Sorption and redox reactions on Sn II and Sn IV at the magnetite/water interface in presence and absence of organic ligands
Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Final Report Abstract
The long-lived fission product 126Sn (105 years) 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 conditions remain dubious. Therefore, we investigated the reactions of Sn" with three Fe-bearing minerals as a function of pH, time, and Sn" loading under anoxic condition with O2 level < 2 ppmv. The uptake behavior of the three mineral surfaces was investigated by batch sorption studies. Tin redox state was investigated by Sn-K X-ray absorption near-edge structure (XANES) spectroscopy, and the local, molecular structure of the expected Sn surface complexes was studied by extended X-ray absorption fine-structure (EXAFS) spectroscopy. Selected samples were also investigated by transmission electron microscopy (TEM) to elucidate the existence and nature of secondary, Fe- and /or Sn containing solids, and by Mössbauer spectroscopy to study Fe" and Fem in the minerals. Based on the such-obtained molecular-level information, surface complexation models (SCM) were fitted to the batch sorption data to derive surface complexation constants. In the presence of the FeIII-bearing minerals magnetite and goethite, Sn" was rapidly removed from solution and oxidized to SnIV The local structure determined by EXAFS showed formation of a tetradentate inner-sphere complexes between pH 3 and 9 for magnetite, and single edge-sharing (1E) and corner-sharing (2C)) complexes at the goethite surface at pH > 3, with the relative amount of 2C increasing with Sn loading. Rietveld refinement of XRD patterns showed an increase of Fen/FeIn ratio in the magnetite structure. For the Sn/goethite system, dissolved Fe" increased with Sn" loading at the lowest pH investigated, indicative of reductive dissolution. At pH >5, spherical and cubic particles of magnetite were observed by TEM, and their number increased with Sn" loading. The sorption data on magnetite were fit with the diffuse double layer model (DLM) employing two different complexes, the first, -14.97±0.35 prevailing from pH 2 to 9, and the second, (Mag sO)4SnIV(OH)2Fe with a logK210 of -17.72±0.50, which forms at pH > 9 by co-adsorption of FeII, thereby increasing sorption at this high pH. The sorption data on goethite were fitted with the charge distribution-multisite complexation model (CD-MUSIC). Based on the EXAFS-derived presence of two different bidentate inner-sphere complexes ((=FeOH)(=Fe3O)Sn(OH)3 (1E) and (EFeOH)2Sn(OH)3) (2C)), sorption affinity constants of 15.5 ±1.4 for the 1E complex and of 19.2 ±0.6 for the 2C complex were obtained. The model is not only able to predict sorption across the observed pH range, but also the transition from a 50/50 distribution of the two complexes at 12.5 gmol/g Sn loading, to the prevalence of the 2C complex at higher loading, in line with the EXAFS data. The retention mechanism of SnII by mackinawite is significantly dependent on the solution pH, reflecting the transient changes of the mackinawite surface in the sorption process. At pH <7, SnII is retained in its original oxidation state, coordinated to four S atoms, and forms a bidentate innersphere surface complex at the mackinawite surface. At pH > 9, Sn" is completely oxidized by and incorporated into an Fen/Fem (hydr)oxide, most likely green rust, forming on the surface of mackinawite. The transition between Sn" and SnIV and between sulfur and oxygen coordination takes place between pH 7 and 8, in accordance with the transition from the mackinawite stability field to more oxidized Fe-bearing minerals. The uptake processes of SnII by mackinawite are largely in line with the uptake processes of divalent cations of other soft Lewis-acid metals like Cd, Hg and Pb. In all three mineral systems and largely independent on the retention mechanisms, inorganic Sn" was strongly retained, with Rd values always exceeding 5, across the relatively wide pH range relevant for the near and far-field of nuclear waste respositories. For the goethite and magnetite systems, the retention could be well modeled with surface complexation models based on the molecular structural data. This is an important contribution to the safety case for future nuclear waste repositories, since such SCMs provide reliable means for predicting the radioactive dose released by 126Sn from nuclear waste into the biosphere across a wide range of physicochemical conditions typical for the engineered as well as natural barriers.
Publications
- The 21nd V.M. Goldschmidt Conference, 14.-19.08.2011, Prague, Czech Republic: Sorption and interfacial redox of Sn(II) under anoxic conditions: Magnetite vs. anatase
Dulnee, S.; Banerjee, D.; Rossberg, A.; Scheinost, A. C.
- Lecture at The 22nd V.M. Goldschmidt Conference, 24.-29.06.2012, Montreal, Canada: Tin sorption to magnetite nanoparticles under anoxic conditions
Dulnee, S.; Banerjee, D.; Rossberg, A.; Scheinost, A. C.
- 2013. Surface complexation and oxidation of Sn" by nanomagnetite. Environ. Sci. Technol. 47, 12852–12859
Dulnee, S., Banerjee, D., Merkel, B. J., and Scheinost, A. C.
(See online at https://doi.org/10.1021/es402962j) - 2014. Surface Reaction of Sn(II) on Goethite (alpha-FeOOH): Surface Complexation, Redox Reaction, Reductive Dissolution, and Phase Transformation. Environ. Sci. Technol. 48, 9341-8
Dulnee, S. and Scheinost, A. C.
(See online at https://doi.org/10.1021/es501923z) - Interfacial Reaction of SnII on Mackinawite (FeS). Journal of Contaminant Hydrology, 2015. Volumes 177–178, June–July 2015, Pages 183-193
Dulnee, Siriwan and Scheinost, Andreas C.
(See online at https://doi.org/10.1016/j.jconhyd.2015.03.012)
