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Citrate complexes are the dominant binding form of trivalent actinides and lanthanides in human urine at pH < 6. Hence, an accurate prediction of the speciation of these elements in the presence of citrate is crucial for the understanding of their impact on the metabolism of the human organism and the corresponding health risks. We studied the complexation of Cm(III) and Eu(III), as representatives of trivalent actinides and lanthanides, respectively, in aqueous citrate solution over a wide pH range using time-resolved laser-induced fluorescence spectroscopy. Four distinct citrate complexes were identified and their stability constants were determined, which are MHCit0, M(HCitH)HCit2−, M(HCit)23−, and M(Cit)25− (M = Cm, Eu). Additionally, there were also indications for the formation of MCit− complexes. Structural details on the EuHCit0 and EuCit− complexes were obtained with FT-IR spectroscopy in combination with density functional theory calculations. IR spectroscopic evidence for the deprotonation of the hydroxyl group of the citrate ion in the EuCit− complex is presented, which also revealed that the complexation of the Eu3+ ion takes place not only through the carboxylate groups, like in EuHCit0, but additionally via the hydroxylate group. In both EuHCit0 and EuCit− the carboxylate binding mode is mono-dentate. Under a very low metal:citrate ratio that is typical for human body fluids, the Cm(III) and Eu(III) speciation was found to be strongly pH-dependent. The Cm(III) and Eu(III) citrate complexes dominant in human urine at pH < 6 were identified to be Cm(HCitH)HCit2− and a mixture of Eu(HCitH)HCit2− and EuHCit0. The results specify our previous in vitro study using natural human urine samples (Heller et al., Chem. Res. Toxicol., 2011, 24, 193–203).
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