Investigating the complex interaction of Technetium with magnetite nanoparticles

Nanoparticles (NPs) are relevant in medicine, catalysis and environmental remediation. Among them, magnetite (Fe(II)Fe(III)2O4) NPs are especially interesting due to their redox and magnetic properties and their tunability of size and surface properties, which makes them suited for the removal of many redox-active pollutants. Tc is of great concern for the safety assessment of nuclear waste repositories, since 99Tc is a fission product with a long half-life (t1/2 = 2.1∙105 years). Under oxidative conditions Tc forms an anionic species, pertechnetate (Tc(VII)O4-), which is mobile due to its weak interactions with minerals. Under anaerobic conditions, pertechnetate is reduced by reducing agents to Tc(IV), which sorbs on minerals, forms insoluble oxides like TcO2, or is structurally incorporated by stable natural minerals. [1]
Previous studies by Yalcintas et al. [2] suggested that Tc(VII) reduction by magnetite resulted in the precipitation and surface adsorption of TcO2-like oligomers at pH 9, i.e. close to the pH of magnetite solubility minimum, while reduction at lower pH of 6 7 resulted in a partial incorporation of Tc(VI) in octahedral Fe sites of magnetite [3]. A working hypothesis was that the incorporation happens only at higher magnetite solubility, while the final retention mechanism remains enigmatic. Thus, our investigations are aimed to carry out a systematic approach covering a wide pH range (3 13), initial Tc concentration ([Tc] = µM-mM) and equilibration time (teq = 1 210 days).
The results show that magnetite removes at least 98 % dissolved Tc. To characterize the molecular geometry of the Tc vicinity, mainly X-ray absorption spectroscopy (XAS) has been used. XANES analysis reveals the predominance of Tc(IV) at all evaluated pH values, supporting that reductive Tc immobilization is the main retention mechanism. A detailed EXAFS analysis with different preparation methods (sorption, coprecipitation, Fe(II)-recrystallization) is currently underway to elucidate the molecular structure of the retained Tc species.
We thank the German Federal Ministry of Economic Affairs and Energy (BMWi) for funding the KRIMI project (02NUK056C).
[1] A.H. Meena, Y. Arai, ENVIRONMENTAL CHEMISTRY LETTERS, 2017, 15, 241.
[2] E. Yalcintas et al., DALTON TRANSACTIONS, 2016, 45, 17874.
[3] T. Kobayashi et al., Radiochimica Acta, 2013, 101, 323.