Selenite retention by nanocrystalline magnetite: Role of adsorption, reduction and dissolution/co-precipitation processes
Introduction
The mobility of selenium in the environment has been of interest for decades. Although selenium is an essential nutrient for humans, animals and plants, it is toxic at higher concentrations, with a narrow gap between toxic and beneficial concentrations (Lakin, 1972, Skorupa, 1998). It is therefore important to understand the processes controlling the distribution of selenium in soil and water.
The foremost processes controlling selenium mobility and bioavailability in the environment are adsorption onto geological materials and the formation of Se minerals (McNeal and Balistrieri, 1989). Selenium chemistry is quite complicated because its mobility strongly depends on both the redox state of the system and the factors influencing its speciation (e.g., pH, presence of organics and kinetics).
Selenium exists in four different oxidation states with very different chemical behaviors: selenide (Se−II), elemental selenium (Se0), selenite () and selenate (). Metal selenides and elemental selenium have a very low solubility and are fairly immobile, but selenite and selenate (both oxoanions) are soluble and mobile. Selenium toxicity is also dependent on its chemical state, with selenite being more toxic than selenate, which is more toxic than selenide.
Oxoanions, in general, are of concern in the context of radioactive waste repositories because of their limited adsorption onto geologic materials (Duc et al., 2003). 79Se, present in spent nuclear fuel and high-level radioactive waste, is an element of interest because of its long half-life, which is reported to be 4.8 × 105 or 1.1 × 106 years (Jiang et al., 1997, Magill et al., 2006).
As anion sorbents, Fe oxy-hydroxides are among the solid phases with the greatest adsorption capacity, especially for selenite. Many detailed studies exist on selenium adsorption onto the most common iron oxides in soil: goethite, hematite and amorphous ferrihydrite (Balestrieri and Chao, 1987, Duc et al., 2006, Hansmann and Anderson, 1985, Hayes et al., 1987, Manceau and Charlet, 1994, Parida et al., 1997, Peak and Sparks, 2002, Su and Suarez, 2000, Zhang and Sparks, 1990). Yet, far fewer studies exist on selenium sorption onto oxides containing FeII, such as magnetite () (Martinez et al., 2006). Due to the presence of FeII, magnetite may be capable of reducing selenite in a coupled redox reaction. In fact, this possibility has been recently confirmed (Scheinost and Charlet, 2008, Scheinost et al., 2008); however, selenite was not reduced by magnetite in another experiment (Loyo et al., 2008). The reason for this discrepancy is, up to now, not known.
Metal containers are the first physical barrier to radionuclide migration in radioactive waste repositories. In the moderate-to-strong reducing environment and neutral-alkaline conditions expected in these repositories, magnetite is the primary stable end product of oxide transformations (Cornell and Schwertmann, 1997). Therefore, a better understanding of radionuclide interactions with magnetite may allow the prediction of radionuclide migration within these waste systems.
We sought to explain the interactions between selenite and a very well characterized nanocrystalline magnetite (Missana et al., 2003a, Missana et al., 2003c), accounting for both adsorption and the possible effects of selenium reduction at the magnetite surface.
To understand the underlying mechanisms of this radionuclide retention, we analyzed in parallel sorption results obtained with a well characterized goethite under similar conditions. Goethite and magnetite differ greatly in their structure, surface area and water solubility, yet an even greater difference is the presence of FeII in magnetite, which potentially triggers reduction reactions, and its absence in goethite.
Batch sorption experiments were performed to obtain sorption data covering a wide range of experimental conditions and suited for surface complexation modeling. The basic parameters, essential for the application of these models, were previously determined (Missana et al., 2003a).
One possible application of our study is in the context of geological repositories of radioactive waste. In the performance assessment (PA) calculation of these repositories, the semi-empiric Kd approach is still used to describe sorption processes. However, in such a context, the application of mechanistic models is recommended. Simple yet effective models for describing sorption could be easily incorporated into PA. Therefore, providing models (as simple as possible) based on batch sorption data and spectroscopic analysis is important for facilitating the inclusion of a mechanistic approach to sorption in PA.
XAS analyses of the system were performed at different pH values to generate and support the model hypotheses. Extended X-ray absorption fine-structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectroscopic measurements were carried out at the Rossendorf Beamline at the European Synchrotron Radiation Facility (Grenoble, France).
Our results revealed that, in the selenite/magnetite system, not only inner-sphere complexation, but also oxide dissolution and co-precipitation processes, play a major role in selenite retention, while selenite reduction was not observed.
Section snippets
Materials and methods
All reagents were of analytical grade and used without further purification. The deionized water (MilliQ–Milliρ system) used to prepare the electrolytes and suspensions was bubbled with N2 and boiled for at least 15 min to minimize CO2 contamination and then stored in an anoxic glove box. The atmosphere in the anoxic glove box was CO2-free nitrogen (O2 < 1 ppm). All experiments were run at room temperature.
Characterization of the oxides
Both magnetite and goethite prepared in our laboratory have been thoroughly characterized previously (Missana et al., 2003a, Missana et al., 2003b, Missana et al., 2003c). Phase identity, purity and morphology were confirmed by XRD, XPS, TEM and SEM analysis. In addition, the oxide batches used in the present work were analyzed with AFM (Fig. 1). Magnetite (Fig. 1, left) is formed by nanocrystals (50–200 nm) with well-defined edges.
As shown in Missana et al., 2003a, Missana et al., 2003c, the
Conclusions
The retention features of selenite on iron oxide magnetite are not directly comparable with other oxides. The selenite retention onto goethite can be modeled over the entire pH range, assuming only the formation of inner-sphere complexes between the solid surface and selenite. The same mechanism explains selenite retention by magnetite only in the neutral-to-alkaline range. However, at acidic pH, selenite retention is dominated by significant dissolution of magnetite, the presence of Se–Fe
Acknowledgments
Victor Fernández and Petri Gil are thanked for their support in laboratory work. The authors thank David R. Peck for English syntactical and grammatical consultations. The authors greatly acknowledge the three anonymous reviewers and the AE Chris Daughney for their useful comments and suggestions for improving the manuscript.
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