Solvent extraction selectivity driven by fine tuning of aggregate size

凝集体サイズの微調整による溶媒抽出選択性

Micheau, C.; 元川 竜平   ; 上田 祐生   ; Bourgeois, D.*; Simonnet, M.*

Micheau, C.; Motokawa, Ryuhei; Ueda, Yuki; Bourgeois, D.*; Simonnet, M.*

Solvent extraction efficiency relies on the metal affinity for extractant complexing part, and extractant aggregate structures. It was recently demonstrated that aggregates contribute to metal size recognition in the bulk impacting the selectivity, while the abundance of extractant/aggregates at the interface contribute to metal transfer in the organic phase impacting extraction kinetics. However, finely adjust the structures formed in the organic phase remain a challenge as the factors influencing their formation are not clearly defined. As the size recognition effect for the separation of Pd(II) and Nd(III) was initially observed using N,N-dibutyl-N,N-dimethyl-2-tetradecylmalonamide extractant (DBMA) in toluene and heptane, it has been decided to investigate mixtures of these two solvents at different volume ratios. Extractant aggregate size in these mixtures contacted to different nitric acid solution was determined using SANS at JRR-3 (Tokai, Japan), and correlated to selectivity coefficients determined using ICP-OES. From the SANS data analysis, three main factors influencing aggregate size have been pointed out: (i) acidity of aqueous phase, (ii) extraction of nitrate ions, and (iii) relative permittivity of the diluent. In this way, bigger aggregates were obtained in heptane contacted to 5M HNO (RG = 4.44 nm), whereas the smallest ones were obtained in toluene contacted to 1M HNO (RG = 0.53 nm), with effects (i) and (ii) becoming more pronounced as the permittivity decreases. Finally, selectivity coefficients were shown to continuously decrease as the size of the aggregate increases. In this study it is demonstrated that the size of the aggregate is directly correlated to the selectivity coefficient which is maximum for small aggregates. In this way selectivity can be finely tuned adjusting either the organic phase or the aqueous phase composition. In a future work, external stimuli will be applied to tune the selectivity on unique chemical systems.