A study of Eu(III)-humate complexation using Eu(III) luminescence spectroscopy

Luminescence excitation spectra (⁷F₀-⁵D₀ transition) of Eu(III) bound to humic acid (HA) in aqueous solution have been studied. Analysis of these spectra and measurements of the luminescence lifetimes confirmed that Eu(III) was bound to HA in two different binding modes with different numbers of car-boxylate groups of HA bound to the Eu(III). The data indicate one binding mode involved three carboxylate groups, and the second involved one or two carboxylate groups. The relative population by Eu(III) of the two types of binding modes was dependent on the Eu(HI)-to-HA concentration ratio and on the contact time of Eu(III) with HA prior to the spectral analysis. The excitation spectra of Eu(III) complexed with humic acids of different molecular size groups is consistent with an increase with the molecular size of HA in the degree of binding of the Eu in the mode involving three carboxylate groups.