Newly designed eco-friendly allyl pentaerythritol crosslinker based pH-responsive hydrogel: Swelling, adsorption behavior, thermodynamic and reusability studies

This study investigates the adsorption of Cu²⁺ and Co²⁺ ions from wastewater using an allyl pentaerythritol crosslinker-based hydrogel (APECLHs hydrogel). Three grades of allyl pentaerythritol crosslinker-based hydrogel (APECLHs-1, APECLHs-2 and APECLHs-3) have been prepared using two monomers viz., acrylic acid (AA) and acrylamide (AAm) via free radical copolymerization method. The APECLHs hydrogels were characterized by FTIR, TGA, ΔpH_PZC and SEM analysis. According to FE-SEM data, the APECLHs hydrogel is highly porous that provides abundant adsorption sites, which are crucial for effective metal ion uptake. The percent swelling ratio (%SR) of APECLHs-1 hydrogel has been found to be 18656, 22455 and 30574 % in 45 h and percent water retention ratio (%WRR) was 77.85, 79.21 and 81.57 % in 48 h at pH values 4, 7 and 11, respectively. The maximum metal ions adsorption of APECLHs-1 hydrogel was obtained as 98.14 % for Cu²⁺ and 95.16 % for Co²⁺ ions. The outcomes presented that experimental data fitted well with Langmuir isotherm (LI) model with an adsorption potential of 370.95 mg/g for Cu²⁺ and 337.53 mg/g for Co²⁺ using APECLHs-1 hydrogel. The kinetic behavior of APECLHs-1 hydrogel towards the HMIs agreed with pseudo second order (PSO) kinetics with a rate constant of 9.9 × 10⁻⁵ g/(mg.min) for Cu²⁺ and 7.2 × 10⁻⁵ g/(mg.min) for Co²⁺ ions. The APECLHs-1 hydrogel showed a remarkable desorption efficiency which facilitates the reuse of the hydrogel many times for Co²⁺ and Cu²⁺ ions adsorption. The percent metal adsorption of APECLHs-1 hydrogel was observed as 87.37 % for Cu²⁺ and 85.11 % for Co²⁺ ions even after the fourth cycle of adsorption of HMIs. Hence, the synthesized APECLHs-1 hydrogel exhibits excellent potential for use as a reusable adsorbent for Co²⁺ and Cu²⁺ ions elimination from water.