Magnetic separation has been proved to be superior to the traditional column separation or filtration especially when nanosized adsorbents are used. In this work, iron phosphate coated nano-Fe₃O₄ particles (Fe₃O₄@FePO₄) were synthesized in aqueous solution through deposition of Fe(III) and phosphate ions onto the superparamagnetic nano-Fe₃O₄ particles that were prepared by coprecipitation. The synthesized nano-particles were applied to the enrichment of trace amounts of Cr(III) for its determination by graphite furnace atomic absorption spectrometry (GF-AAS). The magnetic property and morphology of Fe₃O₄@FePO₄ were characterized by vibrating sample magnetometer (VSM), scanning electron microscope (SEM), transmission electron microscope (TEM). The results indicated that the adsorbent was superparamagnetic. Its saturated intensity of magnetization was 23.0 emu•g^-1. This value implied that the prepared Fe₃O₄@FePO₄ could be efficiently separated from the solutions by magnetic fields. The diameters of the particles evaluated from TEM images were in a range of 2～9 nm, which were consistent with that obtained from dynamic light scattering. X-ray photoelectron spectroscopy (XPS) and Zeta potential measurement were used for determining the composition and surface charge of the adsorbent. Based on XPS results, the amount of FePO₄ in Fe₃O₄@FePO₄ was 33.8%. Cr was found in Fe₃O₄@FePO₄ after Cr adsorption. Zeta potential measurement indicated that the surface of Fe₃O₄@FePO₄ carried positive charge at pH<4 and negative charge at pH>4. The influence of pH, adsorption time and sample volume on the rate of adsorption were systematically investigated. At pH 5.3, 0.0200 g of Fe₃O₄@FePO₄ could quantitatively adsorb Cr(III) from 20.0 mL of sample within 20 min. Freundlich isotherm and pseudo second order adsorption kinetics were observed. Adsorbed Cr(III) could be desorbed with 0.3 mol•L^-1 NH₃-1% (V/V) H₂O₂ for direct GF-AAS analysis. The adsorbent was also used for the enrichment of Cr(III) from large volume samples. After stirring for 2 h, more than 80% of Cr(III) in 800 mL (0.5 μg•L^-1) of aqueous solution could be adsorbed and an enrichment factor of 1125 was attained, which confirmed its suitability for the analysis of trace amounts of Cr(III). The adsorbent was successfully used for the real sample analysis and could be re-used 10 times after proper washing and drying.

Key words: iron phosphate, Fe₃O₄ nanoparticle, enrichment, GF-AAS, chromium (III)