Sodium alginate from Playa del Carmen Sargassum: extraction, characterization, and rheological analysis of solutions and hydrogels.

Since 2011, the Mexican Caribbean has experienced periodic influxes of Sargassum biomass, leading to severe ecological and economic impacts. In this study, sodium alginate was isolated using a multistage methodology that included ultrafiltration to purify and concentrate the biopolymer, yielding 28 %. FTIR analysis confirmed the presence of typical absorption bands for carboxylate groups and acid sugars, with a correlation coefficient of 0.96 when compared to commercial alginate. The Z-potential indicates a negative charge distribution on the molecule, and the average Z-potential value is higher for the commercial alginate than for Sargassum alginate, suggesting a shorter chain length of the latter. The H NMR analysis revealed a mannuronic/guluronic ratio of 0.71. Viscometric analysis showed an intrinsic viscosity of 524.9 mL/g and an estimated molecular mass of 127.4 kg/mol, with a purity of 77 %, being the impurity a complex mixture of substances, such as fucoxanthin, phlorotannin and inorganic salts. A double sequential gelation process, involving acid diffusion and Ca crosslinking, produced stable, stiffer, open-cell hydrogels at a concentration of 2 % (w/v), achieving a compression strength of 80 kPa. The combination of a relatively low molecular mass and low M/G ratio of Sargassum alginate resulted in an elastic, strong hydrogel with reduced syneresis.