Introduction

Polymer-based membranes offer a promising solution for the removal of organic pollutants from various aqueous solutions, including wastewater. These membranes function by selectively allowing the passage of water molecules while blocking the passage of organic pollutants based on size, charge, and affinity1,2. Through techniques such as microfiltration and nanofiltration, polymer membranes effectively remove micropollutants, such as synthetic dyes and emerging contaminants, contributing to cleaner water sources3. Nanofillers, when incorporated into these membranes, enhance their stability and effectiveness in eliminating organic pollutants. Additionally, nanocomposite polymeric membranes, leveraging nanomaterials, further enhance organic micropollutant (OMP) removal by tailoring membrane properties to match specific OMP characteristics. Overall, polymer-based membranes represent a sustainable and efficient approach to combat organic pollution in water sources4.

Metal–Organic Frameworks (MOFs) have emerged as versatile materials with significant potential in adsorption applications. These porous structures, composed of metal nodes connected by organic ligands, offer high surface areas and tunable properties, making them ideal for capturing and storing gases and molecules5,6. Their ability to undergo structural transitions upon adsorption further enhances their adsorption capacities. NH2-MIL-101(Cr), is a MOF renowned for its exceptional adsorption properties. This MOF offers a dual functionality of efficient adsorption and excellent catalytic performance, making it a powerful tool in various adsorption applications7,8,9. However, MOFs also exhibit limitations in adsorption applications. Insolubility, poor processability, and brittleness of pure MOF powders hinder their practical utilization. Moreover, challenges such as the potential for framework collapse under certain conditions, low chemical and thermal stability, and difficulties in maintaining structural integrity during repeated adsorption–desorption cycles pose obstacles in the widespread adoption of MOFs for adsorption purposes10,11,12. These limitations underscore the need for further research and development to address these issues and optimize the performance of MOFs in adsorption applications.

Polyacrylonitrile (PAN) is a widely used polymer in the fabrication of membranes, particularly in the field of water/oil separation and other filtration applications. PAN membranes are popular due to their unique properties, including high surface area and tenability13. These membranes are commonly prepared through techniques like electrospinning, resulting in structures suitable for various filtration purposes. Moreover, a notable advancement involves combining PAN with MOFs to create MOF-PAN composite membranes14,15. MOFs, known for their porous nature, enhance the adsorption and separation capabilities of PAN membranes. This synergy leverages the high surface area of MOFs with the structural integrity of PAN, offering improved performance in areas such as gas separation and water treatment16,17.

Trifluralin, commonly known as Treflan, is a pre-emergence herbicide widely used in agriculture for weed control. It belongs to the dinitroaniline class and is applied before the germination of target weeds, inhibiting their growth. Additionally, concerns exist about the persistence of trifluralin residues in soil, posing risks to subsequent crops and the surrounding environment. Trifluralin may undergo biodegradation, where microorganisms break down the herbicide into simpler and less harmful substances18,19,20.

The process of creating a porous carbon adsorbent starts with synthesizing high internal phase emulsions (HIPEs). These emulsions undergo polymerization, after which the material is subjected to carbonization at elevated temperatures in a nitrogen atmosphere, resulting in the formation of the porous carbon structure21. To augment its adsorption efficacy and recyclability, graphene oxide (GO) and silica nanoparticles were incorporated into the HIPEs. The outcomes revealed the formation of a hyper-crosslinked framework material characterized by a profusion of pores, with an approximate size of 30 μm, thereby ensuring enhanced adsorption and separation capabilities. Remarkably, the adsorption efficiency reached 100%, demonstrating the absorbent’s effectiveness in capturing trifluralin. Furthermore, complete elution of trifluralin from the absorbent was achieved using a mere 2.0 mL of an acetic acid-acetonitrile mixture, underscoring its potential for practical applications in environmental remediation efforts.

Herein, a composite membrane integrating NH2-MIL-101(Cr) and PAN was synthesized meticulously via electrospinning for trifluralin removal from wastewater. NH2-MIL-101(Cr) was chosen due to its functional amino (–NH2) groups, high surface area, and outstanding stability, which are favorable for interactions with polymer matrices such as PAN1,2. The amino groups in NH2-MIL-101(Cr) can facilitate hydrogen bonding with the nitrile (–CN) groups in PAN. Additionally, π–π interactions between the aromatic rings in the MOF structure and the PAN backbone may contribute to the compatibility and stability of the composite material22,23,24. The interaction between trifluralin and the membrane is facilitated by NH2-MIL-101(Cr) and PAN’s active sites and porous structure, ensuring thorough pollutant entrapment. This study emphasizes the critical role of composite membranes in addressing water contamination and suggests promising prospects for NH2-MIL-101(Cr) and PAN composite membranes in environmental restoration efforts. Fig. S1 possesses the structure of trifluralin.

Experimental section

Materials and methods

Polyacrylonitrile, Cr(NO3)3·9H2O, 2-aminoterephthalic acid, sodium hydroxide (NaOH), and dimethylformamide (DMF) were procured and utilized without additional purification. The characterization of the samples involved a range of techniques. Powder X-ray diffraction (PXRD) analysis was conducted using a Philips X′Pert diffractometer equipped with monochromated Cu–Kα radiation (λ = 1.54056 Å). Field emission-scanning electron microscopy (FE-SEM) imaging was carried out using a TESCAN VEGA II instrument from the Czech Republic. Fourier transform infrared (FT-IR) spectroscopy was performed using a Nicolet 100 spectrometer over the wavelength range of 400–4000 cm−1. Surface area and pore size distribution were determined through the BET multilayer N2 adsorption method employing a Micromeritics ASAP 2020 instrument. The electrospinning of nanofibers was accomplished using the Full Option Lab ES model electrospinning instrument provided by Nanoazma Company.

NH2-MIL-101(Cr) synthesis

The NH2-MIL-101(Cr) was synthesized through a mild solvothermal procedure25. Specifically, a solution comprising 3.2 g of Cr(NO3)3·9H2O and 1.44 g of 2-aminoterephthalic acid was gradually introduced into a 60 mL H2O solution containing 0.8 g NaOH. The resulting mixture, stirred at room temperature for 30 min, was subsequently transferred to a 100 mL Teflon-lined stainless-steel autoclave and held at 150 °C for 12 h. Following natural cooling, the resulting mixture was harvested through centrifugation at 7000 rpm for 5 min. The obtained green powder underwent multiple washes with water, DMF, and methanol sequentially, followed by drying at 100 °C overnight. This process yielded the activated NH2-MIL-101(Cr) compound.

Membrane preparation

1000 mg of PAN was dissolved in 10 mL of DMF with agitation for a duration of 12 h. Subsequently, 100 mg of previously synthesized NH2-MIL-101(Cr) was introduced into the solution and stirred for 24 h. The resulting polymer-to-MOF ratio was 10:1. The electrospinning process was employed to fabricate nanofibers using the Full Option Lab ES electrospinning model from Nanoazma Co., located in Tehran, Iran. During electrospinning, the high voltage power supply’s anode was affixed to the syringe needle tip, while the cathode was connected to a metal collector. The solution was then loaded into a 5 mL plastic syringe fitted with a 23-gauge blunt-tip needle and dispensed at a rate of 0.5 mL/h. All PAN@MIL-NH2 nanofibers were produced using a supplied voltage of 15.5 kV, with the distance from the needle tip to the collector surface set at 13 cm. Subsequently, the final membrane was separated from the collector for application in separation processes.

Adsorption investigation

The stock solutions containing Trifluralin at a concentration of 5, 10, 15 and 20 mg/L were individually prepared. Each solution was then subjected to filtration using a pre-prepared membrane. After a single-pass filtration process lasting 150 s, 100 mL of the solution passed through the membrane. The filtrates obtained were subsequently analyzed for adsorption efficiency using UV–Vis equipment.

Results and discussion

Characterization

In FT-IR spectrum of NH2-MIL-101(Cr), which is provided in Fig. 1a, peaks in the range of 3000–3500 cm⁻1 often indicate stretching vibrations of N–H bonds. In the case of NH2-MIL-101(Cr), prominent peaks observed at 1395 cm−1 (symmetric) and 1620 cm−1 (asymmetric) vibrations signify the existence of carboxylate moieties within the framework structure of the MOF. The primary peak associated with the phenyl group is the C=C aromatic stretching vibration, observed in the range of 1600–1585 cm⁻126,27. The FT-IR peak corresponding to Cr–O bonds typically appears in the region of 450–500 cm⁻1, reflecting the vibrational stretching mode of chromium-oxygen linkages within the MOF framework, indicative of the robust structural integrity of the NH2-MIL-101(Cr) network28. FT-IR spectrum of PAN reveals distinctive features that characterize its molecular structure. One prominent vibration peak at 2242 cm−1 is observed, which can be attributed to the nitrile group (C≡N) present in PAN. This peak signifies the stretching vibration of the carbon–nitrogen triple bond29. The coordination of NH2-MIL-101(Cr) metal sites with the nitrile moieties of PAN can be confirmed by a positive shift in the –C≡N stretching frequency in the FT-IR spectrum, a common property of nitriles coordinated to Lewis-acidic metal centers. This interaction involves nitrogen donating its lone pair to chromium, altering the nitrile’s electronic environment and increasing bond polarity. The shift is a characteristic marker of coordination, as observed for nitriles interacting with Lewis acids, and it provides direct evidence of this interaction in the final PAN@MIL-NH2 composite membrane30. Furthermore, peaks in the region of 1470–1375 cm⁻1 are attributed to bending vibrations of C–C and C–H bonds. In the spectrum of PAN@MIL-NH2, all the major peaks of both components are presented, illustrating the accuracy of composite synthesis. The Cr–O peak observed in the FT-IR spectrum, typically within the range of 450–500 cm⁻1, was broadened in the final membrane due to the coordination of the chromium centers with the nitrile moieties of PAN. Despite this broadening, the Cr–O vibrational signature remains distinguishable, providing evidence of the retained structural framework of NH2-MIL-101(Cr) even after the interaction with PAN31,32.

Fig. 1
figure 1

(a) FT-IR spectra and (b) XRD patterns of NH2-MIL-101(Cr), PAN and PAN@MIL-NH2.

Full size image

The X-ray diffraction (XRD) patterns of our produced NH2-MIL-101(Cr) align well with those documented in the literature33, providing evidence for the successful creation of NH2-MIL-101(Cr). The primary Bragg reflection at 2θ = 10° substantiates the effective synthesis of NH2-MIL-101(Cr), affirming its identity as a MOF structure. The XRD pattern of PAN@MIL-NH2 exhibits slight differences in peak intensity and peak position compared to NH2-MIL-101(Cr). These variations can be attributed to the amorphous nature of the PAN matrix in the composite membrane, which can slightly modify the local crystallographic environment of the embedded MOF. However, it is important to note that the characteristic peaks of NH2-MIL-101(Cr) are preserved without significant changes, confirming the structural integrity of the MOF in the composite material34,35, as can be seen in Fig. 1b.

The BET surface area of NH2-MIL-101(Cr) plays a crucial role in determining its adsorption capabilities. Studies have reported that the BET surface area of NH2-MIL-101(Cr) is approximately 2530 m2/g, highlighting its extensive surface for interactions with other molecules36, while the BET surface area of the pristine PAN was 86.512 m2/g. The high surface area of MIL material is attributed to the unique porous structure of the MOF, providing numerous active sites for adsorption. The amine functional groups on NH2-MIL-101(Cr) further enhance its adsorption properties, making it particularly effective in selective adsorption applications. Introducing PAN nanofibers reduces the BET surface area and pore volume, as a result of low BET surface area of PAN moieties. Herein, BET of PAN@MIL-NH2 was measured as 164.94 m2/g. Its N2 adsorption–desorption isotherm follows type II, as can be seen in Fig. 2. In Type II isotherms, adsorption occurs both on the external surface of the material and within the pores. This suggests the presence of mesopores or macropores within the material structure. The initial rise in the isotherm indicates the adsorption of the gas molecules on the external surface, followed by a relatively flat region representing the formation of a monolayer. Subsequently, there might be a gradual increase in adsorption, indicating multilayer adsorption or capillary condensation within the pores37. Furthermore, the pore size distribution of the membrane was also provided in Fig. 2b. Understanding Type II BET isotherm aids in characterizing materials with porous structures and provides insights into their surface area, pore size distribution, and adsorption capacity. Researchers utilize this information for various applications, including catalyst development, gas storage, and environmental remediation. Table. 1 collected the BET plot information of PAN@MIL-NH2 and PAN nanofibers.

Fig. 2
figure 2

(a) BET and (b) BJH plots of PAN@MIL-NH2.

Full size image
Table 1 BET plot information of PAN@MIL-NH2 and PAN nanofibers.
Full size table

Field emission-scanning electron microscopy (FE-SEM) images (Fig. 3) and energy-dispersive X-ray spectroscopy (EDX) and mapping (Fig. 4) provide detailed insights into the structure and elemental composition of the resultant membranes, PAN@MIL-NH2. The left image illustrates the clean and uniform fibrous structure of PAN@MIL-NH2. The right image with higher magnification highlights the incorporation of NH2-MIL-101(Cr) onto the PAN fibers. This composite exhibits a rougher surface due to the deposition of MOF particles. The 500 nm scale bar demonstrates the nanoscale distribution of MOF particles across the fiber surfaces, confirming effective integration without altering the fibrous morphology of the PAN matrix. These SEM images suggest that NH2-MIL-101(Cr) particles are well-dispersed and immobilized on the PAN fibers, ensuring a high surface area for adsorption applications without missing the adsorption sites (i.e. porosity and accessible surface area). Figure 3c also provides the cross-section SEM of the membrane. EDX mapping complements FE-SEM by identifying the distribution of elements within the membrane, highlighting the presence and spatial arrangement of Cr, N, O and C constituents in the membrane structure, uniformly.

Fig. 3
figure 3

(a and b) FE-SEM images of PAN@MIL-NH2 membrane with different magnifications, c) cross-section SEM of the membrane.

Full size image
Fig. 4
figure 4

EDX and mapping of PAN@MIL-NH2 membrane.

Full size image

Thermogravimetric analysis (TGA) of PAN@MIL-NH2 membrane provides valuable insights into its thermal stability and decomposition behavior, as presented in Fig. 5. The TGA curve typically exhibits distinct weight loss events corresponding to different decomposition stages of the membrane components. At lower temperatures, i.e. ~ 100–150 °C, the initial weight loss is attributed to the removal of adsorbed water and DMF molecules from the membrane structure. PAN@MIL-NH2 membrane typically undergoes decomposition of the PAN component and organic linkers of the NH2-MIL-101(Cr) at intermediate temperatures, ~ 290–500 °C. This stage is characterized by a significant weight loss as PAN molecules break down into volatile products such as ammonia and hydrogen cyanide. In another word, above 500 °C, the framework starts to decompose through destroying the organic moieties. After that, chromium oxide is formed after 500 °C. Furthermore, the TGA profile of pristine PAN membrane also demonstrated three distinct stages of weight loss, corresponding to different thermal events. The initial stage, occurring below approximately 200 °C, was attributed to the loss of physically adsorbed DMF. The second stage, observed between 250 and 550 °C, was associated with the onset of thermal degradation of the polymer backbone, including cyclization and decomposition of nitrile groups. The final stage, extending beyond 500 °C, indicated the residual mass suggested the formation of a stable carbon residue. The TGA profile of NH2-MIL-101(Cr) was also provided revealing three distinct weight loss stages. The initial weight loss, occurring approximately at 150 °C, was attributed to the removal of water trapped within the porous structure. The second stage, observed between 280 and 600 °C, corresponded to the decomposition of the organic linkers (amino-functionalized terephthalic acid), which forms the framework of NH2-MIL-101(Cr). This degradation is indicative of the breakdown of the coordination bonds between the chromium centers and the organic ligands. The residual mass after 600 °C confirmed the formation of Cr2O3 as a stable decomposition product. By comparison the TGA data, it can be precisely determined that the higher metal moieties in the pristine NH2-MIL-101(Cr) leads to higher thermal stability which is further confirmed by higher mass of residue in the curve of the MOF. Accordingly, the presence of NH2-MIL-101(Cr) can elevate the thermal stability of PAN in the final membrane composite.

Fig. 5
figure 5

TGA analysis of pristine PAN, NH2-MIL-101(Cr) and PAN@MIL-NH2.

Full size image

Adsorption investigation

The adsorption of Trifluralin using PAN@MIL-NH2 was investigated to assess its efficacy as a removal method for this contaminant. The removal efficiency was determined to be 95.08% within 15 min, indicating the effectiveness of the adsorbent in capturing trifluralin from aqueous solutions. The initial concentration of the analyte in the adsorption process significantly affects both the adsorption capacity and the percentage of removal. Generally, as the initial concentration of the analyte increases, the adsorption capacity of the adsorbent also increases. However, higher initial concentrations may lead to a decrease in the percentage of removal. This is attributed to the saturation of active sites on the adsorbent surface as the concentration rises, limiting further adsorption. In this case, initial concentration of 20 ppm showed the optimum adsorption behavior, and further concentration led to precipitating the trifluralin. Accordingly, we should limit our investigation to 20 ppm. Moreover, the passage of time during the adsorption process also influences the efficiency of adsorption. As time progresses, the amount of adsorption generally increases due to the continued interaction between the adsorbate and adsorbent. However, there may be a point of equilibrium where the rate of adsorption equals the rate of desorption, leading to no further increase in adsorption. The adsorption of trifluralin reached to the equilibrium within 15 min. Understanding these effects is crucial for optimizing adsorption processes for efficient removal of contaminants from solutions. These findings underscore the potential of PAN@MIL-NH2 as a promising adsorbent for the removal of trifluralin from contaminated water sources. Figure 6 provided the adsorption behavior of PAN@MIL-NH2 toward trifluralin.

Fig. 6
figure 6

(a) The removal efficiency of trifluralin with various concentrations within different period of time, (b) Pseudo second order model, which fits with the experimental data, (c) The adsorption capacity of the as-prepared membrane toward trifluralin with various concentrations within different period of time, (d) Freundlich adsorption isotherm, which fits with experimental data.

Full size image

By understanding the specific interactions, the adsorption process can be optimized for efficient removal of trifluralin under various conditions. In the adsorption process of trifluralin by PAN@MIL-NH2, various interactions play crucial roles. Size-selective diffusion facilitates the passage of trifluralin molecules through the membrane based on their size, allowing selective adsorption. Hydrogen bonding between trifluralin and functional groups on the membrane surface enhances adsorption efficiency. The amino (NH2) groups on NH2-MIL-101(Cr) serve as hydrogen bond donors, forming hydrogen bonds with functional groups on trifluralin, such as F and NO2−. Additionally, π–π stacking interactions between aromatic rings of trifluralin and the organic moieties of NH2-MIL-101(Cr) contribute to the adsorption process. Lewis acid–base interactions further facilitate adsorption by forming complexes between Lewis base sites on the membrane, i.e. NH2 functional groups of MIL material and Lewis acid sites on trifluralin molecules, i.e. NO2−. Furthermore, Lewis acid–base interaction between amine groups of the MOF and NO2− can further improve the adsorption process. The hydrophobic nature of both the membrane and trifluralin molecules enhances adsorption efficiency through hydrophobic interactions. The polymer component (PAN) may also contribute to the adsorption process through van der Waals forces or π–π interactions with the aromatic rings of trifluralin. Removal efficiency under specific conditions can be optimized by adjusting parameters such as concentration. For instance, at neutral pH and room temperature, high removal efficiency of trifluralin can be achieved due to favorable interactions between the membrane and trifluralin molecules. In the adsorption process of trifluralin by PAN@MIL-NH2, specific functional groups play crucial roles in interacting with trifluralin molecules38,39. Figure 7 provides the proposed adsorption process scheme.

Fig. 7
figure 7

An illustration of trifluralin adsorption by PAN@MIL-NH2.

Full size image

Adsorption isotherms

In the domain of adsorption isotherms, fundamental frameworks like the Langmuir, Freundlich, and Temkin models serve as crucial tools for elucidating the dynamics of interaction between adsorbate molecules and a solid adsorbent40,41. These models are instrumental in providing insights into the mechanisms governing the adsorption process, thereby facilitating a deeper understanding of the intricate interplay between adsorbate species and the surface of the adsorbent material. The Langmuir adsorption model assumes a monolayer adsorption onto a surface with uniformly distributed sites. It proposes that once an adsorption site is occupied, no further adsorption can occur at that site. The Langmuir equation is described as Eq. (1):

$${q}_{e}=\frac{{q}_{m}{K}_{L}{C}_{e}}{1+{K}_{L}{C}_{e}}$$
(1)

This model’s equation is characterized by qe (mg/g) denoting equilibrium adsorption capacity, qm (mg/g) representing maximum adsorption capacity, and KL (L/mg) symbolizing the model’s constant.

The Freundlich adsorption model is an empirical equation that describes multilayer adsorption onto heterogeneous surfaces. It suggests that the amount of adsorbate adsorbed increases with increasing concentration, and it does not assume a fixed adsorption capacity. The Freundlich equation is given as Eq. (2):

$${\text{log q}}_{{\text{e}}} = {\text{ log K}}_{{\text{F}}} + {\text{log C}}_{{\text{e}}}$$
(2)

where KF (mg/g)/(mg/L) and n denote the Freundlich constant, where the value of n reflects favorable adsorption, typically falling within the range of 1 to 10.

The Temkin adsorption model considers the effect of adsorbent-adsorbate interactions on the adsorption process. It assumes a linear decrease in the heat of adsorption with coverage due to these interactions. The Temkin equation is expressed as Eq. (3):

$$qe ={e}^{-KtCe}\left.\left(\frac{RT}{bT}\right.\right)$$
(3)

Herein, R signifies the universal gas constant, 8.314 J/mol·K, T (K) represents temperature, bT (J/mol) denotes the Temkin adsorption heat constant, and KT refers to the Temkin isotherm equilibrium binding constant in units of L/g.

These models are widely used in understanding and predicting adsorption behavior in various systems and can provide insights into adsorbate-adsorbent interactions and surface properties.

The Freundlich adsorption model describes the adsorption process on heterogeneous surfaces, which is suitable for systems like PAN@MIL-NH2. Significantly, a high R-squared value of 0.99379 was achieved for trifluralin removal, highlighting the effectiveness of the model in explaining the adsorption processes, as demonstrated in Fig. 6b. Adsorption of trifluralin onto PAN@MIL-NH2 involves the attachment of trifluralin molecules onto the surface of the adsorbent. The process is typically multilayered, meaning multiple layers of trifluralin molecules can be adsorbed onto the surface. On the other words, trifluralin molecules also interact with each other while adsorption process proceeds. Freundlich model suggests that the adsorption capacity increases continuously with increasing concentration, reflecting the non-uniform distribution of adsorption sites and varied adsorption energies on the heterogeneous surface of PAN@MIL-NH2. Overall, the adsorption process involves a combination of physical and chemical interactions between trifluralin molecules, PAN, and NH2-MIL-101(Cr), leading to the successful removal of trifluralin from the solution42. Table 2 presents the coefficient of determination (R2) values associated with the Freundlich, Langmuir, and Temkin models applied to trifluralin.

Table 2 Correlation coefficient values for different models of trifluralin adsorption by PAN@MIL-NH2 membrane.
Full size table

Adsorption kinetics

Adsorption kinetics involves the study of the rate at which adsorption processes occur. Different kinetic models, such as the pseudo-first order and pseudo-second order models, are employed to describe and predict adsorption behaviors43. Pseudo-First Order model assumes that the rate of adsorption is directly proportional to the difference between the maximum adsorption capacity and the amount of adsorbate already adsorbed on the surface. Mathematically, it is characterized by the Eq. (4):

$${q}_{t} = {q}_{e}\left.\left(1- {e}^{-i{k}_{l}t}\right.\right)$$
(4)

In this context, qe symbolizes the equilibrium adsorption capacity, qt stands for the adsorption capacity at time t, and k1 represents the rate constant in pseudo-first order kinetics. It is important to recognize that although this model provides valuable insights, its applicability may be limited in accurately representing certain adsorption systems.

Pseudo-Second Order suggests that the rate of adsorption is proportional to the square of the difference between the equilibrium adsorption capacity and the adsorption capacity at time t. Mathematically, it can be expressed as Eq. (5):

$${q}_{t}=\frac{{k}_{2}{q}_{e}^{2}t}{1+{k}_{2}{q}_{e}t}$$
(5)

Understanding adsorption kinetics is crucial for improving a range of processes like wastewater treatment, gas purification, and separation methods. In systems with significant chemical complexities, the rate constant k2 in pseudo-second order kinetics plays a critical role as a key parameter. The pseudo-second order model often provides a more precise depiction of experimental outcomes, particularly in intricate systems with substantial chemical interactions. The calculated R2 correlation coefficient for the pseudo-second-order model in the case of trifluralin was determined to be 0.9979, indicating a robust adherence of the analyte to the pseudo-second-order model, as approved by Fig. 6d. The pseudo-second order model is more widely used and often provides a better fit for experimental data compared to the pseudo-first order model, especially for chemisorption processes. Both models are used to determine the kinetics of adsorption processes and can provide insights into the mechanisms and rates of adsorption onto surfaces. This adsorption process following the pseudo-second order model, which involves several key aspects. The pseudo-second order model suggests a chemisorption mechanism, where the adsorbate molecules (trifluralin) form strong chemical bonds with the adsorbent surface (PAN@MIL-NH2). Chemisorption involves the sharing or transfer of electrons between the adsorbate and the adsorbent, leading to the formation of stable chemical bonds. The rate of adsorption in the pseudo-second order model is proportional to the square of the difference between the equilibrium adsorption capacity and the adsorption capacity at any given time. This suggests that the adsorption rate is influenced by the availability of active sites on the adsorbent surface and the concentration of the adsorbate in the solution. Trifluralin molecules interact with the functional groups (NH2) present on the surface of PAN@MIL-NH2 through chemical bonding, such as hydrogen bonding or dipole–dipole interactions. The presence of these functional groups enhances the affinity of the adsorbent towards trifluralin, facilitating its adsorption onto the surface of the material44. Table 2 collects the R2 values of Pseudo first order and Pseudo Second order models for the analyte.

Stability and reusability

In the regeneration cycles, the used membrane was immersed in fresh deionized water for 30 min before being reused for the subsequent adsorption cycle. This duration was sufficient for the adsorbed pharmaceuticals to release effectively from the membrane’s cavities, as the analytes are water-soluble. Furthermore, the membrane demonstrated excellent stability, maintaining its structural integrity throughout the washing process, which ensures its reusability over multiple cycles. To assess the endurance of the membrane through repeated use, an XRD analysis was conducted on a reused membrane after five cycles. As depicted in Fig. 8, no significant degradation was observed during the adsorption process, indicating the effective preservation of MOF nanoparticles’ stability within PAN nanofibers. Under the optimal condition of 20 ppm initial concentration, minimal reduction in adsorption performance was noted across the five regeneration cycles, showcasing PAN@MIL-NH2 as a promising choice for adsorption applications. Furthermore, the contact angle measurement for the PAN@MIL-NH2 composite membrane was also provided in Fig. 8c for further clarification of the membrane stability toward water. Accordingly, the PAN@MIL-NH2 membrane remains stable in aqueous environments, showcasing its durability and suitability for water-related applications due to the contact angle of ~ 80°. This balance between hydrophilicity and stability highlights the effectiveness of the MOF incorporation in tailoring the membrane’s surface properties. Membrane stability is crucial in various fields, particularly in adsorption processes like wastewater treatment and gas separation, where consistent and reliable performance over multiple cycles is essential for operational efficiency and environmental protection.

Fig. 8
figure 8

(a) XRD patterns of as-prepared membrane and after five cycles, (b) Five regeneration runs of trifluralin removal, (c) Contact angle of PAN@MIL-NH2.

Full size image

Real sample tests

The Table 3 presents the removal efficiency of trifluralin using the NH2-MIL-101(Cr)-based membrane in three different water matrices. The data highlights the variation in adsorption performance based on the type of sample and the initial concentration of trifluralin. Industrial effluent contains a higher concentration of trifluralin, coupled with a likely presence of other industrial pollutants, leading to a greater demand on the membrane’s active adsorption sites. The relatively high pollutant load results in quicker saturation of the active sites, which reduces the adsorption capacity and lowers the removal efficiency to 89.2%. This suggests that the membrane performs efficiently but encounters limitations in matrices with complex contaminant profiles and higher pollutant levels. Urban water contains a minimal concentration of trifluralin and minimal competing pollutants. This allows the membrane to fully utilize its active sites for trifluralin adsorption, leading to a significantly higher removal efficiency of 97.2%. The lower contaminant burden in urban water enables superior performance, as the membrane is not overwhelmed by excessive pollutant interactions or saturation. Agriculture river water exhibits the highest initial trifluralin concentration (15.00 ppm), along with other pesticide and herbicide pollutants. This combination causes significant competition for the active adsorption sites on the membrane, resulting in a lower removal efficiency of 81.6%. The high concentration and complex mixture of contaminants in agriculture effluent challenge the adsorption capacity of the membrane, reducing its overall effectiveness in this matrix. Table 3 collects the real sample test results.

Table 3 Real sample tests.
Full size table

Conclusion

The present investigation introduces a novel composite membrane, composed of NH2-MIL-101(Cr) and polyacrylonitrile, fabricated through meticulous synthesis and electrospinning techniques, resulting in a coherent structure aimed at the efficient extraction of trifluralin from polluted wastewater. The noteworthy removal efficiency of 95.08% achieved within a brief 15-min interval accentuates the membrane’s remarkable adsorption capabilities and swift pollutant sequestration. The intricate interplay between trifluralin molecules and the membrane surface is facilitated by the elaborate network of active sites provided by NH2-MIL-101(Cr) and the porous morphology of polyacrylonitrile, ensuring comprehensive pollutant entrapment. Moreover, the synergistic synergy between NH2-MIL-101(Cr) and polyacrylonitrile not only accelerates adsorption kinetics but also conforms to both Freundlich isotherm and pseudo-second-order kinetic models, elucidating the mechanism of multilayer chemisorption and affirming the membrane’s robust performance under diverse operational conditions. Significantly, the membrane demonstrates notable durability and sustained efficacy over multiple cycles, underscoring its potential for enduring and eco-friendly wastewater treatment solutions. This investigation underscores the pivotal role of composite membranes in effectively addressing water contamination issues and highlights the promising avenues for NH2-MIL-101(Cr) and polyacrylonitrile composite membranes in broad-scale environmental restoration endeavors.