Uranium, like other heavy metals, is formed in supernova explosions. There are many steps from there to the deposits we mine on Earth, however, and it seems we may have been wrong about the last of these. According to a new theory, much of the uranium mined for nuclear power or weapons was processed by bacteria that substitute uranium for oxygen in their biochemical reactions. This could have implications for the practicality of mine restoration and the risk of mine leakage.
Uranium makes up 2-4 parts per million of the Earth's crust, but has sometimes become concentrated, most often in sandstone deposits, making its extraction economically feasible. However, Professor Thomas Borch and Dr Amrita Bhattacharyya of Colorado State University have challenged this idea in Nature Communications.
Uranium can exist in many chemical compounds, including the crystalline mineral uraninite, (a mix of UO2 and U3O8). This has been thought to be the dominant form within deposits, and textbooks refer to this arising from ions transported in groundwater being reduced by elements such as sulfur. Although it is known that bacteria produce organic uranium compounds, and uranium deposits in China have previously been proposed as biological in origin, this has been seen as the exception, not the rule.
In the process of investigating the uranium at some sites in Wyoming, Borch came to question whether some of the uranium he was seeing might be of biological origin. He and Bhattacharyya took 200-meter-long (650-foot-long) samples and subjected them to analysis using techniques that were not available when the uraninite theory was developed. These methods have not been applied since, because everyone thought they knew the answer they would get.
The authors found that 89 percent of the uranium in Wyoming deposits is bound to either organic matter or carbonate. They attribute what they found to bacteria that, instead of using atmospheric oxygen as the basis of chemical reactions, get energy by reducing uranium's oxidation state from U(VI) to U(IV).
Leaving aside the fact that “uranium-breathing” bacteria surely displace arsenate-reducers as the most metal life forms in existence, the work has important implications for miners. Borch noted in a statement that mining permits usually require restoration to pre-mining conditions, but to achieve this “we had better understand those pre-mining conditions. The baseline may not be what we thought it was."
Moreover, non-crystalline uranium converts more easily to water-soluble forms that can get into groundwater, posing hazards to drinking water or the environment.
Several types of uranium-reducing bacteria are known, but which of these is responsible? Since the Wyoming deposits were formed 3 million years ago, it is possible the species that made them are extinct.