Agar xanthate-based smart hydrogel for Cu 2+ and Ni 2+ removal: swelling behavior, adsorption dynamics, and reusability

The study reports the synthesis of three types of novel agar xanthate-based hydrogels (AgX-AH-1, AgX-AH-2, and AgX-AH-3) using acrylamide (Am) as the functional monomer and HEMA as a crosslinker through free-radical graft copolymerization. AgX-AH-2 was further characterized via UV-Visible Spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The highest swelling capacity was found to be 327.542 g/g in distilled water. Under optimum experimental conditions (pH 7.0, adsorbent dose of 2.0 g/L, initial concentration of 1200 mg/L, contact time of 40 min, and temperature of 40 °C), the hydrogel achieved maximum removal efficiencies of 97.24% for Cu2+ and 93.86% for Ni2+ ions. The Langmuir model described the adsorption isotherm data adequately, yielding maximum adsorption capacities (qm) of 656.20 (±26.12) mg/g for Cu2+ and 634.70 (±27.25) mg/g for Ni2+. The adsorption kinetics were best described by the pseudo-second-order model (R2 = 0.988 for Ni2+ and 0.937 for Cu2+), providing the best empirical fit and suggesting the possible involvement of chemisorption. Additionally, AgX-AH-2 exhibited desorption efficiencies of 84.66% and 81.18% for Cu2+ and Ni2+ ions, after the fifth regeneration cycle and retained substantial adsorption performance over five reuse cycles. Overall, this hydrogel is a potentially affordable and sustainable adsorbent for efficiently removing Cu2+ and Ni2+ from contaminated wastewater.