THE RADIOCHEMISTRY OF PLUTONIUM - Sciencemadness.org

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more. Siekierski and Taube, 381 and Taube400 have proposed a system for synergistic
mixtures based on classification of extractors as anionic (A-), neutral (B”), or cationic
(C+). Anionic etiractantfi are acidic compounds such as TTA, DBP, HDEHP, etc. that
act as organic anions in the extracted complex; similarly neutral extractants (TBPJ
TOPO, etc. ) are neutral compounds that form complexes through a basic oxygen atom,
and cationic extractants are strongly basic compounds (’I?OA, TBAN, etc. ) that act as
cations in the extracted complex. In general the synergistic effect is small in mixtures
of compounds of the same class, and may be small or large in mixtures of different
classes. These authors define a synergistic coefficient as
‘1 2, ex
s = 10g D1’ z adj (1)
,,
‘1,
is the experimental
2, exp
distribution coefficient of a mixture of extractants 1 and 2,
and ‘1,
is the calculated
2, add
distribution coefficient based on additivity of the individual
distribution coefficients. Additivity is based on the assumption that (1) no interaction be-
tween the extractants occurs, and (2) no mixed complex of the extracted metal ion with
the two extractants occurs. Both assumptions are commonly not true when extractants
of different types are mixed. A summary table of examples of synergistic mixtures
for U and Pu extraction is given in Table IV- 26, based on this classification system.
Taube400 has made an extensive survey of mixed extraction systems.
Synergism in the system M- TTA-P, where M is an actinide and P is a neutral
phosphate compound has been studied by Irving and Edgington,
196-200 in nitric acid
169 inHCl
and He aly Healy found no evidence of participation of chloride in the ex-
traction. The extraction reaction is thus
‘x +xHT(0) +J’ p(o)
‘ M (T)x (P)y(o) + X H+. (2)
Values of y ranged from 1 to 3 for various di-, tri-, and hexavalent ions. Th(IV) had
a y value of 1 as did U02+, except in the case of TOPO where a value of 3 was also
obtained at high P concentrations.
Irving and Edgington found that 1 or 2 nitrates could enter into the complex,
depending on t$e nitric acid concentration. With tributyl phosphine otide (TBPO) the
species identified were M(IV) T3(N03)P1, M(m) T2(N03)2 P2, and M(~I) T2 (N03)P2,
whereas with P = TBP they were M(IH) T3P2, M(W) T3(N03)P1, and M(VI) T2P1,
where M is an actinide. Thus the complex is influenced by the basic strength of P.
These authors postulate that the reaction mechanism for hexavalent species is the re-
placement of a water molecule in the complex by P, giving a coordinately unsaturated
product, thus
M02T2 H20 + P = M02T2P(0) + H20. (3)
In the case of tetravalent species, one or more chelates are displaced by P molecules,
with nitrates added to preserve electrical neutrality, thus
MT4 + mHN03 + mP = MT4-m(N03)mPm + mHT. (4)
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