The effect of ionic strength on the complexatíon of Am 1+ and Cm 3+ by humic acid is investigated at pH 6 with three different experimental methods: ultrafiltration, UV-Visible spectroscopy, and time resolved laser fluorescence spectroscopy (TRLFS). The ionic strength is adjusted by NaCl0 4 and varied from 0.001 M to 5.0 M. The metal ion concentration is changed from 9.95 X10" 9 mol/L Cm 3+ to 1.04X 10" 5 mol/L Am 3+ , while the humic acid concentration is varied from 5.5 X10" 8 mol/L to 1.87 XI 0~5 mol/L. This assures a wide range of metal ion to humic acid concentration ratios to be examined. At pH 6, the loading capacity (LC) of Gohy-573 humic acid for trivalent actinide ions is found to change with ionic strength : LC = -(0.126+0.012) J~1 + (0.683±0.014).A relatively small effect of ionic strength on the complexatíon constant can be ascertained. The constant at 7=0 evaluated by the SIT model is : log ß(0) = 6.52 ± 0.05 and an average value of the constants determined over the whole range of ionic strength under investigation is: log ßUJ = 6.24±0.14.

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.

The complexation of Am(III) with two humic acids from different origins has been investigated in 0.1 Μ NaC10 4 at pH = 6.0 by UV spectroscopy and radiometric measurement. Both humic acids, extracted from Gorleben groundwater and purchased from Aldrich Co., are purified, protonated and characterized previously. Loading capacities of two humic acids for the Am 3 + ion are determined and taken into account for the evaluation of complexation constants. Experiment verifies a tridentate complexation of Am(III) for both humic acids. The complexation constants are found to be log β = 6.44 + 0.15 Lmol -1 for Gorleben humic acid and log β = 6.39 ±0.14 L moP 1 for Aldrich humic acid.

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.

The complexation of Am(III) is investigated with humic acid from one of the Gorleben groundwaters to describe whether or not the isolation and purification of humic acid influences its complexation behaviour with metal ions. Two different experimental systems are taken for comparison, which comprise (i) natural humic acid separated from groundwater by ultracentrifugation at 6 Χ 10 5 g and (ii) humic acid purified by the standard method of acid-base treatment. 241 Am is used for the complexation study in solutions (0.1 Μ NaC10 4 ) buffered at pH 6.0. Speciation is made by UV spectroscopy with the help of peak deconvolution for the two absorption bands at 503.2 nm and 505.5 nm for the Am" ion and Am humate complex, respectively. The complexation is found to undergo a charge neutralization reaction of the Am 3+ ion with three proton exchanging sites of humic acid. With respect to the complexation reaction, loading capacity and complexation constant, no significant difference is found for the natural and purified humic acids. An average complexation constant is evaluated to be log β = 6.26±0.15. The effect of humic acid purification is also characterized by analysis of inorganic impurities and IR-spectroscopy. The results lead to the conclusion that humate complexation studies made on purified humic acid are directly applicable to natural systems.