Eco-Friendly and Low-Cost Hydrogel Nanocomposite to Efficiently Remove Textile Dye from Aqueous Solution: Thermodynamics and Regeneration Study

INTRODUCTION
Water is considered one of the most important vital resources essential for the sustainability of life for humans, animals, and plants. In recent times, freshwater availability has been severely declining due to pollutants emitted by the textile industry, but also to human activities such as population growth. Industries such as paper, textiles, plastics, and pharmaceuticals are primarily responsible for releasing toxic and hazardous substances such as dyes, pharmaceuticals, heavy metals, radioactive waste, pesticides, and fertilizers into water [1]. The severity of this water pollution is exacerbated by emerging pollutants, such as pesticides, plasticizers, preservatives, personal care products, and food additives, which are constantly released into water. Living organisms are endangered by water pollution, harming the ecosystem and, ultimately, the global economy. Undoubtedly, drinking water pollution is one of the most significant problems facing humanity [2-4].
Textile dyes are a common hazardous contaminant in water. The severity of these textile dyes is determined by their stability and resistance to temperature, light, chemicals, and bacteria. The harmful effects of dyes on living organisms and their inhabitants have been well documented. For example, dyes distort the aesthetics of the environment, reduce transparency, slow photosynthesis, impede sunlight penetration, and disrupt water quality parameters such as dissolved oxygen, pH, and water hardness [5]. They also negatively affect the growth and development of marine organisms such as algae and fish, as well as microbes. Humans are particularly susceptible to the effects of these hazardous and toxic dyes. Depending on their nature, concentration, and duration of exposure, they can cause both mild (such as allergies, skin irritation, vomiting, and nausea) and serious (such as genetic mutations and cancer). Therefore, it is imperative to develop methods to reduce water pollution [6-8].
Hydrogels, due to their highly porous nature, three-dimensional network, and finer, soft texture, are well suited for loading water-soluble materials. Furthermore, hydrogels, which are biodegradable and known as hydrogels, have interconnected pores that are very important for the adsorption process [9]. The porosity can be improved by the following methods: incorporation of nanomaterials, grafting, drying, cross-linking polymerization, and freezing and cryogenic applications. Synthetic hydrogels, with their excellent adsorption capacity, greater mechanical strength, and high swelling capacity, can be used instead of natural hydrogels [10-12].
This research focuses on preparing a highly porous hydrogel from environmentally friendly, non-toxic guar gum using ammonium persulfate as an initiator. The prepared hydrogel was characterized,, and its swelling behavior at different pH levels was tested using sorption experiments to determine its effectiveness.

MATERIALS AND METHODS
Preparation of nanocomposite hydrogel
The GG-grafted ITA in the loading NIPA was prepared using a graft-free radical co-polymerization method with the initiator KPS. In a reaction flask, 0.5 g of NIPA was dissolved in 20 mL of distilled water and sonicated for 30 minutes. Next, 0.5 g of GG and 0.5 g of ITA were added, each dissolved in 10 mL of distilled water, along with 0.05 g of MBA in 5 mL of distilled water. The initiator KPS, at a concentration of 0.005 g in 5 mL, was then added dropwise to the reaction mixture. Nitrogen gas was used to purge the system throughout the process. The copolymerization reaction proceeded for 120 minutes at 75 °C. Finally, the resulting powder was collected to form the hydrogel.

Adsorption Isotherm
Stander solutions of the GRL dye (1.0 g/1000 mL) were prepared, and the range of required concentrations was made by dilutions with distilled water. The conical flask contained an adsorbed concentration of GRL 300 mg/L, and the weight of hydrogel (0.02 -0.08 g) was maintained independently in a shaker water bath at a regulated temperature. The GRL dye concentration will be estimated spectrophotometrically using a spectrophotometer at the 599 nm wavelength equivalent to maximal absorbance, or max (UV–Vis 1700 Shimadzu). Samples are separated using centrifugation at different intervals. After 60 minutes, is established, the absorbance of the solution is measured, and the drug concentration is estimated.

where Co (mg/L) is the initial concentration of GRL, Ce (mg/L) is the concentration of GRL at equilibrium, V (mL) is the volume of the solution, and W(g) is the weight of the hydrogel.

RESULT AND DISCUSSION
FESEM
FESEM images of the hydrogel before and after adsorption are shown in Fig. 1. The hydrogel has a relatively smooth surface before adsorption, while the hydrogel after adsorption has rough surfaces due to crosslinking and polymerization. The structural morphology of the gel changes to a rough surface, appearing as a lumpy surface. This is due to the dye saturating the gel’s active sites. The dye interaction leads to greater crosslinking, which increases the porosity of the polymer matrix. The surface morphology of any gel is highly influenced by the chemical/physical interactions of the hydrogel surface [13].

TEM
Fig. 3 shows the TEM images of the nanocomposite. The hydrogel showed heterogeneous and irregular morphology within the surface, with some patchy shapes, and tended to form chain-like aggregates at 50 nm. Furthermore, the surface of the nanocomposite is covered with a transparent layer, where it is observed that the GG is embedded within the hydrogel. The GG plays a pivotal role in improving stability and increasing the surface area as an essential component for the synthesis [12].

TGA and XRD 
Fig. 4 shows the hydrogel’s thermograms. The hydrogel exhibited three distinct zones of weight loss. The initial weight loss of 9.78% occurred due to moisture loss between 29°C and 274°C. The second phase of weight loss, which accounted for 51.50%, took place within the temperature range of 274351°C , indicating the degradation of the hydrogel backbone[14].
Fig. 3c shows the hydrogel’s XRD spectrum. The spectrum clearly shows a crystalline peak at 20.9° (2θ scale) corresponding to the [001] crystalline plane, confirming the hydrogel’s formation [15].

BET analysis 
BET experiments were carried out to determine the effects of hydrogel inclusion on the surface area and pore volume of the gel matrix of the hydrogel composite. The composite’s BET surface area, pore volume, and pore diameter were 0.1544 m2/g, 0.00223 cm3/g, and 294.42 Å, respectively [16].

Adsorbent dosage
The weight of hydrogel is one of the most important factors known to have the greatest impact on the adsorption process. Using different amounts of hydrogel (0.02-0.08 g) on the adsorption of a dye (300 mg/L dye solution at pH7), while keeping all other parameters constant, the effect of adsorbent dosage on dye removal from aqueous solutions was studied. The prepared solutions were then shaken for 120 minutes at 30 oC [17].
Fig. 4 shows that when the aqueous gel weight increased from 0.02 to 0.08 g, the adsorption efficiency decreased from 1083 to 318.31 mg/g. This was due to the increase in the number of active sites for adsorption into the solution, which led to competition between them and a decrease in adsorption efficiency. The dye removal efficiency increased from 72.3 to 98.6% as the aqueous gel weight increased from 0.02 to 0.08 g. This is because the adsorbent surface contains more active sites that are available for dye adsorption. Therefore, the optimal adsorbent dosage at equilibrium was determined to be 0.05 g [18].

Effect of Contact Time
Equilibrium time is an important and fundamental parameter for water treatment. Fig. 5 illustrates the effect of equilibrium time on dye adsorption. Due to the abundance of unoccupied GRL dye adsorption sites, the dye was rapidly adsorbed during the first 10 minutes of equilibrium time. Rapid dye removal was observed during the first 30 minutes. After 60 minutes, the removal rate stabilized and reached its peak. This may be due to the lack of available active sites for dye adsorption. Therefore, 60 minutes was chosen as the optimal time period for the experiments [19, 20].

Effect of Temperature
Temperature is a crucial factor in the adsorption process, as it significantly impacts adsorption properties. Generally, if the dye adsorption process increases with rising temperature, this indicates an endothermic reaction. This phenomenon can be explained by the fact that higher temperatures accelerate the diffusion rate of the dye and provide the dye molecules with adequate energy to interact with the adsorption sites. However, some research has shown that dye adsorption can also be exothermic, meaning that dye molecules adhere more effectively at lower temperatures. Therefore, a precise understanding of temperature is essential for designing an effective membrane. Studies investigating the effects of temperature on dye adsorption were conducted within a range of 15°C to 40°C, and the results are illustrated in Fig. 6. It is clear that as the temperature increased from 15°C to 40°C, the removal efficiency decreased from 20.76% to 95.53% [21-23].

Effect of pH 
In aqueous solutions, the ionization of functional groups on the hydrogel surface (which represent the charges of the adsorption sites) is clearly affected by acidity (pH), an important parameter that influences dye adsorption behavior. This demonstrates that the dye removal efficiency increased from 50.66% to 98.5% with increasing pH. Hydrogen protons (H+) in the medium competed with the dye molecules for the same binding sites at lower pH, reducing the adsorption efficiency. However, the removal rate increased as the pH of the solution increased, due to the reduced competition between the dye molecules and hydrogen protons (H+) for the adsorption sites. Therefore, when the pH exceeds 8, the dye molecules begin to precipitate, which is important for dye removal [24, 25]
Experiments were conducted using 100 mL of dye solution (300 mg/L) containing 0.05 g of hydrogel over a pH range of 3 to 10 at room temperature for one hour, with other variables held constant[26]. The results are shown in Fig. 7.

Regeneration
Regeneration and recovery tests were performed on the hydrogel sample after washing it multiple times to measure the long-term effectiveness of the nano hydrogel adsorbent in removing dye from the aqueous medium. Four adsorption/stripping cycles were performed, and the dye extraction efficiency was evaluated after each cycle, as shown in Fig. 8.
The regeneration ability of any adsorbent is important and essential for its practical application in many industries. To test the reusability of the nano hydrogel, bound dye molecules were stripped in an acidic or basic solution. The adsorption rate was only 20% in the basic solution. However, the adsorption rate reached 95% when using the acidic solution. Consequently, we used the acidic solution to increase the adsorption of dye from the nano gel surface [9, 27, 28].

CONCLUSION
The prepared hydrogels are promising for the removal of basic dyes from aqueous solutions. Their high adsorption capacity, coupled with their ease of preparation and use, make them a valuable adsorbent for treating textile dye contamination. We thoroughly verified their preparation method and effectiveness using applied technology like FESEM, TEM, XDR, TGA, and BET. Our results indicate that the adsorption equilibrium increases with increasing dye concentration, especially at pH 7. After a contact time of 60 min, a 0.05 g dose of hydrogel showed the highest adsorption capacity at 575.66 mg/g. Furthermore, the success of reusing and regeneration of these granules for water treatment makes them a noteworthy addition to the field of adsorbents.

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