An ionic liquid-based microextraction method for the determination of Cu and Ni in high-salinity produced water from offshore petroleum exploration by GF AAS

Abstract

This work reports the development of a novel method for the ionic-liquid microextraction and determination of Cu and Ni in high-salinity produced water by Graphite Furnace Atomic Absorption Spectrometry (GF AAS). Studies were performed to set a suitable temperature program and to optimize the extraction conditions. Due to the presence of the ionic liquid in the extracts, a drying temperature of 300 °C was chosen for the temperature program. The pyrolysis and atomization curves indicated that the pyrolysis temperature for Cu and Ni should be set at 900 °C, and that the best atomization temperatures were 2100 and 2500 °C for Cu and Ni, respectively. Ionic-liquid microextraction was carried out by mixing 10 mL of sample with 1 mL of a 1% m/v ammonium pyrrolidine dithiocarbamate (APDC) solution and 1 mL of a Britton–Robinson buffer solution (total concentration 0.1 mol L⁻¹) with pH = 4.0. Then, 100 μL of the ionic liquid (1-hexyl-2-methylimidazolium-hexafluorophosphate, C₆MImPF₆) were added and the mixture was shaken manually for 2 min. The obtained suspension was centrifuged for 15 min at 4500 rpm in order to promote the sedimentation of the ionic liquid. The ionic-liquid was separated from the upper aqueous phase using a micropipette and then, absolute ethanol enough to complete 1 mL was added. This final solution was introduced into the graphite tube for the determination of Cu and Ni. The method presented limits of quantification of 0.37 and 0.61 μg L⁻¹ for Cu and Ni, respectively. The calibration of the method was possible using a matrix-matching approach using synthetic saline samples. Recovery tests were performed with the addition of aqueous standard solutions of the metal cations and resulted in the recovery percentages in the range of 76 – 126%.