Evaluation of the Arrhenius behavior of n-dodecane radical cation (RH˙+) reactivity with lanthanide ion-complexed N,N,N',N'-tetraoctyl diglycolamide (TODGA)

Temperature-dependent rate constants for the reaction of the n-dodecane radical cation (RH˙+) with trivalent lanthanide ion-complexed N,N,N',N'-tetraoctyl diglycolamide (TODGA) over the range 10-40 °C have been determined using electron pulse radiolysis/transient absorption spectroscopy techniques. For the free ligand, an activation energy of Ea = 20.4 ± 0.7 kJ mol-1 and pre-exponential factor of ln(A) = 31.23 ± 0.27 were obtained, corresponding to a room-temperature rate constant of k = (9.94 ± 0.52) × 109 M-1 s-1. The RH˙+ reactivity with La(TODGA)3(NO3)3, Nd(TODGA)3(NO3)3, Gd(TODGA)3(NO3)3, Yb(TODGA)3(NO3)3 and Lu(TODGA)3(NO3)3 complexes had rate constants of k = (5.30 ± 0.51) × 1010, (4.23 ± 0.18) × 1010, (2.44 ± 0.13) × 1010, (1.68 ± 0.03) × 1010, and (9.1 ± 0.7) × 109 M-1 s-1, respectively. The corresponding Arrhenius activation energies determined for three (La, Gd, Lu) lanthanide-TODGA complexes showed consistent values of Ea = 35 ± 2.2, 35.3 ± 2.0, and 33.5 ± 3.9 kJ mol-1, respectively. The similar and relatively large barrier energy suggests a common reaction mechanism involving electron abstraction from one of the coordinating nitrate anions, which is consistent with previously reported decreased degradation of TODGA complexes under radiolytic environments.

Radiation-Induced Defects in Uranyl Trinitrate Solids

Actinides are inherently radioactive; thus, ionizing radiation is emitted by these elements can have profound effects on its surrounding chemical environment through the formation of free radical species. While previous work has noted that the presence of free radicals in the system impacts the redox state of the actinides, there is little atomistic understanding of how these metal cations interact with free radicals. Herein, we explore the effects of radiation (UV and γ) on three U(VI) trinitrate complexes, M[UO2(NO3)3] (where M=K+, Rb+, Cs+), and their respective nitrate salts in the solid state via electron paramagnetic resonance (EPR) and Raman spectroscopy paired with Density Functional Theory (DFT) methods. We find that the alkali salts form nitrate radicals under UV and γ irradiation, but also note the presence of additional degradation products. M[UO2(NO3)3] solids also form nitrate radicals and additional DFT calculations indicate the species corresponds to a change from the bidentate bound nitrate anion into a monodentate NO3 • radical. Computational studies also highlight the need to include the second sphere coordination environment around the [UO2(NO3)3]0,1 species to gain agreement between the experimental and predicted EPR signatures.

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.

Impact of lanthanide ion complexation and temperature on the chemical reactivity of N,N,N',N'-tetraoctyl diglycolamide (TODGA) with the dodecane radical cation

The impact of trivalent lanthanide ion complexation and temperature on the chemical reactivity of N,N,N',N'-tetraoctyl diglycolamide (TODGA) with the n-dodecane radical cation (RH˙+) has been measured by electron pulse radiolysis and evaluated by quantum mechanical calculations. Additionally, Arrhenius parameters were determined for the reaction of the non-complexed TODGA ligand with the RH˙+ from 10-40 °C, giving the activation energy (Ea = 17.43 ± 1.64 kJ mol-1) and pre-exponential factor (A = (2.36 ± 0.05) × 1013 M-1 s-1). The complexation of Nd(III), Gd(III), and Yb(III) ions by TODGA yielded [LnIII(TODGA)3(NO3)3] complexes that exhibited significantly increased reactivity (up to 9.3× faster) with the RH˙+, relative to the non-complexed ligand: k([LnIII(TODGA)3(NO3)3] + RH˙+) = (8.99 ± 0.93) × 1010, (2.88 ± 0.40) × 1010, and (1.53 ± 0.34) × 1010 M-1 s-1, for Nd(III), Gd(III), and Yb(III) ions, respectively. The rate coefficient enhancement measured for these complexes exhibited a dependence on atomic number, decreasing as the lanthanide series was traversed. Preliminary reaction free energy calculations-based on a model [LnIII(TOGDA)]3+ complex system-indicate that both electron/hole and proton transfer reactions are energetically unfavorable for complexed TODGA. Furthermore, complementary average local ionization energy calculations showed that the most reactive region of model N,N,N',N'-tetraethyl diglycolamide (TEDGA) complexes, [LnIII(TEGDA)3(NO3)3], toward electrophilic attack is for the coordinated nitrate (NO3-) counter anions. Therefore, it is possible that radical reactions with the complexed NO3- counter anions dominate the differences in rates seen for the [LnIII(TODGA)3(NO3)3] complexes, and are likely responsible for the reported radioprotection in the presence of TODGA complexes.

Study of irradiation decomposition products of PUREX solvents on zirconium metal retention behavior

The metal retention behaviors of several simulated radiolysis products on zirconium metal were investigated, and it was found that acid phosphate radiolysis product HDEHP has the greatest effect. The effects of extraction nitric acid concentration, simulated radiolysis product concentration, metal concentration, and temperature on the zirconium metal retention behavior were also investigated. The results showed that zirconium metal forms complexes with HDEHP resulting in retention in the organic phase. Nitric acid concentration and metal concentration change the morphology of the metal thus affecting the extraction and metal retention behavior. The temperature has almost no effect on metal retention.

Effect of Metal Complexation on Diglycolamides Radiolysis: A Comparison between Ex-Situ Gamma and In-Situ Alpha Irradiation

Radiolytic degradation is an important aspect to consider when developping a ligand or a complexant for radionucleides. Diglycolamide extractants (DGAs) have been playing an important role in many partition processes...