Molecular-scale understanding of diluent effects on ligand assembly for metal ion separations

Use of metal-selective ligands in solvent extraction is instrumental in extraction of critical materials and recycling, yet, diluent effects on extraction performance are not well understood. Experimental and empirical solvent parameters have been proposed to correlate with extraction performance, but are often inadequate predictors. We follow the hypothesis that the diluents' primary influence on extraction efficiency is whether or not it hinders assembly of the bulky extracting ligands into a geometry necessary for metal complexation. This behavior is readily accessible with molecular dynamics (MD), where the atomistic description of molecules can be applied to arbitrary extractant-solvent molecules and their mixtures. Several simulated quantities are considered, from both pairwise and graph theoretical analyses, and compared to experimental distribution ratio data for americium extraction by TODGA in a series of inert, non-interacting diluents. These simple properties, especially the formation of closed triplets corresponding to the 3 : 1 ligand : metal stoichiometric solvate, suggest a potential predictive power of this approach. This methodology provides a path forward to comprehensively understand and predict diluent effects in more complex systems involving different extracting ligands and multi-component diluent mixtures.

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.

Selective actinide(III) separation using 2,6-bis[1-(propan-1-ol)-1,2,3-triazol-4-yl]pyridine (PyTri-Diol) in the innovative-SANEX process: laboratory scale counter current centrifugal contactor demonstration

An innovative-SANEX process for the selective separation of the trivalent actinides americium and curium from a simulated PUREX raffinate solution was successfully demonstrated on the laboratory scale using a 16-stage 1 cm annular centrifugal contactor setup. The solvent was composed of 0.2 mol L−1 N,N,N′,N′-tetra-n-octyl-diglycolamide (TODGA) and 5% v/v 1-octanol in a kerosene diluent. Zr(IV) and Pd(II) co-extraction was prevented using trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid (CDTA) as a masking agent in the feed. The actinide(III) selective back-extraction was achieved using 2,6-bis[1-(propan-1-ol)-1,2,3-triazol-4-yl]pyridine (PyTri-Diol) in 0.45 mol L−1 HNO3 as a CHON alternative to the sulfur-containing stripping agent used in a previous version of the innovative-SANEX process. The new process described in this paper showed excellent performance for the recovery of An(III). An An(III) product with a quasi-quantitative recovery of americium and curium (≥99.9%) and very good separation from fission and activation products was obtained (decontamination factors ≥4000). Only a slight contamination with Zr and Ru was observed. This test demonstrates the successful use of molecules containing only carbon, hydrogen, oxygen, and nitrogen atoms (so-called CHON molecules) for the selective separation of An(III) from a simulated PUREX raffinate solution. By avoiding sulfur- or phosphorous-containing molecules, the generation of secondary radioactive waste during process operation can be reduced drastically.

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...

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 (e_aq^-, 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.

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.

An experimental and computational look at the radiolytic degradation of TODGA and the effect on metal complexation

A combination of Fukui function calculations with experimental characterization gives an improved understanding of the behaviour of TODGA solutions after radiolysis.