophilicity and desalination performance, highlighting the strong potential of PVDF/UHA composite membranes for forward osmosis desalination and wastewater treatment applications.

Keywords

1 Introduction

Poly (vinylidene fluoride) (PVDF) and inorganic materials (IMs) have attracted significant attention over recent decades owing to their broad applicability in advanced technologies such as membrane separation, catalysis, adsorption, and energy-related applications. Their importance arises not only from their individual physicochemical characteristics, but also from the synergistic effects achieved when combined into hybrid systems. PVDF, with the chemical formula (C2H2F2)n, is a semi-crystalline fluoropolymer widely recognized for its excellent thermal stability, chemical resistance, mechanical strength, and processability (Li et al., 2021). These properties originate from the strong dipole interactions between fluorine and carbon atoms along the polymer backbone. In addition, PVDF exhibits remarkable piezoelectric, ferroelectric, and dielectric properties, making it highly attractive for applications in sensors, actuators, and energy harvesting devices (Papež et al., 2022). Despite these advantages, several limitations restrict its broader utilization, particularly in membrane and electronic applications. One of the major challenges is controlling the electroactive β-phase during fabrication, which is essential for achieving optimal functional performance (Jang et al., 2025). Moreover, PVDF membranes often suffer from limited mechanical flexibility under repeated bending or thermal cycling conditions (Guo et al., 2020), while long-term environmental stability may be affected by humidity and ultraviolet exposure (Mohammadpourfazeli et al., 2023). Difficulties associated with surface compatibility and integration with other materials also present additional processing challenges (Zhang et al., 2024).

In the context of water treatment, the increasing scarcity of freshwater resources, rising industrialization, and severe water pollution have intensified the demand for advanced and sustainable membrane technologies (Abdel-Aal et al., 2025; Musie and Gonfa, 2023). Membrane-based separation processes have become essential for desalination, wastewater treatment, and water reuse applications. Among various membrane technologies, forward osmosis (FO) has emerged as a highly promising process because of its low energy consumption, mild operating conditions, high water recovery, and reduced fouling tendency compared with pressure-driven systems such as reverse osmosis (RO) and nanofiltration (NF) (Gomaa et al., 2025; Han et al., 2015; Zhao et al., 2012; Liu et al., 2016). In FO systems, water naturally diffuses from a low-osmotic-pressure feed solution to a high-osmotic-pressure draw solution through a semipermeable membrane without external hydraulic pressure, thereby reducing energy demand and operational complexity.

Although PVDF membranes are widely used in membrane fabrication due to their excellent durability and chemical stability, their intrinsic hydrophobicity often results in low water permeability, severe fouling, and limited separation efficiency (Al Harby et al., 2022). Consequently, substantial research efforts have focused on modifying PVDF membranes through blending, coating, or incorporation of nanomaterials to improve hydrophilicity and membrane performance. Among these approaches, mixed matrix membranes (MMMs) have gained particular interest because they combine the mechanical robustness of polymeric matrices with the superior adsorption and transport properties of inorganic fillers.

UHA represent one of the most promising inorganic materials for membrane modification owing to their high aspect ratio, tunable porous structure, excellent hydrophilicity, biocompatibility, and chemical stability. UHA nanowires possess unique structural characteristics that facilitate strong hydrogen bonding and physical interactions with polymer matrices, thereby enhancing membrane integrity and surface wettability. In addition, UHA has demonstrated excellent performance in applications such as tissue engineering, drug delivery, and environmental remediation, particularly for heavy metal and dye removal (Gomaa et al., 2024). Their incorporation into polymeric membranes has been shown to improve water flux, antifouling behavior, and mechanical stability. The integration of UHA nanowires into PVDF matrices offers a promising strategy to overcome the limitations of conventional PVDF membranes by simultaneously improving hydrophilicity, permeability, selectivity, and fouling resistance. Previous studies have demonstrated that inorganic membranes generally exhibit superior fouling resistance and operational stability compared with purely polymeric membranes (Shukla et al., 2025). Gomaa et al. reported that UHA-based membranes exhibited excellent water flux and operational stability in forward osmosis applications; however, their practical implementation was limited by membrane fragility and fabrication difficulties (Gomaa et al., 2024). Therefore, combining the flexibility and durability of PVDF with the advantageous properties of UHA nanowires may lead to the development of highly efficient hybrid membranes suitable for advanced wastewater treatment applications.

Despite the considerable potential of PVDF/UHA hybrid membranes, several challenges remain unresolved. Achieving homogeneous dispersion of UHA nanowires within the PVDF matrix is critical to ensure consistent membrane performance and long-term stability. Furthermore, excessive inorganic loading may negatively affect the elasticity and mechanical integrity of the resulting composite membranes. Consequently, optimizing fabrication strategies, controlling membrane morphology, and improving interfacial compatibility between PVDF and UHA remain essential for producing reproducible and high-performance hybrid membranes for practical desalination and wastewater treatment applications.

This study focuses on the integration of PVDF with inorganic materials, particularly UHA, to develop advanced hybrid membranes with enhanced separation performance, permeability, and operational stability for water treatment applications.

2 Experimental

2.1 Chemicals

Poly (vinylidene fluoride) (PVDF) with weight average molecular weight of 275,000 (g/mol) and the solvents used in this work N,N-dimethyl formamide (DMF), were purchased from Sigma Aldrich. Sodium hydroxide (NaOH, assay 98%–100%), and ethanol absolute (assay ≥ 99.9%) were obtained from Merck, Germany. Calcium chloride (CaCl2, assay 95%) and sodium dihydrogen phosphate monohydrate (NaH2PO4.H2O, assay 98%) were purchased from Adwic, Egypt. Oleic acid (assay 65%–70%) was purchased from Central Drug House (CDH), India. Sodium Chloride (commercial product grade) was purchased from an Egyptian local market. A bi-distilled water purification system was used to produce purified water for sample preparation and membrane test. All the materials were used without further purification.

2.2 Preparation of UHA

The UHA nanowires were obtained during the calcium oleate major solvothermal method. (Xiong et al., 2017) (Gomaa et al., 2024), and the reaction system contained CaCl2, NaOH, NaH2PO4.2H2O, oleic acid, water, and methanol. NaOH (10.5 g), CaCl2 (3.33 g), and NaH2PO 4·H2O (8.4 g) were dissolved in 150.0 mL of bi-distilled water, respectively. These formed aqueous solutions were separately added into a mixture of ethanol (47.5 g), oleic acid (93.6 g), and bi-distilled water (135 mL) with keeping continuous stirring. The resulted suspension was transferred into a 1 L Teflon-lined stainless-steel autoclave that sealed and heated at 180 °C for 24 h. The autoclave product was separated and washed three times with ethanol and deionized water after cooling to room temperature. The obtained value was determined after drying at a temperature of 80 °C.

2.3 Preparation PVDF membranes

Mixed matrix membranes (MMMs) were fabricated via the non-solvent-induced phase inversion (NIPS) method. Initially, a predetermined amount of UHA nanowires was dispersed in dimethylformamide (DMF) solvent under magnetic stirring for 10 min, followed by ultrasonication for an additional 10 min to obtain a homogeneous suspension. Subsequently, a measured amount of PVDF was gradually added to the prepared solution and dissolved at approximately 70 °C under continuous gentle stirring until a transparent and uniformly dispersed casting solution was obtained. To eliminate entrapped air bubbles and ensure membrane uniformity, the prepared solution was left to stand overnight before casting. The homogeneous polymer solution was then cast onto a clean glass plate using a casting knife adjusted to a thickness of 200 μm. Immediately afterward, the cast film was immersed in a distilled water coagulation bath and maintained for 24 h to complete the phase inversion process and membrane formation. Different concentrations of UHA nanowires were incorporated into the casting solution to prepare various MMM compositions, as summarized in Table 1, and explained in Figure 1. In this study, the same 200-µm thickness blade was used for all membranes, maintaining consistent preparation and drying conditions. The casting thickness was used as a control for the manufacturing process and to ensure comparability between different samples.

TABLE 1. The compositions of PVDF polymer, UHA nanowires, and solvent used for membrane fabrication.

MembraneM0M1M2M3M4M5
UHA content (wt% of PVDF)00.10.30.50.71.0
PVDF (g)20.0020.0020.0020.0020.0020.00
DMF (g)80.0080.0080.0080.0080.0080.00
UHA (g)0.000.020.060.100.140.20
Schematic diagram for preparation of membranes.
Figure 1. Schematic diagram for preparation of membranes.

2.4 Characterizations

Variable pressure scanning electron microscopy (SEM, Hitachi S3400, Japan) was utilized to characterize the surface morphology and cross-sectional microstructure of the fabricated membranes. X-ray diffraction (XRD, Cu Kα radiation, k = 1.54178 Å, D/max 2550 V, Rigaku, Japan) was used to analyze the bond form and composition information of the UHA nanowires, and Fourier transform infrared spectroscopy (ATR-FTIR, Nicolet iN10, United States) were employed to characterize the prepared membranes by examining their functional groups and chemical compositions. The wettability of the membrane surface was evaluated using the CAM 100 (KSV Instruments, Ltd.) through triplicate measurements at various locations on each membrane. The flow distribution of pore size was assessed using POROLUXTM 1000 (Porometer, Belgium).

2.5 Performance evaluation of the membranes

The performance evaluation of the membranes was evaluated using an FO lab-scale device, the feed solution (bi-distilled water) and the draw solution (2.0 mol/L NaCl) circulated with two micropumps (Cole-Parmer, 3.2 LPM). The developed PVDF/UHA membranes were placed between the feed and draw solutions using Sterlitech FO cell (CF016, PTFE, United States), as explained in Figure 1. Equations 1, 2 calculated the water flux and reverse salt flux respectively (Song et al., 2025).where J *v* (L. m -2. h-1, LMH) is the water flux. Δm (g) is the weight change of the feed solution (FS), which was recorded automatically every 1.0 min by an electronic balance (ME3002E, Mettler Toledo) connected to computer assisted by Easy DirectTM license key. A (m2) is the membrane surface area, Δt (h) is the time interval (1.0 min), and ρ *o* is the pure water density (0.997 g/cm3 at room temperature 25 °C).

J v = Δ m A × Δ t × ρ o (1)

J s = C t V t − C 0 V 0 A × Δ t (2)

Js (g.m-2.h -1, gMH) represents the reverse salt flux. The initial and final concentrations of the feed solution (FS) are represented by Co and Ct (g/L), respectively. These concentrations are measured using a conductivity meter manufactured by MIC Meter Industrial Company. Vo (L) represents the initial volume of the FS, while Vt (L) denotes the final volume of the FS. The stability of the membrane was evaluated through a continuous test conducted in mode for a duration exceeding 150 min. The weight was documented at 1-min intervals, and conductivity measurements for the feed solution were taken at both the start and conclusion of the experiment, as shown in Figure 2. The values presented were calculated as the average from a minimum of three replicates. The water fluxes (Jv, L/m-2.h-1, abbreviated as LMH) of the membranes were measured by tracking the mass changes of the DS, which were continuously logged using a digital balance for a period of 3.5 hours. During the interim period, the reverse salt flux (Js, g/m2·h-1, abbreviated as gMH) was quantified by measuring the conductivity of the FS. The side that was in contact with the glass plate during casting faced the draw solution, while the upper side, facing the air blade/doctor, faced the feed solution.

Schematic illustration of the laboratory-scale forward osmosis (FO) system set up.
Figure 2. Schematic illustration of the laboratory-scale forward osmosis (FO) system set up.

3 Results and discussions

3.1 Characterization of membranes

3.1.1 XRD analysis

Figure 3 shows the X-ray diffraction (XRD) results of the films, revealing characteristic peaks of the PVDF polymer, particularly in the region around 2θ ≈ 20°, which are attributed to the polymer’s crystalline phase. With the addition of hydroxyapatite, new peaks appeared at 2θ values ranging from 25° to 35°, characteristic peaks of the hydroxyapatite phase, such as the peak at approximately 25.9° and the strong peak at 31.7° (Abukhadra and Mostafa, 2020). The intensity of these peaks gradually increased from M0 to M5, indicating an increasing UHA content within the membranes. It was also observed that the PVDF peaks did not change significantly, indicating that the polymer retained its crystalline structure. Furthermore, the absence of new peaks indicative of the formation of new crystalline phases confirmed that the interaction between PVDF and hydroxyapatite was a physical reaction without a chemical reaction (Ray et al., 2019).

Shows the X-ray diffraction (XRD) spectra of the prepared membranes (M0, M3, M5).
Figure 3. Shows the X-ray diffraction (XRD) spectra of the prepared membranes (M0, M3, M5).

3.1.2 FTIR analysis

The FTIR spectra of the M0, M3, and M5 membranes are shown in Figure 4, reflecting the chemical composition and the success of hydroxyapatite incorporation into the polymer matrix. The results show a broad band in the 3,200–3,500 cm−1 range, attributed to the (O–H) hydroxyl group vibrations, whether from hydroxyapatite or adsorbed water. The intensity of this band gradually increases from M0 to M5, indicating improved hydrophilicity with increasing UHA content. Peaks were also observed in the 2,850–2,920 cm−1 range, attributed to the characteristic (C–H) vibrations of PVDF polymers (Kachhadiya and Murthy, 2022). Additionally, bands in the 1,200–1,500 cm−1 region were associated with CF2 and CH2 vibrations. These peaks remained remarkably unchanged across all samples, demonstrating the stability of the polymer’s chemical composition and its resistance to the preparation process (Shah et al., 2021). On the other hand, distinct peaks were observed in the range of 875–1,030 cm−1 and at 560–600 cm−1, attributed to the vibrations of the phosphate group (PO4 3-) of hydroxyapatite (Gomaa et al., 2024). The intensity of these peaks increased significantly with increasing M0 to M5, confirming the successful loading and distribution of hydroxyapatite within the membrane structure. A signal also appeared at approximately 1,600–1,650 cm−1, attributed to H–O–H bending vibrations, indicating the presence of bound water within the membrane structure (Cheng et al., 2022). Overall, no new peaks were observed that would indicate the formation of new chemical bonds, demonstrating that the interaction between PVDF and hydroxyapatite is a physical (blending) reaction without any intrinsic chemical reaction, while preserving the basic structure of both components.

Fourier transform infrared (FTIR) spectra of the prepared membranes (M0, M3, M5), at long and focused magnified spectral bands **(a,b)**, respectively.
Figure 4. Fourier transform infrared (FTIR) spectra of the prepared membranes (M0, M3, M5), at long and focused magnified spectral bands (a,b), respectively.

3.1.3 The morphologies and structures

Scanning electron microscopy (SEM) images show a clear difference in the surface structure of PVDF membranes when different proportions of UHA are added compared to the raw membranes, as shown in Figure 5. The surface appears semi-dense with no clear or regular pores. There are smooth areas and some clumps on the surface, along with some micro voids and cracks, likely resulting from the preparation method. A relative change is observed when adding hydroxyapatite (an inorganic material) by mixing (Figure 5b). Many pores are distributed homogeneously, and the distribution is more uniform than in the pristine membrane (Figure 5c). A significant and noticeable difference is observed with increasing overloading of the inorganic material. Here, the effect begins to appear, as the pores become larger, more distributed, and more visible in the presence of the inorganic material (Susanto et al., 2026).

SEM images showing the surface morphology of PVDF membranes containing different concentrations of UHA: pristine PVDF, 0.3 wt% UHA, and 1.0 wt% UHA (**a)**, **(b)**, and **(c)**, respectively).
Figure 5. SEM images showing the surface morphology of PVDF membranes containing different concentrations of UHA: pristine PVDF, 0.3 wt% UHA, and 1.0 wt% UHA (a), (b), and (c), respectively).

Also, when the UHA concentration was increased to 1.0%, SEM images showed a marked change in surface morphology, with the structure appearing tangled and regular, resembling a carpet-like fibrous structure. This appearance reflects the accumulation and interpenetration of particles within the polymer matrix, resulting in a rough surface with distinct agglomerations and regularly distributed pores.

Figure 6 illustrates the cross-sectional area of the membranes, their internal structure, and how their components interlock to form structures resembling a carpet or a grid. At a 1.0% UHA concentration, the pores are relatively open, with larger spaces, due to strong homogeneity and cohesion between the polymer and UHA. These changes resulted in a network structure that provided moderate mechanical support, which is supported by tension and bending. The investigated cross-section of the membrane by SEM showed the average thickness is about 65 µm. Since the same casting conditions were used for all samples, the differences in final thickness are expected to be relatively small and not affect the overall thickness of the prepared membranes. On the other hand, when the UHA nanowires concentration was raised to 1.0%, there was a big increase in nanowires and fillings in the voids between cells. The SEM images, particularly Figure 5c, demonstrate that the incorporation of ultra-long hydroxyapatite nanowires into the PVDF matrix forms an interconnected network, giving rise to a highly porous, nacre-like cross-sectional membrane structure.

SEM cross-sectional analysis for the prepared membranes, UHA 1.0 wt% (M5), at different magnifications (**a,b)**.
Figure 6. SEM cross-sectional analysis for the prepared membranes, UHA 1.0 wt% (M5), at different magnifications (a,b).

3.1.4 Surface wettability

Surface wettability is a physicochemical property that describes the ability of a material’s surface to interact with and diffuse a liquid. It is a direct reflection of the surface energy, chemical composition, and microscopic roughness of the surface. It is experimentally assessed by measuring the contact angle; lower angles indicate higher surface energy and greater affinity between the surface and the liquid, while higher angles indicate weaker interaction and a tendency toward hydrophobicity (Prihandana et al., 2012). The addition of UHA to the overall shape of the membranes prepared with polyvinylidene fluoride (PVDF) only significantly affected the interfacial and surface properties of the prepared membranes, as evidenced by measurements of water flux, porosity, mean flow pore size (MFP), contact angle, and amount of reverse salt rejection. Figure 7 presents result of the mean flow pore size (MFP), which increased from approximately 0.14 µm for the pristine membrane (M0) to a maximum value of about 0.26 µm (M2) at moderate filler loading, UHA, followed by a slight decline at higher concentrations, indicating structural rearrangement and possible pore collapse or particle agglomeration at elevated loadings.

The mean flow pore size (MFP) of the prepared PVDF/UHA membranes.
Figure 7. The mean flow pore size (MFP) of the prepared PVDF/UHA membranes.

Initially, the gradual increase in the amount of hydroxyapatite led to an increase in porosity and average free-flow pore size, thus enhancing water flux. This improvement is attributed to the increased hydrophilicity resulting from the presence of hydroxyl and phosphate functional groups in hydroxyapatite, which facilitates the interaction of water with the membrane and reduces transport resistance. The contact angle gradually decreased with the addition of nanowires, confirming the improved wettability of the membrane surface (Mauter et al., 2010), as shown in Figure 8. This decrease in contact angle was closely associated with increased flux, demonstrating that the surface’s hydrophilicity plays a crucial role in water flux. However, at a moderate loading (M3), a slight structural change was observed, as evidenced by the transient variation in porosity and average water flux. This is likely due to partial pore breakdown or redistribution of the filler within the PVDF polymer mixed matrix PMM, which slightly affected wettability and transport behavior. At a higher hydroxyapatite content (M5), both porosity and average free flow increased again, resulting in the highest recorded flow and lowest contact angle. This indicates improved filler dispersion and enhanced formation of interconnected pore channels. Surface wettability of the prepared membranes was significantly enhanced upon UHA incorporation, as evidenced by the reduction in water contact angle. The presence of hydroxyl and phosphate functional groups increased surface energy, promoting stronger water–membrane interactions and facilitating improved water flux when increasing the amount of added UHA nanowires for membranes (Li et al., 2019).

Contact angle measurements of the prepared PVDF/UHA membranes.
Figure 8. Contact angle measurements of the prepared PVDF/UHA membranes.

3.1.5 Tensile test

The tensile test is a mechanical test used to determine the mechanical behavior of materials under uniaxial tensile force, where a progressive load is applied to the membrane until fracture occurs. This test measures several fundamental mechanical properties, such as tensile strength, Young’s modulus, elongation at break, and yield strength. In polymer membrane applications, the tensile test is an important indicator of the membrane’s durability and its ability to withstand operational stresses. It also reflects the effect of fillers or inorganic additives on the structural bonding within the polymer matrix, whether in terms of enhancing stiffness or affecting elasticity and elongation. The characterization of the mechanical attributes of PVDF membranes holds significant importance in the context of practical applications. The mechanical properties of membranes were examined using a uniaxial tensile tester, in triplicate, and are presented in Figure 9.

Illustrate the dynamic mechanical test for membranes preparation of **(a)** Young’s Modulus and **(b)** Elongation at break.
Figure 9. Illustrate the dynamic mechanical test for membranes preparation of (a) Young’s Modulus and (b) Elongation at break.

The results showed that reinforcing the polyvinylidene fluoride matrix with UHA nanowires significantly altered the mechanical response of the composite membranes. A gradual decrease in Young’s modulus of elasticity was observed with increasing reinforcement percentage, which can be explained by the stress-transfer mechanism within the membranes, from 70 to 25 Mpa (Sorour et al., 2021). In ideal systems, inorganic particles act as reinforcers, efficiently transferring stress from the polymer matrix to them, provided there is strong interfacial adhesion and a homogeneous distribution (Khdier et al., 2024). However, if interfacial adhesion is weak or particle aggregation occurs, stress concentration sites form, acting as starting points for mechanical failure and reducing the overall tensile strength of the membrane. Furthermore, the increased porosity associated with the reinforcement addition may also contribute to a decrease in the stress-bearing effective cross-section, negatively impacting mechanical strength. Regarding elongation at the break, the results showed a significant decrease with increasing hydroxyapatite content from 600% to 250% (Janićijević et al., 2015). This is attributed to the restriction of polymer chain movement due to the incorporation of solid particles into the structure, which reduces the freedom of slippage and molecular rearrangement under load. Consequently, the behavior shifts from a more ductile pattern to one closer to brittleness at higher concentrations of UHA. Despite this relative decrease in mechanical properties, the addition of hydroxyapatite resulted in a marked improvement in physical and hydraulic properties, such as increased porosity, average pore size, and improved surface wettability, reflected in a higher water flux. The incorporation of UHA into the PVDF matrix resulted in a gradual reduction in Young’s modulus and elongation at break. This behavior can be attributed to the formation of stress concentration sites and restriction of polymer chain mobility caused by rigid inorganic particles. Although mechanical flexibility decreased, the structural modifications significantly enhanced membrane porosity and hydrophilicity, leading to improved water flux.

3.2 Performance evaluation of the prepared membranes

The performance of PVDF-based membranes reinforced with UHA nanowires was evaluated using forward osmosis (FO), one of the most energy-efficient membrane separation techniques, as it relies on osmotic pressure differentials rather than hydraulic pressure. This system is well-suited for evaluating the behavior of composite membranes under operating conditions that mimic practical applications in water treatment and desalination. The with draw solution was 2 M NaCl, and the feed solution was distilled water. The unit was operated for 3 h, and the weight change in the feed solution was measured every minute using a Mettler Toledo balance to determine the amount of water flux through the membranes. The conductivity was also measured to indicate the concentration and determine the amount of reverse salt flux. During the tests, a feed solution with a known salt concentration and a high concentration draw solution were used to generate sufficient osmotic pressure differentials to force water molecules across the membrane. Water flux (Jw) was measured periodically by calculating the change in mass or volume of the draw solution over time. The amount of water flux through the membranes was measured and found to range from 2.1 to 3.6 L m-2. h-1, LMH. The reverse salt flux (Js) was also calculated to determine the amount of salt transferred from the draw solution to the feed solution, a crucial factor for evaluating membrane selectivity and actual efficiency, which was found to be equal to 13–66 g.m-2.h-1, gMH in Figure 10. The results showed that adding UHA to the PVDF matrix resulted in a significant improvement in water flux compared to the unmodified membrane, due to several interrelated factors. First, the inorganic particles contributed to increasing the overall porosity and improving the interconnectivity of the internal channels within the membrane’s spongy structure. Second, the improved hydrophilicity (lower contact angle) facilitated water diffusion and reduced the resistance to mass transfer through the active layer. The more open structure also helped minimize internal concentration polarization (ICP), a major challenge in forward osmosis systems. noting that the addition of UHA improved water flux, while some concentrations showed increased reverse salt flux values, indicating the need for further optimization to achieve an optimal balance between water permeability and salt selectivity. The increase was found to be about 70% in the highest concentration of hydroxyapatite compared to the pristine membrane (M0). However, as the UHA concentration increased, the rate of performance improvement began to accelerate. Furthermore, the reinforced membranes demonstrated better operational stability over time compared to the raw PVDF membrane, with no decrease in water flux over time. This indicates improved anti-fouling resistance properties and reduced salt accumulation on the surface. This is associated with increased hydrophilicity and improved pore distribution, which reduces contaminant adhesion and facilitates periodic flushing.

Membrane performance in terms of water flux and reverse salt flux during the forward osmosis process.
Figure 10. Membrane performance in terms of water flux and reverse salt flux during the forward osmosis process.

Indeed, membrane M3 exhibits a significantly lower reverse salt flux value compared to the general trend of the other membranes. This behavior is likely due to achieving an optimal concentration of ultra-long hydroxyapatite (UHA) rods, which improves the homogeneity of the membrane’s internal structure and pore distribution, thus reducing salt transport while maintaining water permeability. Conversely, increasing the UHA content at higher concentrations (M4 and M5) may lead to the formation of additional pathways or interfacial spaces associated with increased inorganic content, facilitating salt transport and resulting in higher reverse salt flux. Therefore, membrane M3 appears to represent a balance between improved water transport and reduced salt transport, which is reflected in its lower reverse salt flux compared to the other membranes.

So, the performance evaluation results using forward osmosis confirm that incorporating UHA into a PVDF matrix is ​​an effective strategy for improving structural and surface properties, reducing internal transport resistance, and enhancing separation efficiency. However, precise control of the addition ratio is crucial to ensure an optimal balance between permeability, selectivity, and long-term stability (Li H. et al., 2017). Table 2, provides a general review of the performance of membranes similar to those prepared in this study, in terms of water flux and reverse salt flux, comparing the operating conditions and the type of support material or additives used. This comparison aims to situate the results of the current study within a clear scientific framework, highlighting the significant performance improvement compared to previously published work.

TABLE 2. Comparison of previously published work and this work results.

No.Type of membraneDraw solutionWater flux (LMH)Ref.
1PVDF/PA1 M NaCl3.1Tian et al. (2013)
2PVDF/PA10% wt. glucose3.3Muratow et al. (2024)
3PVDF/PA FO1 M MgCl21.27Liu et al. (2017)
4PVDF/GO/MWCNTs/PA0.6 M NaCl0.34Yu et al. (2020)
5PVDF flat-sheet1 M NaCl5.6Al-Furaiji et al. (2021)
6PVDF FO membrane0.75 M NaCl4.8Abdel-Kader et al. (2020)
7PVDF Aquaporin FO1 M NaCl8.8Xia et al. (2017)
8PVDF FO membrane/ TFC1 M NaCl4.5Huang et al. (2016)
9PVDF/UHA/FO2 M NaCl3.6This work.

After comparing the results with those of previous studies shown in Table 2, it was found that the values recorded in this study were consistent with those published in the scientific literature for similar membranes, in terms of water flux and reverse salt flux. This reflects the alignment of the current results with current research trends and confirms the efficiency of the preparation method and the performance improvements achieved by adding UHA, with no significant deviation from the standard ranges for similar membranes.

4 Conclusion

In this work, a series of the PVDF-based UHA nanowire composite membranes with various UHA concentrations were prepared through a mixed matrix technique. The results show that the prepared PVDF-based UHA membranes possess an asymmetric structure with a thin, dense skin and a sponge-like shape. The incorporation of hydrophilic UHA enhanced the membrane hydrophilicity of only PVDF flat sheet membranes and increased the porosity and mean pore size of the membrane. At its optimal preparation conditions with 1.0% UHA, the obtained membrane exhibited a pure water flux up to 3.6 LMH and an reverse salt flux of 66 gMH. The membrane exhibited excellent reverse salt rejection performance. Furthermore, the value of the increase reached 70%, which increases the value of the prepared membranes compared to the organic pristine membranes that contain only PVDF, indicating that the inorganic material UHA additive showed excellent performance and improved the performance of the membranes in resisting pollution, treating water, and using them in other applications, such as separating and concentrating precious metals and dyes from their respective solutions.

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Footnotes

Edited by: Abdelaziz Elgamouz, University of Sharjah, United Arab Emirates

Reviewed by: Nur Diyana Suzaimi, University of Malaysia Terengganu, Malaysia

Pengfei Li, Hebei University of Science and Technology, China

Statements

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This work has been carried out within the received fund through joint funded postdoctoral program between Egyptian Academy of scientific Research and Technology and Bibliotheca of Alexandria (ASRT-BA), and the CNR-ASRT (Italy-Egypt) 2022-2023 international Exchanges (project ID: 18775, project title: Flexible Ultrafiltration Inorganic Membrane Based on Ultra-long Hydroxyapatite for Wastewater).

Data availability

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author. The data that support the findings of this study are available from the corresponding author upon reasonable request.

Author contributions

MG: Methodology, Conceptualization, Visualization, Writing – original draft. E-SA-A: Investigation, Funding acquisition, Writing – review and editing, Supervision, Project administration. FR: Investigation, Writing – review and editing, Project administration, Methodology. FG: Methodology, Writing – review and editing, Project administration, Validation. AF: Project administration, Supervision, Writing – review and editing, Funding acquisition. HA-G: Supervision, Investigation, Conceptualization, Project administration, Writing – review and editing, Funding acquisition, Resources.

References

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Citation

Gomaa, M. G., et al. (2026). Ultralong hydroxyapatite nanowire-incorporated PVDF mixed-matrix membranes for enhanced forward osmosis performance. Frontiers in Membrane Science and Technology, Volume 5. https://doi.org/
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5 - 2026

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Copyright © 2026 Gomaa, Abdel-Aal, Russo, Galiano, Figoli and Abdel-Ghafar.

License: https://creativecommons.org/licenses/by/4.0/

*Correspondence: Hamdy Maamoun Abdel-Ghafar

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