Humic substances extraction, characterization and interaction with U(VI) at Ruprechtov site (CZ)

The extent of complexation between natural humic acid (HA) and uranium(VI) has been investigated at the Ruprechtov natural analogue site, Czech Republic. Natural HA was extracted from the clay sediments enriched by organic matter and then characterized for their elemental composition, UV/Vis and infrared spectroscopic properties, exchange capacity and protonation constants. Moreover, the reference sample HA-leonardite was selected and studied for comparison. Complexation between natural HA and U(VI) was studied using the cation exchange technique at pH 5-6. The experimental work was focused on study dependence of U(VI) complexation with HA: (i) on the concentration of HA 1-40 mg/l, and (ii) on the concentration of U(VI) 1 x 10-5-1 x 10-7 M. The charge neutralization model (CNM) was used for evaluation of the experimental data, modified in a way, that the total charge was divided among the individual U(VI) species in solution under given conditions. The stability constant value was determined to be log β = 4.32 ± 0.50, LC=50-60%, being independent on the HA concentration at U(VI) concentrations of 1 x 10-4 M. On the other hand, a dependence on the stability constant for the different U(VI) concentrations was observed. At close to real conditions of the site (U(VI) = 1 x 10-7 M and mHA = 5 mg/l) the stability constant log β=6.16, LC=50.3-52.7%. (orig.)