Parting the waters, molecule by molecule

heavy water molecules in red and white and light water molecules in red and yellow passing through a series of gates with dragonflies at each pinch point, the heavy water passes through more easily
Incorporating dragonfly-shaped gate molecules into the material makes it far more efficient to separate water from heavy water, which has been difficult to separate due to their similar properties. Credit: Mindy Takamiya/Kyoto University iCeMS
November 10, 2022
Ellen Phiddian

Ellen Phiddian

Cosmos science journalist

A team of Chinese and Japanese researchers has figured out how to separate water from… water.

While it may not sound like a dazzling feat of molecular manipulation, this deed actually needs extreme finesse.

The process, described in a paper in Nature, could render key ingredients for nuclear reactors and pharmaceuticals cheaper to make.

A small proportion of water molecules have slightly heavier hydrogen atoms, or isotopes, in them.

This heavy water is referred to as D2O or HDO, as opposed to H2O: the D stands for ‘deuterium’, a hydrogen atom that has more mass than usual.

Heavy water is a key part of nuclear reactors, medical imaging, and some analytical techniques.

D2O, HDO and H2O are also called isotopologues: molecules with the same atomic structure, but atoms with different mass.

“Water isotopologues are among the most difficult to separate because their properties are so similar,” explains co-author Cheng Gu, a materials scientist at South China University of Technology.

Read More

“Our work provided an unprecedented mechanism for separating water isotopologues using an adsorption-separation method.”

The researchers’ technique revolves around a porous crystalline material, made from copper and carbon-based polymers.

When heated, the carbon-based parts of the material can “flip”, acting like a gate that lets molecules in and out of the material’s pores; like a dragonfly flapping its wings.

When the material is cooled, the trapped molecules can’t move.

The researchers found that these so-called “flip-flop dynamics” preferred typical, light, H2O molecules, leaving the two heavier isotopologues to adsorb much more slowly.

This means they can collect heavy water with ease, at room temperature.

“The adsorptive-separation of water isotopologues in our work is substantially superior to conventional methods due to very high selectivity at room temperature operation,” says co-author Susumu Kitagawa, from the Institute for Cell-Material Sciences at Kyoto University, Japan.

Heavy water isn’t the only application here: some medicines work better with heavier isotopologues too.

“We are optimistic that new materials guided by our work will be developed to separate other isotopologues,” says Kitagwa.

Please login to favourite this article.