, Volume 22, Issue 5, pp 921-929

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

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Abstract

Plutonium (Pu), a key contaminant at sites associated with the manufacture of nuclear weapons and with nuclear-energy wastes, can be precipitated to “immobilized” plutonium phases in systems that promote bioreduction. Ferric iron (Fe3+) is often present in contaminated sites, and its bioreduction to ferrous iron (Fe2+) may be involved in the reduction of Pu to forms that precipitate. Alternately, Pu can be reduced directly by the bacteria. Besides Fe, contaminated sites often contain strong complexing ligands, such as nitrilotriacetic acid (NTA). We used biogeochemical modeling to interpret the experimental fate of Pu in the absence and presence of ferric iron (Fe3+) and NTA under anaerobic conditions. In all cases, Shewanella alga BrY (S. alga) reduced Pu(V)(PuO2 +) to Pu(III), and experimental evidence indicates that Pu(III) precipitated as PuPO4(am). In the absence of Fe3+ and NTA, reduction of PuO2 + was directly biotic, but modeling simulations support that PuO2 + reduction in the presence of Fe3+ and NTA was due to an abiotic stepwise reduction of PuO2 + to Pu4+, followed by reduction of Pu4+ to Pu3+, both through biogenically produced Fe2+. This means that PuO2 + reduction was slowed by first having Fe3+ reduced to Fe2+. Modeling results also show that the degree of PuPO4(am) precipitation depends on the NTA concentration. While precipitation out-competes complexation when NTA is present at the same or lower concentration than Pu, excess NTA can prevent precipitation of PuPO4(am).