The adsorption kinetics of Cu²⁺ and Co²⁺ onto the GG/SLS hydrogel were thoroughly investigated to elucidate the mechanism and rate-controlling steps. Experimental data were fitted to both pseudo-first-order and pseudo-second-order kinetic models. The pseudo-second-order model provided a significantly better fit, with correlation coefficients (R²) exceeding 0.998 for both metal ions, indicating that chemisorption—governed by electron sharing or valence bond formation—is the dominant process. The calculated equilibrium adsorption capacities (qe,cal) closely matched experimental values (765.1 mg g⁻¹ for Cu²⁺ and 666.7 mg g⁻¹ for Co²⁺), further validating the model’s applicability. In contrast, the pseudo-first-order model showed lower R² values (0.843 and 0.775), suggesting that physical processes such as diffusion play a minor role. The rapid initial uptake observed within the first 30 minutes indicates abundant accessible active sites on the hydrogel surface, which become progressively occupied over time until equilibrium is reached. This behavior aligns with the porous structure revealed by SEM imaging, allowing swift diffusion of metal ions into the internal network.
Equilibrium adsorption behavior was analyzed using four classical isotherm models: Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R). The Langmuir model exhibited the highest R² values (0.994 for Cu²⁺ and 0.991 for Co²⁺), indicating monolayer adsorption on homogeneous binding sites with no interaction between adsorbed species. The maximum theoretical adsorption capacity (Qmax) derived from this model was 775.2 mg g⁻¹ for Cu²⁺ and 724.6 mg g⁻¹ for Co²⁺, slightly higher than the experimental values, which may be due to slight deviations in real system conditions. The Freundlich model also provided a reasonable fit but with lower R² values, suggesting limited heterogeneity in the adsorbent surface. The Temkin model, accounting for adsorbate-adsorbent interactions, showed moderate agreement, while the D-R model indicated a pore-filling mechanism with energy distribution. Notably, the high Qmax values confirm the material’s exceptional potential for heavy metal removal. The good fit to the Langmuir model, combined with XPS evidence of surface complexation, supports a chemically driven adsorption mechanism involving coordination between metal cations and oxygen-containing functional groups such as -OH, -COO⁻, and -SO₃⁻.
Thermodynamic Analysis and Competitive Adsorption Behavior
Thermodynamic parameters were calculated from temperature-dependent adsorption experiments conducted at 30 °C, 40 °C, 50 °C, and 60 °C. Negative values of Gibbs free energy change (ΔG°) across all temperatures confirmed the spontaneity and feasibility of the adsorption process. The increasingly negative ΔG° with rising temperature indicated enhanced adsorption tendency at higher thermal conditions. Positive enthalpy changes (ΔH°) for both Cu²⁺ (34.76 kJ mol⁻¹) and Co²⁺ (27.11 kJ mol⁻¹) revealed an endothermic nature, consistent with the observed increase in adsorption capacity with temperature. This suggests that heat facilitates ion activation and swelling of the hydrogel matrix, improving accessibility to internal binding sites. The positive entropy change (ΔS°) reflects increased disorder during adsorption, likely due to the release of water molecules from hydration shells upon metal ion binding. These thermodynamic insights collectively support a chemically driven, entropy-favored adsorption process.
In binary systems containing both Cu²⁺ and Co²⁺, competitive adsorption occurred due to shared active sites. At equal initial concentrations, Cu²⁺ exhibited superior uptake (592.5 mg g⁻¹) compared to Co²⁺ (82 mg g⁻¹), confirming its preferential binding. This selectivity correlates with the higher electronegativity of Cu²⁺ (1.9) versus Co²⁺ (1.8), favoring stronger coordination with ligands. The presence of competing anions further influenced performance: Cl⁻ had the strongest inhibitory effect, followed by SO₄²⁻ and NO₃⁻, likely due to differences in ionic strength and hydration energy. Despite competition, the GG/SLS hydrogel maintained significant removal efficiency, demonstrating robustness in complex aqueous environments. These findings underscore the material’s suitability for real wastewater applications where multiple contaminants coexist. Overall, the combination of favorable kinetics, isotherm behavior, thermodynamics, and selectivity establishes the GG/SLS hydrogel as a highly effective and reliable adsorbent for heavy metal remediation.
Surface Complexation Mechanism and XPS Evidence
To probe the underlying adsorption mechanism, X-ray photoelectron spectroscopy (XPS) was employed to analyze the chemical states of elements before and after metal ion adsorption. The C 1s spectrum of GG/SLS displayed peaks corresponding to C–C, C–O, and C=O bonds. After Cu²⁺ and Co²⁺ adsorption, shifts in binding energy were observed: C=O shifted from 287.94 eV to 288.10 eV, and O–H from 530.26 eV to 530.72 eV, indicating electron density redistribution due to coordination. Similarly, the S 2p peak at 167.67 eV (S–C) and 168.88 eV (S–O) shifted to higher values, particularly in GG/SLS-Cu²⁺, confirming sulfur involvement in complexation. Most critically, new peaks appeared in the Cu 2p region at 932.Cytokeratin 5/6 Antibody manufacturer 0 eV (Cu 2p₃/₂) and 952.IRF 1 Antibody web 8 eV (Cu 2p₁/₂), characteristic of Cu²⁺, while Co 2p signals emerged at 781.PMID:35227704 2 eV (Co 2p₃/₂) and 795.8 eV (Co 2p₁/₂), confirming successful adsorption. The absence of metallic Cu or Co peaks ruled out reduction reactions. The shift in O 1s and C 1s components, along with the appearance of metal-specific peaks, provides direct evidence of surface complexation via lone pair donation from oxygen atoms in –COO⁻ and –OH groups to vacant orbitals of Cu²⁺ and Co²⁺. This interaction forms stable chelate-like structures, explaining the high affinity and selectivity of the hydrogel. The proposed mechanism involves multidentate coordination through carboxylate, sulfonate, and hydroxyl groups, forming a robust network capable of immobilizing toxic metal ions effectively.
Practical Application Potential and Reusability Assessment
The practical viability of the GG/SLS hydrogel was evaluated through cyclic adsorption-desorption studies. After five consecutive cycles, the adsorption capacity retained 81% for Cu²⁺ and 79% for Co²⁺, with desorption efficiencies above 90% each time. This demonstrates excellent reusability and structural stability under repeated use. Regeneration was achieved using 1 M HCl, which effectively protonated functional groups and displaced bound metal ions without causing significant degradation of the polymer matrix. The minimal loss in performance over cycles confirms that the functional groups remain intact and active. Moreover, the hydrogel can be easily separated from solution via filtration or centrifugation due to its macroscopic form, facilitating downstream processing. Its synthesis from low-cost, renewable raw materials—guar gum and sodium lignosulfonate—further enhances its economic and environmental sustainability. Compared to conventional methods like precipitation or membrane filtration, this approach offers higher selectivity, lower energy consumption, and reduced secondary waste. The material’s ability to function effectively in multi-component systems and varying pH conditions makes it suitable for treating industrial effluents containing mixed heavy metals. Thus, the GG/SLS hydrogel presents a promising, scalable, and eco-friendly solution for sustainable water purification technologies.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com
