Effects of Redox Cycling of Iron in Nontronite on Reduction of Technetium

Journal Article · Fri Jan 06 04:00:00 EST 2012 · Chemical Geology, 291:206-216

DOI:https://doi.org/10.1016/j.chemgeo.2011.10.013· OSTI ID:1033452

Yang, Junjie; Kukkadapu, Ravi K; Dong, Hailiang; Shelobolina, Evgenya S; Zhang, Jing; Kim, Jinwook

In situ technetium-99 (99Tc) immobilization by Fe(II) associated with clay minerals has been studied and is a potential cost-effective method for Tc remediation at the United States Department of Energy (DOE) sites. Fe redox cycling are common in sedimentary environments, however their effect on Tc reduction and immobilization has not yet been investigated. The objective of this project was therefore to study how multiple cycles of reduction-reoxidation of Fe-rich clay mineral, nontronite, affected its reactivity toward Tc (VII) reduction. Iron-rich nontronite NAu-2 was used as a model clay mineral. NAu-2 suspension was first bioreduced by Shewanella putrefaciens CN32, which consequently was re-oxidized by air. Three cycles of reduction-oxidation were conducted and bioreduced NAu-2 samples from all three cycles were collected and used for Tc(VII) reduction experiments. Each redox cycle resulted in a small fraction of dissolution of small size and/or poorly crystalline NAu-2. The released Fe(II) from the dissolution was likely adsorbed onto NAu-2 surface/edge sites with a high reactivity. Upon exposure to O2, this reactive Fe(II) fraction was oxidized more rapidly than structural Fe(II) and may have accounted for a two-step reoxidation kinetics of NAu-2 associated Fe(II): rapid oxidation over first few hours followed by slow oxidation. Progressive increase of this reactive fraction of Fe(II), from increased dissolution, accounted for the successively higher rate of bioreduction and reoxidation with increased redox cycles. The same Fe redistribution accounted for two-step Tc(VII) reduction kinetics as well. Rapid Tc(VII) reduction in the first few hours may be attributed to a small fraction of highly reactive Fe(II) at the NAu-2 surface/edge sites, and more steady Tc(VII) reduction over longer time may be carried out by structural Fe(II). Similar to the increased rates of Fe(III) reduction and Fe(II) oxidation, the Tc(VII) reduction rate also increased with redox cycles and could be explained by progressive increase of the reactive Fe(II) on NAu-2 surface/edges. These results suggest that iron-rich clay minerals undergo important changes after redox cycles, but eventually reach a steady state with continued reactivity toward heavy metals.

• 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: 1033452
• Report Number(s): PNNL-SA-79180; 25610; 41590; 44687; KP1704020
• Journal Information: Chemical Geology, 291:206-216, Journal Name: Chemical Geology, 291:206-216 Vol. 291; ISSN CHGEAD; ISSN 0009-2541
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
74 ATOMIC AND MOLECULAR PHYSICS
AIR
CLAYS
Cycling
DISSOLUTION
Effects
Environmental Molecular Sciences Laboratory
HEAVY METALS
IRON
Iron
KINETICS
Nontronite
OXIDATION
Redox
Reduction
TECHNETIUM
TECHNETIUM 99
Technetium