Solvent Extraction Principles and Practice, Revised and Expanded

Chemical Kinetics and Reaction Mechanisms in Solvent Extraction: New Trends and Applications

“…In systems controlled by chemical reaction at the interface, the observed reaction rate constant increases linearly with specific interfacial area a ( a = interfacial area Q / volume of the phase V), while no effect is observed on a in systems controlled by reactions in the bulk phase. 6 The values of the rate of extraction k increase with the increase of the specific interfacial area a giving a straight line which passes through the origin indicating that the rate of extraction is dependent on the variation in interfacial area and the rate controlling reaction takes place at the interface rather than in the bulk phase. 15 The rate-determining step of the extraction reaction is the interfacial formation of the complex between the metal ion and the interfacially adsorbed extracting reagent.…”

“…However, this criterion is not always very meaningful, since many chemical reactions occurring in solvent extraction processes exhibit activation energies of less than this value and may be of only a few kilojoules per mole, i.e., in the same order of magnitude as those of diffusion processes. 6 It was found that an experimental plot of lnk vs. 1/T appeared to be approximately linear with a negative slope obeying the Arrhenius equation:…”

“…6 Solvent extraction kinetics can be controlled only by slow chemical reactions or by diffusion through the interfacial films. When one or more of the chemical reactions is sufficiently slow (slow distribution of the predominant species between the aqueous phase and the interface), in comparison with the rate of diffusion to and away from the interface of the various species taking part in an extraction reaction, in this case the solvent extraction kinetics occurs in a kinetic regime.…”

“…In this case, the extraction rate can be described in terms of chemical reactions taking place either in the bulk phase or at the interface. 6 Based on the two films theory, Danesi proposed a model in which he assumed the presence of two stagnant thin layers of finite thickness on the aqueous and organic sides of the interface, shown in Figure 1. When the extractant has low surface activity and a relatively high solubility in the aqueous phase, the reaction takes place in the aqueous phase.…”

“…6 The effect of interfacial area is the parameter usually used to differentiate between chemical reactions taking place in the bulk phase or at the interface. For interfacial chemical control, the interfacial area will be important together with the interfacial activity of the extractant.…”