Mixed Lanthanide-CO3-Fulvic Complex Spectroscopic Evidence

Abstract

International audienceThe formation of mixed complexes between lanthanides and actinides(III), complexing inorganic anions, and humic substances is still not completely settled, particularly, the competition, or mixed complex formation, with CO32- [1,2]. To propose answers on this topic, time-resolved laser-induced luminescence spectroscopy (TRLS) experiments in the system Eu(III)-CO3-FA at pH 7 are done, using Suwannee River fulvic acid (SRFA). Starting from former characterization of the Eu-SRFA system [3], the evolution of both the fluorescence spectra and lifetimes of Eu(III) at pH 7 and varying concentrations of carbonate and FA are obtained. With the use of an optic parametric oscillator, several excitation wavelengths (λexc) were used either to excite selectively Eu(III) through the 5L6←7F0 transition (λexc = 394.7 nm), to take advantage of the antenna effect from the SRFA (λexc = 390 nm), or to minimize the excitation of the fulvic complex (λexc = 532 nm). The evolutions of the Eu(III) luminescence spectra from the 5D0→7F0,2 transitions are showing modifications of the assymmetry ratio (area ratio of 5D0→7F2 transition to the 5D0→7F1 transition) that indicates the presence of several species in addition to the EuCO3+ and Eu-FA complexes. The occurence of bi-exponential decay times, with a faster than Eu3+ component, is typical of the occurrence of fulvic complex [4] even in the presence of carbonates. The analyses of the undirect excitation of the Eu-CO3-SRFA systems are used to build the complexation isotherms varying both SRFA or total carbonate concentrations in the framework of the NICA-Donnan modeling of the Eu(III)-SRFA system [3,5]. In view of the restricted pH span, no strict attribution to the different possible types of sites can be done. Different complex formation are considered and discussed. [1] Morgenstern et al. Radiochim. Acta 88, 7-16. [2] Pourret et al. J. Colloid Interface Sci. 305, 25-31. [3] Kouhail et al. (accepted) Environ. Sci. Technol. [4] Reiller & Brevet, Spectrochim. Acta A 75, 629-636 [5] Kinniburgh et al. (1999) Colloids Surf. A 151, 147-166