Electrophoresis of hexavalent uranium in aqueous solutions containing humic acid (HA) Gohy-573 was studied with the aim to characterize the uranium-humic acid (U-HA) complexes formed. Electrophoretic mobility of uranium towards cathode and anode was determined using a free-liquid electrophoresis, as a function of pH and concentration of humic acid and uranium. Experiments with solutions of 1 X 10~4 mol/1 uranium in absence of humic acid indicated hydrolysis of uranium at pH 3-5. Similar hydrolysis, probably accompanied by the formation of pseudocolloids, was also found for 1 X 10 6 mol/1 uranium. Addition of humic acid in 5.6 X 10~6, 3 X10 5 or 1 X 10 4 mol/1 concentration to 5 X 10" 7 -2 X 10 4 mol/1 solution of uranium at pH 4.0 caused decrease in uranium mobility towards cathode and increase in its mobility towards anode, due to the formation of neutral and/or negatively charged U-HA complexes. The changes were more pronounced at higher ratio [HA]/[U] concentrations. Abundances of U-HA complexes and their mean mobilities towards anode were calculated from the results obtained. It was found that the mobilities decrease with the increasing [U]/HA] ratio due to saturation of negative charge of the humic acid with positively charged uranium species, mainly UO! + .

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Humic acid / Europium / Radiotracer method / Electrophoresis / ComplexationSummary. The interaction of Eu with purified/protonized Aldrich humic acid (HA) was studied by the free liquid/ moving boundary electrophoresis used in combination with the radiotracer method. Introductory experiments were carried out to test electrophoretic behavior of Eu in HA-free solutions, and to determine mobility of Eu 3+ cation. To get the data for evaluation of parameters describing Eu binding to HA, titrations of HA (10 mg/L) with Eu were performed batchwise in the range 3.4 × 10 −8 -1.7 × 10 −3 M of Eu total concentration at pH 3, 4, 5 and 6, and 0.01 M ionic strength (NaClO 4 ). In each batch, the degree of Eu complexation with HA was determined from 152 Eu electrophoretic mobilities in the cathodic direction. The experimental data were fitted with a discrete site model (Scatchard plot-like model) with two types of carboxylic sites. An additional information on the charge characteristics of the Eu-HA complexes has been obtained from the 152 Eu anodic mobilities.

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