Modified polyacrylamide nanofiber membrane for efficient nitrate removal from landfill leachate

Abstract

This study investigates the modification of polyacrylamide (PAM) nanofibers with silver nanoparticles (Ag) and green tea (GT) extract to enhance nitrate removal from landfill leachate collected in Kermanshah, Iran. GT extract was used to synthesize Ag nanoparticles, and 1 wt% of the resulting nanomaterials was added to an 11.7 wt% PAM solution before electrospinning. The fabricated membranes were characterized using scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), and Fourier-transform infrared spectroscopy (FTIR). Their hydrophilicity, water flux, and fouling resistance were also evaluated. Nitrate removal performance was optimized under varying flow rates, pH levels, and pressures using a design of experiments (DOE) approach. The PAM/Ag@GT membrane demonstrated enhanced properties, including reduced fiber diameter, increased porosity (from 26.2 to 58.6%), improved hydrophilicity, and decreased turbidity (from 32 to 3 NTU). Nitrate removal efficiency increased from 32 to 73.25%, with a leachate flux of 1850 L/m²·h and a flux recovery ratio of 84.45%. Additionally, the PAM/Ag@GT membrane exhibited superior tensile strength and elongation compared to unmodified PAM. Under optimal conditions (0.2 bar pressure, pH 7, and a flow rate of 120 L/h), the PAM/Ag@GT membrane demonstrated efficient nitrate removal, enhanced overall performance, and promising potential for sustainable wastewater treatment applications.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-025-05093-y.

Introduction

Landfill leachate is a complex and variable mixture of contaminants formed through the decomposition of waste materials and their interactions under landfill conditions¹. Its composition depends on various factors, including the type of waste, the age of the landfill, climatic conditions, and the presence of diverse chemical compounds. Among the contaminants found in municipal landfill leachate, nitrates are of particular concern due to their environmental persistence and potential health impacts. Effective nitrate removal is essential for protecting water resources², preventing groundwater contamination³, and minimizing ecological and human health risks^4,5. Addressing nitrate pollution in leachate not only preserves aquatic ecosystems but also advances waste management toward sustainability. This highlights the need for innovative, efficient treatment solutions specifically targeting nitrate contamination. Effective management of municipal landfill leachate is crucial to mitigate its adverse impacts, including environmental contamination⁶, groundwater pollution⁷, and associated health risks⁸. Additionally, proper treatment aligns with regulatory compliance⁹ and promotes sustainable waste management¹⁰, as well as resource recovery and energy generation¹¹. Given the growing concern over landfill sustainability, finding efficient and sustainable treatment methods for leachate is paramount¹². Landfill leachate treatment is crucial to mitigate the environmental and health risks posed by the complex mixture of contaminants. A variety of treatment methods have been proposed, including physicochemical processes (coagulation, flocculation^13,14, adsorption¹⁵, and membrane filtration¹⁶), biological treatments (activated sludge¹⁷, anaerobic digestion¹⁸, and constructed wetlands¹⁹), chemical oxidation (advanced oxidation processes (AOPs)²⁰ and wet air oxidation²¹), membrane processes^22–24, and combined methods like sequential batch reactors (SBR)²⁵, membrane bioreactors (MBR)²⁶, and integrated fixed-film activated sludge (IFAS)²². These methods offer varying levels of effectiveness but often face challenges related to cost, efficiency, and long-term sustainability.

Membranes play a key role in leachate treatment and are widely used in water treatment, gas separation, and biomedical applications. Various fabrication methods have been developed to create membranes with specific properties to meet the needs of these applications. These include techniques such as phase inversion²⁷, the sol-gel method²⁸, template-assisted fabrication²⁹, vapor-phase deposition³⁰, melt extrusion³¹, template-free self-assembly³², photolithography³³, and electrospinning³⁴. Among these, electrospinning has gained attention due to its ability to produce highly porous, fine nanofiber membranes with tailored characteristics for enhanced filtration. It is a cost-effective and versatile technique that produces fibers from polymers, minerals, and composites with diameters ranging from tens of nanometers to several micrometers using high voltage. Among the various methods of nanofiber production, electrospinning is considered one of the most efficient. In addition to its scalability and tunability for high efficiency, this process also allows precise control over the size and orientation of nanofibers^35,36. Silver nanoparticles (AgNPs) are widely used to enhance membrane performance due to their antibacterial, catalytic, and antifouling properties. Their incorporation into membranes improves hydrophilicity, reduces biofilm formation, and increases long-term permeability through controlled release of Ag + ions, which inhibit microbial growth and adhesion. Covalent bonding or incorporation methods have been successfully employed to immobilize AgNPs in polymeric membranes, prevent nanoparticle washout, and ensure long-term antimicrobial activity. Recent attention has been focused on the green synthesis of AgNPs using plant extracts as environmentally friendly and cost-effective alternatives to chemical methods. Green tea extracts and other plant materials contain bioactive compounds (e.g. flavonoids, phenolic acids, proteins) that act as both reducing agents and capping agents, facilitating the formation of nanoparticles by increasing their stability and surface functionality^36–39. Using plants as stable and available sources for the synthesis of biocompatible nanoparticles has advantages such as non-toxicity, low cost, biocompatibility, and production of nanoparticles with high purity. Biosynthesis of nanoparticles also minimizes potential risks to humans and ecosystems⁴⁰. Green nanoparticles prepared using biological materials or plant extracts, due to the presence of a large number of functional groups, especially carboxyl groups on their surfaces, provide the possibility of surface modification with organic, inorganic, polymeric, and biological materials. Features such as small dimensions, surface modification capability, and simple, fast, and cost-effective synthesis methods make these nanoparticles an ideal option for modifying the structure of nanofibers⁴¹. Green tea has been used as a natural source for the synthesis of nanoparticles due to its high richness in polyphenolic compounds. Water-soluble phytochemicals in green tea, including catechin and caffeine, act as metal ion-reducing agents to form stable metal nanoparticles coated with biocompounds⁴².

Several studies have demonstrated the effectiveness of electrospun nanofibrous membranes for environmental applications. For example, Ma et al.⁴³ developed functionalized membranes with enhanced bacterial and viral removal efficiency, while Zhang et al.⁴⁴ fabricated polyacrylonitrile/silica composite membranes with high porosity and fine fiber diameter through improved nanoparticle dispersion. Similarly, Li et al.⁴⁵ reported that electrospun nanofibers offer a high surface area suitable for enzyme immobilization, contributing to surface hydrophilicity and functionalization. In the field of nitrate removal, Gholami et al.⁴⁶ modified polyethersulfone membranes using ionic liquids, achieving over 80% nitrate rejection and improved antifouling behavior. Shahmansouri and Bellona⁴¹ emphasized the importance of membrane properties such as surface charge and pore structure in achieving selective nitrate separation. Despite these advancements, limited research has addressed nitrate removal from real landfill leachate using sustainable membrane modification strategies. Our study introduces a novel approach by integrating AgNPs, synthesized through a green method using green tea extract (GT), into polyacrylamide (PAM) electrospun nanofibers. This eco-friendly strategy enhances hydrophilicity, porosity, and fouling resistance—key parameters for improving membrane performance in treating complex leachate matrices. By bridging material innovation with green synthesis, this research aims to fill the gap in nitrate removal technologies and contribute to sustainable and efficient leachate treatment solutions.

This study addresses a critical gap in the development of fouling-resistant, high-flux nanofiber membranes specifically designed for nitrate removal from landfill leachate. While previous research has explored various membrane technologies, few studies have investigated the use of green tea extract-modified silver nanoparticles to enhance both hydrophilicity and fouling resistance in the context of leachate treatment. Conventional membrane materials often face limitations in terms of flux and fouling, particularly when applied to complex, contaminant-rich leachate. By integrating silver nanoparticles functionalized with green tea extract into polyacrylamide (PAM) nanofibers, this study introduces a novel approach to improve membrane efficiency for the selective removal of nitrates from leachate. This strategy is expected to significantly enhance membrane performance by combining the unique properties of silver nanoparticles and green tea extract, resulting in a more sustainable and effective leachate treatment method. Ultimately, this research aims to bridge the existing gap in nitrate removal technologies and contribute to more efficient waste management and environmental protection practices.

Materials and methods

Leachate sample Preparation and characterization

Leachate samples were collected from a municipal solid waste landfill located in Kermanshah, Iran. To ensure that membrane performance was evaluated under realistic yet manageable conditions, the raw leachate was diluted tenfold using distilled water. The physicochemical characteristics of both raw and diluted leachate samples are summarized in Table 1.

Table 1.

Physicochemical properties of Raw and diluted landfill leachate.

Parameters	Raw	Diluted
Turbidity (NTU)	3500–4000	220–280
NO₃⁻ (mg/L)	1400–1500	140–150
COD (mg/L)	20,000–30,000	1800–2500
pH	8–8.5	7–7.5

Preparation of green tea extract

Fresh green tea leaves were washed with distilled water and dried to obtain the green tea (GT) extract (from a local brand, Ghalam Lahijan Green Tea, Lahijan, Iran). Then, 100 g of dried green tea leaves were transferred to an Erlenmeyer flask, and 1 L of 70% ethanol was added. The mixture was stirred at 60 °C for 4 h using a magnetic stirrer and then allowed to cool to room temperature (25 °C). After 48 h, the mixture was filtered using filter paper to separate the tea residue. The resulting filtrate was subjected to rotary evaporation to reduce its volume to 20 mL at 50 °C. The concentrated extract was then dried in an oven at 50 °C, pulverized with a spatula, and further ground in a mortar⁴².

Synthesis of nanoparticles

Silver nanoparticles were synthesized using green tea extract through a green synthesis method. Initially, 0.9 g of green tea hydroalcoholic extract powder was added to 15 mL of distilled water. The mixture was sonicated at 50 °C for 15 min. The extract was then filtered using Whatman filter paper No. 1 to remove any undissolved residue. Subsequently, 50 mL of AgNO₃ solution was added to 11 mL of the green tea extract at 60 °C. Over time, the color of the solution changed from light yellow to dark brown, indicating the reduction of Ag⁺ ions by the green tea extract—acting as the reducing agent—and the formation of silver nanoparticles. Finally, the silver nanoparticles were collected by ultracentrifugation at 70,000 rpm for 30 min at 4 °C using an SW28 rotor (Beckman XL-90 centrifuge). After ultracentrifugation, the supernatant was gently decanted. The resulting dark brown precipitate was then dried in an oven at 60 °C for 2 h, and the modified silver nanoparticles were collected as a powder^42,47. The concentrations of PAM and Ag@GT used in this study were selected based on the optimized formulation reported by Rajati et al.⁴⁸. No additional optimization of polymer or nanomaterial content was performed.

Fabrication of membranes via electrospinning

Electrospun nanofiber membranes were fabricated at a pilot scale using an electrospinning apparatus (Nano Fanavaran Scale Co., Tehran, Iran). The polymer solution was prepared by dissolving 12 g of polyacrylamide (PAM) in 100 mL of formic acid. The solution was stirred for 24 h to ensure homogeneity. Then, 1 wt% of Ag@GT nanoparticles were added and mixed thoroughly to obtain a uniform nanocomposite solution⁴⁹. The prepared solution was loaded into a 1 mL syringe connected to a positive electrode, while a rotating drum collector placed 15 cm away served as the grounded electrode. The syringe was mounted on a programmable syringe pump to maintain a consistent feed rate. Electrospinning was carried out at a voltage of 25 kV, a flow rate of 0.8 mL/h, and a fixed distance of 15 cm between the needle tip and the collector at room temperature (25 °C). The scanner moved at a speed of 15 cm/h. During the process, the electrically charged jet underwent bending and elongation, resulting in the formation of nanofibers with reduced diameter. Upon solvent evaporation, the nanofibers were collected on the collector surface. After electrospinning, the membranes were left to dry at ambient temperature for 24 h to ensure complete solvent removal. All solutions were freshly prepared and electrospinning was performed immediately after solution preparation under ambient conditions. A stationary (non-moving) collector was used in this study unless otherwise specified^50,51.

Physicochemical characterization techniques

The synthesized nanoparticles and fabricated nanofibrous membranes were analyzed using scanning electron microscopy (SEM) and field emission scanning electron microscopy (FESEM). Fourier-transform infrared (FTIR) spectra were recorded in the wavenumber range of 400 to 14,000 cm⁻¹ using a potassium bromide (KBr) pellet at room temperature (Bruker ALPHA, Germany) to study the chemical composition and structural characteristics of the nanoparticles. The water contact angle (WCA), formed at the intersection of the solid-liquid and liquid–gas interfaces, was used to evaluate the hydrophilicity of the fabricated nanofibrous membranes⁴⁹. The static contact angle between the membrane surface and water at 25 °C was measured using a system equipped with a camera for capturing droplet images and software for determining the contact angle between the droplet and the surface⁵².

Permeability and pure water flux

The nanofibrous membranes prepared via electrospinning were evaluated using permeability tests involving the collection of milk powder solution and pure water flux (PWF) measurement. For this purpose, a 200 mL stainless-steel compartment separator system connected to a nitrogen gas line was used. Before testing, each membrane was immersed in distilled water for 10 min. Subsequently, the membrane was placed inside the closed-end container, which was then filled with distilled water. The pure water flux (Jw,1) (L/m²·h) at 25 °C was calculated using Eq. 3. In Eq. 3, M represents the accumulated permeated water volume (L), A is the effective membrane surface area (m²), and Δt denotes the separation duration (h). The overall porosity of the fabricated membranes was determined using the gravimetric method, as shown in Eq. 4 (27). In Eq. 4, ω₁ and ω₂ represent the weights of the wet (after submersion in ethanol for 24 h) and dry membranes, respectively, in kilograms (kg); ρw is the density of ethanol (kg/m³); A is the effective membrane surface area (m²); and L is the membrane thickness (m).

1

2

Fouling characteristics and flux recovery assessment

Following the PWF test and the calculation of Jw,1 (L/m²·h), the membranes were rapidly immersed in a milk powder solution at a concentration of 1000 mg/L to assess their fouling characteristics. In this study, milk powder solution was employed as the fouling agent to evaluate the anti-fouling properties of the prepared membranes²⁷. Subsequently, the permeate flux (Jp) was measured after exposure to the milk solution. The membrane was then rinsed with distilled water for 10 min, and the PWF was measured again as Jw,2 (L/m²·h). The stability of the electrospun membranes prepared in this study was determined using the flux recovery ratio (FRR) parameter. Generally, a higher FRR indicates better performance of microfiltration membranes. This parameter was calculated using Eq. 3 (42).

3

Fouling resistance

Several ratios have been defined to elucidate the fouling process and characterize the fouling resistance of unmodified PAM electrospun membranes and PAM membranes modified with nanomaterials. The total fouling ratio (Rt) is defined by Eq. 4. Jp represents the pure water permeation flux before fouling (L/m²·h). This ratio provides insight into the fouling resistance of the membranes and describes the extent to which the permeation flux is reduced due to fouling. By analyzing Rt, we can better understand the fouling mechanisms and evaluate the effectiveness of the membranes in resisting fouling⁴².

4

Reversible and irreversible fouling ratios

To further analyze the fouling behavior of the membranes, the reversible fouling ratio (Rr) and irreversible fouling ratio (Rir) have been defined in Eq. 5 and Eq. 6, respectively. To evaluate the efficiency of the nanomaterial-modified membrane in removing nitrate from leachate, the membrane was tested under varying conditions of flow rate (60, 120, and 180 L/h), pH (4, 6.5, and 9), and pressure (0, 0.75, and 1.5 bar) using the Design of Experiments (DOE) software. This approach helps minimize systematic errors and optimize experiments, reducing their number. Pressure and pH were selected as the main influential variables, as optimizing them can significantly enhance efficiency and reduce costs in the purification process. To achieve this, Response Surface Methodology (RSM) was employed, using the Central Composite Design (CCD) algorithm in DOE software to create an empirical model and conduct statistical analysis. Each experiment was replicated three times to ensure accuracy and validity⁵³.

5

6

Experimental design and data modeling

The performance of the nanomaterial-modified fiber membranes in nitrate removal was evaluated using Design of Experiments (DOE) software. This method minimizes systematic errors by optimizing experiments and reducing their number. Pressure and pH were selected as the main influential independent variables in this study. Optimizing these variables could significantly improve efficiency and reduce costs in the purification process. For this reason, Response Surface Methodology (RSM) was employed, utilizing the Central Composite Design (CCD) algorithm through DOE software to create an empirical model and perform statistical analysis based on the study’s objectives. During statistical analysis and modeling, the independent variables were examined at three levels. Due to the nature of the process and the need for replication to enhance accuracy and validity, each experiment was repeated three times under constant conditions for statistical analysis and input into the software^53,54.

Statistical analysis

Data analysis was performed using Microsoft Excel (Version 2016) and Design-Expert Software (Version 13, Stat-Ease Inc., Minneapolis, MN, USA). Descriptive results were presented using two-dimensional graphs. The effects of pH, pressure, and flow rate on membrane performance were analyzed using ANOVA in Design-Expert. A three-dimensional response surface plot was used to model the experimental data and visualize the individual and interactive effects of the variables. Statistical significance was evaluated at a 95% confidence level (P ≤ 0.05). All analyses were conducted for both the unmodified PAM membrane and the PAM/Ag@GT modified membrane to directly compare structural and performance differences before and after modification.

Results and discussion

Characteristics of the synthesized nanocomposite

In all characterization and performance evaluations presented in this study, a direct comparison was made between the unmodified PAM membrane and the modified PAM/Ag@GT nanofibrous membrane. This included assessments of morphology, surface hydrophilicity (contact angle), porosity, pure water flux, nitrate rejection efficiency, and anti-fouling properties. Although the unmodified membrane was labeled simply as “PAM” in the figures, it refers to the control membrane without nanoparticle incorporation. This consistent comparison allows for a clear interpretation of the impact of Ag@GT modification on membrane performance. Figure 1 presents scanning electron microscopy (SEM) images illustrating the topography, morphology, and composition of green tea extract-modified silver nitrate nanoparticles (Ag@GT) at scales of 1 μm and 200 nm. The fine grains observed in the Ag@GT structure indicate the presence of silver metal, while the larger grains represent aggregated Ag@GT particles formed due to the adherence of multiple nanocomposites. The inclusion of GT in the Ag@GT structure results in nanoparticles that are smooth, uniformly distributed, and freely assembled. The particle size distribution is also shown in Fig. 1. The FTIR spectra in Fig. 1 illustrate the qualitative and quantitative characteristics of the Ag@GT nanocomposite. The results confirm the effective incorporation of green tea nanoparticles during the synthesis of the nanocomposite. As shown in Fig. 1, the presence of silver nanoparticles (Ag) facilitates the adhesion of smaller particles, resulting in the formation of larger aggregates. This process contributes to the development of structures with specific properties, such as increased surface area and enhanced physical characteristics of the nanocomposite. The functional groups of these nanoparticles are identifiable in the FTIR spectrum. According to the spectrum, two prominent peaks at 3090 cm⁻¹ and 3349 cm⁻¹ correspond to -OH stretching bonds. These bonds are directly associated with the structure of the nanomaterials and indicate the presence of hydrogen bonding in the nanoparticles. Stronger -OH bonds suggest a more uniform nanoparticle structure, which significantly influences the overall properties of the nanocomposite. The peaks observed at 1068 cm⁻¹ and 1164 cm⁻¹ correspond to the symmetric and asymmetric vibrations of OH-OH bonds, indicating the presence of hydroxyl groups in the nanoparticle structure. Additionally, the peaks at 597 cm⁻¹ and 563 cm⁻¹ are associated with the stretching vibrations of Ag-O bonds, reflecting the presence of silver nanoparticles and their interaction with functional groups. These bonds are particularly important in nanocomposite synthesis, as they can influence the physical and chemical properties of the nanoparticles. The peak observed at 1392 cm⁻¹ corresponds to nitrate groups present in the nanoparticle structure and the stretching vibrations of these groups. This indicates that silver nitrate played a role as a precursor in the synthesis of silver nanoparticles. Additionally, the two peaks observed at 1431 cm⁻¹ and 1545 cm⁻¹ are attributed to carboxylate functional groups, which may indicate the presence of carboxyl (-COOH) groups on the nanoparticle surface and their interaction with other functional groups. These groups can significantly influence the biological and chemical properties of the nanocomposites, such as antibacterial activity and physical behavior. Finally, the absorption frequency at 1609 cm⁻¹ corresponds to the functional groups present in the green tea extract used in the synthesis of the nanoparticles. This feature suggests that the bioactive compounds in the green tea extract may play an essential role in stabilizing the silver nanoparticles and enhancing the properties of the nanocomposites. These results are consistent with FTIR spectra reported for nanoparticles synthesized with plant extracts, including green tea, as discussed in various studies⁵⁵.

Fig. 1.

SEM images and FTIR spectra of Ag@GT nanocomposite: (a, b) Surface morphology at different magnifications; (c) FTIR spectrum of Ag nanoparticles; (d) FTIR spectrum of Ag@GT nanocomposite.

FESEM analysis of nanofiber membranes

To determine the morphology, shape, and size of nanoparticles in polyacrylamide (PAM) nanofiber membranes and PAM-based nanocomposite membranes (PAM/Ag@GT), FESEM analysis was performed. Figure 2 presents the FESEM images for PAM (a) and PAM/Ag@GT (b). Previous studies have also investigated the impact of nanostructures on the morphology of nanofibers. For instance, Hota et al. reported that adding boehmite nanostructures to nylon fibers increased the fiber diameter from 300 to 400 nm to 400–850 nm⁵⁶. In another study, Yom et al. demonstrated that the addition of modified nanostructures, even at high concentrations (5% w/w), had no significant effect on the morphology of nylon fibers⁵⁷. Similar to the present study, Kim et al. showed that increasing the concentration of tetraethyl orthosilicate (TEOS) in polyvinylidene fluoride-silica composite membranes resulted in a reduction in the average fiber diameter. This reduction was attributed to changes in solution viscosity and the distribution of surface forces⁵⁸. Additionally, the results of the study by Ahmadi Bonakdar et al. indicated that adding nanoparticles to electrospinning solutions reduced fiber diameter by adjusting the viscosity and improving solution uniformity, which ultimately resulted in smaller fibers with enhanced surface properties⁵⁹. The results of the present study indicate that the addition of Ag@GT led to a decrease in solution viscosity and better control over the electrospinning process, which resulted in reduced fiber diameter and improved surface quality of the nanofibers. Although this observation is consistent with the hypothesis that nanoparticle incorporation reduces the viscosity of the spinning solution, no direct viscosity measurements were performed in this study. The conclusion was drawn based on morphological evidence and previous literature reports. In addition to membrane surface modification, the development of cost-effective, high-flux, and fouling-resistant membranes is essential for achieving durable membrane bioreactor processes⁶⁰. Fouling-resistant membranes should feature a narrow and uniform pore size distribution, as well as high porosity. For this reason, nanofiber-based membranes, which possess exceptionally narrow and uniform pore sizes, are the best option for developing membranes with a consistent and narrow pore distribution⁶¹. Compared to traditional phase separation methods for membrane fabrication, electrospinning produces membranes with uniform pore sizes and high porosity (> 90%)⁶⁰.

Fig. 2.

FESEM analysis and fiber diameter distribution of nanofiber membrane: (a) Unmodified PAM membrane; (b) PAM/Ag@GT membrane; (c) Average fiber diameter comparison; (d) Fiber diameter distribution histogram.

Water contact angle

In Fig. 3, the water contact angle and porosity of the PAM/Ag@GT nanocomposite membrane are shown. The results indicate that modification of the membrane with Ag@GT led to a reduction in the water contact angle by 35.3°, significantly enhancing the hydrophilicity of the membrane. This improvement in hydrophilicity is a notable change for the membrane. Additionally, the results showed that the porosity of the unmodified and modified membranes was 26.2% and 58.6%, respectively. This increase in porosity (32.4%) resulted in better membrane performance in separation processes, associated with increased permeability and reduced fouling. Similar studies have also demonstrated that adding metallic nanoparticles, such as Ag or Ag/Pd, increases the hydrophilicity and porosity of membranes, leading to enhanced water permeability and reduced fouling on the membrane surface⁶². This is attributed to the formation of polar groups, such as hydroxyl and carboxyl groups, on the membrane surface, which interact through hydrogen bonding with water molecules, thereby improving the membrane’s hydrophilicity⁴⁶. Fouling in hydrophobic membranes occurs more frequently than in hydrophilic membranes due to hydrophobic interactions between membrane components and pollutants. Therefore, significant attention has been given to reducing fouling by modifying membranes to make them relatively hydrophilic⁶³.It is worth mentioning that polyacrylamide membranes prepared by electrospinning are more hydrophobic than those produced by phase separation, which is related to their surface roughness, hardness, and chemical structure^64,65. As shown in Fig. 3, the modification of polyacrylamide nanofiber membranes with Ag@GT has improved the hydrophilicity of the membrane. The presence of polar groups and hydrogen bonding interactions between hydroxyl groups on the surface of nanomaterials or carboxyl groups present in the green tea extract structure ensures better performance of the membrane in separation processes⁴⁶.

Fig. 3.

Water contact angle (WCA) and porosity of PAM and PAM/Ag@GT nanocomposite membranes.

Porosity and turbidity

Figure 4 presents the turbidity and nitrate removal results from landfill leachate before and after filtration through nanofiber membranes. The turbidity of the effluent filtered by the unmodified PAM membrane was measured at 32.65 NTU, whereas the PAM/Ag@GT nanocomposite membrane reduced it significantly to 3 NTU. Similarly, nitrate removal efficiency improved from 32% in the unmodified membrane to 73.25% after modification with Ag@GT. This substantial performance improvement can be attributed to several synergistic effects introduced by the incorporation of green tea extract-synthesized AgNPs. First, the PAM/Ag@GT membrane exhibits higher porosity and finer fiber morphology, enhancing the physical entrapment of suspended particles and colloidal matter, which directly contributes to turbidity reduction. Second, the presence of functional groups such as hydroxyl and carboxyl from green tea extract increases the surface hydrophilicity of the membrane, leading to better wettability and stronger interactions with aqueous contaminants. In terms of nitrate removal, the improved efficiency is likely due to a combination of electrostatic attraction between negatively charged membrane surfaces and nitrate ions, as well as enhanced surface area for adsorption. The nanocomposite structure facilitates more effective mass transfer and contact between nitrate molecules and reactive membrane sites. These characteristics collectively demonstrate the superior filtration capability of the PAM/Ag@GT membrane, especially in treating complex leachate matrices⁶⁶.

Fig. 4.

Turbidity and nitrate concentration in leachate before and after filtration by PAM and PAM/Ag@GT membranes (a) and nitrate removal mechanism (b).

The separation mechanism of the PAM/Ag@GT membrane is primarily governed by a combination of size exclusion and electrostatic interactions. Due to the nanoscale pore structure and enhanced surface hydrophilicity provided by electrospinning and Ag@GT modification, water molecules can easily permeate the membrane, while larger contaminants and nitrate ions are selectively retained or repelled. The reduction in water contact angle following Ag@GT incorporation reflects increased hydrophilicity, which not only promotes higher water flux and resistance to fouling but also enhances surface interaction with aqueous ions. This facilitates the electrostatic repulsion of negatively charged nitrate ions, improving selectivity. The surface functional groups introduced by green tea-synthesized silver nanoparticles contribute to a more negative surface charge, which, according to zeta potential analysis (–30 mV, Fig. S1), enhances the repulsion of nitrate ions via Donnan exclusion effects. This negative charge is likely transferred to the membrane structure, supporting selective nitrate separation. Additionally, the presence of a highly porous matrix and increased surface energy reduce the likelihood of fouling and further improve membrane performance in complex matrices such as landfill leachate⁶⁷. The proposed mechanism for nitrate removal using the PAM/Ag@GT membrane is illustrated in Fig. 4(b). The treated leachate in this study is intended for non-potable reuse applications, particularly for the irrigation of non-edible plants and urban green spaces. The final nitrate concentration after treatment (~ 40 mg/L) aligns with acceptable ranges reported by FAO and WHO for restricted irrigation purposes. However, since the evaluation was focused on nitrate and turbidity removal, a full water quality assessment including microbial load, organic content (COD/BOD), and potential toxic residues was not conducted. These additional parameters should be investigated before broader reuse applications are considered.

Pure water flux and fouling resistance properties of membranes

In Fig. 5, the comparison of pure water flux across PAM and PAM/Ag@GT nanocomposite membranes is presented. The results indicate that the pure water flux (Jw,1) of PAM/Ag@GT nanocomposite is higher than that of PAM, which can be attributed to the greater porosity and hydrophilicity of PAM/Ag@GT compared to PAM. In the PAM/Ag@GT membranes, the highest pure water flux was observed in the membrane containing 0.1% w/w nanomaterials.

Fig. 5.

The pure water flux (Jw,1) of the PAM and PAM/Ag@GT nanocomposite membranes.

Membrane properties such as pore size, porosity, surface charge, strength, hydrophilicity, and hydrophobicity are among the key factors influencing performance and, particularly, fouling in membrane bioreactors⁶⁸. The pore size distribution is one of the most significant parameters impacting membrane performance. A narrow and small pore size distribution is preferred for fouling control in both traditional membrane separation processes and membrane bioreactors. Membranes made from different materials typically show varying fouling characteristics due to differences in pore size, morphology, and hydrophobicity. Additionally, membrane permeability increases with higher porosity and increased hydrophilicity⁶⁹.

As shown in Fig. 6, the pure water flux (Jw,1) of PAM/Ag@GT is higher than that of PAM, which is due to the higher porosity and hydrophilicity of PAM/Ag@GT compared to PAM. In PAM/Ag@GT, the highest pure water flux corresponds to the membrane containing 0.1% w/w nanomaterials. Figure 6 shows the permeate flux (Jp), as well as the pure water flux before and after pollutant treatment (Jw,1 and Jw,2). Upon replacing leachate with water in the filtration chamber, the flux decreases, indicating membrane fouling. However, during leachate treatment, this flux remains almost constant. The pollutant flux through the unmodified polyacrylamide membrane was around 1300 L per square meter per hour, which significantly increased to 1850 L per square meter per hour with the addition of nanomaterials. This increase is attributed to the enhanced porosity and hydrophilicity of the modified membranes due to the incorporation of nanomaterials. Overall, the increase in porosity implies more free space within the membrane, leading to an increase in permeate flux. Additionally, on a more hydrophilic surface, the formation of a hydrophilic layer due to water molecule absorption delays contact between the surface and fouling agents, thereby reducing fouling.

Fig. 6.

Fouling resistance parameters of PAM and PAM/Ag@GT membranes, including flux recovery ratio (FRR), reversible resistance (Rr), and irreversible resistance (Rir).

Figure 6 illustrates the anti-fouling performance of the fabricated membranes by evaluating the flux recovery ratio (FRR), reversible fouling (Rr), irreversible fouling (Rir), and filtration resistance parameters, including membrane resistance (Rm), pore resistance (Rf), cake resistance (Rc), and total resistance (Rt). The unmodified PAM membrane exhibited an FRR of 68%, which increased to 84.45% in the PAM/Ag@GT nanocomposite membrane, indicating significantly enhanced fouling resistance⁷⁰. This enhancement is mainly attributed to the increased hydrophilicity and porosity of the modified membrane, as confirmed by WCA and porosity measurements⁷¹. A hydrophilic surface repels hydrophobic foulants, reducing their adhesion, while a more porous structure improves water flow and reduces concentration polarization. Moreover, the smoother fiber morphology in PAM/Ag@GT, as observed in FESEM images, limits foulant entrapment on the surface and facilitates easier cleaning⁷². These structural improvements reduce foulant accumulation and promote backwashing efficiency, directly explaining the higher FRR values. In terms of fouling behavior, reversible fouling which results from weakly attached foulants is dominant in the modified membrane, allowing for efficient removal through hydraulic flushing. The PAM/Ag@GT membrane exhibited a higher Rr and significantly lower Rir (20.38%), indicating that most of the fouling is easily removable without chemical treatment. This reduction in irreversible fouling is particularly important in practical applications, as it extends membrane life and reduces the need for frequent chemical cleaning. Overall, the use of Ag@GT improved the membrane’s surface chemistry and morphology, which together enhanced the anti-fouling performance⁶⁶. These results confirm that the nanocomposite membrane is more suitable for long-term application in treating landfill leachate with reduced maintenance and operational costs.

Figure 7 illustrates the reusability of PAM and PAM/Ag@GT nanocomposite membranes. The results demonstrate that the incorporation of Ag@GT nanoparticles into the polyacrylamide membrane significantly improves resistance to fouling. The irreversible resistance (Rir), which indicates permanent and irreparable membrane fouling, was found to be high at 27.71% for the unmodified polyacrylamide membrane (PAM). However, by using the Ag@GT nanoparticle-modified membrane (PAM/Ag@GT), this value was significantly reduced to 20.38%, indicating a remarkable improvement in fouling resistance. To assess the efficiency and reusability of the membranes, 20 consecutive filtration runs were performed using contaminated leachate. This test specifically focused on membrane performance under practical conditions and their repeatability, evaluating the durability and persistence of their fouling resistance properties. The results showed that the PAM/Ag@GT membrane maintained stable and superior performance throughout the cycle, demonstrating higher resistance to fouling compared to the unmodified membrane. Moreover, reversible fouling played a dominant role in the overall fouling of the modified membranes. After simple washing with water, these membranes were able to maintain high recovery rates. This was not observed in the unmodified membrane, indicating that effective membrane modification enhances efficiency and performance.

Fig. 7.

Reusability performance of PAM and PAM/Ag@GT membranes over multiple filtration cycles.

Mechanical strength

Figure 8 presents a comparison of the mechanical strength of PAM and PAM/Ag@GT membranes. The results show that the tensile strength of the unmodified membrane (PAM) is 4.95 MPa, with a tensile elongation of 2.94 mm. In contrast, the Ag@GT-modified membrane (PAM/Ag@GT) demonstrates better performance, with its tensile strength increasing to 11.23 MPa and tensile elongation reaching 4.31 mm. These results indicate a significant improvement in the mechanical properties of the membrane due to modification with Ag@GT nanoparticles. The increase in tensile strength and elongation in the modified membrane could be attributed to the uniform distribution of nanoparticles in the membrane matrix, which reinforces its structure and enhances resistance to mechanical stress.

Fig. 8.

Mechanical resistance (Tensile strength and elongation) of the PAM and PAM/Ag@GT nanocomposite membrane.

Figure 9 presents three-dimensional (3D) response surface plots illustrating the individual and interactive effects of operating parameters pressure, pH, and flow rate on PWF, FRR, and nitrate removal efficiency for the PAM/Ag@GT membrane. In Fig. 9 (a), PWF is shown to increase with rising pressure, pH, and flow rate; however, pressure exerts the most significant effect. Higher pressure increases the driving force for permeation, enhancing water transport through the membrane pores, which is consistent with previous membrane performance studies⁷³. At higher flow rates, shear stress at the membrane surface may reduce concentration polarization, contributing to improved flux, while a more neutral pH optimizes membrane–water interactions. In Fig. 9 (b), FRR initially improves with moderate increases in pressure but begins to decline beyond a certain threshold. Excessive pressure may compress the fouling layer against the membrane surface, making it harder to remove during cleaning and thereby reducing FRR⁷⁴. The observed FRR values range from 71 to 89%, indicating good but pressure-sensitive anti-fouling behavior. Figure 9 (c) highlights the nitrate removal response, which improves at neutral pH⁷ and moderate pressure. At this pH, nitrate ions remain in a stable ionic form, and the membrane’s negatively charged surface—enhanced by green tea-derived functional groups facilitates electrostatic attraction and adsorption⁷⁵. The role of surface chemistry is especially important here, as the functional groups introduced via Ag@GT contribute to both hydrophilicity and pollutant affinity. Figure 9 (d) integrates the three responses and identifies the optimal operating region (highlighted in yellow), where high PWF, high FRR, and maximum nitrate removal coincide. The optimal conditions were determined to be a pressure of 0.2 bar, pH 7, and flow rate of 120 L/h. These settings balance hydraulic driving force, membrane integrity, and fouling resistance, providing an efficient operational window for practical applications.

Fig. 9.

Three-dimensional response surface plots showing the effect of operational parameters (pressure, pH, flow rate) on: (a) Pure Water Flux (PWF), (b) Flux Recovery Ratio (FRR), (c) Nitrate removal efficiency, and (d) Optimized performance region in the PAM/Ag@GT nanocomposite membrane.

In Table 2, the confirmatory tests for nitrate removal, PWF (kg/m²·h), and FRR under optimal conditions (pH = 7, pressure = 0.2 bar, and flow rate = 120 L/h) are presented. The experimental results indicate that, with an error of less than 2%, there is good agreement with the central composite design optimization.

Table 2.

Confirmatory tests for optimal condition.

Experimental Conditions		Responses
		NO₃⁻ Removal (%)	PWF (kg/m2.h)	FRR (%)
pH = 7, P = 0.2 bar, Q = 120 L/h	Experimental Values	77.80	2002	83.20
	Model Values	78.35	1979	83.77
	(SD)	0.389	16.26	0.403

In Table 3, the confirmation tests under optimal conditions (pH = 7.0, P = 0.2 bar, and Q = 120 L/h) for both modified and unmodified membranes are shown. The results indicate that the nitrate removal percentage in the nanomaterial-modified membrane (80.77%) is higher than that in the unmodified membrane.

Table 3.

Confirmation tests under optimal conditions.

	Nitrate Removal (%)	PWF (kg/m².h)	FRR (%)
PAM	30.45	1478	66.57
PAM/Ag@GT	77.80	2002	83.20

Limitations and future work

In the present study, the concentrations of PAM and Ag@GT were not independently optimized. Instead, we adopted the formulation reported by Rajati et al.⁴⁸, in which the selected concentrations exhibited favorable structural and functional characteristics. This approach enabled us to focus on evaluating the membrane’s performance in nitrate removal from real landfill leachate, which was the primary objective of this research. To assess the validity of the adopted composition, we provided experimental data on pure water flux, nitrate rejection efficiency, porosity, water contact angle, and anti-fouling behavior. These results support the suitability of the selected membrane formulation for practical applications. Nevertheless, future work could involve systematic optimization of polymer and nanomaterial concentrations using statistical tools such as response surface methodology (RSM) to further improve membrane performance under varied operating conditions. Furthermore, the pore size and molecular weight cut-off (MWCO) of the membrane were not directly measured. Instead, FESEM imaging was used to qualitatively assess the nanofibrous structure and pore morphology, which appeared consistent with the microfiltration range. Direct measurement of MWCO would allow for a more accurate correlation between membrane structure and separation performance. In addition, while zeta potential analysis was conducted for the Ag@GT nanoparticles (–30 mV), the surface charge of the final membrane was not evaluated. Since surface charge plays a critical role in electrostatic separation mechanisms such as Donnan exclusion, future studies should aim to directly measure the membrane zeta potential under relevant pH and ionic conditions. Moreover, although fiber morphology changes were observed and linked to possible changes in solution viscosity due to nanoparticle addition, no direct viscosity measurements were performed in this study. This parameter will be considered in future work to enable more accurate correlations between electrospinning solution properties and the resulting fiber structures.

Conclusion

In this study, novel nanofibrous membranes with anti-fouling properties and high flux were designed and evaluated for nitrate removal from leachate at the laboratory scale. The effect of membrane modification with Ag@GT nanomaterials on membrane performance was investigated. The results of the water contact angle tests showed that incorporating nanomaterials increased the hydrophilicity of the membrane surface. This improvement was attributed to the presence of hydrophilic groups in the green tea extract, which can form bonds with water molecules. Furthermore, porosity measurements indicated that the addition of nanomaterials significantly increased the porosity of the fabricated membranes. The performance evaluation revealed that structural modifications led to an increase in both pure water flux (Jw) and fouling flux (Jp). The anti-fouling properties of the modified membranes were assessed by measuring the flux recovery ratio (FRR) and fouling resistances. The results showed that the FRR of the unmodified polyacrylamide membrane was around 68.00%, which increased to 84.45% in the modified membrane. This trend is consistent with the results of the water contact angle and porosity tests, as the incorporation of nanomaterials enhanced hydrophilicity and reduced surface fouling. Additionally, fouling resistance measurements revealed a significant decrease in irreversible fouling resistance (Rir) in the modified nanofibrous membranes, indicating improved performance in minimizing permanent fouling. Overall, the results of this study demonstrate that modifying polyacrylamide nanofibrous membranes with silver nanomaterials and green tea extract enhances anti-fouling properties, hydrophilicity, porosity, and water flux while achieving excellent performance in nitrate removal. These findings highlight the potential for developing advanced membranes for water treatment and other separation processes, where improved fouling resistance and high permeability are essential.

Electronic supplementary material

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Author contributions

All authors contributed to the study’s conception and design. Material preparation, data collection, and analysis were performed by Maryam Moghadam, Habibeh Nasab, Elham Arkan, Arash Dalvand, and Ali Asghar Ebrahimi. The first draft of the manuscript was written by Maryam Moghadam. and Habibeh Nasab. and all authors commented on the previous versions of the manuscript. All authors read and approved the final manuscript. Conceptualization was performed by Elham Arkan, Arash Dalvand, and Ali Asghar Ebrahimi; Methodology by Elham Arkan, Arash Dalvand, and Ali Asghar Ebrahimi; Formal analysis and investigation by Maryam Moghadam; Writing-original draft preparation by Maryam Moghadam, Habibeh Nasab; Writing-review and editing by Maryam Moghadam, Habibeh Nasab, Elham Arkan, Arash Dalvand, and Ali Asghar Ebrahimi.

Funding

No funding was received to assist with the preparation of this manuscript.

Data availability

The supporting data are available from the corresponding authors upon reasonable request.

Declarations

Competing interests

The authors declare no competing interests.

Ethical approval

The present study has the code of ethics IR.SSU.SPH.REC.1400.094 of Shahid Sadoughi University of Medical Sciences, Yazd, Iran.