Bio-based Iota-Carrageenan-g-(acrylamide-co-maleic Acid) Smart Graft Hydrogels with Different Crosslinkers and Their Interaction with Uranyl Ions

In this study, novel smart graft hydrogels (SGHs) were prepared by simultaneous grafting/crosslinking of acrylamide and maleic acid onto iota-carrageenan (CR) in the presence of N,N '-methylenebisacrylamide or 1,4-butanediol diacrylate crosslinkers. The SGHs were characterized by FTIR, SEM, TG, DSC analysis and swelling studies. The sizes of the cavities in the SGHs were determined by digital image analysis using micrographs from the SEM. The average cavity diameters in SGHs are around 10 to 12 mu m and have a heterogeneous distribution. The adsorption of uranyl ion on SGHs was investigated by spectroscopic, kinetic and equilibrium studies. The accuracy of the pseudo-first and pseudo-second-order kinetic models applied to the swelling and adsorption properties of the hydrogels were evaluated by correlation coefficient (R2), mean bias error (MBE), root mean square error (RMSE), reduced chi-square (chi 2) and t-statistic model. The swelling and adsorption kinetics of the hydrogels were found to be consistent with the pseudo-second-order model. The adsorption isotherms of uranyl ion on SGHs were found to be L-type according to the Giles isotherm classification. Freundlich and Langmuir models were applied to these isotherms and adsorption parameters were calculated. From the Langmuir model, the monolayer adsorption capacities of SGH-N and SGH-B were calculated to be 162 and 167 mg g-1, and the maximum fractional occupancy values were calculated to be 99% and 97%, respectively. The pH-responsive swelling and adsorption behavior of the hydrogels was determined and the transition points were determined by applying Saraydin's model to the plotted curves. It was also observed that the amount of uranyl ions adsorbed increased with the increase of adsorbent mass. In conclusion, this study demonstrates that the synthesized SGHs can be used as an effective adsorbent for the removal of uranyl ions from an aqueous solution.