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 us ed here to investigate the kinetics of complex formation between Ni2ions and long chain 5-alkoxypicolinic acids (C-n-PIC, with n = 12, 15, 18) solubilized in different types of micelles, namely cetyl trimethy lammonium bromide (CTAB), hexaethyleneglycoldodecylether (C(12)EO(6)) and CTAB/C(12)EO(6) 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 o f complex formation so as to make possible kinetic separation of mixtu res of metal ions. The observed rate constants for complex formation a t pH 4.5 or 7.0 are indeed much smaller in CTAB micelles than in C(12) EO(6) or mixed micelles, but they still remain too high for the previo us purpose, although the influence of the surfactant concentration dem onstrates, as expected, a much stronger partitioning in the case of CT AB in comparison to C(12)EO(6). On the other hand, it is shown that, o nce complex formation has occurred the removal of Ni2+ ions can be ach ieved using ultrafiltration. The yield of extraction increases with n, with the mole fraction of C(12)EO(6), and with the ligand to metal ra tio.