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alkaline-side aqueous solubility of HDEHP-salt than DMDOHEMA. Consequently,
the solvent used for the extraction step was 0.65 M DMDOHEMA-TPH. Finally, an
overall assessment was made for three candidate organophosphoric acids: HDEHP,
bis(1,3-dimethylbutyl) phosphoric acid, and di(1-hexyl) phosphoric acid (
365
).
Dhami et al. of BARC studied another mixed solvent system, 0.2 M CMPO
−0.3 M HDEHP in
n
-parafin, and a strip solution of 0.4 M hydrazine hydrate-0.4
M formic acid-0.05 M DTPA (
371
). The extraction performance of the process was
also satisfactory.
For the separation of Ans from Lns, many other methods or strategies, including
novel extractants, have been reported (
372-380
). These studies have produced vary-
ing degrees of promise, though progress is still at an early stage. They serve to show
the intensity of interest in the area of An(III)/Ln(III) separations.
1.2.2.2.3 Extraction of Cesium and Strontium
A comprehensive review of the extraction of strontium and cesium was made by Dozol
et a l. (
381
). In the United States, there are many HLW tanks storing alkaline waste solu-
tion and sludge, and thereby energetic and continuing R&D with liquid-liquid extrac-
tion has been devoted to the removal of
137
Cs and
90
Sr, which are the main sources of
soluble radioactivity. In the present article, solvent-extraction methods mostly used for
nitric acid systems are reviewed and summarized (Table 1.4). Some of the reagents
tested are shown in Figure 1.6.
1.2.2.2.3.1 Single-element Separation
Extraction of Cs
+
ion is fairly difficult
due to the small charge density of the atomic surface. Thus, calix-crowns were
preferentially used for the extraction, because they trap Cs
+
ion not only by
coordinating with the crown ring, but also by interaction with the π-electrons of the
phenyl rings of the calixarene (
382, 383
). On the other hand, many reports appeared
concerning extraction of Sr
2+
from acidic solutions by crown ethers (
384
).
Crown ethers
.
Horwitz et al. evaluated 4,4(5)-di-(
t
-butylcyclohexano)-18-crown-6
(D
t
BuCH18C6) in various organic diluents for the removal of Sr from acid solutions
(
385, 386
). The authors have demonstrated a relationship between the value of the
extraction constant of Sr and the solubility of water in the organic diluent. The pres-
ence of water in the diluent obviates the need for complete dehydration of the nitrate
ion associated with Sr
2+
for its transfer into the organic phase. As the diluent of
choice,
n
-octanol was selected for further development. D
t
BuCH18C6 has a low solu-
bility in the aqueous phase and exhibits linearity of its
D
(Sr) versus its concentration.
In 1995, the ANL research group reported the replacement of the 1-octanol in the
SREX process with a hydrocarbon diluent, Isopar L, because low concentrations of
1-octanol, which are carried via the aqueous phase to downstream processes, reduce
the performance of the processes (
387, 388
). This incompatibility is significant when
the SREX process is followed by the TRUEX or PUREX processes. TBP was chosen
as a modifier of the Isopar L diluent, because it showed higher
D
(Sr) values. The
SREX process was efficiently applied to the HLW at INL till 1998 (
389-394
). The
extracted complexes are represented as [SrL
2+
(NO
3
)
2
], where L = crown ether.
Calix-crowns.
In France, exploratory studies of calix-crown molecules have been
conducted by Dozol et al. (CEA) with the cooperation of ligand synthesis by Ungaro
