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 zerovalent 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-1 and 120 min for the uncoated and equal to 2, 1.5 g L-1 and 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(Vl) removal in aqueous medium and capable to justify the obtained results. (C) 2018 Elsevier Ltd. All rights reserved.