Uranyl tricarbonato complexes

Uranium(VI) is converted into uranyl tricarbonato-complexes... 

Leaching with alkali always takes place at high temperatures, either under pressure (5 to 6 bar, 95 to 120°C) or at atmospheric pressure (75 to 80°C). The leaching agent used is sodium carbonate, sodium hydrogen carbonate or ammonium carbonate. Uranium(Vl) oxide is converted in this process into uranyl tricarbonato-complexes ... 

In uranyl(v) solutions, an analogous tricarbonato complex, U02(C03)3, is formed, differing only in one unit of charge from the uranyl(vi) complex [248]. As might be expected from the lower charge of the central ion, the stability is much lower, however, log P3 = 13.3. Any formation of a trinuclear complex (1702)3(003)5 is precluded by the disproportionation of uranyl(v) in the carbonate concentration range where such a complex might exist. 

Uranium occurs in sea water in its highest oxidation state +6 owing to the carbonate content of sea water, uranium predominantly should exist in sea water as the tricarbonato uranylate anion [U02(C03)3]4 , an extremely stable complex with a formation constant of log ji = 22.6. However, there is no experimental evidence for the occurrence of this complex ion in natural sea water due to its extremely low concentration. According to equilibrium constants also other uranium species are expected to occur in sea water (Table 2). 

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