Inner: Versus outer sphere metal-monoamide complexation: Ramifications for tetravalent & hexavalent actinide selectivity

Article abstract of DOI:10.1039/c7nj04851c

Oxidation of americium to the hexavalent state could streamline used nuclear fuel management through a group hexavalent actinide separation process. Monoamides, N,N-dialkyl amides, are being considered in this work due to their selectivity towards hexavalent, over tetravalent, actinides. Hexavalent selectivity arises when the monoamides contain a branched acyl chain. While this selectivity has been known since preliminary studies by Siddall, structures of the extracted species have only recently been investigated. This study expands on other research efforts by examining the structure-extraction functionality of straight and branched monoamides with Pu⁴⁺ and PuO₂²⁺. In both Pu oxidations states, the effect of acyl chain length was investigated by comparing Pu extraction and UV-vis spectra of Pu complexes with N,N-dihexylbutyramide, N,N-dihexylvaleramide, and N,N-dihexylhexanamide. Furthermore, the effects of alkyl chain branching were investigated by comparing N,N-dihexyl(2-methyl)butyramide and N,N-dihexyl(2-methyl)valeramide, as well as ethyl branched N,N-dihexyl(2-ethyl)butyramide and N,N-dihexyl(2-ethyl)hexanamide. These structure-extraction relationships were characterized by collecting UV-vis spectra of the metal-monoamide extracted species and tracer distribution studies. The UV-vis analysis was found to match distribution values collected under radiotracer conditions and indicated Pu was extracted as the tetra- and hexanitrato monoamide complex for Pu⁴⁺ and the bis- or trisnitrato species for PuO₂²⁺. The monoamides' ability to recover Cu³⁺ periodate oxidized Am was also studied using radiotracer methods. Initial efforts with nitric acid pretreated monoamides, salting agents, and synergistic studies with di-(2-ethylhexyl)phosphoric acid (HDEHP) were ineffective. Pretreatment with sodium bismuthate was found to improve Am recovery, and distributions above one were achieved with straight chain monoamides at 3 M HNO₃.

Full text of DOI:10.1039/c7nj04851c

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The role of atropisomers on the photo-reactivity and fatigue of
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Inner versus Outer Sphere MetalꢀMonoamide
Complexation: Ramifications for Tetravalent &
Hexavalent Actinide Selectivity
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Kevin McCann1, Sergey I. Sinkov2, Gregg J. Lumetta2, Jenifer C. Shafer1
1Department of Chemistry, Colorado School of Mines, Golden, Colorado 80401, United States
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2 Pacific Northwest National Laboratory, P.O. Box 999, MSIN P7ꢀ25, Richland, Washington
99352, United States
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Oxidation of americium to the hexavalent state could streamline used nuclear fuel management
through a group hexavalent actinide separation process. Monoamides, N,Nꢀdialkyl amides, are
being considered in this work due to their selectivity towards hexavalent, over tetravalent,
actinides. Hexavalent selectivity arises when the monoamides contain a branched acyl chain.
While this selectivity has been known since its preliminary studies by Siddall, structures of the
extracted species have only recently been investigated. This study expands on other research
efforts by examining the structureꢀextraction functionality of straight and branched monoamides
2+
with Pu4+ and PuO2 . In both Pu oxidations states, the effect of acyl chain length was
investigated by comparing Pu extraction and UVꢀVis spectra of Pu complexes with N,Nꢀ
dihexylbutyramide, N,Nꢀdihexylvaleramide, and N,Nꢀdihexylhexanamide. Furthermore, the
effects of alkyl chain branching was investigated by comparing N,Nꢀdihexyl(2ꢀ
methyl)butyramide and N,Nꢀdihexyl(2ꢀmethyl)valeramide, as well as ethyl branched N,Nꢀ
dihexyl(2ꢀethyl)butyramide and N,Nꢀdihexyl(2ꢀethyl)hexanamide. These structureꢀextraction
relationships were characterized by collecting UVꢀVis spectra of the metalꢀmonoamide extracted
species and tracer distribution studies. The UVꢀVis analysis was found to match distribution
values collected under radiotracer conditions and indicated Pu was extracted as the tetraꢀ and
hexanitrato monoamide complex for Pu4+ and the bisꢀ or trisnitrato species for PuO22+. The
monoamides’ ability to recover Cu3+ periodate oxidized Am was also studied using radiotracer
methods. Initial efforts with nitric acid pretreated monoamides, salting agents, and synergistic
studies with diꢀ(2ꢀethylhexyl)phosphoric acid (HDEHP) were ineffective. Pretreatment with
sodium bismuthate was found to improve Am recovery, and distributions above one were
achieved with straight chain monoamides at 3 M HNO3.
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Introduction
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Used nuclear fuel arising from commercial nuclear power reactors can generally be
managed by using open, partially closed, or fully closed nuclear fuel cycles. In an open nuclear
fuel cycle, used nuclear fuel is disposed of in a geological repository without any chemical
processing. Open nuclear fuel cycles have several limitations, including million year waste
management timelines, lack of viable repository sites and limited demonstration of safe waste
disposal, and only 5% energy consumption of the nuclear fuel originally placed in the reactor. A
partially closed nuclear fuel cycle seeks to recover the remaining energy available in the nuclear
fuel by separating deleterious fission products from U and Pu. The recovered Pu can be
reprocessed into MOX (Mixed Oxide) fuel, which increases the energy recovered from the
original nuclear material. More aggressive, fully closed, fuel cycles are designed to recover and
fission neptunium and americium to decrease the waste storage timelines and high level waste
(HLW) repository capacity. While fully closed nuclear fuel cycle would have several waste
management benefits, the separations technology necessary to enable implementation of a closed
fuel cycle requires further development to decrease costs and increase proliferation resistance.
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Industrial scale separations of use nuclear fuel necessary to enable closed nuclear fuel
cycles have, by in large, remained unchanged since 1954.1 In this year, the Plutonium Uranium
Reduction Extraction (PUREX) was launched at the Savannah River Site in South Carolina. The
PUREX process uses tributyl phosphate (TBP) dissolved in an organic diluent to recover
hexavalent uranium, UO22+, and tetravalent plutonium, Pu4+, from nitric acid solution containing
fission products. In a partially closed fuel cycle, the PUREX process, or a comparable
commercial derivative, is the only separation step. In a fully closed fuel cycle, the selective
recovery of Am and Np is necessary to enable their transmutation to shorter lived isotopes. This
transmutation process significantly shortens nuclear waste management timelines and typically
requires one or two additional separation steps beyond the PUREX process to complete Am and
Np recovery.
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Since neptunium’s chemistry readily parallels that of U or Pu, the PUREX process can be
adjusted to recover neptunium. Recovery of Am is nonꢀtrivial due to the similar chemistry Am
shares with lanthanide fission products. Both Am and the lanthanides share a common trivalent
oxidation state, have similar atomic radii, and tend towards ionic interactions due to the
contraction of the 4f and 5f orbitals with the 6s and 7s valence shells. Americium – lanthanide
separation methods under development use soft donor, such as nitrogen, ligands to selectively
complex Am over the lanthanides since actinides participate in limited covalent interactions with
soft donor ligands.2–6 These approaches are nonꢀideal since they require the removal of U, Np,
and Pu, via a PUREX derivative prior to the Am – lanthanide separation. This increases the cost
associated with used nuclear fuel management through additional capital and operating costs for
the Amꢀlanthanide separation. A more streamlined process, where Am is coꢀrecovered with U,
Np, and Pu, would be preferred to minimize reprocessing cost.
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A potential means of achieving the selective group recovery U, Np, Pu, and Am centers
on the common availability of the hexavalent oxidation state. Actinides readily form the unique
+
2+
linear dioxo cation geometry upon oxidation to the pentavalent (AnO2 ) or hexavalent (AnO2
)
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oxidation state.7–10 Uranium generally forms the linear dioxo cation under the PUREX operating
conditions, and neptunium and plutonium can be readily oxidized to the hexavalent state.11
Americium, on the other hand, requires the use of strong oxidants, such a sodium bismuthate or
Cu3+ periodate, that can overcome the AmO22+/Am3+ 1.68 V standard reduction potential.12
Recent studies have demonstrated the oxidation of Am to the hexavalent state and recovery by
organophosphorus extractants, TBP or diamyl amylphosphonate.8,10,13,14 The over sixty years of
operational precedent associated with the PUREX process have demonstrated the viability of
TBP as an actinide extractant, however, the use of TBP has two major drawbacks. As a
phosphorus based ligand, TBP cannot be incinerated which leads to completely gaseous
byproducts which leads to a secondary lowꢀlevel radioactive waste stream. Secondly, the
radiolytic degradation products of TBP decrease metal stripping efficiency. To circumvent
TBP’s shortcomings, monoamides (Table 1) have been studied due to their innocuous
degradation products and incinerability as a CHON (carbon, hydrogen, oxygen, nitrogen) based
ligand.15
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Extraction studies of UO22+ and Pu4+ using monoamides have shown the chemistry of the
acyl group effects the monoamide’s extraction capabilities. Namely, straight chain monoamides
will effectively extract tetravalent and hexavalent actinides. When the alpha carbon off the acyl
oxygen group becomes more substituted the monoamide becomes more selective for hexavalent
actinides over tetravalent actinides.15–17 Selective recovery of the hexavalent state, over the
tetravalent state, is unusual since the favorability of actinide recovery usually trends as An4+
>
AnO2 >> An3+ > AnO2 due to the decrease in charge density.18,19 While previously reported
literature compared the structural effects to hexavalent U to tetravalent Pu and Th, this
manuscript uses the same metal, Pu, to study structural effects of synthesized straight chained
N,Nꢀdihexylhexanamide, N,Nꢀdihexylvaleramide, N,Nꢀdihexylhexanamide and branched N,Nꢀ
2+
+
dihexyl(2ꢀmethyl)butyramide,
N,Nꢀdihexyl(2ꢀethyl)butyramide,
N,Nꢀdihexyl(2ꢀ
methyl)valeramide, and N,Nꢀdihexyl(2ꢀethyl)hexanamide (Table 1).
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Hexavalent selectivity of branched monoamides is advantageous when considering a
hexavalent group actinide separation process. Oxidation of Am3+ to the hexavalent state (1.68 V
vs. SCE)20,21 and extraction by solvating ligands from molar nitric acid has been demonstrated
using strong oxidants such as sodium bismuthate9 and Cu3+ periodate.10 The strong oxidants are
capable of oxidizing other metals encountered in the back end nuclear fuel cycle, i.e. Ce3+ to
Ce4+. Oxidation of Ce3+ to the tetravalent state would cause Ce to be extracted by solvating
ligands, such as diamyl amylphosonate or triꢀbutylphosphate, instead of remaining with the
trivalent lanthanides.13 Straight chain and hexavalent selective branched monoamides have been
2+
shown to extract AmO2 oxidized by sodium bismuthate.22 The sodium bismuthate study
demonstrated that group hexavalent actinides can be extracted by straight or branched
monoamides, but branched are preferred when considering the presence of Ce4+. The manuscript
also showed hexavalent U and Np had different solvation number (~1.5) compared to Pu and Am
2+
(~2.0). This manuscript assesses the ability for monoamides to extract AmO2 using Cu3+
periodate as an oxidant. Furthermore, UVꢀVis spectra of Pu4+ and PuO22+ monoamide species are
presented, and provide insight into the structure of the extracted species based off variations in
the monoamides structure and plutonium oxidation state.
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Table 1: Structure, name, and abbreviation of straight (left column) and branched (middle and right
column) of monoamides studied in this manuscript. Chain length increases moving down.
Straight chain
Methyl branched
Ethyl branched
O
N
N,Nꢀdihexylbutyramide
(DHBA)
N,Nꢀdihexyl(2ꢀ
methyl)butyramide
(DH2MBA)
N,Nꢀdihexyl(2ꢀ
ethyl)butyramide
(DH2EBA)
N,Nꢀdihexylvaleramide
(DHVA)
N,Nꢀdihexyl(2ꢀ
methyl)valeramide
(DH2MVA)
N,Nꢀdihexylhexanamide
(DHHA)
N,Nꢀdihexyl(2ꢀ
ethyl)hexanamide
(DH2EHA)
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Experimental
Monoamide Synthesis
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All monoamides were synthesized using nucleophilic attack of dialkylamines with acyl
chlorides.23 To make N,Nꢀdihexylbutyramide, 0.131 moles dihexylamine (Acros Organics,
99+%) was diluted with 45.00 mL chloroform (SigmaꢀAldrich, HPLC Plus) and 17.00 mL
triethylamine (Alfa Aesar, 99+%) in a 250 mL round bottom flask with a stirbar, reflux
condenser and addition funnel attached. In the addition funnel, 0.150 moles butyryl chloride
(Alfa Aesar, 98%) were diluted with 18.4 mL chloroform. The reaction apparatus was purged
with nitrogen gas. The round bottom flask was chilled in an ice bath to prevent the reaction from
exceeding 10° C during dropwise addition of butyrl chloride to the dihexylamine solution. The
addition funnel was replaced with a stopper after addition of solutions, and flask was purged with
nitrogen. The solution was heated to 60° C, the boiling point of chloroform, and refluxed for 2
hours. The product was filtered, and the filtrate was washed with 10 weight percent Na2CO3, 1 M
HCl, and distilled water to remove triethylamine chloride and unreacted triethylamine. The
solution was dried with sodium sulfate overnight. To purify the product, chloroform was
removed using a rotovap and finally purified via vacuum distillation. The final product was
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determined to be 99% pure via H NMR and had a 71% yield. In total, N,Nꢀdihexylvaleramide,
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N,NꢀDiheylhexanamide, N,Nꢀdihexyl(2ꢀmethyl)butyramide, N,Nꢀdihexyl(2ꢀethyl)butyramide,
N,Nꢀdihexyl(2ꢀmethyl)valeramide, and N,Nꢀdihexyl(2ꢀethyl)hexanamide were synthesized by
following the same procedure with the desired acyl chloride substituent. All products were 99%
pure by H NMR (JEOL 500MHz) and percent yield ranged from 53% to 77%. Monoamides
were also characterized using Bruker Alpha (ATR) FTIR, SI Figure 1 and 2.
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1H NMR Results:
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N,Nꢀdihexylbutyramide: (CDCl3, 500 MHz), δ = 0.69 – 0.91 (9H, m, CH3), 1.1 – 1.3 (12 H, br,
CH2), 1.33 – 1.50 (4H, br, CH2), 1.50 – 1.64 (2H, m, CH2), 2.15 (2H, t, OCHꢀCH2), 3.01 – 3.26
(4H, 2 t, NꢀCH2)
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N,Nꢀdihexylvaleramide: (CDCl3, 500 MHz), δ = 0.74 – 0.92 (9H, m, CH3), 1.13 – 1.37 (14H, m,
CH2), 1.37 – 1.63 (6H, m, CH2), 2.22 (2H, t, CH2), 3.08 – 3.29 (4H, 2 t, NꢀCH2)
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N,Nꢀdihexylhexanamide: (CDCl3, 500 MHz), δ = 0.76 – 0.93 (9H, m, CH3), 1.15 – 1.35 (16H,
br, CH2), 1.39 – 1.68 (6H, m, CH2), 2.22 (2H, t, CH2), 3.06 – 3.33 (4H, 2 t, NꢀCH2)
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N,Nꢀdihexyl(2ꢀmethyl)butyramide: (CDCl3, 500 MHz), δ = 0.71 – 0.88 (9H, br, CH3), 0.95 –
1.06 (3H, m, CH3), 1.1 – 1.7 (18H, m, CH2), 2.44 (1H, m, CH), 3.02 – 3.34 (4H, m, NꢀCH2)
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N,Nꢀdihexyl(2ꢀethyl)butyramide: (CDCl3, 500 MHz), δ = 0.75 – 0.91 (12H, m, CH3), 1.16 – 1.34
(12H, br, CH2), 1.34 – 1.67 (8H, m, CH2), 2.31 – 2.39 (1H, m, CH), 3.14 – 3.32 (4H, 2 t, NꢀCH2)
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N,Nꢀdihexyl(2ꢀmethyl)valeramide: (CDCl3, 500 MHz), δ = 0.73 – 0.90 (9H, m, CH3), 0.96 –
1.06 (3H, d, CH3), 1.12 – 1.66 (20H, m, CH2), 2.47 – 2.62 (1H, m, CH), 3.04 – 3.36 (4H, m, Nꢀ
CH2)
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N,Nꢀdihexyl(2ꢀethyl)hexanamide: (CDCl3, 500 MHz), δ = 0.72 – 0.90 (12H, m, CH3), 1.0 – 1.64
(24H, m, CH2), 2.31 – 2.45 (1H, m, CH), 3.08 – 3.33 (4H, m, NꢀCH2)
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Extraction Studies
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Extraction studies were completed on 239Pu4+ and 239PuO22+ in HNO3 (Fisher, optima grade) with
1.0 M monoamide in nꢀdodecane (SigmaꢀAldrich). The organic phase was preꢀequilibrated with
HNO3 prior to Pu extraction. The 239Pu stocks were prepared by adding 30% H2O2 dropwise to
25 mM Pu 2 M HNO3 solution to reduce any oxidized species and generate 99.6% Pu4+ solution,
determined using UVꢀVis spectroscopy on a Spectral 420 CCD Array UVꢀVis
2+
Spectrophotometer (SI Figure 3). The PuO2 solution was generated by addition of HClO4 to
oxidize Pu mixed species to PuO22+. To remove HClO4, the solution was slowly evaporated until
2+
PuO2 residue was nearly dry. The residue was dissolved with 0.5 M HNO3. The
2+
evaporation/dilution process was repeated two additional times. The final PuO2 concentration
2+
was 48 mM in 0.5 M HNO3, and was determined to be 100% PuO2 by visible absorbance
spectroscopy (SI Figure 3). To complete tracer level extractions, Pu was added to 2 M HNO3 for
a final concentration of 0.21 mM Pu4+ or 0.40 mM PuO22+, 5.0 µL of either stock was spiked into
500.0 mL of the respective nitric acid solution for triplicate samples. Extractions were completed
by contacting 1 – 5 M HNO3 Pu containing solutions with an equal volume of preꢀequilibrated 1
M monoamide. The solutions were shaken by hand for 30 seconds, and centrifuged to disengage
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the two phases. Each phase was subsampled, added to LSC cocktail (PerkinElmer, Ultima Gold
AB) and quantified using PerkinElmer LSC TriꢀCarb 3110 TR. Organic phase visible absorbance
spectra were also collected after contacting 1.0 M monoamide (preꢀequilibrated with the
2+
appropriate HNO3 solution) with 4.0 mM Pu4+ and 3.0 mM PuO2 in 4.5 and 4.7 M HNO3,
respectively. All visible absorbance spectra are presented as the average of triplicate readings.
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Americium extractions were done using 241Am (Eckert & Ziegler, 1 M HCl), and followed
previous extraction studies that used Cu3+ periodate to obtain AmO22+ 9,13 The stock was diluted
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in 2 M HNO3, and was slowly evaporated until dry and redissolved in 4 M HNO3. The solution
was evaporated until almost dry and diluted with 0.1 M HNO3. The process was repeated a third
time, and final 241Am solution was in 0.1 M HNO3. The 1.0 M monoamide in nꢀdodecane
organic phase was preꢀequilibrated with the corresponding HNO3 prior to Am extraction. To
oxidize Am, 20 mg of Cu3+ periodate (synthesized as previously reported)9 was added to 2 mL
vial and 500 µL 241Am in respective HNO3 was added to the vial. An equal volume of HNO3 preꢀ
equilibrated 1.0 M monoamide was added, and was shaken by hand for 5 seconds and
centrifuged to disengage the two phases. Each phase was subsampled and added to LSC cocktail
(PerkinElmer, Ultima Gold AB) and quantified using PerkinElmer LSC TriꢀCarb 3110 TR.
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17 Results and Discussion
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Tetravalent and Hexavalent Pu-Monoamide Extraction
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Distribution values of Pu4+ with DHBA, DHVA, and DHHA were collected to determine the
effect of acyl chain length on extraction efficiency. As shown in Figure 1, the Pu4+ distribution
values are above 1 at 2 M HNO3 and increase up to the 5 M HNO3 measurement. The increase in
distribution ratio is consistent with monoamide reagents being solvating extractants that require
nitrate to encourage metal recovery.24,25 As the concentration of nitric acid increases the
partitioning of plutonium into the organic phase by two monoamide ligands increases.15 The
extraction of Pu4+ by straight chain monoamide ligands from an acyl chain length 4 carbons
(DHBA) to 6 carbons (DHHA) does not show an overall trend. At 5 M HNO3 the Pu4+
distribution value is 25 ± 3 for DHBA, 15.1 ± 0.5 for DHVA, and 20 ± 2 for DHHA. While the
DHBA extracts Pu4+ more efficiently than the longer acyl chain monoamides, DHVA shows
lower distribution values in general than the six carbon DHHA. It should be noted the range in
distribution values differs from roughly 96.2% DHBA extraction to 93.8% with DHVA, a
difference of only 2.4%.
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Figure 1: Distribution values of Pu4+ at various HNO3
concentrations for straight chain monoamides (sky blue) DHBA
(circle), DHVA (square), DHHA (diamond). Distribution with
methyl branched monoamides (navy blue) DH2MBA (circle)
and DH2MVA (diamond) as well as ethyl branched monoamides
(green) DH2EBA (square) and DH2EHA (triangle).
1
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When the carbon adjacent to the carbonyl group is changed from a secondary (straight chain) to
tertiary carbon (branched) the distribution values of Pu4+ drop significantly across all acid ranges
as shown in Figure 1. The type of branching, i.e. methyl versus ethyl, resulted in a specific
grouping across all acid ranges as well. The methyl branched DH2MBA and DH2MVA
molecules show nearly identical distribution values of 1.09 ± 0.03 and 1.0 ± 0.1, respectively, at
5 M HNO3. Similarly, the DH2EBA and DH2EHA molecules produced distribution values of
0.81 ± 0.06 and 0.77 ± 0.04. The proximity of the distribution values indicates the length of the
primary alkyl chain does not greatly affect the distribution values. On the other hand, when
comparing DH2MBA vs. DH2EBA, where the primary alkyl chain lengths are equivalent, the
distribution values drop from 1.09 to 0.81 for methyl vs. ethyl branching and equates to a 7.4%
change in extraction efficiency. To better understand changes in the extracted species visible
absorbance spectra were collected on Pu4+ extracted by the various monoamides.
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Figure 2: Absorbance spectra of Pu4+ extracted from 4.5 M HNO3 by 1 M
DHBA (sky blue), DH2MBA (navy blue), and DH2EBA (green) in nꢀ
dodecane.
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The visible absorbance spectrum of Pu4+ extracted by the straight chained DHBA from 4.5 M
HNO3, Figure 2, bears a striking resemblance to plutonium hexanitrate, Pu(NO3)6 , absorbance
2ꢀ
4
spectra.26 A spectrum of Pu4+ in 10 M HNO3, producing Pu(NO3)62ꢀ species, was overlayed on
the Pu4+ꢀDHBA spectrum (Figure 3). The two spectra have identical spectral signatures and
show only slight variation in peak intensity at 609, 445, and 405 nm as well as a slight blue shift
with 10 M HNO3 Pu4+ spectrum around 492 nm. Acher et. al. made a similar observation at
elevated nitric acid concentrations, and used EXAFS to determine plutonium hexanitrate was
being extracted by two protonated N,Nꢀdi(2ꢀethyl)hexylbutyramide ligands via an outerꢀsphere
coordination as Pu(NO3)6(HL)2.27 The results presented here support the extraction of plutonium
hexanitrate species by the straight chain monoamides. The reason why the three peaks between
580 and 730 nm are prominent in this work at 4.5 M HNO3 compared to Acher’s 5 M HNO3
spectra in which the three peaks are present but less defined is not obvious. Nonetheless, in
Figure 2 the 609 nm peak has a lower intensity than the 650 and 746 nm peaks for all three
monoamides. A more intense 609 nm peak relative to 650 and 746 nm would indicate a pure
plutonium hexanitrate species is present (Figure 3).28 Therefore, the spectra indicate another
species, plutonium tetranitrate, is present.
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18
19
20
21
The observations in this manuscript and by Acher contrast the common understanding that Pu4+
would be recovered as the charge neutral Pu(NO3)4 species, where the nitrates bind in a bidentate
fashion to Pu4+.25,26 The previous assignment of Pu4+ recovery as a Pu(NO3)4L2 species was
based on radiotracer distribution studies that did not account for the nitrate activity or the
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5
possible presence of protonated extractants participating in the extraction equilibrium. The
ability for commonly available visible spectroscopy to corroborate previous EXAFS findings of
the Pu(NO3)6(HL)2 extracted species suggests that this extracted species should be considered as
the primary mechanism for Pu4+, and possibly tetravalent actinide recovery, by straight chain
monoamide reagents.
Figure 3: Absorbance spectrum of Pu4+ in 10 M HNO3 which takes the form
2ꢀ
of plutonium hexanitrate species, Pu(NO3)6 (dark blue) and spectrum of 4.5
M HNO3 Pu4+ (green) extracted by 1 M DHBA in nꢀdodecane (light blue).
6
7
8
In the Pu4+ 1 M DHBA spectrum (Figure 2), the 650 nm peak is more intense than 609 and 746
nm. This peak intensity pattern indicates some innerꢀsphere Pu(NO3)4L2 species are present in
solution or a dual inner/outer species, Pu(NO3)5L(HL).27,29 Similarly, the most prominent of the
three peaks between 580 – 730 nm spectral region for DH2MBA and DH2EBA occurs at 649
nm. All three peaks are broadened in both branched monoamides with DH2EBA peaks being the
broadest. The broadening is an indication that Pu4+ extracted species is more characteristic of
innerꢀsphere species, Pu(NO3)4L2.27 Furthermore, the peak intensity at 746 nm decreases relative
to the 580 – 730 nm peaks, and the 495 nm peak broadens when moving from the straight to
methyl to ethyl branched monoamide. The experimental uncertainty based off triplicate spectral
measurements showed the monoamide’s spectra were significantly different. The spectra reveal
that the branched monoamides show an increase in innerꢀsphere species, such as Pu(NO3)4L2 or
the dual inner/outer species, Pu(NO3)5L(HL).29 An increase of innerꢀsphere extracted species for
branched monoamides is peculiar as the branching on monoamides induces an increase in steric
hindrance and has been ascribed as the reason tetravalent actinide extraction decreases.15,16,29,30
Further work should consider assessing the complex speciation that seems to persist in these
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tetravalent actinide monoamide systems using a combination of spectroscopic and computational
methods. The data suggests that multiple monoamide extracted species are possible and,
depending on the solution conditions, the equilibrium constants for these species might allow for
multiple species to be present at a given moment. With this information in mind, identifying
model conditions to isolate the presence of an individual monoamide species will be important to
if work assessing the equilibrium constants of such species is to be completed.
7
8
The Pu4+ distribution values were calculated by measuring the Pu4+ concentration at the 476 nm
aqueous phase peak before and after extraction,example shown in SI Figure 4. For 1 M DHBA,
the Pu4+ concentration was below limit of quantitation after mixing owing to its 96% extraction
efficiency measured in Figure 1 by LSC. The 1 M DH2MBA and DH2EBA distribution values
were 1.12 ± 0.05 and 0.79 ± 0.04 respectively and match the values measured in Figure 1. The
decrease in distribution values correlates to a decrease in outerꢀsphere extraction character as
DHBA > DH2MBA > DH2EBA. Intuitively, the branched monoamide would experience less
steric hindrance with an outerꢀsphere species since the extractant is sitting further from the metal
center. The decrease in monoamide protonated outerꢀsphere species, Pu(NO3)6(HL)2, with
branched monoamides for Pu4+ extraction under similar HNO3 concentration suggests branched
monoamides are less apt to protonation than their straight chain homologues. While monoamide
protonation has not been directly measured, Condamines determined HNO3 K11 constant was
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
0.135
for
1
M
N,Nꢀdi(2ꢀethylhexyl)butyramide
in
TPH
while
N,Nꢀdi(2ꢀ
ethylhexyl)isobutyramide had a lower K11 constant of 0.086.24 Condamines’ results support the
protonation hypothesis. Therefore, steric hindrance as well as availability of protonated
monoamides could be contributing to the decreased distribution values observed with branched
monoamides.
24
25
26
27
28
29
30
31
32
33
34
35
Extraction of hexavalent plutonium, PuO22+, by straight chain monoamides displayed a trend
across all acid concentrations with increasing acyl chains. Shown in Figure 4, the distribution
values decrease with increased acyl chain length as 7.9 ± 0.3 > 6.1 ± 0.1 > 5.4 ± 0.1 for DHBA >
DHVA > DHHA from 5 M HNO3. The extraction efficiency is approximately 88.8% for DHBA
and decreases to 84.4% for DHHA. The extraction efficiency is lower than that observed with
Pu4+, but is to be expected due to Pu4+ having a higher charge density than PuO22+. Conversely,
branched monoamides extract PuO2 to a greater extent than Pu4+, Figure 4. The branched
2+
monoamides show distribution values approach 1 at 3 M HNO3 for PuO22+, while Pu4+
distribution values approach 1 at 5 M HNO3 with the same monoamides. At 5 M HNO3, the
PuO22+ distribution values reach 2.27 ± 0.01, 2.5 ± 0.3, 2.8 ± 0.1, and 2.60 ± 0.07 for DH2MBA,
DH2EBA, DH2MVA, and DH2EHA respectively. The values fall within experimental
uncertainty of each other; therefore, no overall trend could be deduced.
36
37
38
39
40
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2+
Figure 4: Distribution values of PuO2 extracted from various
HNO3 solutions by 1 M DHBA (circle), DHVA (square), and
DHHA (diamond) in nꢀdodecane (sky blue). Values for methyl
branched monoamides (navy blue) DH2MBA (circle) DH2MVA
(diamond) and ethyl branched (green) DH2EBA (square) and
DH2EHA (triangle) compared to Pu4+ extracted by DH2MBA
(grey star).
1
2
3
The organic phase absorbance spectra of PuO22+ extraction from 4.7 M HNO3 shows a multitude
of peaks compared to the PuO2 aqueous HNO3 spectrum, Figure 5. Several peaks are present
2+
4
between 450 and 670 nm, and five distinct peaks are observed at 789, broad 801, 809, broad 824,
and 849 nm. The monoamide spectra bear a stark resemblance to UVꢀVis collected by Keder et.
al. of PuO22+ extraction from 4 M HNO3 by triꢀnꢀoctylamine in oꢀxylene ꢀ particularly in the 450
– 670 nm region and with 789 – 824 nm peaks.28 Keder et. al. deduced PuO22+ was extracted as a
trinitrate species, PuO2(NO3)3 , by comparing the PuO2 triꢀnꢀoctylamine spectrum to a
tetraethylammonium trinitratodioxoplutonate(VI) spectrum. Both spectra are consistent with the
aforementioned peaks observed in this work with the exception of the 849 nm peak. Absence of
the 849 nm peak in the tetraethylammonium trinatodioxoplutonate(VI) spectrum suggests
another species is being extracted by the monoamides. Correlating with Keder’s research, one
species could be the plutonyl trinitrate species which would be extracted by a protonated
monoamide as, PuO2(NO3)3·HL.22,25 The second could be the charge neutral dinitrate plutonyl
species, PuO2(NO3)2(L)2, that gives rise to the 849 nm peak.31 These assignments would be
further supported by collecting EXAFS or infrared spectra.
5
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7
⁻
2+
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2+
Figure 5: Absorbance spectra of PuO2 extracted from 4.7 M HNO3 by 1 M
DHBA (dashed), DH2MBA (dark green), DH2MVA (light green), DH2EBA
(navy blue), and DH2EHA (light blue) in nꢀdodecane.
1
2
3
The presence of two species contradicts 1:2 metal:ligand ratio previously reported for
monoamide extraction from 6.5 M HNO3.22 However, the slope analysis method used to
determine the metal:ligand ratio revealed a slope lower than 2. A slope of 2 would be indicative
of ideal conditions where only one extracted species exists, but the less than 2 value indicates a
1:1 species can exist which absorption spectra in this study indicate. Changes of Pu
concentration in the aqueous phase were measured before and after extraction, and showed 2.2
4
5
6
7
2+
8
mM PuO2 was extracted into the organic phase for all branched monoamides. Given the same
2+
9
PuO2 concentrations in each monoamide study, the spectra are distinct from each other based
10
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12
13
14
15
16
17
18
19
20
21
off the type of branching. The methyl branched monoamides exhibit similar relative peak
intensities compared to the ethyl branched monoamides between 450 and 670 nm. In the 780 –
860 nm range the 789, 801, and 809 peaks are similar. The 824 nm peak, however, shows a
broader intensity for the methyl branched while the ethyl branched come to more of a point
around 821 nm. The reason for slight differences in the 824 nm spectral region is not obvious.
The possibility of a protonated outer sphere 1:1 metal:ligand species instead of a inner sphere 1:1
species could be considered. Research by Acher et. al. indicated the presence of a protonated
monoamide ligand. Since protonated monoamides with a 1:2 metal:ligand ratio have been
observed, and a PuO2(NO3)HL has not been observed with monoamides or other types of
solvating ligands, the possibility of a nonꢀprotonated single ligand extracted species is unlikely.
Future studies, using EXAFS, could explicate the type of extracted species and the fine structure
differences observed in the UVꢀVis.
22
Hexavalent Americium Extraction
23
24
25
Extraction efficiency for hexavalent actinides decreases as one moves across the actinide series
(i.e., increasing Z) due to the decrease in charge density.22 Therefore, it was interest to see if the
synthesized monoamides were strong enough to effectively extract hexavalent americium,
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1
2
AmO22+. Prior to studying AmO22+ the extraction of Am3+ by 1 M DHBA was measured for 1 –
5 M HNO3 solutions. Trivalent americium was not extracted by DHBA in quantifiable amounts
from any acid concentration. This is a favorable result as it indicates trivalent lanthanides will
not be extracted in appreciable amounts by monoamides if employed in fuel reprocessing. The
Cu3+ periodate oxidized Am did show extraction, Figure 6, with 1 M DHBA, DHVA, and
DHHA, however, the distribution values are below 1, or less than 50%’ was extracted. The
distribution values increased from 1 to 3 M HNO3, but decreased at higher nitric acid
concentrations. The trend is similar to that observed with diamyl amylphosphonate (DAAP), and
further studies have revealed the decreased values are a result of incomplete Am3+ oxidation as
3
4
5
6
7
8
9
2+
2+
10
11
12
13
14
15
16
well as reduction of AmO2 in higher nitric acid concentrations.10 Reduction of AmO2 also
+
occurs in the organic phase and produces AmO2 and Am3+ species which are poorly extracted
by monoamides.22 Comparing the monoamide chain length, DHBA shows the highest
2+
distribution while DHVA and DHHA extract comparatively. Considering PuO2 had similar
distribution values for all three straight chain monoamides it is thought that AmO22+ experienced
a higher degree of reduction with DHVA and DHHA due to the presence of trace reducing
agents.
Figure 6: Distribution values of Cu3+ periodate oxidized Am in 1 – 5
M HNO3 extracted by 1 M DHBA (blue squares), DHVA (grey
diamonds), and DHHA (green triangles) in nꢀdodecane.
17
18
19
20
21
22
Several parameters were adjusted in attempts to obtain greater than 50% extraction efficiency for
AmO2 with straight chain monoamides. Salting agents can be used to increase the ionic
strength of aqueous phase and promote metal extraction. Using the acid concentration with
highest distribution, 3 M HNO3, distribution values were determined with the addition of 0.1 to
1.0 M LiNO3 and Al(NO3)3 (SI Figure 5). In the absence of LiNO3 distribution value was 0.75 ±
2+
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2
0.02. In the presence of 0.1 M LiNO3, the distribution value increased slightly to 0.81 ± 0.08 and
steadily decreased to 0.71 ± 0.03 at 1 M HNO3. All distribution values fell within experimental
uncertainty of each other; therefore, LiNO3 had no effect on the distribution values. A small
effect was observed when Al(NO3)3 was added to 3 M HNO3 solutions. The distribution
increased from 0.69 ± 0.09 in absence of Al(NO3)3 to 0.86 ± 0.04 with 0.30 M Al(NO3)3. The
distribution values then fell to 0.75 ± 0.8 at 1 M Al(NO3)3. The addition of 1 M Al(NO3) results
3
4
5
6
⁻
⁻
7
in a 3 mole equivalent addition of NO3 . The increased NO3 concentration would promote
formation of the charge neutral AmO2(NO3)2 extracted species, but the high concentration of
8
⁻
⁻
9
NO3 at 1 M Al(NO3)3 could promote formation of anionic species which are only extracted by
protonated monoamides that exist at high molar acid concentrations.22,27 Although Al(NO3)3
showed some improvement the distribution values did not increase in appreciable amounts.
10
11
12
13
14
15
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19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
Mixed ligand separations such as a neutral extractant, diglycolamide (T2EHDGA), and the acidic
extractant 2ꢀethylhexylphosphonic acid monoꢀ(2ꢀethylhexyl) ester (HEH[EHP]) in the ALSEP
system promote higher metal extraction than when the extractants are used individually. The
increased extraction is referred to as a synergistic effect. It was of interest to see if the neutral
monoamide extractants would experience synergism with the acidic extractant diꢀ(2ꢀ
ethylhexyl)phosphoric acid, HDEHP. On its own, HDEHP does not extract Am3+ at high nitric
acid concentrations (SI Figure 6). Extraction of Cu3+ periodate oxidized Am resulted in
distribution values as high as 3.43 ± 0.03 at 2 M HNO3 with 0.5 M HDEHP in nꢀdodecane, SI
Figure 6. The HDEHP result in and of itself is promising. When compared to 1 M diamyl
amylphosphonate (DAAP) studies, a 2.0 ± 0.1 distribution value was achieved, and corresponded
to a AmO22+/Am3+ separation factor of 6.1 at 2 M HNO3 (Am3+ as a Ln3+ surrogate).10 At 2 M
HNO3 a AmO22+/Am3+ separation factor of 1150 was achieved with 0.5 M HDEHP and 3249 at 3
M HNO3. Even higher distribution values were envisioned with a HDEHPꢀmonoamide
synergistic effect. Since the highest monoamide distribution value occurred at 3 M HNO3, the
distribution of oxidized Am was measured at 3 M HNO3 by maintaining an extractant
concentration of 1 M while changing the ratio of DHBA and HDEHP. The results are shown as
a Job’s plot in Figure 7. When no HDEHP is present the distribution is 0.66 ± 0.03 and the
distribution increases with the inclusion of 0.1 and 0.3 M HDEHP. At 0.5 M HDEHP/0.5 M
DHBA the distribution value is 1.7 ± 0.4 which lies within experimental error of the 2.0 ± 0.7
distribution value obtained with 0.5 M HDEHP individually. The similar distribution value
indicates no synergistic effect is observed for HDEHP with DHBA. Based on comparisons with
other solvating reagents,32–36 the inability for monoamide reagents to participate in a synergistic
metal extraction mechanism with HDEHP was unanticipated. In other synergistic systems the
extractants can orient to have a polar head (oxygen donor) and nonpolar tails, akin to a
shuttlecock. The monoamide extractants do not share a similar ability, and the acyl group’s
vicinity to the nonpolar nitrogen alkyl chains could be inhibiting the synergistic coordination.
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Figure 7: Distribution values of Cu3+ periodate oxidized Am in 3 M
HNO3 extracted by varying the ratio of HDEHP and DHBA. Extractant
concentration was maintain at 1 M in nꢀdodecane while the amount of
DHBA decreased from 1 M DHBA (ratio value of 0) to 0.5 M with 0.5
M HDEHP (ratio value of 1) to generate the Job’s plot. Distribution of
oxidized Am in 3 M HNO3 extracted by 0.5 M HDEHP nꢀdodecane is
included for comparison (green square).
1
2
3
The presence of trace reducing species in the organic phase was considered as contributing to the
low extraction since AmO2 would be readily reduced to poorly extracted AmO2 and Am3+
species. To decrease the amount of reducing species, NaBiO3 was added to HNO3 during the preꢀ
equilibration step. Although Cu3+ periodate is used to oxidize Am3+ in the actual extractions,
Cu3+ periodate quickly reduces in the presence of organic phase.10 Due to Cu3+ periodate’s short
lifetime, NaBiO3 was used during preꢀequilibration because it persists for the longer time
durations used to preꢀequilibrate the solutions (2 hours) and has been used elsewhere in
literature.22 The results, shown in Figure 8, show the same trend observed with Am3+ where the
distribution values increase between 1 and 3 M HNO3 but decrease at 4 and 5 M HNO3.
Although the same trend occurs, the distribution values are higher compared to nonꢀoxidative
preꢀtreatment samples. The distributions are noticeably higher between 3 and 5 M HNO3. This
suggests the NaBiO3 did behave as a scavenger to eliminate reducing agents in the organic phase.
2+
+
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Less of a difference between preꢀtreatments with and without NaBiO3 is observed at 1 and 2 M
HNO3 solutions since less Am recovery occurs in the presence of lower nitric acid
concentrations. Higher nitric acid concentrations increase the concentration of AmO2(NO3)2
2+
species causing the increase in distribution values. The sensitivity of AmO2 towards reduction
prevented deduction of an observable trend in distribution values for the increasing monoamide
acyl chain. Distribution values were within close proximity to each other at 3 M HNO3 as 1.03 ±
0.02, 1.16 ± 0.03, and 1.08 ± 0.07 with DHBA, DHVA, and DHHA respectively. At other acid
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2
3
4
5
6
7
concentrations, the values were not statistically different. As seen with PuO22+, distribution
values with branched monoamides are lower than straight chained for hexavalent actinides.
Although NaBiO3 was used to pretreat 1 M DH2MBA and DH2EBA solutions, the extraction
efficiency did not exceed 50%. Previous research has shown that distribution values with
monoamides increase up to 6.5 M HNO3 for NaBiO3 oxidized Am3+.22 If more efficient
oxidation, with less reduction could be achieved at 4 and 5 M HNO3 with Cu3+ periodate, the
distribution values should also increase above 3 M HNO3 concentrations.
Figure 8: Distribution values of Cu3+ periodate oxidized Am over 1 – 5
M HNO3 acid range. The 1 M monoamide in nꢀdodecane solution were
preꢀequilibrated with 60 mg/mL NaBiO3 in the respective acid
concentration. Oxidized americium extraction by straight chain
monoamides (light blue) 1 M DHBA (circle), DHVA (square), and
DHHA (diamond) are compared to branch chain DH2MBA (navy blue
circle) and DH2EBA (green triangle).
8
9
Conclusion
2+
10
11
12
13
14
15
16
17
18
Extraction of Pu4+ and PuO2 with synthesized straight and branched monoamides was
demonstrated. As expected, Pu4+ and PuO2 was efficiently extracted (96% and 89%
2+
respectively) by straight monoamides. No trend was observed over increasing acyl chain length
2+
for Pu4+ extractions, but PuO2 extractions decreased with increasing acyl chain length from
butyramide to hexanamide. Branched monoamides demonstrated the selective extraction of
PuO2 over Pu4+. No trend was observed with PuO2 extractions, but Pu4+ extraction was
grouped based off the branch being either methyl or ethyl branched monoamide. The monoamide
UVꢀVis spectra showed a decrease in the outerꢀsphere extracted species, Pu(NO3)6(HL)2, in the
order of straight > methyl branched > ethyl branched and indicated Pu(NO3)4(L)2 or
2+
2+
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1
2
Pu(NO3)5L(HL) was also extracted. The PuO22+ monoamide UVꢀVis spectra indicated a plutonyl
trinitrate species, mostly likely as PuO2(NO3)3(HL), was extracted as well as PuO2(NO3)2(2L).
Slight variations were observed in the Pu(NO3)3(HL) region between methyl and ethyl branched
monoamides indicating fine structural effects which should be investigated in future studies.
Oxidized americium distribution values were below one for acid pretreated monoamide
solutions. Salting agents LiNO3 and Al(NO3)3 did not significantly improve extraction, and no
synergistic extracted effect was observed with addition of HDEHP. However, 0.5 M HDEHP
itself showed promising results having achieved a 3.43 ± 0.03 distribution value and 1150
separation factor at 2 M HNO3. The use of HDEHP as an extractant for hexavalent group
actinide separations has not been considered in previous oxidized Am studies, and should be
investigated in future studies. Monoamide distribution values above one were achieved in 3 M
HNO3 when straight chain monoamide solutions were pretreated with NaBiO3 to remove
reducing species, but values decreased at 4 and 5 M HNO3 due to incomplete Am3+ oxidation
3
4
5
6
7
8
9
10
11
12
13
14
15
16
2+
and rapid AmO2 reduction. Future studies should focus on determining the structure of
hexavalent monoamide species, which could provide further insight on the branched monoamide
selectivity.
17 Acknowledgements
18
19
20
21
22
23
Work at the Colorado School of Mines was funded by the U.S. Nuclear Energy University
Program under contract DENE0008289. This material is based upon work supported by the U.S.
Department of Energy, Office of Science, Office of Workforce Development for Teachers and
Scientists, Office of Science Graduate Student Research (SCGSR) program. The SCGSR
program administered by the Oak Ridge Institute for Science and Education (ORISE) for the
DOE. ORISE is managed by ORAU under contract DEꢀAC05ꢀ060R23100.
24
25
26
27
28
29
30
31
32
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36
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New Journal of Chemistry
Page 20 of 20
View Article Online
DOI: 10.1039/C7NJ04851C
1
2
TOC Graphic
Extraction of tetravalent and hexavalent
plutonium by straight and branched chained
monoamides was studied using UVꢀVis
spectroscopy. It was determined that anionic
species tended to be extracted, indicating
outerꢀsphere species formed. Branched
monoamides showed more innerꢀsphere type
behavior. Distribution values calculated from
UVꢀVis analysis were compared to tracer
level LSC measurements, and were found to
agree. Distribution values of hexavalent Am
were also assessed. Good distribution values
were achieved with straight monoamides
while
branched
showed
room
for
improvement.
3
20