Analysis of solvent extraction equilibria with consideration of cluster formation of zirconium complexes

Okamura, Hiroyuki   ; Ueda, Yuki   ; Motokawa, Ryuhei   ; Mu, J.*; Masters, A. J.*; Antonio, M. R.*

Recently, cluster formation of metal-extractant complexes in organic solvents has been shown to play important roles in liquid-liquid extraction. Although a number of reaction mechanisms have been proposed for liquid-liquid extraction, application of slope analysis method for evaluating metal-extractant stoichiometry fails when the solute concentrations are high. In this study, we performed the analysis of solvent extraction equilibria for practical liquid-liquid systems with consideration of the formation of cluster aggregates in the organic phase during the extraction process. Molecular dynamics (MD) simulation indicates the formation of aggregated clusters of 1 to 9 Zr(NO)(TBP) complexes in -octane, leading to the determination of composition and molar fraction of each cluster. Furthermore, the extraction equilibrium reactions were studied, and the extraction equilibrium constant K ( = 1-9) for each cluster could be determined from the parameters obtained by the MD analysis. The distribution curve calculated from K agrees well with the experimental values. Therefore, one MD simulation can accurately reproduce the distribution curve in the clustering/aggregation liquid-liquid extraction system, which enabled us to overcome the limitations of slope analysis method.