Retention of redox-sensitive Tc(VII) and Se(IV) on Fe(II)/Fe(III) bearing clay minerals

Clay minerals, as main components of the engineered barrier in deep geological repositories, are not only highly effective sorbents for a wide range of (cationic) contaminants, but due to their structural iron content, they can also modify the mobility and (bio)availability of redox-sensitive elements by changing their oxidation state. Here we will systematically address the retention of the redox-sensitive fission products Tc and Se by FeII/FeIII containing clays. The mobility of Tc and Se strongly depends on their oxidation states. The specific chemical form of Tc and Se is governed by many factors, such as pH, redox potential (Eh), solubility, mineralogical and chemical composition of the environment, biological (microbial) interactions and (redox) reaction kinetics. The most considerable influence, however, is exerted by redox potential, pH and solubility.
Tc(VII) in its anionic form (TcO4-) is highly mobile whereas under reducing conditions the mobility of Tc is decreased when it is reduced to Tc(IV). A few studies have shown that TcVII can be reduced to TcIV by Fe-bearing minerals, mainly forming TcO2.nH2O surface precipitates[1]. Most of these studies, however, were carried out at rather high Tc loadings. The focus of this study is on low to very low Tc loadings, which are more environmentally relevant.
Selenate (SeO42−, Se(VI)) and selenite (SeO32−, Se(IV)) are favoured under oxidizing conditions. Selenate reduction is kinetically hindered and very slow (high activation energy). Hence, the study focuses on selenite. There is little data on the retention of Se(IV) by Fe(II) bearing clay minerals. Reductive precipitation of Se(IV) to nanoparticulate Se(0) was observed when dissolved Fe(II) is sorbed onto synthetic montmorillonite[2].
By combining Tc and Se sorption experiments on native and reduced smectite clay samples with different FeII/FeIII ratios with Tc/Se K-edge extended X-ray absorption fine structure (EXAFS), mediated oxidation and reduction experiments (MEO/MER), Mössbauer spectrometry and microscopy, we aim at discriminating the contributions of the different available Fe sources to the overall adsorption and reduction of Tc and Se and to identify the surface products. The ultimate aim is to correlate mineral properties with respect to their redox reactive iron content, the degree of Tc and Se reduction and the molecular scale surface speciation, in order to improve the understanding of the coupled adsorption and electron transfer reactions contributing to the retention of Tc and Se on FeII/FeIII bearing clay minerals, and thus to contribute to a more reliable prediction of the Tc and Se retention in nuclear waste repositories.

1. Jaisi, D.P., et al., Reduction and long-term immobilization of technetium by Fe(II) associated with clay mineral nontronite. Chemical Geology, 2009. 264(1): p. 127-138.
2. Charlet, L., et al., Electron transfer at the mineral/water interface: Selenium reduction by ferrous iron sorbed on clay. Geochimica et Cosmochimica Acta, 2007. 71(23): p. 5731–5749.