Kinetics of Solvent Extraction of Iron(III) from Sulfate Medium by Purified Cyanex 272 using a Lewis Cell

Abstract

The kinetics of the forward and backward extraction of the title process have been investigated using a Lewis cell operated at 3 Hz and flux or (F) – method of data treatment. The dependences of (F) in the forward extraction on [Fe3+], [H2A2](o), pH, and [HSO4 −] are 1, 0.5, 1, and −1, respectively. The value of the forward extraction rate constant (k f ) has been estimated to be 10−7.37 kmol3/2 m−7/2 s−1. The analysis of the experimentally found flux equation gives the following simple equation: F f =100.13 [FeHSO4 2+] [A−], on considering the monomeric model of BTMPPA and the stability constants of Fe(III)‐HSO4 − complexes. This indicates the following elementary reaction occurring in the aqueous film of the interface as rate determining: [FeHSO4]2++A−→[FeHSO4.A]+. The very high activation energy of 91 kJ mol−1 supports this chemical reaction step as rate-determining. The negative value of the entropy change of activation (−94 J mol−1 K−1) indicates that the slow chemical reaction step occurs via the SN2 mechanism. The backward extraction rate can be expressed by the equation: F b =10−5.13 [[FeHSO4A2]](o) [H+] [H2A2](o) −0.5. An analysis of this equation leads to the following chemical reaction step as rate-determining: [FeHSO4A2](int)→[FeHSO4A]+A(i) −. However, the activation energy of 24 kJ mol−1 suggests that the backward extraction process is intermediate controlled with greater contribution of the diffusion of one or the other species as a slow process. The equilibrium constant obtained from the rate study matches well with that obtained from the equilibrium study.