Core-Shell Bimagnetic Nanoadsorbents for Hexavalent Chromium Removal from Aqueous Solutions

Highlights: • Nanoadsorbents merge several magnetic materials and properties in a single object. • Nanoadsorbents present high adsorption capacity and selectivity for Cr(VI). • The mean size of nanoadsorbents strongly influences their adsorption efficiency. • Nanoadsorbents are magnetic retrievable and reusable for Cr(VI) removal. • Removal process can be extended to other heavy metal ions. - Abstract: Novel nanoadsorbents based on core-shell bimagnetic nanoparticles (CoFe₂O₄@γ-Fe₂O₃) with two different mean sizes were elaborated, characterized and applied as potential sorbents for Cr(VI) removal from aqueous solutions through magnetically assisted chemical separation. The nanoadsorbents were characterized by XRD, TEM, FTIR, XPS, potentiometric-conductometric titrations, BET and vibrating sample magnetometry. The influence of contact time, shaking rate, pH, pollutant concentration, temperature and competing ions on Cr(VI) adsorption were evaluated. The results were analyzed in the framework of Langmuir and Freundlich models to evaluate the maximum adsorption capacity and the extent of affinity. The nanoadsorbents showed a good selectivity for Cr(VI) adsorption and were more effective at pH = 2.5, with a shaking rate of 400 RPM. The adsorption process was spontaneous, endothermic and presented an increased randomness. The contact time required to reach the equilibrium was relatively short and the kinetic date followed the pseudo-second-order model. The maximum adsorption capacity was nearly 40% higher for the nanoadsorbent of smaller mean size due to its higher surface area. Regeneration studies revealed that the nanoadsorbents can be recovered for reuse. These results indicate that prepared nanoadsorbents can be used as a powerful tool for Cr(VI) removal from contaminated water.