Effective removal of Cr(VI) and methyl orange by nano magnetite loaded starch/muscovite biocomposite: Characterization, experiments, advanced modeling, and physicochemical parameters interpretation.

Magnetite nanoparticles (MNPs) synthesized from heated basalt were used to support a biocomposite prepared from muscovite (Mus) and carbohydrate polymer starch (St). The developed Mus/St/MNPs composite was characterized by XRD, FTIR, FESEM, TGA, DSC, and Zeta potential techniques. This multifunctional composite showed outstanding adsorption properties for hexavalent chromium (Cr(VI)) and methyl orange (MO) removal at 25-55 °C and pH 3.0. The adsorption isotherms were fitted to Langmuir model for Cr(VI) and Freundlich equation for MO. To understand microscopically these systems and to analyze their adsorption geometry and interactions mechanism, three statistical physics models were utilized. Theoretical calculations indicated that Cr(VI) ions were adsorbed on composite surface presenting a combination of horizontal and vertical positions, while the aggregated MO molecules displayed a non-parallel adsorption orientation and multi-interactions mechanism. The saturation adsorption capacity increased from 243.37 to 371.59 mg/g for Cr(VI) and 409.29 to 457.62 mg/g for MO at 25 and 55 °C (i.e., endothermic interactions). Cr(VI) and MO adsorption on Mus/St/MNPs was controlled by van der Waals forces, hydrogen bonding, and electrostatic interactions where the calculated adsorption energies were 12.5-30.62 kJ/mol. The utilized adsorbent was easily reactivated and reused several times where regenerated Mus/St/MNPs composite showed nearly 79 % of Cr(VI) and 85 % of MO adsorption capacities even after the fourth adsorption-desorption cycle. This study contributes to understand the physicochemical factors of Cr(VI) and MO adsorption on multifunctional adsorbents like MNPs/carbohydrate polymers/aluminosilicates interface.