Geomicrobiological factors that control uranium mobility in the environment: Update on recent advances in the bioremediation of uranium-contaminated sites

Abstract

Understanding the behavior of uranium (U) in the environment is essential not only for the protection of aquifers from U contamination but also for predicting the fate of U and other actinides disposed of in deep geological settings. It has long been believed that the redox chemistry of U can be simply predicted by thermodynamics and that the development of a low redox potential is a sufficient condition for U reduction. However, recent studies have demonstrated that redox transformations of U are controlled by kinetic factors that are strongly influenced by microbial activity. Although abiological U oxidation proceeds efficiently under oxygenic conditions, abiological reduction of U is inhibited by the formation of negatively charged U(VI)-CO₃ complexes that prevail in nature. Phylogenetically diverse microorganisms are capable of enzymatically reducing U(VI)-CO₃ complexes to form U(IV)-bearing minerals such as uraninite (UO_2+x). The only abiological pathway currently known for the reduction of U(VI)-CO₃ complexes involves the Fe(II) monohydroxo surface complex ≡Fe^IIIOFe^IIOH⁰. This complex is mainly produced by the microbial reduction of Fe(III) in natural systems. Thus, U(VI) reduction is controlled both directly and indirectly, at least in part, by microbial activity. Several mechanisms of U oxidation under anoxic conditions have been revealed recently by laboratory and field studies. U(IV) is abiologically oxidized by Fe(III) and Mn(IV) oxides. Microbial reduction of nitrate to molecular nitrogen, which occurs following the depletion of O₂, produces nitrogen intermediates including nitrite (NO₂⁻), nitrous oxide (NO), and nitric oxide (N₂O). Although the nitrogen intermediates oxidize U(IV), poorly crystalline Fe(III)-oxide minerals resulting from the oxidation of aqueous Fe(II) species by the nitrogen intermediates oxidize U(IV) more efficiently than the nitrogen intermediates alone. Remarkably, the formation of Ca-U(VI)-CO₃ complexes resulting from increased levels of Ca²⁺ and/or HCO₃⁻ leads to the reoxidation of bioreduced U(IV) under reducing conditions. These geomicrobiological factors pose challenges in manipulating and/or predicting the mobility and fate of U in complex and heterogeneous environmental settings.