Fulvic acid complexation of Eu(iii) and Cm(iii) at elevated temperatures studied by time-resolved laser fluorescence spectroscopy

Abstract

The interaction of Eu(III) and Cm(III) with three different aquatic fulvic acids (FA) was studied as a function of the temperature (T = 20–80 °C) in 0.1 M NaCl solution by time-resolved laser fluorescence spectroscopy. The speciation of both trivalent metal ions was determined by peak deconvolution of the recorded fluorescence spectra. For each studied metal ion–FA system only one complexed species is formed under the given experimental conditions. The stability constants at 20, 40, 60 and 80 °C (log β′(T)) were determined according to the charge neutralization model. The log β′ (20 °C) for the different FAs show similar values (log β(20 °C) = 5.60–6.29). The stability constants increase continuously with increasing temperature by approximately 0.3–1.0 orders of magnitude. The reaction enthalpies and entropies are derived from the integrated Van't Hoff equation. The results show that all investigated complexation reactions are endothermic and entropy-driven.