Simulation of interfacial two-step consecutive reactions by diffusion in the mass-transfer kinetics of liquid-liquid extraction of metal cations

Interfacial two-step consecutive reactions (ITSCR) have recently been proposed by us as the rate-controlling steps of many liquid-liquid mass-transfer processes involving strong surface-active extractants. In some instances, however, diffusion can contribute to the measured rate because of a lack of turbulence in one or both of the two phases. A mathematical model has therefore been developed to evaluate the possibility that interfacial film diffusion coupled to a simple one-step reaction (occurring on the aqueous side of the interface) can mimic ITSCR. Analysis of the model indicates that this possibility does indeed exist, especially when fast extraction reactions take place. The developed model has been used to interpret experimental kinetics data obtained at low stirring, such as the extraction of pertechnetate and plutonium nitrate by long-chain quaternary and tertiary alkylammonion salts. Analysis of the data provides reasonable values for the thickness of the diffusion films. The rate constants evaluated in this way seem, however, too low to be representative of bulk aqueous-phase reactions. It is therefore postulated that the chemical reaction which accompanies the mass transfer occurs in a region so close to the interface that it is characterized by physicochemical properties different from those of bulk water. Consequently, rate constants quite different from those expected for a bulk aqueous-phase reaction are found.