THE RADIOCHEMISTRY OF PLUTONIUM - Sciencemadness.org

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M(HN03)\20
Fig. 2. Distribution of Pu(III) and equilibrium
Pu(lII), Pu(IV), and Pu(VI), mixtures
in the presences of several reducing
agents in 40~0 TBP in kerosene and nitric
acid; after Carleson.76
3
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-0.02
364
and Shevchenko and Federov used 20’7.
TBP in kerosene, and Bernstrom and
Rydberg 41 used 1007. TBP. All the
above workers measured the distribution
coefficients of Pu(IV ) and Pu(VI) as a
function of equilibrium aqueous, nitric
acid concentration with similar results,
and several also studied the effect of
U(VI) competition on the extraction of
Pu(IV ) and Pu(VI). Flanary
132
derived
equilibrium constants (defined by Eq.
(2)) for U(VI), HN03, and Pu(IY) of 22,
0.18, and 3 respectively, while Rozen
and Moiseenko334 and Codding
et al 92, 93
—“
get about 1.5 for Pu(IY).
These values illustrate the effective
competition of macro U(VI) over trace
Pu(IV ) in the simultaneous extraction of
these species, resulting in a large re-
duction of the Pu distribution coefficients
(see Fig. 5).
The order of extractability into
TBP from nitric acid solutions for the
actinides is M(IV) > M(VI)
>> M(III)?4’ 258’26 The extractability
of the tetravalent actinides increases
with atomic number, i. e., Th(lY) < Np(lY) < Pu(IV), while that of the hexavalent
actinides decreases with atomic number, i. e., Pu(VI) < Np(VI) < U(VI). 44
Moiseenko and Rozen 281 measured the effect of temperature on the extraction of
Pu(lV) as a function of nitric acid concentration and uranyl nitrate concentration (Figs.
3, 4, and 5). In the absence of uranyl nitrate the distribution coefficient decreases with
temperature below 5 ~ HN03, while at higher acidities it increases. This effect is
explained by a decrease in the equilibrium constant for the distribution reaction with
temperature, with a compensating increase in the activity coefficient of Pu(IY) at higher
acidities. This increase is ascribed to a decreased as:j:iation of Pu(IW) with nitrate
ions at higher temperatures. Shevchenko and Federov have studied the same system
at nitric acid concentrations below 4 ~ with similar results.
Best e~.43 measured the distribution of several tripo sitive actinides and
lanthanides from nitric acid solutions into 1007. TBP. Some of their data are shown
in Fig. 6 plotted as a function of atomic number, along with the lanthanide data of
Hesford ~. ’77 for comparison. The curves for the two homologous series are
approximately superimposable if adjustment is made to compare ions of the same radius
This illustrates the importance of ionic radius on the chemical behavior of these
elements.
31