Extraction of Zn(II), Cd(II), and Hg(II) by dodecyloligo(oxyethylene) carboxylic acids

Abstract

The synthesis of a series of n-dodecyloligo(oxyethylene) carboxylic acids of general formula C12H25(OCH2CH2)nOCH2COOH (n = 0 to 5) and application of these compounds for the separation of Zn(II), Cd(II), and Hg(II) in extraction are reported. Negligible extractability of the metal ions investigated was found using dodecyloxyacetic acid (n = 0) which indicated that complexation resulted from metal–cation interaction with oxygen atoms of the polyoxyethylene chain incorporated into the molecule of complexon. In extractions from aqueous solutions containing ZnCl2, CdCl2, HgCl2, and NaOH (pH regulator), loading of the organic phases increased for extractants with more oxyethylene units. In particular, Hg(II) extractability and molecular structure of the extractant are related. The selectivity of extraction increased drastically if NaCl was added to the initial aqueous phase, as the extractabilities of Cd(II) and, particularly, Hg(II) declined while the extractability of Zn(II) remained high. Selectivity of extraction is produced by the complex chemical equilibria resulted from complexation of the metal cation by an alkyloligoether carboxylic acid in the organic phase and by Cl− and OH− anions in the aqueous phase. Thus, aqueous solution parameters greatly influence the efficiency and the selectivity of the extraction process and suitable experimental conditions are very important.