Removal of Rhodamine B from Aqueous Solutions Using Chitosan-g-poly(Acrylic Acid)/2-(((1E,2E)-1,2-Diphenyl-2-((4(E-1-(Thiazol 2ylimino) Ethyl) Phenyl) Imino) Ethylidene) Amino) Phenol Composite as an Adsorbent

INTRODUCTION
The increasing discharge of hazardous materials into water bodies due to various human activities has resulted in a critical global challenge – contemporary aquatic pollution [1]. This pollution has significant consequences for the environment, economy, and human health [2-4]. Among the major contributors to water toxicity are toxic metal ions and dyes, which have become more prevalent with industrial development [5]. These pollutants negatively impact aquatic life, interfere with plant enzymes, and necessitate immediate action to address this environmental threat and prevent numerous human diseases [6-9].
Rhodamine-B, a synthetic xanthene dye, is widely utilized in textiles, food, cosmetics, and other industries for its bright color, excellent solubility, and high photostability [10, 11]. Its strong absorbance and fluorescence properties make it suitable for various applications such as biological imaging, laser dyes, and tracing water flow in rivers and reservoirs [12]. However, the release of Rhodamine-B into the environment presents significant risks, including water pollution, aquatic life toxicity, and potential human health hazards [13]. Research has shown that Rhodamine-B can be toxic to various aquatic organisms, including fish, algae, and microorganisms, and can bioaccumulate in living organisms, leading to long-term health effects [14]. As a result, there is an urgent need for effective methods to remove Rhodamine-B from wastewater and reduce its environmental impact [15].
Chitosan (CS), a biopolymer derived from chitin, offers unique properties such as biodegradability, biocompatibility, non-toxicity, and excellent adsorption capacity, making it a promising candidate for pollutant removal [16-19]. However, the adsorption capacity of native chitosan is limited by its poor water solubility. To address this limitation, researchers have developed a composite material by grafting poly(acrylic acid) (PAA) chains onto the chitosan backbone, creating CS-g-p(AA)[20-23]. This modification significantly improves chitosan’s solubility and adsorption capacity by introducing carboxyl groups that can form hydrogen bonds with target molecules [24]. Additionally, incorporating Schiff base moieties into the composite further enhances its adsorption capacity by providing more binding sites [25].
 This article aims to offer an extensive overview of the adsorption process of Rhodamine-B onto CS-g-p(AA)/ DEAP composite surface, discussing its synthesis, characterization, and potential application in dye removal.

MATERIALS AND METHODS
Instruments
In this study, several instruments were employed, including a double-beam visible-ultraviolet spectrophotometer, a Fourier Transform Infrared Spectrometer (FTIR), a thermogravimetric analyzer, a FESEM, TEM, and a centrifuge. Additionally, an electric shaker was utilized.

Chemicals
High-purity chemicals were 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), 4-Aminoacetophenone, 2-aminothiazol, 4-amino-2-hydroxy benzoic acid, benzil, Glacial Acetic Acid and Rhodamine B (Rh-B) Fig. 1. All chemicals were supplied by Sigma Aldrich.

Preparation of Schiff base (DEAP) compound
The compound (DEAP) was synthesized in two steps. The first step included the preparation of compound A, which is (E)-4-(1-(thiazo-2-ylimino) ethyl) aniline, which was prepared from dissolving (1.5 g, 1 mmol.) of 2-aminothiazol in (25 ml) of absolute ethanol and by continuing to stir, add to it (1.35 g, 1 mmol.) of 4-aminoacetophenone dissolved in (25 ml) of absolute ethanol. The mixture was refluxed for 8 hours, after which the mixture was cooled, and was observed that a precipitate formed, which was filtered and dried, and then recrystallized from absolute ethanol and then the precipitate was collected, giving a product of (73%) and its melting point (207°C). As for the second step, included the preparation of compound (DEAP) by dissolving compound (A) in an amount of (2.1 g, 1 mmol.) in (15 ml) of absolute ethanol, and by continuous stirring, was added to it a solution (2.1 g, 1 mmol.) of benzil dissolved in (25 ml) of absolute ethanol and (1g, 1 mmol) of 2-aminophenol dissolved in (15 ml) of absolute ethanol and 4-5 drops of glacial acetic acid were added to the mixture. The mixture was refluxed for 8 hours; then, the mixture was cooled, where it was observed that a precipitate formed, which was filtered and dried, and recrystallized from absolute ethanol. The compound 2-(((1E,2E)-1,2-diphenyl-2-((4(E-1-(thiazol 2ylimino) ethyl) phenyl) imino) ethylidene) amino) phenol (DEAP) (Fig. 2) was obtained as solid orange yield 87%, m.p (2270C). 

Preparation of Chitosan-g-poly(acrylic acid)/2-(((1E,2E)-1,2-diphenyl-2-((4(E-1-(thiazol 2ylimino) ethyl) phenyl) imino) ethylidene) amino) phenol composite
Using the free radicals polymerization in an aqueous solution, by dissolving 0.5g of Chitosan (CS) in 20 mL from 1% acetic acid for 30 min with stirring and nitrogen gas for 15 min, the solution temperature was raised to 60° C with the added KPS (0.05g dissolved in 2 mL of distilled water) for 10 min with stirring, and nitrogen gas lasting for 10 min, after which the solution was cooled to 25 ° C, then add 5g of AA to above the solution with continuous stirring, after adding (0.1g) of DEAP for 15 min (dissolved in 2 ml ethanol) to the solution for 10 min; with continuous stirring, add (0.05g) of MBA (dissolved in 2ml distilled water) to the solution for 10 min, and stir with nitrogen gas for 20 min. This solution was transferred to plastic bottles and placed in a water bath at 60 ° C for three hr after the produced composite is cut into small pieces, washed with distilled water to remove interactive material, then dried in the oven at 70°C until a constant weight is obtained for further experiments Fig. 3.

Product characterisation
The prepared surface was characterized using Field Emission Scanning Electron Microscopy (FESEM), thermogravimetric analysis (TGA), and transmission electron microscopy (TEM), which were conducted in the Central Analytical Laboratory of KAC. Infrared spectroscopy (FTIR) analyses were carried out at the University of Al-Qadisiyah laboratories.

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

Determination of Maximum Wavelength and Calibration Curve for (Rh-B) dye
The UV spectrum of (Rh-B) 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 with a 1 cm quartz cell. The λmax for (Rh-B) dye was found to be 554 nm. The calibration curve for (Rh-B) dye was determined by preparing a series of solutions with consecutive dilutions of the standard (Rh-B) dye solution at concentrations ranging from 1 to 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[26]. A calibration curve was obtained by plotting the absorbance values against the concentrations. Effect of Adsorbent Weight on Adsorption
A series of weights ranging from 0.001 to 0.1 g of surface with 10 mL of dye solution (200 mg L-1) [27]. The mixture was then placed in a shaker for 120 minutes at 25°C to reach equilibrium. Absorbance was measured after centrifugation at 6000 rpm for 15 minutes. 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, while varying the time factor [28]. A solution of 200 mg L-1 (Rh-B) dye was prepared, and 10 mL of the dye solution was added to 0.05 g of the composite. The mixture was placed in a shaker at 25°C for varying durations (1-240 minutes). The absorbance of the solutions was measured after separation using a centrifuge 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 minute for 120 minutes for (Rh-B) dye at a temperature of 25°C. The samples were then centrifuged, and their absorbance was measured using a UV-Visible spectrophotometer. The quantity of (Rh-B) dye absorbed at equilibrium qe (mg g-1), dye uptake, were determined using the following Eq. 1 [29-31]:

 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-PAA/ DEAP. By employing equations (1), researchers can evaluate the effectiveness of dye adsorption using CS-g-PAA/ DEAP 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 (Rh-B) dye at temperatures ranging from 15, 20, 25, and 30°C. A 0.05 g of the composite was added to different concentrations (50-500 mg L-1) for Rh-B in 10 mL of the dye solutions. The mixtures were placed in a shaker for 120 minutes for (Rh-B) dye, 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 taking 0.05 gm of the surface and adding it to a dey solution with a concentration of 200 mg L-1 for (Rh-B) dye at different acidity function values (2-10) at a temperature of 25°C and a duration of 120 minutes for (Rh-B) dye. The acidity function was adjusted using 0.1 M of hydrochloric acid solution and 0.1 M of sodium hydroxide solution, and the pH values were measured using a pH meter. The results were analyzed by plotting the amount of adsorbed substance against the acidity function value to determine its effect on the adsorption process.

RESULTS AND DISCUSSION
Characterization of the composite by FT-IR
Infrared spectroscopy was utilized to diagnose the locations of functional group bands and to characterize the interatomic interactions. The FT-IR spectrum of CS-g-pAA Fig. 4a exhibited characteristic bands corresponding to the functional groups present in the polymer. The broad absorption band observed around 3420 cm-1 was assigned to the stretching vibrations of -OH and -NH2 groups in chitosan. The band at 1653 cm-1 indicated the carbonyl stretching of the carboxylic acid groups in poly(acrylic acid), while the band around 1559 cm-1 corresponded to the -NH2 bending vibrations[32]. The FT-IR spectrum Fig. 4b of the Schiff base (DEAP) displayed characteristic bands associated with its molecular structure. 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 [33]. The presence of a broad absorption band around 3329 cm-1 was indicative of the -OH group in the phenol moiety. The FT-IR spectrum Fig. 4c of the CS-g-PAA/ DEAP composite revealed the formation of the composite, as evidenced by the changes in the absorption bands 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 composite formation [34]. Additionally, the appearance of a new band at 1527 cm-1 indicated the presence of the coordinated Schiff base (DEAP). The FTIR spectra Fig. 4d of the CS-g-PAA/DEAP composite after (Rh-B) dye adsorption revealed significant changes in the characteristic absorption bands, indicating interactions between the adsorbent and (Rh-B) dye. The FTIR peaks of (Rh-B) dye after adsorption onto the CS-g-PAA/DEAP appear at 1198 and 1355 cm-1 which are assigned as C-O-C and C-aryl of RhB molecules. Because of the adsorption of Rh-B dye on the CS-g-PAA/ DEAP composite, the stretching vibration of the C–H bond was changed from 2920 cm−1 to 2918 cm−1.Moreover, the C-N peak of (Rh-B) dye after adsorption is observed at 1614 cm-1 which relates the electrostatic adsorption between (Rh-B) dye and composite. In addition to this, FTIR peaks from 1525 to 1355 cm-1 are due to the aromatic ring structure and red-shifted from pure (Rh-B) dye due to π-π interaction [35]. FTIR spectra reveal the main interacting forces like π-π interaction and electrostatic interaction/ hydrogen bonding [36-39].

FESEM and TEM analyses
The Field Emission Scanning Electron Microscopy (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). Furthermore, it facilitated the examination of component homogeneity, distribution on the surface, and interconnection of polymer chains [40]. Upon incorporating the Schiff base (DEAP) into the hydrogel, the resulting composite, CS-g-PAA/DEAP Fig. 5a, demonstrated a morphological alteration characterized by increased roughness, porosity, and irregular structure. The reagent molecules were homogeneously dispersed throughout the hydrogel matrix [41]. The absence of Schiff base aggregations can be ascribed to the interfacial interaction between the schiff base and the hydrogel matrix, facilitated by the functional groups present on both the DEAP and hydrogel surfaces.
Concerning the adsorption of rhodamine dye on CS-g-PAA/ DEAP composite, the FE-SEM images in Fig. 5b 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.
The TEM images reveal valuable insights into the surface composition and response to the adsorption process [42]. The TEM analysis depicted in Fig. 5c demonstrates that CS-g-PAA/DEAP composite exhibit small, dispersed particles that are uniformly distributed and regularly arranged [43]. 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 [44]. This phenomenon is attributed to the interference between dye molecules and surface molecules during the adsorption process [45]. Secondly, the thin layers of overlying materials diminish, likely due to the absorption and accumulation of dye molecules that penetrate the pores and grooves on the surface. The penetrating Transmission Electron Microscope (TEM) technique proves to be an effective method for analyzing the properties of surface CS-g-PAA/DEAP composite. The findings highlight the uniform distribution of particles, the presence of thin layers, and the changes in surface morphology upon the adsorption process Fig. 5d. Further research is recommended to explore the implications of these observations on the functional properties of the materials under investigation [46].

Thermo gravimetric Analysis (TGA)
TGA curve showed from Fig. 6, where the sample was heated in the presence of nitrogen gas within the temperature range of 40-800°C at a rate of 10°C.min-1. The TGA curve of Schiff base (DEAP) exhibited a multi-step decomposition process, indicating the presence of different decomposition stages with varying mass loss profiles[47]. The initial mass loss stage, observed between 100-250°C, can be attributed to the loss of volatile components, such as water or solvent molecules. Subsequent mass loss stages, observed between 250-600°C, are attributed to the decomposition of the compound’s organic moieties, including phenol, imine, and thiazole groups [48]. The final mass loss stage, observed above 600°C, corresponds to the complete decomposition of DEAP and the formation of residual char.The thermal characteristics of CS-g-PAA and CS-g-PAA/DEAP composite demonstrated similar degradation profi les with curves disintegrated in three steps. The first degradation 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 due to degradation of CS. In the third degradation profile in weight reduction occurred in the temperature range of 300–500°C due to the breaking of its carboxyl from acrylic acid groups and degradation of the residual polymer [49]. The TGA curves revealed that the thermal stability and decomposition behavior of hydrogels are strongly influenced by factors such as the nature of the monomers, crosslinking density, and the presence of functional groups [50]. The TGA results showed a significant improvement in the thermal stability of the CS-g-PAA/ DEAP composite hybrid materials compared to the pristine CS-g-PAA.

Effect of Surface Weight
The effect of surface weight on the removal of (Rh-B) dye has been studied by utilizing varying weights of the adsorbent material at a constant concentration of (Rh-B) dye (200 mg L-1) at 25°C Fig. 7. The findings of the study are presented in Fig. 6, which illustrates the impact of surface weight on the adsorption of (Rh-B) dye at 25°C and a pH of 7.

Factors that affect the adsorption of dye
The adsorption isotherm for the (Rh-B) dye was calculated on the surface of the CS-g-PAA/ DEAP composite. The results demonstrated that the adsorption process corresponds to the S4 category according to the Giles Classification, as illustrated in Fig. 8. In this category, the adsorbed molecules tend to orient themselves perpendicularly or obliquely on the adsorbent surface [51]. This type of orientation indicates the occurrence of multilayer adsorption due to the heterogeneous nature of the surface [52].

Effect of the temperature
The results, as shown in Fig. 9, demonstrate that the amount of dye adsorbed on the surface of the composite decreased with increasing temperature, indicating that the adsorption process is exothermic [53]. This is attributed to the fact that rising temperatures reduce the adsorption of dye on the absorbent surface by increasing the solubility of adsorbed dye particles in the solvent. On the other hand, an increase in temperature also increases the kinetic energy of the adsorbed particles, which in turn increases the randomness in the system (increase in system entropy ΔS), thereby weakening the attraction between dye molecules and active sites on the absorbent surface [54-56].
Thermodynamic function values are of great importance since they explain many reactions, particularly in the adsorption process. They enable the determination of the reaction direction and the nature of the controlling forces, and provide a good description of the regularity of the molecules resulting from molecular interactions. The enthalpy change (ΔH) value represents a measure to identify the interaction forces between the adsorbed dye molecules and the adsorbing surface[57]. By plotting lnXm versus the inverse of absolute temperature 1000/T, as shown in Table 1 and Fig. 10, a linear relationship is obtained, and the enthalpy change (ΔH) value for (Rh-B) dye adsorption on the composite surface is determined.
The thermodynamic functions have been calculated to interpret the adsorption processes of Rh-B dye on the surface of the composite. Table 2 presents the thermodynamic function values for the dye adsorption process on the composite surface. The negative sign of the change in enthalpy (ΔH) implies that the adsorption process is exothermic due to the disruption and dissociation of bonds that form between the active centers of the absorbent surface and dye particles, i.e., the adsorption process will decrease with increasing temperature. Meanwhile, the negative value for the change in entropy (ΔS) signifies a reduction in the randomness of the adsorbed particles, meaning they will become less ordered[11, 58]. The results also showed that the adsorption process occurred spontaneously under the reaction conditions, as indicated by the negative value for the Gibbs free energy (ΔG) [59-64]. 
Langmuir, Freundlich, and Temkin adsorption models have been applied to describe the adsorption properties of the adsorbents used in the removal of pollutants, as seen in Table 3 and Fig. 11. It appears that the adsorption of Rh-B dye on the surface of the composite is consistent with the Freundlich model [65-70]. This is clearly demonstrated by the correlation coefficient (R2) for the dye on the composite surface, which equals 0.9749.
The applicability of the Freundlich isotherm to the adsorption of Rh-B dye on the composite surface indicates the heterogeneous nature of the surface and that the active sites on the composite surface have unequal energies. It also suggests that the adsorption is not confined to a single molecular layer, but is multilayered [71-73].

Effect of pH
In this study, the impact of acidity on the adsorption process of Rhodamine B dye on the surface of the CS-g-PAA/ DEAP composite was investigated. The dye concentration was fixed at 200 ppm, and different pH values ranging from 2 to 10 were considered. The other variables, such as time and temperature, were held constant. As depicted in Fig. 12, acidity was found to have a significant effect on the adsorbent material and the composite surface, as well as on the interactions between them. It was observed that the dye adsorption on the composite surface increased with increasing acidity [74]. This can be attributed to Rhodamine B being a cationic dye [75]. At low pH values, the concentration of H+ ions in the solution is very high, leading to competitive interactions between the dye molecules and the H+ ions for the active sites on the composite surface. This results in a decrease in adsorption. Furthermore, the composite surface contains carbonyl, carboxyl, and hydroxyl groups. At low pH values, these groups are protonated, leading to hydrogen bonding between the chains and causing the composite surface to shrink. This makes it difficult for the dye molecules to diffuse into the composite surface. Additionally, electrostatic repulsion occurs between the COOH and OH groups on the surface and the positively charged dye, making it difficult for the dye molecules to access the active sites on the surface and reducing adsorption[76]. On the other hand, at high pH values, the carboxyl and hydroxyl groups on the composite surface deprotonate and become negatively charged. This leads to the electrostatic attraction between the negatively charged surface and the positively charged dye molecules, resulting in increased adsorption.[77] Moreover, at high pH values, the negatively charged functional groups on the composite surface repel each other, causing swelling and expansion. This allows the dye molecules to diffuse into the composite surface, thereby increasing adsorption [78].

CONCLUSION
Based on the experimental study, the CS-g-PAA/DEAP composite surface effectively adsorbed Rhodamine B from its aqueous solution with an optimal equilibrium time of 120 minutes. The adsorption process was found to be exothermic and influenced by acidity. The adsorption of Rhodamine B occurred spontaneously on the composite surface, with Rhodamine B showing a negative entropy change. The adsorption of Rhodamine-B onto CS-g-pAA/DEAP composite surfaces has demonstrated promising potential for the removal of dyes and other pollutants from wastewater. The unique properties of these composites, including their enhanced solubility, adsorption capacity, and affinity towards dye molecules, make them ideal candidates for various wastewater treatment applications. By understanding the synthesis, characterization, and adsorption mechanisms of these materials, researchers can continue to develop novel and efficient adsorbents for environmental remediation and sustainable development.

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