|Fig. 1: Uranium Processing Mill in Colorado. (Source: Wikimedia Commons)|
Uranium is the heaviest element occurring in nature and is an abundant element. It has naturally occurring radioactive isotopes with an average concentration in the earth's crust of about 3 mg/kg. Given this prevalence, there is some concern in regards to the increase of uranium concentration in waters next to Uranium mining operations as they are known to increase the levels by 1ug/dm-3. Given U-238's long half-life of around 4.5 billion years, it is important to find ways to clean it up from the environment and the waters. The uranium removal happens through the following methods, electrochemical treatments, ion exchange and biosorption, direct chemical methods or intracellular sequestration by growing plant, algal and microbial cells. The most practical method financial is often the microbial treatment to remove uranium from waters, and here I will describe the basis in which that happens. 
In Germany, there has been a lot of uranium mining and milling between 1952 and 1989 that produced around 231,000 tons of uranium. This caused the pollution to be studied, focusing on ways that microbial communities could reduce the radionuclides and heavy metals. In Germany, many bacterial strains have been cultivated and their interactions with the metals have been studied, finding that the cells of the strain could bind selectively and reversibly to many heavy metals including uranium, copper, lead, and aluminum. Example of such bacteria is Bacillus sphaericus JG-A12. 
The three-step process described in Kalin et al. is as follows.  First, the uranium is linked with the organic particles whether they are plants, algae, or microbes in the water column. The second step is that the conditions in which these particles sink into the organic sediments is put under observation. Thirdly, the anaerobic conditions in the sediments is also provisioned specifically with the metal-reducing microbial populations. This oxidizes the uranium and the particulates settle and become reduced and bio mineralized. The main benefit of using the living bacteria for the first step in the process is that the process could be repeated multiple times as the bacteria grows. The most common example of an organism that carries out this process is algae, and the right environment must be provided in order to give it the perfect conditions to get rid of the uranium. Moreover, different types of algae are used for different functions. For example, some algae is used for toxicity tests as some are more sensitive to uranium than others. [3,4]
In the US, the main prevalence of uranium contamination occurs in Colorado (example shown in Fig. 1), New Mexico, and Arizona, but has been recently managed by the US Department of Energy Uranium Mill Tailing Remedial Action (UMTRA) program.  This program managed to contain the levels of uranium in groundwater, however, there still exists examples of places where uranium levels remain high. One example discussed by Wall and Krumholz is at Bear Creek Valley site at the Oak Ridge National Laboratory in Tennesse.  There, they have experimented ways where aenaerobic bacteria could be able to reduce the uranium in groundwater to soluble uranium minerals. These studies, however, showed that indigenous microorganism in groundwater are often inhibited by not having enough electron donors. Therefore, it has been found that adding electron donors to supplement the bioreduction of uranium would make the bioremediation better. Examples of electron donors include acetate, lactate or ethanol. Moreover, in this report, it states that for "effective long-term immobilization of uranium through bioreduction, there should be a low probability through bioreduction, there should be a low probability of abiotic or biotic reoxidation of the insoluble U(IV)".  This is because the anoxic conditions turned out to be needed for stability of uranium removal. 
Finally, in Abedlouas et al. we learned that after the addition of electron donors, the uranium is precipitated as hydrated uraninite (UO2:xH2O).  During the reduction other metals could be removes as well such as magnesium (IV) and Iron (III) that comes out of the soil. Some more uranium sulfide is formed that then serves as a redox buffer. Therefore, bacteria shows to work as being a good reducer of Uranium that is prevalent in nature, most commonly being supplemented by sulfate reduces and additional nutrients to the groundwater. 
© Dina Al-Alami. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 J. D. Wall and L. R. Krumholz, "Uranium Reduction," Ann. Rev. Microbiol. 60, 149 (2006).
 K. Pollmann, "Metal Binding by Bacteria from Uranium Mining Waste Piles and Its Technological Applications," Biotechnol. Adv. 24, 58 (2006).
 M. Kalin, W. N. Wheeler, and G. Meinrath, "The Removal of Uranium from Mining Waste Water Using Algal/Microbial Biomass," J. Environ. Radioactiv. 78, 151 (2004).
 M. L. Merroun and S. Selenska-Pobell, "Bacterial Interactions with Uranium: An Environmental Perspective," J. Contam. Hydrol. 102, 285 (2008).
 A. Abdelouas et al., "Biological Reduction of Uranium in Groundwater and Subsurface Soil," Sci. Total Environ. 250, 21 (2000).