Study of salt effects on the kinetics of extraction of cobalt(II) and zinc(II) at trace level by D2EHPA in n-dodecane

The effects of a salt on the extraction kinetics of metal cations Co2+ and Zn2+ by di(2-ethylhexyl) phosphoric acid (D2EHPA) diluted in n-dodecane have been studied using the rotating membrane cell (RMC) technique. The kinetics were investigated for tracer amounts of radiolabelled metal ions (2×10−8 M Co(II) and 10−4 M Zn(II)) and D2EHPA concentration in the range 0.02–0.5 M. A “pre-equilibrated” nitric acid (PENA) solution was used for some experiments. This solution was prepared by contacting a HNO3 solution with a 0.2 M D2EHPA organic phase. First, the effect of the nature of the anion (acetate, Cl−, ClO4−, NO3−) was investigated at pH 3 and 5. The results were compared with those using the D2EHPA-saturated PENA solution. For Co(II), acetate and PENA solutions led to kinetics that were faster than with the other anions. For Zn(II), the kinetics were fast with all anions. For Co(II) with acetate ion and for Zn(II) with acetate, nitrate and perchlorate ions, the results indicate that the reaction cannot be strictly interfacial. The faster kinetics observed with acetate were attributed to the formation of a complex with the metal ion and to a subsequent reaction between this complex and extractant in the aqueous bulk. Next, the effect of adding various amounts of NaClO4 and KNO3 has been studied at pH 3 for salt concentrations between 0 and 1 M. On the assumption of a slow reaction between the metal and the extractant in the aqueous phase, the results for cobalt could be described by a primary electrolyte effect on this reaction.