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Article

Nanostructured Cellulose Acetate Membranes Embedded with Al2O3 Nanoparticles for Sustainable Wastewater Treatment

by
Ines Elaissaoui
1,*,
Soumaya Sayeb
1,
Mouna Mekki
2,
Francesca Russo
2,*,
Alberto Figoli
2,
Karima Horchani-Naifer
1 and
Dorra Jellouli Ennigrou
1
1
Physical Chemistry Laboratory of Mineral Materials and their Applications, National Center for Research in Materials Sciences, Technopark Borj Cedria, P.O. Box 73, Soliman 8027, Tunisia
2
Institute on Membrane Technology (CNR-ITM), Via P. Bucci 17 c, 87036 Rende, CS, Italy
*
Authors to whom correspondence should be addressed.
Coatings 2025, 15(7), 823; https://doi.org/10.3390/coatings15070823
Submission received: 30 May 2025 / Revised: 4 July 2025 / Accepted: 8 July 2025 / Published: 15 July 2025
(This article belongs to the Section Environmental Aspects in Colloid and Interface Science)
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Abstract

Electrospun nanofiber membranes based on cellulose acetate (CA) have gained increasing attention for wastewater treatment due to their high surface area, tuneable structure, and ease of functionalization. In this study, the performance of CA membranes was enhanced by incorporating aluminum oxide (Al2O3) nanoparticles (NPs) at varying concentrations (0–2 wt.%). The structural, morphological, and thermal properties of the resulting CA/Al2O3 nanocomposite membranes were investigated through FTIR, XRD, SEM, water contact angle (WCA), pore size measurements, and DSC analyses. FTIR and XRD confirmed strong interactions and the uniform dispersion of the Al2O3 NPs within the CA matrix. The incorporation of Al2O3 improved membrane hydrophilicity, reducing the WCA from 107° to 35°, and increased the average pore size from 0.62 µm to 0.86 µm. These modifications led to enhanced filtration performance, with the membrane containing 2 wt.% Al2O3 achieving a 99% removal efficiency for Indigo Carmine (IC) dye, a maximum adsorption capacity of 45.59 mg/g, and a high permeate flux of 175.47 L·m−2 h−1 bar−1. Additionally, phytotoxicity tests using Lactuca sativa seeds showed a significant increase in germination index from 20% (untreated) to 88% (treated), confirming the safety of the permeate for potential reuse in agricultural irrigation. These results highlight the effectiveness of Al2O3-modified CA electrospun membranes for sustainable wastewater treatment and water reuse.

Graphical Abstract

1. Introduction

Water pollution has become an escalating global concern due to the combined effects of rapid industrial development, environmental degradation, and population growth [1,2]. Industrial discharges, particularly from sectors like textiles, release harmful compounds such as organic and synthetic pollutants and heavy metals, which degrade water quality and limit its usability [3]. Consequently, addressing wastewater contamination is crucial for environmental protection. Conventional treatment methods, including precipitation, coagulation, biological processes, and filtration, have been widely applied. However, these approaches often face limitations in removal efficiency and operational sustainability [4,5]. In fact, these traditional methods have several limitations that impact their removal efficiency [6,7]. Among different industries, the textile industry frequently causes remarkable significant changes in water color via dye use, which leads to biological and chemical changes causing significant damage to the environment [7,8,9]. Membrane technologies have emerged as a superior alternative due to their high performance, lower energy consumption, and operational simplicity, making them highly suitable for water purification and desalination and adaptable for treating dye-contaminated wastewater [10]. Nevertheless, optimizing membrane properties remains critical, as polymer selection and solvent use during fabrication strongly influence porosity, hydrophilicity, and mechanical performance. The selection of membrane fabrication techniques depends on the choice of polymers used as well as the solvents employed during the preparation process, which play a crucial role in determining membrane properties such as porosity, hydrophilicity, and mechanical stability. Among the various fabrication methods, electrospinning is widely adopted to create fibrous structures using biodegradable polymers and polymer–inorganic composites, particularly for wastewater treatment applications [11,12,13]. The growing application of nanotechnology in water treatment is attributed to the unique advantages of nanomaterials, including their large surface area, enhanced reactivity, rapid dissolution, and superior adsorption properties compared with bulk materials. Furthermore, these materials exhibit excellent compatibility and can be modified with functional additives, though challenges remain in ensuring their stability and preventing nanoparticle release into water systems [11]. Likewise, this technique was an established technique for the fabrication of micro- and nano-fibrous polymer structures. Effectively, the application of nanotechnology is increasing in wastewater treatment owing to the many offered properties such as a high surface-to-volume ratio, increased reactivity, rapid dissolution, and enhanced adsorption, which differ from those of bulk materials [14]. Moreover, nanomaterials demonstrate excellent compatibility and can be easily modified with other functional materials. However, the main limitation is related to the stability of these new materials and avoiding the release of nanomaterials in water.
In recent years, developing environmentally friendly nanofiber membranes has become a priority. Traditional electrospinning solvents raise concerns due to their toxicity and environmental impact [15]. To address this, researchers have introduced greener alternatives such as dimethyl carbonate (DMC), dimethyl sulfoxide (DMSO), and Cyrene, which reduce toxicity while maintaining effective polymer solubility and fiber formation. These solvents further contribute to the adoption of sustainable practices in electrospun membranes fabrication, offering reduced toxicity [16,17]. For instance, cellulose acetate (CA) nanofibers have been successfully produced using less harmful solvents like acetic acid and acetone, with acetic acid offering a safer and more sustainable option than traditional solvents like N,N-dimethylformamide (DMF). Acetic acid, in particular, is considered a less toxic solvent compared with traditional solvents like N,N-dimethylformamide (DMF), making it a safer choice for both electrospinning preparation and the environment.
Cellulose acetate has great potential as a sustainable material in wastewater treatment. Its application in membrane filtration, adsorption, and flocculation processes offers promising environmental benefits, such as biodegradability and a reduced reliance on synthetic polymers [11,18,19,20]. However, challenges such as durability and cost need to be addressed before it can be widely adopted. Ongoing research and technological advancements in cellulose acetate processing are expected to improve its effectiveness and sustainability in wastewater treatment, supporting more eco-friendly water management solutions [21].
Nanoparticle immobilization into natural polymers offers advantages like uniform nanoparticle distribution, improved stability, and ease of reuse. Aluminum oxide (Al2O3) nanoparticles play a vital role in wastewater treatment, offering a variety of properties such as high chemical stability, adsorption, catalytic activity, and filtration capabilities. Further, the Al2O3 nanoparticles have a large surface area, which provides more active sites for adsorbing pollutants, and can often be regenerated and reused multiple times, which makes them cost effective in long-term applications [22]. Its use in heavy metal removal, membrane filtration, and as a coagulating agent demonstrates its versatility [22,23,24,25,26,27]. While challenges like cost and fouling resistance remain, Al2O3 continues to be a promising material for sustainable, efficient wastewater treatment solutions, particularly in industrial and large-scale applications. Based on these insights, this study focused on enhancing CA electrospun nanofiber membranes by incorporating different ratios of Al2O3 nanoparticles to improve dye filtration performance. By varying the Al2O3 content, the aim was to optimize membrane properties such as thermal stability, mechanical strength, hydrophilicity, flux recovery, permeability, and antifouling behavior. The membranes were evaluated in terms of water flux, permeability, water resistance, and dye retention to determine the optimal composition for effective wastewater treatment and water reuse.

2. Materials and Methods

2.1. Electrospun Membrane Preparation

As an initial step, cellulose acetate (CA, Mw 30,000 Da) acquired from Sigma Aldrich (New York, NY, USA) was dissolved in an acetic acid solvent supplied from CHAMLAB and stirred for 48 h at room temperature. The content of CA in the solution was fixed at 16 wt.%. The alumina nanoparticles (Al2O3 NPs, Mw = 101.96 g mol−1) used in this study were purchased from Oxford Lab Chem. They were in the gamma (γ) crystalline phase, with a quasi-spherical morphology and an average particle size of approximately 18 nm. The alumina content in the membranes varied as follows: 0 wt.%, 0.5 wt.%, 1 wt.%, 1.5 wt.%, and 2 wt.%. The modified CA electrospun nanofiber membranes were prepared via the electrospinning process. The solution obtained was loaded into a 10 mL syringe connected to a 23 G metallic needle. While the optimized electrospinning conditions were established based on our previous study [11], the flow rate was set at 2 mL h−1, the applied voltage was fixed at 18 kV, and the distance from the needle tip to the collector was 15 cm, with an ambient temperature of 23 ± 2 °C. A summary of the prepared dope solutions is listed in Table 1. The obtained membranes were immersed in ultra-pure water, which had a resistivity of 18.2 M.cm−1 and was generated by a Millipore Milli-Q water purification system (Burlington, MA, USA), for two hours to effectively separate the fibers from the aluminum foil and obtain CA electrospun nanofiber membranes. Figure 1 provides a summary of the CA electrospun nanofiber membranes preparation procedures used in the present study.

2.2. Membrane Characterization

The presence of Al2O3 NPs in the CA nanofiber membrane was analyzed using FTIR spectroscopy (Perkin Elmer spectrometer (S/N 110400), Waltham, MA, USA), with a resolution of 4 cm−1 and operating in the transmission mode in the 400–4000 cm−1 range.
An X-ray diffractometer (XRD, PANalytical PANalytical, Almelo, The Netherlands) was used to analyze the crystal structure of the CA electrospun nanofiber membranes with a Cu Kα source (λ = 1.5416 nm). X’Pert HighSore Plus was used to further evaluate the diffractograms, which range from 30° to 70° when recorded at the scanning angle (2θ). Curve fitting was performed to identify individual peaks.
The morphological details and the cross-sectional area of the elaborated CA electrospun nanofiber membranes were assessed using scanning electron microscopy (SEM) (Zeiss EVO, MA100, Assing, Italy).
Then, the membrane thickness values was measured using a Digimatic micrometer (Mitutoyo543−561D, Metric Dial indicator, 0→30 mm measurement range, 0.0005 mm, 0.001 mm resolution, 1.5 μm, Japan). The mean thickness and standard deviation were determined from measurements collected in five distinct areas of the membrane.
The pore size and pore size distribution were determined using a POROLUX™ 1000 capillary flow porometer (Porometer NV, Nazareth, Belgium), based on advanced stability algorithms. The instrument employs the pressure step/stability method, which ensures that each data point is only recorded once the pressure and flow reach user-defined stability criteria, guaranteeing that all pores of a given diameter are fully opened. Measurements were carried out by first recording the wet curve (with the membrane saturated by a wetting liquid) and then the dry curve (on the same dry membrane) [28]. This allowed the accurate calculation of key parameters such as the first bubble point, mean flow pore size, smallest pore diameter, and the full pore size distribution.
The thermal characteristics of the blends were investigated using a differential scanning calorimeter (DSC). The measurements were carried out using the DSC 3 (METTLER TOLEDO Module (700/1128), Mettler-Toledo, Columbus, OH, USA) in a pan of aluminum with a pierced lid in a N2 atmosphere at a rate of 10° K/min. The results were noted and examined.
The water contact angle (WCA) is an essential property for examining the surface wettability of CA electrospun nanofiber membranes. A device for measuring contact angles (ATTENSION, THETA, (S/N AAV 100005), Biolin Scientific AB, Gothenburg, Sweden) was used to determine the WCA. The WCA was measured using sliced samples of the CA membrane that were put onto a glass slide. The contact angles that have been reported are the mean of three measurements obtained from randomly chosen points on each sample.

2.3. Filtration Experiment

2.3.1. Pure Water Permeability

The filtration performance of the membranes was evaluated using a laboratory-scale setup, where N2 gas was employed to maintain constant pressure. A Millipore stainless-steel cell was used to assess the permeability and flow of the water. N2 gas was used to maintain consistent pressure in a laboratory-scale setup to assess the membranes’ filtration performance. The test was performed under constant conditions after 30 min of operation time at 4 bar. The pure water permeability (L m−2 h−1 bar−1) was then evaluated using the following Equation (1):
P W P = V p A × t × p
where A corresponds to the membrane area (m2), t for time (h), p is the transmembrane pressure (bar), and Vp is the permeate volume (L).

2.3.2. Porosity of CA Electrospun Nanofiber Membranes

The weight of the small membrane was estimated when the CA electrospun nanofiber membrane was cut into 2 × 2 cm2. After that, the membrane was immersed in 50 mL of room-temperature distilled water for 24 h. After 24 h immersion, the weight of the membrane was determined. The membrane porosity was computed using the following Formula (2) [29,30]:
P o r o s i t y % = M 1 − M 2 V × 100
The weight of the manufactured membranes is denoted by M1, and the weight of the tested membranes following 24 h immersion in water is denoted by M2. V = A × t, where A represents the surface area (cm2) of the tested membrane and t is its thickness (cm).

2.3.3. Color Removal Efficiency

A solution of Indigo Carmine (IC) dye, with the molecular formula C16H8N2Na2O8S2, was used to assess the efficacy of the modified CA electrospun nanofiber membranes. For this measurement, 200 mL of the IC solution was filtered up until a constant permeate flux was found, with a feed concentration of 5 mg L−1. Then, at room temperature, 15 mL of samples were taken at transmembrane pressures of 4 bar.
Using a Thermospectronic UV1 spectrophotometer set to 610 nm, the IC concentrations in the feed and permeate solutions were determined. Equation (3) was used to determine the IC removal efficiency [11]:
C o l o r   r e m o v a l % = A B S 0 − A B S t A B S 0
where the absorbance of untreated dye solution is revealed by ABS0, while the absorbance of the dye following treatment with various CA electrospun nanofiber membranes is indicated by ABSt.

2.4. Phytotoxicity Test

The effectiveness of a wastewater treatment procedure to eliminate toxic compounds can be demonstrated with the use of toxicity tests [31]. Using the root extension of germinated seeds, a number of plant species have been suggested as indicators of terrestrial toxicity [32]. Among these, the Environmental Protection Agency of the United States (1982) has recommended lettuce seeds (Lactuca sativa) [33]. It is an excellent example to highlight the potential negative effects of utilizing irrigation with wastewater. In the context of our research, the most appropriate bioassay test is the phytotoxicity test conducted on lettuce (Lactuca sativa) seeds. The germination index (GI) of Lactuca sativa was measured to assess the phytotoxicity of various samples of raw and treated dye wastewater. Twenty seeds were evenly distributed throughout Petri dishes fitted with a Whatman paper filter. Then, 10 mL of the water samples were distributed equally among the dishes. After being sealed, the dishes were incubated for 7 days at 20 °C ± 2 °C in the dark. Control experiments were performed using distilled water. For every sample, triple phytotoxicity tests were conducted; then, the mean values were noted. Following incubation, the average root length in each sample was measured and the number of seeds that germinated was counted in order to calculate the germination index (GI) in relation to the control treatments. The results are presented according to the equation as follows (4):
G I   ( % ) = N u m b e r   o f   g r o w n   s e e d s   i n   s a m p l e N u m b e r   o f   g r o w n   s e e d s   i n   c o n t r o l × a v e r a g e   s u m   o f   r o o t   l e n g h t s   i n   s a m p l e a v e r a g e   s u m   o f   r o o t   l e n g h t s   i n   c o n t r o l × 100
When a seed’s root length was more than 5 mm, it was considered to have germinated. A seed’s root length of less than 5 mm was deemed to indicate that it had germinated, which was treated as equivalent to 0. In each Petri dish, the average root length was calculated by summing the root lengths of all germinated seeds.

3. Results and Discussion

3.1. FT-IR

To verify the incorporation of Al2O3 NPs in the CA electrospun nanofiber membranes, Figure 2 reports the FT-IR spectra of both the unmodified and modified membranes. The spectra for all samples display characteristic peaks corresponding to the functional groups present in the CA polymer [11,34] at specific wavenumbers: approximately 1739 cm−1 (C-O stretching vibration), between 2700 and 2900 cm−1 (asymmetric C-H stretching), at 2914 cm−1 (symmetric C-H stretching), and at 3484 cm−1 (O-H stretching). After the incorporation of Al2O3 NPs, the spectra displayed distinct changes, particularly a shift to higher wavenumbers (3327 cm−1), which likely indicates an interaction between the hydroxyl group of CA and Al2O3. Likewise, the Al2O3 modification into the CA nanofibers led to the emergence of new peaks at 1635 cm−1 and 2920 cm−1. The band at 1635 cm−1 corresponds to the bending vibrations of water molecules, while the band at 2920 cm−1 is likely attributed to C-H and O-H groups, suggesting an interaction between the hydroxyl groups and CA. Furthermore, the inclusion of Al2O3 NPs into the CA matrix led to significant changes in the intensities of the absorption bands. As the content of Al2O3 NPs in the CA matrix increased, the intensity of the C-O stretching vibration band at 1739 cm−1 decreased. These observations collectively confirm the incorporation of Al2O3 NPs into the CA matrix and support the substantial interaction between the two components.

3.2. X-Ray Diffraction (XRD)

Figure 3 displays the X-ray diffraction patterns of a pristine CA membrane and a modified membrane with different contents of Al2O3 NPs in order to examine the structure and dispersion of Al2O3 NPs within the CA electrospun nanofiber membrane. As shown in Figure 2, the XRD patterns of CA display characteristic cellulose acetate peaks at 2ϴ = 17.9° and 20.4°, corresponding to its semi-crystalline structure [35,36]. The diffraction pattern of Al2O3 NPs, however, shows several characteristic peaks at 2ϴ = 29.1°, 35.8, 39.3, 42.9, 46.8, 47.9, 56.8, and 60.6 [37,38]. It can be seen that the intensity of this peak increases in the functionalized samples with varying molar ratios of Al2O3 NPs.
In CA/Al2O3 nanocomposite membranes, the peaks corresponding to the CA polymer have a low intensity due to the higher crystalline peaks of the Al2O3 NPs. Based on these results, it appears that the Al2O3 NPs are dispersed throughout the CA matrix, which allows us to conclude that the XRD results confirm the formation of the CA/Al2O3 nanocomposite. This observed result is in line with the FT-IR analysis. The dispersion of Al2O3 NPs is expected to enhance the thermal properties of the CA/Al2O3 nanocomposites.

3.3. Thermal Behavior

In order to determine the thermal proprieties of CA electrospun nanofiber membranes, a thermal characterization was performed using DSC analysis. The DSC thermograms are illustrated in Figure 4. A broad endothermic event between ambient temperature and 100 °C is observed for both the CA membrane and the membrane containing 0.5 wt.% Al2O3 NPs. This event is attributed to the desorption of moisture absorbed by the hydrophilic polymer. However, this peak disappeared in the membranes with a higher Al2O3 content (CA1, CA1.5, and CA2), likely due to reduced water retention, possibly caused by stronger polymer–nanoparticle interactions or better packing density within the fiber structure. A second endothermic peak around 228.9 °C is observed, which corresponds to the melting temperature (Tm) of the polymer [39]. This peak remains low in intensity, which is typical for electrospun membranes due to their low crystallinity and rapid solvent evaporation during fiber formation. A slight increase in (Tm) was recorded with increasing Al2O3 content, reaching 230.3 °C for the CA2 membrane. This small shift is consistent with the literature and may be attributed to interfacial interactions between the Al2O3 NPs and CA chains, which can restrict chain mobility and slightly enhance thermal stability [40,41]. It is acknowledged that the observed differences in (Tm) are within a narrow range (<2 °C), and while they may not be statistically significant on their own, they support the trend of improved thermal behavior when considered in combination with the morphological and structural changes observed in SEM images and other characterizations.

3.4. Morphology, Thickness, Contact Angle, Porosity, and Pore Size

The morphology of the electrospun nanofiber membranes with and without Al2O3 NPs was analyzed and reported in Figure 5. The pristine CA membrane exhibits a symmetric structure characterized by a porous nanofiber layer with uniformly distributed fibers.
The presence of Al2O3 nanoparticles was clearly observed within the nanofibers. With increasing nanoparticle concentration, their distribution became more distinct and uniform within the fibrous network, indicating enhanced dispersion and suggesting improved interfacial interactions between the nanoparticles and the polymer matrix. The SEM analysis of the pristine CA nanofibers revealed morphological irregularities, with fiber diameters ranging from 450 ± 50 nm to 950 ± 50 nm. The introduction of Al2O3 nanoparticles, after 48 h of stirring, increased the viscosity of the polymer solution. However, in the case of the CA2 sample, the SEM images showed a notable reduction in fiber diameter, ranging from 500 ± 50 nm down to 200 ± 50 nm, suggesting that the presence of well-dispersed nanoparticles may enhance fiber uniformity and have control over nanofiber size.
The results of the thickness measurements are presented in Figure 6. It was observed that the CA2 membrane, which incorporated aluminum oxide (Al2O3) nanoparticles, exhibited a lower thickness (80.3 μm) compared with the CA membrane (90.2 μm). This reduction in membrane thickness was likely due to the influence of the nanoparticles on the electrospinning process. Specifically, the addition of Al2O3 may have increased the electrical conductivity and dielectric properties of the polymer solution, which enhanced the stretching of the electrospun jet under the applied electric field. As a consequence, thinner fibers were formed and deposited more compactly on the collector surface. Furthermore, the presence of nanoparticles may have altered the viscosity and surface tension of the spinning solution, facilitating better jet elongation and more efficient fiber packing. These combined effects contributed to a more condensed membrane structure and, ultimately, a decrease in thickness.
Similar trends have been reported in the literature. For instance, Bhardwaj and Kundu (2010) [42] demonstrated that the electrical conductivity of the spinning solution plays a critical role in fiber thinning. Likewise, Li et al. (2004) [43] observed that the inclusion of ceramic nanoparticles in polymer matrices can influence solution viscosity and promote jet stability during electrospinning.
The contact angle is a key parameter for characterizing the hydrophilic or hydrophobic nature of a membrane surface [44]. The findings are displayed in Figure 7. For the unmodified CA electrospun nanofiber membrane, the measured contact angle was 107°, indicating the hydrophobic nature of the fabricated membrane [11]. After the incorporation of Al2O3 NPs, the water CA of the modified membranes decreased with the increasing Al2O3 NPs content, reaching a CA of 35° for the CA2 membrane at 2 wt.% of Al2O3 NPs. The reason for this was due to the hydrophilic character of Al2O3 nanoparticles which can influence the surface of the membrane [45].
Figure 8 presented the mean flow pore diameter of the electrospun CA membranes with and without Al2O3 nanoparticles. The pristine CA membrane exhibited the smallest average pore size, approximately 0.08 µm, while the membrane containing 2 wt.% Al2O3 (CA2) showed the largest, around 0.86 µm. This trend was attributed to changes in the viscosity and conductivity of the electrospinning solution induced by the addition of nanoparticles, which affected fiber formation and resulted in a more open membrane structure. The hydrophilic nature of Al2O3 also enhanced the solvent–non-solvent exchange during phase inversion, accelerated phase separation, and promoted the development of larger pores. Moreover, the nanoparticles disrupted the packing of polymer chains, increasing the free volume within the membrane. Consequently, the incorporation of Al2O3 nanoparticles significantly influenced both pore size and overall porosity. As the nanoparticle concentration increases, the porosity in Figure 8 (in red color) increased. The same trend was observed with the pore size and contact angle as well.

3.5. Pure Water Permeability

The pure water permeability (PWP) of CA electrospun nanofiber membranes is a crucial parameter when evaluating their performance in water filtration and separation applications. The pure water permeability of cellulose acetate membranes is generally enhanced by the incorporation of Al2O3 NPs due to improved hydrophilicity, reduced fouling, and better mechanical stability [46,47]. However, the increase in permeability depends on different factors such as nanoparticle concentration, membrane fabrication method, and the specific wastewater treatment conditions. Figure 9 reports the PWP of the pristine and selected modified CA electrospun nanofiber membranes. As clearly observed in Figure 9, PWP increases with the increasing Al2O3 NP concentration to reach 175.47 L m−2 h−1bar−1 for CA2.

3.6. Results of Filtration Tests and Comparison with the Literature

The filtration process using CA electrospun nanofiber membranes involved separating particles and solutes from a liquid stream based on pores size and other properties, such as good toughness and high reflux. The addition of metal–organic frameworks and inorganic nanomaterials to CA electrospun nanofiber membranes can enhance filtration performance by influencing factors such as roughness, water flux, and hydrophilicity [48]. The interaction between cellulose acetate and Al2O3 NPs in wastewater treatment primarily enhances the adsorptive capacity and mechanical properties of the membrane, reduces fouling, and improves filtration efficiency. A filtration test was conducted for the obtained membranes using IC dye as the feed solution at an initial concentration of 5 mg L−1, and the removal efficiency was evaluated. As shown in Figure 10, the IC dye removal efficiency increased slightly as the weight ratios of Al2O3 NPs increased. The removal rate of the membrane with a 2% weight ratio (CA2) reached 99%, indicating that the membrane exhibits excellent filtration properties for IC dye. This can be attributed to the adsorption property of the nanoparticles. As reported in previous studies [29], the Al2O3 nanoparticles possess a high specific surface area and strong adsorptive interaction with dye molecules, enhancing the overall removal efficiency.
Generally, Al2O3 NPs significantly enhance filtration efficiency due to their high surface area, stability, and ability to adsorb a wide range of contaminants. They improve membrane performance by reducing fouling, enhancing permeability, and maintaining long-term stability under harsh conditions, making them highly effective in water and wastewater treatment, as well as other filtration applications. The high surface area of Al2O3 NPs provides more active sites for adsorption, making them efficient at trapping contaminants, including heavy metals, organic compounds, and dyes. This allows for an improved filtration capacity and better performance in removing pollutants. The improved filtration and adsorption properties of the membranes may be attributed to the concentrations of Al2O3 NPs, which accelerate the process, resulting in a denser membrane with a higher impact structure that provides an increased number of activated adsorption sites. Interestingly, this behavior can be attributed to the surface functionalization of the CA polymer induced by the presence of Al2O3 NPs. The CA electrospun nanofiber membranes’ ionic conductivity is improved by the creation of O/OH groups at the grain boundaries, which make it easier to form hydrogen bonds with migrating ionic species [48,49]. This study revealed that Al2O3 NPs enhance water flux and improve membrane performance and antifouling properties by increasing porosity, conductivity, and hydrophilicity. The obtained results with the addition of metal oxide NPs demonstrate improved filtration performance compared with the membrane without NPs. Thus, Al2O3 NPs serve as effective modifying agents for filtration applications to increase the permeability and selectivity of CA electrospun nanofiber membranes.
Table 2 illustrates how comparable compositions have been investigated for dye removal. CA2 is used to fabricate the CA electrospun nanofiber membranes, which exhibit the highest reported adsorption capacity (47.59 mg g−1). Compared with other studies [24,50,51,52,53], the developed CA electrospun nanofiber membranes exhibit a superior adsorption capacity compared with other adsorbents. For instance, Shalan et al. studied the elimination of Methylene Blue (MB) dye using CA nanofiber membranes containing hydroxyapatite co-doped with Ag/Fe. According to the findings, the degradation efficiency increased to 90% after 2 h [50]. It can be observed that Hassan et al. developed an innovative magnetic nanofiber composite by hybridizing nano-magnetite zinc oxide with CA, which was fabricated using the electrospinning technique. The results indicated that the composite nanofiber has the ability to adsorb 64% of phenol within 2 h [51]. Indeed, ZabihiSahebi et al. fabricated cellulose acetate/chitosan/single-walled carbon nanotubes/ferrite/titanium dioxide (CA/chitosan/SWCNT/Fe3O4/TiO2) nanofibers and studied the degradation of Methylene Blue and Congo Red azo dyes and reached higher than 85% removal [52].

3.7. Phytotoxicity Results (Germination Test)

The viability of recycling the modified IC aqueous solution for irrigation was evaluated using lettuce (Lactuca sativa) seed germination. Seed germination assays were conducted to calculate the Germination Index (GI) values for both untreated and treated dye-laden solutions (Figure 11). The results indicated that the GI for the untreated IC solution was 20%, whereas the germination rate for seeds in the IC solution treated with CA2 was 88%, as shown in Figure 10. According to Abdullahi et al., a seed is considered non-phytotoxic if its GI is above 70% and phytotoxic if it is below 70% [54]. Based on the phytotoxicity assessment results, the filtration process using the CA2 membrane had a significant positive effect on the growth parameters and seed germination of L. sativa. Therefore, this process is recommended for use in the agricultural sector, particularly in irrigation.

4. Conclusions

CA electrospun nanofiber membranes modified with different amounts of Al2O3 nanoparticles were successfully fabricated via electrospinning and applied to the removal of Indigo Carmine (IC) dye from wastewater. The synthesized membranes were thoroughly characterized to evaluate their structural, morphological, and thermal properties. The incorporation of Al2O3 NPs into the CA matrix was confirmed through various analytical techniques, revealing a homogeneous dispersion of nanoparticles and strong interactions with the polymer chains. The addition of Al2O3 significantly improved the thermal stability and hydrophilicity of the membranes. As a result, an enhancement in pure water permeability was observed, driven by an increased membrane porosity and surface wettability. Furthermore, the incorporation of Al2O3 NPs induced changes in the electrospinning solution’s viscosity and conductivity, which led to a more open fiber structure and a gradual increase in mean pore size from 0.62 µm for pristine CA to 0.86 µm for the CA2 membrane—contributing positively to the filtration performance. The membrane loaded with 2 wt.% Al2O3 (CA2) exhibited outstanding dye removal efficiency, achieving up to 99% IC rejection at 12 bar with an initial dye concentration of 5 mg L−1. Additionally, the maximum adsorption capacity reached 47.59 mg g−1, indicating the high affinity and effectiveness of the modified membrane in dye capture. To assess the environmental safety of the treated water, a phytotoxicity test using Lactuca sativa seeds was conducted. The germination index improved markedly from 20% in the untreated solution to 88% after filtration with the CA2 membrane. This result, exceeding the 70% non-phytotoxicity threshold, confirmed that the treated effluent is safe and suitable for reuse in agricultural irrigation. Overall, these findings highlight the promising potential of CA/Al2O3 nanofiber membranes as efficient, sustainable, and environmentally friendly materials for wastewater treatment and water reuse applications. Additionally, the positive effects of incorporating Al2O3 nanoparticles can be attributed to both their distribution on the surface of the microfibers, where they enhance hydrophilicity and create additional active adsorption sites, and their encapsulation within the polymer matrix, where they interact with cellulose acetate chains and influence the membrane morphology. This dual localization plays a crucial role in improving water permeability and dye removal efficiency, further confirming the suitability of these membranes for sustainable water treatment applications.
To further enhance the sustainability of membrane fabrication processes, future research will focus on replacing conventional solvents with environmentally friendly alternatives. These green solvents, characterized by low toxicity and high biodegradability, are well suited for techniques such as electrospinning. The emerging trends in this area include the use of bio-based solvents derived from renewable resources, deep eutectic solvents (DESs), and supercritical fluids, all of which offer the potential to significantly reduce environmental impact. Additionally, the implementation of solvent recovery systems and the adoption of closed-loop processing are expected to play a key role in minimizing waste and improving overall process efficiency. Finally, the development of solvent-free or water-based electrospinning methods is gaining traction, offering a promising path toward greener and safer membrane production technologies.

Author Contributions

All authors contributed to the conception and design of the study. I.E. led the data collection and analysis, contributed to the interpretation of the results, and drafted the initial version of the manuscript. S.S. and F.R. provided critical revisions and contributed to the development of the final version of the manuscript. M.M. contributed to the data analysis. K.H.-N., D.J.E., and A.F. supervised the work. All authors have read and agreed to the published version of the manuscript.

Funding

This research was not funded by any program.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors acknowledge the financial assistance provided by the Agency for the Promotion of Scientific Research (ANPR) in Tunisia to carry out this study as a component of the MOBIDOC initiative, which is supported by the European Union under the PROMESSE program. The authors would like to thank the National Research Council (CNR) and the Institute for Physical and Chemical Processes (IPCF) for their support and coordination of the FUTURA-W project, which aims to develop raw materials for the future from non-critical, residual, and renewable sources. This work was funded by the Italian Ministry of University and Research (MUR) under the Riparto Fondo Ordinario Enti di Ricerca—DCM. AD005.081, CUP: B53C23008390005. We extend our sincere gratitude to Lidia Armelao, Director of DSCTM-CNR and Principal Investigator of the project, for her leadership and vision.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Coatings 15 00823 g001
Figure 1. Schematic illustration of the synthesis of CA/Al2O3 membrane.
Figure 1. Schematic illustration of the synthesis of CA/Al2O3 membrane.
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Figure 2. FT-IR spectra of the CA electrospun nanofiber membranes without and with Al2O3 NPs.
Figure 2. FT-IR spectra of the CA electrospun nanofiber membranes without and with Al2O3 NPs.
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Figure 3. XRD patterns of the CA electrospun nanofiber membranes without and with Al2O3 NPs.
Figure 3. XRD patterns of the CA electrospun nanofiber membranes without and with Al2O3 NPs.
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Figure 4. DSC curves of the CA electrospun nanofiber membranes without and with Al2O3 NPs.
Figure 4. DSC curves of the CA electrospun nanofiber membranes without and with Al2O3 NPs.
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Figure 5. SEM images in terms of top surface for the CA electrospun membranes (a) without and with Al2O3 NPs at (b) 0.5 wt.%, (c) 1 wt.%, (d) 1.5 wt.%, (e) 2 wt.%.
Figure 5. SEM images in terms of top surface for the CA electrospun membranes (a) without and with Al2O3 NPs at (b) 0.5 wt.%, (c) 1 wt.%, (d) 1.5 wt.%, (e) 2 wt.%.
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Figure 6. Thickness of the CA electrospun membranes without and with Al2O3 NPs.
Figure 6. Thickness of the CA electrospun membranes without and with Al2O3 NPs.
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Figure 7. Water contact angles of CA electrospun membranes without and with Al2O3 NPs.
Figure 7. Water contact angles of CA electrospun membranes without and with Al2O3 NPs.
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Figure 8. Mean flow pore size and porosity of CA electrospun membranes without and with Al2O3 NPs. The error is less than 2%.
Figure 8. Mean flow pore size and porosity of CA electrospun membranes without and with Al2O3 NPs. The error is less than 2%.
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Figure 9. Water permeability of the CA electrospun membranes without and with Al2O3 NPs.
Figure 9. Water permeability of the CA electrospun membranes without and with Al2O3 NPs.
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Figure 10. Color removal efficiency (%) of the CA electrospun membranes without and with Al2O3 NPs.
Figure 10. Color removal efficiency (%) of the CA electrospun membranes without and with Al2O3 NPs.
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Figure 11. Germination indices of Lactuca sativa seeds before and after treatment of IC dye using CA2 membrane.
Figure 11. Germination indices of Lactuca sativa seeds before and after treatment of IC dye using CA2 membrane.
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Table 1. Dope solutions prepared for CA electrospun nanofiber membranes.
Table 1. Dope solutions prepared for CA electrospun nanofiber membranes.
Membrane CodeCA Content (wt.%)Al2O3 NPs Content (wt.%)
CA160
CA0.50.5
CA11
CA1.51.5
CA22
Table 2. Relevant studies of performance comparisons of modified CA electrospun nanofiber membranes under their application in dye degradation.
Table 2. Relevant studies of performance comparisons of modified CA electrospun nanofiber membranes under their application in dye degradation.
MembraneModification AgentsDyeColor Removal
(%)		Qmax
(mg g−1)		Ref.
CA
CA
CA/chitosan		Al2O3 (2%)IC9945.59Present work
Ag/FeMethylene Blue (MB)90-[50]
Single Walled Carbon Nanotubes/ferrite/
titanium dioxide
(SWCNT/Fe3O4/TiO2)		Methylene blue and Congo Red9994.3/
70.2		[52]
CA/Chitosan
CA/Cadmium sulfate (CdS)		TiO2Methyl Orange9876.22[51]
Gadolinium oxide (Gd2O3) and combined with graphene oxide (GO) nanoparticlesMethylene Blue (MB)91.078.33[53]
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Elaissaoui, I.; Sayeb, S.; Mekki, M.; Russo, F.; Figoli, A.; Horchani-Naifer, K.; Ennigrou, D.J. Nanostructured Cellulose Acetate Membranes Embedded with Al2O3 Nanoparticles for Sustainable Wastewater Treatment. Coatings 2025, 15, 823. https://doi.org/10.3390/coatings15070823

AMA Style

Elaissaoui I, Sayeb S, Mekki M, Russo F, Figoli A, Horchani-Naifer K, Ennigrou DJ. Nanostructured Cellulose Acetate Membranes Embedded with Al2O3 Nanoparticles for Sustainable Wastewater Treatment. Coatings. 2025; 15(7):823. https://doi.org/10.3390/coatings15070823

Chicago/Turabian Style

Elaissaoui, Ines, Soumaya Sayeb, Mouna Mekki, Francesca Russo, Alberto Figoli, Karima Horchani-Naifer, and Dorra Jellouli Ennigrou. 2025. "Nanostructured Cellulose Acetate Membranes Embedded with Al2O3 Nanoparticles for Sustainable Wastewater Treatment" Coatings 15, no. 7: 823. https://doi.org/10.3390/coatings15070823

APA Style

Elaissaoui, I., Sayeb, S., Mekki, M., Russo, F., Figoli, A., Horchani-Naifer, K., & Ennigrou, D. J. (2025). Nanostructured Cellulose Acetate Membranes Embedded with Al2O3 Nanoparticles for Sustainable Wastewater Treatment. Coatings, 15(7), 823. https://doi.org/10.3390/coatings15070823

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