Technetium environmental chemistry: mechanisms for the surface-mediated reduction of Tc(VII)
99Tc is a fission product from 235U and 239Pu found in the earth crust mainly due to human activity. It is of high concern for the nuclear waste management due to its long half-life (2.14 × 105 years) and the high mobility of pertechnetate, Tc(VII)O4−, its most stable compound under aerobic conditions. In the worst case-scenario in the safety case, water could promote the corrosion of the elements of the nuclear waste repository, promoting the mobilization of highly soluble species like pertechnetate. Migration through the biogeosphere and incorporation into the food chain would cause serious environmental and health problems to human being.
Technetium bioavailability depends strongly on it speciation because in contrast to Tc(VII), the reduced Tc(IV) is mainly found as solid, TcO2, and its mobility is limited. Using this fact, several authors have shown that iron minerals, such as magnetite (FeIIFe2IIIO4) or mackinawite (FeS),[2–4] have a remarkable ability to immobilize 99Tc by triggering its reduction and incorporating, sorbing or precipitating Tc(IV). Therefore, one of the goals of this thesis is to study the reductive immobilization of technetium by pyrite (FeS2) as it is a highly abundant redox sensitive mineral, which can be found in the surrounding of nuclear waste repositories like Yucca Mountain. This will be done in two stages. First, batch sorption experiments in a wide range of pH from 3.5 to 10.5 will be used to determine the Tc-scavenging capability of pyrite. Second, a combination of X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS) and Raman microscopy will be used to identify the Tc retention mechanisms.
Figure 1:“New insights into 99Tc(VII) removal by pyrite: a spectroscopic approach” DOI: 10.1021/acs.est.9b05341
The second goal of this thesis deals with better understanding the reduction mechanism from Tc(VII) to Tc(IV) that, even thought is of high relevance for Tc behavior modeling, is not yet well understood.[6,7] In order to study this phenomenon, several electrochemical techniques (cyclic voltammetry, differential pulse voltammetry and chronoamperometry coupled with UV-vis) will be applied varying the background electrolyte, the ionic strength and the pH in order to produce reliable thermodynamic constants that describe the reduction reaction.
This thesis is developed in the frame of VESPA II project (02E11607B), funded by the German Federal Ministry of Energy and Economic Affairs (BMWi).
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