Introduction

The twenty-first century is witnessing significant growth in heavy industries, which is prospering the economy of some countries. Aside from this bloom, the discharged wastewater from these industries tends to be a major catastrophe for the world. Dyes play a quintessential role in industries such as plastic, printing, paper, pharmaceutical, and textiles [1]. Nonetheless, the majority of synthetic dyes are stable and noxious which complicates their removal.

Congo red is one of the most harmful synthetic dyes due to its complicated structure, which enhances its stability in water and thus inhibits light permeability [2, 3]. Therefore, the presence of CR in water poses a serious threat to marine life. The CR dye has deleterious effects on human health, including eye irritation, blood clotting, gene mutation, cancer, diarrhea, and more [4].

Accordingly, various remediation techniques have progressed to remove CR effluents from water, including coagulation [5, 6], adsorption [7], ozonation [8, 9], catalysis [10], and membrane [11, 12]. Notably, the facile operation, low cost, uncomplicated design, and adsorbents’ reusability make adsorption the most preferred remediation method [13].

Obviously, MOFs have gained researchers’ consideration for their unrivaled features comprising outstanding chemical and thermal stability, as well as high porosity and surface area [14, 15]. Interestingly, MOFs are distinguished by their ease of functionalization without experiencing any collapse or partial collapse of their frameworks [16]. Hence, MOFs have exhibited advanced performance in diverse potential applications like catalysis, sensors, gas storage, drug delivery, water purification, and so on [17]. In particular, the MOFs’ stability in aqueous media and their super-high aptitude towards versatile contaminants render them a perfect choice as premium adsorbents. Excessive developments have proceeded on MOFs to enhance their adsorption performances; for instance, fabricating binary MOF [18], bimetallic MOF [19], and MOFs composites [20].

Among the remarkable MOFs, MOF-5, also known as IRMOF-1, stands out with its well-ordered structure, porosity, flexible functionalization, and thermal stability [21]. MOF-5 is constructed from zinc (as the core metal) and 1,4-benzene dicarboxylic acid (as the linker). The applications of MOF-5 have expanded to various fields, including water purification, gas storage, catalysis, gas separation, and drug delivery [22]. Furthermore, MIL-53(Fe) has shown promising results in these applications due to its flexibility, chemical stability, and remarkable adsorption capabilities [23]. MIL-53(Fe) is built using iron as the core metal and 1,4-benzene dicarboxylic acid as the linker. Unfortunately, there are still obstacles in using MOFs as adsorbents, which limits their applications on a large scale, including high cost and poor recyclability.

Chitosan (CTS) is a notable branch in the polysaccharides realm, possessing remarkable features, such as non-toxicity, natural availability, good hydrophilicity, cost-effectiveness, and prime adsorption efficacy [24]. In addition to the ease of preparing CTS from chitin, which is derived smoothly from abundant and cost-effective resources, such as insects, shrimp, fungi, crabs, algae, and so on [25]. Notably, CTS has a unique structure that facilitates its functionalization through the presence of surface-reactive functional groups (viz., OH and NH2), as well as its texture that enables it to take on various forms (such as membranes, spheres, fibers, films, etc.). However, it suffers from low adsorption performance and weak durability when used without proper modifications.

Doping the chitosan film with MOF material enhances its adsorption and durability by increasing the available adsorption sites and introducing metal centers, leading to improved performance as an adsorbent.

Our work intended to exploit the advantages of MOFs to improve the adsorption capacity of CTS. Subsequently, Fe-doped MOF-5 was incorporated into the CTS film, forming an efficacious adsorbent with a good reusability feature for removing CR from aqueous solutions. In detail, Fe-doped MOF-5 was fabricated with Fe ion ratios of 0, 25, 50, 75, and 100% (pure MIL-53(Fe)). Importantly, the optimum ratio of Fe-doped MOF-5 incorporated into the CTS film was examined at a bimetallic MOF ratio of 1–10 wt%. Furthermore, the Fe/MOF-5@CTS composite film was characterized morphologically, thermally, and texturally. A comprehensive batch study of CR adsorption onto the Fe/MOF-5@CTS composite film was conducted. Additionally, the impact of ionic strength on the CR adsorption was assessed, and the selectivity and reusability tests were performed.

Experimental Work

Materials

Text S1 compiles the used materials in the preparation of Fe/MOF-5@CTS composite film.

Preparation of Fe/MOF-5

Dissolving of 1.664 g of Zn(NO3)2·6H2O in 60 mL DMF for 15 min, then 0.352 g of H2BDC was dipped in the Zn2+ solution. After 20 min, a certain proportion of FeCl3⋅6H2O (0, 25, 50, 75, and 100 wt%) was added to the Zn2+/H2BDC solution, and the mixture was stirred for an additional 20 min. Thereafter, the resultant suspension was transferred to an autoclave and kept under heating for 24 h at 120 °C. Ultimately, the obtained MOF was separated, washed by DMF and C2H5OH, and dried at 80 °C overnight.

Preparation of Fe/MOF-5@CTS Composite Film

Fe/MOF-5@CTS composite film was prepared by dissolving CTS (2% wt/V) in 20 mL Dist. H2O (2% V/V AcOH). Then, a calculated mass of Fe/MOF-5 (1–10 wt%) was added bit-by-bit to the CTS solution and kept for 2 h under continuous stirring and sonication for 30 min. Next, Fe/MOF-5@CTS composite film was poured into a petri dish and stored until completely dry of the composite film. To remove Fe/MOF-5@CTS from the petri dish, an aqueous NaOH solution (5% wt/V) was poured onto the sticky composite film. After 1 h, Fe/MOF-5@CTS composite film could be smoothly removed from the petri dish and washed thoroughly by Dist. H2O, and dried at room temperature. Noteworthy, the CTS film was fabricated by the same former procedure without adding MOF. The schematic representation for the fabrication of Fe/MOF-5@CTS composite film is shown in Fig. 1.

Fig. 1
figure 1

Schematic representation demonstrating the synthetic approach for Fe/MOF-5@CTS composite film

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Characterization Instruments

Fe/MOF-5@CTS composite film, CTS film, and pure MIL-53(Fe) and MOF-5 were analyzed by Powder X-Ray Diffraction (PXRD-MAC Science M03XHF, XRD) to scrutinize crystallite phase of the samples. In addition, the morphologies of Fe/MOF-5@CTS composite film and their components were studied by Scanning Electron Microscopy (S4800-Hitachi, SEM), which was further utilized to explore the elemental composition of Fe/MOF-5@CTS composite film via the attached energy dispersive x-ray part. Fourier Transform Infrared Spectrometer was used to define the functional groups of the samples. The surfaces charges were determined by Zeta Potential (Malvern, ZP), and X-Ray Photoelectron Spectroscopy assured the elemental composition of the samples (Thermo-Fisher Sci., XPS). The surface area and pore size of the samples was determined by Brunauer–Emmett–Teller method (Tristar II 3020, BET). Besides, the mechanical properties and thickness of films were investigated using universal testing machine (AG-1 S, Shimadzu, Japan) and a digital micrometer screw gauge (Ajanta, India), respectively.

Water Uptake Study

The water uptake behaviour of the pristine CTS and Fe/MOF-5@CTS films were inspected by soaking 0.1 g of tested film sample in distilled water (pH 7.2) at room temperature. At time intervals (0.5–3 h), the swollen sample was gently separated and blotted between two filter papers to remove the adhered surface water drops and finally followed with weighing in an electronic balance. The water uptake (WU; %) was calculated according to the following equation:

$${\text{WU }}\left( \% \right) = \frac{{{\text{W}}_{{{\text{t}}}} - {\text{W}}_{0} }}{{{\text{W}}_{{\text{0}}} }}{\text{}} \times 100{\text{}}$$
(1)

where, W0 is the initial weight of tested sample and Wt is the weight after time t.

The CR Removal Experiments

The removal process of CR by Fe/MOF-5@CTS composite film proceeded as follows; (i) Assess the pH influence on the removal efficacy of CR at a pH range between 3 and 11. (ii) Test the composite film’s dosage impact on removal% at a range from 0.01 to 0.04 g. (iii) Identify the thermal performance of the CR/composite film system by changing the process temperature of the CR adsorption from 25 to 55 °C. (iv) Evaluate the isotherm of the CR adsorption onto Fe/MOF-5@CTS at a concentration range of 50–300 ppm. At the end of each experiment, a sample was withdrawn and measured via a spectrophotometer at 500 nm, and scrutinize the adsorption performance of Fe/MOF-5@CTS towards CR by applying the following equations:

$${\text{Removal}}\% ,{\text{R}}\% = \frac{{{\text{C}}_{0} - {\text{C}}_{{\text{t}}} }}{{{\text{C}}_{0} }} \times 100$$
(2)
$${\text{Adsorption capacity}},\,{\text{Q}}_{{\text{t}}} = \frac{{({\text{C}}_{0} - {\text{C}}_{{\text{t}}} ) \times {\text{V}}}}{{\text{m}}}$$
(3)

The initial CR concentration is defined as Co, while the final concentration symbolizes Ct. The weight of the used composite film and the CR volume is represented by m and V, subsequently.

Ionic Strength

The influence of ionic strength on the adsorption capacity of CR adsorption onto Fe/MOF-5@CTS was assessed by introducing different concentrations of NaCl (ranging from 0 to 1.0 mol/L) into a solution of CR (20 mL, 100 ppm) and stirring for 5 min. Subsequently, the Fe/MOF-5@CTS composite film was immersed in the CR/NaCl solution. After 2 h samples were withdrawn to measure the remaining CR using a spectrophotometer.

Selectivity Test

The capacity of Fe/MOF-5@CTS composite film towards other anionic dyes like methyl orange (MO) and methyl red (MR) and also cationic dyes like crystal violet (CV) and methylene blue (MB) was assessed to prove the composite film’s selectivity. The selectivity test proceeded by adding 0.02 g of Fe/MOF-5@CTS to 20 mL of the specific dye (100 mg/L). After 2 h, the concentrations of the un-adsorbed dyes were identified using UV–vis spectroscopy.

Cycling Test

The cycling test of Fe/MOF-5@CTS composite film was performed by soaking the CR-loaded composite film in methanol/1 M NaCl for 2 h. Next, the composite film was stored in the air to dry and reused in the next cycle.

Results and Discussion

Characterization of Fe25/MOF-5@CTS

PXRD

Figure 2 A clarifies the crystallite phases of the prepared samples. The PXRD of CTS film indicates its good crystallinity, as evidenced by the prominent peaks with high intensity at 2θ = 10° and 20°, corresponding to (240), and (273) planes [26, 27]. The PXRD profile of MOF-5 exhibits characteristic peaks at 2θ = 6.1°, 9.2°, 14.1°, 15.7°, 17.6°, 26.6°, 28.6°, 30.5°, 32.5°, 35.7°, and 38.9°, corresponding to the (200), (220), (400), (420), (333), (711), (731), (751), (911), (860), and (1022) planes, respectively [28, 29]. Moreover, characteristic peaks of MIL-53(Fe) were observed at 2θ = 9.3°, 12.5°, 17.8°, 25.1°, and 26.5°, corresponding to the (200), (110), (111), (220), and (221) planes, respectively [30]. The PXRD profile of Fe25/MOF-5 exhibits the main characteristic peaks of MOF-5, with a slight shift toward lower 2θ angles, along with the disappearance of some distinctive peaks of MOF-5. This observation confirms the successful doping of Fe ions into MOF-5, in agreement with Abo El-Yazeed et al. [31]. The PXRD profile of Fe25/MOF-5@CTS shows the characteristic peaks of CTS, without the appearance of the corresponding peaks of Fe25/MOF-5. This can be attributed to the low loading proportion of the bimetallic MOF in the CTS matrix and the good homogeneity and dispersion of Fe25/MOF-5 into CTS.

FTIR

Figure 2B illustrates the functional groups of the synthesized samples. The FTIR spectrum of CTS shows absorbance peaks at 1015, 1401, 1558, 2920, and 3417 cm−1, which correspond to C–O–C, C–OH, N–H, C–H, and O–H bonds, respectively [32]. The FTIR spectrum of MOF-5 displays absorbance peaks at 645, 3597, and 1758 cm−1, which are attributed to Zn4O, OH, and C–O bonds, respectively [29]. The peaks at 1495, 1572, and 1653 cm−1 are assigned to asymmetric C=O bonds, while the symmetric C=O bond appears at 1359 cm−1 [33]. The FTIR spectrum of MOF-53(Fe) reveals distinctive absorbance peaks at 518, 778, 1676, and 3342 cm−1, which are attributed to Fe–O, C–H, C = O, and OH bonds. Additionally, peaks related to asymmetric and symmetric C–O bonds are observed at 1464 and 1505 cm−1 [34]. The FTIR spectrum of MOF-5 exhibits characteristic peaks with lower intensity and a peak shift toward higher wavenumbers, suggesting the successful doping of Fe ions into MOF-5. Furthermore, the appearance of peaks at 1470 and 1567 cm−1 reflects the formation of coordination bonds between the carboxylic groups of H2BDC and the central metal ions (Zn and Fe) [35]. The spectrum of Fe25/MOF-5@CTS shows absorbance peaks of Fe25/MOF-5 and CTS with lower intensity, indicating the homogeneity of the composite film.

ZP Measurements

The net carried charges on the surface of Fe25/MOF-5@CTS were evaluated by ZP measurements (Fig. 2C). Obviously, Fe25/MOF-5@CTS is highly charged with positive charges where the ZP attained 56.3 mV at pH 3. The ZP results revealed the suitability of Fe25/MOF-5@CTS to adsorb the anionic pollutants via the electrostatic forces.

Fig. 2
figure 2

A PXRD, and B FTIR of pure CTS film, MOF-5, MIL-53(Fe), Fe25/MOF-5 bimetallic MOF, and Fe25/MOF-5@CTS composite film, and C ZP of Fe25/MOF-5@CTS composite film

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SEM

The morphologies of the fabricated samples were figured out via SEM (Fig. 3). As elucidated in Fig. 3A, the particles of MOF-5 have bipyramid-like morphology. The MIL-53(Fe) particles have a quasi-spherical morphology (Fig. 3B). The SEM image of Fe25/MOF-5 (Fig. 3C) showed poly morphologies of bipyramid and spherical, indicating the formation of the Fe25/MOF-5 bimetallic MOF. The SEM image of the air side of the pristine CTS film (Fig. 3D) revealed a flat and nonporous surface [36, 37]. The SEM of the cross-sectional pure CTS film (Fig. 3E) showed a smooth layer with some cracks. The SEM image of the air side of the Fe25/MOF-5@CTS composite film (Fig. 3G) elucidated a more rigid surface compared to the pristine CTS film, implying the positive role of Fe25/MOF-5 bimetallic MOF in boosting the mechanical strength of CTS film [38, 39]. Obviously, the cross-sectional Fe25/MOF-5@CTS composite film (Fig. 3H) showed distributed particles between the polymeric layers of CTS, inferring the incorporation of Fe25/MOF-5 into the CTS film. Figure 3F, I) points out digital photos of CTS and Fe25/MOF-5@CTS composite film. A slight change in film thickness was observed after the composite formation since the average thickness of the developed composite film recorded about 0.0894 mm compared to 0.081 mm for pure dried CTS.

Fig. 3
figure 3

SEM of A MOF-5, B MIL-53(Fe), C Fe25/MOF-5, D Air side, E Cross-sectional, and F Digital photo of CTS film, G Air side, H Cross-sectional, and I Digital photo of Fe25/MOF-5@CTS composite film

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BET Measurements

Figure 4 A elucidated the nitrogen adsorption/desorption isotherms of CTS, MOF-53(Fe), MOF-5, Fe25/MOF-5, and Fe25/MOF-5@CTS. The isotherms depict that all the fabricated samples are mesoporous substances since the isotherms represent type (IV) with H4. The SBET of CTS, MOF-53(Fe), MOF-5, Fe25/MOF-5, and Fe25/MOF-5@CTS were 41.34, 157.81, 189.26, 645.49, and 516.54 m2/g, respectively. These results confirmed the benefit of adding Fe species to MOF-5 for fabricating a novel bimetallic MOF with a higher surface area than the authentic MOFs. Besides, the remarkable role of Fe25/MOF-5 bimetallic MOF in boosting the inferior SBET of CTS and directly enhancing its adsorption capacity. Notably, the mean pore size (Fig. 4B) of CTS, MOF-53(Fe), MOF-5, Fe25/MOF-5, and Fe25/MOF-5@CTS were 1.89, 2.01, 2.26, 2.98, 2.40 nm, respectively. This finding clarified that Fe25/MOF-5 not only increases the CTS surface area but also improves its porosity.

SEM-EDX

SEM-EDX was utilized for defining the elemental composition of Fe25/MOF-5@CTS as elucidated in Fig. 4C. It was found that Fe25/MOF-5@CTS composed of C, O, Zn, Fe, and N with atomic percent 43.14, 46.83, 3.39, 0.59, and 6.05%. This finding inferred the successful combination between Fe25/MOF-5 and CTS.

Fig. 4
figure 4

BET measurements of CTS, MOF-53(Fe), MOF-5, Fe25/MOF-5, and Fe25/MOF-5@CTS; A Nitrogen adsorption/desorption isotherms, B Pore size distribution, and C SEM-EDX of Fe25/MOF-5@CTS

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XPS

The XPS wide-spectrum (Fig. 5A) implies the composition of Fe25/MOF-5@CTS composite film; C, O, N, Zn, and Fe with characteristic peaks manifested at 285.36, 532.74, 400.03, 1022.82, and 708.19 eV. The high-resolution C1s spectrum elucidated the related peaks to C–O/C–N, C–C/C–H, and O = C–O at 285.03, 284.49, and 288.37 eV (Fig. 5B). The high-resolution O1s spectrum (Fig. 5C) revealed two peaks at 533.60, and 531.77 eV which are accredited to M–O (viz., Zn–O, and Fe–O), and O = C–O. The high-resolution N1s spectrum (Fig. 5D) showed the belonging peaks to N–H at 399.54 eV. The high-resolution Fe spectrum (Fig. 5E) inferred the existence of Fe2+ at 711.19, and 725.54 eV as well as Fe3+ at 713.6 and 731.08 eV. The high-resolution Zn spectrum (Fig. 5F) pointed out the two characteristic XPS peaks to Zn2p1/2, and Zn2p3/2 at 1045.08, and 1022.03 eV.

Fig. 5
figure 5

XPS spectra of Fe25/MOF-5@CTS composite film; A Survey, B C1s, C O1s, D N1s, E Fe2p3, and F Zn2p3

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Water Uptake and Mechanical Properties

Actually, the water uptake process primarily depends on the existence of free hydrophilic functional groups in the matrix. Fig.S1 shows the water uptake profiles of the pristine CTS and Fe25/MOF-5@CTS films. The results implied higher water uptake values of the CTS film compared to Fe25/MOF-5@CTS composite film. The water uptake value of the CTS film gradually increased from 63.56 to 131.07% when the test time increased from 0.5 to 3 h, owing to the hydrophilic nature of CTS. Thus, more water molecules can bind with the free hydrophilic OH and NH2 groups of the CTS film. On the contrary, the incorporation of Fe25/MOF-5 adversely affected the affinity of chitosan towards the absorption of water molecules since the water uptake value was slightly elevated from 50.48 to 58.57% with increasing time from 0.5 to 3 h. These results may be due to the consumption of some hydrophilic groups via interaction with bi-metallic Fe/25MOF-5 during the composite formation. In addition, the mechanical properties of CTS and Fe25/MOF-5@CTS films were scrutinized, as represented in Table S1. Obviously, it was noticed an increase in the rigidity of the CTS film after the impregnation of Fe25/MOF-5. Therefore, the mechanical properties of Fe25/MOF-5@CTS composite film were improved with a maximum force of 33.90 N and maximum strain of 6.70% needed to break Fe25/MOF-5@CTS composite film compared to 29.53 N and 2.70% for neat CTS film. Improvement of the mechanical properties could be associated with the strong interactions between Fe25/MOF-5 and CTS matrix.

Optimization of the CR Adsorption

Comparison Test

The comparison test (Fig. 6) consisted of two pivotal steps; the first step comprised determining the optimal ratio of Fe3+ in Fe/MOF-5 bimetallic MOF. Then, the introduced proportion of Fe3+ was tested in the range of 0–100 wt%. The experimental results showed that the adsorption capacity of CR by MOF-5, Fe25/MOF-5, Fe50/MOF-5, Fe75/MOF-5, and MIL-53(Fe) were 22.14, 78.50, 59.94, 51.06, and 47.64 mg/g, subsequently. Fe25/MOF-5 exhibited higher removal performance toward CR than the pristine MOF-5 and MIL-53(Fe) and the other fabricated bimetallic MOFs did.

Fig. 6
figure 6

Comparison test between the fabricated adsorbents

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Next, in the second step, Fe25/MOF-5 was doped onto the CTS film at a ratio of 0–10 wt%. It was recorded that the adsorption capacity of CR onto pure CTS was 41.81 mg/g, respectively. In addition, increasing the doped proportion of Fe25/MOF-5 from 1 to 5 wt% enhanced Q of CR from 49.79 to 77.06 mg/g, subsequently. Whereas the excessive increase in the doped percent of Fe25/MOF-5 in the film declined the adsorption capacity of CR to 58.39 mg/g, respectively. This decline in the adsorption performance is most probably due to the aggregation of Fe25/MOF-5 in the CTS matrix, blocking its pores and decreasing the surface area.

Consequently, the comparison test reflected the viability of doping the Fe25/MOF-5 bimetallic MOF into the CTS film to increase the adsorption capacity of the CTS film. Besides, decreasing the bimetallic MOF’s cost by using a quite small amount of it and acquiring almost the same result as the pure bimetallic MOF.

The Influence of pH

As a rule, pH controls the pollutant’s ionization form, as well as the net surface charge of the adsorbent. Thence, the aptitude of the CR adsorption onto Fe25/MOF-5@CTS composite film was examined at a broad scale of pH as elucidated in Fig. 7A. It was observed a sharp decline in the adsorption capacity and R% from 77.06 mg/g and 77.74% to 41.22 mg/g and 42.96% when pH of CR solution elevated from 3 to 11, subsequently. Meanwhile, ZP results demonstrated the super high positive charge on the composite film’s surface (56.3 mV) at pH 3, which decreased to 2.2 mV at pH 11 as a result of deprotonation of the composite functional groups. Based on the former results, the electrostatic forces are the predominant mechanism of CR adsorption onto the Fe25/MOF-5@CTS composite film [40]. Thus, the batch adsorption experiments were performed at pH 3. Interestingly, our work is limited to pH values above a certain threshold due to the potential dissolution of the CTS film at pH values below 3.

The Influence of the Dosage of Composite Film

Figure 7B illustrates the influence of the used amount of Fe25/MOF-5@CTS on the efficacy of CR removal. The raising in the weight of the used composite film in removing CR from 0.01 to 0.02 g boosted the CR removal% from 66.06 to 93.98%, subsequently. Such an enhancement in the CR removal aptitude can be explained by increasing the accessible active sites by elevating the composite film proportion [41]. Conversely, the increment in the proportion of Fe25/MOF-5@CTS dwindled the capacity of the adsorbed CR compared to available binding sites and incremented the empty binding sites [42].

The Influence of System Temperature

The thermodynamic performance of the system was deduced at a temperature range of 25–55 °C (Fig. 7C). It was monitored that this elevating in the system temperature sharply declined the R% and Q of CR onto Fe25/MOF-5@CTS from 77.74%, and 77.06 mg/g to 39.95%, and 38.12 mg/g, respectively, reflecting the exothermic behavior of the system. This drop in the CR removal aptitude is most likely due to the acceleration in the kinetic energy of the adsorbed CR, tending to the increase in the CR desorption probability. Furthermore, the elevation in the temperature increments the CR Brownian motion, decreasing the adsorption probability of CR onto Fe25/MOF-5@CTS [43].

The Influence of the CR Concentration

Indeed, it is an essential point during setting up any adsorption system to identify the experimental maximum capacity (Qmax) of the adsorbent toward the targeted pollutants. Thence, the CR adsorption onto Fe25/MOF-5@CTS composite film was scrutinized at various concentrations of CR. Figure 7D implied an elevation in the Qt of CR onto the composite film when the CR concentration uplifted from 50 to 300 ppm, revealing that the experimental Qmax was 201.81 mg/L. The acquired result could be explained by increasing the contaminant’s driving forces toward the adsorbent surface by elevating the contaminant concentrations [44].

Fig. 7
figure 7

Effect of A pH, B Dose, C Temperature, and D Initial concentration on the capacity of the CR adsorption onto Fe25/MOF-5@CTS composite film

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The Adsorption Mechanism of CR

Kinetics Study

For estimating the adsorption mechanism of CR onto Fe25/MOF-5@CTS composite film, the experimental results were analyzed kinetically by which Pseudo-1st -Order (PFO), Pseudo-2nd -Order (PSO), and Elovich (Fig. S2A-D). Table S2 represents the kinetic equations of the applied models. It was inferred the proceeding of the CR adsorption onto Fe25/MOF-5@CTS through the chemisorption pathway since the reckoned R2 by PSO were greater than those of PFO (Table 1). Furthermore, the resemblance between Qe,Exp, and the derived Qe,Cal from PSO is the second evidence of the chemisorption of CR [45]. Moreover, Elovich indicated the adsorption favorability of CR onto Fe25/MOF-5@CTS composite film in which the CR adsorption rate (α = 2.39–33.77 mg/g.min) was much faster than the desorption rate (β = 0.087−0.027 g/mg).

Table 1 Adsorption kinetic model parameters of the adsorption of CR onto Fe25/MOF-5@CTS composite film
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Isotherms Study

Also, the obtained data from the experimental work were modeled isothermally by DR, Freundlich, Langmuir, and Temkin (Fig. S3A-D) to deduce the adsorption pathway of CR onto Fe25/MOF-5@CTS composite film. Table S3 summarizes the isotherm equations of these models. Isotherms analysis concluded the probability to adsorb CR onto Fe25/MOF-5@CTS via both chemisorption and physisorption pathways in which the R2 values of Langmuir ( 0.977) and Freundlich (0.965) are almost similar (Table 2). Langmuir deduced that the Qmax of CR onto the composite film was 219.78 mg/g. In addition, Freundlich confirmed the CR adsorption preference onto the as-fabricated composite film in which n was over 2. Temkin suggested the physisorption mechanism of CR since b was 0.056 KJ/mol [46]. This finding was consistent with DR, which also indicated the proceeding of the adsorption of CR onto Fe25/MOF-5@CTS by physisorption in which E was 0.292 KJ/mol [47].

Table 2 The parameters derived from isotherm models for the adsorption of the adsorption of CR onto Fe25/MOF-5@CTS composite film
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Hence, the acquired findings from kinetics and isotherms investigations denoted the possibility of the CR adsorption onto Fe25/MOF-5@CTS via physisorption and chemisorption pathways. So, Fe25/MOF-5@CTS composite film was analyzed by XPS after the CR adsorption to interpret the CR adsorption mechanisms. The XPS wide-spectrum (Fig. 8A) confirmed the CR adsorption onto Fe25/MOF-5@CTS in which the related peaks to S manifested at 168.22 eV with an increase in the N1s peak intensity. The high-resolution S2p3 spectrum (Fig. 8B) showed the two corresponding peaks to SO3 groups of CR, S = O, and S–O at 167.85 and 169.92 eV. Besides, the high-resolution O1s spectrum (Fig. 8C) illustrated the concomitant peak of S = O at 532.38 eV. The mentioned findings confirmed the occurrence of the adsorption of CR onto the Fe25/MOF-5@CTS. The peaks shifting of M–O and O = C–O from 531.77 and 533.60 to 531.32 and 533.03 eV, suggest the contribution of the metal ions and the oxygen-containing functional groups of Fe25/MOF-5@CTS in the CR adsorption. Moreover, high-resolution Fe2p3 (Fig. 8D) and Zn2p3 (Fig. 8E) spectra elucidated peaks shifting to lower binding energy, which may be due to the formation of coordination bonds between Fe and Zn of Fe25/MOF-5@CTS and SO3 and NH2 of CR. Notably, it was previously deduced via the electron density map that the HOMO orbital’s position in SO3 of CR could interact with NH2-containing adsorbent by host-guest interaction and electrostatic forces [48]. ZP and the impact of the pH medium results implied the vast prominence of the electrostatic force between the anionic CR and the highly positively charged Fe25/MOF-5@CTS composite film (ZP = 56.3 mV at pH 3). Moreover, host-gest interaction between CR and Fe25/MOF-5@CTS could occur via the H-bonding between the provided oxygenated groups on the composite film’s surface (viz., COOH, OH) and H atoms of CR. Also, the SO3 groups of CR could form an H-bonding with the H-atoms of Fe25/MOF-5@CTS. Noteworthy, the aromatic ring in both composite film and CR undoubtedly has an effective role in the CR adsorption onto Fe25/MOF-5@CTS via π-π interaction.

Fig. 8
figure 8

XPS of Fe25/MOF-5@CTS composite film after the CR adsorption; A Survey, B S2p3, C O1s, D Fe2p3, and E Zn2p3

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Shortly, kinetics and isotherms studies confirmed the CR adsorption by both chemisorption and physisorption pathways. In addition, XPS and ZP analyses and the experimental work indicated the proceeding of CR adsorption by physisorption pathway via electrostatic forces. Meanwhile, the chemisorption pathway of the CR adsorption could proceed via host-guest interaction, π-π interaction, and coordination bonds, as illustrated in Fig. 9. Interestingly, these clues suggested the suitability of our perspective that the small amount of the doped bimetallic MOF with varied functional groups could form strong interactions with CR and vastly boost the adsorbability of the CTS film.

Fig. 9
figure 9

The plausible mechanism of the CR adsorption onto Fe25/MOF-5@CTS composite film

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The Impact of Ionic Strength

Figure 10 A exhibits the negative impact of the ionic strength on the capacity of the CR removal by Fe25/MOF-5@CTS composite film. It was observed a drop in the efficacy of the removal of CR since Q and R% declined to 50.95 mg/L and 54.47% by raising the NaCl concentrations to 1.0 mol/L. Such a drop in the CR removal aptitude by Fe25/MOF-5@CTS may be attributed to the competition between Cl− ions and CR for the positive active groups onto the composite film’s surface, which was well consistent with Khan et al. [49].

Selectivity Test

For testing the adsorption selectivity of Fe25/MOF-5@CTS composite film toward CR, the removal capacity of other five common synthetic dyes by the as-fabricated composite film were scrutinized at the same adsorption conditions (Fig. 10B). Surprisingly, Fe25/MOF-5@CTS exhibited a superior removal capacity of CR compared to the other cationic and anionic dyes in which R% of CR, MR, MO, MB, and CV were 78.74, 25.91, 25.13, 11.87, and 13.75%, subsequently. The concrete results reflected the more eminent selectivity of Fe25/MOF-5@CTS towards the anionic dyes, especially CR than the cationic dyes. This selectivity behavior is most likely due to the plenty of positive active groups on the surface of the composite film that enable it to attract the anionic dyes via the electrostatic forces. The higher removal aptitude of Fe25/MOF-5@CTS towards CR that attained 3-folds of the other anionic dyes is most probably owing to the plenty of NH2 onto the CR surface that could be attached to the metals of the composite film via the coordination bonds. While both MO and MR contain only N(CH3)2 group which has a lower affinity to form coordination bonds with Fe25/MOF-5@CTS. In addition to the abundant SO3− in the CR structure that could interact with Fe25/MOF-5@CTS via electrostatic and host-guest interactions. Whereas MO contains less number of SO3− group than CR, thus the interactions between MO and Fe25/MOF-5@CTS are weaker than those of CR. Notably, the difficulty of deprotonation of the COOH group of MR in the acidic medium prevents the occurrence of electrostatic interactions between MO and Fe25/MOF-5@CTS.

Cycling Test

To prove the viability of our strategy to fabricate efficient and recyclable adsorbent with reduced cost, it was an intrinsic issue to assess the renewability of Fe25/MOF-5@CTS composite film. The cycling test (Fig. 10C) revealed the supreme reusability of the as-fabricated composite film for removing CR with a trivial loss in its adsorption capacity was about 18.54 mg/g after ten successive cycles. The inconsiderable decrease in the adsorption capacity of Fe25/MOF-5@CTS may be due to the strong chemical interactions between the composite film and CR that prevent the complete elution of CR. Finally, such auspicious results suggested the excellent efficacy, stability, selectivity, and recyclability of Fe25/MOF-5@CTS composite film and its viability to expand out of lab scale to industrial scale.

Fig. 10
figure 10

A Effect of ionic strength on the CR adsorption, B Selectivity test, and C Cycling test of Fe25/MOF-5@CTS composite film

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Durability Study

To prove the durability of Fe25/MOF-5@CTS, the crystallinity and SBET of the used composite film were characterized using XRD and BET. In addition, the metal leaching was examined by analyzing the authentic and used composite films utilizing SEM-EDX. Figure 11A illustrates the XRD of pristine and used Fe25/MOF-5@CTS composite films. It was noticed the appearance of the same diffraction peaks of Fe25/MOF-5@CTS after the CR adsorption with a slight diminution in the peak intensity. These observations confirmed the stability of the chemical structure of Fe25/MOF-5@CTS, while the slight decline in the Fe25/MOF-5@CTS crystallinity confirmed proceeding the adsorption process of CR onto the composite film’s surface. The BET measurements (Fig. 11B) clarified a slight decrease in the SBET of Fe25/MOF-5@CTS from 516.54 to 414.22 m2/g after the CR adsorption owes to occupying the surface and the pores of the composite film by the CR molecules. SEM-EDX of the used Fe25/MOF-5@CTS composite film (Fig. 11C) revealed inconsiderable diminution in the atomic% of Zn and Fe species by about 0.09 and 0.04%, respectively. In addition, the increase in the atomic% of nitrogen and the manifestation of the distinctive peaks of sulfur, indicate the CR adsorption onto Fe25/MOF-5@CTS. The stable chemical structure, slight decrease in SBET, and low metal leaching after the CR adsorption proved the durability of Fe25/MOF-5@CTS composite film.

Fig. 11
figure 11

A XRD, B BET measurements of fresh and used Fe25/MOF-5@CTS, and C SEM-EDX of used Fe25/MOF-5@CTS

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Conclusion

The study utilized a cost-effective approach to foster the adsorption capacity of the CTS film by decorating it with a small proportion of Fe25/MOF-5 bimetallic MOF, forming an efficient and durable composite film. The ZP results showed the frequently high positive charges onto the Fe25/MOF-5@CTS composite film attained 56.3 mV. The water uptake test exhibited the lower water uptake performance of the Fe25/MOF-5@CTS composite film than the neat CTS. In addition to the advanced mechanical property of the Fe25/MOF-5@CTS composite film, compared to the CTS film. The practical experimental showed that the optimal conditions to adsorb CR onto Fe25/MOF-5@CTS were at pH = 3 and using a composite film dosage of about 0.01 g at room temperature. The selectivity test confirmed the high affinity of the as-fabricated composite film toward the CR molecules. Furthermore, the cycling test affirmed the viability of the used strategy to obtain a durable composite film with recyclability merit, where the loss in the adsorption aptitude of Fe25/MOF-5@CTS composite film after the 10th cycle was 18.54 mg/g. Isotherms study indicated the appropriateness of both Langmuir and Freundlich to represent the CR adsorption onto Fe25/MOF-5@CTS. Whereas, the kinetics study denoted the aptness of PSO to symbolize the adsorption of CR. Host-guest interaction, π-π interaction, electrostatic interaction and coordination bonds were found to be the possible interactions between CR molecules and Fe25/MOF-5@CTS. Ultimately, the characterization tools and the practical experiments proved the viability of this technique to boost the adsorption capacity of CTS by doping its matrix with a small amount with the efficient Fe25/MOF-5@CTS. Interestingly, this strategy renders the adsorption capacity of the cost-effectiveness CTS almost similar to the quite expensive materials like MOFs.