Switchable dispersive liquid–liquid microextraction for lead enrichment: a green alternative to classical extraction techniques

Abstract

A new, innovative and green switchable dispersive liquid–liquid microextraction (SDLLME) method has been introduced as a preconcentration tool for the removal of the toxic metal lead (Pb) in fresh and waste water samples prior to determination with flame atomic absorption spectrometry. We developed a switchable polarity solvent (SPS) system, based on 1,8-diazabicyclo[5.4.0]undec-7-ene and decanol, which was reversibly switched on and off from heterogeneous biphasic nonpolar hydrophobic phase to homogeneous monophasic polar hydrophilic phase in an aqueous medium through exposure to an anti-solvent trigger (CO₂) for a specific interval of time (1–10 min). Then, an SPS of polar microemulsions was switched on from a polar phase through bubbling CO₂ followed by switching off to a nonpolar phase by heating in the range of 40–70 °C in the presence of N₂ gas. The switching phenomenon of SPS from low polarity to high polarity was confirmed by FTIR spectrophotometry and conductivity measurements. SDLLME was successfully applied as an extracting method for the preconcentration of a hydrophobic chelate of Pb with 1-(2-pyridylazo)-2-naphthol (Pb-PAN) from a real water system. Then hydrophobic-enriched Pb-PAN-SPS was treated with a 1.5 mol L⁻¹ HNO₃ and CO₂ purge for different time intervals, to switch back to its miscible polar hydrophilic monophasic state. The recovery of SPS was carried out by heating at 55 °C and purging with N₂ gas. The SPS was easily recycled up to 12 times with >2% loss of efficiency of the developed method. Under the optimized experimental conditions, the limit of detection and the enhancement factor were determined to be 0.25 μg L⁻¹ and 50, respectively.