Mechanism and kinetics of the stepwise voltammetric reduction of pertechnetate in alkaline solution to Tc(VI, Tc(V) and Tc(IV)

A Combined Study of Tc Redox Speciation in Complex Aqueous Systems: Wet-Chemistry, Tc K-/L3-Edge X-ray Absorption Fine Structure, and Ab Initio Calculations

The combination of wet-chemistry experiments (measurements of pH, E_h, and [Tc]) and advanced spectroscopic techniques (K- and L₃-edge X-ray absorption fine structure spectroscopy) confirms the formation of a very stable Tc(V)-gluconate complex under anoxic conditions. In the presence of gluconate and an excess of Sn(II) (at pe + pH ≈ 2), technetium forms a very stable Tc(IV)-gluconate complex significantly enhancing the solubility defined by TcO₂(s) in hyperalkaline gluconate-free systems. A new setup for "tender" X-ray spectroscopy (spectral range, ∼2-5 keV) in transmission or total fluorescence yield detection mode based on a He flow cell has been developed at the INE Beamline for radionuclide science (KIT light source). This setup allows handling of radioactive specimens with total activities up to one million times the exemption limit. For the first time, Tc L₃-edge measurements (∼2.677 keV) of Tc species in liquid (aqueous) media are reported, clearly outperforming conventional K-edge spectroscopy as a tool to differentiate Tc oxidation states and coordination environments. The coupling of L₃-edge X-ray absorption near-edge spectroscopy measurements and relativistic multireference ab initio methods opens new perspectives in the definition of chemical and thermodynamic models for systems of relevance in the context of nuclear waste disposal, environmental, and pharmaceutical applications.

Surprising formation of quasi-stable Tc(vi) in high ionic strength alkaline media

This work demonstrates an aqueous Tc(vi) lifetime 6 orders of magnitude greater than previously suggested.

Reduction of pertechnetate by haloalkaliphilic strains of Halomonas

Abstract It is shown for the first time that haloalkaliphilic bacteria, isolated from soda-lake environments were capable of reducing Tc(VII)O(4) (-) to the Tc(V), Tc(IV) and Tc(III) at pH 10 in carbonate medium, whereas no reduction took place without bacteria or in the presence of dead biomass. After 34 h of incubation, 55% remained as Tc(VII), 36% was found as Tc(IV) and 8% as Tc(V) and after 2 months 80% of the technetium was reduced. Technetium has a toxic effect on bacteria. Reduction of TcO(4) (-) was drastically decreased at concentration above 1.5 mM. The microbial reduction has been suggested as a potential mechanism for the removal of Tc from contaminated environments or waste streams.

Differential pulse voltammetric determination of U(VI)-Tc(VII) and U(VI)-Cr(VI) in alkaline solutions

Differential pulse voltammetric (DPV) technique has been developed for the simultaneous determination of the elements in Tc(VII)-U(VI) and U(VI)-Cr(VI) mixtures in alkaline solutions. The method gave a precision of about 14% to 2% in the range of 1×10-7--1×10-4 for Cr(VI) and Tc(VII) and 1×10-6--1×10-4 for U(VI). The detection limits are 8×10-8 mol l-1 for Tc(VII) and 3×-7 mol l-1 for Cr(VI), and 8×10-7 for U(VI). It has been established that Tc(VII) could be determined in the presence of about 350 excess fold U(VI). The reduction of Tc(VII) with hydrazine has been suggested to eliminate the effect of Tc(VII) ions on the U(VI) determination. U(VI) has been determined in the presence of Cr(VI) up to molar concentration ratio U(VI)/Cr(VI)=1:1, while Cr(VI) could be determined in the presence of U(VI) in the solutions with up to molar concentration ratio Cr(VI)/U(VI)=1:15. The reliability of the determination of the elements of Tc(VII)-U(VI) and U(VI)-Cr(VI) mixtures in alkaline solution has been tested by a standard addition method.