Uranium (VI) speciation diagrams in the UO₂²⁺/CO₃^2-/H₂O system at 25^oC

A. Krestou, D. Panias*

National Technical University of Athens (NTUA), School of Mining and Metallurgical Engineering,
Section of Metallurgy and Materials Technology, Laboratory of Metallurgy,
9, Heroon Polytechniou St., Zografou Campus, 15780 Athens, Greece



Received 16 December 2003; accepted 15 March 2004



ABSTRACT

Uranium is a toxic heavy metal that occurs naturally. Exploitation processes together with the utilization of the ²³⁵U isotope in the nuclear industry produce huge amounts of wastes that are transferred through aqueous phases to the food chain. The kind of U(VI) species that are present under various conditions in an aqueous phase is the dominant parameter for the designing and application of the appropriate treatment technology. Consequently, the speciation of uranium in aqueous systems under several environmental conditions is of great importance for the protection of the environment and of human health. In the present paper the U(VI) aqueous speciation will be studied in the absence as well as in the presence of the CO₃^2- ligand under variable conditions such as ionic strength and total uranium (VI) concentration. In the case where the CO₃^2- ligand is present the simulation was performed in open to the atmosphere and closed systems. The speciation diagrams show that in aqueous solutions and pH values lower than about 3, uranium (VI) is present exclusively in the form of the uranyl cation, UO₆²⁺. In systems that are free of carbonates, an increase of the solution pH favors the formation of positively charged hydroxo-uranium (VI) complexes that are transformed to negatively charged ones with a further pH increase. The same trend in the uranium (VI) aqueous species transformation is followed in open to the atmosphere systems, in which an increase of the solution pH up to about 8 causes the transformation of the uranyl ion to hydroxo- uranium complexes. However, in strongly alkaline conditions the concentration of carbonaceous complexing agents is higher than that of the hydroxide ion and therefore carbonato-uranyl species are formed. In systems that are closed to the atmosphere and contain constant amounts of dissolved carbonates, the transformation sequence of uranium (VI) species with increasing pH includes the formation of carbonato- uranium complexes in the low acidic and neutral region and finally the transition to hydroxo- uranium complexes in the alkaline pH region.


Keywords: Uranium; Aqueous speciation; Carbonato complexes



* Corresponding author
   E-mail : panias@metal.ntua.gr