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Part III: Dissimilatory Bacteria in Uranium Reduction

Last time, we explored how dissimilatory bacteria can be used to generate electricity in devices called Microbial Fuel Cells. In part one, I mentioned that the dissimilatory properties of bacteria like Shewanella can potentially also be used for solving part of the problem of radioactive uranium contaminations. When I mentioned this idea to my friend who is a soil scientist, I realized that I did not know how these bacteria can be used to contain uranium contamination. Curious about how exactly this works, I found a review article titled “Uranium Reduction” in the Annual Review of Microbiology written by Judy D. Wall and Lee R. Krumholz (Vol. 60: 149-166, Oct 2006). Following is a summary of what I found.

Uranium is a metal with the symbol U on the periodic table. It has the atomic number 92 which means that it has 92 protons and electrons. In nature, occurs in three different forms U-238 (~ 99.238 %), U-235 (0.711 %), and U-234 (0.0058%) where each number refers to the combined number of protons and neutrons [1]. All forms are slightly radioactive, but it is the ability of this metal to react with any non-metal that makes this metal chemically very toxic [2].

Most people may not know that uranium contaminations are a problem. Part of it has to do with the fact that uranium is not rare being the 49th most abundant element in the earth crust. In recent years, with the advent of nuclear power, and nuclear weaponry, mining, refining, and nuclear testing has lead to different forms of environmental contamination in some areas. Just within the United States, the Department of Energy (DOE) has identified 120 sites covering 7280 km2 in 36 states and territories. This is about equivalent to the area of the entire urban area of New York City. Mining of uranium in Colorado, New Mexico, and Arizona for example has led to local contaminations of the water as uranium salts leach into the ground water.

The solution to soil and water contaminations is normally to employ microorganisms to bioremediate the contaminated area. Specialized bacteria are able to break toxic contaminants down into less toxic substances.

The problem with uranium and other heavy metals is the ability of these metals to form water soluble complexes and compounds with other non-metallic and organic substances that are often harmful to living organisms when consumed. Heavy metals being elemental also cannot be broken down any further. Because of that the only currently known strategic approach to bioremediation of heavy metals of contaminated soils is to decrease availability of the heavy metal to living organism (called bio-availability).

In water or soil sources, uranium is mostly present as soluble salts in form of uranyl (UO2+2) which has an oxidation state of U(+6). Many metals have the ability to assume different oxidation states. It turns out that when uranium has an oxidation state of +4 only it becomes much less water soluble. By changing the oxidation state, thus, uranium could be precipitated out of the water. As a reminder, dissimilatory bacteria are capable of externally passing electrons onto metal oxides. By gaining electrons, the metal oxide is reduced to a lower oxidation state. It turns out that a growing number of dissimilatory bacteria are capable of reducing uranium. Our friend Shewanella is one of them.

Many issues remain to be solved. Some include the possibility of redissolving precipitated uranium, and the question of how to sequester the precipitated uranium to to contain its spread, but the ability to use microorganisms to precipitate uranium out of groundwater represents an important step because this reduces the bioavailability of uranium to living organisms.

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