Tc(VII) Reduction Kinetics by Titanomagnetite (Fe3-xTixO4) Nanoparticles

Journal Article · Sat Sep 01 04:00:00 EDT 2012 · Geochimica et Cosmochimica Acta, 92:67-81

DOI:https://doi.org/10.1016/j.gca.2012.06.004· OSTI ID:1052511

Liu, Juan; Pearce, Carolyn I; Qafoku, Odeta; Arenholz, Elke; Heald, Steve M; Rosso, Kevin M

Technetium contamination remains a major environmental problem at nuclear reprocessing sites, such as at the Hanford nuclear reservation, Washington, U.S.A. Here we investigate the heterogeneous reduction of the highly soluble pertechnetate anion [Tc(VII)O4-] to sparingly soluble Tc(IV)-bearing solids by a novel and well-characterized set of mixed-valent titanium-doped magnetite nanoparticles, structurally and chemically analogous to titanomagnetites naturally present in Hanford sediments. Titanomagnetite (Fe3 xTixO4) nanoparticles (10-12 nm) with varying Ti content (0 ≤ x ≤ 0.53) were synthesized in aqueous suspension. Reaction with 10 and 30 μM Tc(VII) solution yielded fast exponentially decaying reduction kinetics with rates that increased with increasing solid-state Fe(II)/Fe(III) ratio in the nanoparticles, a characteristic systematically controlled by the Ti-content. Nanoparticles before and after reduction experiments and surface-associated products of Tc(VII) reduction were characterized using transmission electron microscopy (TEM), X-ray absorption nearedge spectroscopy (XANES), extended X-ray absorption fine structure spectroscopy (EXAFS), micro Xray diffraction (μ-XRD), X-ray absorption (XA) and X-ray magnetic circular dichroism (XMCD). A mechanistic reaction model was developed involving reduction of Tc(VII) to form Tc(IV)/Fe(III) solids by structural Fe(II) enriched at the nanoparticle surface, a reactive Fe(II) pool that during reaction is resupplied and sustained by outward migration of Fe(II) from the particle interior with concurrent inward migration of charge-balancing cationic vacancies in a ratio of 3:1. The reaction process was quantitatively linked to mass and electron balanced changes in the Fe3-xTixO4 nanoparticles, and the accessibility of structural Fe(II) from these phases was determined.

• 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: 1052511
• Report Number(s): PNNL-SA-86376; 46203; KP1702030
• Journal Information: Geochimica et Cosmochimica Acta, 92:67-81, Journal Name: Geochimica et Cosmochimica Acta, 92:67-81
• Country of Publication: United States
• Language: English

Related Subjects

Environmental Molecular Sciences Laboratory