A Porous and Amorphous Zn-Doped Fe Metal–Organic Framework for the Efficient Adsorptive Removal of Methylene Blue Dye from Wastewater Samples

Abstract

The remediation of dye-contaminated wastewater is increasingly important due to the growing global dye industry, particularly in developing countries. In this context, porous and amorphous modified metal–organic frameworks have gained significant attention for their potential to remove methylene blue (MB) dye. However, the current challenge is to achieve a delicate balance between maintaining structural stability and ensuring high porosity while deliberately inducing disorder. To address this, an amorphous Zn-doped Fe-MOF, specifically FZM-10:2, was synthesized using a modified hydrothermal method, which involved adjusting the iron-to-zinc ratio to enhance MB removal. The maximum adsorption capacity for MB was determined to be 208.3 mg g^–1 at an Fe/Zn ratio of 10:2 (FZM-10:2). Consequently, the Langmuir isotherm model was used to explain adsorption equilibrium, while the pseudo-second-order model was used to investigate adsorption kinetics. Thermodynamic analysis revealed a positive enthalpy change (+107.6 kJ mol^–1), indicating that the process is endothermic and dominated by chemisorption. Negative Gibbs free energy (ΔG°) values confirmed the spontaneous nature of adsorption. The FZM-10:2 adsorbent exhibited excellent selectivity and stability toward MB in mixed-dye systems. Furthermore, an MB removal efficiency of 98% was achieved in wastewater at pH 8, suggesting a scalable adsorption method.

1. Introduction

Global environmental pollution, which includes the contamination of air, water, and soil, poses a serious threat to the health of all living beings, including humans. − Many industries, such as textiles, leather, dyes, and paper, utilize synthetic dyes and a large amount of water to color their products, subsequently releasing wastewater into the environment and water bodies. − Each year, over 2.8 billion kilograms of wastewater are discharged from the industrial sector. The typical concentration of dye in textile effluent is approximately 0.3 g/L; however, even a small amount is highly undesirable and harmful to the ecosystem. Dyes are nonbiodegradable, chemically stable, and negatively impact the environment and human health. They disrupt natural water supplies and pose serious toxicological and carcinogenic risks. , Furthermore, the main diseases caused by dyes in fish include gill diseases, which can result in conditions such as telangiectasia, structural changes, and the formation of nuclear buds. , Because of their chemical nature and color, dyes are highly conspicuous and can lead to numerous pollution issues. ,

Methylene blue (MB) is a typical colorant that poses health risks, such as eye and skin damage, when consumed directly by humans or animals. It can also negatively effect on living organisms, causing gastrointestinal distress, nausea, loose stools, and abnormal sweating when ingested with food. ,, Therefore, eliminating colorants from contaminated water is a primary environmental issue. , There have been occasional reports of various water treatment techniques employed to reduce and regulate water pollution, including photocatalytic degradation, precipitation, and others. − However, current treatment approaches face several challenges, including high operating and maintenance costs, complex procedures, and the generation of toxic waste and byproducts. In contrast, adsorption using porous materials is inexpensive, simple to use, easy to construct, and effective, making it the most promising method for water purification among those documented. ,

Numerous porous adsorbents, such as organic clay-chitosan composites, charcoal, and polymers, are commonly used to treat wastewater. , Despite significant advancements in this area, there are still inherent disadvantages, including a somewhat challenging synthesis process, selective adsorption, limited adsorption kinetics, and a very restricted adsorption capacity. Consequently, there is a considerable interest in investigating novel materials to address these issues. A new class of materials with pores, known as metal–organic frameworks (MOFs), is considered one of the most promising options for removing pollutants due to their numerous metal sites, large surface area, adjustable porosity, chemical sustainability, and diverse frameworks and arrangements. ,− Recently, amorphous metal–organic frameworks (aMOFs) have emerged as a hot topic of interest due to their potential for large-scale synthesis, processing, and shaping. , They have extensive applications in adsorption and other areas due to their highly modifiable characteristics. − Various techniques for producing MOFs in the laboratory and industry have been documented, ranging from standard solvothermal and hydrothermal synthesis to dry gel conversion and microfluidic synthesis. , By altering manufacturing conditions, preparation methods, and chemical composition, the shapes and characteristics of MOFs can be easily modified.

Iron-based MOFs have attracted interest due to their environmental friendliness, low cost, and capacity to stimulate oxidation processes through redox-active iron centers. However, pure Fe-MOFs often have drawbacks, such as limited or restricted adsorption active sites and moderate or weak stability. Therefore, several modification techniques should be developed to enhance the application of MOFs and improve their adsorptive ability for hazardous chemicals. These techniques include loading, the introduction of uncoordinated functional groups, and doping. The adsorption efficiency of iron-based MOFs for hazardous chemicals can be significantly improved by doping with nonprecious metals. Among these, zinc (Zn) is an appropriate dopant for such modification as it is a transition metal that is both adaptable and relatively nontoxic.

In this study, an amorphous Zn-doped Fe-MOF (FZM-10:2) was synthesized using a straightforward and scalable hydrothermal method. The Fe³⁺/Zn²⁺ ratio was intentionally varied to assess its effectiveness in adsorbing and removing MB dye from aqueous solutions. The unique characteristics, including surface morphology and structure of the as-synthesized materials, were investigated using XRD, FTIR, SEM, XPS, and SEM/EDX techniques. The influences of key working parameters, such as temperature, initial dye concentration, adsorbent dosage, solution pH, and contact time, were systematically examined and optimized. Additionally, the adsorption mechanisms were explored through postcharacterization, thermodynamic studies, kinetics, and in-depth analysis of adsorption isotherm models. Multiple regeneration and reuse cycles were conducted to evaluate the reusability and stability of the adsorbent. This study contributes to the broader application of MOF-based adsorbents in environmental remediation by providing new insights into the development of modified amorphous MOF materials for the effective removal of organic dyes from wastewater.

2. Experimental Section

2.1. Chemicals and Materials

The chemicals used in this study include iron­(III) chloride hexahydrate (99%), zinc chloride hexahydrate (>98%), methylene blue dye (95%), sodium hydroxide (98%), 1,3,5-benzenetricarboxylic acid (trimesic acid, 98%), N,N-dimethylformamide (99%), methanol (99%), and hydrochloric acid (37%), all purchased from Fisher Scientific. A stock solution of methylene blue was prepared by diluting a standard MB solution with deionized water (DI). For calcination, a muffle furnace (model 6104) was used, a digital balance (ESJ-200–4, Japan) was used for sample weighing, and an oven (DHG-9070, Japan) was used for drying the materials. Various sizes of volumetric and conical flasks (50–1000 mL), Whatman filter paper (42.5 mm), and a Systronic digital pH meter (model 802) were used for pH measurements. Micropipettes and appropriately sized volumetric flasks were employed to quantify the volumes of reagents and solutions. All analytical-grade chemicals were commercially available and used without modification to their purity.

2.2. Synthesis of Fe-MOF and Zn−doped Fe−MOFs

In this study, a monometallic Fe-MOF and Zn-doped Fe-MOFs with various Fe³⁺/Zn²⁺ molar ratios (FeZn-MOF-11:1, FeZn-MOF-10:2, FeZn-MOF-8:4, FeZn-MOF-6:6) were prepared using a slightly modified hydrothermal method. For instance, to prepare FeZn-MOF-10:2, 0.27 g of FeCl₃.6H₂O (1.0 mmol) and 0.0489 g of ZnCl₂.6H₂O (0.20 mmol) were dissolved in 16 mL of a 1:1 water/N,N-dimethylformamide mixture and stirred for 25 min to form solution A. Meanwhile, 0.252 g of 1,3,5-benzenetricarboxylic acid (1.2 mmol) was dissolved in 16 mL of a 1:1 water/ethanol mixture to prepare solution B. Solution B was then added to solution A and stirred for 30 min. The resulting mixture was transferred into a 50 mL stainless-steel autoclave lined with Teflon. The mixture in the autoclave was treated at 120 °C for 6 h. Afterward, it was removed from the oven and allowed to cool to ambient temperature. Centrifugation was applied to separate the products, followed by washing with deionized water, ethanol, and DMF three times each, and then drying for 3 h at 50 °C. Similarly, the FeZn-MOF-11:1, FeZn-MOF-8:4, and FeZn-MOF-6:6 materials were synthesized using the same procedure, with Fe and Zn molar ratios of 11:1, 8:4, and 6:6, respectively. The total molar amount of Fe³⁺ and Zn²⁺ was fixed at 12 mmol. The monometallic Fe-MOF was also prepared using a similar approach, excluding the precursor containing ZnCl₂.6H₂O. For convenience, the as-prepared materials in this study, the monometallic Fe-MOF and the Zn-doped Fe-MOFs (FeZn-MOF-11:1, FeZn-MOF-10:2, FeZn-MOF-8:4, and FeZn-MOF-6:6), were denoted as FZM-12:0, FZM-11:1, FZM-10:2, FZM-8:4, and FZM-6:6, respectively. Finally, all synthesized MOFs were stored appropriately for further characterization and application experiments.

2.3. Material Characterizations

To assess the crystallinity of the samples, X-ray diffraction (XRD) was conducted using a wavelength (λ) of 1.5406 Å from Cu Kα radiation. A NEXUS-670 FT-IR instrument was employed to investigate the characteristic functional groups in the material. The elemental composition of the produced adsorbents was analyzed using X-ray photoelectron spectroscopy (XPS), while field emission scanning electron microscopy (SEM) was applied to study their morphology. The zeta potential of the adsorbent was measured using a Malvern Zetasizer Nano ZS90. Finally, a UV–vis spectrophotometer (UV-1800, Shimadzu, Japan), operated at 665 nm, was used to determine the concentration of methylene blue (MB) in both synthetic and real samples.

2.4. Batch Adsorption Process

This process utilized FZM-10:2 as the adsorbent in 0.1 mL Erlenmeyer flasks. A solution was prepared by adding 40 mg of the adsorbent to 50 mL of a 100 mg/L methylene blue solution. The pH was adjusted to 2, 4, 6, 8, 10, and 12 using 0.1 M NaOH and HCl. The mixture was stirred for 70 min at 120 rpm and at ambient temperature. After stirring, 5 mL of the solution was withdrawn and clarified to analyze the MB content using a UV–vis spectrophotometer at 665 nm. The effects of the adsorbent dose, contact time, stirring speed, and initial MB concentration were evaluated by varying these parameters within the ranges of 10–60 mg, 10–110 min, 40–200 rpm, and 10–160 mg/L, respectively. The removal efficiency of MB by FZM-12:0 and FZM-10:2 was determined using eq

$$\%\mathrm{Removal}=\frac{(C_0-C_e)}{C_0}\times 100$$ 1

where C_e is the concentration at equilibrium, while C ₀ is the initial concentration of MB (mg/L). Furthermore, the adsorption capacity of MB on FZM-12:0 and FZM-10:2 was obtained employing eq :

$$q_e=\frac{(C_0-C_e)V}{m}$$ 2

where C_e and C ₀ are the equilibrium and initial concentrations of MB (mg/L), respectively, and m and V stand for the mass of adsorbent (in grams) and batch volume of MB (in liters), respectively.

2.5. Isotherm and Kinetic Studies

The adsorption performance of the prepared materials for MB at concentrations ranging from 10 to 160 mg/L was investigated using several models, including Freundlich, Langmuir, Dubinin–Radushkevich, and Temkin. The assessment was conducted under the following conditions: a stirring speed of 120 rpm, a pH of 8, a temperature of 25 °C, a contact time of 70 min, and an adsorbent dosage of 40 mg. Additionally, pseudo-second-order, pseudo-first-order, intraparticle diffusion, and Boyd models were applied to examine the adsorption kinetics, focusing on MB uptake over various interaction times (see Supporting Information for further details).

2.6. Reusability of FZM-10:2 Adsorbent

Reusability experiments were conducted to evaluate the regeneration and adsorption performance of the FZM-10:2 adsorbent for methylene blue (MB). Desorption studies utilized 0.1 M HCl, 95% methanol, and 0.1 M NaOH as regenerating agents. In a typical adsorption cycle, 40 mg of FZM-10:2 was added to 50 mL of a 100 mg L^–1 MB solution in an Erlenmeyer flask, then agitated at 298 K and 120 rpm for 40 min. The spent adsorbent was recovered by centrifugation, washed with deionized water, and dried at 343 K. Desorption was achieved by immersing the dried FZM-10:2/MB composite in 20 mL of HCl and stirring at 120 rpm for 70 min. The regenerated adsorbent was then separated and dried at 333 K for 2 h before to reuse in subsequent adsorption cycles. All experiments were performed in triplicate.

2.7. Concentrations of MB in Real and Simulated Wastewater Samples

The concentration of MB in wastewater and synthetic samples was determined using UV–vis spectroscopy (UV-1800, Shimadzu, Japan) at a wavelength of 665 nm. A standard concentration of MB at 5, 25, 45, 65, 85, and 105 mg L^–1 was used to construct a six-point calibration curve prior to measuring the concentration of MB in the wastewater samples. To evaluate the selectivity of the synthesized material, a simulated wastewater solution was prepared containing 100 mg/L each of methylene blue, methyl orange (MO), rhodamine 6B (RhB), and procion brilliant blue (PBB). The adsorption experiment involved adding 300 mg of FZM-10:2 to 100 mL of the simulated wastewater at pH 8, a temperature of 298 K, and a contact time of 2.4 h. The concentration of each dye was then measured using UV–vis at the following wavelengths: 554 nm for RhB, 665 nm for MB, 450 nm for MO, and 594 nm for PBB.

3. Results and Discussion

3.1. Structural Studies

The phase and crystal structure of the as-prepared materials were examined using X-ray diffraction (XRD) measurements. Figure a presents the XRD patterns of the monometallic Fe-MOF and Zn-doped Fe-MOFs (FZM-x), which were prepared with varying Fe³⁺/Zn²⁺ molar ratios. All synthesized materials exhibited two broad peaks at 2θ values of 8.6° and 25.5°, corresponding to the (100) and (300) planes of the Fe-MOF, respectively. Although the peaks were broad, their positions closely aligned with the XRD patterns of the simulated Fe-MOF, indicating the successful synthesis of the Fe-MOF with an amorphous structure. Notably, the Fe-MOFs modified with different amounts of Zn displayed characteristics similar to those of the monometallic MOF sample, with no new peaks corresponding to Zn or ZnO detected. This indicates the structural integrity of the zinc dopant within the Fe-MOF framework. Additionally, there was no significant change in the peak features, except for a slight shift to lower values and a decrease in peak intensity as Zn loading increased, suggesting the incorporation of zinc into the Fe-MOF structure.

1.

XRD patterns (a) and FTIR spectra (b) of monometallic Fe-MOF and Zn-doped Fe-MOFs (FZM-11:1, FZM-10:2, FZM-8:4, and FZM-6:6).

Furthermore, FTIR was used to investigate the functional and coordination groups in both the monometallic and Zn-doped Fe-MOFs. Figure b presents the FTIR spectra for the monometallic Fe-MOF and the Zn-doped Fe-MOFs (FZM-11:1, FZM-10:2, FZM-8:4, and FZM-6:6). The spectra for both types of materials exhibit similar vibrational features, indicating that the addition of Zn²⁺ ions does not disrupt the primary MOF framework. The broad band centered at 3154 cm^–1 is attributed to the O–H stretching vibrations of coordinated water molecules and/or surface-adsorbed hydroxyl groups. The peak near 1600 cm^–1 is assigned to the symmetric and asymmetric stretching vibrations of the carboxylate (−COO^–) groups from the organic linker coordinated to the metal centers. The band located at 1067 cm^–1 is associated with C–O stretching vibrations of the carboxylate from the organic linker. The low-wavenumber bands between 400 and 600 cm^–1 are attributed to Fe–O and Zn–O stretching vibrations, characteristic of the metal–oxygen clusters within the MOF. The Fe–O vibration bands shift slightly toward higher wavenumbers (from ∼437 to ∼441 cm^–1) as Zn doping increases, suggesting the formation of Zn–O bonds and a modification of the local coordination environment around the metal centers. Additionally, slight shifts in the carboxylate stretching bands indicate a possible interaction between Zn²⁺ and the organic linker. Crucially, the absence of additional absorption peaks shows that the Zn atoms are integrated into the Fe-MOF framework rather than forming distinct Zn-based phases, such as ZnO, which is consistent with the XRD data.

3.2. Morphological Studies

The morphological behaviors of monometallic Fe-MOF and Zn-doped Fe-MOFs (FZM-x), prepared with different Fe³⁺/Zn²⁺ molar ratios, were examined using SEM at various magnifications (Figure ). The single-metal iron MOFs (Figure a1–a3) exhibit a dense microstructure with low intrinsic porosity, characterized by irregularly shaped aggregates formed by the compact arrangement of ultrafine particles with relatively fine sizes. The binary metal MOF (FZM-11:1) with an Fe³⁺ /Zn²⁺ ratio of 11:1 did not show distinct morphological features compared to the single-metal Fe-MOF (Figure S1), suggesting insufficient charge to induce morphological changes. However, altering the Fe³⁺ /Zn²⁺ molar ratio to 10:2 (FZM-10:2) resulted in a pronounced morphological transformation. Figure b1–b3 shows a three-dimensional, hierarchical, porous structure formed through the interconnection of particles via neck-like structures. This configuration is expected to significantly increase the accessible surface area and diffusion pathways. Changing the molar ratio of Fe³ + /Zn²⁺ to 8:4 (FZM-8:4) resulted in partial particle coalescence (Figure c1–c). While the porous framework remains visible in these SEM images, the interconnectivity of the pores is somewhat reduced, suggesting a moderate decrease in effective porosity, likely due to partial pore collapse or merging during synthesis. Figure d1–d3 presents SEM images of the Zn-doped Fe-MOF at the lowest Fe³ + /Zn²⁺ molar ratio of 6:6 (FZM-6:6). These images reveal extensive particle aggregation, resulting in denser, larger structures. The surface becomes smoother, and the number of visible pores is markedly reduced, indicating a significant loss of porous architecture and diminished pore accessibility.

2.

SEM images of monometallic Fe-MOF (a1-a3), FZM-10:2 (b1-b3), FZM-8:4 (c1-c3), and FZM-6:6 (d1-d3) samples at different magnifications.

3.3. Chemical Analysis

The XPS was used to analyze the chemical composition of the materials. Based on this analysis, Figure a presents the survey spectra of the monometallic Fe-MOF and the Zn-doped Fe-MOFs (FZM-x), prepared with Fe³⁺/Zn²⁺ molar ratios of 10:2 (FZM-10:2). The monometallic Fe-MOF primarily exhibits signals for Fe 2p, C ls, and O 1s, while the Zn-doped FZM-10:2 samples display an additional peak corresponding to Zn 2p orbitals. This finding indicates that Zn was incorporated into the Fe-MOF structure. The high-resolution core-level spectra for Zn 2p, O 1s, and Fe 2p of the Zn-doped Fe-MOF materials and the monometallic Fe-MOF material are shown in Figure . As illustrated in Figure b, the Fe 2p_3/2 and Fe 2p_1/2 peaks of monometallic Fe-MOF are centered at binding energies of 711.3 and 724.9 eV, respectively, indicating that iron exists predominantly in the Fe­(III) oxidation state. The presence of a satellite peak at 719.2 eV, approximately 8 eV higher than the Fe 2p_3/2 main peak, further corroborates the trivalent state of Fe. The Zn-doped Fe-MOF materials (FZM-10:2, FZM-8:4, and FZM-6:6) exhibit Fe 2p spectral features similar to those of monometallic Fe-MOF; however, a systematic shift of the Fe 2p peaks toward lower binding energies is observed upon Zn incorporation. This suggests a modification of the local electronic structure of Fe centers due to the incorporation of Zn⁴³. The Zn 2p high-resolution spectra of the FZM-10:2, FZM-8:4, and FZM-6:6 samples are shown in Figure c. The Zn 2p_1/2 peak appeared at 1021.1 eV and the Zn 2p_3/2 peak appeared at 1044.2 eV are consistent with the presence of Zn²⁺ species. This shows a successful growth of the Zn–Fe-MOF grains, which indicates the Zn-doped Fe-MOF materials exhibit a positive binding energy shift for Zn 2p peaks compared with the Zn reference sample. Moreover, the Zn 2p peak intensity increases as the Fe³⁺/Zn²⁺ ratio decreases from 10:2 to 6:6, demonstrating a comparative increase in Zn content. Quantitative analysis based on XPS peak area ratios reveals that the metal precursor molar ratios and the Fe/Zn atomic ratios in FZM-10:2, FZM-8:4, and FZM-6:6 are in harmony with each other as shown in Table S1. The spectra for O 1s in monometallic Fe-MOF and Zn-doped Fe-MOF (FZM-10:2, FZM-8:4, and FZM-6:6) samples are presented in Figure d. The O 1s spectra of the monometallic Fe-MOF and Zn-doped Fe-MOFs (FZM-10:2, FZM-8:4, and FZM-6:6) MOF samples are presented in Figure d. The dominant peak at ∼529.7 eV is attributed to metal–oxygen coordination bonds (Fe–O and Zn–O) within the framework, while the higher-binding-energy component at ∼531.5 eV corresponds to surface hydroxyl (O–H) species or adsorbed water molecules. Notably, the O 1s peaks of the Zn-doped Fe-MOF samples are shifted toward higher binding energies relative to monometallic Fe-MOF, indicating altered electron density around oxygen atoms due to Zn incorporation and further supporting the formation of bimetallic coordination environments. Although the low-binding-energy O 1s component may resemble oxide-like oxygen species, it is more appropriately associated with metal–oxygen coordination within the MOF structure. Importantly, the Fe 2p spectra do not exhibit features characteristic of bulk iron oxide phases (such as Fe²⁺ signals), suggesting that the coordination environment of Fe³⁺ is largely preserved. Nevertheless, considering the surface-sensitive nature of XPS, minor surface oxidation or defect-related oxygen species cannot be entirely excluded. Therefore, the obtained materials are more accurately described as Zn-doped Fe-MOFs with preserved framework structure and possible surface hydroxylation, rather than strictly phase-pure FeZn-MOFs.

3.

XPS survey spectra of monometallic Fe-MOF and Zn-doped Fe-MOF FZM-10:2 (a); XPS spectra of Fe 2p (b), Zn 2p (c), and O 1s (d) for monometallic Fe-MOF, Zn-doped Fe-MOFs (FZM-10:2, FZM-8:4, and FZM-6:6 samples).

ICP-OES and EDXS were employed to determine the chemical content of the as-prepared materials (Table S1). This analysis reveals that the Zn and Fe in the Zn-doped Fe-MOF materials directly correspond to the metal precursor molar ratios used during preparation. Figure shows the SEM-EDXS elemental mapping of the FZM-10:2 sample, demonstrating a homogeneous micrometer-scale distribution of iron (Fe), zinc (Zn), and oxygen (O). The relative elemental concentrations observed in the EDXS analysis are consistent with the ICP-OES results. In contrast, the monometallic Fe-MOF sample exhibits a nonuniform elemental distribution (see Figure S3), which can be attributed to the absence of Zn. This observation suggests that the modified MOF incorporating Zn²⁺ promotes improved chemical distribution during grain growth. We conducted nitrogen physisorption isotherms to examine the pore size distribution and surface area of the monometallic iron MOF (Fe-MOF) and the FZM-10:2 MOF (Figure S2). The N2 isotherm (Figure S2a) reveals both type II and type IV characteristics, indicating the presence of mesopores and macropores within the MOF structure. The BET surface area was found to be 146.8 m² g^–1 for Cu and 132.7 m² g^–1 for FZM-10:2 and Fe-MOF, respectively. We also evaluated the pore size distribution using the BJH method. Figure S2b shows the BJH pore size distribution for both FZM-10:2 and Fe-MOF, with the main peak for both samples centered at 38.9 nm. The FZM-10:2 exhibits a higher pore volume (0.015 cm³ g^–1) compared to the monometallic Fe-MOF (0.011 cm³ g^–1), further demonstrating the presence of larger mesopores and macropores in the FZM-10:2 structure.

4.

SEM image (a) and corresponding EDX elemental maps for iron (b), oxygen (c), carbon (d), zinc (e), and an overlay image (f) for the FZM-10:2 sample.

3.4. Impact of Adsorption Parameters

3.4.1. Fe-to-Zn Ratio

Monometallic Fe-MOFs and Zn-doped Fe-MOFs (FZM-11:1, FZM-10:2, FZM-8:4, and FZM-6:6) were synthesized to evaluate their effectiveness in absorbing methylene blue from wastewater samples. As illustrated in Figure S4a, a strong correlation was observed between the calibration curve and MB concentration. Under consistent experimental conditions (40 mg of adsorbent, 120 rpm shaking speed, 100 mg/L MB concentration, and a temperature of 25 °C over 70 min of contact time), the efficiencies of pollutant adsorption for monometallic Fe-MOF, FZM-11:1, FZM-10:2, FZM-8:4, and FZM-6:6 were 58.5%, 67.6%, 99.4%, 82.5%, and 41.2%, respectively (Figure S4b). The maximum performance observed is attributed to optimal Zn levels in FZM-10:2, which enhance porosity and the availability of active sites. In contrast, excessive Zn may result in lower efficiency for FZM-6:6, leading to pore blockage or agglomeration. Thus, moderate Zn loading is crucial for maximizing MB removal efficacy using the Zn-doped Fe-MOFs. Introducing Zn²⁺into Fe-based MOFs alters their structural, electronic, and surface properties, significantly improving their adsorption performance. Under optimal conditions, specifically, an Fe/Zn ratio of 10:2, replacing Fe³⁺ with Zn²⁺results in lattice distortion. This results in the formation of defects and unsaturated metal sites, but the framework structure remains stable. This was confirmed through XRD and SEM analysis. XPS analysis revealed that zinc alters the electronic environment of Fe–O clusters, thereby enhancing the Lewis acidity of the metal center and optimizing its interactions with adsorbates. FTIR analysis showed that accessibility to, and reactivity toward, functional groups such as −OH and −COO^– increased. This promotes hydrogen bonding and the formation of surface complexes. An Fe/Zn ratio of 10:2 strikes a balance between the abundance of active Fe sites and defect formation, while also considering the electron-regulating effect of Zn. If the zinc concentration is lower than this, performance improvement is insufficient. However, if the zinc concentration is higher, structural degradation occurs.

3.4.2. pH and Surface Charge Density

We conducted a zeta potential measurement as a function of pH to assess the surface charge density of the FZM-10:2 materials and to determine the point of zero charge (pH_PZC). As shown in Figure a, the pH_PZC of the FZM-10:2 adsorbent was found to be 6.9. This indicates that at a pH of 6.9, the material’s surface charge density is zero, resulting from an equal number of negatively and positively charged sites. At pH levels greater than the pH_PZC, the FZM-10:2 surface carries a negative charge, which enhances the uptake of cationic MB due to electrostatic attraction. On the other hand, when the pH is below the pH_PZC, the FZM-10:2 surface exhibits a net positive charge due to the presence of H⁺ ions, leading to competition with MB and a reduction in dye adsorption. The surface charge of the FZM-10:2 is influenced by the pH of the solution, significantly affecting the adsorption rate. To evaluate this effect, we examined the adsorption of MB onto FZM-10:2 across a pH range of 2 to 12 (Figure b). We observed a lower removal percentage at pH values below 6, likely due to the higher concentration of H⁺ ions on the FZM-10:2 surface under acidic conditions, which inhibits the adsorption of the cationic MB. However, at higher pH levels, the increased availability of negatively charged sites enhances the adsorption of the cationic dye. Therefore, at pH values above the pH_PZC, the negatively charged FZM-10:2 surface favors the binding of positively charged MB molecules. A maximum adsorptive removal of MB was achieved at a pH of 8 in this study, consistent with MB removal using modified bentonite.

5.

Effect of (a) pH_PZC, (b) pH, (c) adsorbent dose, and (d) initial concentration on the removal of MB onto FZM-10:2; (e) coexisting cations, and (f) coexisting dyes on the removal of MB onto FZM-10:2.

3.4.3. Adsorbent Dose

To investigate the impact of dosage, a series of experiments were conducted by varying the dosage from 10 mg to 60 mg (0.2 g/L to 1.2 g/L), while keeping other conditions constant: contact time of 70 min, initial MB concentration of 100 mg/L, stirring speed of 120 rpm, temperature of 298 K, and pH of 8. As the dosage increased from 10 mg to 40 mg, the removal efficiency of MB rose significantly, peaking at 99.3% with a dosage of 40 mg (Figure c). The enhanced surface reactivity from zinc doping contributed to this higher removal efficiency by providing more active sites for MB adsorption. However, beyond a dosage of 40 mg, there was no significant change in removal efficiency, likely due to the overlap of effective adsorption sites and the aggregation of adsorbent particles. Therefore, a dosage of 40 mg was determined to be optimal, which aligns with previously reported research.

3.4.4. Methylene Blue Concentration

To determine the effect of the initial MB concentration on the adsorption efficiency of FZM-10:2, the concentration was varied from 10 to 160 mg/L while maintaining a constant dosage of 40 mg, a pH of 8, a stirring speed of 120 rpm, and a temperature of 298 K. As shown in Figure d, the MB removal percentage decreases slightly as the concentration increases from 10 to 100 mg/L. However, beyond 100 mg/L, the removal percentage drops significantly, likely due to the saturation of available adsorption sites at higher MB concentrations. Therefore, an optimal concentration of 100 mg/L was selected for MB analyses, consistent with findings reported for other adsorbents.

3.4.5. Contact Time

The contact time was adjusted from 10 to 60 min, while maintaining a constant FZM-10:2 dosage (40 mg), pH (8), stirring speed (120 rpm), and temperature (298 K). As shown in Figure S5a, the adsorption efficiency increased rapidly between 10 and 70 min, likely due to the abundance of active sites on FZM-10:2. The adsorption system reached equilibrium after 40 min, at which point the MB removal efficiency plateaued, indicating saturation of the FZM-10:2 active sites. The maximum MB removal efficiency of 99.4% was achieved after 70 min. Further increases in contact time did not significantly enhance dye removal, suggesting that the adsorption process had reached equilibrium. Therefore, 70 min was determined to be the optimal contact time. These results align with prior research on the use of carboxymethyl cellulose-based silica hydrogel nanocomposites as adsorbents.

3.4.6. Rotational Speed

The influence of mixing speed on MB was examined by varying the mixing rate from 40 to 200 rpm while maintaining all other experimental parameters constant. Increasing the rotational mixing speed from 40 to 120 rpm (Figure S5b) enhanced the removal performance from 68% to 99.4%. The lower removal efficiency at lower agitation rates is attributed to limited fluid motion, which restricts the movement of MB from the bulk solution to the adsorbent surface. Conversely, higher agitation rates reduce the boundary layer thickness, promoting rapid mass transfer of dye into the adsorbent surface from the bulk solution, indicating that the adsorption process improves with higher rotational speeds. Consequently, a mixing speed of 120 rpm was applied to achieve maximum performance in the adsorption process (Figure S5b), ensuring sufficient contact between the MB dye and the available active sites on FZM-10:2. However, increasing the rotational mixing speed beyond 120 rpm slightly decreased adsorption efficiency, possibly due to the detachment of weakly adsorbed species or turbulence-induced instability. Therefore, subsequent experiments were conducted at an optimal rotational mixing speed of 120 rpm.

3.4.7. Salinity and Competing Dyes

To investigate the effect of salinity on dye removal, we evaluated the efficiency with which the material removed methylene blue from solutions with increasing concentrations of various inorganic salts: NaCl, MgCl₂, CaCl₂, and AlCl₃ (Figure e). The removal efficiency of MB decreased as the concentration of each salt increased, likely due to competition between cationic MB molecules and hydrated cations for active sites on FZM-10:2. This competition limits the access of MB molecules to the FZM-10:2 surface. An increase in ionic strength weakens the electrostatic interactions between the adsorbent and the adsorbate. Additionally, the increased viscosity caused by high salt concentrations hinders the movement of MB ions toward the adsorbent surface while strengthening hydrophobic interactions. Under these high-salinity conditions, hydrophobic interactions appear to be the primary factor influencing dye removal. In the presence of other salts, the MB removal efficiency follows this order: NaCl > MgCl₂ > CaCl₂ > AlCl₃. We also evaluated the adsorption efficiency of FZM-10:2 in simulated wastewater containing a mixture of the dyes Rhodamine 6B (RhB), Methylene Blue (MB), Methylene Orange (MO), and Procyanin Brilliant Blue (PBB). The removal rates of RhB, MO, MB, and PBB by FZM-10:2 were 58.9%, 26.9%, 99.4%, and 45.7%, respectively (Figure f). These results indicate that FZM-10:2 selectively adsorbed MB over the other dyes in the simulated wastewater, with the order of adsorption efficiency being MB > RhB > PBB > MO. The selectivity of FZM-10:2 in multicomponent dye systems is influenced by a combination of electrostatic interactions, molecular structure, and competitive adsorption effects. Methylene blue (MB), a small and planar cationic dye, demonstrates the highest removal efficiency due to strong electrostatic attraction to negatively charged surface sites and its easy diffusion into accessible pores. In contrast, methyl orange (MO), an anionic dye, exhibits significantly lower adsorption because of electrostatic repulsion. Although rhodamine 6B (RhB) and procyanin brilliant blue (PBB) can interact with the adsorbent through π–π interactions, their larger molecular sizes and steric hindrance limit their access to active sites. Under competitive conditions, MB further dominates adsorption due to its higher affinity and mobility, effectively occupying adsorption sites and suppressing the uptake of competing dye molecules. This synergistic effect results in the observed selectivity order: MB > RhB > PBB > MO.

3.5. Adsorption Isotherms

Figure presents a linear isotherm, while Table S2 details the parameters associated with each model. Results were obtained under optimized conditions, including a dosage of 40 mg, a pH of 8, a contact time of 70 min, an MB concentration of 100 mg/L, a temperature of 298 K, and a mixing speed of 120 rpm. The R² values for the isotherm plots were as follows: 0.999 for Langmuir, 0.949 for Freundlich, 0.969 for Temkin, and 0.939 for Dubinin–Radushkevich. Overall, the best-fit model is ranked as follows: Langmuir > Temkin > Freundlich > Dubinin–Radushkevich, based on R² values. This outcome clearly indicates that the adsorption of MB on FZM-10:2 is monolayer and uniform, as demonstrated by the Langmuir model. Using FZM-10:2, this study achieved an adsorption capacity of 208.3 mg g^–1 (Table S2). Additionally, we evaluated the dimensionless factor (R_L) to assess the feasibility of the reaction. The R_L values ranged from 0.004 to 0.06 for concentration levels between 10 and 160 mg/L, signifying that the adsorption of MB onto FZM-10:2 was highly favorable. An R_L value greater than 1.0 indicates an unfavorable adsorption isotherm, while an R_L value of 1.0 signifies linear behavior, and an R_L value of 0 indicates irreversible adsorption. Since the Freundlich and Langmuir isotherms do not distinguish between chemisorption and physisorption, we used the Temkin and Dubinin–Radushkevich isotherms for further analysis. According to the Temkin model, the binding energy constant (b) exhibited positive values (Table S2), suggesting that the adsorption process is endothermic in nature. From the D–R model, the calculated average adsorption energy was 11.2 kJ mol^–1, signifying that chemisorption is the primary cause for the adsorption of MB on FZM-10:2.

6.

a) Langmuir, b) Freundlich, c) Temkin, and d) Dubinin–Radushkevich linearized isotherm models for MB adsorption onto FZM-10:2.

3.6. Adsorption Kinetics

Kinetic investigations of MB adsorption on FZM-10:2 were performed using four models: pseudo-second-order (PSO), intraparticle diffusion (IPD), pseudo-first-order (PFO), and Boyd kinetic models (Figure ). The tests were conducted by varying the time from 10 to 110 min at 298 K while maintaining a constant MB concentration of 100 mg/L, a pH of 8, a mixing speed of 120 rpm, and an adsorbent dose of 40 mg. The adsorption kinetic models were analyzed using slopes and intercepts from the corresponding graphs (refer to Figure and Table S3). Among the four models, the PSO model yielded the highest R² value of 0.999, indicating that MB adsorption on FZM-10:2 is best described by this model. The adsorption followed the PSO model with a rate constant of 0.0087 g mg^–1·min^–1 for the 100 mg/L MB solution. Additionally, as shown in Table S3, the experimentally obtained q_e value of 123.8 mg/g aligns closely with the PSO model’s q_e values of 120.5 mg/g, confirming a chemisorption process for the adsorption of MB dye onto FZM-10:2. Furthermore, the IPD model was used to analyze the data and evaluate the influence of adsorbent size and surface area on the adsorption process. Key kinetic parameters were determined using the Weber–Morris equation (see Table S3 for details). As shown in Figure c, the fitted curve does not pass through the origin and exhibits two distinct regions: (1) MB diffusion to the external surface of FZM-10:2 through the solution or boundary layer, and (2) MB diffusion into the interior of the FZM-10:2 pore. The calculated C value, which is significantly different from zero (see Table S3), suggests that both intraparticle diffusion and mass transfer contribute to the adsorption rate of MB. However, the IPD results did not clearly indicate which mechanism exerted a greater effect on MB adsorption. Therefore, we further analyzed the experimental data using the Boyd kinetic model to identify the rate-determining step in the adsorption of MB dye onto FZM-10:2 (see Figure d and Table S3). The lack of linearity passing through the origin indicates that external mass transfer, specifically film diffusion, was the dominant rate-determining step Figure d).

7.

Kinetic model analysis fromFZM-10:2 adsorbing MB: a) PFO, b) PSO, c) IPD, and d) the Boyd model.

3.7. Thermodynamic Study

The impact of temperature was studied by varying it from 25 to 40 °C. This change led to a slight increase in the percentage removal of methylene blue as the temperature rose from 298 to 313 K (Figure S6a). To evaluate thermodynamic variables such as standard entropy change (ΔS°), enthalpy change (ΔH°), and free energy change (ΔG°), we employed the van’t Hoff equations (eqs and ) by plotting ln KT against 1/T.

$$\mathrm{\Delta }{G}^o=-RT\ln K_T$$ 3

$$\ln K_T=\frac{\mathrm{\Delta }{S}^o}{R}-\frac{\mathrm{\Delta }{H}^o}{RT}$$ 4

where R (8.314 J mol^–1 K^–1) was the universal gas constant, K_T = q_e /C_e was the distribution coefficient, ΔH (kJ mol^–1) was the enthalpy change, T (K) was the absolute temperature, ΔG (kJ mol^– 1) was the free energy change, and ΔS (J mol^–1 K^–1) was the entropy change. In contrast, the positive ΔH° value (+107.6 kJ mol^–1), which falls within the range commonly associated with chemisorption processes (40–120 kJ mol^–1), , confirms that the adsorption process is endothermic and primarily governed by chemical interactions rather than weak physical adsorption mechanisms. The relatively high ΔH° value may be attributed to the formation of strong interactions between MB molecules and the active sites of the adsorbent, including possible coordination interactions involving Fe or Zn centers and the nitrogen and sulfur atoms of MB. This interpretation is further supported by the XPS and FTIR analyses in the subsequent section, which suggest the involvement of surface functional groups and metal centers during adsorption. The elevated enthalpy values may suggest the presence of strong chemical bonds, which may involve coordinate bonding between a metal center (e.g., Fe or Zn) and nitrogen or sulfur atoms within the MB. Furthermore, the positive ΔS° value (+427.85 J mol^–1 K^–1) suggests an increase in randomness at the solid–liquid interface during adsorption. Although the binding of dye molecules to a surface usually leads to a reduction in entropy, this is counterbalanced by the desolvation phenomenon, which impacts both the dye molecules and the adsorbent surface. When MB⁺ ions undergo a phase transition from liquid to solid, many of the water molecules trapped within their hydration shells are released into the solution. This significantly increases the randomness of the entire system.

3.8. Adsorbent’s Practical Applicability and Comparison with Literature

The effectiveness of the FZM-10:2 adsorbent was evaluated based on its ability to remove methylene blue from real wastewater samples collected at industrial sites. Prior to adsorption experiments, these samples underwent mild pretreatment to remove large suspended particles and textile fibers. This involved allowing the collected samples to settle for 12 h, followed by filtration through Whatman filter paper to eliminate suspended solids. To evaluate the adsorption performance of FZM-10:2 under realistic wastewater conditions in the presence of coexisting interfering species, no further extensive purification was conducted. The pH of the wastewater was then adjusted to the optimized value (pH of 8) obtained from the synthetic solution studies prior to adsorption experiments. The initial MB concentration in the textile wastewater was 20 mg/L, which was reduced to 0.37 mg/L under the optimal conditions established in this study using FZM-10:2 as the adsorbent. This corresponds to a removal efficiency of 98.2%. These findings demonstrate the practical applicability of the adsorbent for real wastewater treatment. In addition, the MB removal efficiency of the developed adsorbent was compared with previously reported adsorbents, as presented in Table . As shown in Table , FZM-10:2 exhibited the highest MB removal efficiency, reaffirming its effectiveness in water purification. Additionally, an average recovery value ranged from 98.8% to 100.5%, with a relative standard deviation (RSD) of less than 4% based on an analysis of 10 mL of wastewater, as shown in Table S5. These results confirmed the method validation for environmental sample analysis containing MB concentrations.

1. Assessment of Adsorption Capacity of FZM-10:2 Using MB Dye with State-of-Art Adsorbents.

No	Adsorbent	qm (mg g–1)	Adsorbent dose (g/L)	MB concentration range (mg/L)	Temperature (oC)	pH	Time (min)	Isotherm	Kinetics	ref.
1	Zn-MOF	2.63	1	3–32	25	7	120	Freundlich	PSO
2	Cu-BTC	39.5	2	60–120	25	8	30	Langmuir	PSO
3	Surfactant-modified bentonite	6.418	0.2	10–60	25	8	60	Langmuir	PFO
4	Chemically modified tomato stems (bio adsorbent)	34.48	0.2 – 5	5–120	25	9	45	Langmuir	PSO
5	Magnetic nanocomposites activated carbon (MWMSC)	303.3	1	40–200	25	6	60	Langmuir	PSO
6	Montmorillonite (MMT)	100	1.4	80–150	32	7	150	Langmuir	PSO
7	Co doped Fe-BDC MOF	23.92	1	1–7	50	4	70	Langmuir	PSO
8	Ceramic filters	258.5	0.2	12–32	25	11	30	Langmuir	PSO
9	P2Mo18/MIL-101(Cr)	312.5	0.6	50–200	25	6	30	Langmuir	PSO
10	Biomaterial prepared from Zizyphus Spina-Christi seed (ZSCS)	666.7	0,6	50–500	45	6	30	Langmuir	PSO
11	Corn husk waste (CHW)	41.06	10	100–1000	21.9	6	60	Langmuir	PSO
12	Ligand-engineered Cu-based triazolate MOFs	352.3	1	30–500	19.9	6.6	720	Langmuir	PSO
13	Physically activated birchwood pellets biocarbon (ABPB)	91	0.5–2.5	100	25	11	1020	Freundlich	PSO
14	Bagasse Fly Ash (BFA)	15.5	2	50 −100	25	9.6	51.5	Langmuir	PSO
15	CS-SBMA	238.1	0.33–1.66	10–750	25	12	180	Langmuir	PSO
16	Didodecyldimethylammonium Bromide-Modified Brown Clay	164	0.6	10–110	55	7	30	Langmuir	PSO
17	Barley Bran and Enset Midrib Leaf	63.2	2.5	10–165	25	5.07	20	Langmuir	PSO
18	Bead-like PAN/ZIF-8 nanofiber membranes	224.4	-	100	25		180		PSO
19	FZM-10:2	208.3	0.8	10–150	25	8	70	Langmuir	PSO	This work

3.9. Reusability and Adsorption Mechanism

To assess the stability and economic viability of the prepared adsorbent, we performed a series of reusability experiments using three solvents: HCl, methanol, and NaOH (Table S6). As shown in Table S6, the first cycle yielded adsorption efficiencies of 99.3%, 98.5%, and 96.3 for HCl, methanol, and NaOH, respectively, indicating that HCl was the most effective solvent for desorbing MB from the FZM-10:2. After four test cycles, the MB removal efficiencies were recorded as 96.6% for HCl, 88.2% for methanol, and 82.5% for NaOH. These results demonstrate that HCl maintained over 97% of its initial removal efficiency after four cycles. Therefore, consistent with previous research findings, FZM-10:2 is a promising, cost-effective, and reusable material for treating wastewater containing MB.

The studies presented in the previous section on pH dependence and surface charge density confirm that electrostatic interactions play a key role in the adsorption mechanism. Furthermore, methylene blue can adsorb onto FZM-10:2 through π-π stacking interactions, as both compounds contain aromatic rings. This significantly enhances the adsorption process (see Scheme S1). To further investigate the adsorption mechanism, FTIR, XPS, and XRD measurements were performed on FZM-10:2 before and after MB adsorption (Figure S7). Figure S7a shows the FTIR spectra of FZM-10:2 before and after MB adsorption. The formation of hydrogen bonds between FZM-10:2 and MB is indicated by the O–H stretching peak, which shifted from 3185 cm^–1 to 3189 cm^–1. Additionally, the peaks near 1600 cm^–1 and 1067 cm^–1, corresponding to the symmetric and asymmetric stretching of carboxylate (−COO^–) groups and C–O, respectively, shifted to 1605 cm^–1 and 1070 cm^–1 in the FZM-10:2/MB spectrum. These shifts, along with a decrease in the intensity of the FTIR peaks following the adsorption process, highlight the significant role of these functional groups.

We used the XPS method to investigate the surface interactions between MB and FZM-10:2. Figure S7b presents the analysis spectra of FZM-10:2 before and after MB adsorption. Prior to adsorption, the spectrum predominantly displayed peaks associated with Fe 2p, Zn 2p, C 1s, and O 1s. After MB adsorption, however, the spectrum revealed S 2p at 164.5 eV and Cl 2p at 198.9 eV, along with signals for Fe 2p, Zn 2p, O 1s, and C 1s. The detection of S and Cl, both constituents of MB, confirms significant adsorption of MB on the surface of FZM-10:2⁴⁸. To further elucidate the adsorption mechanism, we analyzed the high-resolution O 1s and C 1s spectra of FZM-10:2 before and after MB adsorption (Figure S7c and S7d). The C 1s spectrum prior to MB adsorption was deconvoluted into three distinct peaks at 288.1, 286.1, and 284.7 eV, which correspond to OCO, CO/CO, and graphitic carbon functional groups, respectively. After MB adsorption, these peaks shifted slightly to 288.3, 286.5, and 285.2 eV, indicating electron transfer from FZM-10:2 to the MB molecule and supporting the notion of strong interactions between MB and the functional groups of FZM-10:2. The O 1s spectrum of FZM-10:2 before MB adsorption was similarly deconvoluted into three peaks at 530.1, 531.4, and 532.5 eV, corresponding to oxygen in carbonyl groups, hydroxyl/ether groups, and surface-adsorbed oxygen or water molecules, respectively (Figure S7d). Following MB adsorption, these peaks also shifted to higher binding energies, suggesting a redistribution of electron density around the oxygen atoms and confirming their role in chemical interactions with the MB molecules. The XRD patterns of FZM-10:2 after the adsorption of MB displayed similar features, with a slight reduction in intensity and a shift of the peak at 2θ = 25.6° to a lesser quantity, indicating the adsorption of MB onto FZM-10:2 and an enlargement of the d-spacing (Figure S8). To summarize, pH and surface charge density studies indicate that electrostatic attraction plays an important role in facilitating adsorption, particularly via interactions between FZM-10:2’s charged surface sites and MB molecules. However, combined FTIR and XPS analyses reveal π–π stacking interactions and hydrogen bonding as the predominant adsorption mechanisms. The aromatic structure of MB and the carbonaceous framework of FZM-10:2 favor strong π–π interactions (Scheme S1). Additionally, observed shifts in O–H, COO^–, C–O, O 1s, and C 1s signals after adsorption confirm the active participation of oxygen-containing functional groups through hydrogen bonding and electron redistribution. Therefore, the adsorption mechanism is cooperatively governed by electrostatic attraction, π–π stacking, and hydrogen bonding, with π–π interactions and hydrogen bonding dominating under the investigated conditions (Scheme S1). ,

4. Conclusions

This study demonstrates the effective production of a highly stable and efficient amorphous Zn-doped Fe-MOF adsorbent using a modified hydrothermal method. This method effectively breaks down methylene blue into nontoxic chemical species commonly found in industrial effluents. Advanced characterization techniques, including XPS, XRD, FTIR, and SEM, confirmed the formation of a highly amorphous, porous structure enriched with surface functional groups, which significantly enhance adsorption performance. The optimal Fe³⁺/Zn²⁺ ratio (FZM-10:2) exhibited exceptional adsorption capability for MB, achieving a high adsorption capacity of 208.3 mg g^–1 at 25 °C, surpassing many previously reported adsorbents. The adsorption process was best described by the Langmuir isotherm, while the pseudo-second-order (PSO) model was used to analyze the kinetics. Thermodynamic investigations revealed a positive ΔH° value (ΔH° = +106.4 kJ/mol) and a negative ΔG° value, indicating that the adsorption process is endothermic, chemisorptive, and spontaneous. FZM-10:2 displayed remarkable selectivity for methylene blue in mixed-dye systems and demonstrated excellent reusability, maintaining over 95% of its original removal efficiency after four adsorption–desorption cycles using hydrochloric acid (HCl) as the regenerating agent. Furthermore, tests with real wastewater samples showed that the adsorbent achieved an impressive 98.2% MB removal efficiency, highlighting its practical applicability. The adsorption mechanism was primarily governed by van der Waals interactions, hydrogen bonding, and π-π interactions. Additionally, this study indicates the development of a scalable and reusable method for removing organic dye pollutants, contributing to environmental and public health protection.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c01735.

• SEM images of FZM-11:1 sample, molar ratio, ICP and XPS results, nitrogen physisorption isotherms and BJH pore size distributions, SEM images and corresponding EDX elemental maps for the monometallic Fe-MOF sample, calibration curve, MB removal efficiency using different adsorbents, effect of time and rotational mixing speed, isotherm models, kinetic studies, effect of temperature, van’t Hoff plot, thermodynamic studies, recovery test, regeneration efficiencies, and adsorption mechanism (PDF)

The authors declare no competing financial interest.