THREE MEN in safety goggles stare intently at a clear plastic box filling gradually with fog. Droplets begin to form on the walls. They swell and eventually begin to trickle into the base of the fish tank-like container, forming small puddles. Omar Yaghi smiles broadly and congratulates his colleagues.
This seemingly prosaic moment in a laboratory at the University of California, Berkeley, may go down in history as the moment scientists turned the tide against water shortage. “Seeing those water droplets was one of the most amazing experiences of my life,” says Yaghi. “It meant I could create water where there is no water.”
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That was a couple of years ago. Yaghi is now moving beyond drops and puddles, and breaking out of the lab. In his most recent trials, he sucked significant amounts of water from even arid desert-like air. The secret to it all? A sprinkling of extraordinary synthetic crystals based on a form of chemistry he helped pioneer two decades ago.
The potential implications are dramatic. The United Nations says the number of people living in areas of absolute water scarcity, where available supplies are insufficient to meet demand, will rise from 1.2 billion in 2014 to 1.8 billion in 2025. Even places with money to spend on reservoirs, water recycling and technologies like desalination are vulnerable to greater risks of drought as global temperatures and populations rise. Last year, for example, Cape Town in South Africa narrowly avoided “day zero”, the point at which water runs so low that residents are put on survival rations.
Yaghi knows all about water shortage. He was born in Jordan in 1965, the sixth of 10 children. His parents Mwannes and Sadika were Palestinians who fled the fighting following the establishment of the state of Israel in 1948. He lived in one room with his brothers and sisters, and the family’s cattle, in a home without electricity. Water was piped in for a few hours each week, and Yaghi sometimes had to get up at dawn to open a valve to ensure it reached their tank. “You had to think about every drop,” he says. “Water was absolutely precious.”
Mwannes sold the gold jewellery he had given Sadika as a wedding gift and used the proceeds to set up a butcher’s shop. This paid for Yaghi to attend a private Christian missionary school, where his love of chemistry was sparked at the age of 10 when, one lunchtime, he sneaked into the library when it was supposed to be closed. There, in a book, he saw strange diagrams of different sized balls connected by sticks, which he later learned were molecular models. Yaghi sensed he was looking at a template for everything around him, that he had encountered some hidden truth. And the feeling was heightened by his being somewhere he wasn’t supposed to be. “It was like meeting a secret love,” he says.
When Yaghi was 15, his father sent him to join his older brother Khaled in the US, even though he spoke little English and didn’t want to go. Yaghi arrived in New York, where he enrolled in a community college. He excelled and eventually became an assistant professor of chemistry at Arizona State University, where he set about finding new ways to link up those little balls and sticks to make previously unknown substances. Little did he know that they might one day help quench other people’s thirst.
“Even wealthy places will be vulnerable to drought as the planet’s temperature keeps rising”
By this time, in the early 1990s, chemists had made major strides in building organic compounds, which are based on carbon. However, when it came to constructing inorganic compounds, which are based mainly on any of the other elements across the periodic table, they had limited control over the products of their experiments. It was often a case of mixing chemicals in a flask and seeing what happened. Critics described the techniques used as “shake and bake”, “mix and wait” and “heat and beat”.
Among those seeking to move beyond this educated guesswork was the chemist Richard Robson at the University of Melbourne. From 1989, Robson gave the world new ways to design and make compounds called coordination polymers. These are extended arrays of atoms or ions, usually metals, linked together by longer molecules known as ligands. By changing the type of metal, Robson could change how many ligands would bind to it. A metal that bound two ligands might produce a string-like polymer, for instance. A metal that bound six ligands might produce a cubic lattice.
“This will improve access to clean water for millions of people”
Soon, chemists had made a whole family of new materials, each with a different structure and properties. And because they could be made to remain in ordered, crystalline forms, they were easy to study. The only trouble was that Robson’s polymers were liable to react with other substances, making them unstable and not immediately useful.
In the mid-1990s, Yaghi and his team began making coordination polymers consisting of negatively charged ligands joined not by single metal atoms but clusters of them. These materials, which he called metal organic frameworks (MOFs), had stronger bonds than previous coordination polymers, giving them greater stability.
He didn’t have the field entirely to himself. at Kyoto University in Japan and at the Hong Kong University of Science and Technology also did early work on MOFs. But chemists across the world sat up and took note of Yaghi’s work in 1999, when he , a zinc-based polymer. It wasn’t just that it was stable to an impressive 300˚ C. The incredible thing was that the gaps in this material gave it an internal surface area of 2900 square metres – nearly half a soccer pitch – per gram. Gas molecules accumulate and form thin films on surfaces. This means that, bizarre as it sounds, a canister containing MOF-5 can hold far more gas than an empty canister at the same pressure, thanks to the material’s high surface area.
Their super sponge properties made MOFs look useful for a wide range of applications, such as storing carbon dioxide captured from power generators or gas to power cars. No wonder that at least 20,000 different MOFs have been made in the past 20 years.
These materials have been slow to fulfil their potential, however. The German chemicals giant BASF developed vehicles with natural gas tanks containing MOFs with Yaghi’s help, anticipating greater demand for greener fuel. But a plan to launch them in 2015 was shelved because a crash in petrol prices skewered the economic rationale.
The first commercial applications for MOFs emerged in 2016, in the form of cylinders that store toxic gases used in the electronics industry and sachets to release a gas to block the effects of ethylene, a hormone released by fruit and vegetables that speeds ripening. But more widespread applications just haven’t materialised.
The thing that could finally change this is the fact that MOFs aren’t just good sponges, but extremely selective ones. MOFs have internal pores with specific sizes and shapes, making them ideal for taking up certain gases that fit those pores while excluding those that don’t. In 2013, Yaghi was studying how MOFs could separate carbon dioxide from water when he noticed one material that could rapidly take in water vapour, even in low humidity conditions, and then release it again when heated. “Immediately I thought, ‘Wow, this could be used in the desert’,” he says.
When Yaghi went on to , he found one, based on zirconium and called MOF-801, that performed especially well. Its internal pores were the perfect size and shape to let water in and out. Yaghi then worked with engineer at Massachusetts Institute of Technology to develop a palm-sized water harvesting prototype, consisting of MOF-801 crystals pressed into a sheet of copper, encased in a plastic box.
Watching the box
To harvest water, the box is left open overnight, allowing the MOF to suck water molecules from the air into its pores. In the morning, the top is replaced and the sun warms the MOF and the water inside it. This prompts the water to be released and condense on the walls of the box (see “Diagram”). In 2017, Yaghi’s group found the device could harvest water from air of 20 per cent relative humidity, which is similar to the conditions in many deserts. They went on to make a larger device that produced the equivalent of 140 millilitres of water per kilogram of MOF per day in the lab. It was in this device that Yaghi saw those incredible drops of water through his safety goggles.
All this proved that the technology works. But will it really curb our worsening water shortage? There are other ways of extracting water from air, including by simple condensation. This isn’t complicated: a cold surface will cool the air around it and force water vapour to form droplets. But it is power-hungry: think of a fridge with an open door. Even so, a few companies are already marketing devices that do this (see “Condensation stations”).
Another problem with Yaghi’s original devices is that they are based on zirconium. This metal may be resistant to corrosion and high temperatures, but it is expensive, at around $150 a kilogram. A cup of water produced by such a MOF is going to be pricey.
Yaghi, who recently set up a company called ., knows he needs to think about economics as well as chemistry if his plans for a household appliance to help those at threat of water scarcity are to come to fruition. To that end, he has been testing MOF-303, which is based on aluminium, a much cheaper metal. In 2018, he reported devices based on this material . He says he can boost that to more than 2 litres if he connects solar panels to the device and uses them to successively heat and cool it many times in 24 hours, rather than relying on day-night temperature cycles.
Might these refinements turn Yaghi’s dream into a reality? at the University of St Andrews, UK, who is developing wound dressings and catheters containing MOFs to assist healing, says that is still uncertain. “There could be issues around how long the device will last, the build up of bacteria,” he says. “It’s hard to know whether it’s feasible yet, but it’s a neat idea.”
Other leaders in the field think Yaghi isn’t far from cracking it. “Capturing water from the air in places where there is little water is spectacular,” says at Northwestern University in Illinois. “There will be challenges in scaling it up, but I don’t see any showstoppers. I think this will successfully improve access to clean water for millions of people.” If he is right, the day Yaghi watched droplets become drops, and drops become puddles, really will go down as a watershed moment.
Condensation stations
The village of Tojquia in the Cuchumatanes mountains of Guatemala is flanked by 35 towering nets, the largest about twice the size of a car parking space. Each net supplies villagers with up to 200 litres of freshwater a day. As the fog rolls over the high ground each morning, water droplets catch on the mesh, slide down, and then drip gradually into containers.
The Canadian charity FogQuest began exploring whether nets could provide water for the isolated community here 20 years ago. But this technique could be feasible in much less obviously foggy places. “I believe fog harvesting is quickly becoming viable in a wide variety of regions,” says engineer at Virginia Tech.
The design of the nets matters. Too coarse and fog passes through, too fine and the water droplets don’t slide down the net smoothly, clogging the strands. Boreyko and his colleague Brook Kennedy optimised the design, taking inspiration from the redwood trees of north California. They get a lot of their water from coastal fog, which condenses on their needle-like foliage and drips to the ground. Mimicking the parallel arrangement of the trees’ needles, Boreyko and Kennedy’s fog catcher design removes the cross fibres of traditional mesh nets to make . A prototype installed on a farm in Virginia has produced three times as much water as traditional mesh designs.
This level of productivity could make fog capture attractive even in inland locations. A surprisingly large number of places get at least some fog in the mornings, and harps can be built big to increase their productivity. Boreyko cites interest from places as varied as the Mexican plains to the tropics of Bangladesh.
Even places with no fog have water vapour in the air that can be turned into a liquid with the right kit. , a company based in Arizona, promises to do this using large, solar-powered condensers it calls hydropanels. Two can draw around 10 litres of water per day from the air, even in arid climates, it says.
The problem the company faces is the energy required. It takes about 1.4 megajoules to produce a litre of water, roughly the same amount of energy required to boil the water for 40 cups of tea. On the other hand, the tech does away with the need for infrastructure to send water from place to place, which requires energy to install. This system is also readily scalable – to get more water, just add more panels.
You can even tap the water vapour emanating from your sweaty colleagues, if that appeals. Late last year, a company called launched the UK’s first office water cooler that condenses water directly from the air. In future, the firm plans to sell an industrial unit big enough to serve a small town. Frank Swain
