Reduction of pertechnetate by magnetite - influence of pH and time

Magnetite is arguably the most relevant corrosion product when steel canisters corrode anaerobically in future deep geological radioactive waste repositories. 99Tc is of great concern for the safety assessment of these repositories, due to its long half-life (t1/2 = 2.1∙105 years). Under oxidative conditions Tc forms an anionic species, pertechnetate (TcVIIO4−), which is highly mobile due to its high solubility and weak sorption on most minerals. Under anaerobic conditions, TcVII can be reduced to TcIV, which strongly interacts with minerals by sorption, structural incorporation and formation of insoluble oxides like TcO2. FeII-minerals, and among them magnetite, have shown to effectively reduce pertechnetate and play thereby a critical role.[1]
Previous studies by Yalcintas et al.[2] suggested that TcVII 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. In contrast at pH 6-7, the reduction resulted in a partial incorporation of TcIV in octahedral Fe-sites of magnetite.[3] Our initial working hypothesis was that the incorporation is linked to magnetite solubility. To test this, we investigated Tc reaction with magnetite nanoparticles in a wide range of pH (3 - 13), reaction time (1 - 210 days) and varied initial Tc concentration (μM - mM).
To characterize the oxidation state of Tc and its molecular structure, we employed a range of methods including Tc K-edge (21 044 eV) X-ray absorption spectroscopy (XAS) at the Rossendorf Beamline at the ESRF in Grenoble. XANES analysis revealed the predominance of TcIV at all evaluated pH values, supporting that reductive Tc immobilization is the main retention mechanism. EXAFS analysis suggests that two species are formed. At pH 5 and short equilibration times, Tc is retained by forming TcO2∙xH2O polymers, showing the recently reported “zig-zag-chain” structure[4]. In contrast, TcIV substituted magnetite forms within a few hours at pH 7 and 10. At pH 5, it forms only after a few days with the proportion of the Tc-magnetite phase growing at the expense of the TcO2∙xH2O polymers. Therefore, across the pH range 5 to 10, Tc-magnetite seems to be the thermodynamically preferred phase. The initial formation of TcO2∙xH2O polymers at low pH seems to be linked to the release of Fe2+ from magnetite (reductive maghemitization)[5], which leads to substantial Fe2+ concentrations in solution due to the lacking re-adsorption at this pH. The mechanism behind the fast incorporation of TcIV in magnetite, now observed for the first time within days across a wide pH range, remains unsolved. The structural stability of cubic spinel magnetite and the absence of strong evidence for structural uptake of other elements with similar coordination number and ionic
Fig. 1: Schematic process of the interaction of pertechnetate (TcVIIO4−) with magnetite radius like Co, Ni and other 3d metals, point to a complex coupled sorption/redox/incorporation mechanism.
In order to better understand the Tc incorporation into by magnetite, we also conducted density functional theory (DFT) calculations using the PBE[6] functional implemented in AMS/BAND 2022[7], with full optimization of atomic coordinates and lattice parameters. For the finite structures, solvent effects were included with the COSMO[8]. Three charge compensation mechanisms were considered. The calculated incorporation energies and comparison of the Tc coordination structure with experimental EXAFS data indicate that Tc incorporation is most likely to occur either by substitution of two octahedral FeIII sites with a TcIV/FeII pair (Fig. 2, left), or by substitution of two FeII sites with a single TcIV, thus forming a vacancy (Fig. 2, right). The last mechanism, where three TcIV replace four octahedral FeIII (Fig. 2, center), could be excluded.