Diffusion and agglomeration of iron oxide nanoparticles in magnetic calcium alginate beads initiated by copper sorption

Abstract

The contamination of aqueous solutions by heavy metals ions is an important environmental and social problem. During the last decade, cheap and environmentally friendly water treatment technologies based on the adsorption of ions by natural biosorbents were consequently proposed. Among them, magnetic biosorbents are of first interest due to their additional magnetic separation properties. The present study focuses on the bioadsorption of copper ions by magnetic alginate beads and on the associated modification of the beads. The Ferromagnetic behaviour was achieved by including ferromagnetic Fe3O4 nanoparticles in calcium alginate beads. The role of iron oxide on the biosorption process was investigated by kinetic studies and demonstrates the minor role of iron oxide during the adsorption process. Kinetic data are very well fitted by a second-order model and isotherms correspond to the Langmuir model. Morphological and chemical characterizations of the beads after adsorption were performed by Scanning Electron Microscopy (SEM), Fourier Transform InfraRed spectroscopy (FTIR) and X-Ray Photoelectron Spectroscopy (XPS). They demonstrate the presence of copper carboxylate complex in a unidentate form with or without Fe3O4 nanoparticles included in the beads. Moreover, they evidence the migration and the agglomeration of Fe3O4 nanoparticles toward the surface during copper adsorption. This phenomenon irremediably changes the nature of the beads and has to be considered within the context of reusability of the sorbents.