A possible new way to deliver clean water

Australian and Chinese researchers have created an ultrathin sieve membrane from 2D nanosheets they say can filter potentially harmful ions such as lead and mercury from water.

Developed with water-stable Metal Organic Frameworks (MOFs), it has high porosity and the potential to deliver clean water for millions of people through purification and desalination processes. It also can be used in the separation of gases and solvents.

Writing in the journal Science Advances, a team led by Australia’s Monash University and ANSTO (Australia’s Nuclear Science and Technology Organisation) reports that in testing the membrane performed steadily for more than 750 hours using limited energy.

The nanosheets are made from water-stable monolayer aluminium tetra-(4-carboxyphenyl) porphyrin frameworks – termed AI-MOFs –exfoliated to just a nanoscale in thickness: one thousand-millionths of a metre.

That was “a daunting challenge”, says research co-leader Xiwang Zhang, because MOF membranes are “typically thick and suffer from insufficient hydrolytic stability”.

Polymers are the most widespread membrane materials, largely owing to their easy processability and low cost, the study suggests. However, traditional polymeric membranes for ion separation from water usually contain a dense selective layer, leading to limited selectivity.

Nanoporous membranes, where uniform nanopores act as the sieving role, may overcome this limitation.

The new study, involving analysis, suggests that the intrinsic nanopores of Al-MOFs nanosheets facilitate the ion/water separation by creating vertically aligned channels as the main transport pathway for water molecules.

“We use an instrument called the Powder Diffraction beamline at ANSTO’s Australian Synchrotron to understand the difference between the molecular structure of nanosheet samples, and samples at different temperatures, in order to test water purification performance,” says ANSTO’s Qinfen Gu, the co-lead author.

“The technique, called in-situ, high temperature powder X-ray diffraction characterisation, was conducted on the nanosheets, and during the process there were no obvious variations in the samples at increasing temperature, demonstrating their robustness.”