Abstract
Background: The vital importance of trace metal analysis in environmental water samples is widely recognised. In many cases, due to the low levels of trace metals in water samples and the high complexity of the matrixes (i.e. seawater), direct determination of trace metals is not always possible. Atomic absorption spectroscopy (AAS) is a low cost, simple and available technique in many laboratories, but it usually shows saline interferences and insufficient sensitivity for direct Pb determination in water samples. Consequently, a preconcentration and separation step is often necessary prior to lead determination by AAS. Liquid membrane technology is an interesting alternative for separation and enrichment processes, since extraction and stripping processes can be carried out simultaneously and it has been applied in this paper.
Method: The processes of transfer, separation and preconcentration of lead ions from saline water samples take place in a bulk liquid membrane using 2-hydroxybenzaldehyde-N-ethylthiosemicarbazone dissolved in toluene as a carrier. The measurements of Pb(II) ions in the stripping phase are performed by AAS.
Results: Good recoveries can be obtained using ammonium chloride/ammonia buffer solution at pH 8.5 in the feed phase, carrier concentration of 1.42 x 10-3 mol L-1 in the liquid membrane (using 80 mL of toluene) and 0.3 mol L-1 of HNO3 in the stripping phase. The study has evidenced that chloride anions act as coupled ions, being co-transported with the cationic metal complex. The efficiency of the process at optimal conditions is 83.7±2.5% after 7 hours, with a precision of 3.1% at a confidence level of 95%, being the preconcentration factor of 20.45. The detection limit of the method is 0.391 µg L-1 of Pb(II). The liquid membrane system was successfully applied to the determination of lead in spiked saline samples and real seawater collected from the Algeciras Bay (Cadiz, South Spain).
Conclusion: A straightforward method for the preconcentration of trace Pb(II) ions in natural waters with salinity levels (≥10 g L-1) has been developed by a liquid membrane containing 2-HBET in toluene. A lower detection limit of the method in comparison with other liquid membranes for this metal, the use of a cheap commercial organic ligand, as well as the possibility to measure by a common, simple and low cost technique, such as the atomic absorption spectroscopy show the good results obtained. Thus, the separation and preconcentration of lead ions in natural water with saline matrix can be accomplished and the quantitative determination at µg L-1 levels can be achieved by AAS resulting particularly suitable for the analysis of lead ions in seawater samples due to the fact that chloride anions are an active element in the transport process.
Keywords: Bulk liquid membrane, preconcentration, lead, seawater, atomic absorption spectroscopy, toluene.
Graphical Abstract
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