Introduction

Water is an indispensable resource for all living organisms, and its contamination by pollutants poses significant environmental and public health challenges. Among these pollutants, synthetic dyes are of particular concern due to their toxicity, persistence, and ubiquity in industrial effluents. These dyes are extensively utilized in various industries, including textiles, leather, paper, and food processing. Their complex aromatic structures are designed to resist degradation under environmental conditions such as light, temperature, and microbial activity, leading to their persistence in aquatic environments. The discharge of dye-laden wastewater into water bodies not only diminishes the aesthetic quality of these water sources but also poses severe health risks to aquatic life and human populations [1].

Methylene blue (MB) is one of the most commonly used synthetic dyes, particularly in the textile industry, where it is employed for dyeing materials such as cotton, wool, and silk. The widespread use of MB has resulted in its frequent detection in industrial effluents, raising significant environmental and health concerns. As a cationic dye that is highly soluble in water, MB can have toxic effects on aquatic organisms and presents potential health risks to humans through exposure to contaminated water. High concentrations of MB can lead to health issues like eye irritation, respiratory problems, vomiting, severe jaundice, shock, dizziness, necrosis, methemoglobinemia, and sweating [2,3,4,5,6]. Furthermore, the resistance of MB to conventional water treatment processes exacerbates its environmental impact, as it can persist in aquatic ecosystems, leading to long-term contamination. This persistence underscores the urgent need for the development and implementation of more effective and biocompatible methods for the removal of MB and similar pollutants from water sources. Addressing this issue is critical for safeguarding public health and maintaining the integrity of aquatic ecosystems.

Various methods are employed to remove dyes from aqueous solutions, including chemical, physical, biological, and combined techniques. Recently, the use of nanoparticles in eliminating water pollutants has gained prominence. Nonetheless, employing these nanoparticles without a recycling strategy can result in secondary pollution in groundwater posttreatment. Iron nanoparticles (INPs) are popularly used for their high adsorption capacity, particularly for dyes. However, the synthesis of INPs using toxic reducing agents like sodium borohydride poses significant environmental risks [7].

The green synthesis of nanoparticles offers an eco-friendly, cost-effective, and recyclable alternative, gaining importance in contemporary times. This method utilizes aqueous extracts from various plants as reducers and stabilizers in nanoparticle synthesis [8, 9]. Green tea stands out for synthesizing INPs among different plants due to its easy availability and high regenerative properties. Furthermore, using polymer nanofibers as iron nanoparticle carriers can mitigate secondary pollution following treatment [10,11,12]. Nanofibers, in particular, have garnered significant interest for their unique properties compared to traditional fibrous structures [13].

Polymer electrospinning is an economical and practical technique for producing nanofibers, which are characterized by their high specific surface area, high porosity, stability in liquid phases, and excellent support for immobilizing iron nanoparticles [14, 15]. The expansive specific area of these nanofibers enables the iron nanoparticles to offer more reaction sites, thereby enhancing the reduction of pollutants and contributing to a more dispersed and smaller nanoparticle size. Additionally, the high porosity of the nanofibers facilitates effective contact between the iron nanoparticles and raw water, ensuring a relatively stable water flow [16].

PVA, a biocompatible synthetic polymer, is extensively utilized in biomedical sciences and engineering. However, its water solubility is a notable limitation. Consequently, cross-linking becomes indispensable in applications involving direct contact with water or aqueous mixtures to maintain stability and effectiveness [17, 18].

Polyvinyl alcohol (PVA) and its composites with other polymers have been widely recommended as effective carriers for nanoparticles in various studies. For example, Bayat et al. developed a chitosan/PVA nanocomposite embedded with polyaniline/silica hybrid nanostructures for the adsorption of organic dyes [19]. Similarly, Osei et al. utilized green-synthesized silver nanoparticles supported on natural rubber latex/PVA nanofibers for the efficient sorption of mercury. These studies highlight the versatility and efficacy of PVA-based materials in environmental remediation applications [20].

Extensive research has been conducted on the synthesis of nanoparticles on various nanofibers; however, there has been no investigation into the in situ synthesis of iron nanoparticles on polyvinyl alcohol (PVA) nanofibers using a green synthesis approach. Given the affordability and biocompatibility of PVA, coupled with the environmental advantages of green synthesis and the high adsorption capacity of iron nanoparticles, this study focuses on the development of this novel surface adsorbent. The research aims to evaluate its effectiveness in the adsorption of methylene blue, leveraging the synergistic properties of PVA nanofibers and iron nanoparticles.

Materials and methods

Material

Dry green tea was purchased from the herbal store, washed, and then dried. Ferrous sulfate heptahydrate (FeSO4.7H2O), polyvinyl alcohol (MW = 72,000), glutaraldehyde (25% wt), hydrochloric acid, and sodium hydroxide were purchased from Merck Chemicals Co., Ltd. Methylene blue (MB) was supplied by CARLO ERBA Reagents Co., Ltd. Double-distilled water was used in all step. All the reagents were used without further purification.

Leaf extract preparation and synthesis of INPs

Green tea leaves are ground into a powder using a household mill and then sifted through a specific mesh sieve. Five grams of these powdered tea leaves are mixed with 100 mL of double-distilled water in a 250-mL Erlenmeyer flask. This mixture is then heated for 60 min on a hotplate stirrer at a temperature of 80 °C and a stirring speed of 300 rpm. After cooling, the extracts are filtered through filter paper to remove tea sludge. The filtrate is subsequently centrifuged at 10,000 rpm for 10 min to eliminate fine leaf particles. The resulting clear supernatant is stored in the refrigerator for future experiments. The pH of these extracts is adjusted using hydrochloric acid and sodium hydroxide (0.1M), following the design of the experiment data.

For the synthesis of iron nanoparticles, the prepared tea leaf extracts are added to 5 mL of a 0.1M FeSO4.7H2O solution in predetermined volume ratios. This mixture is immediately placed in an incubator shaker set at a speed of 200 rpm. Upon mixing, the formation of zero-valent iron nanoparticles is indicated by the appearance of a black-colored precipitate [6, 18]. The reaction product is then centrifuged, and the resulting black precipitate is washed twice with double-distilled water and once with ethanol. This washing process aims to remove any unreacted plant chemicals and solutes. Finally, the precipitate is suspended in ethanol and left at room temperature for 15 h to dry the nanoparticles.

Fabrication of the PVA/INPs nanofiber

PVA/INPs nanofibers were fabricated by two methods. The production methods are shown in Fig. 1.

Fig. 1
figure 1

PVA/INPs nanofiber fabrication methods: a method 1; and b method 2

Full size image

Method 1: In method 1, the specified amount of INPs was added to double-distilled water and stirred until it reached a homogenous consistency. Subsequently, PVA pellets were introduced and stirred at 80 °C for 4 h to ensure complete dissolution of the polymer. The nanofibers were produced via electrospinning after cooling to room temperature and removing air bubbles through an ultrasonic bath. This process involved a flow rate of 0.8 mL/h, a 9–15 kV voltage range, and a 12 cm distance between the needle and collector. The resultant PVA/INPs nanofibers were then dried at room temperature. Table 1 details the composition of the various nanofibers produced.

Table 1 The required amount of INPs to prepare PVA/INPs solutions
Full size table

The nanofibers were treated with 25% glutaraldehyde and 37% HCL for cross-linking. Initially, a 10:1 mixture of glutaraldehyde and HCL was placed in a Petri dish inside a desiccator. The nanofibers were then positioned in the desiccator above the Petri dish, exposing them to glutaraldehyde vapor for reaction. This setup was placed in an oven, with the temperature first set to 80 °C for one hour, followed by 50 °C for seven hours [21, 22]. After the heat treatment, the cross-linked nanofibers were immersed in water twice, each for a duration of 10 min, and subsequently dried at room temperature.


Method 2: A 10% w/w PVA solution was prepared, electrospun, and cross-linked using method 1. Subsequently, the resultant cross-linked nanofibers were immersed in a 0.1 M FeSO4.7H2O solution for an hour. After this incubation period, tea extract, produced under optimal conditions, was introduced into the solution containing the nanofibers, forming nanoparticles. The PVA/INPs nanofibers were then thoroughly washed—twice with double-distilled water and once with ethanol—to eliminate any excess materials. Finally, these were air-dried at room temperature.

Adsorption study

All experiments were conducted using a batch adsorption process to assess the efficacy of PVA/INPs nanofibers in removing methylene blue (MB). A specific quantity of PVA/INPs nanofibers was introduced into a predetermined concentration of MB solution. This process was carried out for a set duration at room temperature, with constant stirring at 150 rpm. Prior to adding the nanofibers, the pH of the MB solution was adjusted using 0.1 M HCl and NaOH to achieve the desired level. Upon completion of the contact time, the nanofibers were separated from the solution. The concentration of methylene blue in the residual solution was then determined using a UV–visible spectrophotometer at a wavelength of 668 nm. The percentage of dye removal, equilibrium adsorption capacity, and the adsorptive efficiency of the nanofibers were quantified using specific formulas.

$$Removal \,percentage\, \left(\text{\%}\right)=\frac{{(c}_{0}-{c}_{t})}{{c}_{0}}*100$$
(1)
$${q}_{e}\left(mg. {g}^{-1}\right)={(c}_{0}-{c}_{e})\frac{v}{w}$$
(2)
$${q}_{t}\left(mg. {g}^{-1}\right)={(c}_{0}-{c}_{t})\frac{v}{w}$$
(3)

where \({q}_{e}\) is the adsorption capacity at equilibrium, qt is the adsorption capacity at time t, Ct is the dye concentration at time t (mg/L), Ce is the equilibrium dye concentration in the solution, C0 is the initial dye concentration (mg/L), W is the weight of the PVA/INPs nanofiber used (g), and V is the volume of dye solution used (L) [23, 24].

Process optimization

A design of experiments approach was employed to optimize the synthesis of INPs using green tea extract and the removal of methylene blue (MB) from aqueous solutions using PVA/INPs nanofibers. Specifically, the central composite design (CCD) within the response surface method was utilized for this purpose. Four variables were identified for optimization in each process. The levels for these variables were chosen based on insights gathered from preliminary experiments and findings reported in the literature [25,26,27,28]. A series of experiments were delineated using the Design Expert 13.0 software. The specific variables studied and their respective levels are detailed in Table 2. The analysis of the results was also performed using Design Expert 13.0 software.

Table 2 The studied variables and their levels to optimizing the amount of INPs synthesis and removal percentage of MB
Full size table

Characterizations

The structural and morphological characteristics of nanoparticles and nanofibers were extensively analyzed using various techniques. Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) provided molecular and crystalline structure insights. The surface morphology was examined using a field-emission scanning electron microscope (FSEM), while the elemental composition was determined through energy-dispersive X-ray spectroscopy (EDS). Additionally, the size distribution of the nanoparticles was assessed using dynamic light scattering (DLS), and their thermal stability was evaluated through thermal gravimetric analysis (TGA). To measure the concentration of methylene blue (MB) in solutions, a UV–visible spectrophotometer was employed.

Results and discussion

Green synthesis of INPs

Optimization of INPs synthesis

A total of 29 experiments were conducted using the Design Expert software, as outlined in Table S1 in the supplementary material. These experiments closely aligned with the quadratic model, evidenced by a high R2 value of 0.9860, indicating an excellent fit of the data. The results of the ANOVA test are shown in Table S2 and underscored the significance of variables such as extract pH, volume ratio, and time in the synthesis of INPs, as demonstrated by their low p values (less than 0.0001) and high f values. However, the impact of temperature on INP synthesis was comparatively minimal. Analysis revealed that an increase in extract pH, time, and volume ratio leads to a rise in INP production. The developed model predicts that the optimal conditions for maximum INP yield—0.15 g per reaction—are an extract pH of 9.8, a reaction time of 94 min, a volume ratio of 6, and a temperature of 32 °C. This prediction was substantiated by a validation experiment conducted under these optimal conditions, which confirmed the anticipated INP yield. Further details can be found in the Supplementary material.

Characterization of INPs

The physicochemical properties of the INPs, synthesized under optimal conditions, were thoroughly evaluated. Figure 2a presents both the FESEM images and the corresponding size distribution histogram for these green-synthesized INPs. These FESEM images reveal spherical nanoparticles with diameters ranging from 15 to 31 nm, aligning with findings reported by other researchers [7, 27, 29, 30].

Fig. 2
figure 2

a FESEM image; b histogram of particle size distribution; and c DLS of the size distribution of green-synthesized INPs

Full size image

Dynamic light scattering (DLS) analysis was conducted to examine the aggregation tendencies of the INPs in solution [31]. As illustrated in Fig. 2c, the size distribution of the INPs in aqueous solutions ranged between 43 and 489 nm, with an average diameter of approximately 83 nm. This contrasts with the average particle size of 22 nm observed in FESEM images, suggesting a tendency for the INPs to agglomerate. Such agglomeration could be attributed to factors like the solution's form, improper drying, or the Van der Waals forces acting between the particles [30, 31].

Additionally, the zeta potential was measured to assess the stability and surface charge of the INPs in solution. A colloidal dispersion is deemed stable when the particle zeta potential is either above + 30 mV or below -30 mV [32]. The INPs exhibited a zeta potential of -30.4 mV, indicating adequate repulsive forces between particles and confirming their good colloidal stability in an aqueous solution. The combination of good stability and small diameter underscores the effectiveness of the green method used in nanoparticle synthesis.

The TGA and DTG curves of the synthesized iron nanoparticles are presented in Fig. 3a, revealing a significant weight loss trend. The initial weight loss, occurring at temperatures below 160.9 °C, is associated with the evaporation of residual water in the nanoparticle cavities. The weight loss continuing up to 241.5 °C can be attributed to the decomposition of capping materials [27]. A rapid decomposition of organic material at 282.4 °C leads to the main peak in the DTG curve at this temperature. Notably, no weight loss was observed above 500 °C. TGA analysis indicates that the weight percentage of organic materials serving as stabilizers and coatings is approximately 65.56%. EDS analysis Fig. 3b) confirms the presence of organic compounds in the green-synthesized iron nanoparticles (INPs). Besides iron, the spectrum also displays nitrogen, oxygen, and carbon elements, indicating impurities inherent to the green synthesis process. The respective percentages of Fe, C, O, and N are 7.80, 34.37, 42.59, and 7.25%, aligning with other studies using green synthesis methods.

Fig. 3
figure 3

a TGA and DTG curve and b EDS spectrum of green-synthesized INPs

Full size image

The FTIR spectrum of the nanoparticles and the green tea extract are depicted in Fig. 4a. The FTIR peaks of the green tea extract in the 3300 cm−1 range signify O–H stretching vibrations, while the bond at 1637 cm−1 is indicative of polyphenolic compounds arising from the C=C stretching vibration in the aromatic ring. The FTIR spectrum of the nanoparticles also shows stretching vibrations at 3205 cm−1 for O–H, 1610 cm−1 for C=C, and 1074 cm−1 for C–O–C. Peaks at 510 cm−1 and 605 cm−1 suggest Fe–O stretching vibrations [33, 34], indicating that the polyphenols effectively coat the iron particles, which oxidize upon exposure to air and water.

Fig. 4
figure 4

a FTIR spectra; and b X-ray diffraction patterns for green-synthesized INPs, PVA, cross-linked PVA, and PVA/INPs Nanofibers

Full size image

The XRD pattern of the synthesized iron nanoparticles is illustrated in Fig. 4b. The prominent peak at 2θ = 23.4° signals the presence of organic compounds absorbed from the green tea extract, acting as capping and stabilizing agents for the nanoparticles. The peak at 2θ = 44.5° is indicative of zero-valent iron nanoparticles, while the diffraction peak at 2θ = 41.6° corresponds to the (111) plane, resulting from nanoparticle oxidation. The synthesized nanoparticles comprise two components: a core part and an outer layer of iron oxide [31].

PVA/INPs nanofiber fabrication

Effect of PVA/INPs nanofiber fabrication method on adsorption efficiency

To determine a more effective preparation method for PVA/INPs (polyvinyl alcohol/iron nanoparticles) nanofibers, we assessed the adsorption capacity and removal efficiency of methylene blue (MB) for each type of nanofiber under identical conditions, with the results detailed in Table 3 These findings indicate that nanofibers produced using the second method exhibited a higher MB adsorption capacity. This enhanced performance is attributed to the increased presence of INPs on the surface of PVA/INP composite nanofibers prepared by method 2, compared to those prepared by method 1. Consequently, the larger surface area of INPs in contact with methylene blue in the composite nanofibers from method 2 accounts for its superior adsorption effectiveness. Subsequent research focused on PVA/INPs nanofibers fabricated using this second method.

Table 3 Adsorption efficiency comparison of fabricated PVA/INPs nanofibers at initial concentration 50 mg/L, nanofiber dose 1.5 g/L, initial pH 6.4, and contact time180 min
Full size table

Characterization of the PVA/INPs nanofibers

The surface morphologies and diameters of uncross-linked PVA, cross-linked PVA, and PVA/INPs (polyvinyl alcohol/iron nanoparticles) nanofibers were examined using FESEM (field-emission scanning electron microscopy), as shown in Fig. 5. Additionally, a histogram depicting their diameter distribution was constructed. The average diameter of uncross-linked PVA nanofibers was measured at 338 ± 62 nm. In comparison, the average diameters for cross-linked PVA and PVA/INPs nanofibers were found to be 380 ± 76 nm and 374 ± 62 nm, respectively. The process of cross-linking with glutaraldehyde vapor caused the nanofiber surfaces to soften, leading to swelling and an increase in thickness [35, 36]. While the synthesis of nanoparticles on the cross-linked PVA nanofibers did not alter their diameter significantly, it did affect their surface roughness due to the formation of nanoparticles on the nanofibers' surface, as evidenced in Fig. 5c.

Fig. 5
figure 5

FESEM image and diameter distribution of nanofibers for a PVA; b Cross-linked PVA; and c PVA/INPs

Full size image

The FTIR (Fourier transform infrared) spectra of PVA, cross-linked PVA, and PVA/INPs nanofibers, presented in Fig. 4a, were used to assess the cross-linking effectiveness and the accuracy of nanoparticle loading on the nanofibers. Cross-linking predominantly affects the hydroxyl groups in PVA, with cross-linked PVA exhibiting a weaker peak at 3428 cm−1 compared to uncross-linked PVA. This change is due to the acetal reaction between glutaraldehyde and PVA, which limits the availability of hydroxyl groups [37]. In contrast, PVA/INPs nanofibers showed a sharper O–H peak compared to cross-linked PVA, attributable to the presence of organic materials from nanoparticles on the nanofiber surfaces. The absorption peak at 510 cm−1, indicative of the stretching vibration mode of Fe–O bonds, confirms the presence of nanoparticles synthesized on the nanofibers by the green method [38].

XRD (X-ray diffraction) analysis was used to evaluate the crystallinity of uncross-linked PVA, cross-linked PVA, and PVA/INPs nanofibers. The XRD patterns, shown in Fig. 4b, revealed that all three samples exhibited a characteristic peak around 2θ = 18.85°, albeit with varying intensities, which is associated with the PVA matrix. Given that PVA is a semicrystalline polymer, the Scherrer equation can be utilized to estimate the nanofibers' average crystallite size (D).

$$D=\frac{A\lambda }{\Delta \theta cos\theta }$$
(4)

Here, the average crystallite size, D, is calculated using Scherrer's formula where A represents the Scherrer constant (value = 1), λ denotes the x-ray wavelength, and Δθ is the full width at half maximum of the angle. Pure polyvinyl alcohol (PVA) nanofibers exhibited a characteristic amorphous phase peak, with a prominent halo at 2θ = 18.9° and an average crystallite size of 12 nm. The crystallinity of these PVA nanofibers was found to increase upon exposure to glutaraldehyde vapors. In cross-linked PVA, two distinct peaks were observed at 2θ = 19.23° and 40.405°, indicating the formation of a new, more crystalline phase within the PVA structure. The average crystallite sizes for these two phases were measured at 40 nm and 29 nm, respectively. Additionally, PVA nanofibers incorporating iron nanoparticles (PVA/INPs) displayed two peaks at 2θ = 18.85° and 40.835°, with average crystallite sizes of 28 nm and 37 nm, respectively. This change signifies a decrease in crystallinity within the first phase (PVA matrix) and an increase in the second phase, attributed to the synthesis of iron nanoparticles on the nanofibers, thereby confirming the presence of INPs.

MB adsorption by PVA/INPs nanofibers

MB adsorption mechanism

To clarify the adsorption process, the interactions on PVA/INPs nanofibers were studied using cationic methylene blue dye as the adsorbate. Methylene blue, a cationic dye, contains a quaternary amine group, where the cationic nitrogen interacts with the anionic sites on the synthesized adsorbent through electrostatic interactions. In many cases, the adsorption of dyes onto polyvinyl alcohol-based materials is primarily facilitated by hydrogen bonding involving the nitrogen atoms of the dye molecules (Fig. 6). Additionally, since green tea, which is rich in polyphenols, was utilized in the synthesis of iron nanoparticles, it is likely that the nanoparticles contain aromatic rings. As a result, π-π stacking interactions between the aromatic rings of the polyphenols and methylene blue play a crucial role in the adsorption process. Thus, these interactions are responsible for effective dye removal from an aqueous solution.

Fig. 6
figure 6

Proposed mechanism of adsorption of MB dye solution on PVA/INPs nanofibers

Full size image

Optimization MB adsorption by PVA/INPs nanofibers

The response surface methodology (RSM) is an effective technique for determining the optimum conditions of a process. Among various experimental designs for RSM, the central composite design (CCD) is particularly advantageous for exploring a wide surface area to identify the optimum point. This approach was applied to optimize the adsorption of methylene blue, as detailed in Table 4. Analysis of variance (ANOVA) is employed to evaluate the quality of the selected model. ANOVA identified the quadratic model as the most suitable among various mathematical models, including linear, 2-factor interaction (2Fi), quadratic, and cubic. This selection was based on the model's p value being less than 0.005 and an R-squared value of 0.96, indicating a high level of model fit. The ANOVA results for the experimental data are presented in Table 5, where p values below 0.05 signify significant model terms [39]. ANOVA, four variables—pH, initial methylene blue (MB) concentration, contact time, and the dose of nanofibers—are significant in the model. This model is expressed in coded Eq. 5 to predict the efficiency of MB removal.

$$\begin{gathered} MB \,removal\, \left( \% \right)\; = \;74.20\; + \; 14.33 A\; + \; 2.86 B \; - \;5.56 C \hfill \\ \quad \quad \quad \quad \quad \quad \quad \quad \quad + \;13.54D\; - \;0.4387 AB\; + \;4.39 AC \hfill \\ \quad \quad \quad \quad \quad \quad \quad \quad \; - \;2.72 AD \; - \;1.46 BC \; - \;4.54 BD \hfill \\ \quad \quad \quad \quad \quad \quad \quad \quad - \;5.48 CD\; - \;6.30 A^{2} \; - \;2.09 B^{2} \hfill \\ \quad \quad \quad + \;1.19 C^{2} \; - \;2.37 D^{2} \hfill \\ \end{gathered}$$
(5)

where the coded variables A, B, C, and D represent pH, contact time (minutes), initial concentration of methylene blue (MB) (mg/L), and nanofiber dose (g/L), respectively. Utilizing this coding, the model was employed to ascertain the optimal conditions for maximum decolorization using PVA/INPs (polyvinyl alcohol/iron nanoparticles) nanofibers. The model identified that at a pH of 8.6, contact time of 100 min, an initial MB concentration of 30 mg/L, and a nanofiber dose of 1.6 g/L, the color removal percentage reached 89%. Validation experiments conducted under these optimal conditions achieved a maximum dye removal efficiency of 85%, confirming the effectiveness of these identified conditions for achieving maximum decolorization.

Table 4 CCD experimental design and response to the dependent variable MB removal percentage
Full size table
Table 5 The ANOVA results for the response surface quadratic model for MB dye removal
Full size table

This study's findings contribute to a broader range of research exploring different levels of adsorption capacity with various nanofiber composites. For instance, Jiang et al. reported a maximum adsorption capacity of 9.25 mg/g using a sodium alginate/PVA composite nanofiber for MB adsorption [40]. Similarly, He, Yao, et al. documented an adsorption capacity of 60.85 mg/g for a PVA/graphene oxide (GO) composite nanofiber, measured at an initial MB concentration of 50 mg/L and a contact time of 2 h [40]. In comparison, the experiments in this study demonstrated an adsorption capacity of 15.93 mg/g under optimal conditions, which was within the range of these previous findings.

Interaction between influencing factors on adsorption

The interaction between two variables in the removal of MB (methylene blue) was investigated using three-dimensional response surface diagrams, as depicted in Fig. 7. These variables were examined in pairs, with all other variables held constant. The interaction between pH and the initial concentration of MB on dye removal is demonstrated in Fig. 7a. MB is cationic dye characterized by positively charged ions in aqueous solutions. An increase in pH leads to a higher removal percentage of MB. This is because as pH rises, negatively charged deprotonated sites increase, enhancing the electrostatic attraction between the adsorbent and MB. Conversely, a decrease in pH results in more protonated adsorption sites, increasing the electrostatic repulsion between the dye and the adsorbent, thereby reducing the removal efficiency of MB [41, 42].

Fig. 7
figure 7

Response surface plot and interaction between two variables on MB dye removal: a interaction between initial concentration of MB and pH; b interaction between contact time and pH; c interaction between dose of nanofibers and initial concentration

Full size image

As the initial concentration of MB increases, the removal of MB decreases. The overlap of the active surfaces at high dye concentrations and the reduction of the active surface of PVA/INPs nanofibers cause this phenomenon [43]. Therefore, when the concentration of MB is low and the pH is high, the percentage of MB removal is high.

Figure 7b illustrates the interaction between contact time and pH. According to the ANOVA results, this interaction does not significantly influence the removal of MB, as the p value is above 0.05. The removal efficiency of MB improves with increase in contact time, providing more time for the adsorption of reactive dye molecules [44]. Extended contact time at a high pH also improves dye adsorption by PVA/INPs nanofibers.

Figure 7c displays response surface plots that show the dye removal dependency on the dose of nanofiber and the dye concentration in the solution. Increasing the amount of nanofibers makes more adsorbing and active surfaces available for MB adsorption, thus enhancing the dye removal percentage.

Finally, the effect of contact time and the initial concentration of MB on dye removal is presented in Fig. 7d. This figure indicates that more dye is removed as the contact time increases and the initial dye concentration decreases.

Effect of contact time

To determine the equilibrium time in the adsorption of methylene blue (MB) and to examine more accurately the effect of time on the reaction, experiments were repeated with the concentration of MB in the samples being measured at fifteen-minute intervals. Figure 8 reveals that the adsorption of MB on PVA/INPs nanofibers reaches equilibrium in 90 min. Initially, the removal efficiency escalates rapidly due to the abundance of vacant adsorption sites. Subsequently, the rate of increase in removal efficiency slows down as the number of available adsorption sites decreases [45].

Fig. 8
figure 8

Plots a qt versus contact time at temperatures 303, 313, and 323K; and b MB removal percentage versus contact time at temperature 303 K onto PVA/INPs nanofibers

Full size image

Adsorption kinetic

Various kinetic models were applied to the experimental data concerning the adsorption of MB onto PVA/INPs nanofibers, including pseudo-first-order, pseudo-second-order, Elovich, and intra-particle diffusion models. The linear forms of these kinetic equations are presented in Table 6 [31, 41, 46, 47]. A summary of the results derived from these models is provided in Table 7. The findings indicate that the adsorption of MB on PVA/INPs nanofibers adheres to the pseudo-second-order kinetic model, evidenced by an R2 value of 0.9961, which surpasses those of the other models. The Elovich model, which describes the interaction between a solid and liquid interface, suggests that an α value greater than β signifies a lower desorption and higher adsorption rates. Therefore, the experimental results demonstrate a strong affinity of MB for adsorption onto PVA/INPs nanofibers [31, 48].

Table 6 Kinetic models and adsorption isotherms used in this study
Full size table
Table 7 Kinetic parameters for adsorption of the MB dye onto PVA/INPs nanofibers
Full size table

The intra-particle diffusion kinetics, or Weber-Morris model, was employed to identify the reaction pathways, adsorption mechanisms, and predict the rate-controlling step in the process [49].The plot of t0.5 versus q did not yield a single linear slope but instead revealed two distinct linear zones. This observation suggests that intra-particle diffusion of the MB dye solution onto PVA/INPs nanofibers is not the sole rate-controlling step, with other mechanisms also playing a role. The presence of two linear regions in the intra-particle diffusion plot indicates that the sorption process involves both surface sorption and intra-particle diffusion. The initial portion of the plot is indicative of the boundary layer effect, while the second linear portion corresponds to intra-particle or pore diffusion [50].

Adsorption isotherm

The adsorption capacity of PVA/INPs nanofibers was assessed using five different adsorption models: Langmuir, Freundlich, Temkin, Harkins–Jura, and Redlich–Peterson, as detailed in Table 8. The linearized equations of these models are presented in Table 6 [41, 51]. The experimental data showed a strong correlation with the Langmuir model, indicated by the highest correlation coefficient (R2) among the models. This result suggests that MB molecules are adsorbed onto the surface of the nanofibers through monolayer adsorption. The maximum adsorption capacity (max) determined by this model was 24.509 mg/g.

Table 8 Isotherm parameters for adsorption of the MB dye onto PVA/INPs nanofibers
Full size table

Temkin model considers that the heat of adsorption of the molecules in the layer reduces linearly with the coverage because of the interactions between adsorbent and adsorbate. From the data presented in Table 8, the Temkin isotherm model yielded an R2 value of 0.984, indicating a strong correlation with the experimental data. The Temkin isotherm equilibrium binding constant (A) was determined to be 4.538. Additionally, the Temkin constant (B), which is associated with the heat of methylene blue (MB) adsorption on PVA/INPs nanofibers, was calculated to be 5.221 J/mol. The fact that the B value obtained from the Temkin model is greater than 1 suggests that electrostatic interactions are significant in the adsorption process. Furthermore, it implies that pore heterogeneity on the surface of the PVA/INPs nanofibers plays a crucial role in the adsorption of MB [52].

The Redlich–Peterson (R–P) isotherm model describes a homogeneous monolayer adsorption process, characterized by constant enthalpies and adsorption energies. This model effectively addresses the limitations associated with the two-parameter Langmuir and Freundlich isotherm equations in certain adsorption systems. The three-parameter R–P isotherm equation simplifies to the Langmuir isotherm when the isotherm exponent (α) approaches 1, whereas when the b value approaches 0, it becomes complementary to the Freundlich isotherm [53]. In this study, fitting the experimental data to the R–P isotherm model resulted in a high correlation coefficient (R2) of 0.971, suggesting a strong fit between the model and the observed data. The isotherm exponent (α) was found to be 0.865, which indicates that the adsorption of methylene blue (MB) onto PVA/INPs nanofibers aligns more closely with the Langmuir model than with the Freundlich model. This suggests that the adsorption process is likely dominated by monolayer coverage on a relatively uniform surface, where all adsorption sites are equivalent in terms of energy [54].

Thermodynamic studies

To ascertain the thermodynamic parameters of the adsorption process of MB by PVA/INPs nanofibers, experiments were conducted at varying temperatures, 30, 40, and 50 °C, under optimal conditions. The results are depicted in Fig. 8a. For this purpose, the plot of ln(qe/ce) versus T−1 was drawn based on Eq. 6 using the experimental data. The values of ΔH (enthalpy change) and ΔS (entropy change) for the adsorption process were calculated from the slope and intercept of this plot.

$$\mathit{ln}\frac{{q}_{e}}{{c}_{e}}=-\frac{\Delta H}{RT}+\frac{\Delta S}{R}$$
(6)

The value of ΔG for different temperatures was obtained from Eq. 7, and the thermodynamic parameters are reported in Table 9.

$$\Delta G=\Delta H-T\Delta S$$
(7)

where R represents the universal gas constant (8.314 J/mol K) and T denotes the absolute temperature (K). ΔG is the Gibbs free energy of sorption (kJ/mol), while ΔH (kJ/mol) and ΔS (J/mol·K), respectively, signify the enthalpy and entropy of sorption. The results revealed that ΔG was negative at all three temperatures, suggesting that methylene blue (MB) adsorption onto PVA/INPs nanofibers occurs spontaneously. The nature of the MB adsorption process is exothermic, as indicated by the negative enthalpy value. A negative ΔS value implies a decrease in randomness due to the interaction between the adsorbate and adsorbent [45].

Table 9 Thermodynamic parameters for MB adsorption on PVA/INPs at different temperatures
Full size table

Reusability

The reusability and recovery of the synthesized adsorbent are critical factors in assessing the potential of PVA/INPs for industrial applications. The dye-loaded PVA/INPs adsorbents were regenerated by rinsing with an ethanol solution. The recyclability of the used adsorbents, as depicted in Fig. 9, indicates that the removal efficiency of methylene blue (MB) by PVA/INPs adsorbent decreases slightly with each adsorption cycle. This slight decline suggests effective regeneration of the spent adsorbents. Notably, even after five adsorption cycles, the regenerated PVA/INPs adsorbent maintained a relatively high removal efficiency of 72% for cationic MB dye, demonstrating the practicality and durability of PVA/INPs adsorbents for repeated use in industrial settings.

Fig. 9
figure 9

Regeneration and reusability for PVA/INPs nanofibers after five cycles using ethanol solution

Full size image

Evaluation of performance

In Table 10, the adsorption capacities (qe) of various PVA-based polymer composite adsorbents for methylene blue (MB) removal, as reported in the literature, are summarized. It is noteworthy that the nanofiber adsorbent developed in this study demonstrates a respectable adsorption capacity (qe), particularly given the simplicity of its fabrication, in comparison with other recently reported adsorbents. However, the qe values calculated in this work are lower than those observed for PVA/PEI nanofibers, polyvinyl alcohol nanofibers containing sericin/β-cyclodextrin, imprinted sericin/polyvinyl alcohol adsorbents, and PVA/chitosan nanofibers embedded with Ce-UiO-66 nanoparticles. Despite this, an analysis of the cost (Table S3) and ease of fabrication of this type of adsorbent indicate that its economic efficiency could potentially offset the lower adsorption capacity when compared to the aforementioned alternatives.

Table 10 Comparison of the equilibrium adsorption capacity for methylene blue dye on PVA-based adsorbents reported in the literature
Full size table

Conclusions

This study presents a novel approach for the preparation of polyvinyl alcohol (PVA) nanofibers embedded with iron nanoparticles (INPs) through an environmentally friendly green synthesis method. Iron nanoparticles were synthesized using green tea leaf extract, and their production was optimized through response surface methodology (RSM) based on central composite design (CCD). The model predicted a maximum production of 0.15 g of iron nanoparticles per reaction (4.285 mg/ml of the reaction mixture), achieved under optimal conditions: volume ratio (6) (5 ml of ferrous sulfate heptahydrate 0.1 M and 30 ml green tea extract), pH (9.8), reaction time (98 min), and temperature (32 °C). The synthesized nanoparticles had an average diameter of 22 nm. The PVA/INPs nanofibers, with an average diameter of 389 nm, achieved an MB removal rate of 85% from aqueous solutions under optimal conditions, including a pH of 8.6, an initial MB concentration of 30 mg/L, a nanofiber dose of 1.6 g/L, and a contact time of 100 min. The kinetic model analysis revealed an equilibrium adsorption capacity of 14.492 mg/g, with a maximum adsorption capacity of 24.509 mg/g. The negative values of Gibbs free energy and adsorption enthalpy indicate that the adsorption process is both spontaneous and exothermic. These results demonstrate that the PVA/INPs nanofiber membranes developed through this innovative green synthesis method are highly effective for the removal of methylene blue from wastewater. This advancement offers a promising solution for addressing dye pollution in the textile and dye industries.