Solvent Extraction Preconcentration of Trace Metal Ions from Natural Waters with an Alkylated Oxine Derivative

Abstract

<p> A method for the simultaneous preconcentration by solvent extraction of a group of trace metal ions from natural waters has been developed. The procedure makes use of a proprietary "liquid cation-exchanger", Kelex 100, the primary component of which is an alkylated oxine (8-quinolinol) derivative, 7-(4-ethyl-1-methyloctyl)-8-quinolinol (HL). After purification of HL from the commercial mixture, the extraction of ten environmentally-significant trace metal ions from artificial seawater into toluene solution was studied as a function of pH. From these investigations, the optimal conditions for the extraction of Cd(II), Co(II), Cu(II), Mn(II), Ni(II), Pb(II) and Zn(II) from natural waters were established. The conditions for quantitative back-extraction of the metal ions were then investigated. With the exception of cobalt, the metal ions were quantitatively back-extracted into a small volume of nitric acid, simplifying the matrix and providing additional analyte enrichment. The optimized forward- and back-extraction technique was subsequently applied to the determination of total (soluble) Cd, Cu, Mn, Ni and Pb in a coastal seawater reference standard by graphite-furnace atomic absorption spectroscopy (GFAAS). The quantitative recovery of the analytes and the uncomplicated matrix of analysis enabled quantitation to be carried out by external calibration. Compared to the method of standard additions, external calibration has advantages in overall analysis time and sample consumption. Satisfactory agreement was obtained between the experimental and reference values, although Cu(II) blanks were high due to trace Cu(II) contamination of HL and the stability of the Cu(II)-HL chelate.</p> <p> The lipophilicity of HL and its metal chelates provided high metal chelate distribution ratios which, in turn, permitted preconcentration factors of up to 500 in a single batch-extraction. Additionally, studies on the recovery of radiotracer spikes from lakewater and seawater suggested that HL is an effective extractant for stripping metal ions from variously-bound forms from natural waters.</p>