Removal of Rhodamine B from Aqueous Solutions Using CS-g-p(AA-co-DEAP)/MWCNTs-COOH Hydrogel Nanocomposite as an Adsorbent

INTRODUCTION
Human activities in various industries, such as textiles, contribute to water pollution, through the discharge of dyes [1, 2]. These pollutants pose serious threats to both humans and other creatures. Rhodamine B (RhB), a highly toxic dye prevalent in textile wastewater, is valued for its high stability and non-degradability in the textile industry as a textile dye [3-5]. RhB also finds usage in ballpoint pens, paints, leather, dye lasers, carbon paper, stamp inks, explosives, and fireworks[6-9]. Classified as a carcinogenic, neurotoxic dye, RhB induces respiratory and skin infections, gastrointestinal irritation, and eye infections[10-13]. It exhibits developmental toxicity and mutagenicity in animals and humans. Intensive use of RhB is toxic via inhalation and ingestion, causing liver and thyroid damage, and irritations to the eyes and skin. Hence, the dye’s removal or reduction in water solutions is crucial[14]. Adsorption, a simple, easy, efficient, and relatively inexpensive method, is one of various ways to remove dyes [15-17]. In recent years, substantial research has been directed towards the use of nanoparticles as adsorbent materials [18-20] Nanomaterials have responded well to the need for high-surface-area materials, high adsorption capacity, and low cost that can successfully remove pollutants even at low concentrations[21-23]. Specific materials, particularly hydrogel nanomaterials, are used to overcome traditional adsorbents’ limitations[24]. The next generation of adsorbents (nano-adsorbents) for water treatment systems, with exceptional physical and chemical properties, make them superior adsorbent materials compared to their similar bulk counterparts[25]. MWCNTs-COOH have attracted significant interest in various fields, such as nanotechnology, materials science, and biomedical applications, due to their unique combination of the inherent properties of carbon nanotubes (strength, conductivity, etc.) and the reactivity of carboxyl groups[26-28]. The carboxyl groups can serve as attachment points for a variety of other molecules, enhancing the versatility and utility of these functionalized nanotubes. Moreover, MWCNTs-COOH can serve as effective adsorbents in environmental remediation due to their high surface area and the chemical reactivity of the carboxyl groups, which can interact with various contaminants[29]. These properties, among others, make MWCNTs-COOH a promising material for a multitude of applications[30].This article aims to offer an extensive overview of the adsorption process of RhB onto CS-g-p(AA-co-DEAP)/MWCNTS-COOH hydrogel nanocomposite surface, discussing its synthesis, characterization, and potential application in dye removal.

MATERIALS AND METHODS
Chemicals
High-purity chemicals used in this study, such as Acrylic acid (AA), Chitosan (CS), acetic acid, hydrochloric acid, sodium hydroxide, N,N-methylene-bis-acrylamide (MBA), potassium persulfate (KPS), 2-(((1E,2E)-1,2-diphenyl-2-((4(E-1-(thiazol 2ylimino) ethyl) phenyl) imino) ethylidene) amino) phenol (DEAP), Carboxylated multi-walled carbon nanotubes (MWCNTs-COOH) and Rhodamine B (RhB) were supplied by Sigma Aldrich.

Synthesis of CS-g-p(AA-co-DEAP)/MWCNTs-COOH hydrogel nanocomposite
The polymerization in an aqueous solution was conducted by dissolving 0.5g of CS in 20 mL 1% acetic acid for 30 min with stirring and nitrogen gas for 15 min. The temperature of this solution was then raised to 60°C. After that, KPS (0.05g dissolved in 2mL distilled water) was added during 10 min, in which stirring was continued and nitrogen gas was administered for an additional 10 min. Post this procedure, the solution was cooled to 25°C. Sequentially, 5g of AA was introduced to the solution with continuous stirring. This was followed by the addition of 0.1g of DEAP (dissolved in 2mL ethanol) for 15 min and 10 mL of MWCNTs-COOH (0.1%), with constant stirring. Subsequently, 0.05g of MBA (dissolved in 2ml distilled water) was added to the solution over 10 min, while stirring and administering nitrogen gas for an additional 20 min. This prepared solution was transferred into plastic bottles and placed in a water bath at 60°C for three hours. The resulting compound was cut into small pieces, washed with distilled water to remove any reactive substance, and then oven-dried at 70°C until a constant weight was achieved for further experiments. This process is illustrated in Fig. 1.

Characterisation
The prepared samples was characterized using several instruments including field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and X-ray diffraction (XRD), which were conducted in the Central Analytical Laboratory of KAC. Fourier transform infrared spectroscopy (FTIR) analyses were carried out at the University of Al-Qadisiyah laboratories.

Preparation of standard solutions of RhB dye
The standard solution of RhB dye used in this study was prepared at a concentration of 200 mg L-1 by dissolving 0.2 g of the dye in 1 L of deionized water. From this, various solutions with different concentrations were prepared.

Determination of maximum wavelength and calibration curve for RhB dye
The UV spectrum of RhB dye solutions in water at concentrations of 10 mg L-1, was recorded. Absorption spectra were obtained using a UV-Vis spectrophotometer within the range of 200-800 nm. The λmax for RhB dye was found to be 554 nm. The calibration curve for RhB dye was determined by preparing a series of solutions with consecutive dilutions of the standard RhB dye solution at concentrations ranging 1-10 mg L-1, using a UV-Visible spectrophotometer. The absorbance of these solutions was recorded at the maximum absorption wavelength of 554 nm for the dye[31]. A calibration curve was obtained by plotting the absorbance values against the concentrations. Effect of adsorbent weight on a series of weights ranging from 0.001 to 0.1 g of surface with 10 mL of dye solution (200 mg L-1) was studied [32]. The mixture was then placed in a shaker for 120 min at 25°C to reach equilibrium. Absorbance was measured after centrifugation at 6000 rpm for 15 min. The results were plotted as the amount adsorbed against the weight of the adsorbent to determine its effect on the adsorption process.

Equilibrium time for adsorption process
The equilibrium time for dye adsorption on the adsorbent surface was determined by fixing all conditions, including temperature (25°C), pH, dye concentration, and surface weight [33]. A solution of 200 mg L-1 RhB dye was prepared and 10 mL of the dye solution was mixed with 0.05 g of the composite. The mixture was placed in a shaker at 25°C under various durations (1-240 min). The absorbance of the solutions was measured after separation to determine the optimal time for the adsorption process.

Adsorption isotherms
Different concentrations of the dye (5-200 mg L-1) were prepared and 10 mL of each was added to 0.05 g of the adsorbent. The mixtures were placed in a shaker at a speed of 120 rotations per min (120 min) at a temperature of 25°C. The samples were then centrifuged, and their absorbance was measured using a UV-Visible spectrophotometer. The quantity of RhB dye uptake at equilibrium qe (mg g-1) was determined using the following Eq. 1 [34]:

                                                                                     (1)

Where Co (mg L-1) refers to the initial concentration of the dye. Ce (mg L-1) represents the dye concentration at equilibrium. V (L) stands for the solution volume. m (g) is the weight of CS-g-p(AA-co-DEAP)/MWCNTS-COOH hydrogel nanocomposite. By employing equation (1), researchers can evaluate the effectiveness of dye adsorption using CS-g-p(AA-co-DEAP)/MWCNTs-COOH under various experimental conditions.

Effect of temperature
To determine the effect of temperature on the amount of substance adsorbed from aqueous solutions, the study was conducted on the RhB dye at diverse temperatures such as 15, 20, 25, and 30°C. 0.05 g of the composite was introduced into different concentrations (50-500 mg L-1) of RhB in 10 mL of the dye solutions. The mixtures were placed in a shaker for 120 min, separated using a centrifuge and the absorbance was measured using a UV-Visible spectrophotometer.

Effect of acidity function
The effect of acidity function on the adsorption process was studied by employing 0.05 g of the sample and a concentration of 200 mg L-1 of RhB dye at different acidity function values (2-10) at a temperature of 25°C and duration of 120 min. The acidity function was adjusted using 0.1 M of hydrochloric acid solution and 0.1 M of sodium hydroxide solution. The pH values were measured using a pH meter. The results were analyzed by plotting the amount of adsorbed substance against the acidity function values.

RESULTS AND DISCUSSION
FT-IR
The hydrogel nanocomposites prepared before and after adsorption of RhB dye were analyzed using FT-IR spectra within a range between (400-4000 cm-1)[35]. It also showed the synergy between the atoms around the sites of the packages shown by the active functional groups, such as the carbonyl and hydroxyl groups in the acidic carboxyl group, the amine and carbonyl groups in the amide group, and other effective functional groups[36-38]. The characteristic bands of MWCNTs-COOH were observed in the composite’s FT-IR spectrum of Fig. 2a. The bands at 1719 cm-1 and 1197 cm-1, corresponding to the stretching vibrations of C=O and C-O-C, respectively, indicating the presence of carboxyl groups on the surface of CNT. FT-IR spectrum in Fig. 2b displayed characteristic bands associated with the Schiff base (DEAP). The absorption bands at 1643 cm-1 and 1529 cm-1 were attributed to the stretching vibrations of the C=N and C=C groups, respectively[39-43]. The presence of a broad absorption band around 3329 cm-1 was indicative of the -OH group in the phenol moiety. FTIR spectrum of chitosan in Fig. 2c showed the appearance of the hydroxyl group at 3440 cm-1 with an overlap with the NH group, as for the bending frequencies of the methylene and methyl, they appeared at 1380 cm-1 and 1427 cm-1, respectively.
The FT-IR spectrum of the CS-g-p(AA-co-DEAP)/MWCNTs-COOH composite in Fig. 3a revealed the formation of the composite, as compared to the individual components. A shift in the -OH and -NH2 stretching vibrations to lower wavenumbers, along with a decrease in the intensity of the carbonyl stretching band, suggested the involvement of these functional groups in complex formation. Additionally, the appearance of a new band at 1557 cm-1 indicated the presence of the coordinated Schiff base DEAP [44]. The FTIR spectra of the CS-g-p(AA-co-DEAP)/MWCNTs-COOH hydrogel nanocomposite before and after RhB adsorption revealed significant changes in the characteristic absorption bands, indicating interactions between the adsorbent and RhB. The shifts in the -OH, -NH2, and carbonyl stretching vibrations suggested the involvement of these functional groups in the adsorption process. Based on the FTIR analysis, the adsorption of RhB onto the composite occurred via hydrogen bonding, electrostatic interactions, and π-π stacking between the dye molecules and the adsorbent’s functional groups. The presence of MWCNTs-COOH in the composite contributed to the enhanced adsorption capacity due to their large surface area and strong affinity for RhB molecules[45]. The observed changes in the absorption bands in Fig. 3b supported the proposed adsorption mechanism, highlighting the potential of the synthesized composite in RhB removal from aqueous solutions.

FESEM and TEM analyses
The FE-SEM technique was employed to investigate the surface characteristics of synthesized materials, enabling the assessment of particle shape, size, aggregation, and surface nature (porous or smooth) [46, 47]. Furthermore, it facilitated the examination of component homogeneity, distribution on the surface, and interconnection of polymer chains[48]. The incorporation of MWCNTs-COOH into the CS-g-p(AA-co-DEAP) appeared to enhance the porous structure, which may contribute to the composite’s increased adsorption capacity for RhB. The FE-SEM images in Fig. 4(a and b) indicate an increase in surface smoothness resulting from the complete filling of surface pores by dye molecules. Consequently, the surface becomes entirely covered by the dye molecules, confirming the adsorption process[49]. The TEM images reveal valuable insights into the surface composition and response to the adsorption process[50]. The TEM analysis depicted in Fig. 4(c and d) demonstrates that CS-g-p(AA-co-DEAP)/MWCNTs-COOH hydrogel nanocomposite is dispersed particles that are uniformly distributed and regularly arranged[51]. Additionally, the presence of some agglomerates is noted. The surfaces are characterized by the formation of thin layers, which contribute to their overall structure. Upon the occurrence of the adsorption process, several changes in surface morphology are observed. Firstly, an increased presence of agglomerates with larger sizes is detected[52]. This phenomenon is attributed to the interference between dye molecules and surface molecules during the adsorption process[53]. Secondly, the thin layers of overlying materials diminish, due to the absorption and accumulation of dye molecules that penetrate the pores and grooves on the surface[54].

TGA Analysis 
TGA curves of the sample heated in the presence of nitrogen gas within the temperature range of 40-800°C at a rate of 10°C min-1 are showed in Fig. 5 [55, 56]. The TGA curve of MWCNTs-COOH was thermally stable, and the mass loss was only 6.4% in the temperature range of 60–700 °C, which can be regarded as the content of carboxyl functional groups[57].The thermal characteristics of CS-g-p(AA-co-DEAP) and CS-g-p(AA-co-DEAP)/MWCNTs-COOH hydrogel nanocomposite demonstrated similar degradation profiles with curves disintegrated in three steps. The first degradation of 7.1% occurred in the temperature range of 60–200 °C due to the loss of water residual. In the temperature region of 200–300°C, there was a second decomposition stage of about 28.4% due to degradation of CS. In the third degradation profile, about 40.3% weight reduction occurred in the temperature range of 300–500°C due to the breaking of its carboxyl from acrylic acid groups and the cleavage of Schiff base linkages and degradation of the residual polymer[58]. The TGA curves revealed that the thermal stability and decomposition behaviour of hydrogels are strongly influenced by factors such as the nature of the monomers, crosslinking density, and the presence of functional groups[59]. The TGA results showed a significant improvement in the thermal stability of the CS-g-p(AA-co-DEAP)/MWCNTs-COOH hybrid materials compared to the pristine CS-g-p(AA-co-DEAP)[60]. The decomposition process occurred in multiple stages, corresponding to the degradation of the chitosan backbone, the loss of grafted poly(acrylic acid) chains, DEAP and the decomposition of MWCNTs-COOH. The enhanced thermal stability could be attributed to the strong interactions between the copolymer and MWCNTs-COOH, as well as the efficient dispersion of the nanotubes within the matrix[61].

XRD results
XRD measurement were used to study the crystal structure of MWCNTs-COOH and CS-g-p(AA-co-DEAP)/MWCNTs-COOH hydrogel nanocomposite [62, 63]. Generally, it is clear from the Fig. 6a that the MWCNTs-COOH exhibit three peaks; one at 26.1 ° (d =3.41 Å), is attributed to the distance between walls in MWCNTs-COOH  and others at 42.7° (d = 2.10 Å), and at 44.5° (d = 2.02 Å) are corresponded to the MWCNT interwall spacing, which can be indexed as the (002) plane of the graphitic carbon (JCPDS-ICDD No. 751621), (100) and (101) reflections of the carbon atoms, respectively[64]. Fig. 6b displays the XRD pattern of the CS-g-P(AA-co-DEAP)/MWCNTs-COOH composite hydrogels. The MWCNTs-COOH diffraction values suggest a well-dispersed and uniform distribution of MWCNTs-COOH in the composite hydrogel matrix. This dispersion alters the crystalline nature to a non-crystalline state, as evidenced by the decreased diffraction peak and the increased interlayer distance. The increase in interlayer distance is attributed to the penetration of organic molecules into the interlayer spaces, confirming the capacity of the nanostructures to accommodate structural deformations and swelling[65, 66].

Adsorption process
Effect of catalyst weight
Various weights were taken from the prepared CS-g-PAA\DEAP composite ranging from)0.1 g to 0.005 g), at a constant concentration of 200 ppm for the RhB dye. 10 mL of the dye solution was added to the prepared weights at a temperature of 25°C. The results in Fig. 7 indicated that the removal percentage (Re%) of RhB dye increases with an enhancement in the weight of the adsorbent material, due to an increase in the surface area and consequently an increase in the number of active sites for the adsorbent material.
The impact of the MWCNTs-COOH concentrations (ranging from 0.01 to 0.3 wt%) used in the synthesis of the CS-g-PAA/DEAP/MWCNTS-COOH composite on the adsorption process was examined. At a constant weight of the composite (0.05g) at 25˚C, 10mL of RhB dye solution with a fixed concentration of 200ppm was added. The results, as depicted in Fig. 8, indicate that the optimal concentration of MWCNTs-COOH is 0.1%. An increase in the proportion of MWCNTs-COOH in the composite leads to an increase in hydrophilic aggregates. Furthermore, the uniform dispersion of MWCNTs-COOH within the hydrogel would affect the structural composition of the hydrogel network due to its nanoscale structural nature. Consequently, a gradual increase in the quantity of adsorption occurs as the concentration of MWCNTs-COOH increases until it reaches a point where the amount of adsorption decreases. This decline can be attributed to the fact that excessive augmentation in MWCNTS-COOH substrates leads to the formation of agglomerates. These agglomerates tend to weaken the interaction between MWCNTS-COOH and the hydrogel network.

Equilibrium time
The appropriate equilibrium time for the removal of a specific concentration of RhB dye by the adsorbent surface was studied at 25˚C, pH=7, a constant adsorbent weight of 0.05g, and dye concentration of 200ppm for different time intervals ranging from 1 to 240 min. According to the results depicted in Fig. 9, the necessary equilibrium time for RhB dye is 120 min. During the adsorption process, the quantity of adsorbed dye rapidly increases, followed by a gradual increment until the equilibrium time is reached. This quick increase in the adsorbed dye quantity is due to the large number of unoccupied active centers at the beginning of the adsorption process, which is sufficient for dye adsorption. Following this, the adsorption process becomes slower and more challenging due to all the active centers of the adsorbent surface being occupied by dye molecules.

Adsorption isotherms
Adsorption isotherms were studied to describe the relationship between the amounts of dye adsorbed (Qe) on the composite surface and its equilibrium concentration (Ce). Based on the information obtained from the study and according to Giles classification, the results indicated that the adsorption process for the RhB dye corresponds to the L4 type, also known as Langmuir type 4, where adsorption occurs in a monolayer, and the orientation of the adsorbed molecules is horizontal on the adsorbent surface[67]. Langmuir, Freundlich, and Temkin isotherm models were applied to describe the adsorption properties of the adsorbent materials used for the removal of pollutants from the environment. As shown in Table 1 and Fig. 10, Freundlich’s isotherm shows a high degree of correspondence for the adsorption of RhB dye, as evidenced by the correlation coefficient (R2) value of 0.9884, which indicates the heterogeneous nature of the adsorbent surface. According to Freundlich’s equation, the adsorption is not confined to a single molecular layer, but it is multilayered.

Effect of temperature and calculation of thermodynamic functions of the adsorption process
The adsorption process of RhB dye was performed on the surface of the composite at varying temperatures (15, 20, 25, and 30˚C), and at concentrations within the range of 25-200 mg L-1. It has been noted that temperature plays a significant and principal role in the adsorptive ability of the composite surface to adsorb dye from its aqueous solutions[68]. The obtained results suggest a decline in the amount of adsorbed dye (RhB) as temperatures escalate, as depicted in Fig. 11 and Fig. 12. It is deduced that the adsorption process is exothermic. This can be attributed to the rise in temperature, which enhances the solubility of adsorbed dye particles in the solvent, consequently diminishing the dye particles’ adsorption affinity towards the absorbent surface. Furthermore, increasing temperatures boost the kinetic energy of adsorbed molecules towards the adsorbent surface, leading to an augmentation in the system’s entropy (∆S). This corresponds to a decline in the system’s order, which refers to the movement of adsorbed substance molecules on the adsorbent surface. This results in a weaker attraction between dye molecules and active sites on the adsorbent surface[69].
Thermodynamic functions were calculated by studying the impact of temperature on the adsorption process (∆G free energy, ∆H enthalpy, ∆S entropy), due to their significance in understanding the adsorption process[70, 71]. These functions provide a detailed description of the dye molecule arrangement resultant from molecular interactions, as observed by measuring ∆S entropy changes. Additionally, by measuring ∆H enthalpy changes, it becomes possible to identify the dominant forces, whether chemical or physical and the reaction’s direction. Table 2 illustrates thermodynamic function values for the dye adsorption process. The negative value of the heat content ∆H indicates that the RhB dye adsorption process is an exothermic process. This signifies that the mutual action between the adsorbent surface and dye molecules is expected to decrease with increase in temperature factor, which can be attributed to the disruption of bonds formed between the active centers of the adsorbent surface and dye molecules[72]. The negative value of the free Gibbs energy also indicates that the RhB dye adsorption process is spontaneous under the conditions in which the adsorption was conducted. Moreover, the negative value for the entropy change in the RhB dye adsorption suggests a reduction in the degrees of freedom for the adsorbed molecules, implying that these molecules are more constrained and ordered. The RhB dye adsorption is considered physical adsorption because the ∆H value is less than 40 KJ/mol[73].

Effect of pH function on adsorption
The influence of acidity function on the adsorption process of RhB dye on the prepared adsorbent composite surface at a concentration of 200 mg L-1 was studied. Acidity function values ranged from 2 to 10 while temperature and equilibration time conditions were fixed. The results, as shown in Fig. 13, indicate that the amount of dye adsorbed on the composite surface increases with the rise of acidity function[74]. This is attributed to the ionization of -COOH, and OH groups at high pH, making the composite surface negatively charged. This induces an electrostatic attraction between the positively charged dye molecules and the composite surface, enhancing the adsorption process. Also, repulsion between negative groups on the composite surface increases swelling, allowing dye molecules to diffuse into the composite surface, thereby enhancing adsorption[75]. Conversely, at low pH, high concentrations of H+ ions in the solution lead to competition between them and the positively charged dye molecules for active sites, decreasing adsorption[76]. Furthermore, at low pH, the composite surface groups -COOH, OH are protonated, leading to hydrogen bonding between polymer chains and carbon nanotubes, which makes the surface contract, hindering the diffusion of dye molecules into the composite [77]. Additionally, repulsion between the -COOH, OH groups on the composite surface and the positive charge of the dye hinders the molecules’ access to the surface, resulting in a decrease in the adsorption process[78].

CONCLUSION
Upon analyzing hydrogel nanocomposites, it was identified that the incorporation of MWCNTs-COOH enhanced the composite’s porous structure, increasing adsorption capacity for RhB dye. The synthesized materials were analyzed using TEM, XRD, and TGA techniques. The FESEM results demonstrated increased surface smoothness due to dye molecule absorption. The TGA results showed the thermal stability of the hybrid materials was significantly improved compared to the CS-g-p(AA-co-DEAP) alone. XRD patterns confirmed a well-dispersed and uniform distribution of MWCNTs-COOH in the composite hydrogel matrix, confirming the capacity of nanostructures to accommodate structural deformations. Composite weight and MWCNTs-COOH concentration were investigated to identify optimal concentrations in the adsorption process. A Freundlich isotherm provided the best fit for the adsorption data, suggesting a multilayered, heterogeneous nature of the adsorbent surface. The effect of temperature on the adsorption process was noted, indicating an exothermic process, with a reduction in the degrees of freedom for adsorbed RhB dye molecules.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.