Uranium(VI) complexation with citric, humic and fulvic acids

The binding of uranium(VI) by Suwannee River humic and fulvic acids was studied at pH values of 4.0 and 5.0 in 0.10 M NaClO₄ using an ion-exchange technique. Few data sets currently exist for metal binding to different molecular weight fractions from the same source. The complexation of U(VI) by citric acid was also studied under the same experimental conditions in order to "calibrate" the experimental and modeling approaches. For the citric acid system, the experimental results were analyzed using Schubert´s ion-exchange method, which indicated the formation of only a 1:1 uranyl-citrate complex. Close agreement was found for the values of log β_1,1 (6.69±0.03 at I = 0.10) determined from nonlinear regression of data collected at pH values of 4.0 and 5.0. This value represents a more direct measurement of the binding constant for the 1:1 uranyl-citrate complex than do other existing literature values derived from experimental data requiring the simultaneous consideration of 1:1 and 2:2 species.

Both humic and fulvic acids were demonstrated to strongly bind U(VI), with humic acid forming slightly stronger complexes and exhibiting greater pH dependence. Analyses of the data for the humic and fulvic acid systems using the Schubert´s equation previously applied to the citrate system result in an apparent nonintegral number of ligands binding the uranyl ion. Schubert´s method is only appropriate for interpreting mononuclear complexes with integral moles of binding ligands. Thus, a more elaborate binding model was required and the data were interpreted assuming either: (1) a mixture of 1:1 and 1:2 uranyl-ligand complexes or (2) a limited number of high affinity sites forming a 1:1 complex. While both of these modeling approaches are shown to provide excellent fits to the data, the second is deemed more appropriate given the large size of humic and fulvic acid molecules as well as previous results obtained with other metal cations, such as Cu(II).