Abstract:We have determined the identity of the complexes extracted into the ALSEP process solvent from solutions of nitric acid. The ALSEP process is a new solvent extraction separation designed to separate americium and curium from trivalent lanthanides in irradiated nuclear fuel. ALSEP employs a mixture of two extractants, 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) and N,N,N′,N′-tetra(2-ethylhexyl)diglycolamide (TEHDGA) in n-dodecane, which makes it difficult to ascertain the nature of the extra… Show more
“…The organic extractant HEHEHP (Carbosynth) was purified to 98% HEHEHP/2% bis(2-ethylhexyl)phosphoric acid (HDEHP) using the third phase purification method as previously described. 49 The secondary organic extractant TEHDGA (99%, Eichrom) was used as received. HEDTA (99%, Sigma Aldrich), citric acid (99.5%, Sigma Aldrich), and sodium nitrate (99% Sigma Aldrich) were used as received to prepare the aqueous phases.…”
Section: Methodsmentioning
confidence: 99%
“…Neodymium solutions were prepared from a standardized stock solution of Nd(NO 3 ) 3 as described previously. 49 Americium solutions were prepared from a radiochemically pure Am-241 stock solution at 2 μCi/μL in 1 M HNO 3 (Am was sourced from Eckert & Zeigler). Except for the liquid–liquid extraction experiments used to verify the citrate and HEDTA stoichiometry of the aqueous complexes, the ionic strength of the aqueous solutions was controlled by adding 1 M NaNO 3 to minimize changes in activity coefficients.…”
The actinide lanthanide
separation (ALSEP) process is a modern
solvent extraction approach used for the separation of the minor actinides
americium and curium from the lanthanide fission products for transmutation,
a process that can significantly reduce the long-term radioactivity
and heat loading of nuclear waste. This process, inspired by existing
chemistry, uses the aminopolycarboxylate
N
-(2-hydroxyethyl)ethylenediamine-
N
,
N′
,
N′
-triacetic
acid (HEDTA) to selectively separate the actinides by stripping them
from the organic phase while leaving the lanthanides behind. HEDTA
is used in this separation as it has been shown to exhibit faster
extraction kinetics than other aminopolycarboxylates, but its lower
coordination number can allow for the formation of higher order complexes
with the typically 8- to 9-coordinate f-elements. ALSEP uses a carboxylic
acid buffer in the aqueous phase to control the pH of the system during
metal stripping, and this buffer has the ability to complex actinide(III)
and lanthanide(III) ions. The presence of a previously uncharacterized
ternary lanthanide-HEDTA-citrate complex was detected during single-phase
spectroscopy experiments. A combination of partitioning experiments
and spectrophotometric titrations led to the identification of a 1:1:1
complex containing a partially protonated citrate ligand and determination
of the stability constant of its neodymium complex.
“…The organic extractant HEHEHP (Carbosynth) was purified to 98% HEHEHP/2% bis(2-ethylhexyl)phosphoric acid (HDEHP) using the third phase purification method as previously described. 49 The secondary organic extractant TEHDGA (99%, Eichrom) was used as received. HEDTA (99%, Sigma Aldrich), citric acid (99.5%, Sigma Aldrich), and sodium nitrate (99% Sigma Aldrich) were used as received to prepare the aqueous phases.…”
Section: Methodsmentioning
confidence: 99%
“…Neodymium solutions were prepared from a standardized stock solution of Nd(NO 3 ) 3 as described previously. 49 Americium solutions were prepared from a radiochemically pure Am-241 stock solution at 2 μCi/μL in 1 M HNO 3 (Am was sourced from Eckert & Zeigler). Except for the liquid–liquid extraction experiments used to verify the citrate and HEDTA stoichiometry of the aqueous complexes, the ionic strength of the aqueous solutions was controlled by adding 1 M NaNO 3 to minimize changes in activity coefficients.…”
The actinide lanthanide
separation (ALSEP) process is a modern
solvent extraction approach used for the separation of the minor actinides
americium and curium from the lanthanide fission products for transmutation,
a process that can significantly reduce the long-term radioactivity
and heat loading of nuclear waste. This process, inspired by existing
chemistry, uses the aminopolycarboxylate
N
-(2-hydroxyethyl)ethylenediamine-
N
,
N′
,
N′
-triacetic
acid (HEDTA) to selectively separate the actinides by stripping them
from the organic phase while leaving the lanthanides behind. HEDTA
is used in this separation as it has been shown to exhibit faster
extraction kinetics than other aminopolycarboxylates, but its lower
coordination number can allow for the formation of higher order complexes
with the typically 8- to 9-coordinate f-elements. ALSEP uses a carboxylic
acid buffer in the aqueous phase to control the pH of the system during
metal stripping, and this buffer has the ability to complex actinide(III)
and lanthanide(III) ions. The presence of a previously uncharacterized
ternary lanthanide-HEDTA-citrate complex was detected during single-phase
spectroscopy experiments. A combination of partitioning experiments
and spectrophotometric titrations led to the identification of a 1:1:1
complex containing a partially protonated citrate ligand and determination
of the stability constant of its neodymium complex.
“…A model of trivalent f-element cation extraction by the ALSEP organic solvent under acidic conditions was previously developed to determine the effects of nitric acid on metal distribution. 2 The model considers variations in the activity coefficients of aqueous solutes and the activity of water and accounts for the corresponding changes in the degree of metal-nitrate complex formation and nitric acid extraction equilibria. The activity coefficients of organic phase species were considered to be independent of the aqueous phase composition and thus constant for our nitric acid dependence experiments.…”
Section: Thermodynamic Model For Alsep Extraction Applied To the Nitr...mentioning
confidence: 99%
“…(HEH[EHP])2 were previously described by Picayo et al 2 and the values used are summarized in Table S2. log 1 = 1.33…”
Section: Thermodynamic Model For Alsep Extraction Applied To the Nitr...mentioning
Chemical structures for ALSEP extractants, 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) and N,N,N',N '-tetra(2-ethylhexyl) diglycolamide (TEHDGA) and truncated extractants employed in the computational analysis, ethylphosphonic acid monoethyl ester (HE[EP]) and N,N,N',N'-tetraethyldiglycolamide (TEDGA).
“…Furthermore, to be applied under the harsh conditions of used nuclear fuel recycling, innovative processes should be robust, use conventional and commercially available equipment and chemicals, and show good hydrolytic and radiolytic stability. For this purpose, the Actinide Lanthanide Separation Process (ALSEP) process was developed and proved to fulfill these requirements [18,[23][24][25][26][27][28][29]. The optimized ALSEP solvent is composed of 0.5 mol L −1 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP], Figure 1) and 0.05 mol L −1 N,N,N ,N -tetra-(2-ethylhexyl)-diglycolamide (T2EHDGA, Figure 1) in n-dodecane [27].…”
An Actinide Lanthanide Separation Process (ALSEP) for the separation of trivalent actinides (An(III)) from simulated raffinate solution was successfully demonstrated using a 32-stage 1 cm annular centrifugal contactor setup. The ALSEP solvent was composed of a mixture of 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) and N,N,N′,N′-tetra-(2-ethylhexyl)-diglycolamide (T2EHDGA) in n-dodecane. Flowsheet calculations and evaluation of the results were done using the Argonne’s Model for Universal Solvent Extraction (AMUSE) code using single-stage distribution data. The co-extraction of Zr(IV) and Pd(II) was prevented using CDTA (trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid) as a masking agent in the feed. For the scrubbing of co-extracted Mo; citrate-buffered acetohydroxamic acid was used. The separation of An(III) from the trivalent lanthanides (Ln(III)) was achieved using citrate-buffered diethylene-triamine-N,N,N′,N″,N″-pentaacetic acid (DTPA), and Ln(III) were efficiently back extracted using N,N,N′,N′-tetraethyl-diglycolamide (TEDGA). A clean An(III) product was obtained with a recovery of 95% americium and curium. The Ln(III) were efficiently stripped; but the Ln(III) product contained 5% of the co-stripped An(III). The carryover of Am and Cm into the Ln(III) product is attributed to too few actinide stripping stages, which was constrained by the number of centrifugal contactors available. Improved separation would be achieved by increasing the number of An strip stages. The heavier lanthanides (Pr, Nd, Sm, Eu, and Gd) and yttrium were mainly routed to the Ln product, whereas the lighter lanthanides (La and Ce) were mostly routed to the raffinate.
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