Synergistic effect of various metal ions on the mechanical, thixotropic, self-healing, swelling and water retention properties of bimetallic hydrogels of alginate

Abstract We demonstrate ultrafast gelation between alginate (Alg) and a series of metal ions Fe3+, Cu2+, Co2+, Ni2+, and Mn2+ in their single and binary mixtures. The hydrogels exhibit remarkable mechanical properties, thixotropy, and self-healing ability. The mixing of Fe3+ or Cu2+ metal ions to the Alg hydrogels formed by the Mn2+, Co2+ and Ni2+ metal ions that possess lower mechanical strength and higher stretchability were observed to enhance their mechanical strength, reduce the stretchability and decrease the water retention ability. These properties of divalent metal ion alginate hydrogels involving Co2+, Ni2+, and Mn2+ are significantly improved in their mixed state. The self-healing ability is not seen in Fe-Alg and all the bimetallic hydrogels containing Fe3+ as one of the metal ions. However, all the other monometallic or bimetallic hydrogels show excellent self-healing ability. Bimetallic hydrogels containing Fe3+ or Cu2+ as one of the components do not show the swelling property. But the bimetallic hydrogels containing Mn2+, Co2+, and Ni2+ not only show high swelling property but also high water retention ability. The synthesized hydrogels were observed to be multi-stimuli-responsive and depict sol-gel transition in response to EDTA and applied shear strain. Both single, as well as mixed metals, could be removed from hydrogel by changing the pH indicating their reusability and their prospective use in wastewater treatment. The hydrogels also possess good processability and excellent moldability which form rigid 3D shapes. This study highlights the importance of using the mixed metal ions for tunning the rheological, thermal, swelling, self-healing, and water retention properties of the alginate hydrogels.