Graphene flakes soak up radioactive waste

RICE (US) — Researchers are “amazed” by graphene oxide’s ability to quickly remove radioactive material from contaminated wastewater.

Microscopic, atom-thick flakes of graphene oxide bind quickly to natural and human-made radionuclides and condense them into solids. The flakes are soluble in liquids and easily produced in bulk.

The discovery, reported in the journal Physical Chemistry Chemical Physics, could be a boon in the cleanup of contaminated sites like the Fukushima nuclear plants damaged by the 2011 earthquake and tsunami, says Jim Tour, professor of mechanical engineering, materials science, and computer science at Rice University.

It could also cut the cost of hydraulic fracturing for oil and gas recovery, and help reboot American mining of rare earth metals, he says.

A new method for removing radioactive material from solutions developed by chemists. The vial on the left holds microscopic particles of graphene oxide in a solution. On the right, graphene oxide is added to simulated nuclear waste, which quickly clumps for easy removal. (Credit: Anna Yu. Romanchuk/Lomonosov Moscow State University)

Graphene oxide’s large surface area defines its capacity to adsorb toxins, according to Stepan Kalmykov, professor of radiochemistry at Lomonosov Moscow State University. “So the high retention properties are not surprising to us. What is astonishing is the very fast kinetics of sorption, which is key.”

“In the probabilistic world of chemical reactions where scarce stuff (low concentrations) infrequently bumps into something with which it can react, there is a greater likelihood that the ‘magic’ will happen with graphene oxide than with a big old hunk of bentonite,” says Steven Winston, an expert in nuclear power and remediation who is working with the researchers. “In short, fast is good.”

Determining how fast was the object of experiments. The lab tested graphene oxide synthesized with simulated nuclear wastes containing uranium, plutonium, and substances like sodium and calcium that could negatively affect their adsorption.

Even so, graphene oxide proved far better than the bentonite clays and granulated activated carbon commonly used in nuclear cleanup.

Graphene oxide introduced to simulated wastes coagulated within minutes, quickly clumping the worst toxins, Kalmykov says. The process worked across a range of pH values.

“To see Stepan’s amazement at how well this worked was a good confirmation,” Tour says.

The researchers focused on removing radioactive isotopes of the actinides and lanthanides—the 30 rare earth elements in the periodic table—from liquids, rather than solids or gases.

“Though they don’t really like water all that much, they can and do hide out there,” Winston says. “From a human health and environment point of view, that’s where they’re least welcome.”

Naturally occurring radionuclides are also unwelcome in fracking fluids that bring them to the surface in drilling operations, Tour says.

“When groundwater comes out of a well and it’s radioactive above a certain level, they can’t put it back into the ground,” he adds. “It’s too hot. Companies have to ship contaminated water to repository sites around the country at very large expense.”

The ability to quickly filter out contaminants on-site would save a great deal of money, he says.

Tour sees even greater potential benefits for the mining industry. Environmental requirements have “essentially shut down US mining of rare earth metals, which are needed for cell phones,” he says.

“China owns the market because they’re not subject to the same environmental standards. So if this technology offers the chance to revive mining here, it could be huge.”

Capturing radionuclides does not make them less radioactive, says Tour, just easier to handle.

“Where you have huge pools of radioactive material, like at Fukushima, you add graphene oxide and get back a solid material from what were just ions in a solution,” he says. “Then you can skim it off and burn it. Graphene oxide burns very rapidly and leaves a cake of radioactive material you can then reuse.”

The low cost and biodegradable qualities of graphene oxide should make it appropriate for use in permeable reactive barriers, a fairly new technology for in situ groundwater remediation.

The Office of Naval Research Multidisciplinary University Research Initiative and the Air Force Office of Scientific Research funded the work at Rice. The Ministry of Education and Science of the Russian Federation and the Russian Basic Research Foundation funded the research at Moscow State.

Source: Rice University