Synthesis and Characterization of Tc(I) Carbonyl Nitrosyl Species Relevant to the Hanford Tank Waste: FY 2016 Status Report

Abstract

Among long-lived radioactive constituents in the Hanford tank waste, Tc presents a unique challenge in that it exists predominantly in the liquid phase, generally in the anionic form of pertechnetate, TcO 4 -, which is highly volatile at low-activity waste (LAW) vitrification melter temperatures and mobile in the Hanford site's subsurface environment. The complex behavior of Tc under storage, treatment, and immobilization conditions significantly affects its management options, which to-date remain uncertain.In strongly alkaline environments, Tc exists as pertechnetate, TcO 4 -(oxidation state +7), and in the reduced forms (oxidation state < +7) collectively known as non-pertechnetate species. Pertechnetate is a well-characterized, anionic Tc species that can be removed from LAW by anion exchange or solvent extraction methods. There is no definitive information on the origin of the non-pertechnetate Tc species, nor is there a comprehensive description of their composition and behavior. It has been recently proposed that the non-pertechnetate species can comprise Tc(I) metal center and carbonyl or mixed carbonyl nitrosyl ligands stabilizing low-valent Tc. Recent work by our group has significantly expanded this previous work, generating a series of Tc(I) carbonyl compounds and demonstrating that they can be generated from reduction of TcO 4 in the simulated Hanford tank waste in presence of CO at elevated temperature (Levitskaia et al. 2014). These results are consistent with the previous proposal that [Tc(CO) 3 ] + species can be present in the Hanford tank waste and suggest that the low Tc(I) oxidation state is stabilized by the π-accepting ability of the CO ligands. The continuation work has been initiated to develop model Tc carbonyl nitrosyl compounds and investigate their potential presence in the Hanford tank wastes. This report summarizes our to-date results. Synthesis of the low-valent Tc carbonyl nitrosyl complexes was performed using two Tc(I) tricarbonyl precursors, namely monomeric [Tc(CO) 3 Cl 3 ] 2and tetrameric [Tc(CO) 3 (OH)] 4 species, in methylene chloride solvent using NOBF 4 as the nitrolysation reagent. Both pathways generated [Tc(CO) 2 (NO)] 2+ species as evident from the Tc-99 nuclear magnetic resonance (NMR) and infrared (IR) spectroscopic characterization of the reaction products. The reaction yield using [Tc(CO) 3 Cl 3 ] 2starting material was about 70% with the Tc carbonyl nitrosyl species partitioned between the liquid and solid reaction fractions, and about 30% of starting Tc(I) oxidized to Tc(VII). The reaction yield using [Tc(CO) 3 (OH)] 4 starting material was nearly quantitative with Tc carbonyl nitrosyl product forming insoluble precipitate.The obtained results suggest that the Tc carbonyl nitrosyl product contained one monomeric [Tc(CO) 2 (NO)Cl 3 ]and two trans Cl-bridged dimeric [Tc(CO) 2 (NO)(μ-Cl)Cl] 2 species. However, this assignment should be validated by the future studies. Density functional theory (DFT) computational modeling supported the spectroscopic characterizatio...

Spectroscopic Characterization of Aqua [fac-Tc(CO)3]+ Complexes at High Ionic Strength

“…(Et 4 N) 2 [fac-Tc-(CO) 3 Cl 3 ] was prepared by a two-step reduction procedure, which involved (a) an initial reduction of ammonium pertechnetate to a Tc(V) isolated as (Bu 4 N)[TcOCl 4 ] solid and (b) its subsequent reduction in the presence of CO and BH 3 /THF to Tc(I) in the form of [fac-Tc(CO) 3 Cl 3 ] 2− which was isolated as the (Et 4 N) 2 [fac-Tc(CO) 3 Cl 3 ] product by precipitation followed by washing with cold water-free ethanol. 20 For the preparation of aqueous [fac-Tc(CO) 3 ] + species, the nonwashed (Et 4 N) 2 [fac-Tc(CO) 3 Cl 3 ] solid was used to generate analytically pure tetrameric [fac-Tc(CO) 3 (OH)] 4 species according to a modified literature procedure 21 by dissolution in aqueous 0.1 M NaOH solution and extraction of the product into diethyl ether. Using the nonwashed (Et 4 N) 2 [fac-Tc(CO) 3 Cl 3 ] intermediate does not appear to affect the purity of the final product while allowing for higher overall yield.…”

“…Cl 3 ] product by precipitation 18. It was used to generate analytically pure tetrameric [Tc(CO) (OH)] 4 species according to modified literature procedure by dissolution in aqueous 0.1 M NaOH solution and extraction of the product into diethyl ether.…”

Oxidative Stability of Tc(I) Tricarbonyl Species Relevant to the Hanford Tank Waste

“…Characterization of this product by a variety of methods was also explored and a report summarizing the FY 2013 work was issued (Rapko et al 2013b). In FY 2014-2015, the optimization of the synthesis and purification of the Tc-tricarbonyl starting precursor was completed (Levitskaia et al 2014;Hall et al 2015). The experimental work continued on understanding various aspects of the nature and stability of various Tc-tricarbonyl species under simple alkaline conditions, alkaline conditions in the presence of the complexing agent sodium gluconate, high-ionic-strength alkaline conditions in the presence and absence of gluconate, and in the presence of alkaline Hanford tank solution simulants (Levitskaia et al 2014;Levitskaia et al 2015).…”

Characterization of Non-pertechnetate Species Relevant to the Hanford Tank Waste

“…These species will be referred to as the [Tc(CO) 2 (NO)] 2+ species. While the NMR and EPR analyses on these species were reported previously (Hall et al 2015), this year they were subjected to XAS and XPS measurements.…”