| Summary: | In this study, we reveal an Eu(III) extraction mechanism at the interface between HNO3 and tributyl phosphate (TBP) solutions using fluorescence microspectroscopy. The mass transfer rate constant at the interface is obtained from the analysis of fluorescence intensity changes during the forward and backward extractions at various HNO3 and TBP concentrations to investigate the reaction mechanism. This result indicates that one nitrate ion reacts with Eu(III) at the interface, whereas TBP molecules are not involved in the interfacial reaction, which is different from the results obtained using the NaNO3 solution in our previous study. We demonstrate that the chemical species of Eu(III) complex with nitrate ion and TBP in the aqueous solution play an important role for the extraction mechanism. The rate constants of the interfacial reactions in the forward and backward extractions are (4.0–5.0) × 10–7 m M−1 s−1 and (3.2–3.3) × 10–6 m s−1, respectively. We expect that our revealed mechanism provides useful and fundamental knowledge for actual solvent extraction. Graphical abstract: [Figure not available: see fulltext.] © 2022, The Author(s), under exclusive licence to The Japan Society for Analytical Chemistry.
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