Synthesis and characterization of chemically functionalized novel and reusable D-mannitol xanthate derived hydrogel for capturing of toxic metal ions from aqueous solutions

Current study focuses on the development of a promising technique to prepare D-mannitol xanthate-based hydrogel (DMXAGAHs hydrogel) using a free radical co-polymerization technique for eliminating Cu²⁺ and Ni²⁺ ions from aqueous solutions. The UV, FTIR, TGA, ΔpH_PZC and SEM analysis have been used for the characterization of the synthesized hydrogel. The percent swelling (%SR) of the prepared DMXAGAHs-3 hydrogel has been found to be 28,123, 13,433 and 11,177% and water retention ratio (%WRR) was 78.02, 78.84 and 79.22% for distilled water, tap water and gray wastewater. Additionally, the maximum metal ion removal was obtained as 96.9 for Cu²⁺ ions and 94.1% for Ni²⁺ ions by DMXAGAHs-3 hydrogel at optimum adsorbent dosage of 3.125 g/L, pH of 7, temperature 45 °C, initial concentration 1000 mg/L and contact time 60 min, respectively. The experimental data were found to be well-fitted with the Langmuir isotherm model. The optimal adsorption capacities of DMXAGAHs-3 hydrogel, as per the Langmuir model, were determined to be 371.74 mg/g for Cu²⁺ ions and 296.73 mg/g for Ni²⁺ ions, respectively. The pseudo-second-order kinetic model has been found to be the most suitable model for describing the kinetic behavior of the adsorption process. The rate constant according to this model has been found to be 3.0 × 10⁻⁴ g/(mg.min) for Cu²⁺ and 3.2 × 10⁻⁴ g/(mg.min) for Ni²⁺ ions, respectively. The negative ΔG values (− 5.567 kJ/mol for Cu²⁺ and − 4.647 kJ/mol for Ni²⁺ ion) at optimum temperature of 45 °C indicates that the adsorption process is spontaneous in nature along with positive ΔH values (79.80 and 25.38 kJ/mol for Cu²⁺ and Ni²⁺ ion) and ΔS values (267.81 and 100.51 J/Kmol for Cu²⁺ and Ni²⁺ ion) also suggests adsorption process is endothermic in nature and results in the increase in the degree of randomness of metal ions on the DMXAGAHs-3 hydrogel during the adsorption process. In addition, the DMXAGAHs-3 hydrogel has shown a remarkable desorption efficiency with 82.36 and 80.21% desorption for Cu²⁺ and Ni²⁺ ions even on 5th cycle and the hydrogel can be utilized many times for the adsorption of heavy metal ions.