Extraction kinetics and ultrafiltration removal of Nickel (II) by long chain alkoxypicolinic acids in cationic and mixed micelles

Micellar particles can solubilize lipophilic extractants similarly to the organic phase in classical biphasic extraction. This analogy is used here to investigate the kinetics of complex formation between Ni²⁺ ions and long chain 5-alkoxypicolinic acids (C_n-PIC, withn=12, 15, 18) solubilized in different types of micelles, namely cetyl trimethylammonium bromide (CTAB), hexaethyleneglycol-dodecylether (C₁₂EO₆) and CTAB/C₁₂EO₆ mixed micelles. In the case of CTAB micelles, the interaction between the carboxylic function of the extractant and the polar head of surfactant molecules was expected to decrease the rate of complex formation so as to make possible kinetic separation of mixtures of metal ions. The observed rate constants for complex formation at pH 4.5 or 7.0 are indeed much smaller in CTAB micelles than in C₁₂EO₆ or mixed micelles, but they still remain too high for the previous purpose, although the influence of the surfactant concentration demonstrates, as expected, a much stronger partitioning in the case of CTAB in comparison to C₁₂EO₆. On the other hand, it is shown that, once complex formation has occurred the removal of Ni²⁺ ions can be achieved using ultrafiltration. The yield of extraction increases withn, with the mole fraction of C₁₂EO₆, and with the ligand to metal ratio.Institut Nancéien de Chimie Moléculaire (I.N.C.M.)