Complexation of Curium(III) and Europium(III) with Urea

To understand the toxicity, transport, deposition and elimination of man-made radioactive elements in the human organism, it is crucial to elucidate their chemical behavior and properties on a molecular level. The present work is part of a project to determine the speciation of curium(III) and its lanthanide analog europium(III) in human urine in order to identify potential decontamination agents, to discover possible metabolism pathways in the organism and to detect differences and similarities in the chemistry of trivalent lanthanides and actinides. The aim of this work was to investigate the complexation of curium(III) and europium(III) with urea, the main component of mammal urine. Since both elements exhibit unique luminescence properties, the complexation was studied by time-resolved laser-induced fluorescence spectroscopy with an excitation wavelength of 395 nm. The europium(III) and curium(III) concentration was 3x10-5 M and 3x10-7¬ M, respectively, in all series. The urea concentration was varied between 0.001 and 5 M, the pH ranged from 1 to 8. All measurements were carried out in a glove box under nitrogen atmosphere.

In aqueous solution the luminescence spectrum of europium(III) shows two typical peaks at 585-595 and 610-620 nm, respectively. Each of these peaks is slightly split, the first one into two peaks with emission maxima at 588 and 591nm, respectively, the second one into two peaks with luminescence maxima at 612 and 616 nm, respectively. Upon complexation with urea the position of all peaks remains unaltered but the ratio of the split peaks in the 610-620 nm range changes. Furthermore luminescence in this wavelength range is significantly increased by complexation. The lifetime of the europium(III)-aqua-ion was determined to be 110 µs. In contrast the complex exhibits a considerably longer lifetime (> 120 µs). Analysis of the time-resolved spectra indicated the formation of a 1:1 complex. At an urea concentration of 1 M the complex is stable till pH 6 but starting at pH 7 the luminescence spectrum changes due to formation of europium(III) hydroxides. Using the factor analysis program SPECFIT the stability constant of the complex log ß110 was determined to be -0.08.

In contrast to europium(III), the luminescence spectrum of curium(III) in aqueous solution shows only one emission peak at 593 nm. Upon complexation with urea the peak is red-shifted to 598 nm and the luminescence intensity decreases. The lifetime of the curium(III)-aqua-ion was determined to be 68 µs but the lifetime of the complex is considerably longer (> 75 µs). Analysis of the time-resolved spectra indicated the formation of a 1:1 complex. At an urea concentration of 1.5 M the complex is already formed at pH 1 and till pH 5 the luminescence spectra are identical. Starting at pH 6 the luminescence intensity decreases significantly due to the formation of curium(III) hydroxides. Using the factor analysis program SPECFIT the single spectra of the different species were calculated and the stability constant of the complex log ß110 was determined to be between -0.8 and -0.9.

Comparing the complexation of urea with europium(III) and curium(III), it seems that the latter is complexed inferior to the first one although it is obvious that both form very weak complexes. This can be explained with the structure of urea in aqueous solution over the whole pH range investigated in this study. The results obtained indicate that urea unlikely plays a role in heavy metal binding in urine and that other substances will have greater complexing potential.