Preparation and characterization of amino-functionalized nano-Fe₃O₄ magnetic polymer adsorbents for removal of chromium(VI) ions

Four kinds of NH₂-functionalized nano magnetic polymer adsorbents (NH₂-NMPs) coupled with different diamino groups, i.e., ethylenediamine (EDA), diethylenetriamine (DETA), triethylentetramine (TETA), and tetraethylenepentamine (TEPA), named as EDA-NMPs, DETA-NMPs, TETA-NMPs, and TEPA-NMPs, respectively, have been prepared and characterized by transmission electron microscopy (TEM), X-ray diffractometer (XRD), vibrating sample magnetometer (VSM), elementary analyzer (EA), Brunauer, Emmett, Teller surface area analyzer (BET), and Fourier transform infrared spectroscopy (FTIR). The sorptive characteristics of the NH₂-NMPs intended for removal of chromium(VI) was investigated. Batch adsorption studies were carried out to optimize adsorption conditions. The evaluation of the adsorption kinetics, isotherm, and thermodynamics was deeply investigated. The results showed the adsorptive properties of the NH₂-NMPs were highly pH dependent. Adsorption of Cr(VI) reached equilibrium within 30 min. The data of adsorption kinetics obeyed pseudo-second-order rate mechanism well. The adsorption data for Cr(VI) onto NH₂-NMPs were well fitted to the Langmuir isotherm. The maximum adsorption capacities (q _m) of the NH₂-NMPs to Cr(VI) were 136.98, 149.25, 204.08, 370.37 mg g⁻¹, for EDA-NMPs, DETA-NMPs, TETA-NMPs, and TEPA-NMPs, respectively. Thermodynamic parameters like ΔH ^θ, ΔS ^θ, and ΔG ^θ for the adsorption of Cr(VI) onto the NH₂-NMPs have been estimated, which suggested that the adsorption processes of Cr(VI) onto the NH₂-NMPs were endothermic and entropy favored in nature. The adsorption mechanism studies showed that the adsorption of Cr(VI) onto the NH₂-NMPs could be related with electrostatic attraction, ion exchange, and coordination interactions.