Ultra thin graphene membrane can purify fracking water

Engineers at the University of South Carolina have constructed a graphene oxide membrane less than 2 nanometers thick with high permeation selectivity between hydrogen and carbon dioxide gas molecules.

The selectivity is based on molecular size, the team reported in the journal Science. Hydrogen and helium pass relatively easily through the membrane, but carbon dioxide, oxygen, nitrogen, carbon monoxide and methane permeate much more slowly.

“The hydrogen kinetic diameter is 0.289 nm, and carbon dioxide is 0.33 nm. The difference in size is very small, only 0.04 nm, but the difference in permeation is quite large” said Miao Yu, a chemical engineer in USC’s College of Engineering and Computing who led the research team. “The membrane behaves like a sieve. Bigger molecules cannot go through, but smaller molecules can.”

The advance has a range of potential applications. With widespread concerns about carbon dioxide as a greenhouse gas, the efficient separation of carbon dioxide from other gases is a high research priority.

Moreover, hydrogen represents an integral commodity in energy systems involving, for example, fuel cells, so purifying it from gas mixtures is also an active area of interest.

Yu also notes that the dimensions of the molecular sieve are on the order of the size of water, so, for example, purifying the copious amounts of tainted water produced by hydraulic fracturing (fracking) is another possibility.
Being able to reduce membrane thickness – and by an order of magnitude – is a big step forward, Yu said.

“Having membranes so thin is a big advantage in separation technology,” he said. “It represents a completely new type of membrane in the separation sciences.”