Layer-by-layer assembly of iron oxide magnetic nanoparticles decorated silica colloid for water remediation

• Layer-by-layer assembly of magnetic nanoparticles, polyelectrolyte and silica colloid nanocomposite.
• Nanocomposite with good colloidal stability, exhibits magnetic and catalytic bifunctionalities.
• Catalytic degradation dominates over electrostatic interactions for dye removal.
• Nanocomposite remained catalytically active after 6 months of storage.

Silica colloid–polyelectrolyte–iron oxide nanocomposite with both magnetic and catalytic properties has been synthesized via layer-by-layer assembly. Dynamic light scattering (DLS) and electrophoretic mobility measurements were employed to monitor the evolution of these structures from silica colloid to silica colloid–polyelectrolyte–iron oxide composite. In addition to DLS, transmission electron microscope was used to investigate the morphology of nanostructure synthesized at each stage. The final structure formed show good colloidal and catalytic stability and real time magnetophoretic response under low magnetic field gradient. Here we demonstrated the potential environmental engineering application of this nanocomposite by taking organic dye, Methylene Blue (MB) and Methyl Orange (MO), as our model system. The experiment was conducted by testing the capability of nanomaterials synthesized at each stage, namely silica colloid, polyelectrolyte-functionalized silica colloid (silica–PDDA), and silica colloid–polyelectrolyte–iron oxide composite (silica–PDDA–IOMNPs), for dye removal. By taking into account the electrostatic interactions between the dye molecules and the as-synthesized nanomaterials, we verified that silica colloid–polyelectrolyte–iron oxide composite is superior for pollutant removal from aqueous environment mainly due to its catalytic property. We rationalized our finding by performing (1) Langmuir and Freundlich adsorption analysis, and, (2) pseudo-first-order and pseudo-second-order kinetic study for all three species of aforementioned nanomaterials. The reusability of silica–PDDA–IOMNPs nanocomposite was tested by subjecting this nanomaterial for multiple cycle of dye removal process. This hybrid material remained catalytically active after six months of storage.

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