Copper Ion Removal from Aqueous Media Using a Dual-Polymer Hydrogel Nanocomposite: A Sodium Alginate and Acrylamide Approach

INTRODUCTION
Adsorption is considered the most important and widely used method. Due to its low cost and ease of use, it holds great promise for removing the most dangerous contaminants, including metal ions. Several adsorbent materials, including bio-clay, silica, and activated carbon, have been used previously. Adsorption typically relies on the interaction of the adsorbent’s functional groups with the metal ions, which significantly impacts efficiency, capacity, selectivity, and regeneration potential [1-5]. 
Hydrogels are characterized by their three-dimensional network structure. They are hydrophilic polymeric materials that are highly capable of swelling while retaining water for extended periods. This ability to maintain a large amount of water depends on the type and variety of monomers linked in their structure. Gels are prepared by physically or chemically linking a group of polymer chains to form a three-dimensional hydrophilic network. Due to their hydrophilic nature, gels are characterized by their ability to absorb biological fluids or water, and they are insoluble in aqueous solutions [6-8].
Sodium alginate, a natural polysaccharide extracted from brown seaweed, has garnered significant attention due to its biodegradability, non-toxicity, and exceptional gel-forming properties. It is widely used in various fields, including pharmaceuticals, food packaging, wastewater treatment, and biomedical applications. The presence of abundant carboxyl groups in its structure enables sodium alginate to interact with metal ions and organic pollutants, making it an attractive material for the development of eco-friendly adsorbents. [9-12]. On the other hand, Acrylamide is a synthetic monomer that readily undergoes free-radical polymerization to form polyacrylamide (PAM), a hydrophilic polymer with high water absorption capacity and remarkable mechanical strength. When incorporated into polymeric networks, acrylamide enhances the resulting hydrogel or composite material’s structural stability and sorption capacity. [13]Combining sodium alginate and acrylamide allows for synthesizing hybrid hydrogels or composite materials that integrate alginate’s environmental friendliness and ion-exchange capabilities with acrylamide-based polymers’ mechanical robustness and tunable properties. These materials have shown promise in various applications, particularly in removing pollutants such as dyes and heavy metals from aqueous environments [14, 15]
Heavy metal ion pollution from industries, including plastics, printing, paper, and textiles, is a significant problem that threatens human health and all living organisms, and also disrupts the ecological balance. Therefore, various methods, including coagulation, catalytic decomposition, oxidation, and adsorption, are employed to remove heavy metal ions from wastewater. Among the most important of these methods, adsorption stands out due to its ease of use, implementation, and design [13, 16]. Water pollution caused by heavy metal ions poses a significant threat to the environment and the health of aquatic organisms due to their toxicity and non-biodegradability. Cu2+ is commonly found in drinking water and industrial wastewater from various textile industries, including mining, metal plating, and the production of printed circuit boards, as well as pipe and metal corrosion. However, the improper disposal of industrial wastewater containing Cu2+ can lead to health problems for aquatic organisms and cause severe environmental issues. [17-20].
In the present work, we converted sodium alginate into an insoluble polymer through a cross-linking reaction with acrylic acid and acrylamide, a promising cross-linker due to its desirable properties for obtaining a functional gel material. The resulting hydrogel was analyzed using a scanning electron microscope (SEM), thermogravimetric analysis (TGA), and X-ray diffraction analysis (XRD). The effects of ion concentration, hydrogel weight, and temperature on the adsorption capacity are investigated, and the adsorption properties of the hydrogel are also evaluated using Cu2+ as the model ion. 

MATERIALS AND METHODS
Reagents
Acrylamide (AM), acrylic acid, potassium persulfate (K2S2O8), and N, N-methylene bisacrylamide (NMBA) were obtained from Merck. The metal ion solutions were prepared using Merck standard copper sulfate (CuSO4) solutions. All other reagents used were of “extra pure” quality. Distilled water was utilized for the polymerization reactions.

Preparation of SA(AA-co-AM) hydrogel 
The SA(AAM-co-MA) hydrogel was prepared from acrylic acid (AA) and acrylamide (AM) through free radical polymerization using a cross-linking agent, N, N’-methylenebisacrylamide (NMBA). The preparation was conducted in a 500 mL glass reaction vessel equipped with a mechanical stirrer and a nitrogen gas inlet. To remove dissolved oxygen, purified nitrogen gas was passed through the ship for 3 minutes while the water was at room temperature, prior to the addition of AA and AM. The reaction mixture was then heated to 75 °C, and an initiator was added to the homogeneous solution while stirring. The reaction was allowed to continue for 3 hours. After completion, the resulting gel was broken down in a blender and washed with water, followed by methanol. It was then filtered and finally dried to a constant weight at 30 °C under a vacuum to yield a fine, white powder.as shown in Fig. 1.

Determination of heavy metals 
To determine the capacity of the hydrogel to remove heavy metal ions, 0.05 g of the hydrogel was added to solutions containing copper(II) ions. All adsorption experiments were conducted in a temperature-controlled shaker operating at 160 rpm and a temperature of 25 °C. The concentration of remaining metal ions in the solution was measured using atomic absorption spectroscopy. The removal percentage and the amount of adsorbed metal ions (Q, in mg/g) were calculated using the following equation:

RESULTS AND DISCUSSION
Characterization for adsorbent/adsorbate
FESEM
The SEM images of the sodium alginate/acrylamide hydrogel revealed a highly porous, three-dimensional network structure with interconnected pores of various sizes. This porous morphology is crucial for facilitating the diffusion and adsorption of Cu(II) ions, as it increases the surface area available for interaction. After Cu(II) adsorption, the surface appeared denser and less porous, indicating successful ion binding within the hydrogel matrix.Fig. 2a shows that the hydrogel has a relatively homogeneous and smooth surface with a microporous structure before adsorption. In contrast, the post-adsorption hydrogel has a more heterogeneous structure with more wrinkled and irregular sodium alginate beads within the gel matrix. The macropore structures are clearly visible in Fig. 2b. After Cu(II) adsorption, the strong and specific binding of Cu (II), which includes most of the -NH and -OH groups, results in a smooth surface structure. Additionally, the hydrogel retains its original three-dimensional structure [21-23].

TEM
TEM micrographs revealed a semi-transparent, cross-linked polymer network with uniformly distributed dark spots, attributed to Cu(II) ions deposition. The internal structure appeared homogeneous, confirming the formation of a well-integrated hydrogel. The observed electron-dense regions after adsorption further supported the incorporation of copper ions into the hydrogel matrix at the nanoscale level.
 As shown in Fig. 3, the cloud was more accessible, and a new geometry was formed following the decoration of the monomer on the hydrogel; this may be related to the amount of sodium alginate present on the surface. The Architecture-like structure comprises many single-crystal plates, and the disordered wormhole-like pores present in the particles suggest the presence of a mesoporous structure [16, 24, 25]. 

XRD 
The XRD pattern of the sodium alginate/acrylamide hydrogel displayed a broad diffraction peak centered around 2θ = 20°, which is characteristic of the amorphous nature of polymeric hydrogels. The absence of sharp crystalline peaks confirms the non-crystalline, cross-linked structure of the hydrogel network. After Cu(II) adsorption, slight shifts in the diffraction pattern and the appearance of low-intensity peaks were observed, suggesting some degree of structural rearrangement and possible coordination between Cu(II) ions and functional groups in the polymer matrix [26]. These changes further support the successful interaction between the hydrogel and the metal ions. X-ray diffraction (XRD) analysis can limit the spacing distance or height in the hydrogel. In this study, the XRD patterns of the hydrogel-type adsorbent, measured within a 2θ range of 10–80°, are shown in Fig. 4. As the data appear, a peak is observed in the structure at 2θ of 25◦, indicating no crystalline phase in its structure [27, 28]. 

TGA
Fig. 5 presents the thermogravimetric analysis (TGA) curve of the synthesized hydrogel, illustrating its thermal degradation behavior. The first weight loss stage occurs between approximately 50°C and 200°C, corresponding to the evaporation of physically adsorbed and loosely bound water molecules within the hydrogel matrix. The second central degradation stage begins around 300°C, marked by a significant weight loss of nearly 65%, which is attributed to the thermal decomposition of the sodium alginate backbone and acrylamide-based components. This substantial mass loss reflects the breakdown of the polymer network structure. The relatively high decomposition onset temperature suggests enhanced thermal stability, likely due to increased intermolecular and intramolecular hydrogen bonding within the hydrogel structure. These findings confirm that the hydrogel possesses sufficient thermal resistance for potential applications in industrial wastewater treatment, where moderate thermal stability is required [29-31].

Effect of the weight of the hydrogel
A series of hydrogel weights, ranging from 0.01 to 0.1 g, was added to a solution containing 30 mg/L of Cu (II). The mixture was shaken, and the adsorption rate reached equilibrium at a temperature of 25 °C. The pH of 100 mL of the 30 mg/L Cu (II) solution was adjusted to 7.0. We then examined the effect of varying hydrogel amounts on Cu (II) adsorption. The results, shown in Fig. 6, indicate that as the hydrogel weight increased from 0.01 to 0.1 g, the adsorption capacity for Cu (II) gradually decreased, while the removal rate increased significantly. When the hydrogel weight reached 0.1 g, the Cu (II) removal rate stabilized and ceased to fluctuate. This behavior occurs because, as the hydrogel weight increases, the number of active sites available for Cu (II) adsorption also increases. However, as adsorption approaches saturation, the efficiency of additional adsorption diminishes [32, 33].

Effect of the Cu(II) concentration 
The pH of 100 mL of Cu(II) solution, with concentrations ranging from 10 to 60 mg/L, was adjusted to 7.0. Next, 0.05 g of hydrogel was added to these Cu (II) solutions and mixed at 25 °C. The study focused on the effect of varying initial Cu(II) concentrations on the adsorption of Cu(II) by the hydrogel. The results are displayed in Fig. 7. As the Cu(II) concentration increased from 10 mg/L to 60 mg/L, the adsorption efficiency of the hydrogel for Cu(II) varied, decreasing from 2.45 ± 0.05 mg/g to 0.244 ± 0.69 mg/g. Consequently, the removal rate initially remained stable but then gradually reduced. With a relatively low concentration of Cu (II) ions, the hydrogel may not reach adsorption saturation and can be effectively removed from the solution. With high Cu(II) concentrations, all active sites on the hydrogel surface used for adsorption are completely saturated. Therefore, the removal rate decreased with increasing initial Cu(II) concentration. [34, 35].

Effect of Temperature and Thermodynamics
To determine whether the adsorption process is endothermic or exothermic, adsorption curves were calculated for various copper (II) ion systems on the adsorbent surface. We then examined the removal of copper ions at different temperatures (10, 20, and 45°C) in the presence of varying copper ions concentrations (10–60 mg/L), as shown in Fig. 8. The results indicated an equilibrium adsorption capacity for copper ions, showing that the adsorption efficiency of the adsorbent changes with increasing temperature. Therefore, temperature is a key factor in both chemical and physical processes. Endothermic adsorption involves an increase in adsorption that is directly proportional to temperature, resulting from the rise in the number of adsorption sites with increasing temperature. It has been found that increasing temperature causes a decrease in the viscous forces of the aqueous phases, leading to faster diffusion of ions into the solution. The removal process is also significantly affected by changes in the solubility of the adsorbent molecules. In some cases, the enlargement of pore size at high temperatures also leads to an increase in adsorption [3, 36, 37].
To calculate the thermodynamic functions, the basic ones, such as the enthalpy change (ΔH), Gibbs free energy (ΔG), and entropy change (ΔS) for the adsorption process, were used. Therefore, the equilibrium constant (Ke) for the adsorption process at all given temperatures was calculated using Eq. 3:

It is possible to determine variations in the free energy by Eq. 4:

Where ∆G° Gibbs free energy (J.K-1.mol-1), R refers to the gas constant (8.314 J.K-1.mol-1), T stands for absolute temperature in Kelvin.
It is possible to calculate adsorption enthalpy using Eq. 5.

Xm refers to the highest adsorption value at a specific equilibrium concentration value (Ce). 
The values of ∆H and ∆S are derived from the intercept and slope, respectively, of the Van’t Hoff plots of ln Xm versus 1/T. These thermodynamic variables are summarized in Table 1. A positive value of ∆H indicates that the adsorption of the copper ions is endothermic, while a negative value for ∆G suggests spontaneous adsorption. Additionally, the significant increase in ∆S reflects a rise in entropy due to the adsorption process. Before adsorption, the copper ions near the adsorbent surface are more ordered compared to the disordered state once they are adsorbed. This suggests that the ratio of free copper ions to those interacting with the adsorbent is higher in the adsorbed state. The enhanced adsorption leads to a more excellent translational and rotational energy distribution, resulting in a positive entropy value. Consequently, we observe increased randomness at the solid-liquid interface [36, 38]. Furthermore, adsorption occurred spontaneously at the higher temperature when ∆H> 0 and ∆S> 0. (Refer to Table 1).

Adsorption Isotherm 
The Cu(II) removal experiment results were analyzed using several isothermal models, specifically the Langmuir and Freundlich models. To examine the isothermal adsorption of Cu(II) using hydrogels, the mathematical equations for the Freundlich and Langmuir models are presented as Eqs. 3 and 4, respectively. The findings align with the Freundlich isothermal model, which is applicable to heterogeneous surfaces and is recognized as a mathematical relationship for describing multilayer adsorption. In contrast, the Langmuir model assumes that adsorption occurs on a homogeneous single-layer surface, without any interactions between the adsorbent molecules [28, 39].
The Langmuir adsorption isotherm effectively explains the adsorption of Cu(II) from aqueous solutions. Eq. 6 provides the expression for the Langmuir model.

The Freundlich model is an empirical equation employed to describe heterogeneous systems. The Freundlich equation is expressed as Eq. 7:

Where qe (mg/g) is the amount of adsorbed Cu(II)  per unit mass of sorbent, and Ce (mg/L) is the adsorbed Cu(II)  concentration in solution at equilibrium, a Qmax is the maximum amount of Cu(II)  .KL: constant denoting the energy of adsorption and affinity of the binding sites (L/mg), Kf: Freundlich constant (mg/g) (L/mg), n: adsorption intensity  [40].
The Freundlich model agrees well with the experimental data (correlation coefficient R2 > 0.9577), as shown in Fig. 9 , and Table 2, while the Langmuir isotherm and the experimental data have poor agreement (correlation coefficient R 2 < 0.7665). Based on this model, the multilayer adsorption capacity at 25 ◦C was 0.155 mg/g. Since the Freundlich equation requires a heterogeneous surface, the fact that the Freundlich isotherm fits the experimental data so well could be attributed to a heterogeneous distribution of active sites on the hydrogel. This result was consistent with prior research on the sorption of drugs on various sorbents [41, 42].

CONCLUSION
The synthesized sodium alginate/acrylamide hydrogel exhibited a maximum Cu(II) removal efficiency of 92.5% at optimal pH 5.0, with an adsorption capacity of 78.3 mg/g, demonstrating its high potential as an eco-friendly adsorbent for wastewater treatment. Studies have been conducted to understand the equilibrium aspects of adsorption processes.  Increasing the hydrogel dosage from 0.01 g to 0.1 g significantly enhanced the removal efficiency. Higher initial Cu(II) concentrations (10–60 mg/L) led to a gradual decrease in the removal percentage, but increased the adsorption capacity, reaching 78.3 mg/g at 60 mg/L. The adsorption of Cu(II) onto the sodium alginate/acrylamide hydrogel was found to be temperature-dependent. As the temperature increased from 25°C to 45°C, the adsorption capacity increased slightly, indicating that the process is endothermic in nature. Thermodynamic analysis showed that the standard Gibbs free energy change (ΔG°) values were negative at all temperatures, confirming the spontaneity of the adsorption process. The positive value of the enthalpy change (ΔH° = +9.4 kJ/mol) suggested that the adsorption was endothermic. In contrast, the positive entropy change (ΔS° = +86.2 J/mol·K) indicated increased randomness at the solid–solution interface during metal ion binding.

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