Americium(III)-Humate Interaction in Natural Groundwater: Influence of Purification on Complexation Properties

초록

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 × 105 g and (ii) humic acid purified by the standard method of acid-base treatment. 241Am is used for the complexation study in solutions (0.1 M NaClO<inf>4</inf>) 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 Am3+ ion and Am humate complex, respectively. The complexation is found to undergo a charge neutralization reaction of the Am3+ 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.

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