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. 2025 Feb 21;10(8):7822–7830. doi: 10.1021/acsomega.4c08506

Influence of Metal Ion Complexation on the Radiolytic Longevity of Butyramide Extractants under Direct Dissolution Process Conditions

Amy E Kynman †,‡,*, Anh N Dang §, Travis S Grimes †, Stephen P Mezyk §, Joseph R Wilbanks †, Christopher A Zarzana †, Rupali G Deokar ∥, Andrew R Cook ∥, Daria Boglaienko ⊥, Gabriel B Hall ⊥, Gregory P Horne †,*
PMCID: PMC11886914  PMID: 40060798

Abstract

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The direct dissolution of voloxidized used nuclear fuel (UNF) into an organic solution—comprised of diluent and specialized extractants—poses a promising alternative to the traditional liquid–liquid solvent extraction approach to reprocessing UNF. However, moving to direct dissolution removes the presence of a concentrated nitric acid aqueous phase, which has been shown to significantly influence the radiolytic longevity of extractants in conventional extraction flowsheets. Given the limited knowledge of radiation effects under direct dissolution conditions, here we present a time-resolved and dose-accumulation study on the impact of direct dissolution conditions on the radiolytic longevity of two candidate butyramide extractants, N,N-di(2-ethylhexyl) butyramide (DEHBA) and N,N-di(2-ethylhexyl)isobutyramide (DEHiBA), in pre-equilibrated n-dodecane solvent in the presence and absence of process-relevant metal ions, specifically, uranium and rhenium. Loss G(DEHBA) and G(DEHiBA) values were found to be comparable to each other, with an average of 0.37 ± 0.02 μmol J–1, and to previous data from the γ irradiation of DEHBA and DEHiBA under conventional solvent extraction conditions. Rhenium, and by extension technetium, extraction had a modest decrease (∼10%) in the overall radiolytic stability of DEHiBA only, despite >2× observed increases in chemical kinetic reactivity of the corresponding complexes with the n-dodecane radical cation. Uranium loading, on the other hand, significantly improved the lifetime of both ligands (>30%) under γ irradiation, with a greater stabilization observed for DEHBA over DEHiBA. The observed radioprotective effect afforded by uranium loading is fortuitous for the longevity of direct dissolution solvent.

Introduction

Minimizing the quantity of high-level radioactive waste directed toward final disposal in geological repositories is essential for securing the long-term success of nuclear power, by both reducing its environmental footprint and maximizing the usage efficiency of natural resources such as uranium. Nuclear reactors in the United States currently operate an open fuel cycle, yet a partially closed fuel cycle, which instead recovers uranium and plutonium from irradiated fuel for refabrication, would both improve energy security and reduce the volume of high-level waste streams.1 Although many liquid–liquid separation flowsheets have been developed for the extraction and recovery of uranium and plutonium from used nuclear fuel (UNF), the industrial standard remains the Plutonium Uranium Reduction EXtraction (PUREX) process.2

The PUREX process employs tributyl phosphate (TBP, at ∼30% by volume) dissolved in hydrocarbon solvent (e.g., odorless kerosene or n-dodecane) which, upon contacting with an aqueous phase of UNF dissolved in concentrated nitric acid (HNO3), can extract TBP complexes of uranium and plutonium.3 Under these conditions, the minor actinides and fission products remain in the aqueous phase. The large volumes of concentrated HNO3 required for PUREX and PUREX-style processes pose significant drawbacks and increase the volume of secondary low-level waste streams.4,5 In addition, TBP is susceptible to third phase formation6 and undergoes substantial radiolytic degradation, affording the formation of problematic degradation products.7−11 Accumulation of TBP degradation products deteriorates process performance, through decreasing mass transfer coefficients and worsening both phase separation and fission-product/actinide separation factors.12 As such, two significant research aims in this area are to (i) establish alternate extractants with greater robustness and/or separation efficiency under radiation fields relative to TBP; and (ii) investigate alternative separation flowsheets that remove, or reduce, the need for large volumes of HNO3.

Regarding suitable alternate reprocessing extractants, two butyramides, N,N-di(2-ethylhexyl) butyramide (DEHBA) and N,N-di(2-ethylhexyl)isobutyramide (DEHiBA), are being considered as potential replacements for TBP. Relative to TBP, these butyramides show improved radiolytic stability and uranium selectivity and generate degradation products that cause less interference in separations.13,14 Furthermore, both DEHBA and DEHiBA have the additional benefit of only containing carbon, hydrogen, oxygen, and nitrogen (CHON), which, unlike phosphorus-containing TBP, allows for more facile waste management via incineration.

DEHiBA, at a concentration of 1.0 M, serves as the extractant in the first cycle of the EURO-GANEX process, which selectively separates uranium prior to the coextraction of the remaining actinides in the second cycle. The EURO-GANEX process has been demonstrated successfully with irradiated fuel15−17 but did generate a higher volume of aqueous high-level waste per kilogram of uranium processed in comparison to PUREX. This difference in waste generation when using DEHiBA is driven by a lower thermodynamic driving force for the extraction of uranium, as compared to TBP and consequently either a larger ratio of organic:aqueous or a diluted feed is necessary to achieve adequate separation. With the goal of reducing high-level waste through process intensification, researchers have been investigating increased concentrations of butyramides. Hall et al. have recently demonstrated the efficient extraction of uranium using 1.5 M DEHiBA with organic phase loadings comparable to that of the PUREX process, without a significant increase in the extraction of transuranic elements or fission products.18 The exception is technetium, which coextracts more strongly in DEHiBA-based systems than in TBP-based systems.

Removing the HNO3 dissolution step in UNF reprocessing would both reduce the volume of waste streams and improve reprocessing plant cost efficiency.5 A promising alternate strategy is the direct dissolution of UNF that has been pretreated by volumetric oxidation (voloxidation/volox) into the envisioned reprocessing solvent. Volox treatment is a proposed head-end process designed to remove tritium and other volatile fission products from UNF and reactor cladding by applying heat in the presence of an oxidizing gas.19 To date, a hybrid voloxidation-direct dissolution approach has been successfully implemented using 30–50% volume TBP in n-dodecane diluent with and without concentrated HNO3 pre-equilibration to dissolve both uranium oxide, a mixture of actinide oxides and UNF.20−22 Such studies have elucidated uranium dissolution kinetics and have shown that uranium is completely dissolved while neptunium and plutonium remain as a precipitate. Alternative solvents (methyl butyl ketone and N,N′-dimethyl-N,N′-dioctylhexylethoxymalonamide) have been investigated for the dissolution of both uranium and plutonium oxides,23 and more recently these dissolution studies have been expanded to evaluate the effectiveness of DEHBA and DEHiBA in dissolving uranium oxide under these pre-equilibration conditions.

However, the removal of the aqueous HNO3 phase from the envisioned direct dissolution reprocessing system has the potential to drastically change the suite of radiation-induced processes occurring. For example, it has been shown in several liquid–liquid reprocessing solvent systems that the presence of an aqueous HNO3 phase has a significant stabilizing effect upon the radiation robustness of the ligands employed.24−26 Moreover, some radiation-induced ligand degradation products are less soluble in the organic phase and, thus, migrate into the aqueous phase, removing their contribution from the overall process as solvents are exchanged—as postulated for bis-2-ethylhexylamine (b2EHA) following the irradiation of DEHBA in 0.1 and 3.0 M HNO3.25 However, the fate and impact of these degradation products on direct dissolution process performance is unknown. The impact of minor actinide and fission-product metal ions on organic phase radiation-induced complexation chemistry can also not be discounted. These species are, for the most part, rejected by traditional PUREX-style process flowsheets, but there is evidence for their extraction under unoptimized direct dissolution conditions. For example, technetium, as the pertechnetate anion (TcO4–), is coextracted as the counterion to uranium and plutonium at low-to-moderate HNO3 concentrations.27 The impacts of metal ion complexation on the radiation robustness of separations technologies are only now being scrutinized and have been shown to have ranging effects from none to significant (orders of magnitude) exacerbation or inhibition of ligand radiolysis, and thus, process longevity and efficiency.28−31

Understanding the changes in radiolytic behavior due to the nonaqueous-solvent paradigm shift in pursuing volox-direct dissolution approaches is essential to support the development of this technology for next generation fuel cycles. To bridge the previously described knowledge gaps and support the continued development of volox-direct dissolution methods, this study has investigated the impact of envisioned uranium loading on the radiation robustness of 1.5 M DEHBA and DEHiBA in n-dodecane solvent pre-equilibrated with 6.0 M HNO3—acid loading is required to dissolve the otherwise insoluble metal oxide, and, in the case of uranium, to generate the predominant neutral [UO2(NO3)2L2] complex (where L is the corresponding organic ligand) species, alongside water.32 The influence of γ radiation on these ligands in the presence and absence of envisioned loading amounts of volox-treated uranium (solvent loading of ≥80%, ∼0.35 M uranium) has been studied, and we have also elucidated the impact of rhenium, a surrogate for technetium, extraction effects on radical reaction kinetics, and butyramide ligand longevity under direct dissolution conditions through electron beam irradiation studies.

Materials and Methods

Caution: Depleted uranium (DU), which contains uranium-238 (t1/2 = 4.5 billion years) as its major isotope, was used in this study. As DU is a weak α-emitter and can be hazardous in large quantities, its storage and handling was performed in dedicated radiological and nuclear facilities using well-established radiological safety protocols.

Materials

Volox-processed uranium33 (ε-UO3) was provided by Pacific Northwest National Laboratory (PNNL). Rhenium oxide (ReO3, 99.9%) was purchased from Strem Chemicals (Newburyport, MA). DEHBA (99%), DEHiBA (99%), and N-(2-ethylhexyl)butyramide (MEHBA) (99%) were supplied by Technocomm Ltd. (Wellbrae, Scotland, U.K.). Bis-2-ethylhexylamine (b2EHA) (>98.0%) was purchased from TCI America (Philadelphia, PA), and N-(2- ethylhexyl)isobutyramide (MEHiBA) (>99.0%) was supplied by Marshallton Research Laboratories Inc. (King, NC). Dichloromethane (CH2Cl2, 99%), iron(III) sulfate heptahydrate (FeSO4·7H2O, ≥99%), n-dodecane (≥99% anhydrous), nitric acid (HNO3, ≥99.999% trace metals basis), sodium chloride (NaCl, 99.999% trace metals basis), and sulfuric acid (H2SO4, 99.999%) were sourced from MilliporeSigma (Burlington, MA). All chemicals were used as received. Ultrapure water (Milli-Q quality, >18.2 MΩ), generated in house using a Thermo Fisher Scientific (Waltham, MA) Barnstead E-Pure 4-Module water purification system, was used for the preparation of all aqueous solutions.

Direct Dissolution Solvent Preparation

Each ligand (DEHBA and DEHiBA) was prepared as a 1.5 M solution in n-dodecane, which was then pre-equilibrated with 6.0 M HNO3 solution, in line with currently envisioned direct dissolution conditions.32 Pre-equilibration was achieved by contacting the aforementioned organic and aqueous phases in a 1:1 ratio by volume, agitating with a VWR (Radnor, PA) standard vortex mixer for 2 min, and then separating by centrifugation using a Labnet (Edison, NJ) Hermle Z206A centrifuge for 2 min at 3150 rpm. The organic phases were then recovered and pre-equilibrated twice more using the same procedure with fresh 6.0 M HNO3 solution. After the third pre-equilibration, the organic phases for each ligand were then recovered and combined, giving two final stock solutions that were used for all of the following sample preparation.

Uranium-Loaded Samples

Pre-equilibrated organic solution (10 mL) was used to dissolve 1.00 g of volox-treated ε-UO3 powder in a radiological glovebox at the Idaho National Laboratory (INL) Radiochemistry Laboratory to yield a uranium concentration of nominally 0.35 M. Following dissolution, each uranium-loaded solution was filtered and separated into screw-cap scintillation vials, each containing 0.25 mL of solution. One vial of each ligand’s sample series was reserved as a zero-dose control measurement, while the remaining vials were used for cobalt-60 γ irradiation.

Rhenium-Loaded Samples

Pre-equilibrated organic solutions (10 mL) were used to dissolve 0.1 M equivalent of solid ReO3 under agitation, which was accompanied by a brief evolution of gas and a color change from colorless to red-orange (attributed to the conversion of ReO3 → HReO4, see below). No ReO3 dissolution was observed in the absence of ligands or for n-dodecane that had not been pre-equilibrated. The resulting ReO3-loaded solutions were then used to prepare samples for time-resolved and dose-accumulation electron pulse irradiations.

Cobalt-60 γ Irradiation

The INL Center for Radiation Chemistry Research’s cobalt-60 γ irradiation instruments—a Nordion (Ottawa, Canada) Gammacell 220E and a Foss Therapy Services (Pacoima, CA) Model 812 were used to irradiate the aforementioned uranium-loaded samples and corresponding controls (i.e., no ε-UO3 loading) in triplicate. Dosimetry for both irradiators was determined using Fricke solution (1 mM FeSO4·7H2O and 1 mM NaCl in 0.4 M H2SO4)34,35 and colocated Agilent (Santa Clara, CA) Cary UV–vis spectrophotometers. The resulting γ dose rates were corrected for the radioactive decay of cobalt-60 (τ1/2 = 5.27 years, Eγ1 = 1.17 MeV and Eγ2 = 1.33 MeV) and solution electron density (0.80) vs water.36 Note that the solution electron density is higher than that for neat n-dodecane to account for the significant quantity of butyramide present. Samples were irradiated to a maximum absorbed dose of 1 MGy under ambient irradiator sample chamber temperature conditions, as determined using a calibrated National Instruments (Austin, TX) USB-TC01 Single Channel Temperature Input Device equipped with a K-type thermocouple: 23 ± 1 °C in the Nordion Gammacell 220E and 40 ± 1 °C in the Foss Therapy Services Model 812. Note, although initially aerated, samples were considered deaerated upon exposure to relatively low absorbed γ doses due to radiolytic oxygen consumption to produce relatively inert peroxyl radicals.36 Post-irradiation, all samples were analyzed by gas-chromatography (GC), see below.

Electron Beam Irradiation

The ReO3-loaded samples were irradiated by using a 2 MeV Van de Graaff (VdG) electron accelerator at Brookhaven National Laboratory (BNL). Samples comprised of 125 μL of solution pipetted into a Teflon-capped 5 × 5 mm2 quartz cuvette (Spectrocell Laboratories, TX). A low repetition rate (1 μs electron pulses at 2 Hz) was chosen to ensure no major sample heating occurred during irradiation. Samples were also manually mixed after each 80 kGy of absorbed dose. Dosimetry was achieved using fresh Fricke solution and single 1 μs electron pulses, affording an average of ∼183 Gy per pulse when corrected for solution electron density vs water.

Dose-Accumulation Sample Analysis

Post-irradiation, all samples were analyzed by GC at INL. Ligand quantification was achieved using an Agilent 7890 Gas Chromatograph equipped with an Agilent 7693 autosampler, a flame ionization detector (FID), and an Agilent J&W HP-5 GC column (30 m × 0.32 mm ID × 0.25 μm df). Prior to analysis, each sample was diluted by a factor of 100 in 2-propanol three separate times and then vortex mixed to yield three dilution replicates. Each dilution replicate was injected four times, subject to the following procedure: injector temperature of 350 °C and an initial oven temperature of 100 °C, held for 1 min and ramped at a rate of 15 °C min–1 up to 275 °C before being held for 1 min. The dilution replicate injection order was randomized to differentiate systematic instrument drift from genuine sample trends, although quadruple injections per dilution were performed sequentially to reduce perturbations due to diluent evaporation. Quality control (QC) standards at four different concentrations were measured after every 10 samples as well as the beginning and the end of each analysis to confirm the validity of each calibration curve. Each QC standard was placed in a separate vial to other QC standards to reduce the effects of solvent evaporation. Ligand and degradation product concentrations were derived from calibration curves composed of six calibration points and a blank, with standards prepared from neat DEBHA, DEHiBA, MEHBA, MEHiBA, or b2EHA in 2-propanol. Uranium-loaded calibration curves were generated from the sequential dilution of the original ligand/uranium stock solutions of known concentration. While metal-loading was found to influence absolute instrument response, changes in metal ion concentration within calibration standards caused no perturbation of the linearity of the response over the concentration range studied. However, metal-loaded degradation calibration curves were not able to be prepared, as intact ligand is required to solubilize uranium. As such, degradation product analyses of uranium-loaded solutions are reported by peak area rather than absolute concentration; see Supporting Information.

Time-Resolved Electron Pulse Irradiation

To determine the impact of ReO3-loading on the radiation-induced chemistry of DEHBA and DEHiBA, chemical kinetics were measured for the reaction of the n-dodecane radical cation (RH•+) with DEHBA/DEHiBA in pre-equilibrated 0.5 M CH2Cl2/n-dodecane solutions in the absence and presence of extracted ReO3 at 23 ± 1 °C. Note, CH2Cl2 was added to scavenge the solvated electron (eS–), thereby increasing the available yield and the absorption of RH•+. Measurements were performed using a 15:1 ligand:ReO3 concentration ratio throughout. These experiments were performed using the BNL Laser Electron Accelerator Facility (LEAF), the optical detection system for which has been previously described.37 Aerated sample solution was pipetted into either 5 or 10 mm Suprasil Starna Scientific Ltd. (Ilford, U.K.) cuvettes sealed with Teflon stoppers. Dosimetry was determined using a standard solution (deaerated water containing 20% methanol by volume and 1.0 M NaOH),38 affording an average dose per pulse of 8–12 Gy water equivalent. The time-resolved changes in the absorption decays of RH•+ were measured near its maximum absorption at 800 nm by using an FND-100 silicon diode detector and Teledyne Lecroy HDO6104-MS oscilloscope (1 GHz, high-definition mixed signal oscilloscope, 2.5 GS/s) digitizer. The quoted rate coefficient (k) errors (1σ) are a quantitative combination of measurement precision (∼4%) and sample concentration (initial concentration (∼9%) and dilution (<1%)) errors.

Results and Discussion

A summary of the radiolytic yields—G-values (μmol J–1)—determined by this study for the loss of ligand as a function of absorbed radiation dose are given in Table 1.

Table 1. Summary of G-Values for the Loss of Ligand from the Irradiation of Deaerated 1.5 M DEHBA or DEHiBA in 6.0 M HNO3 Pre-Equilibrated n-Dodecane solution in the Absence and Presence of Uranium or Rhenium under Envisioned Direct Dissolution Conditions.

system G-value (μmol J–1)
1.5 M DEHBA/n-dodecane (cobalt-60) 0.38 ± 0.03a
1.5 M DEHBA/n-dodecane (electron beam) 0.30 ± 0.01a
1.5 M DEHBA/n-dodecane + 0.35 M uranium (cobalt-60) 0.18 ± 0.01
1.5 M DEHBA/n-dodecane + 100 mM rhenium (electron beam) 0.31 ± 0.02
1.5 M DEHiBA/n-dodecane (cobalt-60) 0.36 ± 0.01a
1.5 M DEHiBA/n-dodecane (electron beam) 0.29 ± 0.05a
1.5 M DEHiBA/n-dodecane + 0.35 M uranium (cobalt-60) 0.24 ± 0.01
1.5 M DEHiBA/n-dodecane + 100 mM rhenium (electron beam) 0.33 ± 0.04
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a

The slight discrepancies between cobalt-60 and electron beam results are attributed to insufficient solution mixing at the VdG beamline. However, the observed trends are internally consistent.

In the absence of metal ions, G(DEHBA) and G(DEHiBA) values are comparable to one another and to previous data from the cobalt-60 γ irradiation of 1.0 M DEHBA and DEHiBA in n-dodecane solutions in the presence and absence of HNO3: G(DEHBA) ∼0.41 ± 0.02 μmol J–1, and G(DEHiBA) ∼0.53 ± 0.02 μmol J–1.39,40 Moreover, the predominant observable degradation products for each butyramide were also consistent (Figures S1 and S2), with a decrease in the yield of b2EHA from DEHBA following HNO3 pre-equilibration. This was shown previously for DEHBA irradiated in contact with a HNO3 aqueous phase of varying concentration.25 Our pre-equilibration observations suggest that reduction in the yield of b2EHA is due to acid hydrolysis rather than migration into the aqueous phase as previously hypothesized. The resulting lower molecular weight hydrolysis products were likely not detectable by using the analytical methods employed here. While no absolute quantitative analysis of radiolytically induced DEHiBA degradation products is currently available in the literature, results are consistent with those acquired for DEHBA and with electrospray ionization mass spectrometry analysis of irradiated DEHiBA.39

In the presence of an envisioned loading amount of voloxidized uranium, the rates of DEHBA and DEHiBA radiolysis significantly decreased, by 53 and 33%, respectively (Table 1). The normalized concentration of each ligand as a function of absorbed γ dose with and without uranium loading is shown in Figure 1.

Figure 1.

Figure 1

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Normalized loss of DEHBA (A) and DEHiBA (B) in deaerated, 6.0 M HNO3 pre-equilibrated n-dodecane solution in the presence and absence of ε-UO3 as a function of absorbed cobalt-60 γ dose.

Upon direct dissolution in HNO3 pre-equilibrated organic solvent, ε-UO3 is expected to form the uranyl ion (UO22+) and subsequently the corresponding butyramide complex, [UO2(NO3)2L2]:41

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Under irradiation, a combination of the following processes is expected to occur:

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Of these anticipated radiolytic processes, we have reported on the impact of UO22+ complexation on the radiation-induced chemical reactivity of DEHBA and DEHiBA with RH•+ (eq 5).28 The RH•+ is an important transient radiolysis product with significant reactivity for separations ligands and is the species predominantly attributed for the formation of their degradation products:36,37

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We previously found that UO22+ complexation afforded a 2.6× and 1.4× factor increase in the second-order rate coefficients for the reaction of RH•+ with [UO2(NO3)2(DEHBA)2] and [UO2(NO3)2(DEHiBA)2], respectively, vs the corresponding non-complexed ligands.28 This difference in the chemical reactivity of the butyramides with uranium complexation was attributed to subtle changes in structure, target size, and electron distribution that enhanced the favorability of electron transfer (eq 6a). These time-resolved observations suggest that uranium loading should lead to enhanced ligand radiolysis (i.e., a larger G-value), which is the inverse of our findings (Table 1 and Figure 1). The inverse relationship observed here between the increase in chemical reactivity enhancement (k[UO2(NO3)2(DEHBA)2]/k[UO2(NO3)2(DEHiBA)2] = 1.86) and the decrease in radiolytic yield (G[UO2(NO3)2(DEHBA)2]/G[UO2(NO3)2(DEHiBA)2] = 1.61) suggests that strong complexation of the butryamides by uranium—as indicated by our previous calculations28—stabilizes the initial butyramide radical cation (L•+) long enough to propagate a fraction of radiation-induced damage elsewhere prior to fragmentation. Considering the hexavalent oxidation state of the coordinated UO22+ ion, which is unlikely to be further oxidized by the bound L•+, it is more likely that some of the initial radical-induced damage is propagated from the amide N atom—the most reactive center within the molecule when complexed to UO22+—onto either the coordinating nitrate counteranions (NO3–) or the surrounding HNO3 or water molecules extracted during pre-equilibration. That said, such a mechanism would generate the corresponding nitrate (NO3•) and hydroxyl (•OH) radicals, both powerful oxidants.42,43 However, the fate of these species in organic solvents of relevance to UNF reprocessing is only just being investigated.40

Note, while uranium dissolution decreases the pre-equilibrated nitrate concentration in the organic solvent to ∼0.7 M, the observed radioprotective effect is predominantly due to the difference in chemistry afforded by the [UO2(NO3)2(L)2] complexes and not the change in extracted HNO3 concentration. The presence of a 4.0 M aqueous HNO3 phase was shown to decrease the radiolytic degradation of DEHiBA,39 and although the HNO3 phase was shown to increase the susceptibility of DEHBA to radiolysis (∼70%),25 our G-value for pre-equilibrated DEHBA-solvent in the absence of uranium (0.38 ± 0.03, Table 1) is very close to our previously reported value for the irradiation of DEHBA under “organic only” (i.e., n-dodecane that had not been pre-equilibrated with HNO3) conditions (0.31 ± 0.02). This similarity suggests that HNO3 has only a modest influence on the radiolysis of butyramide ligands under single-phase conditions.

It was hoped that the relative distributions of butyramide degradation products arising from the γ irradiation of uranium-loaded DEHBA and DEHiBA samples could provide some insight into the mechanism of degradation of each ligand when complexed to uranium. While the analysis of such products suggests that the presence of uranium does not lead to significant changes in the formation of b2EHA from DEHBA or DEHiBA (Figure S3), it appears that the fragmentation pathways for each ligand to generate MEHBA or MEHiBA, respectively, are altered upon complexation. A growth and decay of MEHBA and MEHiBA was observed for uranium-loaded butyramide samples as compared to an exponential growth of these products when no metal was present (Figure S4), though further study of the mechanisms underpinning the radiolysis of metal ion complexes in relevant organic media is again required.

Irrespective, our butyramide findings are consistent with previous observations for the radiolysis of N,N,N′,N’-tetraoctyl diglycolamide (TODGA) in the presence and absence of complexed metal ion—where radical reactions with NO3– counteranions dominate the difference in rates seen for TODGA metal ion complexes and are likely responsible for the observed radioprotective effects relative to the non-complexed TODGA molecule.29 Overall, the observed radioprotective effect afforded by the direct dissolution of uranium by DEHBA/DEHiBA is fortuitous for the longevity of direct dissolution solvent.

Interestingly, the direct dissolution of ReO3, and by extension technetium oxides (TcO2/Tc2O7), had a negligible impact upon the radiolytic longevity of DEHBA but appeared to increase (∼10%) the susceptibility of DEHiBA to radiolysis (Table 1). The normalized concentration of each ligand as a function of absorbed γ dose with and without ReO3-loading is shown in Figure 2.

Figure 2.

Figure 2

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Normalized loss of DEHBA (A) and DEHiBA (B) in 6.0 M HNO3 pre-equilibrated n-dodecane solution in the presence and absence of ReO3 as a function of absorbed VdG electron beam dose.

Upon direct dissolution in HNO3 pre-equilibrated organic solvent, ReO3 is expected to hydrolyze into perrhenic acid (HReO4), an analogue for pertechnetic acid (HTcO4):44

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By analogy to reported technetium chemistry, under the high HNO3 concentrations (6.0 M) used here for pre-equilibration, the [HReO4(HNO3)L2](org) species is expected to dominate (>90%) for both DEHBA and DEHiBA ligands.45−47 Given the irradiation conditions used to attain the ReO3-loaded data in Table 1 and Figure 2, the following radiation-induced processes are anticipated in addition to eq 2:

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To provide greater insight into the trends observed in Figure 2 we measured chemical kinetics for eq 13. The kinetic data observed for the decay of RH•+ in the presence of [HReO4(HNO3)(DEHBA)2] and [HReO4(HNO3)(DEHiBA)2] are given in Figure 3A,B, respectively.

Figure 3.

Figure 3

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Transient absorption decay kinetics at 800 nm for the reaction of RH•+ with [HReO4(HNO3)(DEHBA)2] (A) and [HReO4(HNO3)(DEHiBA)2] (B) for the electron pulse irradiation of pre-equilibrated n-dodecane solution containing 0.5 M CH2Cl2 at 25 °C for 0 (Inline graphic), 1.0 (Inline graphic), 2.0 (Inline graphic), 3.0 (Inline graphic), and 4.0 (Inline graphic) mM extracted ReO3 equivalent. The concentration of the ligand was held at 15× that of ReO3. Solid red curves shown are double-exponential decay kinetics. Insets: Second-order rate coefficient determination for the RH•+ reaction with [HReO4(HNO3)(DEHBA)2] (Inline graphic) and [HReO4(HNO3)(DEHiBA)2] (Inline graphic) is performed using the faster exponential component of the double-exponential decay fits. Solid lines are weighted linear fits, corresponding to k(RH•+ + [HReO4(HNO3)(DEHBA)2]) = (2.72 ± 0.26) × 1010 M–1 s–1 (R2 = 0.94) and k(RH•+ + [HReO4(HNO3)(DEHiBA)2]) = (3.03 ± 0.44) × 1010 M–1 s–1 (R2 = 0.92).

These measured RH•+ decays were fitted using a double-exponential decay function:

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starting at 2 ns after the electron pulse to allow for the instrument response. Here, kobs is the overall rate of decay (s–1), Ai are the optical density amplitudes, ki are the pseudo-first-order rate coefficients (s–1), and t is the time (s) following the electron pulse. The faster exponential decay (A1 and k1 parameters) corresponds to the total reaction of RH•+ with both [HReO4(HNO3)L2] and the remaining fraction of non-complexed ligand:

graphic file with name ao4c08506_m016.jpg 15

The second exponential decay (A2 and k2 parameters) accounts for the slower absorption decrease as seen previously.48 Subtraction of the non-complexed ligand’s reactivity from the total fitted k1 values was performed using the values ky = (1.04 ± 0.02) × 1010 and (1.52 ± 0.11) × 1010 M–1 s–1 for DEHBA and DEHiBA, respectively.25 Plotting this difference against the concentration of [HReO4(HNO3)L2] afforded the desired second-order rate coefficients of kx = (2.72 ± 0.26) × 1010 and (3.03 ± 0.44) × 1010 M–1 s–1 for [HReO4(HNO3)(DEHBA)2] and [HReO4(HNO3)(DEHiBA)2], respectively. (eq 13), as shown in the inset plots of Figure 3. These rate values are equivalent within experimental error, and both are faster than for their non-complexed rates. This behavior is consistent with that observed previously for other metal-complexed separations ligands.28,29,31,49

The increase in RH•+ reactivity for the rhenium complexes compared to the non-complexed ligands (2.6× and 1.5× for DEHBA and DEHiBA, respectively) is consistent with that measured for [UO2(NO3)2L2]—with the chemical reactivity of DEHBA being enhanced to a greater extent than DEHiBA—and yet, here we did not observe ligand radioprotection (Table 1). In lieu of complementary structural calculations for the rhenium complexes, the absence of radioprotection in these systems suggests a difference in chemical bonding between the butyramide ligands and the two different metal coordination environments, HReO4 and UO22+. For the [UO2(NO3)2L2] complexes, the coordinated L•+ ligand is purportedly stabilized in part by the direct interaction of the butyramide carbonyl oxygen with the UO22+ center via sigma-type localized molecular orbitals with 8% uranium contribution.28 Whereas for the [HReO4(HNO3)L2] complexes, we speculate that the ligand-to-metal interaction occurs indirectly via H-bonding from the HReO4 center to the butyramide—concordant with the development of rhenium50,51 and technetium extractants that make the use of hydrogen bonding and electrostatic interactions between the ligand and rhenium/technetium moiety to achieve separation.52,53 Given our observations, we propose that this alternative bonding mode does not allow for the sufficient stabilization of L•+ to prevent a fraction of ligand fragmentation in place of damage propagation onto the surrounding HNO3 or water molecules. Furthermore, given the similarities in reaction kinetics behavior, proposed differences in chemical bonding, and comparable ionic radii of U(VI) (rcov = 0.69 Å) and Re(VII) (rcov = 0.68 Å),54 our findings suggest that the observed enhancement in chemical reactivity of the butyramide complexes with RH•+ is mainly due to the increased size of the complexes vs the non-complexed ligands. These postulations will be evaluated by follow-on structure calculations of the bonding and average local ionization energies for the [HReO4(HNO3)L2] complexes.28,55 Overall, rhenium extraction had a lower impact on the radiolysis of DEHBA/DEHiBA under direct dissolution conditions, as compared to uranium loading (Table 1).

Conclusions

The γ and electron beam irradiation of butyramide-based direct dissolution solvents in the presence of important UNF metal ions (uranium and rhenium, the latter as a technetium surrogate) has been accomplished. Uranium loading was found to provide significant radioprotection (>30%) to both DEHBA and DEHiBA systems, whereas rhenium extraction provided a modest increase (∼10%) in the rate of DEHiBA radiolysis.

Although a small fraction of other materials (fission products and transuranic elements) will ultimately be dissolved alongside uranium under envisioned real-world direct dissolution reprocessing conditions, uranium will always be present in significant excess, as it is typically 95% of the metal content in UNF. Therefore, the radiation-induced chemistry of butyramide-based direct dissolution technologies will be governed by competition kinetics involving [UO2(NO3)2L2](org) complexes. This is especially true for DEHiBA-based solvents, which have a greater affinity for hexavalent actinides (uranium) over lower oxidation state actinides (plutonium, ∼1% by mass in UNF and neptunium, ∼0.2% by mass in UNF) by traditional solvent extraction. Regardless, the observed radioprotective effect is promising for advanced UNF reprocessing strategies as it suggests greater recyclability of the direct dissolution solvent and thus increased cost-effectiveness.

That said, there are several radiolytic factors that must still be addressed, including the influence of radiation quality (type and energy) on the radioprotective mechanism. The presented data are all for low linear energy transfer (LET) radiation, i.e., γ rays and high energy electrons, which are synonymous with β-particles. However, there are several radioisotopes in UNF that are α emitters, including the reprocessing-relevant actinides uranium, neptunium, plutonium, americium, and curium. The radiation chemistry induced by energy deposition from high LET α particles is different, owing to changes in the distribution of radical and molecular radiolysis products. Consequently, the response of [UO2(NO3)2L2](org) complexes to α radiolysis and then multicomponent radiation fields (α, β, and γ) must be investigated.

Another complicating factor is that rhenium/technetium may also be coordinated within the uranium-butyramide complexes, i.e., [UO2(NO3)n(ReO4/TcO4)2–nL2](org), as is typical in traditional solvent extraction technologies.45 Given that we attribute the radioprotective effect of uranium loading to the propagation of radiation damage from the coordinated ligands onto the NO3– counteranions, the question still remains as to what extent would ReO4–/TcO4– substitution interfere with this mechanism?

Acknowledgments

This research has been funded by the U.S. Department of Energy (DOE) Assistant Secretary for Nuclear Energy, under the Material Recovery and Waste Form Development Campaign. Idaho National Laboratory is operated by Battelle Energy Alliance LLC for the U.S. DOE under DOE-Idaho Operations Office Contract DE-AC07-05ID14517. Pacific Northwest National Laboratory is operated by Battelle Memorial Institute for the U.S. DOE under Contract DE-AC05-76RL01830. Cook, Deokar, and electron beam experiments at the BNL Accelerator Center for Energy Research were supported by the U.S. DOE Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences under Contract DE-SC0012704. The authors would also like to thank Gregg Lumetta of PNNL for insightful discussions, and Kazuhiro Iwamatsu of Hunter College, New York for assistance with VdG dose-accumulation experiments.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.4c08506.

  • Data for the observable radiation-induced degradation products arising from DEHBA and DEHiBA radiolysis under direct dissolution conditions. (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao4c08506_si_001.pdf (146.3KB, pdf)

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