Abstract
The pertechnetate ion Tc(VII)O(4) (-) is a nuclear fission product whose major issue is the high mobility in the environment. Experimentally, it is well known that Fe(3)O(4) can reduce Tc(VII)O(4) (-) to Tc(IV) species and retain such products quickly and completely, but the exact nature of the redox process and products is not completely understood. Therefore, we investigated the chemistry of Tc(VII)O(4) (-) and Tc(IV) species at the Fe(3)O(4)(001) surface through a hybrid DFT functional (HSE06) method. We studied a possible initiation step of the Tc(VII) reduction process. The interaction of the Tc(VII)O(4) (-) ion with the magnetite surface leads to the formation of a reduced Tc(VI) species without any change in the Tc coordination sphere through an electron transfer that is favored by the magnetite surfaces with a higher Fe(II) content. Furthermore, we explored various model structures for the immobilized Tc(IV) final products. Tc(IV) can be incorporated into a subsurface octahedral site or adsorbed on the surface in the form of Tc(IV)O(2)·xH(2)O chains. We propose and discuss three model structures for the adsorbed Tc(IV)O(2)·2H(2)O chains in terms of relative energies and simulated EXAFS spectra. Our results suggest that the periodicity of the Fe(3)O(4)(001) surface matches that of the TcO(2)·2H(2)O chains. The EXAFS analysis suggests that, in experiments, TcO(2)·xH(2)O chains were probably not formed as an inner-shell adsorption complex with the Fe(3)O(4)(001) surface.