In situ formation of a Fe3O4@HgS nanocomposite for green preconcentration of Hg followed by detection with a direct mercury analyzer

A novel method with a high greenness profile for the selective preconcentration and detection of Hg at ultratrace levels that involves magnetic nanoparticles has been developed. The ultrasonic decomposition of l-cysteine generated sulphide anion, which acts as a precursor for the formation of Fe₃O₄@HgS structures resulting in the efficient trapping of Hg(II) onto magnetite. Ultrasonic energy can be considered as green energy and l-cysteine is more environmentally friendly reagent than other sulphide precursors such as sodium sulphide. The magnetic sorbent allowed an easy phase separation using a magnet, thus avoiding centrifugation or filtration steps. A direct mercury analyzer allowed the direct and sensitive determination of the Hg collected onto the magnetic phase. A preconcentration factor of up to 91 and a limit of detection of 0.04 μg L^-1 Hg can be achieved using a sample volume of 20 mL. Three reference materials were analyzed after application of the developed preconcentration method obtaining recoveries in the range 96-100 %. Additionally, water samples, including mineral water, tap water, seawater, fountain water and river water, were spiked at two levels of concentration, 5 and 10 μg L^-1, recoveries being in the range of 91-110 %.