In an area that contains high concentrations of natural organic matter, it is expected that it plays an important role on the behavior of rare earth elements (REE), like europium, and of trivalent actinides. Competitive interactions with H~+, inorganic species, major cations, e.g. Ca(ll) or Mg(ll), could influence these metals transport and bioavailability. Competitive experiments between cations, which can bind differently to humic substances and Eu~(3+), will bring an improved understanding of the competitive mechanisms. The aim of this study is to acquire data for Eu(lll)/Cu(ll) and Eu(lll)/Ca(ll) competitive binding to a sedimentary originated humic acid (Gorleben, Germany). The NICA-Donnan parameters for Ca~(2+), Cu~(2+), and Eu~(3+) obtained from competitive binding experiments using Ca(2+) or Cu~(2+) ion selective electrodes were used to model time-resolved laser fluorescence spectroscopy (TRLFS) measurements. Eu~(3+) and Cu~(2+) are in direct competition for the same type of sites, whereas Ca~(2+) has an indirect influence through electrostatic binding.
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