Surface Complexation and Oxidation of Sn^II by Nanomagnetite

Abstract

The long-lived fission product 126Sn is of substantial interest in the context of nuclear waste disposal in deep underground repositories. However, the prevalent redox state, the aqueous speciation as well as the reactions at the mineral-water interface under the expected anoxic and reducing conditions are a matter of debate. We therefore investigated the reaction of Sn(II) with a relevant redox-reactive mineral, magnetite (Fe(II)Fe(III)2O4) at <2 ppmv O2, and monitored Sn uptake as a function of pH and time. Tin redox state and local structure were investigated by Sn–K X-ray absorption spectroscopy (XAS). We observed a rapid uptake (<30 min) and oxidation of Sn(II) to Sn(IV) by magnetite. The local structure determined by XAS showed two Sn–Fe distances of about 3.15 and 3.60 Å in line with edge and corner sharing arrangements between octahedrally coordinated Sn(IV) and the magnetite surface, indicative of formation of tetradentate inner-sphere complexes between pH 3 and 9. Based on the EXAFS-derived surface structure, we could successfully model the sorption data with two different complexes, (Magn_sO)4Sn(IV)(OH)2–2 (logK(2,0)(–2) −14.97 ± 0.35) prevailing from pH 2 to 9, and (Magn_sO)4Sn(IV)(OH)2Fe (logK(2,1)(0) −17.72 ± 0.50), which forms at pH > 9 by coadsorption of Fe(II), thereby increasing sorption at this high pH.

Phase Dynamics on Conversion-Reaction-Based Tin-Doped Ferrite Anode for Next-Generation Lithium Batteries

“…34 In the Sn K-edge XANES spectra of Figure 1e, the oxidation state of tin in the Fe 2.76 Sn 0.24 O 4 /rGO and Fe 2.64 Sn 0.36 O 4 /rGO composites possesses a similar oxidation state of Sn 4+ . 23 The peaks between 0.5 and 2 Å and 2 and 4 Å of all the composites in Figure 1f the typical feature of SnO 2 , in the Fe 2.76 Sn 0.24 O 4 /rGO and Fe 2.64 Sn 0.36 O 4 /rGO composites, the tin is coordinated by six oxygen atoms at the first coordination shell, 39,40 which indicates that the tin atoms in the Fe 3−x Sn x O 4 composite adopt the octahedral site in the inverse cubic spinel structure and also demonstrates that the substitution of Fe 3+ by Sn 4+ is achieved at the octahedral site. 39,40 Considering the occupancy of the octahedral and tetrahedral site (as noted above, all of the Fe 2+ and half of Fe 3+ , and the remaining Fe 3+ is located at the octahedral and tetrahedral site, respectively) and the size of the ionic radius (as noted above, 0.78 and 0.65 Å for Fe 2+ and of Fe 3+ , respectively), the octahedral site is more voluminous than the tetrahedral site, and, as a consequence, it is occupied by the Sn 4+ .…”

Plutonium Retention Mechanisms by Magnetite under Anoxic Conditions: Entrapment versus Sorption

“…The release of Fe­(II) from hydrogen-driven steel corrosion and subsequent precipitation of magnetite may create a highly dynamic system where one could envision that Pu is incorporated by magnetite following either a coprecipitation process or by overgrowth of an already existing magnetite surface with adsorbed Pu. Such a process has been observed, for instance, for Tc­(IV) and Sn­(IV), two cations with ionic radii (0.65 and 0.69 Å, respectively) similar to those of Fe­(II) or Fe­(III) (0.78 and 0.65 Å, respectively), which therefore can easily replace Fe in octahedral sites. ,, More surprisingly, from a crystal-chemical point of view, this has also been demonstrated for the larger actinide U. − In this case, U is preferentially incorporated in its pentavalent oxidation state, by adopting an uranate-type local structure characterized by relaxation of the multiply bonded “-yl” oxygen ligands.…”

“…Such a process has been observed, for instance, for Tc(IV) and Sn(IV), two cations with ionic radii (0.65 and 0.69 Å, respectively) similar to those of Fe(II) or Fe(III) (0.78 and 0.65 Å, respectively), which therefore can easily replace Fe in octahedral sites. 32,37,38 More surprisingly, from a crystal-chemical point of view, this has also been demonstrated for the larger actinide U. 39−41 In this case, U is preferentially incorporated in its pentavalent oxidation state, by adopting an uranate-type local structure characterized by relaxation of the multiply bonded "-yl" oxygen ligands.…”

Surface Reaction of SnII on Goethite (α-FeOOH): Surface Complexation, Redox Reaction, Reductive Dissolution, and Phase Transformation

“…When looking for the elemental distribution of Sn with TEM, one finds that Sn is predominately associated with goethite and only to a smaller part with the newly formed magnetite nanoparticles. This suggests that Sn remains bound to goethite after oxidation, in line with the EXAFS spectra, which show neither the structure of the tetradentate sorption complex at the surface of magnetite nor Sn IV structurally incorporated by magnetite (Figures and ) . Therefore, there is no substantial re-distribution of sorbed Sn from goethite to magnetite within the given reaction time.…”

“…Previous studies have shown that Fe minerals with a low band gap such as magnetite efficiently catalyze redox reactions at their surface . This has been confirmed for Sn II , which is rapidly oxidized at the surface of magnetite particles to Sn IV , which forms a tetradentate inner-sphere sorption complex . The formation of this complex can be adequately predicted with a double-layer surface complexation model.…”