Chemistry Reference
In-Depth Information
reported extraction of disolvate species, with the log
K
ex
values as 8.11, 5.27, and
4.12 for HNO
3
, HCl, and H
2
SO
4
, respectively (
58
).
Recently, another class of neutral organophosphorus compounds, namely,
N,N
-
dialkyl carbamoyl methyl phosphonate (CMP) (
59
) and its phosphine oxide analog
(CMPO) have received attention due to their ability to extract even trivalent actinides
from acidic solutions along with the hexa- and tetravalent actinide ions. These biden-
tate phosphorus-based neutral extractants are reported to be stronger extractants as
compared to TOPO (
59- 62
). Pu(IV) and U(VI) are extracted as per the following
extraction equilibria:
Pu
4
+
+
4
NO
−
+
2
CMPO
Pu(NO
)·
2
MPO
(2.7)
3
(o)
3
4
UO
2+
+
2
NO
−
+
2
CMPO
UO
(
NO
) ·
2
CMPO
(2.8)
3
(o)
2
32
Due to the ease of formation of third phase with CMPO, many literature reports are
based on the use of the mixture of 0.2 M CMPO + 1.2 M TBP as the solvent (
63
). This
mixture has also been used in the TRUEX process recommended for the partitioning
of minor actinides from HLW. Some applications of CMPO for the separation of Pu
include its recovery from assorted laboratory wastes and oxalate supernatant (
64, 65
).
Due to several distinct advantages over TBP (see Section 2.8.2), dialkylamides
are being considered as alternatives to TBP for process applications. Basic studies
on the effect of the structure of
N,N
-dialkylamides with varying alkyl substituents
on the carbon atom adjacent to carbonyl group, namely, di-(2-ethylhexyl)acetamide
(D2EHAA), di-(2-ethylhexyl)propionamide (D2EHPRA), di-(2-ethylhexyl)isobutyr-
amide (D2EHIBA), and di-(2-ethylhexyl)pivalamide (D2EHPVA), on the extraction of
U(VI) and Th(IV) have been carried out (
66, 67
). The conditional extraction constants
(
K
ex
) for uranium are found to vary in the order: D2EHAA (34.0 ± 2.0) > D2EHPRA
(5.6 ± 0.4) > D2EHIBA (1.2 ± 0.1) > D2EHPVA (0.51 ± 0.03). A third phase has been
observed in the case of 1 M D2EHAA (≥1 M HNO
3
) and 1 M D2EHPRA (≥3 M
HNO
3
) in the presence of macro concentrations of thorium. The separation factor
(SF =
D
U
/
D
Th
) for D2EHPVA is distinctly larger as compared to other amides as well
as to TBP. Studies have revealed that successive alkylation of the C
c
atom adjacent to
the carbonyl group greatly suppresses the extraction of tetravalent actinides and fis-
sion products as compared to the hexavalent metal ions and, therefore, holds promise
for the separation of U(VI) from Th(IV). Large numbers of amides were evaluated for
their extraction behavior with respect to U/Th separation (Table 2.2) (
66-72
).
Vidyalakshmi et al. studied the influence of the molecular structure of amides on
the extraction of uranium and nitric acid (
73
). Gupta et al. showed that from moder-
ately acidic medium (3.5 M HNO
3
), U(VI) and Pu(IV) are extracted by amides (viz.
dihexylhexanamide, DHHA; dihexyloctanamide, DHOA; and dihexyldecanamide,
DHDA) via the solvation mechanism similar to TBP. The log
K
ex
values for the sol-
vated species involving amides were lower than TBP for U(VI), but higher than TBP
for Pu(IV) (
74 -76
). At higher nitric acid concentration, the amides undergo protona-
tion (HAmide
+
) and extract U(VI) and Pu(IV) as ion pairs of the type [UO
2
(
NO
3
−
]
)
[HAmide
+
] and [Pu
(
NO
3
2−
] [HAmide
+
]
2
. Thus, the amides differ from TBP (sol-
vated species) with respect to the nature of extracted species at high acidity.
)
