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Complexation of the Uranyl Ion with Aquatic Humic Acid

The complexation of the uranyl(VI) ion with aquatic humic acid from Gorleben groundwater (Gohy-573) is studied in 0.1 Μ NaC104 at pH 4 under argon atmosphere. The concentration range investigated is from 1 Χ 10" to 1 X 10~ mol/1 for the uranyl ion and from 1 Χ10" to 1.12 Χ 10" mol/1 for the humic acid. Three different experimental methods are applied to separate the uncomplexed uranyl ion from the uranyl humate complex, using differences of the two species in size by ultrafiltration at the pore size of about 1 nm, in charge by anion exchange separation and in spectroscopic properties by time resolved laser fluorescence spectroscopy (TRLFS). No reduction of U(VI) to U(IV) is observed in the course of the humate complexation process. The experimental results confirm that the prevailing complexation reaction is charge neutralization of the UC>2 ion with two proton exchanging sites of humic acid. At pH 4 in 0.1 Μ NaC104, where the hydrolysis of the uranyl ion appears negligible, the fraction of proton exchanging sites of the humic acid that can be occupied by the uranyl ion (called the loading capacity (LC) of the uranyl ion) is determined to be 0.185 ±0.003. The complexation constant evaluated by taking the loading capacity into account is found to be log/? = 6.16±0.13, which is a grand average of all values determined by the three experimental procedures under consideration. This value is valid for the humate complexation of the non-hydrolyzed uranyl ion, being independent of pH and ionic strength.