Determination of cobalt in high-salinity reverse osmosis concentrates using flame atomic absorption spectrometry after cold-induced aggregation microextraction

Abstract

The main objective of this paper is to evaluate the applicability of cold-induced aggregation microextraction (CIAME), followed by flame atomic absorption spectrometry (FAAS) for the determination of cobalt in high-salinity reverse osmosis concentrates. In this method, NaPF₆ was added to the sample solutions (50 °C) containing very small amounts of 1-hexyl-3-methylimidazolium tetrafluoroborate ([Hmim][BF₄]). After cooling, a cloudy solution containing the fine droplets of the hydrophobic and robust extraction phase ([Hmim][PF₆]) was formed. Through centrifugation, the fine droplets of the extractant phase were settled to the bottom of the conical-bottom glass centrifuge tube. The high density and low solubility of the extraction phase in the presence of an ion pairing agent (PF₆⁻) led to successful phase separation even in the high salinity solution (NaCl, 40% w/v). α-Nitroso β-naphthol was chosen as a complexing agent. Experimental parameters were investigated and optimized. Under the optimum conditions and by using 10 mL of sample, a limit of detection (LOD) of 0.8 μg L⁻¹, an inter-day relative standard deviation (R.S.D) of 1.78% at a concentration of 20 μg L⁻¹ and an enhancement factor of 95 were obtained. Validation of the method was performed by analysis of a certified reference material and comparison of results with those obtained by the ASTM standard method (D 3558). In addition, the concentrate sample was spiked to assess the matrix effect. The relative recovery at a spiking level of 20 μg L⁻¹ was 94.7%, showing no matrix interference.