Characterization of competitive binding of Eu(III)/Cu(II) and Eu(III)/Ca(II) to Gorleben humic acid

Complete text of publication follows: In an area that contains high concentrations of natural organic matter, it is expected to play an important role on the speciation of trivalent radionuclides. Competitive interactions with H+ and major cations, e.g. Ca2+ or Mg2+, could influence these metals transport and bioavailability. Competitive experiments between Eu3+ and cations which can bind differently to humic substances, would bring an improved understanding of the competitive mechanisms. The aim of this study is to acquire data for Eu(III)/Cu(II) and Eu(III)/Ca(II) competitive binding to a sedimentary-originated humic acid (Gorleben, Germany). The NICA-Donnan parameters [1] for Ca(II), Cu(II), and Eu(III) obtained from competitive binding experiments using Ca2+ or Cu2+ ion selective electrodes (ISE), were used to model time-resolved luminescence spectroscopy (TRLS) measurements. Then the TRL spectra and decay times were interpreted to check the consistency of the modelling. From ISE data, Eu(III) and Cu(II) are in direct competition for the same type of sites, whereas Ca(II) has an indirect influence through electrostatic binding. The spectroscopic interpretation of the competition experiments showed two strikingly different environments for the Eu(III)/Cu(II) and Eu(III)/Ca(II) systems. Cu(II) seems to expel more effectively Eu(III) into an aqueous like environment within the humic acid structure, i.e., the Donnan phase, and to the aqueous phase as free Eu3+. This is evidenced both from the spectra as well as from the decrease in the luminescence decay times. Moreover, Ca(II) causes a slighter modification of the chemical environment of the humic-complexed Eu(III). [1] Kinniburgh et al. (1999) Colloids Surf. A 151, 147-166