Novel magnetically separable anhydride-functionalized Fe3O4@SiO2@PEI-NTDA nanoparticles as effective adsorbents: synthesis, stability and recyclable adsorption performance for heavy metal ions

In this paper, a novel adsorbent, Fe3O4@SiO2@PEI-NTDA, was first prepared by the immobilization of an amine and anhydride onto magnetic Fe3O4@SiO2 nanoparticles with polyethylenimine (PEI) and 1,4,5,8-naphthalenetetracarboxylic-dianhydride (NTDA) for the removal of heavy metal ions from aqueous solutions. The structure of Fe3O4@SiO2@PEI-NTDA was systematically investigated; the results confirmed that amine and anhydride groups were successfully covalently grafted onto the surface of Fe3O4@SiO2, which showed a homogenous core–shell structure with three layers of about 300 nm diameter (Fe3O4 core: 200 nm, nSiO2 layer: 20 nm, and PEI-NTDA layer: 20 nm). The adsorption performance of Fe3O4@SiO2@PEI-NTDA NPs was evaluated for single Pb2+ and coexisting Cd2+, Ni2+, Cu2+, and Zn2+ ions in an aqueous solution in a batch system. The amine and anhydride groups may have a synergistic effect on Pb2+ removal through electrostatic interactions and chelation; Fe3O4@SiO2@PEI-NTDA NPs exhibited preferable removal of Pb2+ with maximum adsorption capacity of 285.3 mg g−1 for Pb2+ at a solution pH of 6.0, adsorbent dosage of 0.5 g L−1, initial Pb2+ concentration of 200 mg L−1 and contact time of 3 h. The adsorption mechanism conformed well to the Langmuir isotherm model, and the adsorption kinetic data were found to fit the pseudo-second order model. Fe3O4@SiO2@PEI-NTDA NPs could be recovered easily from their dispersion by an external magnetic field and demonstrated good recyclability and reusability for at least 6 cycles with a high adsorption capacity above 204.5 mg g−1. The magnetic adsorbents showed high stability with a weight loss below 0.65% in the acid leaching treatment by 2 M HCl solution for 144 h. This study indicates that Fe3O4@SiO2@PEI-NTDA NPs are new promising adsorbents for the effective removal of Pb2+ in wastewater treatment.