Hemimicelles of Alkyl Carboxylates Chemisorbed onto Magnetic Nanoparticles: Study and Application to the Extraction of Carcinogenic Polycyclic Aromatic Hydrocarbons in Environmental Water Samples

In this work, magnetic nanoparticles (MNPs) coated with hemimicelles of alkyl (C10−C18) carboxylates, which have traditionally been used to prepare ferrofluids for technological applications, were used for the first time to extract organic contaminants from environmental water samples. The nanoparticles were coated by following a simple, rapid procedure (30 min at 85 °C) that uses commercially available magnetite of 20−30 nm mean particle diameter. The strong chemical bonding between the surfactant and magnetite relative to the electrostatic forces involved in conventional hemimicelle-based sorbents precludes leaching of the surfactant and facilitates its reuse and the obtainment of surfactant-free extracts. Tetradecanoate hemimicelles were used here to extract carcinogenic polycyclic aromatic hydrocarbons (CPAHs) prior to analysis by liquid chromatography (using a C18 Supelcosil LC-PAH column and a gradient elution program with water and acetonitrile) and fluorescence detection. The procedure involved stirring filtered aqueous samples (350 mL) with 200 mg of tetradecanoate-coated MNPs for 15 min, isolating the sorbent with a Nd−Fe−B magnet and eluting the CPAHs with a mixture of acetonitrile and tetrahydrofuran. The extraction efficiency was not influenced by salt concentrations up to 1 M and pH values over the range 2.5−9. The preconcentration factor was 116. No cleanup of the extracts was needed, and the method proved matrix-independent. The limits of quantitation thus obtained, 0.2−0.5 ng L−1, meet the stringent water quality requirements established by the recently amended European Water Framework Directive 2000/60/EC and also the U.S. EPA for the determination of CPAHs in surface and ground waters. The proposed method was successfully applied to the determination of these pollutants in surface and ground environmental water samples collected from various places in southern Spain. Recoveries from samples spiked with CPAHs at concentrations of 1−10 ng L−1 ranged from 85 to 94% and relative standard deviations from 1 to 7%. CPAHs were only present in the sample from the Navallana reservoir, at concentrations over the range 0.42−0.96 ng L−1.