Kinetics and Mechanism of Backward Extraction of Mn2+ from Mn2+-Cyanex 272 Complex Dissolved in Kerosene by Acidic Sulfate-Acetato Solution Using the Technique of Single Drop

摘要

The rate of acidic sulfate-acetato solution stripping of Mn2+ from kerosene solution of Mn2+-Cyanex 272 complex (MnA(2)) is investigated using the falling single drop technique. To study the kinetics of this process, the reaction orders and the value of backward extraction rate constant (k(b)) have been determined to get the Mn2+-transfer flux equation in backward extraction as: F-b = 10(-4.88) [MnA(2)]((o)) [H(2)A(2)]((o))(-1/2) (1+0.002 [H+](-1))(-1) (1+5.13 [SO42-]). The energy of activation (E-a), entropy variation on activation (Delta S-double dagger), and the enthalpy variation on activation (Delta H-double dagger) have also been determined. It is noticed that the reaction orders with respect to [H+] and [SO42-], and the values of E-a, Delta S-double dagger, and Delta H-double dagger depend on the concentration regions of H+ and SO42- used in stripping. The analysis of flux equation at low-concentration region of H+ and SO42- points out that the dissociation of A(-) from MnA(+) is a rate-controlling chemical reaction step. On the other hand, at high-concentration regions of H+ and SO42-, the rate determining chemical reaction step is the replacement of A(-) in MnA(+) by SO42-. In other conditions, the process is either diffusion or intermediate control. High negative Delta S-double dagger values indicate that the rate controlling chemical reaction steps occur via S(N)2 mechanisms. Rate data have been compared with the equilibrium data for the Mn2+-Cyanex 272 extraction system.