Hydroxylated carbon-60 molecules might offer an environmentally friendly way to remove metals from hydrofracking and acid mining fluids. (Credit: Barron Group/Rice University)

carbon

Buckyballs pull toxic metals from liquids

Treated buckyballs not only remove valuable but potentially toxic metal particles from water and other liquids, but also reserve them for future use.

“It’s all very well to say I can take metals out of water, but for more complex fluids, the problem is to take out the ones you actually want,” says Andrew Barron, a chemist at Rice University.

Hydroxylated carbon-60 molecules
(Credit: Jeff Fitlow/Rice University)

He and colleague found that when buckyballs (carbon-60 fullerenes) go through the chemical process known as hydroxylation, they can aggregate into pearl-like strings as they bind to and separate metals—some better than others—from solutions.

Potential uses of the process include the environmentally friendly removal of metals from acid mining drainage fluids, a waste product of the coal industry, as well as from fluids used for hydraulic fracturing in oil and gas production.

How it works

Barron says the treated buckyballs handled metals with different charges in unexpected ways, which may make it possible to pull specific metals from complex fluids while ignoring others.

“Acid mining waste, for example, has large amounts of iron and aluminum and small amounts of nickel and zinc and copper, the ones you want. To be frank, iron and aluminum are not the worst metals to have in your water, because they’re in natural water, anyway,” Barron says.

“So our goal was to see if there is a preference between different types of metal, and we found one. Then the question was: why?”

The answer was in the ions

An atom or molecule with more or fewer electrons than protons is an ion, with a positive or negative charge. All the metals the Barron’s lab tested were positive, with either 2-plus or 3-plus charges.

“Normally, the bigger the metal, the better it separates,” Barron exlains, but experiments proved otherwise.

Two-plus metals with a smaller ionic radius bound better than larger ones. (Of those, zinc bound most tightly.) But for 3-plus ions, large worked better than small.

“That’s really weird,” Barron says. “The fact that there are diametrically opposite trends for metals with a 2-plus charge and metals with a 3-plus charge makes this interesting. The result is we should be able to preferentially separate out the metals we want.”

A dozen metals

Previous research in Barron’s lab showed that hydroxylated fullerenes (known as fullerenols) combined with iron ions to form an insoluble polymer.

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The recent experiments show that fullerenols combined with a dozen metals, turning them into solid cross-linked polymers. In order of effectiveness and starting with the best, the metals were zinc, cobalt, manganese, nickel, lanthanum, neodymium, cadmium, copper, silver, calcium, iron, and aluminum.

Barron says fullerenols act as chelate agents, which determine how ions and molecules bind with metal ions. Experiments with various metals showed the fullerenols bound with them in less than a minute, after which the combined solids could be filtered out.

The choices of aluminum, zinc, and nickel for testing were due to their co-presence with iron in acid mining drainage water. Similarly, cadmium was tested for its association with fertilizer and sewage sludge and copper with mining discharge. Nickel, lanthanum, and neodymium are used in batteries and drive motors in hybrid vehicles.

Barron says the research shows the versatility of the buckyball, discovered at Rice in 1985 by Nobel Prize winners Rick Smalley, Robert Curl, and Harold Kroto. It also points the way forward.

“The understanding we now have is allowing us to find alternatives to C-60s to design ways in which we can separate out metals more efficiently,” he says.

The Robert A. Welch Foundation and the Welch Government Sêr Cymru Programme supported the research, which appeared in the journal Dalton Transactions.

Source: Rice University

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