In the near-field of a deep underground high-level radioactive waste repository the production of H2 and the corrosion of iron canisters will release large amounts of ferrous iron (Fe(II)) into the interstitial porewater of the engineered barrier (bentonite) as well as into the argillaceous host rock. Fe(II) will compete with radionuclides for sorption sites, but will also influence the redox state of the radionuclides through various associated redox reactions. In this study the uptake of Fe2+ by several montmorillonites was investigated by batch sorption experiments, surface complexation modelling, Moessbauer and X-ray absorption spectroscopy (XAS). We used four montmorillonite samples, a synthetic montmorillonite with no structural Fe(III), two natural montmorillonites with 0.5 wt.% (STx) and 2.9 wt.% (SWy) structural Fe, and a dithionite-treated SWy where all structural Fe(III) was reduced to Fe(II). All sorption experiments were carried out under either anoxic (O2 <0.1 ppm) or electrochemically controlled reducing conditions. The Fe(II) sorption behaviour on the Fe-free, Fe-low (STx) and dithionite treated SWy is similar and comparable to that of other divalent metals, e.g. Zn(II), and could be successfully modelled with the two-site protolysis non-electrostatic surface complexation and cation exchange (2SPNE SC/CE) sorption model [1]. In contrast, SWy with 2.9 wt.% structural Fe, sorbed considerably higher amounts of Fe(II), and the Fe(II) uptake could be reproduced only after introducing an additional surface reaction involving an electron transfer to the surface-bound Fe(II). Moessbauer and XAS spectroscopy confirmedthe oxidation of the surface-bound Fe(II) to Fe(III) on SWy. At low Fe-loading, Fe(III) retains its local structure as inner-sphere sorption complex whereas at higher loadings a Fe-(hydr)oxides-like precipitate was formed like observed in [2]. Fe(II) sorbed on the Fe-low and dithionite-reduced clays is predominantly present as Fe(II).The outcome contributes to an improved understanding of the role of iron-bearing clay minerals in the uptake mechanism of redox-sensitive contaminants in natural and engineered systems. 1. Bradbury, M.H. and B. Baeyens, A mechanistic description of Ni and Zn sorption on Na- montmorillonite. 2. Modelling. Journal of Contaminant and Hydrology., 1997. 27 (3-4): p. 223-248.2. Schaefer, M. V.; Gorski, C. A.; Scherer, M. M. Spectroscopic Evidence for Interfacial Fe(II)-Fe(III) Electron Transfer in a Clay Mineral. Environmental Science & Technology 2011,  45 (2), 540-545.

Fe(II) uptake on Montmorillonites: Surface complexation and oxidation

248th National Meeting (2014)

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