Magnetite nanoparticles with aminomethylenephosphonic groups: synthesis, characterization and uptake of europium(III) ions from aqueous media

Two adsorbents with covalently bound aminomethylenephosphonic acid functions (and referred to as MNPs/AMPA and MNPs/SiO₂-AMPA) were synthesized from two types of amino-functionalized magnetic nanoparticles (MNPs) via Moedritzer-Irani reaction. The sorbents with anchored dopamine ligand (MNPs/dopa) or aminopropyl groups (MNPs/SiO₂-NH₂), and the MNPs/AMPA were characterized by X-ray diffraction, FTIR, transmission electron microscopy and vibrating sample magnetometry. Surface modification does not adversely impact the physical properties of the starting magnetite. Compared to the size of the unmodified Fe₃O₄ (magnetite) nanoparticles (7–12 nm), the average size of functionalized nanoparticles is increased to 10–16 nm. Similarly, the magnetic saturation decreased from 67.5 emu g^-1 to 42.0 emu g⁻¹, and the surface area is increased up to 205 m² g⁻¹ for MNPs/SiO₂-AMPA. The kinetics of the adsorption of Eu(III) on the sorbent is ultra-fast, and equilibria are attained within 5–10 min at room temperature. The adsorption kinetics can be described by a pseudo-second-order model. Adsorption and desorption conditions were tested with respect to the removal of Eu(III) ions from water solution. The adsorption capacities for Eu(III) at pH 7.0 are 77 mg g⁻¹ and 69 mg g⁻¹ for MNPs/AMPA and MNPs/SiO2-AMPA nanoparticles, respectively. Eu(III) was quantified by ICP-MS. The limit of detection (LOD) for Eu(III) is 0.05 ng L⁻¹ (based on the 3σ criterion), with an enrichment factor of 150. The selectivity over ions such as Tb(III), Fe(III), Zn(II), Cu(II), and Ca(II) ions was studied. Under optimal condition the distribution coefficient for Eu(III) relative to these ions is near 10⁵ mL g⁻¹. The sorbents can be easily retrieved from even large volumes of aqueous solutions by magnetic separations. The method was tested for spiked water samples (with recoveries from 96.6–102.5%) and for rock minerals.

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