According to a news release from the University of South Carolina, an ultra-thin membrane could transform gas and water purification.

Engineers at the school have built a graphene oxide membrane less than two nanometers thick with high penetration selectivity between hydrogen and carbon dioxide gas molecules. According to engineers, the selectivity rests on molecular size. Hydrogen and helium move somewhat easily through the membrane, but carbon dioxide, oxygen, nitrogen, carbon monoxide and methane penetrate much more leisurely.

“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 team leader Miao Yu, a chemical engineer in University of South Carolina’s College of Engineering and Computing. “The membrane behaves like a sieve. Bigger molecules cannot go through, but smaller molecules can.”

The membrane is built on the surface of a permeable aluminum oxide support. Flakes of graphene oxide, with widths of 500 nm but only one carbon atom thick, were placed on the support to develop a circular membrane approximately two square centimeters in area.

The membrane is like an overlapping collage of graphene oxide flakes. Gas molecules are searching for holes anywhere they can be located, and in a membrane constructed of graphene oxide flakes, there could be holes within the flakes or holes between the flakes.

To overcome the issue of holes between flakes, microporous membranes have usually been extremely thick. “At least 20 nm, and usually thicker,” Miao noted.

The engineers came up with a way of adapting a membrane without those “inter-flake” leaks. They scattered graphene oxide flakes in water and utilized sonication and centrifugation methods to adapt a dilute, homogenous slurry. These flakes were then placed on the support by simple filtration.

Their thinnest membrane was 1.8 nm thick. This particular membrane only allowed gas molecules to move through holes in the graphene oxide flakes themselves, according to the engineers. They discovered by atomic force microscopy that a single graphene oxide flake had a thickness of about 0.7 nm.

The technology has a wide range of uses. For example, the efficient separation of carbon dioxide from other gases is an extremely high research priority due to worries about carbon dioxide as a greenhouse gas.

According to Miao, the membrane could also be used to purify the large quantities of tainted water generated by fracking.

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

The study’s findings are described in greater detail in the journal Science.