New material separates water from heavy water

A research team led by Susumu Kitagawa of Kyoto University’s Institute of Cellular Material Science (iCeMS), Japan, and Cheng Gu of South China University of Technology, China, have made a material that can effectively separate heavy water from normal water at a temperature room.

Until now, this process has been very difficult and energy-consuming. The findings have implications for industrial—and even biological—processes that involve the use of different forms of the same molecule. The scientists reported their results in the journal Nature.

Isotopes are molecules that have the same chemical formula and whose atoms are bonded in similar arrangements, but at least one of their atoms has a different number of neutrons than the parent molecule. For example, a water molecule (H₂C) is formed from one oxygen atom and two hydrogen atoms.

The nucleus of each of the hydrogen atoms contains one proton and no neutrons. In heavy water (D₂O), on the other hand, deuterium (D) atoms are isotopes of hydrogen with nuclei containing a proton and a neutron. Heavy water has applications in nuclear reactors, medical imaging and biological research.

“Isotopes of water are among the most difficult to separate because their properties are so similar,” explains materials scientist Cheng Gu. “Our work provided an unprecedented mechanism to separate water isotopes using an adsorption-separation method.”

Gu and chemist Susumu Kitagawa, along with their colleagues, based their separation technique on a porous copper-based coordination polymer (PCP). PCPs are porous crystalline materials formed by metal nodes linked by organic linkers. The team tested two PCPs made with different types of connectors.

What makes their PCPs particularly important for isotopic separation is that the ligands flip when moderately heated. This flipping motion acts like a gate, allowing molecules to pass from one “cage” in the PCP to another. Movement is blocked when the material cools.

When the scientists exposed their “dynamic flip-flop crystals” to steam containing a mixture of normal, heavy, and semi-heavy water and then heated it slightly, they adsorbed the normal water much faster than the other two isotopologists did. Mainly, this process occurred within room temperature limits.

“The adsorptive separation of water isotopes in our work is substantially superior to conventional methods due to the very high selectivity in room temperature operation,” says Kitagawa. “We are optimistic that new materials inspired by our work will be developed to separate other isotopologists.”