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) CO 3 complexes that prevail in nature. Phylogenetically diverse microorganisms are capable of enzymatically reducing U(VI) CO 3 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) CO 3 complexes involves the Fe(II) monohydroxo surface complex Fe III OFe II OH 0 . 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 O 2 , produces nitrogen intermediates including nitrite (NO 2 -), nitrous oxide (NO), and nitric oxide (N 2 O). 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) CO 3 complexes resulting from increased levels of Ca 2+ and/or HCO 3 -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.

Removal of nitrate and hexavalent uranium from groundwater by sequential treatment in bioreactors packed with elemental sulfur and zero‐valent iron

“…It is well established that a large variety of microorganisms catalyze the reduction of U VI to U IV coupled to the oxidation of external electron donors (such as ethanol, acetate, lactate, and H 2 ) under anaerobic conditions (Suzuki and Suko, 2006; Wall and Krumholz, 2006). Microorganisms with this capability include Fe 3+ ‐reducing bacteria such as Geobacter metallireducens and Shewanella putrefaciens (Fredrickson et al, 2000; Holmes et al, 2002), and sulfate‐reducing bacteria, such as Desulfovibrio spp.…”

“…Uranium occurs in two major valences in the environment, either as hexavalent uranium (U VI ) which is soluble, mobile, and occurs commonly as ${\rm UO}_{{\rm 2}}^{{\rm 2 + }}$ , or as tetravalent uranium (U IV ), which is insoluble, immobile, and occurs as the mineral uraninite (UO 2 ) (Abdelouas, 2006). Many bacteria are known which are capable of reducing U VI to insoluble U IV (Merroun and Selenska‐Pobell, 2008; Suzuki and Suko, 2006; Wall and Krumholz, 2006). This bioconversion, leading to the reductive precipitation of uranium, is being considered as a bioremediation strategy for eliminating uranium from contaminated groundwater (Anderson et al, 2003; Madden et al, 2009; Suzuki and Suko, 2006).…”

“…Many bacteria are known which are capable of reducing U VI to insoluble U IV (Merroun and Selenska‐Pobell, 2008; Suzuki and Suko, 2006; Wall and Krumholz, 2006). This bioconversion, leading to the reductive precipitation of uranium, is being considered as a bioremediation strategy for eliminating uranium from contaminated groundwater (Anderson et al, 2003; Madden et al, 2009; Suzuki and Suko, 2006). However, the viability of reductive precipitation as a bioremediation method, depends on safeguarding biogenic U IV from re‐oxidation.…”

Modeling U(VI) biomineralization in single‐ and dual‐porosity porous media

“…As yet, no radionuclide‐contaminated sites have been proposed for bioaugmentation [ Hazen and Tabak , 2005], and further research is needed to determine the capabilities and limitations of the technique. Additionally, reoxidation of immobilized uranium should be considered in future work in order to assess the long‐term success of bio‐immobilization strategies [ Suzuki and Suko , 2006].…”

Constraints in the colonization of natural and engineered subterranean igneous rock aquifers by aerobic methane‐oxidizing bacteria inferred by culture analysis

“…For applied processes such as the deep geological disposal of spent nuclear fuel (SNF), the corrosive properties of oxygen could potentially impair the long-term integrity of deposited canisters (Pedersen, 2002). From a geochemical perspective, oxygen when dissolved in groundwater could influence the solubility of the radioactive metal uranium and its chemical complexes (Suzuki & Suko, 2006;Roelofs et al ., 2007); consequently influencing their mobility and dispersal patterns while from an environmental point of view, radioactive metals released from leaky SNF canisters reaching the far field may have disastrous implications.…”