Selenium(IV) Uptake by Maghemite (γ-Fe 2 O 3 ) Norbert Jordan, †, * Aline Ritter, † Andreas C. Scheinost, †,‡, * Stephan Weiss, † Dieter Schild, § and Rene ́ Hü bner ∥ † Institute of Resource Ecology, Helmholtz-Zentrum Dresden-Rossendorf e.V., 01328 Dresden, Germany ‡ The Rossendorf Beamline at ESRF, P.O. Box 220, F-38043 Grenoble, France § Institute for Nuclear Waste Disposal, Karlsruhe Institute for Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ∥ Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf e.V., Bautzner Landstrasse 400, 01328 Dresden, Germany * S Supporting Information ABSTRACT: The mechanism of selenium(IV) uptake by maghemite was investigated on both the macroscopic and the molecular level. Maghemite nanoparticles exhibited fast adsorption kinetics toward selenium(IV). Batch experiments showed a decreased sorption with increasing pH (3.5−11). Ionic strength variations (0.01 to 0.1 M NaCl) had no significant influence on selenium(IV) uptake. Electrophoretic mobility measurements revealed a significant shift toward lower values of the isoelectric point of maghemite upon selenium(IV) uptake, suggesting the formation of inner-sphere surface complexes. At the molecular level, using X-ray Absorption Fine-Structure Spectroscopy (EXAFS), the formation of both bidentate binuclear corner-sharing ( 2 C) and bidentate mononuclear edge-sharing ( 1 E) inner-sphere surface complexes was observed, with a trend toward solely 1 E complexes at high pH. The absence of a tridentate surface complex as observed for arsenic(III) and antimonite(III) might be due to the relatively small size of the Se IV O 3 unit. These new spectroscopic results can be implemented in reactive transport models to improve the prediction of selenium migration behavior in the environment as well as its monitoring through its interaction with maghemite or maghemite layers at the surface of magnetite. Due to its chemical stability even at low pH and its magnetization properties allowing magnetic separation, maghemite is a promising sorbing phase for the treatment of Se polluted waters. ■ INTRODUCTION Although Se is an essential element for animals and humans, 1 it is toxic in excess. 1−3 A teratogenic effect and poisoning (birth deformity and mortality) of fish and wildlife were observed at Kesterson National wildlife refuge in California. 4−6 Selenium- 79, a long-lived (t 1/2 ≈ 3.27 × 10 5 years 7 ) and radiotoxic radionuclide present in spent nuclear fuel, is of high relevance in the context of nuclear waste management, according to safety assessments. 8−10 Although the most important exposure route to Se for humans is food, 1,11 selenium leads to severe health effects when present even at low concentrations in drinking water. 12,13 Adsorption and heterogeneous reduction on iron, alumina, titanium oxides, and so forth were shown to be mechanisms able to retard the migration of selenium in the environ- ment. 14−17 Maghemite, the red-brown γ polymorph of Fe 2 O 3 , also belongs to the wide range of naturally occurring iron oxides. It is found in tropical and subtropical soils, and is commonly formed from the oxidation of lithogenic magnet- ite. 18−21 Other reported formation pathways are the dehydrox- ylation of lepidocrocite or heating of goethite in the presence of organic matter. 18 It was also identified as a corrosion product of steel waste canisters 22 and iron archeological analogues. 23−25 Recently, nanomagnetite particles (10−20 nm) were shown to be very promising sorbents for the removal of selenite from aqueous solutions, with a final concentration less than 5 μg L −1 , 16,26 thus below the concentration recommended by the World Health Organization. The process responsible for these low concentrations is the reduction of Se(IV) to Se(−II) by magnetite and subsequent precipitation of highly insoluble FeSe. 16 However, such nanomagnetite particles are transformed to maghemite either by aerial oxidation or by interfacial ionic and/or electron transfers depending on the pH. 27 Indeed, magnetite is thermodynamically unstable with respect to maghemite (γ-Fe 2 O 3 ) and is slowly oxidized to maghemite even at room temperature in the presence of oxygen. 28 As already stated by Tang et al. 28 and Morin et al., 29 the oxidation of magnetite to maghemite is therefore a process of high Received: October 13, 2013 Revised: January 14, 2014 Accepted: January 14, 2014 Published: January 14, 2014 Article pubs.acs.org/est © 2014 American Chemical Society 1665 dx.doi.org/10.1021/es4045852 | Environ. Sci. Technol. 2014, 48, 1665−1674