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which was supported by absorption spectrophotometry (
98
). In another report, the
extraction behavior of technetium and actinides such as thorium, uranium, neptu-
nium, plutonium, americium, and curium from nitric acid medium was investigated
using Aliquat-336 in 1,3-diisopropyl benzene as the extractant (
99
). Distribution data
obtained are modeled by anion exchange (technetium) and ion-pair formation mech-
anisms (actinides) with the extraction of nitric acid included to account for the lower-
ing of the free extractant concentration. Takahashi et al. developed a selective and
very effective concentration method for uranium(VI) by the homogeneous liquid-
liquid extraction method based on the ion-pair separation of perfluorooctanoate ion
(PFOA
−
) with tetrabutylammonium ion (TBA
+
) and acetate as the complexing agent,
which formed anionic U-bearing species such as UO
2
(CH
3
COO)
-
(
100
). There are
several other reports on the ion-pair extraction of actinide ions that involved neutral
donor ligands. In those studies, diluents played an important role in deciding the
nature of extracted species. Mohapatra et al. studied the ion-pair extraction behav-
ior of uranyl ion from aqueous picrate solutions (pH 3.0) employing several crown
ethers such as benzo-15-crown-5 (B15C5), 18-crown-6 (18C6), dibenzo-18-crown-6
(DB18C6), and dibenzo-24-crown-8 (DB24C8) in chloroform (
101
). The stoichiom-
etry of the extracted species corresponded to [UO
2
(crown ether)
n
]
2+
· [pic
−
]
2
, where
n
= 1.5 for B15C5 and 1 for 18C6 as well as DB18C6. Adducts of DB24C8 could not
be observed, as practically no extraction was possible using this reagent. Interestingly,
in this report, trivalent lanthanides were extracted to a much larger extent than the
uranyl ion. In another study, Am
3+
was extracted to a much higher extent than UO
2+
ion, when 18C6 in CHCl
3
was used as the extractant and picrate ion was used as
the counteranion (
102
). The same authors have observed similar unusual extraction
behavior of Am
3+
and UO
2+
using TBP and DOSO as extractant and picrate as the
counteranion. The inner-sphere water molecules and their substitution by the oxo
donor molecules appeared to influence the extraction constants of these metal ions,
which was corroborated with the help of thermodynamic parameters (
103
).
2.3.4 S
y n e r g i S t i C
e
X t r a C t i of n
“Synergism” refers to the phenomenon where the extraction of metal ions in the
presence of two or more extractants is more than that expected from the sum total
of the individual extractants. Though solvent extraction of actinides using a single
extractant is discussed above, there are numerous applications of synergistic extrac-
tion using a combination of suitable extractants. Major advantages of the synergistic
extraction include low ligand inventory and the possibility of extraction from a high
concentration of acids or complexing agents. Well-known examples of synergistic
extraction are the extraction of U(VI) and Pu(IV) from nitric acid medium by a
mixture of a beta-diketone (such as HTTA) and neutral oxo donors such as TBP and
TOPO (Figure 2.4) (
23
). A linear correlation between the adduct formation constant
and ligand basicity was observed (
24
). No role of stereochemistry of auxiliary ligand
(substituted amides) was observed for U(VI) extraction with primary ligands such as
HTTA, HPMBP, or HPBI (
104-106
). The synergistic extraction systems are influ-
enced by side reactions in the aqueous phase, side reactions in the organic phase,
and interactions between the primary and auxiliary ligands wherein diluent plays
