trans-K(3)[TcO(2)(CN)(4)]

Identification and Quantification of Technetium Species in Hanford Waste Tank AN-102

Technetium-99 (Tc), a high yield fission product generated in nuclear reactors, is one of the most difficult contaminants to address at the U.S. Department of Energy Hanford, Savannah River, and other sites. In strongly alkaline solutions typifying Hanford tank waste, Tc exists as pertechnetate (TcO₄^-) (oxidation state VII) as well as in reduced forms (oxidation state < VII), collectively known as non-pertechnetate (non-TcO₄^-) species. Designing strategies for effective Tc management, including separation and immobilization, necessitates understanding the molecular structure of the non-TcO₄^- species and their identification in actual tank waste samples. Identification of non-TcO₄^- species would facilitate the development of new treatment technologies effective for dissimilar Tc species. Toward this objective, a spectroscopic library of the Tc(I) [fac-Tc(CO)₃]⁺ and Tc(II, IV, V, VII) compounds was generated and applied to the characterization of the actual Hanford AN-102 tank waste supernatant, which was processed to adjust Na concentration to ∼5.6 M and remove ¹³⁷Cs by spherical resorcinol-formaldehyde (sRF) ion-exchange resin. Post ¹³⁷Cs removal, the cesium-loaded sRF column was eluted with 0.45 M HNO₃. As-received AN-102, Cs-depleted effluent, and sRF eluate fractions were comprehensively characterized for chemical composition and speciation of Tc using ⁹⁹Tc nuclear magnetic resonance spectroscopy and X-ray absorption spectroscopy. It was demonstrated for the first time that non-TcO₄^- Tc present in the AN-102 tank waste is composed of several low-valent Tc species, including the Tc(I) [fac-Tc(CO)₃]⁺ and Tc(IV) compounds. This is the first demonstration of multiple non-TcO₄^- species co-existing in the Hanford tank waste, highlighting their importance for the waste processing.

Redox-Based Electrochemical Affinity Sensor for Detection of Aqueous Pertechnetate Anion

Rapid, selective, and in situ detection of pertechnetate (TcO₄^-) in multicomponent matrices consisting of interfering anions such as the ubiquitous NO₃^- and Cl^- or the isostructural CrO₄^2- is challenging. Present sensors lack the selectivities to exclude these interferences or the sensitivities to meet detection limits that are lower than the drinking water standards across the globe. This work presents an affinity-based electrochemical sensor for TcO₄^- detection that relies on selective reductive precipitation of aqueous TcO₄^- induced by a 1,4-benzenedimethanethiol capture probe immobilized on an electrode platform. This results in a direct decrease in the electron transfer current, the magnitude of the decrease being proportional to the amount of TcO₄^- added. Using this approach, a detection limit of 1 × 10^-10 M was achieved, which is lower than the drinking water standard of 5.2 × 10^-10 M set by United States Environmental Protection Agency. The proposed approach shows selectivity to the TcO₄^- anion, allowing detection of TcO₄^- from a multicomponent groundwater sample obtained from a well at the Hanford site in Washington (well 299-W19-36) that also contained NO₃^-, Cl^-, and CrO₄^2-, without discernably affecting the detection limits.