Water from Sydney Harbour has been purified with the help of graphene coated onto a synthetic fluoropolymer membrane, researchers have reported. Graphene is usually water repellent, so to get around this problem the researchers at government research organisation CSIRO created a film with microscopic nano-channels that let water pass through, but not pollutants.
‘Commercial membranes exhibited rapid and significant degradation in membrane properties when contaminants such as oil or chemicals were introduced into saline water systems in the absence of graphene,’ explains study author Michael Seo at CSIRO in Australia. Once graphene was added, no clogging was seen and contaminants such as oils, chemical, dissolved salts were rejected with 99% plus efficiency, the team reported (Nature Commun., doi: 10.1038/s41467-018-02871-3).
The inability of membranes to handle a wide spectrum of pollutants is an unsolved problem for water treatment.
Many previous studies have used graphene or graphene oxide to filter water via a pressure gradient. However, the scientists in Australia relied on moving water vapour across a porous membrane due to a temperature gradient as low as 10°C. Water vapour moves to the cold side of the membrane, leaving behind dissolved salts, organics and waste.
Membrane distillation is viewed as an emerging water purification technology. Here, the same fluoropolymer membrane filters out salt but not organic contaminants, which are stopped by the added graphene layer. ‘That is the unique beauty of this special graphene,’ says Seo.
Nanochannels in the graphene film allow only water molecules to pass through, whereas dissolved chemicals or oils are much bigger molecules and so get rejected. A membrane of 4cm2 processed half a litre of harbour water a day, running for 72 hours.
‘It is difficult to think that a graphene membrane can achieve such a good permeability to water vapour, but the experimental results show it is now possible,’ comments material scientist Mauricio Terrones at Penn State University. Previous reports with graphene laid down by chemical vapour deposition showed it to be highly impermeable. This is the first time ‘a pure 2D graphene membrane was used,’ says Terrones.
Over 2bn people do not have access to clean and safe drinking water, which means millions of children die each year from diseases linked to inadequate water supply, sanitation and hygiene. ‘This technology can create clean drinking water, regardless of how dirty it is, in a single step,’ says Seo. ‘All that’s needed is heat, our graphene, a membrane filter and a small water pump. We are hoping to commence field trials in a developing world community next year.’