Subsurface bio-mediated reduction of higher-valent uranium and plutonium

Abstract

Bio-mediated reduction of multivalent actinide contaminants plays an important role in their fate and transport in the subsurface. To initiate the process of extending recent progress in uranium biogeochemistry to plutonium, a side-by-side comparison of the bioreduction of uranyl and plutonyl species was conducted with Shewanella alga BrY, a facultative metal-reducing bacterium that is known to enzymatically reduce uranyl. Uranyl was reduced in our system, consistent with literature reports, but we have noted a strong coupling between abiotic and biotic processes and observe that non-reductive pathways to precipitation typically exist. Additionally, a key role of biogenic Fe 2+ , which is known to reduce uranyl at low pH, is suggested. In contrast, residual organics, present in biologically active systems, reduce Pu(VI) species to Pu(V) species at near-neutral pH. The predominance of relatively weak complexes of PuO 2 + is an important difference in how the uranyl and plutonyl species interacted with S. alga. Pu(V) also led to increased toxicity towards S. alga and is also more easily reduced by microbial activity. Biogenic Fe 2+ , produced by S. alga when Fe(III) is present as an electron acceptor, also played a key role in understanding redox controls and pathways in this system. Overall, the bioreduction of plutonyl is observed under anaerobic conditions, which favors its immobilization in the subsurface. Understanding the mechanism by which redox control is established in biologically active systems is a key aspect of remediation and immobilization strategies for actinides when they are present as subsurface contaminants.

Actinide (III) solubility in WIPP Brine: data summary and recommendations

“…Most general reviews and studies (Runde et al 2002;Choppin 2006;Rard 1997;Silver 2002) acknowledge the possibility of Pu(III) species, but experimentally, we routinely only identify Pu(IV) solid and aqueous species in WIPP-related studies (Reed et al 2006). Anaerobic bacteria typical of low ionic strength groundwater systems have been shown to reduce Pu(V) to Pu(III) when complexants prevent Pu(IV) precipitation (Reed et al 2007). The redox distribution of plutonium under WIPP-related conditions is the subject of ongoing ACRSP studies.…”

Bacterial Pu(V) reduction in the absence and presence of Fe(III)–NTA: modeling and experimental approach

“…by biogenically produced Fe 2? (Reed et al 2007). Figure 4A compares the model-calculated and experimental results for the bioreduction of PuO 2 ?…”

“…and Fe 2? ); grow S. alga; and carry out anaerobic experiments are the same as reported in Reed et al (2007). For this work, we added spectroscopic analyses of the reduced Pu solid formed.…”

“…Our group published work on bioreduction of higher-valent uranium and plutonium (Reed et al 2007), where part of that work investigated Pu(V) reduction in the absence and presence of Fe 3? and NTA.…”

“…In this study, we use modeling analyses to understand the mechanisms governing the previously observed slow reduction of Pu(V) in the presence of iron (Reed et al 2007). We also discuss new spectroscopic results that expand our ability to interpret the experimental findings.…”