Poly(N-isopropylacrylamide-co-acrylic acid) hydrogels for copper ion adsorption: Equilibrium isotherms, kinetic and thermodynamic studies

Hydrogel was successfully synthesised by incorporating acrylic acid (AA) as copper ion (Cu2+) chelator into the thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) via emulsion polymerisation. The stimuli-responsive properties and the Cu2+ adsorption capacity of the cross-linked PNIPAM-co-AA hydrogels were evaluated. The hydrodynamic diameter of the PNIPAM-co-AA hydrogel particles was studied using dynamic light scattering (DLS) method. It was found that the volume phase transition temperature (VPTT) of hydrogels shifted from 32 °C to 27 °C after Cu2+ adsorption and the hydrodynamic diameter decreased with increasing temperature. The size of the PNIPAM-co-AA hydrogel particles and the incorporation of AA functional group were confirmed by transmission electron microscopy (TEM) and Fourier transform infrared (FT-IR), respectively. Equilibrium isotherms, kinetic and thermodynamic studies have also been evaluated for its copper ions adsorption. The adsorption capacity (qm) of PNIPAM-co-AA hydrogels for Cu2+ was found to be 67.25 mg g−1 with best fit to Langmuir isotherm and the adsorption mechanism follows the pseudo-second-order model. Gibbs free energy analysis shows that the adsorption was spontaneous and it exhibited endothermic chemisorption properties. Adsorption of copper ions on PNIPAM-co-AA hydrogels was found to be more thermodynamically driven.