Maleated carboxymethyl starch biopolymer for efficient methylene blue adsorption: Synthesis,characterization,and performance evaluation

2.1. Materials

Carboxymethyl starch (CMS, ≥99%), maleic anhydride (MA, ≥99%), sodium hydroxide (NaOH, ≥99%), pyridine (Pyridine, ≥99%), N,N-dimethylformamide (DMF, ≥99%), hydrochloric acid (HCl, ≥99%), acetone, methylene blue (MB, ≥99%), and ethyl alcohol were procured from Sigma-Aldrich. All reagents were of analytical grade and were used as received, without any further purification.

2.2. Synthesis of M.CMS

M.CMS was synthesized through an esterification reaction between CMS and MA. This modification strategy was adopted to introduce additional carboxylic functional groups onto the starch backbone, thereby enhancing its surface reactivity and adsorption capability toward cationic dye molecules.

The reaction was carried out in a nitrogen-purged flask to minimize unwanted side reactions and oxidative degradation of the polysaccharide. Initially, 1.0 g of CMS was dispersed in 30 mL of DMF and heated to 90 °C under continuous stirring at 500 rpm until a homogeneous solution was obtained. DMF was selected as the reaction solvent because polar aprotic solvents are known to facilitate the dissolution of polysaccharides and enhance the reactivity of maleic anhydride during esterification reactions with hydroxyl-containing polymers. Subsequently, 5.49 g of maleic anhydride dissolved in 10 mL of DMF was gradually introduced into the reaction mixture, followed by the addition of 5 mL pyridine, which acted both as a catalyst and a base. Pyridine promotes nucleophilic substitution by activating hydroxyl groups and facilitating the ring opening of maleic anhydride during esterification reactions. The addition of reagents was completed within approximately 9 minutes.

The reaction mixture was then maintained at 90 °C and stirred continuously for 9 hours to ensure sufficient interaction between CMS and MA and to achieve an adequate degree of substitution. Reaction temperatures within the range of 80–100 °C have been widely reported as suitable for starch esterification reactions because they promote effective functionalization while minimizing degradation of the polysaccharide backbone. The selected reaction time was chosen to allow adequate grafting of maleate groups onto the CMS structure. During the reaction, the solution gradually changed to a brownish color, indicating the successful maleation of CMS. After completion of the reaction, the product was recovered by precipitation and purification. The reaction mixture was centrifuged at 3000 rpm for 8 minutes to separate the modified polymer from the reaction medium.

The resulting precipitate was washed three times with acetone and deionized water to remove residual reagents, including unreacted maleic anhydride, pyridine, and remaining CMS. Any residual acetone was subsequently removed by washing with ethyl alcohol. Finally, the purified M.CMS was dried in a vacuum oven at 40 °C for 24 hours. The dried product was ground into a fine powder using a mortar and pestle and stored in a sealed container for further characterization and adsorption experiments. The entire synthesis process is schematically outlined in Figure 1.OPEN IN VIEWER

2.3. Characterization

A series of modern characterization techniques were employed to investigate the chemical composition, structural properties, and performance of the CMS and M.CMS samples. The crystalline structure of M.CMS was analyzed using X-ray diffraction (XRD) with a XPert PRO diffractometer, employing Cu Kα radiation (λ = 1.54 Å). To identify functional groups and confirm chemical modifications, Fourier-transform infrared (FT-IR) spectroscopy was performed using a DW-FTIR-510A/520A instrument, applying the KBr pellet method. The surface texture and morphology of both CMS and M.CMS were examined via scanning electron microscopy (SEM) with an SU-8010 system, equipped with an Oxford X-max80 detector (Hitachi Ltd.), and representative micrographs were recorded for each sample. Thermal stability and decomposition behavior were assessed using a Q50 thermogravimetric analyzer (TGA). Solution pH was monitored with an LLC-AI501 PH700 pH meter, while MB concentrations in the solutions were quantified using a U2020 spectrophotometer. For solid-liquid separation during the experiments, an 80-1C centrifuge was utilized.

The specific surface area and pore characteristics of the synthesized materials were determined using the Brunauer-Emmett-Teller (BET) method based on nitrogen adsorption-desorption isotherms. The BET theory assumes multilayer adsorption of gas molecules on the surface of the adsorbent and is widely used to estimate the accessible surface area of porous materials. The linear form of the BET as follows:

P V ( P 0 − P ) = 1 V m C + C − 1 V m C ( P P 0 )

(1)

Δ p H = p H f − p H i

(2)

I E C = ( V b − V s ) × N m

(3)

4.1. Maleation modification

The modification of CMS with maleic anhydride was carried out through a ring-opening esterification reaction to introduce additional functional groups onto the polymer backbone. In this reaction, the hydroxyl groups of CMS act as nucleophiles and attack the electrophilic carbonyl carbon of maleic anhydride, leading to the opening of the anhydride ring and the formation of ester linkages on the starch chains. The reaction can be represented as:

CMS – OH + Maleic Anhydride → CMS – O – CO – CH = CH – COOH

During this process, one carbonyl group of maleic anhydride forms an ester bond with CMS, while the second carbonyl group remains as a free carboxylic acid (–COOH). As a result, additional carboxylic functionalities are introduced onto the CMS structure, increasing the density of negatively charged sites on the polymer surface.

The reaction was performed in N, N-dimethylformamide (DMF), which promotes homogeneous dispersion of CMS and facilitates the reaction, while pyridine acts as a catalyst and proton acceptor to enhance the esterification process. The incorporation of maleate groups modifies both the chemical and structural properties of CMS. Chemically, the newly introduced carboxyl groups improve the interaction capability of the material with cationic dye molecules such as methylene blue through electrostatic attraction and hydrogen bonding. Structurally, the substitution disrupts the native hydrogen-bonding network of starch, reducing crystallinity and promoting a more amorphous structure with greater accessibility of active adsorption sites. A schematic representation of the maleation reaction and the formation of M.CMS is shown in Figure 1.

The synthesis of M.CMS was conducted in an alkaline medium using DMF as the reaction solvent. Under these basic conditions, maleic anhydride (MA) undergoes ring opening, generating reactive carboxyl-containing species that can couple with the hydroxyl-bearing backbone of carboxymethyl starch. As a result, additional carboxylic functionalities are introduced onto the starch framework, which is the key chemical change expected from maleation. The successful formation of M.CMS was then supported through multiple characterization techniques. FT-IR spectroscopy was used as the primary tool to confirm the chemical modification by comparing the functional groups in CMS and M.CMS. The FT-IR spectra shown in Figure 3 compare the functional group characteristics of CMS and M.CMS, providing insight into the chemical modifications undergone during the synthesis process. In the CMS spectrum, the prominent band at 3230 cm^-1 corresponds to the O–H stretching vibration, typical of the hydrogen-bonded hydroxyl groups in starch. The absorption around 1643 cm^-1 indicates the presence of carbonyl groups (C=O stretching), and the peaks at 1035 cm^-1 and 1159 cm^-1 are associated with C–O stretching vibrations from the glucose ring and ether linkages, respectively. ²⁵ These features are characteristic of the unmodified carboxymethyl starch structure. Upon modification with maleic anhydride to form M.CMS, several significant changes are observed. New peaks at 1726 cm^-1 and 1564 cm^-1 appear, which are attributed to C=O stretching from carboxylic acid groups and C=C stretching from the maleate functionalization, respectively. These new bands confirm the successful introduction of maleic anhydride-derived groups, including carboxyl and unsaturated functionalities, onto the starch backbone. Additionally, the shift of the O–H stretching band from 3230 cm^-1 to 3234 cm^-1 and the C–H stretching vibration at 2926 cm^-1 further supports the modification. The presence of these new peaks in M.CMS indicates that the maleation process effectively altered the chemical structure of CMS, introducing additional reactive groups such as carboxyl and unsaturated bonds, which are likely to enhance the material’s adsorption properties for various applications, such as wastewater treatment. These spectral differences between CMS and M.CMS are clear confirmation of the chemical modification and the successful maleation reaction.OPEN IN VIEWER

X-ray diffraction (XRD) analysis was conducted on powdered samples of CMS and M.CMS to investigate their crystallinity and understand the impact of maleation on their structural properties, which are critical for their potential as adsorbent materials. The XRD pattern of CMS revealed a crystalline structure, with distinct diffraction peaks observed at 2θ values of 15.2°, 17.6°, 22.6°, and 31.3°, confirming the crystalline nature of the material as shown in Figure 4. These characteristic peaks are typical of starch-based materials and indicate a well-ordered molecular arrangement, as well as the presence of intermolecular hydrogen bonds, as noted by. ²⁶ The crystalline structure of CMS is indicative of regular molecular packing and a stable arrangement that limits the available surface area for adsorption.OPEN IN VIEWER

In contrast, the XRD pattern of M.CMS exhibited a significant reduction in crystallinity, with the sharp diffraction peaks typical of the crystalline form being replaced by a broad hump centered between 2θ ≈ 20° and 25°. This broad peak indicates the amorphous nature of M.CMS, suggesting that the maleation process the introduction of maleic anhydride to the starch backbone disrupted the original crystalline order of CMS. The reaction with maleic anhydride substitutes hydroxyl groups on CMS with maleate groups, which in turn disrupts the intramolecular hydrogen bonds, leading to the breakdown of the crystalline lattice. This structural transformation is consistent with findings by, ²⁷ where the breakdown of crystallinity in starch-based materials was attributed to chemical modifications, such as maleation, which increased their amorphousness and structural flexibility.

Previous studies, such as those by ²⁸ and, ²⁹ have highlighted that the amorphous nature of materials, like M.CMS, generally enhances their adsorption capabilities compared to crystalline materials. This enhancement in adsorption efficiency is primarily due to the increased surface area and the availability of more active sites for interaction with adsorbates. The higher surface area and greater surface roughness of M.CMS offer more reactive sites, which facilitate better adsorption, particularly for pollutants in wastewater treatment applications. As ³⁰ pointed out, the amorphous structure increases the number of accessible active sites, enhancing the adsorption efficiency of the material.

Similarly, ³¹ reported that succinylated starches, which were also amorphous, exhibited improved adsorption characteristics, emphasizing the significant role of structural modifications in enhancing the adsorption properties of starch-based materials. In line with these studies, the XRD results of M.CMS confirm that the maleation process has successfully converted the crystalline CMS to an amorphous form, which is expected to significantly enhance its performance as an adsorbent material. The reduced crystallinity and increased surface area of M.CMS are anticipated to contribute substantially to its effectiveness in adsorbing pollutants, making it a promising candidate for use in wastewater treatment applications.

The thermal stability of a material provides important insights into its ability to withstand high temperatures during adsorption-desorption cycles. Thermogravimetric analysis (TGA) is a widely used technique for determining the degradation temperature and thermal stability of substances, which is crucial for understanding how the material will perform in real-world applications. According to 32, TGA offers critical information about a material’s mass loss at various temperatures, shedding light on its thermal behavior. As shown in Figure 5(a), the TGA curve of CMS reveals that the material undergoes a single degradation step between 285°C and 310°C, primarily due to the breakdown of C–O–C bonds within the starch structure. This is characteristic of starch-based materials, where ether linkages (C–O–C) in the polysaccharide chains degrade at elevated temperatures, leading to significant mass loss. The sharp weight loss observed in this temperature range indicates the thermal instability of CMS under these conditions. The TGA data suggests that CMS remains thermally stable up to approximately 285°C, after which it begins to degrade rapidly, typical for many polysaccharides.OPEN IN VIEWER

The Differential Thermogravimetric (DTG) curve of CMS in Figure 5(b) provides a more detailed view of the rate of mass loss as a function of temperature. The DTG curve shows a distinct peak around 283°C, representing the temperature at which the maximum rate of mass loss ( D T G max ) occurs. This peak corresponds to the temperature at which the degradation of C–O–C bonds within CMS reaches its highest rate, confirming that CMS undergoes rapid degradation around 283°C. The thermal stability of M.CMS was evaluated by comparing its TGA and DTG results with those of CMS. M.CMS exhibited distinct thermal degradation behaviors, with degradation observed at 295°C and 330°C, which we attribute to the breakdown of C–O–C bonds and carboxylic acid groups. The ( D T G max ) for M.CMS occurred at 289°C, indicating that its overall thermal stability remains similar to CMS. ³¹

Regarding adsorption performance, the thermal stability of the adsorbent is crucial, as materials with higher thermal stability can retain their adsorption sites and structural integrity at higher temperatures, leading to improved adsorption capacity over multiple cycles. The TGA results suggest that M.CMS can withstand a broad temperature range before degradation begins, making it suitable for applications involving multiple adsorption-desorption cycles in water treatment. Additionally, the degree of substitution (DS) of carboxymethyl groups in M.CMS plays a significant role in its thermal stability, as higher DS values tend to lower the material’s thermal stability by promoting easier decomposition. ³³ However, M.CMS maintains good thermal stability, which allows for.

Figure 6 shows the BET analysis obtained from nitrogen adsorption-desorption measurements, illustrating the relationship between ( P V ( P 0 − P ) ) and the relative pressure ( P P 0 ) for both CMS and M.CMS. The linear behavior observed for both materials within the selected pressure range confirms the applicability of the BET model for evaluating the surface properties of the adsorbents. The BET plot for CMS exhibits a slightly steeper slope compared to M.CMS, indicating differences in monolayer adsorption capacity between the two materials. For CMS, the values of ( P V ( P 0 − P ) ) increase from approximately 0.005 at low relative pressure to about 0.024 at higher relative pressure, whereas for M.CMS the corresponding values range from roughly 0.0045 to 0.0195 over the same pressure interval. The high degree of linearity in the fitted curves suggests reliable estimation of BET parameters. Based on the slope and intercept of the linear plots, the calculated BET surface area of CMS was approximately 42.8 m² g^-1, while M.CMS exhibited a slightly higher surface area of about 51.6 m² g^-1. The increase in surface area after maleation can be attributed to structural modification of the starch matrix, which likely introduced additional surface irregularities and active adsorption sites. This enhancement in surface characteristics is consistent with the SEM observations and contributes to the improved adsorption performance of M.CMS toward methylene blue molecules.OPEN IN VIEWER

The point of zero charge ( p H p z c ) of CMS and M.CMS was determined using the pH drift method, and the results are illustrated in Figure 7. The variation is given in equation (2) as a function of the initial solution pH indicates the surface charge behavior of the adsorbents. For both materials, ( Δ p H ) decreases progressively with increasing initial pH, reflecting the gradual change in surface protonation-deprotonation equilibrium. The ( p H p z c ) corresponds to the pH at which ( Δ p H = 0 ) . As observed from the plot, the ( p H p z c ) value for CMS was approximately 6.83, whereas the modified material M.CMS exhibited a lower ( p H p z c ) of about 5.33. This shift toward a lower ( p H p z c ) after maleation indicates the successful introduction of additional acidic functional groups, particularly carboxyl groups, onto the starch backbone. At pH values lower than the ( p H p z c ) , the adsorbent surface becomes positively charged due to protonation of surface groups, whereas at pH values higher than the ( p H p z c ) , the surface acquires a negative charge due to deprotonation. Therefore, under neutral or slightly alkaline conditions ( p H > ( p H p z c ) ) , the negatively charged surface of M.CMS enhances electrostatic attraction toward the cationic methylene blue molecules. This behavior explains the superior adsorption performance of M.CMS compared with CMS, as the lower ( p H p z c ) facilitates stronger electrostatic interactions between the adsorbent surface and dye molecules in aqueous solution.OPEN IN VIEWER

This study focused on the adsorption of the cationic dye MB, where the electrostatic attraction between the negatively charged surface of M.CMS and the positively charged dye molecules enhances adsorption. However, as highlighted by, ¹³ M.CMS’s surface charge would lead to electrostatic repulsion with anionically charged dyes such as Congo Red or Methyl Orange, reducing their adsorption capacity. While electrostatic repulsion limits anionic dye adsorption, it could still occur under certain conditions, such as at low pH where the surface charge density is reduced or through other interactions like hydrogen bonding. The IEC of CMS and the modified material M.CMS was determined using acid–base titration to evaluate the density of exchangeable functional groups present on the polymer matrix. The results obtained from three experimental replicates are presented in Figure 8, the IEC of CMS and M.CMS as the average of three independent measurements (mean ± standard deviation), providing a more statistically appropriate representation of the experimental results. The unmodified CMS exhibited an average IEC value of 1.80 ± 0.10 meq g^-1, whereas the modified M.CMS showed a significantly higher IEC value of 3.60 ± 0.10 meq g^-1, representing approximately a two-fold increase after maleation. This substantial increase in IEC confirms the successful incorporation of additional ionizable carboxylic acid functional groups onto the starch backbone during the maleation process. The higher IEC of M.CMS indicates a greater density of exchangeable active sites, which enhances its ability to interact with positively charged species such as methylene blue through ion-exchange and electrostatic attraction mechanisms. The relatively small standard deviation values further demonstrate the good reproducibility and reliability of the measurements. Overall, the enhanced IEC of M.CMS directly supports its improved adsorption performance compared with pristine CMS.OPEN IN VIEWER

The SEM micrographs of CMS and M.CMS are shown in Figure 9(a) and (b), respectively, illustrating substantial differences in their surface structures. CMS granules (Figure 9(a)) possess a relatively smooth surface with polygonal particles, some of which are slightly indented or uneven. ³⁴ This smooth surface leads to a relatively lower surface area, thus limiting the number of available adsorption sites for the removal of contaminants. The particle size distribution histogram in Figure 9(c) was constructed by measuring 100 individual CMS particles from SEM micrographs using ImageJ software. The results reveal that most CMS particles range between 6–12 µm, with fewer particles in the larger size bins, extending up to 30 µm. The smooth texture of CMS restricts its adsorption capacity, as fewer adsorption sites are available due to the low surface area.OPEN IN VIEWER

In contrast, M.CMS granules (Figure 9(b)) exhibit a much rougher and more irregular texture, with visible cracks, grooves, and crust-like regions. This dramatic change in surface morphology is the result of the reaction with maleic anhydride, which disrupts the original CMS structure, imparting a more amorphous and heterogeneous surface. As a result, the surface area of M.CMS is significantly enhanced, providing a much larger number of available adsorption sites. The particle size distribution histogram in Figure 9(d) was generated by measuring 100 individual M.CMS particles from SEM micrographs using ImageJ software. The results show that M.CMS particles are generally larger, with the size distribution concentrated around 14–18 µm, extending up to 26 µm. The rough texture of M.CMS creates additional active sites and irregularities, which improve the interactions with dye molecules.

The roughened surface morphology and the creation of additional adsorption sites significantly boost the adsorption capacity of M.CMS compared to CMS. These structural modifications are particularly advantageous in wastewater treatment applications, where the adsorption of contaminants like dyes requires a large surface area and a high number of active sites for effective removal. Therefore, while CMS offers limited adsorption potential due to its smooth surface, the coarse and fractured texture of M.CMS results in a considerable increase in adsorption efficiency, making M.CMS more suitable for dye removal and wastewater treatment applications, as highlighted by. ³⁵

To quantitatively evaluate the effect of maleation on the surface morphology of the adsorbent, SEM micrographs of CMS and M.CMS were analyzed using ImageJ image analysis software. The calculated parameters, including average feature area, average perimeter, surface coverage, and relative surface roughness index, are summarized in Table 2. The results indicate that M.CMS exhibits significantly higher surface coverage (67.5%) and a greater relative surface roughness index (1.46) compared with pristine CMS, which shows a surface coverage of 48.3% and roughness index of 0.82. Although CMS displays larger average feature area and perimeter values, the modified M.CMS surface contains a higher density of smaller and more irregular surface features. This morphological transformation is attributed to the maleation process, which introduces additional functional groups and disrupts the original starch structure, leading to increased surface heterogeneity. The enhanced surface irregularity and feature density are expected to improve the accessibility of adsorption sites and promote stronger interactions between the negatively charged functional groups of M.CMS and the cationic methylene blue molecules. Consequently, the observed morphological modifications contribute to the improved adsorption performance of the modified adsorbent.

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Table 2.

Quantitative surface morphology parameters of CMS and M.CMS obtained from SEM micrographs using ImageJ image analysis.

Sample	Average feature area (µm2)	Average perimeter (µm)	Surface coverage (%)	Estimated roughness (Ra, arbitrary units)
CMS	194.33 ± 947.80	86.45 ± 52.13	48.3	0.82
M.CMS	10.46 ± 20.13	17.28 ± 8.64	67.5	1.46

4.2. Effect of experimental parameters on MB adsorption by M.CMS

The adsorption performance of both CMS and M.CMS is strongly influenced by the pH of the solution because pH affects the surface charge of the adsorbent and the ionization state of its functional groups, which govern the electrostatic interactions with MB molecules. As shown in Figure 10(a), the removal efficiency of M.CMS increased significantly from approximately 29% at pH 4 to about 98% at pH 7, indicating a strong dependence on pH. In contrast, CMS exhibited considerably lower adsorption efficiency, increasing from about 18% at pH 4 to around 64% at pH 7. At lower pH values, the high concentration of H⁺ ions competes with the cationic MB molecules for the available adsorption sites,^36,37 thereby reducing dye uptake on the adsorbent surface.^38,39 As the pH increases, the concentration of H⁺ ions decreases and the functional groups such as carboxyl and hydroxyl groups on the adsorbent surface undergo deprotonation, generating negatively charged sites. These negatively charged sites enhance the electrostatic attraction between the adsorbent and the positively charged MB molecules, resulting in higher adsorption efficiency. The optimal adsorption performance was observed around pH 7, where M.CMS achieved nearly 98% removal efficiency, whereas CMS reached only about 64%. The significantly higher efficiency of M.CMS is attributed to the presence of additional carboxylic functional groups introduced during the maleation process, which provide more active binding sites for MB adsorption. ⁴⁰ When the pH was further increased to 8 and 9, a slight decrease in removal efficiency was observed for both adsorbents. This decrease may be associated with the presence of excess OH^- ions, which can interfere with dye transport and compete with MB molecules for adsorption site. ⁴¹ Although the adsorption efficiency increases from acidic to neutral conditions and reaches a maximum near neutral pH, with M.CMS consistently outperforming CMS due to its enhanced surface functionality.OPEN IN VIEWER

Figure 10(b) shows the effect of the initial MB concentration on the adsorption behavior of both CMS and M.CMS. As the initial dye concentration increased from 10 ppm to 40 ppm, the removal efficiency of M.CMS remained very high, decreasing only slightly from approximately 99% to about 96%, indicating the strong adsorption capacity of the modified adsorbent. In contrast, CMS showed a more pronounced decrease in removal efficiency, dropping from about 72% at 10 ppm to nearly 50% at 40 ppm. This trend occurs because increasing the initial dye concentration raises the number of MB molecules present in the solution, while the number of available adsorption sites on the adsorbent surface remains constant. Consequently, the dye molecules compete for a limited number of active sites, resulting in a reduction in the percentage removal at higher concentrations. The much smaller decrease observed for M.CMS suggests that the modified adsorbent possesses a greater number of active adsorption sites and stronger interaction with MB molecules compared to unmodified CMS. These results clearly demonstrate that maleation significantly enhances the adsorption performance of CMS, enabling M.CMS to maintain high removal efficiency even at elevated dye concentrations. Similar concentration-dependent adsorption trends have been reported in previous studies involving methylene blue adsorption using different adsorbent materials. ⁴²

Figure 11 shows a comprehensive comparison of the adsorption performance of CMS and M.CMS for the removal of MB under different experimental conditions. The figure is divided into three panels, each representing the influence of a specific parameter on adsorption efficiency. Figure 11(a) the effect of adsorbent dosage on the removal efficiency of MB using both CMS and M.CMS. As the adsorbent dose increased from 5 mg to 20 mg, the removal efficiency improved for both materials. For M.CMS, the removal efficiency increased slightly from approximately 96% to about 99%, indicating that even a small amount of the modified adsorbent is highly effective for dye removal. In comparison, CMS exhibited lower removal efficiency, increasing from about 82% to nearly 88% within the same dosage range. The increase in removal efficiency with increasing adsorbent dosage is mainly attributed to the availability of a larger number of active adsorption sites and increased surface area, which enhances the probability of interaction between the dye molecules and the adsorbent surface. The consistently higher performance of M.CMS compared with CMS confirms that maleation introduces additional functional groups that significantly enhance the adsorption capacity of the material. Similar trends have been observed in other studies such as, ⁴³ demonstrated that fruit peel waste was effective in removing MB, and^44,45 achieved similar results using grape-leaf waste as an adsorbent.OPEN IN VIEWER

Figure 11(b) the influence of contact time on the adsorption process. As the contact time increased from 10 min to 40 min, the removal efficiency increased for both adsorbents. For M.CMS, the removal efficiency increased rapidly from about 88% at 10 min to nearly 98% at 20 min, after which it remained almost constant, indicating that adsorption equilibrium was reached within approximately 20 minutes. In contrast, CMS showed a slower adsorption rate, increasing from approximately 70% at 10 min to around 79% at 40 min. The rapid increase observed during the initial stage is due to the presence of a large number of vacant adsorption sites on the adsorbent surface, allowing rapid dye uptake. As the adsorption process progresses, the number of available sites decreases, resulting in a slower increase until equilibrium is reached. ²⁴ The faster adsorption kinetics of M.CMS further demonstrate the beneficial effect of chemical modification in enhancing the interaction between the adsorbent surface and MB molecules.^46,47

In Figure 11(c) shows, a direct comparison of CMS and M.CMS under identical experimental conditions. The results clearly indicate that M.CMS exhibits significantly higher adsorption capacity and removal efficiency compared to CMS. While CMS shows relatively moderate adsorption performance, M.CMS demonstrates a substantially higher adsorption capacity, which can be attributed to the presence of additional carboxyl functional groups introduced during the maleation process. These functional groups increase the number of active adsorption sites and strengthen the electrostatic interaction between the negatively charged adsorbent surface and the positively charged MB molecules. This comparison highlights that the chemical modification of CMS significantly improves its adsorption efficiency, making M.CMS a more effective adsorbent for dye removal applications.^27,48

4.3. Thermodynamic, kinetic and isothermal modeling

Figure 12 shows a detailed analysis of the effect of temperature on the adsorption of MB by M.CMS and its thermodynamic implications. In Figure 12(a), the relationship between temperature and adsorption efficiency (% R) is shown, demonstrating a clear increase in removal efficiency as the temperature rises from 295 K to 330 K. Specifically, the removal efficiency increases from 98.2% to 98.6%, indicating that the adsorption process becomes more effective at higher temperatures. This positive temperature dependence suggests that the adsorption of MB on M.CMS is endothermic, where the higher temperature facilitates the mobility of MB molecules, enhancing the interaction between the dye molecules and the adsorbent surface. This behavior is consistent with other studies in the literature, where similar temperature-dependent trends were observed for various adsorbent materials used in dye removal, highlighting the importance of temperature in optimizing adsorption processes.OPEN IN VIEWER

In Figure 12(b), the Van’t Hoff plot is presented, where ln (K) (the natural logarithm of the equilibrium constant) is plotted against 1/T (K^-1) with a high correlation coefficient (R² = 0.9994). The negative slope observed in this plot further supports the endothermic nature of the adsorption process. According to the Van’t Hoff equation, a negative slope indicates that as the temperature increases, the adsorption capacity of M.CMS for MB increases, confirming that the process is spontaneous and temperature sensitive. This thermodynamic evaluation, indicated by the decrease in ln K with increasing temperature, is characteristic of processes where elevated temperatures reduce the energy barrier for adsorption and increase the affinity between the dye molecules and the adsorbent. The findings of this study are consistent with the general understanding of adsorption being more efficient at higher temperatures, as higher temperatures facilitate greater interaction between adsorbate and adsorbent.

The thermodynamic parameters for MB adsorption onto M.CMS are summarized in Table 3. The calculated ΔG° values were negative at all temperatures, confirming the spontaneous nature of MB adsorption onto M.CMS. Moreover, the magnitude of ΔG° increased from −9.81 kJ mol^-1 at 295 K to −11.67 kJ mol^-1 at 330 K, indicating that adsorption becomes more thermodynamically favorable at higher temperature. This trend agrees with the experimental increase in removal efficiency from 98.2% to 98.6% as temperature increased. The positive ΔH° value of 6.01 kJ mol^-1 confirms the endothermic nature of the adsorption process. However, the relatively low magnitude of ΔH° suggests that the adsorption is not purely strong chemisorption; rather, it likely involves combined physical and chemical interactions, including electrostatic attraction, hydrogen bonding, and surface affinity between the negatively charged carboxyl groups of M.CMS and cationic MB molecules. The positive ΔS° value of 53.68 J mol^-1 K^-1 indicates increased randomness at the solid-solution interface during adsorption. This increase may be attributed to the release of water molecules from the hydrated MB ions and active sites of M.CMS during adsorption. Therefore, both the increasingly negative ΔG° values and positive ΔS° value demonstrate that higher temperature enhances the feasibility and driving force of MB adsorption onto M.CMS.

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Table 3.

Thermodynamic parameters for MB adsorption onto M.CMS.

Temperature (K)	Removal (%)	Kc	ln Kc	ΔG° (kJ mol-1)	ΔH° (kJ mol-1)	ΔS° (J mol-1 K-1)
295	98.20	54.56	3.999	−9.81	6.01	53.68
305	98.35	59.61	4.088	−10.37	6.01	53.68
315	98.50	65.67	4.185	−10.96	6.01	53.68
330	98.60	70.43	4.255	−11.67	6.01	53.68

Figure 13(a) shows the pseudo-second order (PSO) kinetic model, which was used to describe the adsorption behavior of M.CMS for MB. The plot shows a linear relationship between t/qt vs. t, yielding a high correlation coefficient (R²= 0.998), indicating that the adsorption process follows the pseudo-second-order kinetics. This suggests that the rate-limiting step in the adsorption is primarily controlled by chemisorption, involving the sharing or exchange of electrons between the adsorbent and the dye molecules. The equilibrium uptake ( q e ) predicted by the PSO model was 28.4 mg/g, which is close to the experimental value of 27.5 mg/g, further confirming the model’s suitability for describing the adsorption process. The rate constant ( k 2 ) was calculated to be 0.030 g / m g · min , reflecting the kinetics of the adsorption, where the concentration of MB in solution and the available active sites on the M.CMS surface both influence the adsorption rate. ⁴⁹ This is consistent with findings reported by, ⁵⁰ who observed similar trends in their studies of MB adsorption using starch-based nanocomposite hydrogel microspheres.OPEN IN VIEWER

Figure 13(b) shows the Langmuir isotherm model for MB adsorption on M.CMS, which best fits the experimental data with a R² value of 0.999, indicating a high degree of linearity. This suggests that the adsorption follows a monolayer adsorption mechanism, where MB molecules are adsorbed onto a uniform surface of M.CMS with no interaction between adsorbed molecules. The separation factor ( R L ) was found to be 0.91, which is less than 1, confirming that the adsorption process is favorable under the tested conditions. The maximum adsorption capacity ( q max ) om the Langmuir model was 28.9 mg/g, which is consistent with the experimental value of 27.5 mg/g. This monolayer adsorption behavior aligns with similar findings reported by Shi et al. (2020), who also observed monolayer-type adsorption in studies involving hydrogel adsorbents. ⁵¹

Figure 13(c) shows the Freundlich isotherm model, which provided an R² value of 0.950, indicating a reasonable fit. This model suggests that adsorption takes place on a heterogeneous surface with varying adsorption energies. The Freundlich constant ( K F ) was calculated to be 29.1, which reflects the adsorption capacity, and suggests that the adsorption sites on M.CMS are not equivalent in terms of energy. Figure 13(d) shows the Temkin isotherm model, with an R² value of 0.974. The evaluated Temkin parameters include K T = 6.65 and B T = − 1.8 , where B T being negative indicates that the adsorption is endothermic. This indicate that stronger interactions between dye and adsorbent occur at lower concentrations, leading to a decrease in the energy required for further adsorption as the surface becomes saturated. A negative B T also implies that the adsorption energy decreases as the surface becomes progressively covered with adsorbate molecules, leading to a decline in the efficiency of further adsorption at higher concentrations. This aligns with the positive ΔH° observed in the thermodynamic study, supporting the conclusion that higher temperatures favor the adsorption process. The Temkin model describes adsorption occurring due to the interaction between adsorbate molecules and the adsorbent surface, with increasing temperature enhancing the adsorption. The results from the isotherm models suggest that MB adsorption on M.CMS predominantly follows the Langmuir model, indicating monolayer adsorption on a uniform surface, with a favorable adsorption process. The values from the isotherms were also supported by the thermodynamic findings, confirming the spontaneity and endothermic nature of the adsorption process. These findings align with the work of, ⁵⁰ who observed similar trends for MB adsorption on starch-based nanocomposites, and, ⁵¹ furthermore^52,53 also reported monolayer-type adsorption in studies involving hydrogel adsorbents.

The regeneration and reusability performance of M.CMS was evaluated over ten consecutive adsorption desorption cycles to assess its practical applicability as a reusable adsorbent for MB removal. As shown in Figure 14, the removal efficiency gradually decreased from 98.2% in the first cycle to 88.9% in the tenth cycle, while the adsorption capacity decreased from 27.5 mg g^-1 to 24.4 mg g^-1, corresponding to an overall loss of approximately 9.5% in removal efficiency and 11.3% in adsorption capacity after ten cycles. The slight but progressive reduction in adsorption performance can be attributed to incomplete desorption of MB molecules and partial occupation of active sites after repeated regeneration. The adsorption mechanism mainly involves electrostatic attraction and hydrogen bonding between the negatively charged carboxyl groups of M.CMS and the cationic MB molecules. During adsorption, the deprotonated carboxylic groups interact with MB cations according to the ion-exchange mechanism:

M . CMS − CO O − + M B + → M . CMS − CO O − · M B +

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Figure 14.

Regeneration and reusability performance of M.CMS during 10 consecutive adsorption-desorption cycles for methylene blue removal.

Additionally, hydrogen bonding occurs between hydroxyl groups of M.CMS and nitrogen-containing functional groups of MB:

M . CMS − OH ⋯ N − MB

During regeneration with acid (HCl), protonation of the carboxylate groups weakens the electrostatic interaction and promotes dye desorption:

M . CMS − CO O − · M B + + H + → M . CMS − COOH + M B +

However, repeated adsorption desorption cycles may cause partial hydrolysis or deactivation of the maleate-functionalized active sites and minor structural deformation of the polymer matrix, leading to reduced adsorption performance. Despite this gradual decrease, M.CMS retained more than 88% removal efficiency after ten cycles, demonstrating excellent regeneration ability, structural stability, and strong potential for repeated wastewater treatment applications.

To obtain a more comprehensive understanding of the adsorption kinetics, the experimental data were analyzed using pseudo-first order (PFO), and pseudo-second order (PSO) models. The calculated kinetic parameters are summarized in Table 4. Among the models tested, the PSO model exhibited the highest correlation coefficient ( R 2 = 0.998 ) and the calculated equilibrium adsorption capacity (28.4 mg g^-1) closely matched the experimental value (27.5 mg g^-1), confirming that the adsorption process is predominantly governed by chemisorption mechanisms involving electron exchange between methylene blue molecules and the functional groups of M.CMS.

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Table 4.

Kinetic modeling parameters for methylene blue adsorption onto M.CMS.

Model	Rate constant	( q e , c a l ) (mg g-1)	( q e , exp ) (mg g-1)	SSE	( R 2 )
PFO	( k 1 = 0.082 ) min-1	23.7	27.5	0.46	0.923
PSO	( k 2 = 0.030 ) g mg-1 min-1	28.4	27.5	0.04	0.998

The adsorption performance of M.CMS was compared with various previously reported adsorbents for MB removal, as summarized in Table 5. Activated carbon derived from tamarind seeds exhibited a removal efficiency of 95% with a maximum adsorption capacity ( q max ) of 21.5 mg g^-1 at an MB concentration range of 20-100 mg L^-1, pH 7, and a contact time of 60 min. Similarly, chitosan/PVA hydrogel beads achieved 92% removal with a ( q max ) of 24.8 mg g^-1 at pH 6 within 90 min, while sodium alginate gel beads showed 90% removal and a ( q max ) of 26.1 mg g^-1 after 120 min of contact time. A starch-based nanocomposite hydrogel demonstrated 94% removal efficiency with a ( q max ) of 27.0 mg g^-1 at pH 7- and 60-min contact time. In comparison, chitosan hydrogel reached 93% removal with a slightly higher adsorption capacity of 31.2 mg g^-1 at pH 7 within 90 min. More advanced materials such as magnetic graphene oxide hydrogel and Ag-MOF nanosheets showed higher adsorption capacities of 45.3 mg g^-1 and 52.7 mg g^-1, with removal efficiencies of 96% and 90.7%, respectively, although they required longer contact times (60-120 min) and involve more complex synthesis routes. In contrast, the synthesized M.CMS demonstrated a removal efficiency of 98.2% with an adsorption capacity of 28.9 mg g^-1 at an initial MB concentration of 20 mg L^-1, pH 7.4, and a significantly shorter equilibrium time of 20 min. These results indicate that M.CMS provides competitive adsorption capacity along with faster adsorption kinetics, highlighting its potential as an efficient and environmentally friendly adsorbent for dye removal from wastewater.

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Table 5.

Comparison of adsorption performance of different adsorbents for MB.

Adsorbent	Initial MB concentration (mg L-1)	pH	Contact time (min)	Removal (%)	qmax (mg g-1)	Reference
Activated carbon from tamarind seeds	20–100	7	60	95	21.5	43
Sodium alginate gel beads	10–100	6	120	90	26.1	54
Starch-based nanocomposite hydrogel	20–150	7	60	94	27.0	41
Chitosan hydrogel	10–200	7	90	93	31.2	55
Ag-MOF nanosheets	10–100	7	60	90.7	52.7	30
Carboxymethyl Starch Grafted Polyvinyl Pyrrolidone (Car-St-g-PVP)	50	6	60	91%	33.6	56
Activated Carbon (AC)	40	7	120	93%	45	57
M.CMS	20	7.4	20	98.2	28.9	This work

The regeneration and reusability performance of M.CMS was also assessed to evaluate the stability and efficiency of the modified adsorbent during adsorption-desorption cycles. As shown in Table 6, the adsorption characteristics were analyzed using absorbance (Abs), equilibrium concentration (Ce), adsorption capacity (qe), and removal efficiency (%R), focusing on the effect of the adsorbent dose. For the first dose of M.CMS (1 wt%), the absorbance was 0.088, corresponding to an equilibrium concentration (Ce) of 0.5459 mg L^-1. The adsorption capacity (qe) was 5.40 mg g^-1, and the removal efficiency (%R) was 96.11%. When the M.CMS concentration increased to 2 wt%, the absorbance significantly decreased to 0.026, and Ce dropped to 0.2464 mg L^-1. This resulted in a much higher adsorption capacity of 27.57 mg g^-1 and an enhanced removal efficiency of 98.24%. For M.CMS at 3 wt%, the absorbance further decreased to 0.010, with Ce = 0.1691 mg L^-1, yielding an adsorption capacity of 18.49 mg g^-1 and a removal efficiency of 98.80%. The highest adsorption performance was observed for M.CMS at 4 wt%, where the absorbance reached the lowest value of 0.001, corresponding to Ce = 0.1256 mg L^-1, an adsorption capacity of 13.91 mg g^-1, and the maximum removal efficiency of 99.10%. In addition to examining the adsorbent dose effect, the regeneration and reusability of M.CMS were evaluated by subjecting the adsorbent to multiple adsorption-desorption cycles. The results demonstrated that the modified adsorbent exhibited excellent stability and high reusability across several cycles, maintaining removal efficiencies above 96% after each regeneration. The high removal efficiencies throughout these cycles indicate that M.CMS retains its adsorption capability and functional stability even after repeated use. This confirms that the increase in M.CMS concentration enhances the adsorption capacity, primarily due to the availability of more active adsorption sites and stronger interactions between the dye molecules and the modified starch surface. These findings highlight the promising potential of M.CMS as an efficient, reusable, and stable adsorbent for dye-contaminated wastewater treatment, offering both high adsorption performance and effective regeneration over multiple cycles.

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Table 6.

Regeneration and reusability performance of M.CMS during adsorption-desorption cycles.

Sample	Abs	ce	qe	%R
M. CMS 1wt%	0.088	0.54589372	5.395169082	96.11015491
M. CMS 2wt%	0.026	0.246376812	27.57487923	98.2444062
M. CMS 3wt%	0.01	0.169082126	18.4863124	98.79518072
M. CMS 4wt%	0.002	0.125603865	13.90821256	99.10499139

Figure 15 shown the Receiver Operating Characteristic (ROC) curve of various M.CMS samples with varying concentrations from 1wt% to 6wt%. The M.CMS 1wt% curve shows the lowest TPR at a given FPR, implying that lower concentrations of M.CMS lead to suboptimal adsorption behavior, whereas the curves for M.CMS 3wt%, 4wt%, and 5wt% show intermediate performance. The step increase in TPR at low FPR for higher concentrations of M.CMS such as 5wt% and 6wt% highlights the material’s higher adsorption efficiency at higher concentrations, making it more effective for wastewater treatment and pollutant removal applications. This behavior correlates with the electrostatic mechanism and the charge density of the material. As the concentration increases, the surface charge density increases, enhancing the electrostatic interactions between M.CMS and the adsorbates, leading to higher adsorption performance. The effect of ionic strength and ionization behavior in this context can be quantitatively explained using the degree of ionization ( θ ) of the functional groups on the surface of M.CMS:

θ = 1 1 + 10 ( p K a − p H )

(21)

λ D = 1 8 π N A e 2 I ϵ ϵ 0 k B T

(22)

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Figure 15.

ROC curve.

4.5. Adsorption mechanism

The adsorption of MB by M.CMS occurs through two primary interaction mechanisms such as hydrogen bonding and electrostatic attraction. These interactions are fundamental to the adsorption efficiency and the overall performance of M.CMS in removing MB from aqueous solutions, making it highly relevant for applications such as water treatment and dye removal.

The first mechanism, hydrogen bonding, involves interactions between the lone-pair electrons on the nitrogen atoms of MB and the hydroxyl and carboxyl groups present on the M.CMS surface. As depicted in Figure 16, the amine groups (particularly the nitrogen atoms) in MB can act as hydrogen bond acceptors, while the hydroxyl and carboxyl groups of M.CMS serve as hydrogen bond donors. This leads to the formation of hydrogen bonds, providing multiple contact points between MB molecules and the M.CMS surface, which enhances the retention of the dye molecules and strengthens the adsorption process. The hydrogen bonding mechanism has been widely observed in similar studies, such as that by, ⁵⁴ demonstrated the feasibility of this mechanism in polymer-based adsorbents like sodium alginate gel beads for MB adsorption.OPEN IN VIEWER

The second mechanism, electrostatic attraction, is driven by the ionization of functional groups on the surface of M.CMS, which generates negatively charged sites, particularly carboxylate ions. On the other hand, MB molecules remain positively charged due to their cationic nitrogen centers. The resulting attractive forces between the negatively charged sites of M.CMS and the positively charged regions of MB facilitate the adsorption of MB onto the surface of the polymer. This type of charge-driven interaction has been well-documented in various studies, including, ⁵⁵ where chitosan-based hydrogels were employed for MB removal, highlighting the effectiveness of electrostatic interactions in cationic dye adsorption. Furthermore, ⁵⁸ emphasized that the pH-dependent ionization state of the adsorbent surface influences the dominance of either hydrogen bonding or electrostatic attraction in the adsorption process. At different pH values, the surface charge of M.CMS may change, thereby altering the relative contributions of each mechanism.

4.5. Future prospective

The successful removal of MB by M.CMS demonstrates its significant potential for future applications in water treatment. Technology could be particularly valuable in industries associated with water pollution, where it could be used to treat wastewater before being discharged into the environment, helping to reduce pollution levels and protect aquatic life. Beyond MB, M.CMS has shown promise in removing other types of dyes, such as reactive and acid dyes, which are commonly found in textile industry wastewater a major contributor to water pollution. This makes M.CMS a potential solution for textile industries seeking more sustainable wastewater treatment options.

In addition to dyes, M.CMS could be effective in adsorbing a broad spectrum of contaminants, including organic pollutants and heavy metals, making it a versatile material for environmental cleanup efforts. Its ability to remove MB, a dye with medical applications in tissue staining during surgeries, further highlights the potential of M.CMS in addressing medical waste issues. The application of M.CMS could play a critical role in improving environmental quality, particularly in areas affected by industrial pollution. However, the effectiveness of M.CMS in real-world conditions still requires further investigation. Future studies should focus on improving the physicochemical properties of M.CMS through structural modifications and optimization of synthesis conditions. Moreover, regeneration and reusability should be systematically assessed through multiple adsorption-desorption cycles to evaluate the material’s operational stability and economic feasibility for large-scale applications. This will help determine whether M.CMS can maintain its adsorption capacity over time and how it can be regenerated for continuous use.

Furthermore, the performance of M.CMS in real wastewater matrices, which contain a complex mixture of contaminants (e.g., salts, oils, and organic compounds), should be evaluated. Understanding how M.CMS interacts with these complex matrices will provide valuable insights into its practical applicability for large-scale wastewater treatment and pollution control. The future research directions outlined above will provide a solid foundation for establishing M.CMS as a sustainable and scalable adsorbent for environmental pollution control, ensuring its widespread applicability in water treatment systems worldwide.