The complexation of Np(V) with humic acid, which is extracted from one of the Gorleben groundwaters, has been investigated by spectrophotometry in the pH range from 6 to 9 in 0.1 Μ NaC10 4 . By the spectroscopic speciation, the complexed and uncomplexed species are precisely quantified and only 1:1 complex is observed under present experimental conditions. Constants determined at each pH are: log/? = 2.28+0.06 at pH 6; log/? = 2.45+ 0.03 at pH 7; log0 = 2.71 ±0.09 at pH 8; logß = 3.10 + 0.07 at pH 9. By introducing the loading capacity of the humic acid for the NpOj ion at each pH, which is determined parallely in this experiment, a new constant is evaluated, i.e. logß* = 3.66 ±0.02, which remains constant independent of pH and ionic strength.

The complexation of technetium with well characterized humic acid, which was extracted from one of the Gorleben groundwaters in northem Germany, was studied. It was found that technetium humate was produced as a brownish-black precipitate by addition of Sn^^ as a reductant in a mixture Solution of pertechnetate and humic acid at pH 4 in 0.1 mol P' NaC104. This precipitate could be distinguished from the TCO2 with respect to the dissolution behavior at higher pH. The produced amount of the formed technetium humate linearly increases with the added amount of Sn^ * with a slope of ca. 0.5, showing that the valence State of technetium is 3 + . With decrease of the concentration of humic acid in the Solution, the formation of TCO2 became prominent.The stock Solution was warmed after addition of H2O2 to guarantee complete oxidation to pertechnetate. The radioactivity measurements were carried out using a liquid scintillation counter (Aloka LC700). Proton exchange capacity, 5.38 meq g"' [5], of the humic acid (Gohy 573 HA) was used to indicate the HA concentration.

A direct speciation of Cm(III) traced in three different Gorleben groundwaters is performed by time resolved laser fluorescence spectroscopy (TRLFS) in very dilute concentrations from 7.8 χ 10~9 mol 1" 1 to 6.3 χ IO" 8 mol Γ 1 , which are below the solubility of Cm(III) at the given aquatic neutral pH. As spectroscopic references, fluorescence spectra are produced for individual hydroxides, carbonates, húmate and fulvate of Cm(III). Parallel to the spectroscopic speciation, a thermodynamic speciation is also made based on complexation constants of the predominant aquatic chemical reactions, i.e. hydrolysis, carbonate complexation and húmate complexation. The speciation results from thermodynamic calculation and spectroscopy are compared with one another.

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