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

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 (Fe(3+)) is often present in contaminated sites, and its bioreduction to ferrous iron (Fe(2+)) 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 (Fe(3+)) and NTA under anaerobic conditions. In all cases, Shewanella alga BrY (S. alga) reduced Pu(V)(PuO(2) (+)) to Pu(III), and experimental evidence indicates that Pu(III) precipitated as PuPO(4(am).) In the absence of Fe(3+) and NTA, reduction of PuO(2) (+) was directly biotic, but modeling simulations support that PuO(2) (+) reduction in the presence of Fe(3+) and NTA was due to an abiotic stepwise reduction of PuO(2) (+) to Pu(4+), followed by reduction of Pu(4+) to Pu(3+), both through biogenically produced Fe(2+). This means that PuO(2) (+) reduction was slowed by first having Fe(3+) reduced to Fe(2+). Modeling results also show that the degree of PuPO(4(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 PuPO(4(am)).

In situ and ex situ bioremediation of radionuclide-contaminated soils at nuclear and norm sites

“…For example, the following conversions by microorganisms have been reported: selenate, selinite, tellurate, and tellurite to elemental forms; vanadate to a vanadyl compound; molybdate and molybdenum trioxide to a molybdenum blue compound; arsenate to arsenite; mercuric chloride to elemental mercury; chromate ions to chromic ions that are precipitates at neutral pH; pentavalent and trivalent bismuth to an elemental form; lead dioxide to its divalent state; osmium tetroxide to the osmate ion; osmium dioxide and trivalent osmium to the metal; and hexavalent uranium to the tetravalent state; hexavalent Pu to the tetravalent form, petavalent neptunium to the tetravalent form, and heptavalent technetium to tetravalent form (Mohagheghi, Updegraff, & Goldhaber, 1985;Lovley, 1993;Woolfolk & Whitely, 1962;Kauffman, Laughlin, & Baldwin, 1986;Francis, Dodge, Gillow, & Cline, 1991a;Francis, Dodge, & Gillow, 1991b;Francis, Dodge, Lu, Halada, & Clayton, 1994;Francis, Dodge, & Gillow, 2008;Neu, Icopini, & Boukhalfa, 2005;Deo, Rittmann, & Reed, 2011). The reduction of an element from a higher to a lower oxidation state or to elemental form affects its solubility, so resulting in the precipitation of several metals.…”

“…Due to high ionic charge, it can undergo hydrolysis and convert to a polymeric form at pH > 2. Information is limited on the influence of microbes on the solubility of Pu (Deo & Rittmann, 2012;Deo et al, 2011). The chemical speciation of Pu can be influenced by the soil's pH, redox conditions, organic content, mineralogy, and microbial activities (Francis, 2007).…”

A biogeochemical framework for bioremediation of plutonium(V) in the subsurface environment

“…Modeling domain and utilized feature of CCBATCH We develop our biogeochemical framework for Pu bioremediation for a batch system using previously reported experimental (Reed et al 2007) and modeling (Deo et al 2011) results. Table 1 describes the components of the medium used for the batch reactions.…”

“…to Pu(III) by direct (enzymatic) reduction reaction (Deo et al 2011;Reed et al 2007). The yield and stoichiometry for all components involved in Pu(V)O 2 ?…”

BioRadBase: A database for bioremediation of radioactive waste

“…Similarly, differential bioreduction of Pu at contaminated sites could occur through biotic and abiotic means. However, biotic reduction is largely done by bacteria, while abiotic reduction was shown to be dependent on availability of complexing agent in soil (Deo et al, 2011). Bacterial cell surface plays a significant role in biocrystallization of uranium phosphate (Macaskie et al, 1996).…”