3-Methyl pyridine as extraction solvent and chelating agent in extraction and preconcentration of some heavy metals in aqueous samples based on heat-induced homogeneous liquid-liquid extraction

Heavy metal ions are present in relatively low concentrations in the environment. They are widely used in many industries. Cobalt and nickel are typical metal ions in environmental samples and have important roles in many physiological functions. However, high concentrations of these metal ions may be toxic and lead to side effects [1, 2]. Prior to assess low concentration, performing separation and preconcentration techniques are compulsory to eliminate or minimize matrix effects which leads to low detection limit and improved sensitivity of detection techniques towards metals.Homogeneous liquid-liquid extraction (HLLE) is based on the usage of a low dielectric constant organic solvent such as acetonitrile, methanol, acetone, etc., which is soluble in water. An agent is utilized as a phase separation phenomenon in a homogeneous solution to produce two distinct phases. In this method an analyte (organic / inorganic compound) in the homogeneous aqueous solution is extracted in the separated organic phase. In HLLE the contact area between the phases is practically infinitive compared to liquid liquid extraction and dispersive liquid liquid microextraction facilitating fast mass transfer. Moreover, it possesses the advantage of having a short extraction time with a high enrichment factor due to the absence of obstacles from surface contact between the organic phase and the aqueous phase during extraction. Picoline (PL) is soluble in water at low temperatures while its solubility in water is decreased by heating. Therefore heating can be used as phase separation agent to form a two-phases system from a homogenous solution containing water and PL. In the proposed method, initially to 5 mL standard solution or real sample (pH= 5) in 8-mL glass tube, µL-level PL was added to form cation-PL complexes in a homogeneous solution. Then 0.9 g sodium chloride was added to the solution. After manual shaking, the tube was placed in a water bath with 70°C for 6 min. A cloudy solution was formed due to the decrease of PL solubility at 70°C compared to room temperature. After centrifugation for 5 min at 5000 rpm, the fine droplets of PL (extraction solvent) containing the formed complexes were collected as the upper phase. In order to investigate the extraction efficiency of the method, the collected phase was removed and injected into GFAAS. Under the optimum conditions the calibration curves were linear in the ranges of 10-150 ng L-1. Repeatability of the proposed method ranged from 3-5% (n=6, C=100 ng L-1). Moreover, the detection limits of the selected analytes were obtained in the range of 4-7 ng L-1. Finally, the proposed method has been successfully applied for the simultaneous analysis of the selected analytes in environmental water samples.