Unmodified calcium alginate beads from Sargassum muticum brown algae: A high-capacity, reusable adsorbent for...

This study reports a high-yield extraction of sodium alginate from Sargassum muticum brown algae collected along the Algerian coastline and its transformation into calcium alginate (ALC) beads for efficient methylene blue (MB) adsorption from aqueous media. An enhanced extraction strategy combining ultrasonication with EDTA-assisted alkaline treatment achieved an alginate yield of 45.17%, surpassing most values reported in the literature. The structural and textural properties of the synthesized ALC beads were comprehensively characterized by FTIR, XRD, BET, and ¹H NMR, confirming the formation of a chemically stable, amorphous, and mesoporous biopolymeric network. Adsorption experiments revealed a strong dependence on pH and temperature, with a maximum MB removal efficiency of 93% at pH 12. Equilibrium data were best described by the Langmuir model (R² = 0.992), indicating monolayer adsorption with a high maximum capacity of 403.58 mg g^-1. Kinetic analysis showed excellent agreement with the pseudo-first-order model (R² > 0.99), suggesting that electrostatic interactions and physical adsorption govern the process. Thermodynamic parameters (ΔG° < 0, ΔH° = 8.8 kJmol^-1, ΔS° = 24.78 Jmol^-1K^-1) confirmed the spontaneous and endothermic nature of adsorption. Notably, the ALC beads exhibited outstanding reusability, retaining over 95% of their initial adsorption capacity after eight regeneration cycles using 0.4 M HCl. These findings highlight Sargassum muticum-derived calcium alginate beads as a highly efficient, stable, and sustainable adsorbent for advanced wastewater treatment applications.