On the removal of hexavalent chromium by olive stones coated by iron-based nanoparticles: Equilibrium study and chromium recovery

In this work hexavalent chromium, Cr(VI), removal in aqueous solution by olive stones coated by zero-valent iron and magnetite nanoparticles was investigated. The Cr(VI) removal test results showed that the coated biomass adsorption capacity significantly increased (2.54 and 4.11 mg Cr g−1) when compared to the uncoated one (1.48 mg Cr g−1). Optimal removal conditions were reached by fixing the operating parameter values for pH, mass concentration and contact time equal to 3, 2.5 g L−1and 120 min for the uncoated and equal to 2, 1.5 g L−1and 120 min for the coated biomasses, respectively. Both Cr(VI) and Cr(III) recovery, after 5 cycles of use, was investigated using different solvents and operative conditions (with and without sonication), showing that remarkable Cr recovery was possible using the more environmentally friendly washing procedure. A possible Cr(VI) removal mechanism will be here proposed and discussed, considering both reduction and adsorption contribution towards Cr(VI) removal in aqueous medium and capable to justify the obtained results.