Green Synthesis and Characteristics of Cellulose Nanocrystal/Poly Acrylic Acid Nanocomposite Thin Film for Organic Dye Adsorption during Water Treatment

Nanocellulose shows potential as an effective natural adsorbent for removing harmful contaminants from wastewater. This paper describes the development of innovative nanocellulose thin films made of cellulose nanocrystals (CNCs), polyacrylic acid (PAA), and active carbon (AC) as adsorbent materials for absorbing azo dyes from wastewater. The CNCs were recovered from sugarcane bagasse using alkali treatment and acid hydrolysis. The composition and processing parameters of the thin films were optimized, and their adsorption capacity was determined using thermodynamic isotherms and adsorption kinetics. Adsorption characteristics such as the methylene blue (MB) dye concentration, contact time, temperature, and pH were investigated to determine how they affected adsorption. The results show that the adsorption process follows pseudo-second-order kinetics. At an adsorbent mass of 50 mg, dye concentration of 50 ppm in 50 mL, and contact period of 120 min at 25 °C, the thin film comprising 64 wt% CNC, 16 wt% PAA, and 20 wt% AC showed high dye removal efficiency (86.3%) and adsorption capacity (43.15 mg/g). The MB removal efficiency increased to 95.56% and the adsorption capacity to 47.78 mg/g when the medium’s pH was gradually increased from neutral to alkaline. The nontoxicity, low production cost, water stability, easy recovery, and high adsorption capacity of these membranes make them suitable for water treatment systems.


Introduction
The presence of dyes in effluents is a major problem due to the negative effects they have on many different types of life. Dye discharge into the environment is a source of concern for both toxicological and aesthetic reasons [1]. Textile, leather, paper, plastics, and other industries utilising dyes consume large amounts of water. As a result, they produce a significant volume of coloured wastewater [2]. More than 100,000 commercially accessible dyes are expected to be available, with over 7 × 105 tonnes of dyestuff produced annually [3][4][5]. It is widely acknowledged that the colour of water has a significant impact on public impressions of its quality. Colour is the first pollutant found in wastewater. Even trace levels of dyes in water-less than 1 ppm for some dyes-are highly visible and unwanted [6,7]. MB is the most commonly used dyeing agent for cotton, wood, and silk. It can induce eye burns, which can result in irreversible damage to human and animal eyes. It can cause short periods of fast or difficult breathing when inhaled, whereas absorption through the mouth causes a burning feeling, nausea, vomiting, intense perspiration, mental confusion, and methemoglobinemia [8][9][10]. Because of the negative effects on the receiving waterways, the treatment of wastewater containing such dye is of importance. Our work showed that CNC/PAA/AC thin films with tunable CNC/PAA compositions can improve MB's adsorption behaviour. Their adsorption activities, including kinetics, isotherms, thermodynamics, and pH effects, were thoroughly examined. Several mathematical models were used to evaluate and analyse the kinetics and isotherms of the adsorption process to explain the interaction mechanisms between MB and thin films. Finally, desorption tests were used to assess the thin film's reusability.

Extraction of Cellulose Nanocrystals (CNCs) from Sugarcane Bagasse
CNC synthesis was carried out in many stages. Initially, sugarcane bagasse was sundried for 24 h before being cut into small pieces and de-waxed by heating 20 g dry weight in 1000 mL of water to 90 • C for 2 h with constant stirring and repeating the technique twice to thoroughly remove the wax. Second, the powder was then acid treated for 2 h at 60 • C with 400 mL of 10% HCl to remove inorganic minerals. Third, the demineralized powder was washed with water until it reached a neutral pH to remove any leftover acid and minerals. Fourth, 350 mL of 4 w/v% sodium hydroxide (NaOH) was added to the pulp and agitated for 2 h at 60 • C to remove lignin and hemicellulose. The pulp was then washed in distilled water until the pH reached neutral. Fifth, the pulp was bleached for 2 h at 60 • C with mechanical agitation with 250 mL of 24 v/v% H 2 O 2 and then rinsed with distilled water until the pH was neutral. Sixth, the pulp was bleached at 90 • C for 4 h with mechanical stirring with 200 mL of 2 w/v% NaOCl, then washed, filtered, and dried to yield chemically pure pulp. Lastly, CNCs were created by acid hydrolysing the resulting cellulose fibres for 2 h at 45 • C with vigorous stirring in 180 mL of 50 v/v% H 2 SO 4 . The CNCs were then rinsed several times until the pH was neutral before being centrifuged for 15 min at 4000 rpm and drying overnight at 50 • C.

Fabrication and Characterization of Thin Film
Five thin films with varying dry weight concentrations of 0-64 wt% CNC, 16-80 wt% PAA, and 0-20 wt% AC were produced ( Table 1). The thin film's solid composition (CNC, PAA, and AC) was adjusted to 1 g in 20 mL of water. First, CNC, PAA, and AC were ultrasonically mixed for 30 min before being magnetically swirled for 60 min. Next, the mixture was agitated for 15 min with a mechanical stirrer before being allowed to sit overnight to remove air bubbles. Then, the mixture was placed onto 9 cm diameter Petri dishes and air-dried for 48 h. Finally, the dried thin film was heated to 140 • C for 30 min to accelerate the reaction between the components (CNC, PAA, and AC) until crosslinking occurred and then washed with deionized water to remove unreacted reagents.
Various approaches were used to characterize the physicochemical parameters of AC, CNCs, and the produced thin films. The CNCs' crystalline phase was determined by X-ray diffraction (XRD; Bruker AXS D8, Germany). Changes in the functional groups of the CNCs and PAA after crosslinking were assessed using Fourier transform infrared (FTIR) spectrophotometry (JASCO model 6700, USA). The particle size and shape of CNCs and AC particles were examined using transmission electron microscopy (TEM; JEOL-JEM-2100, Japan). The thin films' surface morphology and elemental composition were examined using field emission scanning electron microscopy (FE-SEM; Japan, EOL JSM-6510LV QSEM, Japan). Thin films (1 × 1 cm) were immersed in 50 mL of water for 24 h at room temperature to assess their swelling properties. The thin films' dry weight was determined before and after water immersion using the equation: swelling (%) = (W 2 /W 2 − W 1 ) 100), where W 1 is the dry weight of the thin film, and W 2 is its swollen weight after immersion.

Adsorption Kinetics and Isotherm
The MB dye's adsorption capacity was determined as a function of contact time using adsorption kinetics with a thin film mass of 50 mg, initial MB dye concentration of 50 ppm, pH of 7, and ambient temperature of 25 • C. The pseudo-first-order and pseudosecond-order adsorption kinetic models were then used to fit the experimental kinetic data. Equation (3) shows the linear version of the pseudo-first-order kinetic model of adsorption solid-liquid systems: Equation (4) shows the pseudo-second-order kinetic model based on the MB chemisorption hypothesis: where q t (in mg/g) is the adsorption capacity at the given contact time t, q e (in mg/g) is the adsorption capacity at the equilibrium time, and k 1 and k 2 are the rate constants for pseudo-first-order and pseudo-second-order models, respectively.

Thermodynamic Isotherm
The thin film (C 4 ) with the best favourability, reversibility, and energy for MB dye adsorption was investigated using adsorption thermodynamics. The Langmuir and Freundlich isotherms were used to study the interactions between the MB dye and the best thin film [44]. Then, the Van't Hoff equation (Equations (5) and (6)) was used to estimate thermodynamic parameters such as changes in Gibbs free energy (∆G • ), enthalpy (∆H • ), and entropy (∆S • ). ln K = −∆G • /RT (5) where the equilibrium constant/distribution coefficient K = Q e /C e , T is the solution's temperature (in K), and R is the universal gas constant (8.314 J/mol K). The changes in enthalpy (∆H • ) and entropy (∆S • ) were calculated from the slope and intercept of the ln(K) vs. 1/T plot, respectively.

Crosslinking and Thin Film Formation
CNC (C6H10O5)n is a natural carbohydrate with several glucose units. Its OH groups and hydrolysis resistance are the most crucial properties for adsorbing cationic dyes and heavy metals from wastewater. CNCs can be enhanced by mixing them with PAA, a hydrophilic, nontoxic, and safe polymer with carboxyl functionalities groups [45].
Ester production occurs due to the thermally induced crosslinking between the hydroxyl and carboxylic acid groups. The procedure is simple, effective, and quick, producing no hazardous byproducts. Because of the many hydroxyl groups present in

Crosslinking and Thin Film Formation
CNC (C 6 H 10 O 5 ) n is a natural carbohydrate with several glucose units. Its OH groups and hydrolysis resistance are the most crucial properties for adsorbing cationic dyes and heavy metals from wastewater. CNCs can be enhanced by mixing them with PAA, a hydrophilic, nontoxic, and safe polymer with carboxyl functionalities groups [45].
Ester production occurs due to the thermally induced crosslinking between the hydroxyl and carboxylic acid groups. The procedure is simple, effective, and quick, producing no hazardous byproducts. Because of the many hydroxyl groups present in CNCs, they interact and crosslink with PAA in the thin film ( Figure 2). benefits, emphasizing the need to optimize the CNC:PAA ratio inside the nanocomposite films [46]. Anhydride molecules are produced during the heating of cellulosic materials (CNC and PAA). Figure 3 depicts the creation of cyclic anhydride due to further esterification of the molecule.  The crosslinking between CNC and PAA in the thin film (C4) was examined using FTIR analysis ( Figure 3). The CNC spectrum showed peaks for O-H stretching (3700 cm −1 ), a characteristic reflecting hydroxyl groups remaining after the hydrolysis reaction, asymmetric CH2 stretching (2987 cm −1 ), a characteristic of distributed cis ranglap bonds, and C=O stretching (1639 cm −1 ). The strong peak centred at 1636.67 cm −1 was assigned to the H-O-H stretching mode, a characteristic of H2O bending and the C=C bond of the aromatic ring at 1531 cm −1 [47]. PAA's typical IR peaks were detected for O-H stretching (3752 cm −1 ), asymmetric CH2 stretching (2900 cm −1 ), C=O stretching (1627 cm −1 ), C=C stretching (1523 cm −1 ), and C-O-C stretching of the polysaccharide skeleton (1000-1200 cm −1 ). The efficient crosslinking of the thin film was confirmed by a new peak at 1724 Crosslinking has been used successfully to improve CNC qualities such as mechanical, thermal, and barrier properties. Furthermore, preparing nanocomposite sites with the highest feasible CNC concentration has significant environmental and economic benefits, emphasizing the need to optimize the CNC:PAA ratio inside the nanocomposite films [46]. Anhydride molecules are produced during the heating of cellulosic materials (CNC and PAA). Figure 3 depicts the creation of cyclic anhydride due to further esterification of the molecule. benefits, emphasizing the need to optimize the CNC:PAA ratio inside the nanocomposite films [46]. Anhydride molecules are produced during the heating of cellulosic materials (CNC and PAA). Figure 3 depicts the creation of cyclic anhydride due to further esterification of the molecule.  The crosslinking between CNC and PAA in the thin film (C4) was examined using FTIR analysis ( Figure 3). The CNC spectrum showed peaks for O-H stretching (3700 cm −1 ), a characteristic reflecting hydroxyl groups remaining after the hydrolysis reaction,  The crosslinking between CNC and PAA in the thin film (C4) was examined using FTIR analysis ( Figure 3). The CNC spectrum showed peaks for O-H stretching (3700 cm −1 ), a characteristic reflecting hydroxyl groups remaining after the hydrolysis reaction, asymmetric CH 2 stretching (2987 cm −1 ), a characteristic of distributed cis ranglap bonds, and C=O stretching (1639 cm −1 ). The strong peak centred at 1636.67 cm −1 was assigned to the H-O-H stretching mode, a characteristic of H 2 O bending and the C=C bond of the aromatic ring at 1531 cm −1 [47]. PAA's typical IR peaks were detected for O-H stretching (3752 cm −1 ), asymmetric CH 2 stretching (2900 cm −1 ), C=O stretching (1627 cm −1 ), C=C stretching (1523 cm −1 ), and C-O-C stretching of the polysaccharide skeleton (1000-1200 cm −1 ). The efficient crosslinking of the thin film was confirmed by a new peak at 1724 cm −1 , which was attributed to the characteristic stretching band of the C=O group linked through ester bonds in the network. This distinctive absorption peak indicated the effective production of the crosslinked thin film.
The thin film's morphology (C 4 ) was evaluated using SEM microscopy, and its elemental composition after MB dye adsorption was examined by energy-dispersive X-ray (EDX) spectroscopy.
After MB adsorption, the atomic composition of the C 4 thin film was 37% carbon, 28% oxygen, 11% nitrogen, and 3% sulfur. Carbon (in red) is the most prevalent element in this composition based on the EDX data, followed by oxygen ( Figure 4). Carbon atomic concentrations most likely derive mainly from the CNC, PAA, AC, and the carbon tab of the SEM sample holder. To confirm the adsorption of MB, the sulfur and nitrogen distributions corresponding to the MB dye were obtained using EDX-based element mapping. Sulfur and nitrogen could be used to determine the MB distribution in a thin film. through ester bonds in the network. This distinctive absorption peak indicated the effective production of the crosslinked thin film.
The thin film's morphology (C4) was evaluated using SEM microscopy, and its elemental composition after MB dye adsorption was examined by energy-dispersive X-ray (EDX) spectroscopy.
After MB adsorption, the atomic composition of the C4 thin film was 37% carbon, 28% oxygen, 11% nitrogen, and 3% sulfur. Carbon (in red) is the most prevalent element in this composition based on the EDX data, followed by oxygen ( Figure 4). Carbon atomic concentrations most likely derive mainly from the CNC, PAA, AC, and the carbon tab of the SEM sample holder. To confirm the adsorption of MB, the sulfur and nitrogen distributions corresponding to the MB dye were obtained using EDX-based element mapping. Sulfur and nitrogen could be used to determine the MB distribution in a thin film.

Crosslinked Thin Film Swelling Effect
One of the most critical properties of polymer thin films is their ability to swell in water. Water molecules diffuse slowly into the thin film at first, resulting in a bloated thin film. Bulk swelling refers to the swelling of a thin film over an area of 1 mm 2 , whereas microscopic swelling refers to minor dimensional changes. CNC and PAA have negative

Crosslinked Thin Film Swelling Effect
One of the most critical properties of polymer thin films is their ability to swell in water. Water molecules diffuse slowly into the thin film at first, resulting in a bloated thin film. Bulk swelling refers to the swelling of a thin film over an area of 1 mm 2 , whereas microscopic swelling refers to minor dimensional changes. CNC and PAA have negative surface charges, compatible with their high hydrophobicity. The thin film's swelling capacity is affected by the density of crosslinks, pH, and temperature. The amount of swelling in specific liquids is determined by the thin film's degree of crosslinking, pore volumes, and functionality. Figure 5 shows 206% swelling of the CNC/PAA thin film after 60 min of immersion in distilled water. Within 60-120 min, the thin film swelled from 206% to 210%. At 240 min, it had reached its maximum value (216%). The CNC/PAA/AC thin films absorb water due to their hydrophilic functional groups (e.g., OH, COOH) linked to the CNC and PAA chains. Surprisingly, thin films with suitable mechanical qualities are favoured for water treatment.

Effect of Thin Film Composition on MB Dye Removal
Adsorption is a common approach for treating dye-contaminated wastewat most common issue for sustainable adsorbents is achieving a high dye adsorptio bility. Five thin films were produced with varying CNC, PAA, and AC concentra obtain a high dye removal capacity ( Table 1). The five thin films were evaluated in of solutions with a constant initial MB dye concentration (50 mg/L) and pH (7) at perature of 25 °C (Table1 . Since the CNC:PAA ratio in C0 and C4 increased from 75%, the adsorption capacity increased marginally from 76.53% to 86.30% (Figure evident that increasing the CNC:PAA ratio had only a minor effect on dye remov tentially because it resulted in more negatively charged groups that enabled grea dye adsorption. AC particle loading and thin film composition adjustment also he improve MB dye removal. With an initial MB concentration of 50 ppm, an initial p a temperature of 25 °C, and a contact period of 2 h, the best thin film (C4) removed of the MB dye and had a dye adsorption capacity of 43.15 mg/g.

Effect of Thin Film Composition on MB Dye Removal
Adsorption is a common approach for treating dye-contaminated wastewater. The most common issue for sustainable adsorbents is achieving a high dye adsorption capability. Five thin films were produced with varying CNC, PAA, and AC concentrations to obtain a high dye removal capacity ( Table 1). The five thin films were evaluated in 50 mL of solutions with a constant initial MB dye concentration (50 mg/L) and pH (7) at a temperature of 25 • C (Table 1. Since the CNC:PAA ratio in C 0 and C 4 increased from 0% to 75%, the adsorption capacity increased marginally from 76.53% to 86.30% ( Figure 6). It is evident that increasing the CNC:PAA ratio had only a minor effect on dye removal, potentially because it resulted in more negatively charged groups that enabled greater MB dye adsorption. AC particle loading and thin film composition adjustment also helped to improve MB dye removal. With an initial MB concentration of 50 ppm, an initial pH of 7, a temperature of 25 • C, and a contact period of 2 h, the best thin film (C 4 ) removed 86.30% of the MB dye and had a dye adsorption capacity of 43.15 mg/g. 75%, the adsorption capacity increased marginally from 76.53% to 86.30% ( Figure 6). It is evident that increasing the CNC:PAA ratio had only a minor effect on dye removal, potentially because it resulted in more negatively charged groups that enabled greater MB dye adsorption. AC particle loading and thin film composition adjustment also helped to improve MB dye removal. With an initial MB concentration of 50 ppm, an initial pH of 7, a temperature of 25 °C, and a contact period of 2 h, the best thin film (C4) removed 86.30% of the MB dye and had a dye adsorption capacity of 43.15 mg/g.

Effect of Initial MB Dye Concentration on Its Removal
The initial MB dye concentration and thin film mass are crucial in determining dye removal effectiveness from water. For example, dye removal efficiency usually improves with increasing thin film mass since the number of sorption sites increases as thin film mass increases [48]. Therefore, additional MB adsorption experiments were conducted on the best thin film in this study (C 4 ). The initial dye concentration ranged from 10 to 100 ppm, while the dye solution volume (50 mL), thin film mass (50 mg), and pH (7) remained constant, and the thin film was agitated at 400 rpm for two hours. Figure 7 shows that as the MB dye concentration increased from 10 to 50 ppm, the adsorption capacity increased from 5.66 to 86.3 mg/g, and dye removal increased from 56.61% to 86.3%. This finding reflects the many adsorption sites available at a low dye concentration of 50 ppm. Increasing the dye concentration from 75 to 100 ppm decreased dye removal from 73.95% to 70.85%, reflecting the formation of two-dimensional cationic dye aggregates, making the adsorption process more difficult and reducing the amount of MB dye removed.

Effect of Initial MB Dye Concentration on Its Removal
The initial MB dye concentration and thin film mass are crucial in determining dye removal effectiveness from water. For example, dye removal efficiency usually improves with increasing thin film mass since the number of sorption sites increases as thin film mass increases [48]. Therefore, additional MB adsorption experiments were conducted on the best thin film in this study (C4). The initial dye concentration ranged from 10 to 100 ppm, while the dye solution volume (50 mL), thin film mass (50 mg), and pH (7) remained constant, and the thin film was agitated at 400 rpm for two hours. Figure 7 shows that as the MB dye concentration increased from 10 to 50 ppm, the adsorption capacity increased from 5.66 to 86.3 mg/g, and dye removal increased from 56.61% to 86.3%. This finding reflects the many adsorption sites available at a low dye concentration of 50 ppm. Increasing the dye concentration from 75 to 100 ppm decreased dye removal from 73.95% to 70.85%, reflecting the formation of two-dimensional cationic dye aggregates, making the adsorption process more difficult and reducing the amount of MB dye removed.

Effect of Contact Time on MB Dye Removal
The dye removal rate increased with contact time due to an increase in the likelihood of the dye adhering to the thin film's accessible adsorption sites. The time required to reach equilibrium showed the thin film's maximum adsorption capacity. Adsorption studies were conducted with contact times from 0 to 180 min to study the effects of contact time. The results showed that adsorbed MB dye gradually increased with contact time (Figure 8). The adsorption capacity increases with contact time in the first 120 min until an equilibrium is established [49]. Increasing contact time from 0 to 120 min increased adsorption capacity from 7.14 to 42.57 mg/g and dye removal from 14% to

Effect of Contact Time on MB Dye Removal
The dye removal rate increased with contact time due to an increase in the likelihood of the dye adhering to the thin film's accessible adsorption sites. The time required to reach equilibrium showed the thin film's maximum adsorption capacity. Adsorption studies were conducted with contact times from 0 to 180 min to study the effects of contact time. The results showed that adsorbed MB dye gradually increased with contact time (Figure 8). The adsorption capacity increases with contact time in the first 120 min until an equilibrium is established [49]. Increasing contact time from 0 to 120 min increased adsorption capacity from 7.14 to 42.57 mg/g and dye removal from 14% to 85.14%. These findings are consistent with Somsesta et al. [50], who created AC/cellulose composite sheets for MB dye removal and investigated the effect of exposure time. They found that as contact time increased, the adsorption capacity increased until equilibrium was reached.

Effect of Temperature on MB Dye Removal
The dye solution's temperature is one of the most important factors influencing adsorption capacity. Adsorption tests were conducted at 25 °C, 40 °C, and 60 °C to investigate the effects of temperature. Figure 9 shows that as temperature increased, MB adsorption decreased from 43.15 to 37.52 mg/g, while dye removal decreased from 86.3% to 75.04%. The decreased MB dye removal with increasing temperature may reflect the weakening of the adsorption forces between the MB dye molecules and the thin film's active sites and between nearby molecules in the adsorbed phases [51]. These findings are consistent with those of Mohammed et al. [52], who developed CNC/alginate hydrogel beads and investigated the influence of temperature on MB dye removal. They found that increasing the temperature from 25 °C to 55 °C reduced dye removal efficacy from 77% to 64%.

Effect of pH on MB Dye Removal
The pH of the dye solution regulates the number of electrostatic charges transferred from ionized dye molecules, influencing the dye adsorption capacity [53]. Cationic dye adsorption is best at high pH, whereas anionic dye adsorption is less efficient. Because

Effect of Temperature on MB Dye Removal
The dye solution's temperature is one of the most important factors influencing adsorption capacity. Adsorption tests were conducted at 25 • C, 40 • C, and 60 • C to investigate the effects of temperature. Figure 9 shows that as temperature increased, MB adsorption decreased from 43.15 to 37.52 mg/g, while dye removal decreased from 86.3% to 75.04%. The decreased MB dye removal with increasing temperature may reflect the weakening of the adsorption forces between the MB dye molecules and the thin film's active sites and between nearby molecules in the adsorbed phases [51]. These findings are consistent with those of Mohammed et al. [52], who developed CNC/alginate hydrogel beads and investigated the influence of temperature on MB dye removal. They found that increasing the temperature from 25 • C to 55 • C reduced dye removal efficacy from 77% to 64%.

Effect of Temperature on MB Dye Removal
The dye solution's temperature is one of the most important factors influencing adsorption capacity. Adsorption tests were conducted at 25 °C, 40 °C, and 60 °C to investigate the effects of temperature. Figure 9 shows that as temperature increased, MB adsorption decreased from 43.15 to 37.52 mg/g, while dye removal decreased from 86.3% to 75.04%. The decreased MB dye removal with increasing temperature may reflect the weakening of the adsorption forces between the MB dye molecules and the thin film's active sites and between nearby molecules in the adsorbed phases [51]. These findings are consistent with those of Mohammed et al. [52], who developed CNC/alginate hydrogel beads and investigated the influence of temperature on MB dye removal. They found that increasing the temperature from 25 °C to 55 °C reduced dye removal efficacy from 77% to 64%.

Effect of pH on MB Dye Removal
The pH of the dye solution regulates the number of electrostatic charges transferred from ionized dye molecules, influencing the dye adsorption capacity [53]. Cationic dye adsorption is best at high pH, whereas anionic dye adsorption is less efficient. Because

Effect of pH on MB Dye Removal
The pH of the dye solution regulates the number of electrostatic charges transferred from ionized dye molecules, influencing the dye adsorption capacity [53]. Cationic dye adsorption is best at high pH, whereas anionic dye adsorption is less efficient. Because the electrostatic attraction between the positively charged MB dye and the thin film surface increases with increasing pH, the adsorption capacity increases [54]. The ionization of the thin film's functional surface groups and the ionization of MB dye molecules are affected by pH changes. Experiments were performed with 50 mg of thin film support mass, 50 mL of dye solution, an initial MB dye concentration of 50 ppm, and an ambient temperature of 25 • C to explore the effects of pH. The solution was adjusted to an initial pH of 3, 5, 7, 9, and 12 with dilute HCl and NaOH. Figure 10 shows that at a pH of 3, the adsorption capacity was 22 mg/g, and the removal efficiency was 44%, while at a pH of 7, dye removal reached up to 86.3%. Increasing the initial pH from 7 to 12 achieved an excellent dye adsorption capacity of 47.78 mg/g and a dye removal efficiency of 95.56%. Therefore, pH values >8 favoured MB dye adsorption by the prepared thin film. These results agree with Oyewo et al. [55], who prepared sawdust-based CNCs containing zinc oxide nanoparticles and investigated the influence of the initial pH. Changing the initial pH from 2 to 10 increased dye removal from 60% to 90% [56].
Polymers 2023, 15, x FOR PEER REVIEW 12 of 18 mass, 50 mL of dye solution, an initial MB dye concentration of 50 ppm, and an ambient temperature of 25 °C to explore the effects of pH. The solution was adjusted to an initial pH of 3, 5, 7, 9, and 12 with dilute HCl and NaOH. Figure 10 shows that at a pH of 3, the adsorption capacity was 22 mg/g, and the removal efficiency was 44%, while at a pH of 7, dye removal reached up to 86.3%. Increasing the initial pH from 7 to 12 achieved an excellent dye adsorption capacity of 47.78 mg/g and a dye removal efficiency of 95.56%. Therefore, pH values >8 favoured MB dye adsorption by the prepared thin film. These results agree with Oyewo et al. [55], who prepared sawdust-based CNCs containing zinc oxide nanoparticles and investigated the influence of the initial pH. Changing the initial pH from 2 to 10 increased dye removal from 60% to 90% [56].

Adsorption Kinetics and Thermodynamic Isotherm
Adsorption kinetics was used to determine MB dye adsorption capabilities as a function of time at a constant dye concentration. Adsorption kinetic studies were performed with a C4 thin film mass of 50 mg, an initial MB dye concentration of 50 ppm in 50 mL of dye solution, an initial pH of 7, a temperature of 25 °C, and a contact time of 2 h. The experimentally determined adsorption capacity was 43.15 mg/g. Pseudo-first-order and pseudo-second-order kinetic models were used to fit the experimental kinetic data (Figure 11a,b). The adsorption capacity at equilibrium time (qe = 42.13 mg/g), correlation coefficient (R 2 = 0.998), and reaction constant (k2 = 0.011184513 g/mg.min) obtained with the pseudo-second-order model were close to those obtained with the other published models. However, the adsorption capacity at equilibrium time (qe = 14.39 mg/g), correlation coefficient (R 2 = −0.180), and reaction constant (k1 = −1.47 × 10 −5 min −1 ) obtained with the pseudo-first-order model were far from those obtained with the other published models. These findings suggest that the adsorption of the investigated adsorbates occurs via a chemisorption process and that the pseudo-second-order mechanism predominates in the MB dye adsorption processes [57]. Fitting the experimental data gathered at the equilibrium time point with Langmuir and Freundlich isotherm models yielded thermodynamic adsorption isotherms. These models demonstrate the relationship between adsorption capacity and equilibrium concentration at a constant temperature [58]. According to the Langmuir adsorption model, a saturated monolayer of dye molecules on the adsorbent surface corresponds to the maximum restricted absorption [59]. The Freundlich adsorption isotherm assumes that adsorption occurs on a heterogeneous surface via a multilayer adsorption mechanism and that the amount adsorbed increases

Adsorption Kinetics and Thermodynamic Isotherm
Adsorption kinetics was used to determine MB dye adsorption capabilities as a function of time at a constant dye concentration. Adsorption kinetic studies were performed with a C 4 thin film mass of 50 mg, an initial MB dye concentration of 50 ppm in 50 mL of dye solution, an initial pH of 7, a temperature of 25 • C, and a contact time of 2 h. The experimentally determined adsorption capacity was 43.15 mg/g. Pseudo-first-order and pseudo-second-order kinetic models were used to fit the experimental kinetic data (Figure 11a,b). The adsorption capacity at equilibrium time (q e = 42.13 mg/g), correlation coefficient (R 2 = 0.998), and reaction constant (k 2 = 0.011184513 g/mg.min) obtained with the pseudo-second-order model were close to those obtained with the other published models. However, the adsorption capacity at equilibrium time (q e = 14.39 mg/g), correlation coefficient (R 2 = −0.180), and reaction constant (k 1 = −1.47 × 10 −5 min −1 ) obtained with the pseudo-first-order model were far from those obtained with the other published models. These findings suggest that the adsorption of the investigated adsorbates occurs via a chemisorption process and that the pseudo-second-order mechanism predominates in the MB dye adsorption processes [57]. Fitting the experimental data gathered at the equilibrium time point with Langmuir and Freundlich isotherm models yielded thermodynamic adsorption isotherms. These models demonstrate the relationship between adsorption capacity and equilibrium concentration at a constant temperature [58]. According to the Langmuir adsorption model, a saturated monolayer of dye molecules on the adsorbent surface corresponds to the maximum restricted absorption [59]. The Freundlich adsorption isotherm assumes that adsorption occurs on a heterogeneous surface via a multilayer adsorption mechanism and that the amount adsorbed increases as the dye concentration increases [60,61].

Conclusions
Thin films composed of CNC, PAA, and AC have received considerable att due to their unique properties, including high specific surface area, low weight, n icity, chemical reactivity for surface modification, biocompatibility, high hydroph and environmental friendliness. This study produced an environmentally friend sorbent thin film composed of CNC, PAA, and AC to remove azo (MB) dyes. T portance of this study lies in how to extract nanomaterials such as cellulose nanoc from agricultural waste such as sugarcane bagasse and use it with certain ad The linear forms of the studied isothermal models are shown in Equations (7) and (8): log q e = 1 n log C e + log p (8) where q e is the amount of MB dye adsorbed at equilibrium (in mg/g), C e is the dye equilibrium concentration in solution (in mg/L), q m is the maximum adsorption capacity of the thin film of a given mass, and k L and k F are the Langmuir and Freundlich constants, respectively. According to the correlation coefficient (R 2 ), the Langmuir isotherm model best described the MB dye adsorption process (Figure 11d). The MB dye's theoretical maximum adsorption amount (q max ), obtained with the Langmuir isotherm model, was 76.6 mg/g, close to the experimental values (76.5 mg/g). The better fitting of the Freundlich isotherm model means that MB dye molecules cover the membrane surface by multilayer adsorption on heterogeneous membrane sites. Figure 9e shows that the effect of temperature on MB dye adsorption at 25 • C (298 K), 45 • C (318 K), and 60 • C (333 K), and the thermodynamic parameters including ∆S, ∆H, and ∆G, were calculated based on the adsorption capacity q e (in mg/g) and concentration C e (in mol/L). The results showed that as the temperature increased from 25 • C (298 K) to 45 • C (318 K) to 60 • C (333 K), ∆G increased from −0.37 to 0.63 to 0.79 kJ/mol, respectively. This finding indicates that high temperature is not beneficial for spontaneous MB dye adsorption, similar to the experimental results in Figure 9. The ∆H value for MB dye adsorption was −3.93 kJ/mol, indicating exothermic physical adsorption of MB dye molecules on the C 4 thin film. The negative ∆S of −14.23 JK −1 /mol indicated that randomness at the interface between the thin film and the MB dye solution decreased during adsorption [62].

Conclusions
Thin films composed of CNC, PAA, and AC have received considerable attention due to their unique properties, including high specific surface area, low weight, nontoxicity, chemical reactivity for surface modification, biocompatibility, high hydrophilicity, and environmental friendliness. This study produced an environmentally friendly adsorbent thin film composed of CNC, PAA, and AC to remove azo (MB) dyes. The importance of this study lies in how to extract nanomaterials such as cellulose nanocrystals from agricultural waste such as sugarcane bagasse and use it with certain additives (PAA, AC) to adsorb the largest possible amount of methylene blue dye. The effect of crosslinking on increasing the thin film's hydrolytic stability was shown in the swelling investigation. The C 4 thin film formulation (64% CNC, 16% PAA, and 20% AC) showed excellent adsorption with an alkaline pH, increasing from 86.3% at neutral pH to 95.56% at pH 7-12. This finding indicates that at pH values > 7, the surfaces of the thin film's components (CNCs and PAA) were negatively charged due to OH − ions. Therefore, electrostatic attraction forces rapidly adsorb the MB dye cations on these surfaces. Consequently, pH values > 7 are beneficial for MB dye adsorption on the produced thin film. Adsorption studies showed that the produced thin films effectively removed MB dye from aqueous solutions. Adsorption equilibrium was achieved in about two hours. Furthermore, temperature effects on adsorption uptake were used to calculate its thermodynamic parameters. These findings show that the C 4 thin film could be a good adsorbent for removing dyes from contaminated water sources, which is different than other similar work present in the literature in the following ways: Firstly, the study used the extraction of cellulose nanocrystals from bagasse, which is considered an agricultural waste. Secondly, it used different concentrations of cellulose nanocrystals to determine the extent of its adsorption of the dye. Thirdly, it employed the addition of activated carbon to this composition. Fourthly, the results indicated the high adsorption of methylene blue using this composite. Notably, it has a higher adsorption capacity for the MB dye than other published adsorbents ( Table 2).  Data Availability Statement: Detailed data supporting this study's findings are included in the article. The originally collected data are available upon reasonable request from the corresponding author.