AMERICIUM(III)-HUMATE INTERACTION IN NATURAL GROUNDWATER - INFLUENCE OF PURIFICATION ON COMPLEXATION PROPERTIES

The complexation of Am(III) is investigated with humic acid from one o f the Gorleben groundwaters to describe whether or not the isolation a nd purification of humic acid influences its complexation behaviour wi th metal ions. Two different experimental systems are taken for compar ison, which comprise (i) natural humic acid separated from groundwater by ultracentrifugation at 6 x 10(5) g and (ii) humic acid purified by the standard method of acid-base treatment. Am-241 is used for the co mplexation study in solutions (0.1 M NaClO4) buffered at pH 6.0. Speci ation is made by UV spectroscopy with the help of peak deconvolution f or 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 unde rgo a charge neutralization reaction of the Am3+ ion with three proton exchanging sites of humic acid. With respect to the complexation reac tion, loading capacity and complexation constant, no significant diffe rence is found for the natural and purified humic acids. An average co mplexation constant is evaluated to be log beta = 6.26 +/- 0.15. The e ffect of humic acid purification is also characterized by analysis of inorganic impurities and TR-spectroscopy. The results lead to the conc lusion that humate complexation studies made on purified humic acid ar e directly applicable to natural systems.