Sodium alginate-modified alkali-activated eggshell/Fe3O4 nanoparticles: A magnetic bio-based spherical adsorbent for cationic dyes adsorption

Herein, a mixture of eggshell (ES) and magnetite nanoparticles (MNPs) was alkali-activated using NaOH/Na2SiO3 solution and then, impregnated with sodium alginate (SA) to prepare a magnetic bio-based adsorbent (namely SAAES/SA/MNPs) for the decontamination of water containing basic dyes, in particular, methylene blue (MB) and crystal violet (CV). The physicochemical properties of magnetic spheres of SAAES/SA/MNPs were characterized using XRD, FTIR, FESEM, EDX, elemental mapping, TEM, and zeta potential techniques. Dye adsorption equilibrium was studied experimentally at pH 8.0 and 25–55 °C, and a statistical physics multilayer model was applied to understand the removal mechanism of these dyes including the adsorption orientations on the adsorbent surface. The number of adsorbed dye molecules per functional group (n) of this bio-based adsorbent ranged from 0.70 to 0.91, indicating the presence of vertical and horizontal adsorption orientations for these organic molecules at all tested solution temperatures. The calculated saturation adsorption capacities (Qsat) were 332.57–256.62 mg/g for CV and 304.47–240.62 mg/g for MB, and an exothermic adsorption was observed for both adsorbates. The estimated adsorption energies (∆E) were