More than 300 million people around the world depend on drinking water extracted from the sea, but turning saltwater into freshwater isn鈥檛 always efficient. Computer pioneer Alan Turing had an idea more than 50 years ago that is just now being put to use to improve the process.
Two basic desalination methods exist: boil sea water and collect the evaporated pure water, or pump sea water through membranes that extract the salt. This process, called reverse osmosis, is favoured everywhere except in the Middle East, where boiling is cheaper. But with existing membranes there is always a trade-off between the speed the water flows through and how much salt they capture.
Advertisement
Now, inspired by mathematician Alan Turing鈥檚 only paper on chemistry published in 1952, 聽at Zhejiang University in Hangzhou, China and his colleagues have overcome this trade-off to make filtration membranes that allow water through much faster without sacrificing salt retention.
Although made from a conventional plastic called polyamide, the membranes have unusual surface patterns called Turing structures that the legendary codebreaker theorised in 1952. Some of the new membranes are covered with tiny welts or bubbles. Others have longer, ridge-shaped welts that give a stripy appearance when magnified from above.
These nanoscale bumps are made by simultaneously accelerating and decelerating a chemical reaction. In this case, during the process that creates polyamide plastic from long chains of monomeric building blocks.
Speed bumps
Zhang and his team add one extra substance that activates polymerisation and another that blocks it. Crucially, one diffuses through the reaction mixture faster than the other, so polymers form in some zones but are blocked in others, resulting in the surface welts or ridges, depending which reaction conditions are chosen.
These welts are the secret to the membranes鈥� improved output. 鈥淭he membrane with the Turing structures can remove salts from water up to five times faster than conventional filtration membranes,鈥� says Zhang. 鈥淏ut it has the same selectivity for removing salt.鈥�
The team put water laced with gold nanoparticles through the membrane and found that the nanoparticles collected around the bubbles and stripes, indicating the water was passing through there. Larger particles like salt are trapped in the uneven surface structures. When pitted against 16 commercially available desalination membranes, the ones with the Turing structures outperformed them all.
California, a state that has been blighted by prolonged droughts, ordered eight new desalination facilities in January. And as natural freshwater sources dwindle due to global warming and over-extraction, the numbers of people reliant on the sea for drinking water are likely to rise. Turing鈥檚 belated gift could mean they can make it much faster in the years to come.
Science
Read more: Sticky yet slick material pulls water from foggy or humid air
