Novel Preparation of Stable Hydrogels Based on the Red Alga Porphyridium cruentum and Their Use as Adsorbent

A stable hydrogel beads based on the Porphyridium cruentum polysaccharide in combination with chitosan and a trivalent ion, Al³⁺ or Fe³⁺ were synthesized for the first time. Comparisons of the FTIR results for the beads obtained with Fe³⁺ with the spectra of neat polysaccharides or the spectrum of the complex formed from chitosan and Fe³⁺ showed changes in the infrared bands that reflect physical interactions among the components between the oxygen on the COO⁻ within the Porphyridium polysaccharide and between the NH₂ groups within chitosan and Fe³⁺. XPS analysis of the beads showed that the typical bonds of each polysaccharide component were observed on the bead’s surface and that Fe-Oxides were formed. SEM analysis showed that bead morphology was characterized by a non-porous, smooth surface. In addition, viscosity showed extremely high viscosity at low shear rates, indicating a highly structured, viscoelastic soft solid. Rheological frequency sweeps further demonstrated that G′ ≫ G″ across the entire frequency range, confirming the hydrogel comprised a predominantly elastic, solid-like network that remains stable over time. Methylene blue sorption experiments revealed that equilibrium was reached after approximately 45 min, with a maximum sorption yield of 86% obtained at 200 ppm methylene blue, whereas increasing the concentration to 500 ppm led to a decrease in sorption yield. The hydrogel beads were successfully reused for five consecutive adsorption–desorption cycles. Kinetic analysis showed that the pseudo-second-order model provided the best fit, and the adsorption isotherms were well described by both the Langmuir and Freundlich models.