U(VI) Sorption and Reduction Kinetics on the Magnetite (111) Surface

Journal Article · Tue Apr 03 04:00:00 EDT 2012 · Environmental Science and Technology

DOI:https://doi.org/10.1021/es203878c· OSTI ID:1040673

Singer, David M.; Chatman, Shawn M.; Ilton, Eugene S.; Rosso, Kevin M.; Banfield, Jillian F.; Waychunas, Glenn A.

Sorption of contaminants onto mineral surfaces is an important process that can restrict their transport in the environment. In the current study, uranium (U) uptake on magnetite (111) was measured as a function of time and solution composition (pH, [CO₃]T, [Ca]) under continuous batch-flow conditions. We observed, in real-time and in situ, adsorption and reduction of U(VI) and subsequent growth of UO₂ nanoprecipitates using atomic force microscopy (AFM) and newly developed batch-flow U LIII-edge grazing-incidence X-ray absorption spectroscopy near-edge structure (GI-XANES) spectroscopy. U(VI) reduction occurred with and without CO₃ present, and coincided with nucleation and growth of UO₂ particles. When Ca and CO₃ were both present no U(VI) reduction occurred and the U surface loading was lower. In situ batch-flow AFM data indicated that UO₂ particles achieved a maximum height of 4–5 nm after about 8 h of exposure, however, aggregates continued to grow laterally after 8 h reaching up to about 300 nm in diameter. The combination of techniques indicated that U uptake is divided into three-stages; (1) initial adsorption of U(VI), (2) reduction of U(VI) to UO₂ nanoprecipitates at surface-specific sites after 2–3 h of exposure, and (3) completion of U(VI) reduction after ~6–8 h. U(VI) reduction also corresponded to detectable increases in Fe released to solution and surface topography changes. Redox reactions are proposed that explicitly couple the reduction of U(VI) to enhanced release of Fe(II) from magnetite. Although counterintuitive, the proposed reaction stoichiometry was shown to be largely consistent with the experimental results. In addition to providing molecular-scale details about U sorption on magnetite, this work also presents novel advances for collecting surface sensitive molecular-scale information in real-time under batch-flow conditions.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1040673
• Report Number(s): PNNL-SA-83907; 47433; 39953; 42318; KP1702030
• Journal Information: Environmental Science and Technology, Journal Name: Environmental Science and Technology Journal Issue: 7 Vol. 46; ISSN 0013-936X
• Publisher: American Chemical Society (ACS)
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
ABSORPTION SPECTROSCOPY
ADSORPTION
ATOMIC FORCE MICROSCOPY
AVAILABILITY
Environmental Molecular Sciences Laboratory
KINETICS
MAGNETITE
NUCLEATION
SORPTION
SPECTROSCOPY
TOPOGRAPHY
TRANSPORT
URANIUM