Sorption and interfacial redox of Sn(II) under anoxic conditions: Magnetite vs. anatase


Sorption and interfacial redox of Sn(II) under anoxic conditions: Magnetite vs. anatase

Dulnee, S.; Banerjee, D.; Rossberg, A.; Scheinost, A. C.

The long-lived fission product 126Sn is of substantial interest in the context of nuclear waste deposition in deep underground repositories. However, the redox state (di- or tetravalent) under the expected anoxic conditions is still a matter of debate. We therefore investigated the stability of Sn(II) in the presence of a highly redox-reactive mineral, magnetite (FeIIFeIII2O4), in comparison to a non-redox-reactive, anatase (TiO2).
Sorption experiments were performed at < 2 ppm O2, and redox state and local structure was monitored over time by X-ray absorption spectroscopy (XAS).
We found a rapid (< 30 min) oxidation of Sn(II) to Sn(IV) in the presence of magnetite. Although solubility calculation predicted the precipitation of SnO2, 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 inner-sphere complexation. The structure of the complex remained largely unchanged up to an equilibration time of 1 month.
After 30 min reaction with anatase, Sn(II) was conserved. However, even with the redox-inert anatase, Sn(II) oxidized to Sn(IV) over time, forming an Sn(IV) inner-sphere complex with Sn-Ti distances at 3.24 and 3.53 Å. Therefore, our results clearly indicate that Sn(IV) is the most relevant oxidation state to be considered even under reducing conditions, and that inner-sphere complexation is a relevant retention mechanism.

Keywords: Sn; Sorption; Magnetite; Anatase; Redox; XAFS; Molecular structure

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