Kinetically revealed transfer mechanism of europium (III) in tributyl phosphate microdroplet/NaNO3 aqueous solution system by fluorescence microspectroscopy

Abstract

In the present study, we investigate the mechanism of the extraction of Eu(III) with a tributyl phosphate (TBP) microdroplet using fluorescence microspectroscopy. The TBP microdroplet is immobilized in a microcavity, and an interface between TBP and an NaNO3 aqueous solution is formed by immersion of the droplet-immobilizing device. The fluorescence intensity change in the TBP microdroplet during Eu(III) extraction is monitored in real time. As a result, we demonstrate that the rate-determining process is the interfacial reaction of Eu(III) in the TBP/aqueous solution system. To elucidate the reaction mechanism of Eu(III) at the interface, the dependences of the mass-transfer rate constants on the NO3− and TBP concentrations are investigated. Consequently, the mechanism of Eu(III) extraction, in which two nitrate ions and one TBP molecule are involved in the interfacial reaction, is quantitatively revealed. The interfacial reaction rate constant is determined to be (1.3–1.9) × 10−6 m M−3 s−1 for the forward reaction and (3.0–3.6) × 10−6 m s−1 for the backward reaction. The present approach allows us to analyze the extraction kinetics in a manner that is not possible in a bulk solution and will provide an effective strategy for developing solvent extraction.