Microbial Transformations of Uranium Complexed with Organic and Inorganic Ligands

Abstract

Abstract. Biotransformation of various chemical forms of uranium present inwastes, contaminated soils and materials by microorganisms under different process conditions such as aerobic and anaerobic (denitrifying, iron-reducing, fermentative, and sulfate-reducing) conditions will affect the solubility, bioavailability, and mobility of uranium in the natural environment. Fundamental understanding of the mechanisms of microbial transformations of uranium under a variety of environmental conditions will be useful in developing appropriate remediation and waste management strategies as well as predicting the microbial impacts on the long-term stewardship of contaminated sites.

Dissolution of Uranium-Bearing Minerals and Mobilization of Uranium by Organic Ligands in a Biologically Reduced Sediment

“…Speciation calculations also indicate that Fe−EDTA or Al−EDTA complexes dominate in either KCl or KHCO 3 solutions (Supporting Information Table S2). These results suggest that U(IV) dissolution by EDTA result mainly from the dissolution of U(IV)-bearing Fe- and Al- minerals. ,, On the other hand, a relatively high molar concentration of citrate (0.7−1.4 mM) was available and could have caused the dissolution of uraninite by forming biligand mononuclear U(IV)−citrate complexes. ,, …”

“…6,7,26 On the other hand, a relatively high molar concentration of citrate (0.7À1.4 mM) was available and could have caused the dissolution of uraninite by forming biligand mononuclear U-(IV)Àcitrate complexes. 19,20,31 The dissolution experiments also indicate that a substantial amount of oxidized U(VI) species existed in the sediment and was dissolved in the presence of either citrate or EDTA (Figure 2e, f, g, h). The observation of oxidized U(VI) is consistent with previous studies, which showed the persistence of some forms of U(VI) species in the bioreduced sediments from the same research site with similar mineralogical 5,40 A particularly high concentration of phosphate was observed in the sediments and thought to be responsible for the formation of stable U(VI)-phosphate minerals.…”

“…This concern was evident by observations that more than 80% of U(IV) remained in the aqueous phase when citrate was used as an electron donor for the bioreduction . Furthermore, the formation of soluble U(IV)−citrate complexes was confirmed by spectroscopic techniques including ultraviolet absorption and extended X-ray absorption fine structure (EXAFS) spectroscopy. , Remobilization of U(IV) by complexing organic ligands could thus be a potential concern in the stabilization of uranium through biological reductive precipitation or immobilization.…”

Dissolution and Mobilization of Uranium in a Reduced Sediment by Natural Humic Substances under Anaerobic Conditions

“…Perhaps it is only in the presence of strong complexing agents such as humic acid and synthetic organic ligands such as ethylenediaminetetraacetic acid (EDTA) and citrate that the dissolution and mobilization of reduced U(IV) could become a significant concern under anaerobic environments. These synthetic organic ligands commonly occur as cocontaminants with metals and radionuclides such as uranium, technetium (Tc), and chromium (Cr) at many contaminated sites , , although their effects on the dissolution and mobilization of reduced Cr(III) and Tc(IV) are yet to be determined. Seasonal, hydrogeological, and land-management factors , might further contribute to changes in the groundwater chemistry such as modifications in the concentrations of dissolved oxygen and organic ligands, thereby influencing the long-term stability and mobility of these contaminants in the subsurface environment.…”

“…Similarly, in a study of the effects of humic materials on the biological reduction of uranium, Gu et al found that reduced U(IV) formed soluble complexes with humics and thus remained in the solution phase. The mechanisms of complexation between U(IV) and synthetic ligands, such as citrate, also were studied by Francis and colleagues , , who showed that U(IV) was present as a biligand mononuclear U(IV)−citrate complex. All of these previous investigations thus indicate that the reduced U(IV), even under strict anaerobic conditions, could become complexed and solubilized in the presence of organic ligands, such as natural humic substances.…”