Plutonium carbonates, formation

Two of the study systems, Lake Michigan and Pond 3513, exhibit cyclic behavior in their concentrations of Pu(V) (Figure 2 and 3). The cycle in Lake Michigan seems to be closely coupled with the formation in the summer and dissolution in the winter of calcium carbonate and silica particles, which are related to primary production cycles in the lake(25). The experimental knowledge that both Pu(IV) and Pu(V) adsorb on calcium carbonate precipitates(20) confirms the importance of carbonate formation in the reduction of plutonium concentrations in late summer. Whether oxidation-reduction is important in this process has not been determined. 

In the presence of mineral phases containing anions that would form sparingly soluble compounds (e.g. POt - and F for the lower oxidation states) an enhanced plutonium uptake due to chemisorption can be expected (57). For plutonium in the higher oxidation states the formation of anionic carbonate complexes would drastically reduce the sorption on e.g oxide and silicate surfaces. 

The existence of such a process can be indirectly evidenced by the irregular change of decrement observed in free carbonate (also bicarbonate) concentrations near pH = 10 as illustrated in Figure 3. This may be ascribed to the consumption of free carbonate ions for the formation of the plutonium hydroxocarbonate precipitate. The reactions involved in this process are interpreted as follows ... 

The bicarbonate ion, HC03, is a prevalent species in natural waters, ranging in concentrations up to 0.8 X 10 3. As was indicated previously, carbonate ions have the ability to form complexes with plutonium. Starik (39) mentions that, in an investigation of the adsorption of uranium, there was a decrease in the adsorption after reaching a maximum, which was explained by the formation of negative carbonate complexes. Kurbatov and co-workers (20) found that increasing the bicarbonate ion concentration in a UXi (thorium) solution decreased the amount of thorium which formed a colloid and became filterable. This again was believed to be caused by the formation of a soluble complex with the bicarbonate. 

This method was applied to produce carbides of titanium, zirconium, lead, and bismuth (Barcicki Myrdzik, 1974). Applications ofthe method are limited becanse of insufficient contact between solid particles in the presence of plasma flow if the solid mixtnre is not bricked or sintered. The efficiency of the process arranged in bricks or sintered solid mixture is limited by radiation heat losses and insufficient heat transfer inside of the brick, especially taking into account the decrease of density dne to CO formation (7-92). The most effective condensed-phase synthesis of carbides (7-92) is that from melt containing carbon compounds (Tumanov, 1981). A relevant example is the synthesis of carbides of uraninm and plutonium from a melt containing their nitrites and carbon compoimds (Coppinger Johnson, 1969). 

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