Puzzled by uranium

© 2011 EPFL

In sites scattered across the Earth, the extraction and processing of uranium for nuclear fuel and weapons has left aquifers contaminated. Immobilizing the contaminant through the action of bacteria has been considered a potential solution to limit its transport away from the contamination site. A recent article by Professor Rizlan Bernier-Latmani and collaborators, published last week in Geochimica et Cosmochimica Acta, points to new unknowns and paints a more complex picture.

Less than a year ago, the nuclear power industry had the wind in its sails, thanks to increasing global energy consumption and public support for carbon neutral power generation. Then came Fukushima. Although the future of nuclear energy has become less certain - Switzerland and Germany have decided to abandon it altogether - demand for uranium is not expected to decrease; besides its use as an energy source, uranium is in high demand for its military and medical applications. And while a significant fraction of uranium used today is recovered from decommissioned Russian nuclear submarines, those stocks will eventually become exhausted, renewing the need to mine for the radioactive metal.

Uranium is a naturally occurring radioactive element that can be found in low concentrations throughout the earth’s crust. Extracted from uranium ore, most commonly uraninite (UO2), it becomes a hazardous contaminant when mined and processed, since this produces soluble oxidized uranium, U(VI). Once released, it quickly reaches groundwater, threatening potential subsurface sources of drinking water. Ultimately, it makes its way to rivers. So how could the uranium be kept from reaching the groundwater? Sealing off the area, especially underground, doesn’t seem like a viable option. But what if there were a way to make it insoluble, stopping it in its tracks?

Nature, the mother of all inventions, appears to be equipped to deal with the problem of soluble uranium. It turns out that different types of bacteria found beneath the earth’s surface process uranium while carrying out their metabolism. It has been observed that, in doing so, the bacteria transform the soluble uranium into insoluble uraninite (UO2), the common form of uranium ore. This mechanism may have been at play in the formation of the uranium ore deposits mined today.

Just like our bodies, the earth’s subsurface is awash with bacteria. Underground, microbial populations spend much of their time in a dormant state. With the right kind of “food”, bacteria could be forced into an active state, where, through the side effects of their own metabolism, they would immobilize the uranium by transforming it into uraninite. For a while, there was a consensus that this approach would work. Problem solved, or so it seemed.

In their recent publication, Professor Rizlan Bernier-Latmani and her collaborators present research that shows that things are not quite that simple. Using laboratory experiments, they were able to show that the bacteria process uranium in multiple ways. Critically, and contrary to expectation, only a fraction of the uranium appears to be immobilized as uraninite. The dominant fraction winds up in a different, unexpected, and until today, poorly understood form: as monomeric U(IV). Bad news, since it turns out that monomeric U(IV) is much more soluble than uraninite, increasing its chances of entering the water cycle.

What exactly is monomeric U(IV), and how does it form? These are the questions that are keeping Bernier-Latmani and her research group busy. “We don’t know exactly what causes its formation [...], because thermodynamically, uraninite should form.”, she explains. Analyses show that the uranium appears to bind to hair-like strands that stick out of the bacteria like tentacles. But much still remains to be learned.

With a full quiver of experimental techniques, from synchrotron spectroscopy to genome-based analysis methods, to the newest addition, Scanning Transmission X-ray Microscopy (STXM), Bernier-Latmani is determined to learn more about the fate of monomeric U(IV) and understand the mechanisms underlying its creation. “Ideally we would find a way to control which product is formed [...] to make sure uraninite is formed immediately, and not monomeric U(IV).”