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Chapter 8 : Direct and Indirect Processes Leading to Uranium(IV) Oxidation
Category: Applied and Industrial Microbiology; Environmental Microbiology
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Microbial activity has shown to catalyze the reduction of the soluble and mobile contaminant uranium (VI) to the sparingly soluble, nanoparticulate mineral uraninite (UO2). Many of the oxidants can be formed biologically, and these various redox-active compounds can also react with each other. Thus, the potential oxidation of uranium (IV) in the subsurface is a complex and intricate series of direct and indirect coupled biological redox reactions. This chapter presents a comprehensive view of the collective understanding of the processes potentially responsible for uranium (IV) oxidation in the subsurface. Most studies have focused on the most widely reported product of microbial U(VI) reduction, namely the mineral uraninite. In particular, the comparison of two studies allows an evaluation of the effect of sulfate on the reoxidation of U(IV) by oxygen. Although the studies focused on the abiotic oxidation of U(IV) by molecular oxygen, a single study reported the aerobic and enzymatic oxidation of U(IV). Accounting for this catalytic UO2 oxidation coupled to Mn, cycling is needed to evaluate the long-term stability of U(IV) at remediated sites. A major aspect of the mechanism(s) of abiotic U(IV) oxidation by Fe(III) and Mn(III,IV) oxides remains poorly understood: both reactants are solid phases and it is unclear how the relative localization of the two minerals influences the U(IV) oxidation.
High-resolution transmission electron microscopy (HRTEM) (left) and Fourier-filtered HRTEM (right) image of biogenic nanoparticles of UO2 obtained from S. oneidensis MR-1 under nongrowth conditions. 10.1128/9781555817190.ch8.f1
Redox potentials of species discussed in the text. Thermodynamic data were obtained from Ginder-Vogel et al. (2006 ) and Morel and Hering (1993 ). All calculations were carried out at pH 7. 10.1128/9781555817190.ch8.f2
Schematic of the direct and indirect biological pathways involving oxygen for U(IV) oxidation. In this and subsequent figures, the boxed text corresponds to the electron acceptor in the processes considered. 10.1128/9781555817190.ch8.f3
Schematic of the direct and indirect biological pathways involving N oxides for U(IV) oxidation. The product of nitrite reduction by U(IV) is taken to be NO2 – ( Senko et al., 2005a ) by analogy to Fe(II) ( Cooper et al., 2003 ). Only NO2 – was considered as the product of heterotrophic denitrification for the sake of simplicity. In reality, N2O and NO are also formed and also oxidize U(IV) abiotically (especially N2O). For the anaerobic U(IV) oxidizer, the bacterium is capable of complete denitrification, but this was not measured (H. Beller, personal communication). In this and subsequent figures, dashed and shaded boxes are intended for purely aesthetic reasons to aid in the visual separation of the various processes in the diagram. 10.1128/9781555817190.ch8.f4
Schematic of the direct and indirect biological pathways involving iron oxides for U(IV) oxidation. The product of coupled Fe(II) oxidation and nitrate reduction is most commonly N2 ( Straub et al., 2001 ). Fe(III) labeled in bold corresponds to the most reactive form of Fe(III) produced by the processes considered. 10.1128/9781555817190.ch8.f5
Schematic of the direct and indirect biological pathways involving Mn oxides for U(IV) oxidation. 10.1128/9781555817190.ch8.f6