COMPLEXATION KINETICS AND ULTRAFILTRATION REMOVAL OF NICKEL(II) BY LONG-CHAIN ALKOXYPICOLINIC ACIDS AND ALKOXYPYRIDINE ALDOXIMES IN MICELLAR MEDIA

In view of the development of new surfactant-based extraction processes avoiding the use of organic solvent, we have investigated the kinetics of complex formation between Ni2+ ions and long-chain 5-alkoxypicolinic acids (C-n-PIC, with n = 12, 15, 18) and 5-alkoxypyridine aldoximes (C-n-PAX, with n 12, 15) solubilized in different types of micelles. Stopped-flow kinetic experiments were performed in cetyltrimethylammonium bromide (CTAB), hexaethylene glycol dodecyl ether (C(12)EO(6)), and CTAB/C(12)EO(6) mixed micelles. For C-n-PAX derivatives at neutral pH, the apparent rate constants for complex formation are some 2 orders of magnitude lower than for the low molecular weight analogue, pyridine aldoxime (PAX), with dividing factors of 100 and 350 in C(12)EO(6) and CTAB, respectively. In the case of CTAB, a strong effect of added ionic strength was observed, and its origin is discussed. Concerning C-n-PIC, the strong ion-pairing interaction between the carboxylic function of the extractant and the polar head of CTAB molecules introduces a new feature responsible for a specific behavior: the apparent rate constant for complex formation is 2-3 orders of magnitude lower in CTAB micelles than in C(12)EO(6) micelles. The rate in mixed micelles is extremely sensitive to the value of the mole fraction of each surfactant. This observation is discussed in relation to the variation of the electrostatic potential at the surface of mixed micelles, and a theoretical prediction of the variation of the observed rate constant is attempted. Finally, the practical removal of Ni2+ ions was investigated using ultrafiltration. The yield of extraction at pH 7 is shown to be as high as 95-98% in CTAB micelles when the ligand to metal ratio (L/M) is high enough. For low L/M values the extraction yield increases with addition of C(12)EO(6) and with increasing alkyl chain length of the extractant.