Mass Transfer Rate with Liquid Ion Exchangers. The Role of Interfacial Chemical Reactions in the Extraction Kinetics by Dinonylnaphthalenesulphonic Acid and Trilaurylammonium Salts

Abstract

AbstractKinetics and interfacial tension studies concerning the liquid cation exchanger dinonylnaphthalene sulphonic acid (HD) and the liquid anion exchangers trilauryl‐ammonium chloride and nitrate (TLAHCI and TLAHNO3), performed in the laboratory, are reviewed. The interfacial tension studies have shown that, due to the very strong surface active properties of these compounds, even at very low bulk organic concentrations, the interfacial plane gets completely saturated with a strongly adsorbed monomolecular layer of the exchanger. The structure of the interface is such that the ion‐exchange group faces the aqueous phase while its organic part is completely immersed in the organic diluent. Extraction kinetic experiments performed with a quiescent interface cell (Lewis cell) as a function of the stirring speed, interfacial area and volume of the phases have shown that (a) a region always exists where the extraction kinetics is independent of the stirring speed and (b) the exchange kinetics is directly proportional to the interfacial area and inversely proportional to the volume of the phases. These two facts, together with the knowledge of the interfacial structure of the exchanger, are a strong indication that interfacial chemical reactions can have a predominant effect in determining the extraction kinetics. Experiments performed at relatively high stirring speeds of the phase as functions of the chemical composition of the system have produced a set of interfacial chemical reactions which completely describe the kinetics of the liquid ion‐exchange process. This set of reactions consists basically of the following steps: (a) formation of an interfacial complex between the interfacially adsorbed molecules of ion‐exchanger and the metal cation or complex present in the aqueous solution, (b) replacement at the interface of the interfacial complex with bulk organic molecules of the ion‐exchanger; this process, with a rate which is proportional to the organic concentration of the exchanger, occurs through the reaction with bulk molecules of the extractant, and (c) the reverse steps occur when organic to water transfer takes place. By applying the stationary condition to the interfacial complex its concentration has been evaluated and a rate expression derived which has led to the calculation of the rate constants of the chemical reactions. The following metal ions have been studied experimentally: Fe+3, Eu+3, Ce+3, Gd+3, Tm+3 with HD; FeCln3‐n with TLAHCI and Pu(NO3)n4‐n with TLAHNO3. By using the derived rate expressions and rate constants, distribution laws have been obtained to calculate equilibrium distribution data in good agreement with experimental results.