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 processe s avoiding the use of organic solvent, we have investigated the kineti cs of complex formation between Ni2+ ions and long-chain 5-alkoxypicol inic acids (C-n-PIC, with n = 12, 15, 18) and 5-alkoxypyridine aldoxim es (C-n-PAX, with n 12, 15) solubilized in different types of micelles . Stopped-flow kinetic experiments were performed in cetyltrimethylamm onium 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 orde rs of magnitude lower than for the low molecular weight analogue, pyri dine 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 adde d ionic strength was observed, and its origin is discussed. Concerning C-n-PIC, the strong ion-pairing interaction between the carboxylic fu nction of the extractant and the polar head of CTAB molecules introduc es a new feature responsible for a specific behavior: the apparent rat e constant for complex formation is 2-3 orders of magnitude lower in C TAB micelles than in C(12)EO(6) micelles. The rate in mixed micelles i s extremely sensitive to the value of the mole fraction of each surfac tant. This observation is discussed in relation to the variation of th e electrostatic potential at the surface of mixed micelles, and a theo retical prediction of the variation of the observed rate constant is a ttempted. 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 w ith addition of C(12)EO(6) and with increasing alkyl chain length of t he extractant.