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ACS Omega
ACS Omega
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ACS Omega
2470-1343
American Chemical Society

10.1021/acsomega.4c01150
Article
Cellulose Acetate Supported MOF-5/Crystalline Nanocellulose Composite Film as an Adsorbent Material for Methylene Blue Removal from Aqueous Solutions
Manamela Lebogang
https://orcid.org/0000-0002-9946-2568
Nombona Nolwazi *
Department of Chemistry, University of Pretoria, Private Bag X20, Hatfield 0028, Gauteng, Pretoria 0002, South Africa
* Email: nolwazi.nombona@up.ac.za.
27 08 2024
10 09 2024
9 36 3762137635
06 02 2024
09 08 2024
01 07 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

In this study, a novel, low-cost, and efficient adsorbent film was fabricated by a solvothermal method. The adsorbent film was developed to be hydrolytically stable, not vulnerable to aggregation in aqueous environments, and not prone to secondary contamination. The adsorbent consists of cellulose acetate (CA) as a support embedded with a MOF-5/crystalline nanocellulose (CNC) composite material. The CA-supported MOF-5/CNC film was characterized using a variety of techniques, including X-ray diffraction, thermal gravimetric analysis, scanning electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy, which revealed hydroxyl and carbonyl functional groups on the adsorbent film. The film was evaluated for the adsorptive removal of methylene blue (MB) from an aqueous solution. Adsorption was characterized by a rapid increase in MB adsorption during the first hour with equilibrium achieved within 4–5 h into the adsorption process. The maximum adsorption capacity was determined to be 4.29 mg/g and the maximum dye removal efficiency was 77%. The MB adsorption process best fitted the Freundlich isotherm and pseudo-second-order kinetic models. Thermodynamic studies showed that the adsorption was exothermic and feasible. The adsorbent film showed admirable regeneration ability, demonstrating its cost-effectiveness and its potential as a promising material for wastewater treatment.

National Research Foundation 10.13039/501100001321 NA document-id-old-9ao4c01150
document-id-new-14ao4c01150
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pmcIntroduction

The textile industry has been listed as one of the emerging contributors of CO2 emissions and, unfortunately, the textile sector is also among the largest contributors of wastewater with an annual worldwide dyestuff production of over 7 × 105 tons.1,2 One of the ways in which dye effluents enter waterbodies such as rivers and lakes is due to the inability of dye mixtures to completely attach to intended textiles.3 Synthetic dyes are toxic by nature,4 stable,5 and pose a risk to the environment and human health.6,7

The removal of organic dyes from aquatic systems is crucial in eliminating harmful effects from reduced sunlight penetration caused by dye pollution, which often leads to a decrease in photosynthesis. Methylene blue (MB) is frequently used in the textile industry to print cotton and dye leather. Several dye removal techniques have been reported and these include membrane separation,8 coagulation/flocculation,9 and ozonation.10 The most preferred method among these has been physical adsorption using solid materials, as it is highly efficient in the removal and treatment of contaminants in wastewater.11−13 This low-cost technology is a separation process that concentrates molecules (contaminants) on the surface of a solid adsorbent.14 Various materials have been employed as adsorbents with activated carbon being the most common commercial adsorbent.15−17 Despite being the most established adsorbent, activated carbon is expensive and has high reactivation costs, and this suggests the need for cheaper, efficient, and reusable adsorbents.18−21

Metal–organic frameworks (MOFs) are reported to play an important role in the adsorption of organic pollutants due to their high surface area, high porosity, and biocompatibility.22−24 Zr-based MOFs have been reported to have high adsorption capacities for the removal of organic dyes.12,25−27 These MOFs are stable in aqueous media and can withstand harsh chemical environments, moisture and temperature changes.28 However, their high cost of synthesis makes them uneconomical as adsorbents for organic dye removal; as a result, there is a need for cheaper MOF alternatives. One of the major challenges that cheaper MOFs encounter when applied in water remediation technologies is poor hydrolytic stability.29 To increase water stability, high oxidation metals have been incorporated in the MOF structure with linker groups such as pyrazole to increase the metal-linker bonding strength.30 Coating of the MOFs’ internal or external surface with hydrophobic materials has also been shown to increase hydrolytic stability.31 It was previously demonstrated that the external surface hydrophobization of MOFs has the potential to lower surface area and enhance water stability.32 Ding et al. developed a polymerization approach to coat the surface of HKUST-1, ZIF-67, and MIL-125 with 2,2,2-trifluoroethyl methacrylate and 3-methacryloxypropyltrimethoxysilane to synthesize water-stable MOF composites without significant change of their pore features.33 The incorporation of styrene to MOFs has been shown to improve the hydrolytic stability of MOFs, and their performance34 hence, it is important to fabricate a MOF-based material with hydrolytic stability without compromising functionality. While these modifications improve hydrolytic stability, they do not address potential aggregation, which might minimize surface area for adsorption and cause secondary contamination. To manage these challenges, MOFs have been embedded in membrane support materials,22,35−37 and they have been incorporated in the matrix of polymeric materials.38 This approach was found to address secondary contamination; however, MOF particles could still aggregate on the membrane support.22,37 Aggregation on polymer membrane supports occurs due to the attraction of MOF particles to one another coupled with poor MOF adhesion to most polymers. Duan et al. reported MOF aggregation on polymeric membrane surfaces when using an adsorption-filtration technique for MOF immobilization, resulting in secondary contamination.22

This study explores the use of MOF-5 as an adsorbent material for the removal of MB from aqueous solutions. MB was selected as a model contaminant, as it is frequently used in the textile industry and is an abundant pollutant in the aquatic system. Its cationic nature enables it to interact and penetrate the negatively charged cellular membranes of marine animals and human beings and this can lead to bioaccumulation of MB, which can cause illnesses such as cancer.39 In this work, MOF-5 was used as an active material for the adsorptive removal of MB from water due to its relatively high surface area, which provides active sites for adsorption, it is cheaper compared to most MOFs, can be used multiple times for adsorption, and can be tailored to have high affinity for cationic dyes. The drawbacks of using MOFs for water remediation are that they have low water stability, they aggregate in aqueous solution, are sensitive to acidic/alkaline environments, and are hard to recycle. The functionalization of MOFs with various materials has been shown to overcome some of these challenges. To maximize the strength of the adsorbent, crystalline nanocellulose (CNC) was combined with MOF-5 to form a powdered-crystalline composite (MOF-5/CNC) for improved hydrolytic stability. The MOF-5/CNC was embedded in a cellulose acetate (CA) matrix to form an adsorbent film (CA-supported MOF-5/CNC) to avoid secondary contamination when it was applied for the adsorptive removal of MB from water. The multi-component film also acts to eliminate aggregation suffered by MOFs and renders the film recyclable. The impact of initial concentration, pH, contact time, and temperature on the adsorption process was examined. The adsorption isotherms and adsorption kinetics were evaluated by using models.

Experimental Section

Materials

Acetone (99.9%), N,N-dimethylformamide (DMF, 99.9%), chloroform (CH3Cl, 99%), and sulfuric acid (H2SO4, 98%) were purchased from Radchem. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 98%), microcrystalline cellulose (MCC, 11 wt %), CA (39.3–40.3%), MB (98%), and terephthalic acid (BDC, 99%) were purchased from Sigma-Aldrich.

Synthesis of a CA-Supported MOF-5/CNC Adsorbent Film

Pristine MOF-5 was prepared following a reported method.40 Briefly, 6.5 g of zinc nitrate hexahydrate and 1.2 g of BDC were dissolved in 180 mL of DMF, and the mixture was stirred for 30 min. Thereafter, the mixture was heated in an oven at 105 °C for 24 h. After being cooled to room temperature, the white powder was washed with DMF and repeatedly washed with chloroform. The product, MOF-5, was vacuum-dried at 60 °C for 12 h.

CNC was synthesized using a reported procedure.41 MCC (1 g) was added to 30% H2SO4 under vigorous stirring for 1 h. Distilled iced water was used to quench the mixture, and the solution was sonicated for 10 min followed by centrifugation for 30 min. The aqueous supernatant was discarded, and fresh distilled water was added to the pellet, followed by ultrasonication before further centrifugation. This washing process was repeated until the pH of the supernatant was neutral. The resulting pellet was freeze-dried to yield powder CNC.

14.7 wt % MOF-5/CNC was synthesized by adding 1.2 g of BDC to 90 mL of DMF. The mixture was stirred for 10 min, and CNC (50 mg) was added to the reaction mixture. In a separate reaction vessel, 5 g of Zn(NO3)2·6H2O was added to 90 mL of DMF. The mixtures were stirred separately for 15 min, followed by ultrasonication for 15 min. The mixtures were combined, and the resulting mixture was stirred for 15 min, followed by ultrasonication for 15 min and heating at 50 °C under vigorous stirring for 2 h. The solution was cooled to room temperature and centrifuged to isolate the solid material. The product was washed with DMF and vacuum-dried at 60 °C for 6 h to yield MOF-5/CNC.

The synthesized MOF-5/CNC composite was supported on CA by dissolving 7.5 g of CA in 76 mL of DMF followed by stirring for 18 h at room temperature.42 The solution was degassed with nitrogen at room temperature overnight. In a separate reaction vessel, MOF-5/CNC in DMF was stirred and sonicated for 10 min. Thereafter, 3.3 mL of CA solution was added to the reaction vessel, and the resulting mixture was stirred vigorously for 1 h, followed by sonication for 30 min to obtain a viscous gel. The gel was drop-cast on a glass slide until a thickness of 0.2 mm was obtained. The coated glass slide was placed in a water bath at 6 °C for 4 h. The coated glass slide was removed from the water bath, and another glass slide was placed on top of the newly formed mat; subsequently, a force of 50 N was exerted on the film for 30 min. The film was air-dried overnight.

Adsorption Experiments

Solutions of MB (1–6 mg/L) were prepared from a 400 mg/L stock solution. The adsorbent mats (7 mm × 15 mm) were immersed in 3 mL of MB solutions and agitated in a bath shaker for 6 h. The adsorption behavior of the material at different parameters, such as contact time, solution pH, temperature, and initial MB concentration, was investigated. The percentage of MB removal was calculated using eq 1 and adsorption capacity, qe, was calculated using eq 2:1

2

where Ci is the initial concentration of MB solution (mg/L), Ce is the equilibrium concentration of MB solution (mg/L), qe is the equilibrium adsorption capacity (mg/g), V is the volume of the solution (mL), and m is the mass of the adsorbent (g).

To assess the reusability of the film, recyclability tests were performed. The adsorbent film was removed from the MB solution after an adsorption cycle and immersed in methanol to desorb the dye. The desorption solvent (methanol) was shaken for 4 h, and the film was removed to air-dry for 1 h, ready for the next adsorption cycle.

Adsorption Isotherms

Adsorption isotherms were used to establish the effectiveness of the adsorption process and assess the affinity of the adsorbent for the adsorbates, thus describing the interactions between the adsorbent and the adsorbates. In this study, the adsorbents were analyzed using Langmuir and Freundlich isotherms. The Langmuir isotherm operates under the assumption that the adsorption process is monolayer adsorption and occurs on a homogeneous surface while the Freundlich isotherm assumes the adsorption process is multilayer and occurs on a heterogeneous surface. Equations 3 and 4 show the linearized Langmuir and Freundlich adsorption isotherm models, respectively:3

4

From the equations, qmax is the maximum adsorption capacity of the adsorbent (mg/g), and KL is the Langmuir constant, which is related to adsorption affinity. The larger the values of KL, the stronger the interactions between the adsorbent and adsorbate. KF is the Freundlich isotherm constant. The gradient of the linear form of the equation, 1/n, is a function of the strength of the adsorption process. The normal adsorption process occurs when . When , cooperative adsorption is taking place. RL is a separation factor and is calculated using eq 5 below:5

where KL is the Langmuir constant and Ci is the initial adsorbate concentration. The adsorption nature is not favored when RL > 1, linear when RL = 1, favorable when 0 < RL < 1, and irreversible when RL = 0.

Adsorption Kinetics

Adsorption kinetics provide a platform to understand the adsorption mechanisms, such as mass transfer and chemical reactions during adsorption. In this study, the kinetic data obtained was analyzed by pseudo-first-order and pseudo-second-order models, using eqs 6 and 7, respectively:6

7

where qe is the adsorption capacity at equilibrium, qt (mg/g) is the adsorption capacity at time t, t (minutes) is the contact time, k1 is the first-order equilibrium constant, and k2 is the second-order equilibrium constant. The diffusion mechanisms of the adsorption process were studied using the intraparticle diffusion rate equation, eq 8:8

where kid is the intraparticle diffusion rate constant (mg/g·min–1/2) and I is the thickness of the boundary layer. The Bangham diffusion model was studied to understand the adsorption mechanism using eq 9:9

where kb and α are constants. The pore diffusion model, the Boyd model, was employed to identify the rate-limiting step using eq 10:10

where F represents the fraction of MB adsorbed at any time t (min) and is calculated by eq 11:11

Statistical Test

The nonlinear chi-square (χ2) statistic test was performed in addition to the correlation coefficient (R) to discriminate the best model fit to the experimental data. A small χ2 value demonstrates similarity between the modeled and experimental data, while a larger χ2 demonstrates disparity. χ2 was calculated using eq 12:12

where qe is the experimental equilibrium adsorption capacity and qe,cal is the model equilibrium adsorption capacity.

Characterization Techniques

X-ray diffraction (XRD) analysis was conducted by using a Bruker D2 PHASER-e diffractometer using Cu-Kα radiation (0.15418 nm). Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were carried out on a cross-beam 540 FEG SEM microscope from Zeiss. UV–vis absorption measurements were conducted on a CARY 100 BIO UV–vis spectrophotometer. Fourier transform infrared spectroscopy (FTIR) spectroscopy was done on a Brunker Alpha Fourier transform spectrometer with platinum attenuated total reflectance sampling accessory. Thermal gravimetric analysis (TGA) was done using an SDT Q600 V20.9 Build 20 Module DSC-TGA Standard. N2 absorption/desorption isotherms were acquired using autosorb iQ model:7, ASiQWin version 5.2× at −196 °C, operating within a relative pressure (P/Po) range of 0–1.0. Prior to measurement, the samples were predegassed at 130 °C for 18 h under vacuum. The Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda models were utilized to determine surface area and pore volume. X-ray photoelectron spectroscopy (XPS) was conducted on a Thermo model: ESCAlab 250Xi using a monochromatic Al Kα X-ray source.

Results and Discussion

Material Characterization

XRD

Figure 1a illustrates the reference XRD pattern of MOF-5; Figure 1b shows the XRD pattern of the synthesized MOF-5 with peaks matching the reference XRD. The synthesized MOF-5 pattern is also consistent with the previously reported MOF-5 diffraction pattern.43 The diffraction pattern exhibits peaks at 2-theta = ∼8.5°, 9.9°, 14.9°, 15.8°, 17.8°, and 19.3°, which are characteristic of MOF-5 confirming successful synthesis of the material.43−45 The crystallite size of MOF-5 was calculated using the Scherrer equation, shown in eq 13:13

where D is the crystallite size, K is the Scherrer constant with a value of 0.9, β is the full width at half-maximum, and θ is the peak position. The size was calculated using peaks at 2θ positions 9.9°, 14.9°, 17.8°, and 19.3° and an average of 30.82 nm was considered the crystalline size of MOF-5.

Figure 1 XRD patterns of (a) reference MOF-5, (b) MOF-5, (c) CNC, (d) MOF-5/CNC, (e) CA, and (f) CA-supported MOF-5/CNC.

Figure 1c shows the XRD pattern of CNC. This XRD pattern was observed to be consistent with previous reports and in line with JCPDC file no. 03-0226.46,47 The diffraction pattern exhibited peaks at 2-theta = 15.2°, 16.7°, 22.6°, and 34.4°. These peaks are indexed as (110), (110), (200), and (004), respectively. Figure 1d depicts the MOF-5/CNC composite. This diffraction pattern exhibits a mixture of peaks from both the MOF-5 diffractogram and the CNC diffractogram, which is desired, as it suggests that the two materials coexist within the composite. The figure displays peaks at 2-theta = 9.9°, 14.9°, and 28°, which belong to MOF-5; and the peaks at 15.2° and 22.6° belong to CNC. This observation suggests that the phase and the crystal structure of the individual materials are maintained during the formation of the composite. No new phases or crystal structures were observed when the two materials were combined. Figure 1e shows the XRD pattern of the highly amorphous CA material, with no well-defined peaks. The XRD pattern of the CA-supported MOF-5/CNC (Figure 1f) displayed broad peaks at 10° and 22.8°, which are characteristic of CA and the MOF-5/CNC nanocomposite.

FTIR

The synthesized materials were characterized by using FTIR spectroscopy. Figure 2a shows the CNC spectrum with functional group vibrations consistent with previous reports for CNC.41,46,47 The peak at 1634 cm–1 is due to C=O stretching while the vibrations of the C–O–C pyranose ring are responsible for the peaks at 1430, 1374, and 1313 cm–1. The peak observed at 892 cm–1 is because of glycosidic C1–H deformation within the ring, which is characteristic of glycosidic linkages linking glucose monomers in the cellulose polymer. The FTIR spectra of MOF-5 (Figure 2b) show two peaks at 1381 and 1573 cm–1 due to asymmetric and symmetric stretching of carboxylate groups, respectively. The peak observed at 450 cm–1 is due to Zn–O stretching. The peaks observed at 743 and 815 cm–1 are due to C–H vibrations of the benzene ring. This FTIR spectrum coincides with MOF-5 spectra from previously reported studies confirming the successful synthesis of the material.43,45Figure 2c shows the spectrum of the MOF-5/CNC composite with peaks characteristic of CNC at 1648, 1310, and 1024 cm–1. The peaks characteristic of MOF-5 are observed at 1583, 1364, 819, and 742 cm–1. Figure 2d,e shows the FTIR spectra of CA and the adsorbent film, respectively. The peak at 1737 cm–1 in Figure 2d is due to carbonyl (C=O) stretching, and the peaks at 1028 and 1219 cm–1 are due to C–O–C symmetrical and asymmetrical vibrations, respectively. For the adsorbent film only peaks arising from CA are exhibited, this has been previously observed for CA nanocomposites.42 The carbonyl (C=O) stretch in Figure 2e was shifted to 1740 cm–1, indicating interaction between the MOF-5/CNC and the CA.

Figure 2 FTIR spectra of (a) CNC, (b) MOF-5, (c) MOF-5/CNC, (d) CA, and (e) CA-supported MOF-5/CNC.

SEM

The surface morphology of the materials was studied by using SEM. Figure 3a shows MOF-5 as mostly cubic crystals with particle sizes under 2 μm, consistent with previous reports.40,43−45 The SEM image of CNC showed rod-like structures or nanowhiskers, which was the expected morphology.46−48 The nanowhiskers have lengths of ca. 300 nm with diameters between 10 and 20 nm. The SEM image of MOF-5/CNC (Figure 3c) shows that the cubic crystals of MOF-5 were maintained; however, they appear to be coated with a thin layer of CNC. Figure 3d,e shows the SEM micrographs of the CA-supported MOF-5/CNC surface and cross-section, respectively. The surface of the adsorbent film is porous, and the adsorbent exhibits an irregular cross-section structure of dense skin layers and finger-like microcavities, which has been reported for CA-supported materials and membranes.42,49 The film was synthesized via phase inversion, which results in a porous top layer and a finger-like sublayer. The film was measured to be 131.5 μm thick. Figure 3f shows the EDX spectrum of the film, and the elements observed, Zn and S, are from MOF-5 and CNC further confirming the successful synthesis of the adsorbent film.

Figure 3 SEM images of (a) MOF-5, (b) CNC, (c) MOF-5/CNC, (d) CA-supported MOF-5/CNC film surface, (e) CA-supported MOF-5/CNC film cross-section, and (f) EDX spectrum of CA-supported MOF-5/CNC film.

TGA

TGA was used to assess the thermal stability of the adsorbent film. For all the materials (Figure 4a–d), the weight loss at 100 °C was attributed to loss of adsorbed water, and weight loss due to the removal of DMF occurred between 150 and 260 °C. The decomposition of the MOF-5 framework is shown in Figure 4a with weight loss in the range of 410–500 °C corresponding to the decomposition of the bridge between the Zn4O carboxylate and the BDC ligand leaving a 51% zinc metal weight content. The CNC TGA curve shown in Figure 4b exhibits a second mass loss between 260 and 320 °C due to the pyrolysis of cellulose and hemicellulose. Further breakdown of the intermediates occurred beyond 340 °C until all the organic material was completely degraded. Figure 4c shows the weight loss of MOF-5/CNC. The weight loss between 410–500 °C is due to the decomposition of the composite, and 27% of zinc metal remains. The TGA curve of the CA-supported MOF-5/CNC film (Figure 4d) is slightly similar to the CNC curve since CA is a derivative of cellulose. Decomposition of the MOF-5 framework occurred between 360 and 500 °C, revealing a 5% zinc metal content.

Figure 4 TGA curves of prepared (a) MOF-5, (b) CNC, (c) MOF-5/CNC, and (d) CA-supported MOF-5/CNC film.

BET and XPS

Figure 5 shows the N2 adsorption–desorption isotherm of the adsorbent film. It can be observed that a type I adsorption–desorption isotherm was exhibited, and the specific surface area was determined to be 19.87 m2/g. This renders the adsorbent microporous with a pore volume of 0.27 cm3/g and a pore diameter of 1.17 nm. The elemental distribution of the adsorbent surface was studied by XPS. The presence of the O and C elements and their electronic state in the film are shown in Figure 6a. Figure 6b shows the deconvoluted carbon region depicting five peaks at 284.9, 284.2, 286.2, 286.2, and 287.5 eV assigned to sp3-hybridized C–C, sp2-hybridized C–C, C–O, and C=O bonds, respectively. The oxygen deconvoluted into two peaks at 531.9 and 532.9 eV (Figure 6c) assigned to C–O and C=O respectively.

Figure 5 N2 adsorption/desorption isotherm for the CA-supported MOF-5/CNC film.

Figure 6 (a) XPS survey and (b) C 1 s and (c) O 1 s spectra of the CA-supported MOF-5/CNC adsorbent film.

Effect of pH on Adsorption of MB

The point of zero charge (pHpzc) was used to determine the pH at which the net surface charge of the adsorbent film is zero. Figure 7 shows the plot of pH vs pHi for the adsorbent and this study helps in understanding the surface chemistry involved during adsorption when the film interacts with MB. The figure shows the pHpzc of the film to be 7.0. This means that the surface of the adsorbent film has a net positive charge in acidic pH and a net negative charge in alkaline pH.

Figure 7 Plot of ΔpH vs pHi.

Figure 8a shows the effect of pH on equilibrium adsorption capacity (qe), and Figure 8b shows the effect of pH on % MB removal. pH has been observed to be a controlling parameter in the adsorption of MB.50 The adsorption efficiency generally increases with increasing pH because, at an acidic pH, the surface of the adsorbent is positively charged, resulting in electrostatic repulsions between the cationic adsorbent and the cationic MB, minimizing adsorption. In addition, at low pH, there are excess H+ ions that compete with the cationic MB dye for adsorption sites making adsorption unfavorable. This is true at pH 5; however, there is an anomaly at pH 3. The error limit of pH 3 is within the error limit of pH 5, and as a result, the actual adsorption behavior is valid from pH 5 and above. At pH > pHpzc, the negatively charged adsorbent is electrostatically attracted to the cationic MB, and this favors adsorption. This adsorbent is versatile, as it has the ability to adsorb cations or anions depending on the application. In this study, the analyte of interest is cationic, and the highest adsorption efficiency was observed at pH 11.

Figure 8 (a) Equilibrium adsorption capacity vs pH, and (b) % MB removal vs pH.

Effect of Contact Time on MB Adsorption

The optimized contact time was determined at optimum pH and at initial concentrations (1–6 mg/L). Figure 9 shows the effect of contact time on the adsorption capacity and percentage of MB removal. Adsorption occurred in three phases; the first hour was the initial phase, where adsorption took place at a relatively high rate as illustrated by the sharp increase in equilibrium adsorption capacity and percentage removal. Similar behavior has been observed for dyes on biosorbents.51 This occurs because of the availability of vacant active sites on the adsorbent film. The second phase occurs when contact time increases to 4–5 h; at this stage, adsorption happens at a lower rate due to the gradual saturation of active sites on the surface of the adsorbent film, and adsorption equilibrium is reached after 5 h. The adsorption capacity at equilibrium was 4.29 mg/g, and this corresponds to a percentage removal of 77.4%.

Figure 9 (a) Effect of contact time on adsorption capacity at initial concentrations of 1–6 mg/L, and (b) effect of contact time on percentage MB removal at initial concentrations of 1–6 mg/L.

Effect of Initial Concentration on MB Adsorption

The dependence of equilibrium adsorption capacity and % MB removal on initial MB concentration was studied at pH 11, contact time of 6 h, at 25 °C. Figure 10a shows the equilibrium adsorption capacity increases (qe) linearly with initial concentration from 0.75 to 4.29 mg/g, while the % MB removal (Figure 10b) decreases in a parabolic fashion from 75 to 71.6%. The increasing MB adsorption at higher initial concentrations was influenced by mass transfer, these conditions result in the saturation of active sites causing a reduction in % MB removal. From these results, the initial concentration of 6 mg/L was considered as the optimal concentration.

Figure 10 Plots of (a) equilibrium adsorption capacity vs initial concentration and (b) % MB removal vs initial concentration.

Effect of Temperature on MB Adsorption

The influence of temperature on qe and % MB removal was investigated under the optimized conditions, which include the initial concentration of 6 mg/L. When the temperature was increased from 25 to 45 °C equilibrium adsorption efficiency decreased from 4.29 to 3.02 mg/g (Figure 11a) and the % MB removal dropped from 71 to 51% (Figure 11b). The results show that MB interacts less with the adsorbent film as the temperature increases. This decrease in activity implies that the adsorption process was exothermic in nature.

Figure 11 Effect of temperature on (a) equilibrium adsorption capacity and (b) % MB removal.

Adsorption Isotherms

Langmuir's and Freundlich's adsorption isotherm models were used to determine the adsorption parameters for the adsorption process. Figure 12 illustrates the linear plots, and Table 1 lists the resulting parameters along with correlation coefficients (R2) and nonlinear chi-square (χ2) values. This study provides insight into the adsorbate and adsorbent surface interaction during the adsorption process. The Langmuir isotherm model, shown in Figure 12a, assumes the surface of the adsorbent is homogeneous, and the maximum adsorption process occurs when monolayer coverage is achieved.51 From Table 1, the R2 value of the isotherm was 0.9738 and the χ2 value was 0.011. The Langmuir constant, KL, is dimensionless and represents the affinity of the adsorbent for the adsorbate. The KL value in this study was 0.25, which is relatively high, suggesting that the interaction between the adsorbate and the adsorbent’s surface is strong, implying that the adsorption process is favorable. The maximum adsorption capacity, qmax was calculated to be 13.23 mg/g and the separation factors, RL, for all the initial concentration values, as illustrated by Figure 12c, was between 0 and 1 indicating a favorable adsorption process. The Langmuir model showed a decent fit of the experimental data; however, the maximum monolayer capacity overestimated (13.23 mg/g) the experimentally determined equilibrium capacity value of 4.29 mg/g. The Freundlich isotherm model assumes adsorption occurs on heterogeneous active sites with varying surface energies while following a multilayer coverage (Figure 12b). The R2 value of the isotherm was 0.9674, and the χ2 value was 0.031. From this curve, the heterogeneity factor, 1/n, was between 0 and 1, indicating favorable adsorption. The Freundlich constant KF was determined to be 2.77, and the equilibrium solid phase concentration was calculated using eq 14 to be 4.39 mg/g, which was closer to the experimental value (4.29 mg/g):14

Figure 12 (a) Langmuir and (b) Freundlich isotherms of MB adsorption onto CA-supported MOF-5/CNC, and (c) RL values from the Langmuir isotherm.

Table 1 Parameters for MB Adsorption on a CA-Supported MOF-5/CNC Film

Langmuir	Freundlich	pseudo-first-order	pseudo-second-order	intraparticle diffusion model	Bangham model	pore diffusion	
qmax (mg/g) 13.23	qe (mg/g) 4.39	qe (mg/g) 13.21	qe (mg/g) 5.65	kid 0.2073	KB 0.098	1st segment slope 0.008	
 	 	 	 	 	 	2nd segment slope 0.04	
KL (L/mg) 0.25	KF (mg/g) 2.77	k1 0.00005	k2 0.0021	 	 	1st intercept −0.31	
R2 0.9738	R2 0.9674	R2 0.9201	R2 0.9864	R2 0.9709	R2 0.9726	2nd intercept −7.67	
Χ2 0.011	Χ2 0.031	Χ2 0.036	Χ2 0.023	Χ2 0.028	Χ2 0.044	 	

The Freundlich constant presented a higher value compared to that of the Langmuir constant; this suggests that the adsorption mechanism may be described by the Freundlich isotherm. This indicates that the adsorption of MB on CA-supported MOF-5/CNC may conform to a mechanism involving multilayer coverage on a heterogeneous surface. Similar results have been observed on cellulose-based adsorbents for MB removal, showing better fitting with Freundlich isotherm.52−54

Adsorption Kinetic Models

Kinetic studies were used to investigate the potential rate-controlling step, as well as the adsorption mechanism. The adsorption kinetics are governed, among various steps, by reaction processes and diffusion.55 The adsorption data were fitted to the linearized pseudo-first-order kinetic model, and Figure 13a shows it was not applicable over the whole contact time and the calculated qe was determined to be 13.21 mg/g. In most cases, the first order applies to the initial phase of the adsorption process and not to the rest of the contact time.50,56Figure 13b shows the experimental data fitting favorably over the whole adsorption range for the pseudo-second-order kinetic model. The calculated qe from the pseudo-second-order model was determined to be 5.65 mg/g, which was closer to the experimentally obtained qe (4.29 mg/g). The adsorption reaction can be described by pseudo-second-order kinetics with chemical interactions dominating the adsorption process.57,58 The kinetic parameters and corresponding R2 and χ2 values are listed in Table 1. Adsorption can proceed through various pathways, including external diffusion, intraparticle diffusion, and pore diffusion.58 The intraparticle and/or pore diffusion typically influence the overall rate of the adsorption process. Figure 13c–e shows the fitting of experimental data for the diffusion models. The Weber–Morris plot (qt vs t1/2) in Figure 13c shows that the plot is linear with R2 > 0.9 and χ2 < 0.05, indicating that intraparticle diffusion is a possible pathway for MB adsorption. For this model the linear fit has to pass through the origin; this suggests that intraparticle diffusion is not the only controlling mechanism in this adsorption study but that the adsorption is a combination of multiple adsorption processes.55,58 The intercept of the Weber–Morris plot has been reported to serve as an indicator of the thickness of the boundary layer, i.e., a large intercept implies a large boundary layer.55,59 The Bangham plot shown in Figure 13d was used to check the possibility of pore diffusion as the rate-limiting step. The plot was found to be linear with R2 > 0.9 and χ2 < 0.05, implying pore diffusion governs the adsorption process. The Boyd plot (Figure 13e) was used to assess pore diffusion, and it shows multilinearity. The multilinearity represents succeeding adsorption processes.57Figure 13e can be described as two segments, the first between 60 and 180 min with an intercept close to zero; this indicates that adsorption in this period could be described by pore diffusion. In the second segment, the intercept suggests that other mechanisms described above have taken over the adsorption process.

Figure 13 (a) Pseudo-first-order, (b) pseudo-second-order, (c) intraparticle, (d) pore diffusion, and (e) Boyd model plots of the experimental data for MB adsorption on the CA-supported MOF-5/CNC adsorbent film.

Adsorption Thermodynamics

The Arrhenius equation, eq 15, was used to gain more insight into the type of adsorption:15

where k is the rate constant, A is the Arrhenius factor, Ea is the activation energy for the adsorption process, R is the gas constant (8.314 J/K.mol), and T is the temperature in Kelvin. The activation energies for physisorption are categorized between 5 and 40 kJ/mol and are between 40 and 800 kJ/mol for chemisorption.60 The slope of Figure 14 was used to determine Ea, which was found to be 41.2 kJ/mol for the adsorption of MB on the adsorbent film, indicating chemisorption. Similar values have been observed for cellulose composites used for dye adsorption.61

Figure 14 Plot of ln k vs T–1: for the estimation of activation energy, Ea.

Thermodynamic parameters which include standard entropy (ΔS°), standard enthalpy (ΔH°), and standard free energy (ΔG°) were determined to assess the thermal nature of MB adsorption on the adsorbent film according to previous reports.51,57 ΔS° and ΔH° were calculated from the van’t Hoff plot as depicted in Figure 15 according to eq 16:16

ΔG° was determined using eq 17:17

Figure 15 van’t Hoff plot for MB adsorption on the CA-supported MOF-5/CNC adsorbent film.

Table 2 shows thermodynamic parameters derived from Figure 15. The ΔGo values became less negative with increasing temperature, indicating the occurrence of saturation on the adsorbent surface at increased temperatures; however, the process was thermodynamically spontaneous at higher temperatures. Furthermore, ΔS° was negative, and ΔHo was negative, signifying an exothermic adsorption process.

Table 2 Thermodynamic Parameters for MB Adsorption on the CA-Supported MOF-5/CNC Adsorbent Film

T/K	ΔGO (kJ/mol)	ΔHO (kJ/mol)	ΔSO (J/K.mol–1)	activation energy (Ea) (kJ/mol)	
298	–2.21838	–34.44158	–107.749	41.2	
308	–1.05112	
318	–0.03525	

Proposed Adsorption Mechanism and Reusability Studies

The interaction of MB molecules with CA-supported MOF-5/CNC can be understood by using Figure 16. The presence of functional groups on the adsorbent film guarantees intermolecular interactions with dye molecules. The proposed adsorption mechanism of MB on the surface of the adsorbent occurs through several ways. FTIR showed the presence of hydroxyl and carbonyl groups on the adsorbent film. These groups can interact electrostatically with MB.62 The interactions include ion-dipole forces, dipole–dipole, hydrogen bonding, and π–π interactions, similar mechanisms have been proposed in the literature for MB adsorption.63 During adsorption, since pH > pHpzc, the negatively charged adsorbent is electrostatically attracted to the cationic MB, and this favors adsorption. In addition, the hydrogen atom from the hydroxyl groups can form hydrogen bonds with the nitrogen from MB as illustrated in Figure 16. π–π interactions can also occur between the cellulose polymer chain and the MB aromatic ring.64

Figure 16 Adsorption mechanisms of CA-supported MOF-5/CNC to MB.

Adsorption efficiency was evaluated, as seen in Figure 17. Cycle 1 represents MB adsorption with a fresh adsorbent and cycles 2–5 represents the times the adsorbent has been desorbed in methanol and readsorbed in fresh MB solution. A 7% drop in the adsorption capacity was observed from cycle 1 to cycle 2. A 13% decrease in adsorption capacity was observed for cycle 3 followed by a 20% decrease for cycle 4. These results support the thermodynamic data that allude to MB adsorption being driven by chemisorption resulting in reduced removal efficiency. There was no significant decrease after the fourth cycle (21% decrease for cycle 5), suggesting the binding sites were relatively fixed after the fourth cycle.

Figure 17 MB uptake over five adsorption/desorption cycles on CA-supported MOF-5/CNC.

Simultaneous Adsorption of Methylene Blue and Methyl Orange (MO) and Treatment of Dye-Containing Wastewater

Dye-containing wastewater constitutes a complicated mixture of dyes, which include neutral, cationic, and anionic dyes. The simultaneous adsorption of anionic and cationic dyes is important for remediation hence the CA-supported MOF-5/CNC film was evaluated for the simultaneous adsorption for MB and MO at varied pH. As shown in Figure 18, MO had better adsorption under acidic conditions with the highest adsorption capacity of 2.65 mg/g obtained at pH 3. pH 11 remained the optimum pH for MB adsorption for the adsorbent film with an adsorption capacity of 2.9 mg/g, and MO had an adsorption capacity of 1.86 mg/g at this pH. The MB qe values were lower than those reported without MO, this shows the influence of competing species for active sites.

Figure 18 Simultaneous adsorption of MB and MO on the CA-supported MOF-5/CNC adsorbent film.

The efficiency of the adsorbent film for wastewater treatment was examined by using domestic-industrial effluent spiked with MB. The pH of the wastewater sample was determined to be 8.35. Figure 19 shows a decrease in the absorption spectra for MB post adsorption. The % removal of MB in the real water sample was determined to be 42%. The reduced efficiency was due to other contaminants interfering with the adsorption process.

Figure 19 Adsorption of MB in domestic-industry effluent spiked with MB.

Economic Assessment

In the development of any technological method applied in wastewater treatment, it is important to provide information about the cost of the treatment process. Depending on the treatment method, cost analysis is generally done by considering the costs of energy and material as major cost items.65 For a material for which the purpose of the study is to investigate its efficiency in the removal of a dye, the simplified approach used in this study is considered sufficient.

In calculating the operating cost (OC), eq 18 was used:18

where Cenergy and Cchemicals are the energy consumed (in kWh/m3) and the chemical consumption (in kg/m3), respectively. The letter a is the electrical energy price USD 0.105 kWh for the South African market as of March 2024, and the letter b is the price of the chemicals used in the preparation of the adsorbent film material (USD/kg or USD/m3).

In this study, the synthesized material could treat 0.030 L of wastewater in the first cycle; however, the calculated operating cost was done on the basis of treating 1 m3 of the synthesized aqueous solution containing methylene blue following a reported approach.66 The study considered the power rating in watts for all of the equipment used and the duration for which it was operated as well as the cost of the chemicals. Therefore, the total energy consumption was 110.8 kWh/m3, which equates to USD 11.67/m3, and the total chemical cost was USD 31.93/m3. The operating cost was calculated to be USD 43.60. The operating cost for Cchemicals was 2.8 times more expensive than that of Cenergy.

Comparison with Other Adsorbents

A comparison of the maximum adsorption capacities for MB adsorption of MOF-cellulose composite adsorbents is shown in Table 3. An adsorbent based on MOF-derived porous carbon (MOF-DPC) nanoparticles supported on CA had a qe value of 41.36 mg/g at an initial MB concentration of 50 mg·L–1 at 65 °C.42 A cellulose aerogel fabricated from CA and ZIF-8 had a qe value of 90.18 mg/g.67 The MOF was grown in situ on the surface of the adsorbent. A qe value of 227 mg/g was determined from nitrogen-doped carbon aerogels fabricated from the carbonization of bacterial cellulose@ZIF-8 composite aerogels.68 While these adsorbents show better qe values, the fabrication processes require extreme temperatures that may affect the overall cost-effectiveness of the adsorbent.

Table 3 MB Maximum Adsorption Capacity of MOF-Cellulose-Based Adsorbents

adsorbent	qe (mg/g)	ref	
CA/MOF-DPC	41.36	(42)	
ZIF-8 (CA@ZIF-8)	90.18	(67)	
C-BZ-1000	227	(68)	
CA-supported MOF-5/CNC	4.29	this study	

Conclusions

A CA-supported MOF-5/CNC adsorbent film was successfully fabricated by using solvothermal and ultrasonication methods for methylene blue adsorption in aqueous media. The film showed a porous morphology with a measured film thickness of 131.5 μm. The integration of CNC and MOF into the CA support was confirmed by FTIR, SEM-EDX, TGA, and XPS. The CNC provided the adsorbent with hydrolytic stability, while the CA support secured the MOF. Various factors that influence MB batch adsorption were assessed, and the maximum adsorption capacity and % removal were determined to be 4.29 mg/g and 77.4%, respectively, at 25 °C, at an initial concentration of 6 mg/L, at pH 11 and at 14.7 wt % MOF loading. The adsorption process was spontaneous and exothermic as indicated by negative ΔGo and ΔHo values. The Freundlich and pseudo-second-order models were the best-fit adsorption isotherm and kinetic models, respectively, and the activation energy was consistent with chemisorption. Electrostatic interactions, such as dipole–dipole, π–π interactions, and hydrogen bonding, were proposed as chemical interactions between methylene blue and the film to influence adsorption. After being regenerated five times, the film maintained 79% activity, indicating its value for reusability. The cost analysis for the adsorption was established. There was a 42% removal of MB when the film was applied for the treatment of wastewater. Overall, this work established a simple synthetic route for a MOF adsorbent film for water remediation.

Author Contributions

L.M. designed the project with the guidance of N.N. L.M. prepared the manuscript and N.N. proofread and made the contributions and suggestions to the structure and contents of the manuscript.

The authors declare no competing financial interest.

Acknowledgments

The authors would like to acknowledge the University of Pretoria, Chemistry department for providing a conducive environment to conduct this study and the National Research Foundation for funding this study. We would like to thank the National Metrology Institute of South Africa for the XPS measurements, Tshwane Municipality for providing the industrial-domestic wastewater, and Dr Samuel A. Iwarere from the Department of Chemical Engineering at the University of Pretoria for the cost analysis calculations.
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