Superabsorbent hydrogels for removal of divalent toxic ions. Part I: Synthesis and swelling characterization

In this study, co-polymeric hydrogels, synthesized by polymerization of acrylamide and sodium acrylate in aqueous medium, have been characterized with respect to their swelling behaviour and structural parameters. The gels exhibited appreciable water uptake in distilled water and the swelling exponent has been found to be in the range 0.60-0.80, thus indicating non-Fickian mechanism. The gels demonstrated second-order kinetics as confirmed from the linear nature of the plots obtained by fitting the experimental data to Schott equation. To obtain better model for above 60% water uptake, the Beren-Hopfenberg differential model was applied, which also enabled to calculate the relaxation constants. The value of apparent activation energy for the sample containing 44 percent mole fraction of sodium acrylate was found to be 13.85 kJ mol−1 as determined from the Arrhenius plot, while the sample without sodium acrylate exhibited higher activation energy of 17.00 kJ mol−1. The enthalpy of mixing was determined using the Gibbs-Helmholtz equation. The values of ΔHmix were found to be −0.827 and 1.021 kJ mol−1 for the samples with 44 and 29 percent mol fraction of sodium acrylate. Finally, the swollen gels demonstrated a sharp volume phase transition when put in the solutions of divalent ions, i.e., Ni2+ and Ba2+ ions, thus proving their candidature for being used in removal of toxic divalent ions from their aqueous solutions via divalent-monovalent ion-exchange mechanism.