Selective adsorption of Cr(VI) from aqueous solution by EDA-Fe3O4 nanoparticles prepared from steel pickling waste liquor

•Fe3O4 nanoparticles prepared from waste liquor were functionalized with EDA.•EDA-Fe3O4 nanoparticles had high adsorption capacity and selectivity for Cr(VI).•The adsorption kinetics, thermodynamics and isotherm were studied.•Electrostatic attraction and complexation were postulated as adsorption mechanisms.•EDA-Fe3O4 nanoparticles retained high capacity after several adsorption processes. In this study, Fe3O4 nanoparticles (NPs) prepared from steel pickling waste liquor were functionalized with ethylenediamine (EDA) to form EDA-Fe3O4 NPs for engineering applications. The obtained EDA-Fe3O4 NPs were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface analyzer and Fourier-transform infrared (FTIR) spectroscopy. The results showed that the EDA-Fe3O4 NPs had a crystalline structure with a particle size range of 20–50nm and a BET surface area of 28m2g−1. Functionalization with EDA was able to improve the adsorption selectivity of Fe3O4 for Cr(VI) in Cr(VI)/Cl−or Cr(VI)/SO42− double-mixture systems. The adsorption isotherm data fitted better to the Langmuir adsorption model, and the adsorption kinetics was better described by the pseudo-second order equation. The spontaneous and endothermic characteristics of this adsorption reaction were confirmed by thermodynamic study. Based on the results of X-ray photoelectron spectroscopy (XPS), electrostatic attraction and surface complexation between Cr(VI) and EDA-Fe3O4 NPs are postulated as mechanisms for the adsorption of Cr(VI) from aqueous solution. The EDA-Fe3O4 NPs retained a high adsorption capacity after several consecutive adsorption–desorption processes, indicating that EDA-Fe3O4 NPs serve as an excellent regenerable adsorbent for Cr(VI).