Direct and Indirect Processes Leading to Uranium(IV) Oxidation

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.