Azur C Dye Removal using GO/P(CMC-Co-Am) Nanocomposite: Adsorption and Kinetic Studies

INTRODUCTION 
The accessibility of clean and safe water is one of the most critical concerns in modern society. The key factor responsible for polluting water is industrial effluent particularly the effluent coming from textile industry [1]. Wastewater coming out from the textile sectors constitutes variety of pollutants including metals, ions, dyeing agents and other inorganic substances [2]. Among these, dyes are the common pollutants that are visible even at very less concentration and are resistant to degradation [3-8]. These dye when present in water, responsible for deteriorating water quality and all life forms on earth both directly and indirectly [9]. Azure C (Basic Violet 3) is a cationic dye that find extensive uses in dying process with its chemical formula as C13H12ClN3S (λmax = 611.5 nm) [10, 11]. Inspite of its beneficial uses, this dye is responsible for affecting both environment and humans when it gets discharged into waterbodies without any treatment [12]. Keeping in view the toxicity caused by dye, this study is devoted to the adsorption of Azure C dye from water by adsorption technique. The simple and effective nature of adsorption process and easy availability of adsorbing agents [13-16] make this technique as superior to other available water treatment techniques [3-8, 13-15, 17-24]. The adsorbent used in the study was graphene oxide-carboxymethyl cellulose-co-acrylamide (GO/P(CMC-Co-Am)) nanocomposite that was synthesized by free radical copolymerization. The key objectives of the study include (i) synthesis of graphene oxide-carboxymethyl cellulose-co-acrylamide (GO/P(CMC-Co-Am)) nanocomposite by free radical copolymerization, (ii) characterization of prepared nanocomposite by FTIR, XRD and FESEM techniques, (iii) batch adsorption study for investigating the effect of pH (on both adsorption of dye and swelling ratio of adsorbent), time and salt on Azure C dye adsorption, (iv) investigating the kinetic modeling of dye adsorption process, providing valuable insights into the rate of adsorption. 

MATERIALS AND METHODS
Materials and chemicals used 
The materials and chemicals that were utilized for adsorption of Azur C dye include sodium chloride, potassium chloride, graphite (5μm), hydrochloric acid, calcium chloride, potassium persulfate, bis-acrylamide, acrylamide, carboxymethyl cellulose sodium salt, nitrogen gas, sodium nitrate, potassium permanganate, hydrogen peroxide and barium chloride.

Synthesis of hydrogel P(CMC-Co-Am)  
To synthesize P(CMC-Co-Am) hydrogel, free radical copolymerization method was used. For this, 0.05 g sodium carboxymethyl cellulose (CMC) was dissolved in 50 mL of water that was followed by addition of 12g acrylamide to solution (in three-necked round-bottom flask) equipped with a condenser, a separatory funnel along with nitrogen gas inlet. This was followed by stirring of mixture and addition of 0.18g N, N’-methylenebisacrylamide (MBA), two drops of TAMAD and 0.12 grams of potassium persulfate (KPS) that serve as the initiator. The reaction mixture was then stirred continuously under a nitrogen atmosphere at 60°C for the time period of two hours. The resulting hydrogel was washed repeatedly with water to remove impurities (for 6 hours with regular replacement of water after every 30 min) followed by drying in an oven at 60 °C (Fig. 1). 

Synthesis of GO/P(CMC-Co-Am) nanocomposite 
After the synthesis of hydrogel was done, the next step is to synthesize its nanocomposite via free radical copolymerization. For the synthesis of nanocomposite, CMC was firstly dissolved in water, followed by addition of acrylamide in a three-necked round-bottom flask equipped with a condenser, a separatory funnel, and a nitrogen gas inlet. The mixture was then stirred continuously and pre-dissolved graphene oxide (GO) was added to above mixture that was then followed by addition of KPS to mixture with continuous stirring under nitrogen at 60°C for the time period of 2 hours. The resulting nanocomposite was then cut into smaller pieces, washed and oven-dried at 60°C till a constant weight was obtained (Fig. 2). 

Characterization of adsorbent  
The prepared nanocomposite then undergoing characterization analysis by using three different techniques i.e., (i) Fourier Transform Infrared (FTIR, Shimadzu 8400s spectrophotometer within range of 500 to 4000 cm⁻¹ for functional group determination [3, 22, 25]), (ii) Field Emission Scanning Electron Microscopy (FESEM, TESCAN MIRA3 at 25 kV voltage for morphological study [26, 27]) and (iii) X-ray diffraction (XRD, Shimadzu XRD-6000 at 2θ of 10° to 80° for crystallographic analysis of adsorbent [28, 29]). 

Adsorption experiments 
For studying the effect of equilibrium time, the time for adsorption varied from 1 to 240 min using 0.1g of adsorbent dose with neutral pH for 100 mg/l solution. The effect of pH on adsorption of Azur C dye studied via changing solution pH (from 1.2 to 10) for 50 mg/l dye solution while using 0.1g of adsorbent dose. The optimization of adsorbent dose was carried out by using solution concentration of 50 ppm at variable adsorbent doses from 0.01g to 0.08g while the study of salt (NaCl, KCl and CaCl2) was carried out with 50 mg/l dye solution using 0.1g of adsorbent dose. All experiments were conducted at 20°C temperature with shaking speed of 130 rpm for 120 min (unless otherwise specified). After equilibrium time, adsorption capacity was calculated by using Eq. 1: 

here  and  denotes initial and equilibrium dye concentrations (mg/l),  and  denotes volume (mL) of solution as well as dose of adsorbent (g), correspondingly. The point of zero charge (pHPZC) of nanocomposite determination was carried out via pH titration method. A NaCl solution was prepared and adjusted to different pH values, then mixed with the nanocomposite. The final pH of solutions measured. The graph of pH change against initial pH was plotted. The pHPZC was identified as pH of intersection on this plot [30]. For studying the effect of pH on swelling percentage of prepared nanocomposite, weighed amount of adsorbent (0.1g) was immersed in solutions with variable pH ranging from 3-10. After the immersion period, the nanocomposite was filtered to remove excess water and reweighed. Swelling ratios were subsequently calculated according to the specified Eq. 2:

where,  and  refers to swollen as well as dry weight (g) of nanocomposite correspondingly. The data from contact time study was applied to kinetic models. The pseudo-first-order model assumes that the rate of adsorption and number of active sites of adsorbent have direct relation with each other. Pseudo-second model presumes that rate of adsorption is proportional to square of number of adsorbent’s active sites. The mathematical expressions for pseudo-first and pseudo second kinetic models are expressed in Eqs. 3 and 4 respectively: 

where  (mg/g) and  (mg/g) refers to amount of dye getting adsorbed at time  (min) and at equilibrium, correspondingly while  (1/min) and  (g/mgmin) refers to rate constant for pseudo-first and pseudo-second model correspondingly [31].

RESULTS AND DISCUSSION 
Characterization results 
The FTIR spectrum of the GO/P(CMC-Co-Am) nanocomposite before adsorption (Fig. 3, black line) exhibited characteristic peaks at approximately 3400 cm⁻¹ (hydroxyl stretching), 1700 cm⁻¹ (carbonyl stretching), and 1600 cm⁻¹ (aromatic C=C stretching). After the adsorption of Azure C dye (Fig. 3, red line), several changes were observed [32-34]. The intensity of 3400 cm⁻¹ peak decreased, suggesting the involvement of hydroxyl groups in interactions with dye. The 1700 cm⁻¹ peak shifted slightly, indicating potential interactions with carbonyl groups. The 1600 cm⁻¹ peak showed minor changes, signifying π-π stacking interactions between aromatic rings of dye and GO. In fingerprint region (1000-1500 cm⁻¹), several peaks underwent changes in intensity or wavenumber, indicating the participation of other functional groups, such as C-O and C-N, in adsorption process [10, 29, 35-40].  
The XRD pattern of GO/P(CMC-Co-Am) nanocomposite (Fig. 4) showed a broad peak centered around 2θ of 20-30°, indicating an amorphous or semi-crystalline structure. The absence of sharp peaks at higher angles confirmed the limited crystallinity of the nanocomposite, with the polymer contributing to its overall amorphous nature. This pattern indicated the successful incorporation of GO into polymer matrix [41-43].   
The FESEM images of GO/P(CMC-Co-Am) nanocomposite before adsorption (Fig. 5a) showed a rough and porous surface morphology, which is favorable for adsorption. The nanocomposite exhibited clusters of irregularly shaped particles in nanometer range (59.59 nm) and a high surface area, which is advantageous for facilitating dye adsorption. After adsorption, however, the FESEM images revealed significant changes in surface morphology (Fig. 5b), with the previously observed rough and porous surface appearing partially covered or filled. The post-adsorption images exhibited a more compact structure with reduced pore visibility, indicating that dye molecules had occupied the available surface area, leading to decreased porosity (27.87 nm). Overall, the results of study confirmed successful adsorption, characterized by clear surface modifications and filling of the porous structure following dye interaction [5]. 

Kinetic results  
The graph (Fig. 6 and Table 1) revealed that the adsorption of Azure C dye onto GO/P(CMC-Co-Am) nanocomposite initially increases rapidly with time, then slows down and eventually reaches equilibrium. This is because the available active sites on nanocomposite surface are rapidly occupied by the dye molecules at the beginning. As the adsorption process continues, the number of available adsorption sites decreases, leading to a slower rate of adsorption [44]. The nanocomposite achieved an equilibrium adsorption capacity of approximately 9.14 mg/g within 120 minutes. Although a slight increase to 9.28 mg/g was observed after an additional 120 minutes, this change was deemed insignificant. Therefore, 120 minutes was determined to be the optimal adsorption time. 
The straight line pseudo first order model (Fig. 7a and Table 2) with R² value of 0.9284 indicates that adsorption of Azure C dye onto GO/P(CMC-Co-Am) nanocomposite is favorable. However, while comparing the values of calculated qe (mg/g) i.e., 3.06 mg/g with experimental qe (mg/g) i.e., 9.14 mg/g, it was observed that greater variation exists between these values. These results highlight that some other models also required for understanding the adsorption process. The results of pseudo second order model (Fig. 7band Table 2) revealed higher R² value of 0.9997 with minor variation between calculated qe (mg/g) i.e., 9.26 mg/g and experimental qe (mg/g) i.e., 9.14 mg/g. These results highlight that adsorption of Azure C dye onto the GO/P(CMC-Co-Am) nanocomposite follows pseudo-second model. 

Effect of pH 
The adsorption of azure C dye onto GO/P(CMC-Co-Am) nanocomposite is significantly influenced by the solution’s pH (Fig. 8 and Table 3). Under acidic conditions, the nanocomposite’s surface exhibits higher adsorption capacity (mg/g) compared to neutral or alkaline pH. As the pH increases, the nanocomposite’s surface charge becomes less positive or even negative, reducing electrostatic attraction and hindering dye adsorption. Additionally, hydroxide ions compete with dye molecules for available adsorption sites at higher pH values. The decrease in adsorption capacity in basic pH suggests a shift in interaction mechanism, likely related to changes in the charge properties of both nanocomposite and dye. In general, adsorption of azure C dye onto GO/P(CMC-Co-Am) nanocomposite is most effective in acidic environments [44].   
The point of zero charge (pHPZC) is pH at which the surface charge of a material is neutral. In this study, the pHPZC can be estimated by finding the pH at which the curve intersects the pH axis. Based on the results obtained, pHPZC of material was around pH 5. This means that at pH value below 5, the surface of material has a net positive charge, while at pH above 5, the surface has a net negative charge. At pHPZC, surface charge is neutral (Fig. 9 and Table 4).  
The effect of pH was also explored on the swelling ratio of adsorbent and graph (Fig. 10 and Table 5) illustrates the influence of pH on swelling ratio of material. As pH increases from 3 to 10, the swelling ratio continuously increases. This suggests that the material’s swelling behavior is sensitive to changes in pH. At lower pH values, the material exhibits a relatively low swelling ratio, indicating a more compact structure. As the pH increases, the material’s structure likely undergoes changes that allow for greater water uptake, resulting in a significant increase in swelling ratio from 97.5% to 1648% when pH increases from 3 to 10 respectively [45].  
Effect of nanocomposite dose and salt 
Results of adsorbent dose (Fig. 11 and Table 6) on Azure C dye adsorption revealed that adsorption capacity (mg/g) of nanocomposite is the highest (32.40 mg/g) at the lowest adsorbent dose (0.01g). This is because there is a greater number of available active sites on the nanocomposite surface to adsorb dye molecules. However, as the adsorbent dose continues to increase, the adsorption capacity begins to decrease (5.85 mg/g while using 0.08g of adsorbent dose). This is likely due to the aggregation of nanocomposite particles at higher concentrations, which reduces the effective surface area available for adsorption. Additionally, the competition for adsorption sites between the dye molecules may also become more intense at higher adsorbent doses, leading to a decrease in adsorption capacity [45, 46].  
Results of the effect of salt concentration on adsorption of Azure C dye onto prepared GO/P(CMC-Co-Am) nanocomposite are outlined in Fig. 12 and Table 7. Study highlights the increase in adsorption capacity with increasing salt concentration for all three salts. However, the rate of increase varies among different salts. CaCl2 shows the highest increment in adsorption capacity with increasing salt concentration, followed by NaCl and KCl. This trend can be attributed to the salting-out effect, where the addition of salts to a solution reduces the solubility of the adsorbed substance, thereby increasing its adsorption onto the studied adsorbent. The higher charge density of CaCl2 in comparison to NaCl and KCl majorly contributes to its stronger salting-out effect thereby resulting in an increase in adsorption capacity (mg/g). 

CONCLUSION
The research focuses on exploring the adsorptive capacity of a graphene oxide-carboxymethyl cellulose-co-acrylamide (GO/P(CMC-Co-Am)) nanocomposite for adsorption of Azure C dye from solution. Characterization techniques, such as FTIR, XRD, and FESEM, confirmed the successful synthesis of nanocomposite. Batch adsorption results revealed the equilibrium time to be 120 minutes and the process follows pseudo-second-order model, demonstrating a chemical adsorption mechanism. The nanocomposite can effectively adsorb dye even at low adsorbent doses. The presence of salts, particularly CaCl2, enhanced adsorption capacity through the salting-out effect. Overall, the results revealed the promising potential of GO/P(CMC-Co-Am) as an efficient adsorbent for adsorptive removal of cationic dyes like Azure C dye from water. 

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