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The complexation of trivalent metal ions with humic acid has been studied at pH 4 and 5 in 0.1 Μ NaC10 4 by three different experimental methods, i.e. UV spectroscopy, time resolved laser fluorescence spectroscopy (TRLFS) and ultrafiltration. The direct speciation of the metal ion and its humate complex in the reaction process has been made by UV spectroscopy for Am(III) in the micromolar concentration range and by TRLFS for Cm(III) in the nanomolar concentration range. The ultrafiltration is used with the lowest pore size of filter (ca. 1 nm) to separate the uncomplexed metal ion from its complexed species. The concentrations of both metal ion and humic acid are varied in such a manner that the effective functional groups of the humic acid becomes loaded with metal ions from 1 % to nearly 100%. The loading capacity of the humic acid for the trivalent metal ion, determined separately at each pH, is introduced into the evaluation of complexation constants. The variation of the metal ion concentration from 6 χ 10" 8 mol/1 to 4 χ 10" 5 mol/1 does not show any effect on the complexation reaction. The three different methods give rise to constants being comparable with one another. The average value of the constants thus determined is log β = 6.24 + 0.28 for the trivalent actinide ions.

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A spectroscopic study has been carried out at pH 6 in 0.1 M NaClO« for the complexation of trivalent transuranium ions with a fulvic acid from the Gorleben aquifer system. For comparison of methods, two different spectroscopic procedures are applied, i.e. conventional UV/Vis absorption spectroscopy for Am(III) and time-resolved laser fluorescence spectroscopy (TRLFS) for Cm(III). Both absorption and emission spectroscopy result in comparable constants of the fulvate complexation; log)3 = 5.78±0.07 for Am(III) and log;S=5.90±0.11 for Cm(III). These values, being independent of pH and ionic strength, are evaluated by taking into account the loading capacity of the fulvic acid, which amounts 64.9% of the total proton exchange capacity at the given pH and ionic strength. A large concentration variety of the metal ions, i.e. from 4.6x 10"'mol r' to 5.6x10"® moir\ does not show any concentration effect on the complexation reaction and hence leads to nearly the same complexation constants.