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

Abstract

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.

Scheme 1. The diagram of the synthesis process of Fe₃O₄@SiO₂@PEI-NTDA NPs and their application in Pb²⁺ removal.

Fig. 1. SEM (a–c) and TEM (d–f) images of Fe₃O₄ NPs, Fe₃O₄@SiO₂, and Fe₃O₄@SiO₂@PEI-NTDA NPs, respectively; EDXS analysis (g and h) of Fe₃O₄@SiO₂@PEI-NTDA.

Fig. 2. XRD patterns (a), FT-IR spectra (b), TGA curves (c) and magnetization curves (d) of the samples.

Fig. 3. Adsorption capabilities of Fe₃O₄@SiO₂, Fe₃O₄@SiO₂@PEI, and Fe₃O₄@SiO₂@PEI-NTDA for removal of Pb²⁺ from water. Adsorption conditions: sorbent dosage of 0.5 g L⁻¹, solution pH value of 6.0, equilibrium time of 3 h, temperature of 298 K.

Fig. 4. Effects of (a) dosage, (b) pH, (c) contact time and initial Pb²⁺ concentration, and (d) coexisting heavy ions on Pb²⁺ adsorption of Fe₃O₄@SiO₂@PEI-NTDA NPs.

Fig. 5. Pseudo-first order model (a) and pseudo-second order model (b) for the adsorption of Pb²⁺ on Fe₃O₄@SiO₂@PEI-NTDA at dosage = 0.5 g L⁻¹, pH = 6.0, Pb²⁺ concentration = 200 mg L⁻¹, contact time = 3 h, and temperature = 298 K.

Fig. 6. Langmuir (a) and Freundlich (b) simulations for the adsorption of Pb²⁺ on Fe₃O₄@SiO₂@PEI-NTDA at dosage = 0.5 g L⁻¹, pH = 6.0, Pb²⁺ concentration = 200 mg L⁻¹, and contact time = 3 h at three different temperatures of 298 K, 303 K, and 308 K.

Fig. 7. (a) The leaching percentages of Fe from Fe₃O₄@SiO₂@PEI-NTDA in different HCl solutions; T = 298 K, dose = 0.5 g L⁻¹. (b) Recycling of Fe₃O₄@SiO₂@PEI-NTDA in the removal of Pb²⁺; T = 298 K, dose = 0.5 g L⁻¹, pH = 6.0.