Chemistry of the Interaction and Retention of Tc VII and Tc IV Species at the Fe 3 O 4 (001) Surface.

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 · x H 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 · x H 2 O chains were probably not formed as an inner-shell adsorption complex with the Fe 3 O 4 (001) surface.