Chemistry of the Interaction and Retention of TcVII and TcIV Species at the Fe3O4(001) Surface

The pertechnetate ion TcVIIO4– is a nuclear fission product whose major issue is the high mobility in the environment. Experimentally, it is well known that Fe3O4 can reduce TcVIIO4– to TcIV 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 TcVIIO4– and TcIV species at the Fe3O4(001) surface through a hybrid DFT functional (HSE06) method. We studied a possible initiation step of the TcVII reduction process. The interaction of the TcVIIO4– ion with the magnetite surface leads to the formation of a reduced TcVI species without any change in the Tc coordination sphere through an electron transfer that is favored by the magnetite surfaces with a higher FeII content. Furthermore, we explored various model structures for the immobilized TcIV final products. TcIV can be incorporated into a subsurface octahedral site or adsorbed on the surface in the form of TcIVO2·xH2O chains. We propose and discuss three model structures for the adsorbed TcIVO2·2H2O chains in terms of relative energies and simulated EXAFS spectra. Our results suggest that the periodicity of the Fe3O4(001) surface matches that of the TcO2·2H2O chains. The EXAFS analysis suggests that, in experiments, TcO2·xH2O chains were probably not formed as an inner-shell adsorption complex with the Fe3O4(001) surface.


INTRODUCTION
Technetium is a major concern due to its radiotoxicity, high fission yield in nuclear reactors, long half-life, and long mobility in the environment. The β-emitting 99 Tc isotope is especially concerning. With a formation yield of ca. 6% in both 235 U and 239 Pu nuclear reactors and a half-life of ca. 2.1 × 10 5 years, 1 99 Tc will be the main radiation emitter 10 4 −10 6 years after the production of the nuclear fuel waste.
In the absence of complexing agents besides oxygen and water, technetium assumes VII and IV oxidation states. 2 In oxidizing conditions, Tc VII is preferred and forms the pertechnetate ion (Tc VII O 4 − ), which is highly soluble and mobile in the environment due to its weak interaction with mineral surfaces. 3 On the other hand, in nonoxidizing conditions, technetium is reduced to Tc IV , precipitating as Tc IV O 2 ·xH 2 O or forming adsorption complexes with mineral phases containing Fe II , which participate in the Tc VII reduction. 4,5 Magnetite (Fe II Fe 2 III O 4 ) plays an important role in the immobilization of technetium in nuclear waste. In a typical geological nuclear waste repository, the spent nuclear fuel is enclosed in steel containers, which are then deposited in stable geological sites hundreds of meters below the surface; once full, the repository is sealed with bentonite clay and cement. 6 In such an environment, magnetite forms as one of the main products of the anoxic corrosion of steel containers. 7 It has been demonstrated that Fe II in solid phases can quickly reduce Tc VII O 4 − to Tc IV species, 4,5 whereas Fe III solid phases can adsorb and incorporate Tc IV , 2,8 hence the importance of magnetite in preventing the diffusion of Tc into the environment. It has been shown that Tc IV remains adsorbed on or incorporated in the oxidized magnetite. 2,8 However, the exact structure of the redox products has not been completely elucidated and is affected by several factors, such as pH, initial Tc concentration, and redox conditions of the aqueous phase, among others.
In 2016, Yalcintaşet al., 9 based on X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) measurements, found that the end product of TcO 4 − reduction by magnetite is related to the initial Tc content in solution, with higher concentrations (2 × 10 −4 M) favoring adsorption of dimeric Tc IV oxides onto the magnetite surface and lower concentrations (2 × 10 −5 M) favoring incorporation of Tc IV into the magnetite lattice. A gradual transition from exclusively adsorbed to exclusively incorporated Tc was also observed with decreasing Tc concentration. On the other hand, when using mackinawite (FeS) instead of magnetite, Yalcintaşet al. 9 obtained noncrystalline TcO 2 ·xH 2 O precipitates, for which two distinct linear chains of edge-sharing TcO 6 octahedra with the H 2 O groups at the trans positions could be fitted to the EXAFS spectra; in the first structure, the Tc atoms are equally spaced along the chains (as proposed by Lukens et al.), 10 whereas in the second, the Tc−Tc distances alternate between shorter and longer values, as in the TcO 2 crystal structure. 11 In 2022, Oliveira et al. 12 used density functional theory (DFT) calculations and EXAFS data to show that the precipitates are more likely formed by zigzag chains with terminal H 2 O at cis positions. Thus, it is clear that the interpretation of EXAFS spectra for these Tc systems is rather complex and can benefit from the aid of quantum chemical calculations.
In this work, we use a hybrid DFT functional method to explore the interaction of various Tc species with magnetite, starting from the adsorption of Tc VII O 4 − onto the Fe 3 O 4 (001) surface and proceeding with possible products of the full Tc VII O 4 − reduction, namely, Tc IV incorporated into the magnetite lattice and adsorbed Tc IV O 2 ·2H 2 O chains. The Tc VII O 4 − adsorption is analyzed in terms of spin densities, charges, and electronic density of states, whereas the structures of the Tc IV species are discussed in terms of relative energies and simulated EXAFS spectra.

METHODS AND MODELS
2.1. Computational Methods. All DFT calculations were carried out with the HSE06 13,14 hybrid exchange−correlation functional using the CRYSTAL17 package. 15,16 This methodology has been shown to give a good description of the structural, electronic, and magnetic properties of magnetite systems. 17 The Kohn−Sham orbitals were expanded in Gaussian-type orbitals: the all-electron basis sets are H| 511G(p1), O|8411G(d1), Fe|86411G(d41), and Tc|976311-(d631f1) according to the scheme previously used for Fe 3 O 4 . 17−20 The irreducible Brillouin zone was sampled with a 3 × 3 × 1 k-point grid generated with the Monkhorst−Pack scheme. 21 The convergence criterion of 4.5 × 10 −4 hartree/ bohr for atomic force was used during geometry optimization, and the convergence criterion for total energy was set to 10 −6 hartree for all of the calculations. All structures (see details below) were constructed in such a way as to keep inversion symmetry (e.g., by adding adsorbate molecules in specific locations above and below the slab models) in order to minimize the appearance of artificial dipole moments.
The EXAFS spectra were simulated for optimized structures with FEFF9.6.4 22−24 using the self-consistent field mode with a global Debye−Waller factor of 0.003 Å, amplitude reduction factor of 0.9, and ΔE 0 = 0.
2.2. Models of the Fe 3 O 4 (001) Surface. The (001) termination is one of the most stable magnetite surfaces. 25,26 Under the alkaline conditions of geological repositories, 8 it is expected to be one of the most exposed surfaces in nanostructures, 27,28 according to the Wulff construction. 29 For these reasons, we have used this surface in our model. In the [001] direction, the Fe 3 O 4 consists of alternating planes containing tetrahedral iron (Fe Tet ) atoms and octahedral iron (Fe Oct ) coordinated to oxygen atoms. The most recent and reliable models for the (001) termination are based on a bulk truncation at the Fe Oct and O plane. The distorted bulk truncation (DBT) model consists of a simple bulk truncation, 30 whereas the subsurface cation vacancy (SCV) model shows a reconstruction that consists of an extra interstitial Fe Tet atom in the second layer replacing two Fe Oct atoms from the third layer (per the (√2 × √2)R45°unit cell). 31 The DBT and SCV models for the Fe 3 O 4 (001) surface are shown in Figure S1 in the Supporting Information. Their relative stability is highly dependent on the concentration of adsorbing molecules in the environment: an increasing amount of carboxylic acid or water molecules adsorbed onto the surface is found to favor the DBT structure. 18,32−34 In this work, both models were constructed as a (1 × 1) 17-layer slab with inversion symmetry, as previously done by some of us. 18,35 The five layers in the middle of the slab were kept fixed at the bulk positions, whereas the other layers were free to relax.

Models for the Adsorption of TcO 4
n− onto the Fe 3 O 4 (001) Surface. Different complexes were constructed by adsorbing or embedding TcO 4 n− into different sites of the DBT and SCV surface models. The SCV surface being more oxidized (fewer Fe II centers) than the DBT one, it is interesting to compare the reducing power of both surface models. Since the DBT and SCV surfaces have identical terminating layers, exposing four penta-coordinated Fe Oct atoms per unit cell, the models were built with the same criteria. The coordination shell of the superficial undercoordinated Fe Oct atoms was saturated with either molecular or dissociated water, based on experimental and computational results. 18,35,36 To balance the total charge, the most reactive superficial oxygen atoms were decorated with a proper number of hydrogen atoms. 37−39 All structures were optimized, and, for each surface, the two lowest energy structures were selected for further analysis. Here, only the models associated to the two lowest energy structures are described in detail. The first model was built by adsorbing a TcO 4 species onto two superficial undercoordinated Fe Oct atoms, a H 2 O molecule, and an OH group onto the two remaining superficial undercoordinated Fe Oct atoms. Two superficial oxygen atoms were decorated with two hydrogen atoms. The second model was built by attaching a TcO 2 fragment to two superficial oxygen atoms, forming a TcO 4 species embedded into the surface. The four superficial undercoordinated Fe Oct atoms were saturated by one H 2 O molecule and three OH groups. No superficial oxygen atoms were decorated with hydrogen atoms. These two models have the same number of atoms for each element.

Models for the Incorporation of Tc IV into the Fe 3 O 4 (001) Surface.
Four models of Tc IV incorporation were considered, one based on the SCV surface and three on the DBT surface. In all of them, an Fe Oct atom from the third layer was replaced by a Tc atom. In two of the DBT-based models, an Fe vacancy was created in addition to the Tc-for-Fe substitution (i.e., two Fe Oct atoms were replaced with one Tc and one vacancy). Several DBT-based models were created with the Fe vacancy at different positions with respect to the Tc atom, but only the two structures with the lowest energies were selected for further analysis. We considered both SCV and DBT surface models because (i) the SCV, being more oxidized than the DBT and characterized by the presence of iron vacancies, bears stronger resemblance to maghemite, which is expected to be one of the main final products of the magnetite oxidation by TcO 4 − , and (ii) recent experimental and theoretical findings show that under certain circumstances, the diffusion of other transition-metal atoms could reverse the SCV reconstruction, restoring a DBT surface, which presents the diffusing transition-metal atom (Tc in this case) instead of Fe occupying an octahedral site in the third layer. 40−42 The Journal of Physical Chemistry C pubs.acs.org/JPCC Article 2.5. Models for the Adsorption of TcO 2 ·2H 2 O Chains onto the Fe 3 O 4 (001) Surface. Three models were considered for the TcO 2 ·2H 2 O infinite chains, based on the work by Oliveira et al. 12 Each chain consists of edge-sharing TcO 6 octahedra with terminal H 2 O groups occupying two corner positions. In α-TcO 2 ·2H 2 O, the TcO 6 octahedra form a linear chain with the terminal H 2 O in trans configuration and Tc−Tc nearest neighbors alternating longer and shorter distances along the chain as in the TcO 2 (P2 1 /c) crystal structure. 11 In β-TcO 2 ·2H 2 O, the TcO 6 octahedra form a zigzag chain, similar to ReO 2 (Pbcn) 43 �note that Re is regarded as a Tc analogue�with the H 2 O groups at cis positions and identical distances for the Tc−Tc nearest neighbors. The last model, γ-TcO 2 ·2H 2 O, differs from α-TcO 2 ·2H 2 O for having identical Tc−Tc nearest distances along the chain, as in ReO 2 (P4 2 /mnm). 44 Oliveira et al. 12 found β-TcO 2 ·H 2 O to be the most energetically favored structure, with γ-TcO 2 ·2H 2 O being the least favored.
The adsorption complexes were constructed by removing the H 2 O groups from one side of the TcO 2 ·2H 2 O chains and placing the resulting structures on the Fe 3 O 4 (001) bare surface at bonding distance. Different models were constructed for each chain to explore different orientations on the surface. All structures were optimized, and the lowest energy structure of adsorbed α, β, and γ chains was used for further analysis. The investigation was restricted to the SCV surface because, being more oxidized than the DBT and characterized by the presence of iron vacancies, it bears stronger resemblance to maghemite, which is expected to be one of the main final products of the magnetite oxidation by TcO 4 − , as already discussed in Section 2.4. Furthermore, the SCV differs from the DBT only in the structure of the second and third layers and in the Fe II /Fe III ratio, and it is reasonable to suppose that these differences do not influence the adsorption properties, especially when no redox reactions involve the Fe II /Fe III pair, as in this case.

Adsorption of TcO 4
n− onto the Fe 3 O 4 (001) Surface. In the first part of this study, we simulated the interaction of TcO 4 n− species with the Fe 3 O 4 (001) surface by considering that the ions may either just adsorb by binding to undercoordinated surface Fe ions or become involved in surface reactivity leading to their surface embedding. The details of the models are described in the Methods and Models section. For both DBT and SCV surfaces, we have selected the two lowest-energy adsorption complexes, shown in Figure 1. In the models reported in the left panels of Figure 1  n− becomes embedded in the surface through two μ 4 -O (i.e., fourfold coordinated oxygen) bridging atoms. This second adsorption site is the same that is generally preferred by single metal atoms adsorbed on the magnetite (001) surface, according to several recent studies. 40,45,46 The (Tc VII O 4 ) − /DBT and (Tc VII O 4 ) − /SCV models are characterized by the presence of technetium in its VII oxidation state. As we can see in the PDOS in Figure 2, there are no Tc 4d states (Figure 2, cyan curve) in the valence band. All technetium 4d orbitals are located in the conduction band. Furthermore, Tc is characterized by almost null spin polarization. These findings are compatible with a Tc VII species, corresponding to the electronic configuration [Kr]. On the other hand, in (Tc VI O 4 ) 2− /DBT and (Tc VI O 4 ) 2− /SCV models, we observe that Tc VII is reduced to Tc VI while one Fe II (in the fifth and in the seventh layer, respectively) of the magnetite surface is oxidized to Fe III . The reduction of Tc VII to Tc VI is in line with the Mulliken charge decrease of 5% and with a new Tc 4d contribution to the valence band in the spindown channel of the PDOS (Figure 2, cyan curve)  more oxidized SCV one; second, the electron transfer from one Fe II center to the Tc VII ion implies the formation of an electric dipole which is smaller on the DBT than on the SCV surface, since the distances between the Tc VII ion and the Fe II centers involved in its reduction to Tc VI are found to be about 6 and 8 Å, respectively.
The reduction of Tc VII to Tc VI by a simple electron transfer (from the magnetite surface to the technetium atom) is likely the first step of a complex redox process, which is known to proceed rapidly, producing Tc IV end-members at slightly alkaline pH. The process involves the oxidation of Fe II close to the Tc adsorbate, changing the Tc geometry from tetrahedral to octahedral 47 48 Given the complexity of the process and the lack of more specific information regarding the chemical species involved, the simulation of the full Tc VII O 4 − reduction is out of the scope of this work. Therefore, we restrict our study to hypothetical final products: incorporation of Tc IV in the magnetite slab and formation of Tc IV O 2 ·xH 2 O chains adsorbed on the magnetite surface.

Incorporation of Tc IV into the Fe 3 O 4 (001) Surface.
In Figure 3, four models for the incorporation of Tc are shown, where we either only replaced a subsurface Fe Oct in the third layer with a substitutional Tc atom (Tc S ) ((Tc S )@DBT) or we also introduced a Fe Oct vacancy (Fe V ) ((Tc S + Fe V 5L )@DBT, (Tc S + Fe V 3L )@DBT, and (Tc S )@SCV). Note that SCV differs by one net Fe vacancy with respect to DBT, as detailed in Section 2.2.
The (Tc S )@DBT model presents a Tc in the IV oxidation state in place of a Fe III , as confirmed by the Mulliken charge value, which is lower than those found for Tc VII and Tc VI , and almost identical to that obtained for Tc in the rutile phase of TcO 2 . The charge balance of the system is achieved by the reduction of a Fe III ion to Fe II , indicated by the decrease of the Mulliken charge (15%) and spin density (from 4.2 to 3.7 μ B ). This incorporation scheme consists of two Fe III ions being replaced with a Tc IV -Fe II pair, as already observed in previous computational studies investigating Tc incorporation in bulk hematite and magnetite. 49,50 Similarly to (Tc S )@DBT, the (Tc S + Fe V 5L )@DBT, (Tc S + Fe V 3L )@DBT, and (Tc S )@SCV models also present Tc in the IV oxidation state. However, in this case, the Mulliken charges indicate that one Tc IV ion replaces two Fe II ions with respect to the pristine DBT surface, keeping the charge neutrality of the system, as previously observed for Tc-doped bulk magnetite. 50 These models resemble what is observed in the oxidation process from magnetite (Fe 3 O 4 ) to maghemite (γ-Fe 2 O 3 ), which have the same structure, but the Fe II ions in magnetite are replaced by Fe III ions and vacancies in maghemite. 51 Among the oxidized systems (namely, (Tc S + Fe V  )@DBT, and (Tc S ) @SCV become less stable. However, no stability inversion is observed down to experimentally feasible low O 2 pressure (ca. 10 −20 atm). In particular, (Tc S )@SCV is the most stable model at all considered values of oxygen chemical potential.

Adsorption of TcO 2 ·2H 2 O Chains onto the Fe 3 O 4 (001)
Surface. An alternative surface reactivity discussed in the literature would lead to the formation of hydrated Tc IV O 2 dimers or chains on the magnetite surface. 8,9,52 To study this possibility, we first investigated a freestanding TcO 2 ·2H 2 O chain, as described in the Methods and Models section. The β-TcO 2 ·2H 2 O chain was found to be the most stable chain, with Tc−Tc and Tc−O distances of ca. 2.4 and 1.9 Å, respectively. The α-TcO 2 ·2H 2 O chain was found to be less stable by +758 meV per Tc atom (as reported in Table  1), with alternating Tc−Tc distances of ca. 2.2 and 3.3 Å.
Consequently, also Tc−O distances present alternating values: 1.9 Å when O is bridging Tc−Tc at a smaller distance and 2.1 Å when bridging the Tc−Tc at a longer separation. The γ-TcO 2 ·2H 2 O transformed to the α chain during the geometry optimization.
As a next step, we investigated the interaction between the TcO 2 ·2H 2 O chains with the magnetite surface. The periodicity of the magnetite surface and, in particular, of the alternating O−O distances along the [1̅ 10] direction, matches that of the α-TcO 2 ·2H 2 O chain. The adsorbed α-TcO 2 ·2H 2 O chain ( Figure 5, α-TcO 2 ·2H 2 O/SCV) presents only one kind of Tc ( Figure 5, red beads) that is six-coordinated by four O from the chain itself ( Figure 5, blue beads), one O shared with magnetite, and one O from a water molecule ( Figure 5, white beads). Half of the O bridges in the chain (indicated with a yellow star in Figure 5) interacts with exposed undercoordinated Fe ( Figure 5, green beads). The adsorption is driven by two types of interactions: one between Tc IV and magnetite O, and the other between surface Fe III and O belonging to TcO 2 ·2H 2 O chains. α-TcO 2 ·2H 2 O/SCV presents different alternating Tc−Tc distances with respect to the freestanding chain: ca. 2.8 and 3.1 Å versus 2.2 and 3.3 Å. This )@DBT, and (Tc S )@SCV). In the DBT cases, the dashed circle indicates the Fe V , whose location among the layers is given by the 3L (third layer) and 5L (fifth layer) apexes. The white, green, and red beads represent O, Fe, and Tc, respectively. The black arrows indicate the crystallographic directions.   Figure 5, γ-TcO 2 ·2H 2 O/SCV) presents only one kind of Tc, whose coordination sphere is analogous to the one in α-TcO 2 ·2H 2 O/SCV. Still, in analogy to α-TcO 2 ·2H 2 O/ SCV, half of the O atoms in the chain are interacting with superficial undercoordinated Fe. These structural similarities are translated into comparable energies: the total energy difference between the adsorbed γ and α chain is only 16 meV per Tc atom (as reported in Table 1), in favor of the former. The free-standing β chain has a shorter lattice parameter than the α and γ chains due to its zigzag configuration. Consequently, it is not possible to efficiently adsorb the β-TcO 2 ·2H 2 O along the diagonal direction of the cell as previously done for the α and γ one. Therefore, we studied the adsorption of the β-TcO 2 ·2H 2 O along the [100] direction ( Figure 5, β-TcO 2 ·2H 2 O/SCV). In this case, two different kinds of Tc are present: one farther from the surface and one closer to it, labeled as Tc up and Tc down in Figure 5, respectively. Both Tc up and Tc down species are six-coordinated. Each Tc up is coordinated by four O from the chain itself and by two O from two different water molecules, whereas each Tc down is coordinated by four O from the chain itself and by two O shared with magnetite ( Figure 5, dashed lines). Tc−Tc distances and other structural parameters of the adsorbed chain are not significantly different from what is observed for the free-standing chain. This configuration is found to be less favored than the α and γ one by +327 and +343 meV per Tc atom (see Table 1), respectively. This is an unexpected result, since the β-TcO 2 ·2H 2 O chain is the most stable one in vacuum. This finding can be understood in terms of the (i) lower number (half compared to α and γ cases) of chain O atoms interacting with the magnetite surface and (ii) weaker Tc−O interactions (2.3−2.4 versus 1.9−2.0 Å for the α and γ chains) between the chain and the magnetite surface.
Finally, we compared the experimental EXAFS spectra for the sorption complex by Yalcintaşet al. 9 (Figure 6, black dashed curve) and for the aged TcO 2 ·xH 2 O precipitate by Oliveira et al. 12 (Figure 6, black dotted curve) with the calculated EXAFS spectra obtained for the simulated TcO 2 · 2H 2 O chains adsorbed on the magnetite (001) surface just described. Regarding the experimental sorption complex curve, there is no match with the calculated curves of the simulated TcO 2 ·2H 2 O chains models. We also modeled a magnetite/ TcO 2 -dimer complex (shown in Figure S2 in the Supporting Information) in line with that suggested by Yalcintaşand collaborators. 9 However, also in this case, the simulated EXAFS spectrum does not match the experimental one for the  sorption complex. These results suggest that TcO 2 chains (or dimers) are probably not formed as an inner-shell adsorption complex with the Fe 3 O 4 (001) surface, at least not immediately. Regarding the aged TcO 2 ·xH 2 O precipitate curve, there are few similarities with the calculated β-TcO 2 ·2H 2 O/SCV curve ( Figure 6, blue curve). In particular, the positions of the first and second peaks are in fair agreement, as well as the presence of a small shoulder on the right of the second peak, but the third peak in the computed curve finds no correspondence in the experimental one. This result suggests that β-TcO 2 ·2H 2 O chains might be formed in solution, not as an inner-shell adsorption complex with magnetite, and only afterward might precipitate and adsorb on the surface. Indeed, the formation of β-TcO 2 ·2H 2 O chains is energetically favored over that of the α and γ ones in vacuum, not on the Fe 3 O 4 (001) surface. However, the agreement between the experimental aged precipitate curve and the computed β-TcO 2 ·2H 2 O/SCV one is not good enough to definitively sustain this hypothesis. Therefore, the comparison between the experimental and the calculated data suggests the possibility that in the experiments, more radically modified and reconstructed Fe 3 O 4 (001) surfaces or even completely different surfaces, such as the (111) and (110), not considered in this work, might be involved.

CONCLUSIONS
In this work, based on a comprehensive hybrid DFT study, we investigated the chemistry of the interaction and retention of Tc VII and Tc IV species at the Fe 3 O 4 (001) surface.
As a first step, we studied the interaction and reactivity of the Tc VII O 4 − ion with the magnetite surface. We suggest a possible initiation step for the reduction of Tc VII to Tc IV upon contact with the Fe 3 O 4 (001) surface. The adsorption of the Tc VII O 4 − ion onto 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 possible final products of the full reduction from Tc VII to Tc IV at the Fe 3 O 4 (001) surface: Tc IV incorporation or adsorption in the form of Tc IV O 2 ·2H 2 O chains. The replacement of a Fe atom with a Tc atom in an octahedral site in the subsurface leads to the presence of an incorporated six-coordinated Tc IV , which is more stable in the SCV than in the DBT surface. Regarding the adsorption of TcO 2 ·2H 2 O chains on magnetite, we propose three model structures that are characterized by three different symmetries. The periodicity of the Fe 3 O 4 (001) surface matches that of the TcO 2 ·2H 2 O chains, and the adsorption is driven by two types of interactions: one between Tc IV and magnetite O, and the other between surface Fe III and O belonging to TcO 2 ·2H 2 O chains. However, the comparison between the experimental and computed EXAFS spectra 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.
To summarize, we have demonstrated that the Fe 3 O 4 (001) surface can adsorb and reduce Tc VII complexes and retain Tc IV species. In particular, we propose an initiation step for the reduction of Tc VII and two retention mechanisms, i.e., Tc IV ions incorporation into octahedral subsurface sites and adsorption in the form of TcO 2 ·2H 2 O chains. Our results furnish a solid basis for any future study whose aim is to elucidate the steps of the complex reduction of Tc VII to Tc IV and, on the basis of the EXAFS analysis, could stimulate further investigations to understand whether the formation of TcO 2 ·xH 2 O chains could take place in solution or even at other Fe 3