Environ. Sci. Technol. 2010, 44, 4493–4498
Uptake and Release of Cerium During Fe-Oxide Formation and Transformation in Fe(II) Solutions S . N E D E L , * ,† K . D I D E R I K S E N , †,‡ B. C. CHRISTIANSEN,† N. BOVET,† AND S. L. S. STIPP† Nano-Science Center, Department of Chemistry, University of Copenhagen, Denmark, and Department of Earth and Planetary Science, University of California, Berkeley, Cailfornia
Received October 15, 2009. Revised manuscript received April 23, 2010. Accepted May 7, 2010.
Fe-oxides are ubiquitous in soils and sediments and form during Fe(0) corrosion. Depending on redox conditions and solution composition, Fe-oxides such as ferrihydrite, goethite, magnetite, and green rust (GR) may form. These phases typically have high surface area and large affinity for adsorption of trace components. Further, Fe(II)-Fe(III) (hydr)oxides are redox active. Cerium, a member of the lanthanide family, can be used as an analogue for the tri- and tetra-valent actinides found in radioactive waste, expected to be stored in subsurface repositories. In experiments with ferrihydrite, Ce(III) was effectively scavenged from Fe(II)-bearing solutions within 5 min at pH 7. During transformation of ferrihydrite to green rust, however, all Ce(III) was released to solution. By varying initial solution Fe(II): Fe(III) ratio, magnetite and goethite formed together with GRNa,SO4, resulting in decreased Ce(III) release. X-ray photoelectron spectroscopy revealed Ce(III) adsorbed on magnetite. When Feoxides were synthesized by air oxidation of Fe(II) solutions at pH 7, GRNa,SO4 played a catalytic role in the oxidation of Ce(III) to Ce(IV) by O2, removing more than 90% of the dissolved Ce. Transmission electron microscopy revealed that it formed discrete nanocrystals of CeO2(s). These results demonstrate that Fe-oxide interaction with radionuclides is likely to depend strongly on the local redox conditions. By analogy with Ce, the trivalent actinides are not expected to be sequestered by preformed GR in anoxic environments. Our results also suggest that trivalent actinides and lanthanides are released when dissimilatory iron reduction of Fe(III)-oxides leads to GR formation However, under oxidizing conditions, GR may influence radionuclide mobility by catalyzing their transformation to a higher oxidation state.
Introduction Several nations plan the construction of subsurface repositories, where radioactive waste will be stored for more than 100 000 years. A common model uses copper canisters with iron liners where spent fuel rods will be isolated under dry, reducing conditions. During this very long time span, fracture of engineered barriers and corrosion of canisters could lead to radionuclide leakage and migration. Several studies have * Corresponding author phone: (45)3532038; e-mail: sorin@ nano.ku.dk. † University of Copenhagen. ‡ University of California. 10.1021/es9031503
2010 American Chemical Society
Published on Web 05/24/2010
been carried out on the behavior and mobility of transuranic elements (1-3) but such investigations require extensive equipment to prevent dangerous exposure, so nonradioactive analogues have been used to define some boundaries for contaminant behavior, which can then more easily be tested in hot-lab facilities. Often called the rare earth elements (REE), the lanthanides are convenient analogues for the actinides because of their similar electronic configuration, ionic radius,and valence. For example, La(III), Nd(III), and Eu(III) are often used as trivalent representatives for Am(III) and Cm(III) (4-6). Among the lanthanide series, cerium, Ce, can exist both in the +III and +IV oxidation states, making Ce(IV) a good analogue for Pu(IV). The migration of radionuclides from waste is controlled by interaction with minerals in the near and far field. Iron(oxyhydr)oxides, with their high surface area, effectively sorb heavy metals (7, 8). Several authors have shown that Feoxides scavenge REE by adsorption (9-11). A number of parameters that influence adsorption have been examined: the dependence of pH and ionic strength (12), and complex formation (13, 14). Other studies have shown that REE can be incorporated into the Fe-oxide structure. Aging ferrihydrite in the presence of dissolved Lu(III) and Eu(III) at 70 °C and pH 13 for a week produces goethite with 5 at% substitution by Lu(III) and 2 at% by Eu(III) (15). The same researchers demonstrated that 0.5 at% Lu(III) was incorporated into a hematite-like phase, produced by aging ferrihydrite at 70 °C (16). Although these experiments provide a good background for understanding REE behavior, they offer no information about the processes occurring in reduced environments; such as are expected in the repository near field, where dissolved Fe(II) dominates. Dissolved Fe(II) promotes ferrihydrite conversion, within a day, to more stable phases such as goethite, magnetite, and green rust even at room temperature (17). Green rust belongs to the structural family of layered double hydroxides (LDH) and consists of brucite-like layers of Fe(II),Fe(III)-hydroxide, [FeII(1 - x)FeIIIx(OH)2]x+, with interlayers of anions and water molecules to compensate for excess positive charge from Fe(III). Several types of GR exist, depending on the interlayer anion: green rust sulfate (GRSO4), carbonate (GRCO3), chloride (GRCl), etc. In a recent study of the GRSO4 structure, Christiansen et al. (18) demonstrated that monovalent cations present in solution are also incorporated in the interlayer, producing, for example, GRNa,SO4, NaFe(II)6Fe(III)3(OH)18(SO4)2 · xH2O, from a sodium sulfate solution. Formed by both abiotic and biotic processes, GR is stable at circumneutral pH and under anoxic conditions (19). Observing GR in nature is difficult because it oxidizes quickly when exposed to air, transforming within minutes to iron(III)-bearing (oxyhydr)oxides: magnetite (Fe3O4), goethite (R-FeOOH), lepidocrocite (γ-FeOOH) or ferrihydrite (Fe5HO8,4H2O) (20, 21). The end-product depends on many factors such as pH, oxidation rate (22), and the presence of other ions (23, 24). GR has been identified as a corrosion product of water supply pipes (25) and steel (26). It has also been reported in Fe(II)-rich groundwater (27) and in Fe(0) reactive barriers (28). The high reactivity of GR is not limited to reaction with O2. Because it contains both Fe(II) and Fe(III) and it has relatively high surface area, GR reacts with many redox-sensitive contaminants common in groundwater, reducing, for example, Cr(VI) (29, 30), NO3-, NO2- (31, 32), chlorinated solvents (33, 34), Tc(VII) (35), U(VI) (36) and it adsorbs arsenic, a common groundwater contaminant in India and Bangladesh (37). These studies demonstrate that GR controls the transport of a range of contaminants. Our purpose was to investigate VOL. 44, NO. 12, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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how formation and transformation of GRNa,SO4 affects dissolved Ce(III) concentrations. In this work, cerium serves as a model for tri- and tetra-valent actinides to improve fundamental understanding of actinide interaction with Feoxides present in the near and far field of radioactive waste repositories and to provide analogue data for safety assessment.
Methods and Materials Preparation of the Ce(III) Solution and the Radioactive Tracer. A stock solution of 10-1 M Ce(III) was prepared by dissolving CeCl3 · 7H2O (analytical grade) in acidified (pH ∼ 3, with 6 M HCl) deoxygenated (by bubbling N2 for two hours) Milli-Q water (conductivity
