Chemistry Reference
In-Depth Information
studies have shown that solutions of methylamine carbonate and DTPA or
methylamine carbonate and nitrilotriacetic acid (NTA) were as efficient as
the guanidine carbonate and DTPA solution to strip the target extracted
elements. Furthermore, methylamine carbonate is easily regenerated by
distillation.
The UNEX solvent seems to be of limited utility for reprocessing acidic
•
solutions containing large quantities of lanthanides and/or actinides such
as dissolved spent nuclear fuel solutions. These constraints are primar-
ily attributed to the limited solubility of Ph
2
-CMPO and of its metallic
complexes in the UNEX solvent. That is why diamide derivatives of
dipicolinic acid Figure 3.5) have recently been suggested as alterna-
tive An + Ln(III) extractants. The tetrabutyl derivative shows the most
promising results for the extraction of trivalent elements, but it requires
the presence of CCD and PEG-400 as synergistic agents in the FS-13
diluent (
123, 125
).
A procedure based on condensation with phenol and paraform (used as formalde-
hyde source) was developed to convert spent UNEX solvent (CCD, PEG-400, Ph
2
-
CMPO, and FS-13) into a solid infusible resin for disposal. The resulting material is
insoluble in aqueous alkali and acidic solutions and organic solvents. Incorporation
of FS-13 in the cross-linked polymer was confirmed by physicochemical methods.
Resistance of the cured resin to high temperatures was proven by thermogravimetry
(
126
). This procedure is assumed to be applicable to other organic wastes containing
weakly reactive aromatic compounds, such as Fluoropole-732, 1,2-dichlorobenzene,
and nitrobenzene.
3.3.1.1.3.4 Diphosphine Dioxides
BNOPCs, such as diphosphine dioxides, have
been considered for the recovery of transplutonium elements and REEs from HAWs
(
127
). These chelating compounds exhibit enhanced extractive power compared with
CMPO or malonamide compounds (
128
) due to their bidentate coordination poten-
tial. “Anomalous aryl strengthening effects” (due to an increase in substituent elec-
tronegativity, but a decrease in diphosphine dioxide basicity) are often observed when
NOPCs coordinate metal cations in a bidentate mode. The resulting six-membered
ring (Figure 3.6,
n
= 1) has actually a greater acceptor power, and the phenyl substitu-
ents at the phosphorus atoms turn from electron acceptors into electron donor moieties
(
129
). Therefore, the extraction constants of bidentate NOPCs are higher by a factor
of 10
5
than those of monodentate NOPCs, such as trioctyl phosphine oxide (TOPO).
R
1
R
1
N
N
R
2
R
2
N
O
O
FIGURe 3.5
2,6-Pyridinedicarboxamide derivates investigated for the UNEX process.
