Radiolytic evaluation of a new technetium redox control reagent for advanced used nuclear fuel separations.

Technetium is a problematic radioisotope for used nuclear fuel (UNF) and subsequent waste management owing to its high environmental mobility and coextraction in reprocessing technologies as the pertechnetate anion (TcO4-). Consequently, several strategies are under development to control the transport of this radioisotope. A proposed approach is to use diaminoguanidine (DAG) for TcO4- and transuranic ion redox control. Although the initial DAG molecule is ultimately consumed in the redox process, its susceptibility to radiolysis is currently unknown under envisioned UNF reprocessing conditions, which is a critical knowledge gap for evaluating its overall suitability for this role. To this end, we report the impacts of steady-state gamma irradiation on the rate of DAG radiolysis in water, aqueous 2.0 M nitric acid (HNO3), and in a biphasic solvent system composed of aqueous 2.0 M HNO3 in contact with 1.5 M N,N-di-(2-ethylhexyl)isobutyramide (DEHiBA) dissolved in n-dodecane. Additionally, we report chemical kinetics for the reaction of DAG with key transients arising from electron pulse radiolysis, specifically the hydrated electron (eaq-), hydrogen atom (H˙), and hydroxyl (˙OH) and nitrate (NO3˙) radicals. The DAG molecule exhibited significant reactivity with the ˙OH and NO3˙ radicals, indicating that oxidation would be the predominant degradation pathway in radiation environments. This is consistent with its role as a reducing agent. Steady-state gamma irradiations demonstrated that DAG is readily degraded within a few hundred kilogray, the rate of which was found to increase upon going from water to HNO3 containing solutions and solvents systems. This was attributed to a thermal reaction between DAG and the predominant HNO3 radiolysis product, nitrous acid (HNO2), k(DAG + HNO2) = 5480 ± 85 M-1 s-1. Although no evidence was found for the radiolysis of DAG altering the radiation chemistry of the contacted DEHiBA/n-dodecane phase in the investigated biphasic system, the utility of DAG as a redox control reagent will likely be limited by significant competition with its degradation by HNO2.

Transient Radiation-Induced Berkelium(III) and Californium(III) Redox Chemistry in Aqueous Solution.

Despite the significant impact of radiation-induced redox reactions on the accessibility and lifetimes of actinide oxidation states, fundamental knowledge of aqueous actinide metal ion radiation chemistry is limited, especially for the late actinides. A quantitative understanding of these intrinsic radiation-induced processes is essential for investigating the fundamental properties of these actinides. We present here a picosecond electron pulse reaction kinetics study into the radiation-induced redox chemistry of trivalent berkelium (Bk(III)) and californium (Cf(III)) ions in acidic aqueous solutions at ambient temperature. New and first-of-a-kind, second-order rate coefficients are reported for the transient radical-induced reduction of Bk(III) and Cf(III) by the hydrated electron (eaq-) and hydrogen atom (H•), demonstrating a significant reactivity (up to 1011 M-1 s-1) indicative of a preference of these metals to adopt divalent states. Additionally, we report the first-ever second-order rate coefficients for the transient radical-induced oxidation of these elements by a reaction with hydroxyl (•OH) and nitrate (NO3•) radicals, which also exhibited fast reactivity (ca. 108 M-1 s-1). Transient Cf(II), Cf(IV), and Bk(IV) absorption spectra are also reported. Overall, the presented data highlight the existence of rich, complex, intrinsic late actinide radiation-induced redox chemistry that has the potential to influence the findings of other areas of actinide science.

Complexation of Lanthanides and Heavy Actinides with Aqueous Sulfur-Donating Ligands.

The separation of trivalent lanthanides and actinides is challenging because of their similar sizes and charge densities. S-donating extractants have shown significant selectivity for trivalent actinides over lanthanides, with single-stage americium/lanthanide separation efficiencies for some thiol-based extractants reported at >99.999%. While such separations could transform the nuclear waste management landscape, these systems are often limited by the hydrolytic and radiolytic stability of the extractant. Progress away from thiol-based systems is limited by the poorly understood and complex interactions of these extractants in organic phases, where molecular aggregation and micelle formation obfuscates assessment of the metal-extractant coordination environment. Because S-donating thioethers are generally more resistant to hydrolysis and oxidation and the aqueous phase coordination chemistry is anticipated to lack complications brought on by micelle formation, we have considered three thioethers, 2,2'-thiodiacetic acid (TDA), (2R,5S)-tetrahydrothiophene-2,5-dicarboxylic acid, and 2,5-thiophenedicarboxylic acid (TPA), as possible trivalent actinide selective reagents. Formation constants, extended X-ray absorption fine structure spectroscopy, and computational studies were completed for thioether complexes with a variety of trivalent lanthanides and actinides including Nd, Eu, Tb, Am, Cm, Bk, and Cf. TPA was found to have moderately higher selectivity for the actinides because of its ability to bind actinides in a different manner than lanthanides, but the utility of TPA is limited by poor water solubility and high rigidity. While significant competition with water for the metal center limits the efficacy of aqueous-based thioethers for separations, the characterization of these solution-phase, S-containing lanthanide and actinide complexes is the most comprehensively available in the literature to date. This is due to the breadth of lanthanides and actinides considered as well as the techniques deployed and serves as a platform for the further development of S-containing reagents for actinide separations. Additionally, this paper reports on the first bond lengths for Cf and Bk with a neutral S donor.

Radiation-induced effects on the extraction properties of hexa-n-octylnitrilo-triacetamide (HONTA) complexes of americium and europium.

The candidate An(iii)/Ln(iii) separation ligand hexa-n-octylnitrilo-triacetamide (HONTA) was irradiated under envisioned SELECT (Solvent Extraction from Liquid waste using Extractants of CHON-type for Transmutation) process conditions (n-dodecane/0.1 M HNO3) using a solvent test loop in conjunction with cobalt-60 gamma irradiation. The extent of HONTA radiolysis and complementary degradation product formation was quantified by HPLC-ESI-MS/MS. Further, the impact of HONTA radiolysis on process performance was evaluated by measuring the change in 243Am and 154Eu distribution ratios as a function of absorbed gamma dose. HONTA was found to decay exponentially with increasing dose, affording a dose coefficient of d = (4.48 ± 0.19) × 10-3 kGy-1. Multiple degradation products were detected by HPLC-ESI-MS/MS with dioctylamine being the dominant quantifiable species. Both 243Am and 154Eu distribution ratios exhibited an induction period of ∼70 kGy for extraction (0.1 M HNO3) and back-extraction (4.0 M HNO3) conditions, after which both values decreased with absorbed dose. The decrease in distribution ratios was attributed to a combination of the destruction of HONTA and ingrowth of dioctylamine, which is capable of interfering in metal ion complexation. The loss of HONTA with absorbed gamma dose was predominantly attributed to its reaction with the n-dodecane radical cation (R˙+). These R˙+ reaction kinetics were measured for HONTA and its 241Am and 154Eu complexes using picosecond pulsed electron radiolysis techniques. All three second-order rate coefficients (k) were essentially diffusion limited in n-dodecane indicating a significant reaction pathway: k(HONTA + R˙+) = (7.6 ± 0.8) × 109 M-1 s-1, k(Am(HONTA)2 + R˙+) = (7.1 ± 0.7) × 1010 M-1 s-1, and k(Eu(HONTA)2 + R˙+) = (9.5 ± 0.5) × 1010 M-1 s-1. HONTA-metal ion complexation afforded an order-of-magnitude increase in rate coefficient. Nanosecond time-resolved measurements showed that both direct and indirect HONTA radiolysis yielded the short-lived (<100 ns) HONTA radical cation and a second long-lived (µs) species identified as the HONTA triplet excited state. The latter was confirmed by a series of oxygen quenching picosecond pulsed electron measurements, affording a quenching rate coefficient of k(3[HONTA]* + O2) = 2.2 × 108 M-1 s-1. Overall, both the HONTA radical cation and triplet excited state are important precursors to the suite of measured HONTA degradation products.

Exploring Soft Donor Character of the N-2-Pyrazinylmethyl Group by Coordinating Trivalent Actinides and Lanthanides Using Aminopolycarboxylates.

The trivalent f-element coordination chemistry of a novel aminopolycarboxylate complexant was investigated. The novel reagent is an octadentate complexant that resembles diethylenetriamine-N,N,N',N″,N″-pentaacetic acid (DTPA), but a single N-acetate pendant arm was substituted with a N-2-pyrazinylmethyl functional group. Thermodynamic studies of ligand protonation and trivalent lanthanide, americium and curium, complexation by N-2-pyrazinylmethyldiethylenetriamine-N,N',N″,N″-tetraacetic acid (DTTA-PzM) emphasize the strong electron withdrawing influence of the N-2-pyrazinylmethyl group. Specifically, DTTA-PzM is more acidic compared to a N-2-pyridinylmethyl-substituted structural equivalent, DTTA-PyM, with a substantial lowering of pK7, corresponding to the protonation of a second aliphatic amine site. The participation of the pyrizyl nitrogen in the metal ion coordination sphere is evident from the fluorescence lifetime decay measurements of metal hydration and the interpretation of the stability constants for ML- and MHL(aq) complexes. The overall conditional stability constants for the trivalent f-element complexation by DTTA-PzM complexes decrease, relative to DTTA-PyM, as expected based on the lower basicity of pyrazine in water relative to pyridine. Replacement of the N-2-pyridinylmethyl group with N-2-pyrazinylmethyl, while enhancing the total acidity of DTTA-PzM, also reduces its softness, as manifested by a small lowering of ß101Am/Nd and liquid-liquid separation of trivalent lanthanides from trivalent americium. Despite this, the 4f/5f separation is doubled when DTTA-PzM replaces DTPA as an aqueous complexant in solvent extraction.

Does addition of 1-octanol as a phase modifier provide radical scavenging radioprotection for N,N,N',N'-tetraoctyldiglycolamide (TODGA)?

To mitigate third phase formation in next generation used nuclear fuel reprocessing technologies, the addition of 1-octanol has been trialed. However, contradictory reports on the radiolytic effect of 1-octanol incorporation on separation ligand degradation need to be resolved. Here, 50 mM N,N,N',N'-tetraoctyldiglycolamide (TODGA) dissolved in n-dodecane was gamma irradiated in the presence and absence of 1-octanol (2.5-10 vol%) and a 3.0 M HNO3 aqueous phase. Radiation-induced TODGA degradation exhibited pseudo-first-order decay kinetics as a function of absorbed gamma dose for all investigated solution and solvent system formulations. The addition of 1-octanol afforded diametrically different effects on the rate of TODGA degradation depending on solvent system formulation. For organic-only irradiations, 1-octanol promoted TODGA degradation (d = 0.0057 kGy-1 for zero 1-octanol present vs.∼0.0073 kGy-1 for 7.5-10 vol%) attributed to a favourable hydrogen atom abstraction reaction free energy (-0.31 eV) and the ability of 1-octanol to access a higher yield of n-dodecane radical cation (RH˙+) at sub-nanosecond timescales. This was rationalized by determination of the rate coefficient (k) for the reaction of 1-octanol with RH˙+, k = (1.23 ± 0.07) × 1010 M-1 s-1. In contrast, irradiation in the presence of 1-octanol and a 3.0 M HNO3 aqueous phase afforded significant radioprotection (d = 0.0054 kGy-1 for zero 1-octanol present vs.≤ 0.0044 kGy-1 for >2.5 vol%) that increases with 1-octanol concentration, relative to the single phase, organic-only solutions. This effect was attributed to the extraction of sufficiently high concentrations of HNO3 and H2O into the organic phase by TODGA and 1-octanol as adducts which interfere with the hydrogen atom abstraction process between the 1-octanol radical and TODGA. Our findings suggest that the addition of 1-octanol as a phase modifier will enhance the radiation robustness of TODGA-based separation technologies under envisioned solvent system conditions in the presence of aqueous HNO3.

Influence of a Pre-organized N-Donor Group on the Coordination of Trivalent Actinides and Lanthanides by an Aminopolycarboxylate Complexant.

The thermodynamic influence of a pre-organized N-donor group on the coordination of trivalent actinides and lanthanides by an aqueous aminopolycarboxylate complexant has been investigated. The synthesized reagent, N-2-methylpicolinate-ethylenediamine-N,N',N'-triacetic acid (EDTA-Mpic), resembles ethylenediamine-N,N,N',N'-tetraacetic acid (EDTA) with a single acetate pendant arm replaced by a 6-carboxypyridin-2-ylmethyl group. The rigid N-donor picolinate functionality has a profound impact on ligand protonation and trivalent f element complexation equilibria, as demonstrated by potentiometric, spectroscopic, and liquid/liquid metal-partitioning studies as well as by molecular dynamics calculations. Relative to diethylenetriamine-N,N,N',N'',N''-pentaacetic acid (DTPA), the ability to preferentially bind trivalent actinides over trivalent lanthanides was moderately lowered due to the presence of the N-(6-carboxypyridin-2-ylmethyl) substituent. The structural modification substantially amplifies the total ligand acidity of EDTA-Mpic. As a result the complexant sustains the metal complexation and efficient An3+ /Ln3+ differentiation in aqueous mixtures of unprecedented acidity for this class of reagents.

Influence of a Heterocyclic Nitrogen-Donor Group on the Coordination of Trivalent Actinides and Lanthanides by Aminopolycarboxylate Complexants.

The novel metal chelator N-2-(pyridylmethyl)diethylenetriamine-N,N',N″,N″-tetraacetic acid (DTTA-PyM) was designed to replace a single oxygen-donor acetate group of the well-known aminopolycarboxylate complexant diethylenetriamine-N,N,N',N″,N″-pentaacetic acid (DTPA) with a nitrogen-donor 2-pyridylmethyl. Potentiometric, spectroscopic, computational, and radioisotope distribution methods show distinct differences for the 4f and 5f coordination environments and enhanced actinide binding due to the nitrogen-bearing heterocyclic moiety. The Am3+, Cm3+, and Ln3+ complexation studies for DTTA-PyM reveal an enhanced preference, relative to DTPA, for trivalent actinide binding. Fluorescence studies indicate no changes to the octadentate coordination of trivalent curium, while evidence of heptadentate complexation of trivalent europium is found in mixtures containing EuHL(aq) complexes at the same aqueous acidity. The denticity change observed for Eu3+ suggests that complex protonation occurs on the pyridyl nitrogen. Formation of the CmHL(aq) complex is likely due to the protonation of an available carboxylate group because the carbonyl oxygen can maintain octadentate coordination through a rotation. The observed suppressed protonation of the pyridyl nitrogen in the curium complexes may be attributed to stronger trivalent actinide binding by DTTA-PyM. Density functional theory calculations indicate that added stabilization of the actinide complexes with DTTA-PyM may originate from π-back-bonding interactions between singly occupied 5f orbitals of Am3+ and the pyridyl nitrogen. The differences between the stabilities of trivalent actinide chelates (Am3+, Cm3+) and trivalent lanthanide chelates (La3+-Lu3+) are observed in liquid-liquid extraction systems, yielding unprecedented 4f/5f differentiation when using DTTA-PyM as an aqueous holdback reagent. In addition, the enhanced nitrogen-donor softness of the new DTTA-PyM chelator was perturbed by adding a fluorine onto the pyridine group. The comparative characterization of N-(3-fluoro-2-pyridylmethyl)diethylenetriamine-N,N',N″,N″-tetraacetic acid (DTTA-3-F-PyM) showed subdued 4f/5f differentiation due to the presence of this electron-withdrawing group.

Thermodynamic, Spectroscopic, and Computational Studies of f-Element Complexation by N-Hydroxyethyl-diethylenetriamine-N,N',N″,N″-tetraacetic Acid.

Potentiometric and spectroscopic techniques were combined with DFT calculations to probe the coordination environment and determine thermodynamic features of trivalent f-element complexation by N-hydroxyethyl-diethylenetriamine-N,N',N″,N″-tetraacetic acid, HEDTTA. Ligand protonation constants and lanthanide stability constants were determined using potentiometry. Five protonation constants were accessible in I = 2.0 M (H+/Na+)ClO4. UV-vis spectroscopy was used to determine stability constants for Nd3+ and Am3+ complexation with HEDTTA. Luminescence spectroscopy indicates two water molecules in the inner coordination sphere of the Eu/HEDTTA complex, suggesting HEDTTA is heptadentate. Luminescence data was supported by DFT calculations, which demonstrate that substitution of the acetate pendant arm by a N-hydroxyethyl group weakens the metal-nitrogen bond. This bond elongation is reflected in HEDTTA's ability to differentiate trivalent actinides from trivalent lanthanides. The trans-lanthanide Ln/HEDTTA complex stability trend is analogous to Ln/DTPA complexation; however, the loss of one chelate ring resulting from structural substitution weakens the complexation by ∼3 orders of magnitude. Successful separation of trivalent americium from trivalent lanthanides was demonstrated when HEDTTA was utilized as aqueous holdback complexant in a liquid-liquid system. Time-dependent extraction studies for HEDTTA were compared to diethylenetriamine-N,N,N',N″,N″-pentaacetic acid (DTPA) and N-hydroxyethyl-ethylenediamine-N,N',N'-triacetic acid (HEDTA). The results indicate substantially enhanced phase-transfer kinetic rates for mixtures containing HEDTTA.

Thermodynamic and Spectroscopic Studies of Trivalent f-element Complexation with Ethylenediamine-N,N'-di(acetylglycine)-N,N'-diacetic Acid.

The coordination behavior and thermodynamic features of complexation of trivalent lanthanides and americium by ethylenediamine-N,N'-di(acetylglycine)-N,N'-diacetic acid (EDDAG-DA) (bisamide-substituted-EDTA) were investigated by potentiometric and spectroscopic techniques. Acid dissociation constants (K(a)) and complexation constants (ß) of lanthanides (except Pm) were determined by potentiometric analysis. Absorption spectroscopy was used to determine stability constants for the binding of trivalent americium and neodymium by EDDAG-DA under similar conditions. The potentiometry revealed 5 discernible protonation constants and 3 distinct metal-ligand complexes (identified as ML(-), MHL, and MH2L(+)). Time-resolved fluorescence studies of Eu-(EDDAG-DA) solutions (at varying pH) identified a constant inner-sphere hydration number of 3, suggesting that glycine functionalities contained in the amide pendant arms are not involved in metal complexation and are protonated under more acidic conditions. The thermodynamic studies identified that f-element coordination by EDDAG-DA is similar to that observed for ethylenediamine-N,N,N',N'-tetraacetic acid (EDTA). However, coordination via two amidic oxygens of EDDAG-DA lowers its trivalent f-element complex stability by roughly 3 orders of magnitude relative to EDTA.

Coordination Chemistry and f-Element Complexation by Diethylenetriamine-N,N″-bis(acetylglycine)-N,N',N″-triacetic Acid.

Potentiometric and spectroscopic techniques were used to evaluate the coordination behavior and thermodynamic features of trivalent f-element complexation by diethylenetriamine-N,N″-bis(acetylglycine)-N,N',N″-triacetic acid (DTTA-DAG) and its di(acetylglycine ethyl ester) analogue [diethylenetriamine-N,N″-bis(acetylglycine ethyl ester)-N,N',N″-triacetic acid (DTTA-DAGEE)]. Protonation constants and stability constants of trivalent lanthanide complexes (except Pm3+) were determined by potentiometry. Six protonation sites and three metal-ligand complexes [ML2-, MHL-, and MH2L(aq)] were quantified for DTTA-DAG. Four protonation sites and one metal-ligand complex [ML(aq)] were observed for DTTA-DAGEE, consistent with the presence of two ester groups. Absorption spectroscopy was utilized to measure the stability constants for complexation of trivalent neodymium and americium by DTTA-DAG and trivalent neodymium by DTTA-DAGEE. The coordination environment of trivalent europium in the presence of DTTA-DAG was investigated at various acidities by luminescence lifetime measurements. Decay constants indicate one water molecule in the inner coordination sphere across the 1.0 < pH < 5.5 range, presumably due to octadentate coordination by DTTA-DAG. A trans-lanthanide pattern of complex stabilities for DTTA-DAG was found to be analogous to that observed for DTPA, with a ∼106 reduction of the complex stability. The lessened strength of complexation, relative to DTPA, was attributed to significant reduction of the total ligand basicity for DTTA-DAG due to the electronic influence of amide functionalization. When DTTA-DAG is used as an aqueous holdback complexant in liquid-liquid distribution experiments, the preferential coordination of Am3+ in the aqueous environment offers efficient An/Ln differentiation. The separation extends to pH 2 conditions, where the kinetics of phase transfer in such liquid-liquid systems are aided by the acid-catalyzed dissociation of a metal/aminopolycarboxylate complex.

Effect of f-element complexation on the radiolysis of 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP]).

A systematic study of the impact on the chemical reactivity of the oxidising n-dodecane radical cation (RH˙+) with f-element complexed 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) has been undertaken utilizing time-resolved electron pulse radiolysis/transient absorption spectroscopy and high-level quantum mechanical calculations. Lanthanide ion complexed species, [Ln((HEH[EHP])2)3], exhibited vastly increased reactivity (over 10× faster) in comparison to the non-complexed ligand in n-dodecane solvent, whose rate coefficient was k = (4.66 ± 0.22) × 109 M-1 s-1. Similar reactivity enhancement was also observed for the corresponding americium ion complex, k = (5.58 ± 0.30) × 1010 M-1 s-1. The vastly increased reactivity of these f-element complexes was not due to simple increased diffusion-control of these reactions; rather, enhanced hole transfer mechanisms for the complexes were calculated to become energetically more favourable. Interestingly, the observed reactivity trend with lanthanide ion size was not linear; instead, the rate coefficients showed an initial increase (Lu to Yb) followed by a decrease (Tm to Ho), followed by another increase (Dy to La). This behaviour was excellently predicted by the calculated reaction volumes of these complexes. Complementary cobalt-60 gamma irradiations for select lanthanide complexes demonstrated that the measured kinetic differences translated to increased ligand degradation at steady-state timescales, affording ∼38% increase in ligand loss of a 1 : 1 [La((HEH[EHP])2)3] : HEH[EHP] ratio system.

Understanding the solution behavior of minor actinides in the presence of EDTA4-, carbonate, and hydroxide ligands.

The aqueous solution behavior of An(III) (An = Am or Cm) in the presence of EDTA(4-) (ethylenediamine tetraacetate), CO3(2-) (carbonate), and OH(-) (hydroxide) ligands has been probed in aqueous nitrate solution (various concentrations) at room temperature by UV-vis absorption and luminescence spectroscopies (Cm systems analyzed using UV-vis only). Ternary complexes have been shown to exist, including [An(EDTA)(CO3)](3-)(aq), (where An = Am(III) or Cm(III)), which form over the pH range 8 to 11. It is likely that carbonate anions and water molecules are in dynamic exchange for complexation to the [An(EDTA)](-)(aq) species. The carbonate ion is expected to bind as a bidentate ligand and replaces two coordinated water molecules in the [An(EDTA)](-)(aq) complex. In a 1:1 Am(III)/EDTA(4-) binary system, luminescence spectroscopy shows that the number of coordinated water molecules (N(H2O)) decreases from ~8 to ~3 as pH is increased from approximately 1 to 10. This is likely to represent the formation of the [Am(EDTA)(H2O)3](-) species as pH is raised. For a 1:1:1 Am(III)/EDTA(4-)/CO3(2-) ternary system, the N(H2O) to the [Am(EDTA)](-)(aq) species over the pH range 8 to 11 falls between 2 and 3 (cf. ~3 to ~4 in the binary system) indicating formation of the [An(EDTA)(CO3)](3-)(aq) species. As pH is further increased from approximately 10 to 12 in both systems, there is a sharp decrease in N(H2O) from ~3 to ~2 in the binary system and from ~2 to ~1 in the ternary system. This is likely to correlate to the formation of hydrolyzed species (e.g., [Am(EDTA)(OH)](2-)(aq) and/or Am(OH)(3(s))).

Radiolytic Evaluation of 3,4,3-LI(1,2-HOPO) in Aqueous Solutions.

The octadentate hydroxypyridinone ligand 3,4,3-LI(1,2-HOPO) (abbreviated as HOPO) has been identified as a promising candidate for both chelation and f-element separation technologies, two applications that require optimal performance in radiation environments. However, the radiation robustness of HOPO is currently unknown. Here, we employ a combination of time-resolved (electron pulse) and steady-state (alpha self-radiolysis) irradiation techniques to elucidate the basic chemistry of HOPO and its f-element complexes in aqueous radiation environments. Chemical kinetics were measured for the reaction of HOPO and its Nd(III) ion complex ([NdIII(HOPO)]-) with key aqueous radiation-induced radical transients (eaq-, H• atom, and •OH and NO3• radicals). The reaction of HOPO with the eaq- is believed to proceed via reduction of the hydroxypyridinone moiety, while transient adduct spectra indicate that reactions with the H• atom and •OH and NO3• radicals proceeded by addition to HOPO's hydroxypyridinone rings, potentially allowing for the generation of an extensive suite of addition products. Complementary steady-state 241Am(III)-HOPO complex ([241AmIII(HOPO)]-) irradiations showed the gradual release of 241Am(III) ions with increasing alpha dose up to 100 kGy, although complete ligand destruction was not observed.

Tuning aminopolycarboxylate chelators for efficient complexation of trivalent actinides.

The complexation of trivalent lanthanides and minor actinides (Am3+, Cm3+, and Cf3+) by the acyclic aminopolycarboxylate chelators 6,6'-((ethane-1,2-diylbis-((carboxymethyl)azanediyl))bis-(methylene))dipicolinic acid (H4octapa) and 6,6'-((((4-(1-(2-(2-(2-hydroxyethoxy)ethoxy)ethyl)-1H-1,2,3-triazol-4-yl)pyridine-2,6-diyl)bis-(methylene))bis-((carboxymethyl)azanediyl))bis-(methylene)) dipicolinic acid (H4pypa-peg) were studied using potentiometry, spectroscopy, competitive complexation liquid-liquid extraction, and ab initio molecular dynamics simulations. Two studied reagents are strong multidentate chelators, well-suited for applications seeking radiometal coordination for in-vivo delivery and f-element isolation. The previously reported H4octapa forms a compact coordination packet, while H4pypa-peg is less sterically constrained due to the presence of central pyridine ring. The solubility of H4octapa is limited in a non-complexing high ionic strength perchlorate media. However, the introduction of a polyethylene glycol group in H4pypa-peg increased the solubility without influencing its ability to complex the lanthanides and minor actinides in solution.

Curium(iii) radiation-induced reaction kinetics in aqueous media.

Insight into the effects of radiolytic processes on the actinides is critical for advancing our understanding of their solution chemistry because the behaviour of these elements cannot be easily separated from the influence of their inherent radiation field. However, minimal information exists on the radiation-induced redox behaviour of curium (Cm), a key trivalent transuranic element present in used nuclear fuel and frequently used as an alpha radiation source. Here we present a kinetic study on the aqueous redox reactions of Cm(iii) with radicals generated through the radiolysis of aqueous media. In particular, we probe reaction kinetics in nitric acid solutions that are used as the aqueous phase component of used nuclear fuel reprocessing solvent systems. Second-order rate coefficients (k) were measured for the reaction of Cm(iii) with the hydrated electron (eaq-, k = (1.25 ± 0.03) × 1010 M-1 s-1), hydrogen atom (H˙, k = (5.16 ± 0.37) × 108 M-1 s-1), hydroxyl radical (˙OH, k = (1.69 ± 0.24) × 109 M-1 s-1), and nitrate radical (NO3˙, k = (4.83 ± 0.09) × 107 M-1 s-1). Furthermore, the first-ever Cm(ii) absorption spectrum (300-700 nm) is also reported. These kinetic data dispel the status quo notion of Cm(iii) possessing little to no redox chemistry in aqueous solution, and suggest that the resulting Cm(ii) and Cm(iv) transients could exist in irradiated aqueous solutions and be available to undergo subsequent redox chemistry with other solutes.

Dataset and kinetic model reaction compilation for the radical-induced degradation of formic acid and formate in aqueous solution.

This article contains the individual datasets and complete reaction kinetics compilation for the formic acid/formate component of the kinetic model described in "Radiolytic Degradation of Formic Acid and Formate in Aqueous Solution: Modeling the Final Stages of Organic Mineralization under Advanced Oxidation Process Conditions" [1]. Gamma irradiation data were collected for aqueous sodium formate solutions under pH = 1.5 and 9.0 conditions. To determine the optimum conditions necessary to effectively mineralize formic acid/formate in an Advanced Oxidation Process utilized for water treatment, several solution compositions were evaluated: air, nitrogen, and nitrous oxide saturation. Data were collected by a combination of high performance liquid chromatography and gas chromatography. These measured values were used to construct a kinetic computer model, by combining with published literature rate coefficients and optimizing specific important rate coefficients to afford the best agreement with experimental data.

Radiolytic degradation of formic acid and formate in aqueous solution: modeling the final stages of organic mineralization under advanced oxidation process conditions.

The successful use of advanced oxidation processes to treat aqueous solutions containing undesirable organic species requires the degradation of these species to lower molecular weight, lower hazard compounds. Safe application of this technology requires a thorough understanding of the mechanisms of degradation. These oxidative transformations are mainly initiated by the reactions of reactive oxygen species, particularly hydroxyl radicals. These react with organic molecules to generate carbon-centered radicals. In the presence of dissolved oxygen, the carbon-centered radicals are next converted to peroxyl radicals, which then decay to lower molecular weight species by multiple mechanistic pathways. Formic acid and its conjugate base formate are the last stable chemical species produced immediately before the complete mineralization of any organic molecule undergoing oxidative degradation in aqueous solution. Once understood, the radical-induced chemistry of formic acid/formate under these conditions has wide applicability in all advanced oxidation technologies. To develop this quantitative knowledge, we have performed a series of 60Co gamma irradiation studies on aqueous formic acid/formate over different pH and solution conditions. The measured species concentration changes, as a function of applied dose, are compared with the predictions of a kinetic computer model constructed from literature reactions and reported rate coefficients. The excellent agreement found between the results and modeling gives confidence in the mechanism presented here and provide the first complete computer model for the radiolytic degradation of formic acid in water.

Synthesis and characterization of a novel aminopolycarboxylate complexant for efficient trivalent f-element differentiation: N-butyl-2-acetamide-diethylenetriamine-N,N',N'',N''-tetraacetic acid.

The novel metal ion complexant N-butyl-2-acetamide-diethylenetriamine-N,N',N'',N''-tetraacetic acid (DTTA-BuA) uses an amide functionalization to increase the total ligand acidity and attain efficient 4f/5f differentiation in low pH conditions. The amide, when located on the diethylenetriamine platform containing four acetate pendant arms maintains the octadentate coordination sphere for all investigated trivalent f-elements. This compact coordination environment inhibits the protonation of LnL- complexes, as indicated by lower K111 constants relative to the corresponding protonation site of the free ligand. For actinide ions, the enhanced stability of AnL- lowers the K111 for americium and curium beyond the aptitude of potentiometric detection. Density functional theory computations indicate the difference in the back-donation ability of Am3+ and Eu3+ f-orbitals is mainly responsible for stronger proton affinity of EuL- compared to AmL-. The measured stability constants for the formation of AmL- and CmL- complexes are consistently higher, relative to ML- complexes with lanthanides of similar charge density. When compared with the conventional aminopolycarboxylate diethylenetriamine pentaacetic acid (DTPA), the modified DTTA-BuA complexant features higher ligand acidity and the important An3+/Ln3+ differentiation when deployed on a liquid-liquid distribution platform.

Optical Absorption Characteristics For [Formula: see text] and [Formula: see text] Transitions of Trivalent Americium Ion in Aqueous Electrolyte Mixtures.

Optical absorption features for [Formula: see text] and [Formula: see text] transitions of trivalent americium ion were investigated in a wide range of aqueous combinations of perchloric and nitric acids (0.1-6.0 mol L-1). The developed qualitative matrix of extinction coefficients measures the cumulative impact of increasing electrolyte content, changes in the hydration zones of americium ion, and inner-sphere perturbation by nitrate on the absorbance. The effects of growing complexity of aqueous electrolyte medium were highlighted for the [Formula: see text] transition. Spectroscopy indicates the perturbation of the inner hydration sphere of trivalent f-element by one nitrate ligand.