Allyl sorbitol crosslinker-based novel hydrogel for the removal of toxic metal ions from water

The rising concentration of toxic metal ions in waterbodies threatens human and ecological health. Adsorption, especially using hydrogels, is an effective method for removing these pollutants. The newly designed allyl sorbitol crosslinker-based hydrogel (ASCLHs-hydrogel) has been prepared via free radical copolymerisation technique. The synthesised ASCLHs-hydrogels have been characterised by several analytical techniques, viz. Fourier transform infra-red (FTIR) spectroscopy, thermogravimetric analysis (TGA), derivative thermogravimetry (DTG), point of zero charge (Delta pHPZC) analysis, gel permeation chromatography (GPC) analysis, scanning electron microscope (SEM)-energy dispersive X-ray spectroscopy (EDS) analysis and X-ray photoelectron spectroscopy (XPS). Three grades of ASCLHs-hydrogel have been prepared and utilised for the elimination of Ni2+ and Co2+ ions from wastewater. The properties of ASCLHs-hydrogel, i.e. swelling and water retention ratio (WRR), have been studied in distilled water. The maximum swelling of the best grade ASCLHs-1 hydrogel was found to be 367.52 g/g, and WRR of ASCLHs-1 hydrogel was found to be 74.37% in distilled water. Under optimised conditions, the removal efficiencies of the best grade of ASCLHs-1 hydrogel were found to be 96.2% for Co2+ and 94.5% for Ni2+ ions. The Langmuir and Freundlich isotherm models give excellent fitting with experimental data. The maximum adsorption capacities for ASCLHs-1 hydrogel have been found to be 209.20 mg/g for Co2+ and 182.14 mg/g for Ni2+ ions, respectively. The kinetic behaviour of the adsorption between hydrogels and heavy metal ions (HMIs) was found to be controlled by pseudo-second-order (PSO) kinetic model with a rate constant 1.0 x 10-3 g/(mg.min) for Co2+ and 1.2 x 10-3 g/(mg.min) for Ni2+ ions, respectively. The reusability of the ASCLHs-1 hydrogel has been studied for five cycles and the hydrogel showed remarkable desorption efficiency even after the fourth cycle. Overall, these findings suggest that the ASCLHs-1 hydrogel works as an economical, and efficient adsorbent for the removal of Ni2+ and Co2+ ions from aqueous solutions.