The Stability of Uranium(VI) Complexes of Humates and Fulvates in Biphasic Systems

Abstract

Slight momentary heating of aqueous solutions containing humic substances (HSs) and uranyl ions (UO2²⁺) yielded humic flocs that carried the bulk of the ion down with them. These biphasic systems were at near-neutral pH, and after returning to ambient temperatures they remained intact and were analyzed by determining the free uranyl concentration in the aqueous phase. Phosphate enhanced UO2²⁺ fluorescence was used for this, allowing the determination of the stability constants of HS-uranyl complexes in the floc. The neutral pH of the systems and the relatively unencumbered nature of the analysis provided a realistic picture of the complex formation in natural environments. The log K values of the complexes were found to lie in the 5-7 range and were independent of the total acid content of the humates and fulvates considered. Stability constants obtained for different size fractions of a humic acid were generally smaller than those of the whole material and decreased with molecular size. A comparison of humates and fulvates showed similar trends. The observations suggested that larger HSs polyanions, generally having a greater aliphatic content, provide a cage effect and are able to sequester UO2²⁺ more effectively than smaller size fractions.

Permanent URL: http://hdl.handle.net/2047/d10014974