Microbes May Be Mobilizing Uranium From an Abandoned Australian Mine's Waste Piles

In 1944, rocks from the Mount Painter mine in the middle of Australia were shipped to the United States for experiments tied to the Manhattan Project. The mine's later history was quieter. It closed in 1965 and was eventually folded into the Arkaroola Wilderness Sanctuary, and the uranium and rare earth elements left in its dry waste piles, known as tailings, were thought to be more or less safely locked away.

Recent samples from the surface of the tailings at the mine's Number 6 Workings suggest that assumption may be wrong. The material has supposedly lain undisturbed for more than 80 years. Yet something has been carrying potentially dangerous minerals up toward the surface of the piles, where they have built up at unexpectedly high concentrations.

In a study published in the Journal of Hazardous Materials, the researchers propose that microbes living in the rubble are responsible. They suggest the organisms are moving these metals into the top 10 centimeters of the waste in the form of polymetallic nanoparticles. Geoscientist Joël Brugger says that scientists have long assumed such toxic metals were securely held by nature in arid environments, but that these tiny organisms are proving otherwise. He describes them as microscopic factories that break down stable minerals and mobilize elements like uranium into the surrounding ecosystem.

The finding comes with a caveat. Further studies are needed to confirm that the microbes are actually behind the changes in the tailings. For now it is the team's best guess, because there are few other good explanations. Researchers elsewhere in the world have also documented radiation-resistant microbes interacting with uranium waste.

The main worry is chemical. The microbes appear to convert minerals from insoluble phosphates into colloidal forms that travel easily through the environment. Rain is rare in semi-arid Arkaroola and tends to arrive as sudden summer thunderstorms. Colloids of uranium and similar minerals are readily suspended in rainwater, which could carry them much further from the mine, particularly during the flash floods that sometimes strike the area. Uranium is highly radioactive, and exposure, for example through contaminated drinking water or inhaled fine particles, can seriously harm humans and other living things.

The paper calls for a serious rethink of how mining waste is handled. Arkaroola is protected from further mining, but its radioactive legacy is clearly not finished. Mount Painter's Number 6 Workings is just one of many mines abandoned after the spread of nuclear weapons in the twentieth century, and uranium production continues around the world.

Demand is also climbing. Brugger and colleagues write that global development, the shift of energy systems away from fossil fuels, and the electrification of transport are all pushing up demand for base metals such as copper, critical minerals such as rare earth elements, and energy minerals such as uranium. Mining waste is expected to grow quickly, especially as ore grades fall. Even when uranium is not a mine's target, many rare earth deposits are bound up with uranium and thorium, which end up in the waste stream.

Environmental microbiologist Santonu Sanyal says the volume of waste will rise significantly as operations scale up to meet unprecedented demand. Understanding exactly how native microbes interact with that waste, he says, is vital to preventing unseen, long-term contamination and to developing the sustainable mine of the future. The next step is the confirmation work that would show whether the microbes really are the cause.