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    How do you make salty water drinkable? The hunt for fresh solutions to a briny problem

    High levels of sodium in drinking water have been linked to increased rates of pre-eclampsia in Bangladesh.Credit: Paula Bronstein/Getty

    People have been separating salt from water for millennia, harvesting both salt and fresh drinking water from salty seawater. But there are limits to what can be done — sometimes with drastic consequences. When people in ancient Mesopotamia couldn’t work out how to desalinate their irrigation water and prevent salts from accumulating in their soils, their society collapsed. “It’s kind of the world’s oldest, most boring, but serious problem,” says Sujay Kaushal, a hydrologist at the University of Maryland in College Park.This problem is now growing more pressing, as salinity levels creep up in fresh waters for a slew of reasons. Rising sea levels are pushing salt into coastal groundwaters, while excessive groundwater extraction in other places is drawing deeper, saltier waters up into aquifers. And human activities — from deicing roads to washing clothes and fertilizing fields — are polluting surface waters with many kinds of salt. Last October, Kaushal and his colleagues reported that salt levels in major streams and rivers around the world are booming; some bodies of water are now several times saltier than they were a few decades ago1. Freshwater salinization is a massive global problem, not just a regional one, he says.A second, related issue is the growing burden of problematic waste brines. A variety of industries — from oil and gas extraction to the desalination plants that produce drinking water — create salty waste waters that are costly to dispose of. “We need to do something with the brine,” says Menachem Elimelech, an environmental engineer at Yale University in New Haven, Connecticut.
    New desalination technique yields more drinkable water
    On the flip side of these problems is the salt-mining industry. Hundreds of millions of tonnes of mineral salts are extracted every year from rock or briny waters. The Dead Sea is a major source of potassium; the Great Salt Lake in Utah, magnesium. Mining companies seeking supplies of lithium, a metal crucial for batteries and green technologies, are turning to brines around the world.Researchers who sense opportunities in this field are hoping that they can extract salts from waste brines, turning a problem into a profit while squeezing out more fresh water.To do all that, scientists are now exploring techniques to separate salt from water more efficiently, using electricity, new materials and solvents. With a wide range of brine chemistries to tackle and a host of different goals, there isn’t one “killer” technology, says Shihong Lin, an environmental engineer at Vanderbilt University in Nashville, Tennessee. “This is like a thousand different problems,” says Lin.Super saltyIt’s clear that extremely salty water isn’t healthy for humans, animals or plants. Drinking seawater, which contains about 3.5% salt, most of it sodium chloride, can shut down our kidneys and be fatal. Brackish groundwater, which contains around 0.1–1% salt, is typically desalinated before drinking. But the health impacts from less-salty water are murky. “Whether you can drink it for 30 years without any problem, not increasing any risk of disease, nobody knows,” says Lin.There’s no agreed limit for a safe level of salt in drinking water. The World Health Organization suggests that drinking water should have sodium levels below 200 milligrams per litre (0.02%) and chloride levels below 250 mg/L (0.025%), but these guidelines are based mainly on taste. Kaushal points to a study that has linked drinking water with sodium levels of more than 0.03% to increased rates of pre-eclampsia and gestational hypertension among pregnant people in Bangladesh2. Salty water could also pose indirect harms, for instance by helping heavy metals to leach out of soils or plumbing as these metals swap places with salt in the water, says Allison Lassiter, a social scientist at the University of Pennsylvania in Philadelphia.With salinity on the rise, specialists agree that the top priorities should be to stem salt-polluting human activities, conserve fresh water and reuse waste waters. But many researchers, including Lassiter and Kaushal, expect desalination to be increasingly needed as a buffer against freshwater scarcity.

    Lithium mines in Chile spread brines over vast areas to evaporate the water. Researchers are seeking alternative, more compact methods of extracting lithium salts.Credit: Cristobal Olivares/Bloomberg/Getty

    A standard method for desalination involves heating seawater to evaporate the water, then condensing the vapour; this basic principle is used today in a large number of the world’s desalination plants, especially those that dot the coasts of the Gulf in the Middle East. But this method consumes a lot of energy.A more energy-efficient technique emerged in the 1960s, using physical pressure to force water molecules through tiny pores of a thin membrane while leaving dissolved salt ions behind. This process, called reverse osmosis, is the gold standard for desalination plants today.The trouble is that reverse osmosis has a limit. As fresh water is extracted, the source waters get ever-saltier, making it harder and harder to continue the separation process. This is an “inescapable problem”, says Christopher Fellows, a chemist at the Saline Water Conversion Corporation (SWCC) in Jubail, Saudi Arabia. All forms of desalination leave a waste brine that needs to be managed.Mine the brineSome waste brines are simply put into the ocean; in California, a pipe system called the Brine Line carries waste brines that are produced more than 100 kilometres inland out to sea. In other places, the most economical solution is to inject waste brines underground, in a spot far enough away from utilized groundwater. This technique has been criticized for its potential to cause micro-earthquakes.
    Sustainable implementation of innovative technologies for water purification
    Alternatively, brine can be spread out in ponds to evaporate under the Sun, and the leftover salts collected — a land- and time-consuming strategy that also demands an amenable climate. A faster, more compact way to concentrate brine entails heating it and compressing the water vapour to accelerate evaporation. But this requires a staggering amount of energy, says environmental engineer Ngai Yin Yip at Columbia University in New York City, as well as expensive alloys that can withstand corrosive hot brine.Paying for the safe disposal of brines can be exorbitant. Communities that have brackish groundwater, for example, sometimes can’t afford desalination because of the costs of brine disposal, and so must find fresh water elsewhere. Researchers who have suggested desalinating California’s Salton Sea — which is growing so salty that it threatens the wildlife living in it — are also contending with high brine-management costs.Rather than throwing it away, some researchers are thinking about mining waste brines for minerals. For environmental engineer Jason Ren at Princeton University in New Jersey, this idea aligns better with his opinion that clean drinking water should be a human right: desalination companies, he says, should profit from selling salts, not clean water. “For many years, we’ve missed the point,” says Ren. “We focused on the water as the product; in my view, water should be a by-product of the other resources.”Ren and others have their sights set on one particularly valuable mineral: lithium. Today, a large chunk of the world’s lithium supplies comes from natural brines in arid South America. The brine is spread out in sprawling ponds, evaporating under the Sun for many months. As researchers identify other lithium-rich brines — including waste waters from the oil and gas industry — they are realizing they need new techniques to suit places where there isn’t enough land or the right climate for evaporation ponds.

    Researchers at Princeton University in New Jersey use the principle of chromatography to separate lithium from water and other salts along vertical strings.Credit: Bumper DeJesus/Princeton University

    Other brines could provide an unconventional source of sodium chloride. Today, ironically, some companies use fresh water to mine salt: they puncture underground salt deposits with pipes carrying fresh water to dissolve sodium chloride. This highly pure brine is then pumped up and piped or transported to chlor-alkali plants — more than half of all chemicals produced rely on these chemical refineries.Even if waste brines don’t contain any particularly valuable salts, water researchers have other reasons to champion the idea of mining waste brines: removing salt from brine liberates more fresh water, and the cost of disposal drops when volumes of waste brine are smaller.Fresh solutionsIn Saudi Arabia, recognizing the opportunity to bring in extra revenue while producing more fresh water, the government-owned SWCC is now building a demonstration plant to harvest sodium chloride, among other salts, from seawater desalination wastes.The plant, scheduled to break ground at the end of this year in Haql, Saudi Arabia, uses an emerging salt-sorting technique called nanofiltration as part of a long string of processes, says Fellows. Like reverse osmosis, nanofiltration works by pushing water molecules across a membrane. But the membrane has larger pores that also allow some salt ions through: dissolved salt ions carrying only one electrical charge, such as sodium, potassium and chloride, can cross the barrier, whereas those with two or more charges, such as magnesium and calcium, stay behind. The SWCC’s key challenge is to produce sodium chloride that is pure enough for the chlor-alkali market.The final stage at the SWCC plant entails boiling the hot brine until pure sodium chloride crystallizes. This energy-intensive step is far from ideal, says Fellows. His team has begun exploring other strategies for this stage, including freezing desalination. This approach is inspired by the fact that sea ice is composed of fresh water, even though seawater is salty. It is alluring, Fellows says, because it takes one-seventh of the energy to freeze ice-cold water as it does to evaporate boiling water. “I don’t know what’s the winning [strategy] at the moment, but it will be different for the different separations we want to do,” says Fellows.Many groups are focusing on an alternative strategy that uses electricity, rather than pressure, to do the work of separation. In this technique, an electric current is used to pull dissolved salt ions across specialized ion-exchange membranes, which permit the movement of ions in one direction only. As the ions pass through these membranes, the brine they started out in becomes more dilute, or fresher. Researchers expect the technique to be useful for pre-diluting extremely salty brines so that conventional reverse osmosis can then be used to squeeze out more fresh water.In one variant of these electricity-based techniques, Lin’s team tried to let the concentrations of salt ions that have crossed the ion-exchange membranes build until they form solid crystals3. This attempt to crystallize salts without evaporating the water worked well for certain salts — such as sodium sulfate, which is commonly found in power-plant waste water, Lin says — but not for the most abundant salt in waste brines, sodium chloride. Sodium and chloride ions hold on to water molecules so tightly that they also drag the water across the membrane, Lin says.

    To avoid both evaporation and the use of membranes, Yip’s team members at Columbia are instead looking to chemical solvents4 (see video). One promising candidate is an off-the-shelf solvent called diisopropylamine5. The solvent floats on top of a salty brine and — at low temperatures — selectively sucks water molecules into itself, leaving most of the salt ions behind. At higher temperatures, diisopropylamine switches to repelling water and spontaneously expels the water that it has absorbed, so the water can be recovered and the solvent reused.Yip says his team has used this method to recover fresh water from brine samples that are up to ten times as salty as seawater — an impossible task for standard reverse osmosis. The fresh water portion might not be potable until further steps are taken to remove contaminant solvent and salt, the researchers say. But the technique could aid industries that are seeking to recycle water from their waste brines. The researchers are currently participating in a prize challenge organized by the US Department of Energy to build a small pilot that would use solar heat for the water-expelling step.Ren and his colleagues have taken an entirely different approach6, inspired by trees. Trees can draw water up several metres against gravity, emitting clean water vapour from their leaves while trapping dissolved compounds in their tissues. His team’s approach mimics trees by using long strings of fibres with one end soaking up salt water. As the brine travels upwards, the salts are separated leveraging the common principle of chromatography — different compounds move at different speeds through a medium.Ren’s main target, lithium chloride, is extremely soluble and small, so its ions move quickly up the string, ahead of larger sodium ions. Ren has successfully used this method to recover lithium from natural brine samples from Chile, using less energy and space than conventional evaporation. The team is designing an enclosed module incorporating stacks of these strings. The researchers aim to extract lithium from waste brines produced from oil and gas operations, while recovering the evaporated water.Yet more inspiration could be found in nature: highly-selective channel proteins embedded in cell membranes. One type of ion channel allows just one sodium ion to pass through for every thousand potassium ions, says Elimelech. His team is currently working on membranes that mimic these channel proteins, although for now they are in the earliest stages of development.Price barrierWhether any of these ideas will take hold depends on economics. If the SWCC mined all the available sodium chloride from Saudi Arabia’s seawater desalination brines, Fellows notes, it would be enough to supply one-third of the world’s market. Meanwhile, waste brines left over from brackish-water desalination could offer the plentiful mineral gypsum, but it’s unlikely that unconventional brine mining could compete economically with conventional quarrying from rock.New markets, such as the advent of salt-fuelled technologies including zinc–bromine batteries, could create fresh demand for certain salts, says Fellows. Regulations could also play a part, either by making it more expensive to dispose of waste brines or by incentivizing the use of brine-sourced salts in various applications, for instance brine-sourced gypsum in road salts.One thing is clear: freshwater needs are increasing. Addressing the current limits of desalination with new technologies is important, researchers say. But it isn’t an alternative to the still-essential step of conserving fresh water. It will always take energy, time or land space to separate salt from water, so there will always be a price to pay for desalination. “There’s no magic there,” says Elimelech. More

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    Digging deep for India’s water solution

    “In this photo, I’m visiting a once-abandoned well, known as the Three Trees Well, north of Bengaluru, India. In the past, this type of well would have been filled with concrete and the land used to build properties, but thanks to community outreach efforts it has been protected and cleaned by local community members. Their work has been rewarded by the well reconnecting with the below-ground aquifer and bringing water to the community again. It now contains around 3 metres of water during the peak summer months.This is a success story, and we need more of those. People on the city’s outskirts are struggling, and the state’s government has responded by arranging tankers to bring water from the Kaveri River, 100 kilometres away. Unfortunately, this kind of event is taking place across much of urban India. Around 50% of the supply in Indian cities comes from groundwater and, without it, life grinds to a halt.I work as an adviser for the Biome Environmental Trust in Bengaluru and develop policies related to sustainable water management. I am also an adjunct faculty member at Azim Premji University in Sarjapura, where I teach courses on water conservation and management.Here in Bengaluru, I’ve been working with the traditional well-digging community, known as the Mannu Vaddar or Bhovi, to complete works in the city and surrounding rural areas. If the culture of the well can be revived in India, it might help people to find a path to sustainability in the face of climate change.Since our success at the Three Trees Well, the government has funded a further programme to rejuvenate wells and shallow aquifers in 10 cities across India. Now, the government wants to replicate that programme across a further 5,100 urban areas in the country. I am a small part of this journey, and it all began with Bengaluru’s wells.” More

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    How to address agriculture’s water woes

    Almond farmers in California are under pressure to reduce the amount of water they use.Credit: Ed Young /Design Pics Editorial/Universal Images Group/Getty

    From the Dust Bowl era of the 1930s in North America to the droughts in Ethiopia in the 1980s, Australia in the early 2000s and Syria, Iraq and Iran in 2020, the spectre of water shortages has long hung over the world’s farmlands. When rain fails to arrive season after season, crops wither and cattle starve, and famine and conflict often follow. Climate change brings a whole new level of unpredictability to the rainfall that farmers rely on, either to water their crops directly, or to feed the rivers, lakes, ground water and snowpack from which they draw water for irrigation. And that means agriculture is having to adapt — quickly.But crops that have been cultivated in the same place or cattle that have occupied the same rangelands for centuries can’t just be packed up and moved to a new area when rain patterns change. Instead, producers — and the governments that rely on them to nourish the populace and the economy — are having to rethink what is really involved in future-proofing an industry that humanity can’t live without. Some of the solutions lie in engineering crops to be more drought tolerant, or choosing crop varieties that are inherently better able to survive in drier conditions. But they also lie in an approach that requires economic and agricultural flexibility.
    Nature Spotlight: Agricultural sciences
    The Intergovernmental Panel on Climate Change has forecast that the percentage of the world’s population exposed to extreme drought will increase from 3% to 8% by 2100. If global warming exceeds 3 °C above pre-industrial levels, around 170 million people — mostly in low- and middle-income countries — face extreme drought.“I can see that Africa, Latin America and the Mediterranean continues to become drier,” says Hideki Kanamaru, a natural-resources researcher at the United Nations Food and Agriculture Organization in Rome. “These are particular areas of concern [because they] overlap with the historical trend of droughts.”Modelling suggests that rainfall will generally increase at higher latitudes — towards the poles — but decrease over subtropical areas. Over the past century, there has been a trend towards more rainfall in eastern North and South America, northern Europe, and northern and central Asia. However, there has been less rainfall overall in the Sahel, southern Africa, the Mediterranean and southern Asia. Climate change is also likely to alter monsoon rain patterns, which many agricultural regions rely on for predictable rainfall.However, this isn’t the only water-scarcity threat faced by producers. Amal Talbi, a hydrogeologist and water-resources management specialist at the World Bank in Washington DC, says that drought can also arise from economic water scarcity.Whereas physical water scarcity is when there is not enough water to meet the needs of agriculture or other uses that need fresh water, economic water scarcity is when, “you have the water, but you don’t have access to the water because you don’t have the infrastructure,” Talbi says. This distinction is important because the approach to solving these problems is very different.Flexible food strategiesPhysical water scarcity can be tackled in several ways. The first is to use less water overall: “Either you reduce your irrigation area, or you change the crops, so you use crops that use less water,” Talbi says. The second is to boost water sources with methods such as wastewater reuse or desalination plants.Another method is to be flexible with what crops are grown and when, and then use this to make the most of both water and market demand. This is the approach taken by Jordan, one of the most water-scarce nations. Receiving less than 50 millilitres of rainfall per year, the country is facing an even drier future, with its freshwater supplies per person now just 3% of what they were two decades ago, owing in part to climate change. Despite this, agriculture contributes around 30% of the country’s gross domestic product.

    Severe droughts in eastern Australia in 2019 meant farmers had to feed cattle with fodder imported from the other side of the country.Credit: David Gray/Getty

    Jordan’s answer to worsening water shortages is to focus on growing high-value, water-intensive crops for export, such as strawberries and tomatoes, in the central and northern Jordan Valley region. Although this area gets some rainfall, farmers also have access to the Jordan River and the King Abdullah Canal, an irrigation project that provides water to the Jordan Valley.It might seem illogical to grow water-hungry crops in a water-deprived landscape, but Talbi says it makes more sense than growing a crop such as wheat. “For the same land, what you would get in terms of these foods — exporting them, getting that money and then buying wheat — you will have much more than if you were using wheat in that area,” she says. Jordan also has another advantage: its climate means that those high-value seasonal products ripen earlier than they do in regions such as Spain and Portugal, so Jordan gets them to European markets ahead of other producers. “In a way, it is among the best countries in the region in terms of managing the water scarcity, given that they have so little options,” Talbi says.Morocco has a more complex water scenario to negotiate because different parts of the country experience different rainfall. Its largest crop is wheat, followed by barley, but it also produces high-value, water-hungry crops such as tomatoes, potatoes, citrus fruits and watermelon. Farmers and businesses there, like those in Jordan, grow high-value crops in irrigated areas where the water supply can be more carefully controlled and is therefore reliable, and save the less water-hungry crops for the rain-fed regions. “Roughly 50% of the time Morocco has a low rainfall, 50% it has good rainfall, so it has high variability,” Talbi says. When rainfall is good, they plant wheat and grains, and when it isn’t they maximize their irrigated high-value crops and use this money to buy wheat and to compensate the grain farmers.Change in the timesAnother factor that influences physical water availability is changes to the timing of previously predictable climatic patterns. In the northwest United States — Oregon, Washington and Idaho — wheat, tree fruit such as apples and cherries, and potatoes are the dominant crops. These are watered by a combination of rain and irrigation, the latter of which relies on the annual snowpack melting and delivering a flush of water to rivers and lakes in the Columbia River basin.But rainfall patterns are changing, says Georgine Yorgey, the associate director of the Center for Sustaining Agriculture and Natural Resources at Washington State University in Mount Vernon. “We’re going to hold less water in snowpack, more precipitation falling as rain at shoulder times of year and in shoulder elevations, and then also earlier snowmelt,” Yorgey says. And that has implications for planting and harvesting. “We have more of a mismatch between when the water comes and when the water is needed.”The timing of a crop’s sensitivity to water stress — when it is likely to have the greatest impact — varies between crops, Kanamaru says. “The last stage — ripening through harvest stage — they are not so sensitive to water stress,” he says. “The next critical stage is planting to early vegetation and the most critical stage is during reproduction.” If rainfall patterns change, it could mean that the timing of planting and harvesting of crops has to change. It’s not a new strategy in agriculture, but one that is being considered much more broadly in the face of shifting temperatures and rainfall patterns. One study has found evidence that the sowing of spring crops such as maize (corn), rice, sorghum and soya bean can shift by 10–30 days across different regions (S. Minoli et al. Nature Commun. 13, 7079; 2022). Another project in Australia found that moving the planting window for sorghum forward by four weeks reduced the risk of high summer temperatures causing heat stress during flowering (see, being flexible and tailoring each year’s agricultural focus to rainfall works only with crops that are planted and harvested in yearly cycles. It’s less viable for longer-lived crops, such as tree nuts, as California’s almond industry is discovering. The almond sector has quadrupled in size in the past 20 years, and is now the fourth largest agricultural commodity in the state, supplying around 80% of the world’s almonds. This expansion comes at a water cost: in 2021, the crop consumed 520 billion gallons more water than it did in 2017.

    Strawberry farmers in Morocco grow the crop in irrigated areas where the water supply is carefully controlled but reliable.Credit: Youssef Boudlal/Reuters

    In the past two years, drought has forced a reckoning, and there are now calls for the almond industry to reduce in size to preserve the state’s water supply in times of shortage. An almond tree can take around seven years to become fully productive, so it’s an industry that can’t just turn on a dime. As a result, producers are facing some tough decisions about its future viability in a drier, hotter climate.Cattle are a lot more mobile than an almond tree, but even in a country with grazing lands as vast and expansive as Australia’s, droughts have had devastating effects on this agricultural sector. “There were genuine shortages of feed for livestock. We had farmers in the eastern side of Australia with very hungry livestock, having to pay very high prices to ship grain and fodder from the other side of the country because there was none in eastern Australia,” says Neal Hughes in Geelong, Australia, who is an economist at the Australian Bureau of Agricultural and Resource Economics and Sciences — a national government research agency.Australia is usually a significant exporter of grain around the world, accounting for around 13% of all global wheat exports. But during the last devastating drought, which culminated in the Black Summer bush fires of 2019 and 2020, Australia’s contribution to wheat exports dropped drastically, and the nation even ended up having to import small amounts of grain to meet domestic needs, Hughes says. It was a shot across the bow of a country with an economy that is heavily dependent on its natural resources, warning that climate change could threaten a long-cherished status quo.An issue of accessEconomic water insecurity is a very different challenge, because solutions require a cross-disciplinary approach. A big issue is that the water exists, but requires efficient and affordable irrigation to enable farmers to get to it. In regions such as West Africa and the Sahel, the cost of irrigation is astronomical compared with that in other nations, Talbi says. For example, to irrigate one hectare in the Sahel can cost up to US$20,000, whereas doing the same in China might be around $600–700 per hectare, she says.One reason is that the supply chain for irrigation equipment is not yet established in Africa, so these products must be imported. Getting irrigation set up not only where it’s needed, but how it’s needed is also a challenge. Pumps and infrastructure can’t simply be parachuted in for free, Talbi says. Those systems have to be built from the ground-up if they are to be sustainable in the long term.Water isn’t the only challenge that agriculture faces in a climate-changed future, but historically it has been the most devastating, accounting for at least half of agricultural losses, Kanamaru says. And that’s only going to get worse. “Climate change is an additional amplifier to the long-standing problems of managing water,” he says.Finding solutions will require a holistic approach. “There are many parameters: variables we can modulate in this complex balance between demand and supply of water,” he says. “But I think we need to take a step back and look at the water budget of the whole hydrological cycle.” More

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    How to achieve safe water access for all: work with local communities

    More than two billion people worldwide lack access to reliable, safe drinking water. Challenges around managing water resources are complex and wide-ranging. They are interlinked with those affecting land and food systems and are exacerbated by the climate crisis. Four scholars propose ways to prompt progress in water governance — and highlight just how crucial it is for local communities to be involved.

    Farhana Sultana approaches research on environmental harms and social inequities in tandem.Credit: Wainwright Photos

    FARHANA SULTANA: Collaborate to advance water justice Throughout my childhood in Dhaka, Bangladesh, the frantic call ‘Pani chole jaitese!’ (‘The water is running out!’) prompted my family, along with the entire neighbourhood, to scramble to fill pots and buckets with water before the taps ran dry. I witnessed women and girls walk long distances to secure this basic necessity for their families, long before water governance became central to my academic career. Amid water insecurity, the opposite extreme was just as familiar — going to school through devastating floods and experiencing the fall-out from disastrous cyclones and storm surges.Municipal water services in Dhaka also struggled to meet the growing demands of a rapidly urbanizing and unequal megacity. Access to electricity — needed to run water pumps — was sporadic, and there weren’t enough treatment plants to ensure clean water for millions of residents.These early experiences fuelled my dedication to tackling water injustices. Today, as an interdisciplinary human geographer with expertise in Earth sciences, and with policy experience gained at the United Nations, I approach environmental harms and social inequities in tandem — the root causes that connect both must be addressed for a just and sustainable future. My research also encompasses climate justice, which is inextricably linked with water justice. Climate change intensifies water-security concerns by worsening the unpredictability and severity of hazards, from floods and droughts to sea-level rise and water pollution.Such events hit marginalized communities the hardest, yet these groups are often excluded from planning and policymaking processes. This is true at the international level — in which a legacy of colonialism shapes geopolitics and limits the influence of many countries in the global south on water and climate issues — and at the national level.However, collaborative work between affected communities, activists, scholars, journalists and policymakers can change this, as demonstrated by the international loss-and-damage fund set up last year to help vulnerable countries respond to the most serious effects of climate-related disasters. The product of decades of globally concerted efforts, this fund prioritizes compensation for low-income countries, which contribute the least to climate change but often bear the brunt of the disasters.I also witnessed the value of collaboration and partnership in my research in Dhaka. Community-based groups, non-profit organizations and activists worked with the Dhaka Water Supply and Sewerage Authority to bring supplies of drinking water at subsidized prices to marginalized neighbourhoods, such as Korail, where public infrastructure was missing.Globally, safe water access for all can be achieved only by involving Indigenous and local communities in water governance and climate planning. People are not voiceless, they simply remain unheard. The way forward is through listening.

    Tara McAllister is exploring the interface between Mātauranga Māori (Māori Knowledge) and non-Indigenous science.Credit: Royal Society of New Zealand

    TARA MCALLISTER: Let Māori people manage New Zealand’s water I have always been fascinated by wai (water) and all the creatures that live in it. Similar to many Indigenous peoples around the world, Māori people have a close relationship with nature. Our connection is governed by geneaology and a concept more akin to stewardship rights than to ownership. This enables us to interact with our environment in a sustainable manner, maintaining or improving its state for future generations.I was privileged to go to university, where I studied marine biology. I then moved to the tribal lands of Ngāi Tahu on Te Waipounamu, the South Island of New Zealand, which triggered my passion for freshwater ecosystems. Intensive agriculture is placing undue pressure on the whenua (land) and rivers there. Urgent work was required. Undertaking a PhD in freshwater ecology, I studied the causes of toxic benthic algal blooms in rivers. For me, there is no better way to work than spending my days outside, with my feet in the water.

    A worker fills people’s water containers from a tanker in Kolkata, India.Credit: Rupak De Chowdhuri/Reuters

    Having just started a research position at Te Wānanga o Aotearoa, a Māori-led tertiary educational institution, I am now exploring the interface between Mātauranga Māori (Māori Knowledge) and non-Indigenous science, and how these two systems can be used alongside each other in water research. I have also been working on nurturing relationships with mana whenua, the community that has genealogical links to the area where I live, so that I can eventually work in the community’s rivers and help to answer scientific questions that its members are interested in.Despite a perception that Aotearoa (New Zealand) is ‘clean and green’, many of its freshwater ecosystems are in a dire state. Only about 10% of wetlands remain, and only about half of rivers are suitable for swimming. Water resource management is challenging, because of a change this year to a more right-wing government. The current government seems intent on revoking the National Policy Statement for Freshwater Management, established in 2020.This policy has been crucial in improving the country’s management of freshwater resources. Although not perfect, it does include Te Mana o te Wai — a concept that posits that the health and well-being of water bodies and ecosystems must be the first priority in such management. It is now in danger of being repealed.I think that, ultimately, our government’s inability to divulge control and power to Māori people to manage our own whenua and wai is what limits water resource management. More than any change in policy, I would like to see our stolen lands and waters returned.

    Suparana Katyaini calls for more policy support for Indigenous-led water management.Credit: Milan George Jacob

    SUPARANA KATYAINI: Consider water, food and land together Growing up in New Delhi, I always had easy access to drinking water — until the summer of 2004, when a weak monsoon triggered a water crisis and the city had to rely on water tankers. I realized then that good management of water resources supports our daily lives in ways we take for granted until we experience scarcity.My professional journey in research and teaching has been motivated by this experience. During my environmental studies of water poverty in India, I noticed that the field relied largely on quantitative data over qualitative insights — the degree of water-resources availability, access and use are typically assessed through metrics such as the water-availability index or the water-demand index. But in many places, Indigenous and local communities, including farmers and women in any occupation, have collectively developed skills to weather periods of water scarcity. Paying attention to these skills would lead to better water management. For example, the issue of food and nutritional insecurity in water-scarce areas in the state of Odisha, India, is being solved by Bonda people through revival of the crop millet, using varieties that are nutritious, water-efficient and climate-resilient.But these efforts need more policy support. My current work at the Council on Energy, Environment and Water explores how water, food and land systems are interlinked in India, and how better understanding of these relationships can inform policies. I am looking to identify similarities and differences in objectives of national and regional policies in each sector, as well as exploring whom they affect and their intended impacts. The aim is to move towards unifying water, food and land governance.

    Michael Blackstock examines climate change from a water-centred perspective.Credit: Mike Bednar

    MICHAEL BLACKSTOCK: Shift attitudes towards water In 2000, I conducted an ethnographic interview with Indigenous Elder Millie Michell from the Siska Nation in British Columbia, Canada, that transformed my interest in water from intellectual curiosity to passion. She passed a torch to me that fateful day. During our conversation for my research about the Indigenous spiritual and ecological perspective on water, she asked me: “Now that I shared my teachings and worries about water, what are you going to do about it?” She died of a stroke a few hours later.As an independent Indigenous scholar, I went on to examine climate change from a water-centred perspective — drying rivers, downpours, floods and melting ice caps are all water. This approach, for which I coined the term ‘blue ecology’, interweaves Indigenous and non-Indigenous ways of thinking. It acknowledges water’s essential role in generating, sustaining, receiving and, ultimately, unifying life on Mother Earth. This means changing our collective attitude towards water.In 2021, I co-founded the Blue Ecology Institute Foundation in Pavilion Lake, Canada, which teaches young people in particular to acknowledge the spiritual role of water in nature and in our lives, instead of taking it for granted as a commodity or ecosystem service. Giving back to nature with gratitude is also crucial. Such restrained consumption — taking only what is needed — would give abused ecosystems time to heal.A focus on keeping water healthy can help to guide societies towards more sustainable environmental policies and climate-change resilience — and ensure that future generations will survive with dignity. Critics say, ‘Blue ecology is kinda out there.’ In my view, however, ‘here’ is not working. More

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    The Solar System has a new ocean — it’s buried in a small Saturn moon

    Striped by its rings’ shadows, Saturn (light blue; artificially coloured) looms behind its moon Mimas (grey sphere), which conceals a liquid ocean underneath its surface.Credit: NASA via Alamy

    There’s a newfound ocean in the outer Solar System, and it’s in a very surprising place1. Mimas, a mid-sized moon of Saturn, turns out to have an ocean beneath its icy surface — despite looking too geologically inert to have water sloshing inside.Mimas joins a growing list of icy moons that are also ocean worlds. The fact that boring-looking Mimas has an ocean means that “you could have liquid water almost anywhere”, says Valéry Lainey, an astronomer at the Paris Observatory.That’s important because interactions between ocean water and rock, which would occur where a buried ocean meets a moon’s rocky core, can generate enough chemical energy to sustain living organisms. If there are more stealth ocean worlds out there similar to Mimas, there are greater chances of extraterrestrial life.Peek-a-boo oceanThe discovery, reported today in Nature by Lainey and his colleagues, largely resolves the long-standing question of whether Mimas has an ocean. Many researchers hadn’t expected it to: Mimas’s geology does not display signs of a possible buried ocean, such as the icy rafts that jostle on Jupiter’s moon Europa or the geysers that spew from Enceladus, another icy moon of Saturn.
    Pluto’s dark side spills its secrets — including hints of a hidden ocean
    But in 2014, a team that included Lainey and that was led by Radwan Tajeddine, an astronomer then at the Paris Observatory, analysed images taken by NASA’s Cassini spacecraft, which explored Saturn and its moons between 2004 and 2017. By studying how the 400-kilometre-wide Mimas wobbled in its orbit around Saturn, the researchers concluded that it had either a buried ocean or a rugby-ball-shaped core2. As more scientists studied how an ocean could have formed and evolved, it became harder to explain the geology of Mimas without invoking an ocean3.In the 2024 study, Lainey and his colleagues seem to have nailed the case. They went further than they had in 2014, by analysing not just the orbit’s wobble but also how Mimas’s rotation around Saturn changed over time. The team combined Cassini observations with simulations of Mimas’s interior and its orbit to conclude that there must be an ocean 20–30 kilometres below Mimas’s surface.Solid evidenceThe work is the best evidence yet for an ocean in Mimas, says Alyssa Rhoden, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, who will report similar conclusions at a conference next month in Texas. “I am happy to move Mimas from the ‘maybe possibly an ocean world’ category to the ‘yeah it really could be an ocean moon’ category,” she says.
    Cassini’s 13 years of stunning Saturn science — in pictures
    But it seems to be a young ocean — having formed in the last 25 million years, compared with almost 4 billion years ago for Earth’s first ocean. If the ocean had been around for longer, it would have begun to exert its influence on Mimas’s icy surface by now, for example by fracturing it. At some point in the recent past, Lainey says, Mimas was probably travelling on a stretched-out orbit that caused it to gravitationally interact with other Saturnian moons. That tidal interaction would have heated up Mimas, melting its interior and creating the ocean.Ultimately, the pockmarked Mimas could evolve to look similar to smooth Enceladus, which is coated in ice created by water spraying through cracks in its shell. And beyond Saturn, the discovery suggests that several moons of Uranus could also be hiding oceans of their own, despite looking static and frozen on their surfaces.“There are no boring moons,” Rhoden says. More

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    Groundwater decline is global but not universal

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    How to take ‘forever’ out of forever chemicals

    Water-treatment firm Aquagga’s ‘forever’ chemical destruction unit in Fairbanks, Alaska, uses a technique called hydrothermal alkaline treatment.Credit: Gus Millevolte

    Selma Thagard watched in astonishment as the indestructible chemicals did the one thing that they shouldn’t do — fall apart.A chemical engineer at Clarkson University in Potsdam, New York, Thagard was developing a plasma reactor for water treatment in 2016 when an environmental-engineer colleague suggested she add chemicals known as PFAS to the water she was testing. These per- and polyfluoroalkyl substances, also commonly referred to as forever chemicals, are made up of chains of carbon and fluorine atoms held together by some of the strongest chemical bonds in nature. They don’t break down naturally, and many decontamination techniques can’t touch them either. The PFAS wouldn’t be destroyed by Thagard’s plasma reactor, her colleague told her, but might act as a useful reference sample.But it didn’t play out that way. In just a few minutes, the chemicals were no more. “When plasma degraded PFAS so rapidly, within minutes, he told me: ‘That’s not right. Nothing can degrade PFAS,’” Thagard says. She ran the test seven or eight more times, and each time the chemicals disappeared.Thousands of variations of PFAS chemicals have been used for decades in a wide variety of products, including food packaging, stain-resistant textiles and firefighting foam. Their widespread use, combined with their inability to break down naturally, means that they have spread to water, soil and wildlife. Thagard’s colleague was studying the accumulation of the chemicals in fish in North America’s Great Lakes, but they are present all around the globe.The substances also accumulate in people, and are thought to contribute to reproductive issues, impaired immune function and even cancer. Over the past two decades, concern about these forever chemicals has grown, leading to the imposition or proposal of regulations to cap their presence in water in the United States, the European Union and the United Kingdom.But ‘forever’ might be a shorter time than previously thought. Scientists, including Thagard, are developing methods to break down PFAS into fluoride and carbon dioxide, which are not dangerous in the small amounts produced. These approaches to degrading the molecules have arisen in the past few years and could become widely available in just a few more. The big questions are where in the water cycle to deploy them, and which method makes the most economic sense.Treatment technologies by themselves won’t completely solve the problem of PFAS pollution. For one thing, the number of possible molecules based on the carbon–fluorine bond is vast, making it difficult to know for certain whether a particular method can tackle each one. “There are new ones being put on the market each year,” says Timothy Strathmann, a civil and environmental engineer at the Colorado School of Mines in Golden. It can also be difficult to measure some of these molecules, especially at low concentrations. The sheer number of possible molecules, plus their stealthiness, are an ongoing challenge, Strathmann says. “This is why we need to also keep up with our ability to detect and sense these chemicals. Because if you don’t know what you’re looking for, you don’t find it.”Electrical zappingThagard’s water-treatment technique relies on electrical discharge plasma1. She puts water contaminated with PFAS in a reactor and pushes bubbles of argon gas through it. The PFAS is attracted to the interface between the water and the bubbles, and rides them to the surface of the water. The atmosphere above the water is also argon — chosen because it has a high density of electrons. High-voltage pulses of electricity flow between electrodes near the surface of the water, knocking electrons loose from the argon atoms and turning the insulating gas into a conducting plasma.

    Environmental biochemist Susie Dai is using a fungus to help break down ‘forever’ chemicals.Credit: Michael Miller, Texas A&M AgriLife

    The process delivers enough energy to break the carbon–fluorine bonds. If any PFAS is left, it’s at concentrations too low to detect, below the parts-per-billion level. The fluoride and carbon dioxide that are produced from the disintegration of PFAS are absorbed by the water, but in amounts that Thagard says are too small to be concerning. However, the mechanism that causes the bonds to break — something that was never expected to happen — is still unclear. “The science is largely unknown,” she says. “We are doing extensive research from the fundamental side.”Thagard and her colleagues carried out a field test on PFAS-polluted water at Wright–Patterson Air Force Base, outside Dayton, Ohio, in 2019 and showed that they could treat 4 litres of water and reduce the amount of PFAS to below the health-advisory level of the US Environmental Protection Agency in a couple of minutes2. That was using a crude system, she says; an optimized reactor could treat about 40 litres per minute. The US military has been funding research, including Thagard’s work, into cleaning up PFAS because the long-time use of firefighting foams has contaminated many bases.Thagard is chief executive of DMAX Plasma, a start-up firm she founded in Potsdam to commercialize the technology. The start-up has sold small systems to military and industrial customers. Its standard treatment unit, the company says, requires less electricity than most household electric ovens. With some engineering work, Thagard says that the systems could be scaled up to meet the needs of water-treatment plants.Under pressureAnother effort to destroy PFAS is being led by Aquagga, a water-treatment start-up company in Tacoma, Washington, in collaboration with Strathmann. It is using a technique called hydrothermal alkaline treatment (HALT), which involves adding an alkaline substance such as sodium hydroxide to the PFAS and heating it to 350 °C under high pressure (roughly 160 times atmospheric pressure)3. Under these conditions, the hydroxide draws the fluorine to itself and destabilizes the PFAS molecules. Using high-resolution mass spectrometry, the researchers found that after treating a sample of water containing PFAS they had extracted as much fluoride as should have been bound up in the PFAS to begin with — suggesting it had all been broken down.In the absence of destructive technologies, PFAS in water systems has been filtered out and sent to landfill or an incinerator. But even burning doesn’t destroy all the PFAS, which can be spread by smoke or ash from the incinerator or leach out of landfill. Neither process results in the substances being removed from the environment permanently, the way that the destructive approach does.Some sort of filtration or separation process to increase the pollutant-to-water ratio will probably be a step in any PFAS-destructive technology. “You’re not going to treat a million gallons with the destructive process,” Strathmann says. Indeed, the HALT method that Aquagga is developing works with PFAS that has been caught by an activated-carbon filter. So far, the pilot versions can treat only around 4–8 litres of concentrated PFAS per hour, but the company is working to scale that up. It’s taking orders for systems that can treat up to 75 litres per hour and developing ones that will treat nearly 600 litres per hour.Some attempts to destroy PFAS have only succeeded in breaking long-chain molecules into smaller ones with fewer than six carbon atoms. The HALT method seems to be more versatile. “This process applies across the full spectrum, from the very shortest chains, with only one carbon, all the way to the longest chain we’ve tested”, with ten carbon atoms, Strathmann says. That means it should destroy any PFAS, even those that regulatory agencies have not yet listed as of concern4,5.A sound techniqueIn addition to heated chemicals or bright plasma, high-frequency sound waves might also provide the energy needed to break up the molecular chains, by knocking the fluorine atoms loose. Jay Meegoda, a civil and environmental engineer at the New Jersey Institute of Technology in Newark, is among those working on this approach — known as sonolysis. He sends sound waves at a frequency of about 1 megahertz into a concentrated solution of PFAS6. This ultrasound creates bubbles in the water that are only a few nanometres across.Meegoda keeps pouring acoustic energy into the solution until the bubbles become unstable and implode. That releases a burst of energy, raising the temperature of the water that immediately surrounds the bubbles to 5,000 °C for about 10 nanoseconds. Although brief, the heating “is good enough to break all the molecular bonds”, Meegoda says. Everything in the immediate vicinity of the bubble gets broken down to individual atoms, even the water. Hydrogen and oxygen atoms quickly recombine as water. Carbon atoms from the PFAS join with oxygen to become carbon monoxide, and then carbon dioxide. Fluorine atoms form fluoride ions.Meegoda is working with Tetra Tech, an engineering services company in Pasadena, California, and hopes to run a pilot project with his technology in 2024. He expects to see some sort of PFAS degradation technology, whether his own or another, on the market in about two years.Meegoda, Strathmann and Thagard, along with many other researchers, are focused on degrading PFAS at existing water-treatment facilities, where the chemicals would have to be concentrated before destruction. But Michelle Crimi, a civil and environmental engineer and a colleague of Thagard’s at Clarkson, is taking the attack closer to the source. She wants to use a version of sonolysis to handle polluted ground water. Her idea is to build horizontal wells at contaminated areas, such as air-force bases or industrial sites, where there is already a high concentration of PFAS. “We don’t want to treat extremely low concentrations and huge volumes of drinking water indefinitely,” Crimi says. “That’s super expensive.” As the ground water slowly flows through the well — it could take two days to traverse a 46-centimetre well — an ultrasound system hits it with sound waves at frequencies in the mid-kilohertz range7. In the same way as Meegoda’s sonolysis system, the sound waves deliver enough energy to create bubbles in the water and break apart the PFAS molecules. The water would then continue on its natural course, into rivers, lakes or the aquifers that feed more-familiar vertical wells. “Our goal is to stop the contaminated water from reaching the drinking-water wells,” Crimi says.Crimi has co-founded a start-up company — RemWell in Potsdam — to commercialize her technology. She launched a field test in late October at Peterson Space Force Base in Colorado Springs, Colorado, to gather data on how well the system works.Let it rotLeaning towards a more naturalistic approach, Susie Dai, an environmental biochemist at Texas A&M University in College Station, is working on a technique using bioremediation, which relies on living organisms to break down the PFAS. “Bioremediation is typically cheaper than any other chemical or mechanical process, because you have an organism that’s growing by themselves do the work,” she says.Dai starts with maize (corn) stover — the leaves, stalks and cobs that remain after the maize is harvested. She separates its two main components: the cellulose that makes up plant cell walls and fibres, and the lignin that gives the stalks their stiffness. She then modifies the lignin by treating it with polyethylenimine to add functional groups, then mixes it back together with the cellulose to form a fibrous, organic filter material that can catch and hold the PFAS molecules8. Finally, Dai adds a fungus called Irpex lacteus, or white-rot, that commonly grows on fallen trees. The fungus devours the PFAS, using enzymes to break it down into more benign molecules. It also eats the filter material.Dai still needs to measure whether the fungus fully breaks down all of the contaminant and produces pure fluoride, or leaves behind some chains. “I think it’s pretty promising if PFAS are disappearing from the environment,” she says. “It is still important for us to know what the degraded products are, but it’s less important than the removal of the parent molecule.” She is looking for a site where she can test her technology under real-world conditions.Crimi would like to see the producers of PFAS pollution take further steps to shoulder the costs of cleaning up the problem, which tends to disproportionately affect lower-income communities. “It’s tricky with PFAS, because the solutions are really just emerging. There’s still a lot of work to do to really inform what is the most sustainable and cost-effective way to address the big problem,” she says.Still, she’s optimistic that the world won’t be stuck with forever chemicals eternally. “I always say, ‘Forever no more.’” Scaling up the various techniques now under development would turn that hope into a cleaner-water reality. More

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    Fresh water from thin air

    Illustration: Sam Falconer

    In late summer, Death Valley National Park earns its name. The heat in this region of California and Nevada is relentless. Record temperatures are set, and the air is often bone dry. The 22 August 2022 was no exception, with an average temperature during daytime of 51.6 °C and humidity of just 14% in the location aptly known as Furnace Creek.Despite the heat and aridity, there was a slow but steady drip of water into the collection vial of Omar Yaghi’s device, an assembly of components loosely resembling a telescope. By the end of the day, this system had collected only a few millilitres of water — barely enough for a refreshing sip. But these results, published in July1, nevertheless represent a landmark in the field of atmospheric water harvesting (AWH).Given the extremity of the testing conditions, the results suggest that the key ingredient in this device — a water-absorbing compound called MOF-303 — has the potential to deliver life-sustaining volumes of clean water to regions that currently struggle to access it. “The vision there is to have something like a village-scale device,” says Yaghi, a chemist at the University of California, Berkeley. “If you’ve got a tonne of MOF-303, you could deliver about 500 litres of water a day, every day for five to six years.”By current estimates, roughly two billion people lack access to clean drinking water. Desalinated seawater can meet some of this need, but the technology required remains costly and is limited to communities with coastal access. This explains the growing enthusiasm for alternative solutions that extract clean water from the air. The US Geological Survey estimates that Earth’s atmosphere contains nearly 13,000 cubic kilometres of water — more than six times the volume of the world’s rivers. “You cannot deplete it — it’s always replenished by natural evaporation from a larger water body,” says Tian Li, a materials scientist at Purdue University in West Lafayette, Indiana. And although many of the most promising AWH technologies are still at the stage of lab demonstrations or proof-of-concept devices, the field is quickly building momentum towards real-world systems that produce plentiful amounts of water at low cost.Searching for suitable sorbentsThere are already several commercially available AWH systems. In mountainous, foggy regions, it is possible to literally cast a net to collect water from ever-shifting cloud masses. Such installations are producing water from the air in South America, India and parts of Africa, according to Thomas Schutzius, a mechanical engineer at the University of California, Berkeley. There are also systems for collecting the water that accumulates overnight as dew. But both fog and dew harvesting are limited to high-humidity areas. And for dew, only modest volumes of water can be produced even under optimal conditions.

    Dry (left) super-moisture-absorbent gels swell as they absorb atmospheric water (right).Credit: Guihua Yu, University of Texas at Austin

    Systems that condense water from ambient air offer a more generally useful solution. Several companies have already developed electrically powered ‘active’ AWH machines for this purpose. In most cases, these use fans to draw warm, moisture-bearing air into an apparatus that directly cools the air and collects the resulting water condensate; in some cases, this water is also subject to filtration and additional treatment. These systems can produce considerable volumes. The Maximus system from the firm SkyH2O in Irvine, California, for example, can produce more than 10,000 litres of purified water per day. But this system is complex and massive — weighing around 13 tonnes — and requires continuous external power to run. It is also priced at a costly US$395,000. Such systems could be a solution in wealthy water-deprived regions — the southwestern United States, for example, or Saudi Arabia — but they are a non-starter in locations with limited budgets or unreliable electrical infrastructure.The need for more affordable options has spurred interest in ‘passive’ AWH systems that use moisture-hungry sorbent compounds to collect water. The small amounts of power that such systems require could, ideally, be supplied by the Sun. Typically, these sorbents are exposed to the air overnight, when temperatures are cooler and moisture is more abundant. They collect the airborne moisture as liquid in a process known as adsorption. When day breaks, the sorbents are transferred to a device that uses solar energy to drive the release of water. This water is then condensed and collected. These passive systems are tricky, however, because they require sorbents that bind water strongly — but not so strongly that they refuse to yield their bounty without a fight. “That’s an energy penalty that you need to pay,” says Guihua Yu, a materials scientist at the University of Texas at Austin.The field got a big boost in 2017 when Yaghi, along with engineer Evelyn Wang at the Massachusetts Institute of Technology in Cambridge and their colleagues, described a solar-powered system that could extract nearly 3 litres of water per day per kilogram of sorbent — an unprecedented feat at the time2. “I was inspired by that paper,” says Peng Wang, an environmental scientist at Sun Yat-sen University in Guangzhou, China. “This is how I got into this field.”The leap in performance was thanks to the use of a different kind of sorbent — a metal–organic framework or MOF. These porous compounds, developed in Yaghi’s lab, offer a vast surface area for water to bind to, and can be readily chemically modified to further enhance their capacity and water affinity. “It takes up water even at as little as 5% relative humidity,” says Yaghi about his current sorbent of choice, MOF-303. Equally important is that little heat is needed to drive the water back out, with temperatures of 40–45 °C typically proving sufficient. Moreover, Yaghi says, MOFs remain stable throughout years of continuous use.Other promising sorbents are also emerging. Polymers known as hydrogels are a low-cost and highly customizable class of materials that can potentially achieve even greater capacity for moisture capture than MOFs. This is especially true if these gels are loaded with water-absorbing salts such as lithium chloride. Hydrogel-based AWH systems are not yet as efficient as their MOF-based counterparts at capturing and releasing water — particularly under ultra-dry conditions — but they are steadily improving. In September, Yu’s team described a microgel formulation that offers a much larger water-binding surface area than other hydrogel designs, and incorporates a heat-sensitive component to induce water release at lower temperatures3. This allows water to be cleared from the gel in about 20–30 minutes — three to four times faster than previous iterations of his team’s hydrogel-based system, Yu says. This is still about ten times slower than the release from MOF-303, however.Even simpler materials are also being explored. Li and her colleagues have been developing specialized fabrics based on cellulose, a plant-derived fibrous molecule that can absorb water4. In addition to being abundant and inexpensive, says Li, cellulose “has the nanoscale features already there without you doing anything”. Her group is exploring ways to extend the capabilities of cellulose. Impregnating the fabric with lithium salts, for example, has been shown to boost its water-harvesting capacity by more than five-fold relative to the salt-free version5.But cellulose-based systems yield a substantial amount of water only when the relative humidity is at least 60%. By comparison, the MOF-303-based system operates effectively at relative humidity of 20% or less, as shown in the Death Valley field test. And Yu’s microgels could achieve reasonably fast uptake of meaningful volumes of water at 30% relative humidity — although, of course, the water yield will always be lower in such conditions owing to the limited moisture available.Preparing for the harvestA good sorbent is only a starting point. Wang says that most passive AWH systems that have been described so far have the capacity for only one round of water absorption and release every 24 hours. This single-cycle operation can squander the potential output of a material that saturates quickly.

    A water harvester containing MOF-303 can collect water from desert air with high efficiency and without power.Credit: Yaghi Laboratory, UC Berkeley

    To address this, many researchers are using batch-process systems, which require swapping the sorbent beds between an air-exposed state for water absorption and an enclosed state for Sun-assisted water release. Most of these are active systems that require external sources of electrical power. That’s not necessarily a deal breaker, however — such systems could prove cost-effective. “If you just have a battery that can open a door and close it, you can triple your delivery because now you can do more than one cycle a day,” says Yaghi. In a 2019 study, his group demonstrated a compact device6 that used batteries to power multiple cycles of atmospheric water collection throughout the day. These batteries could be fully recharged by solar power during daylight hours, allowing the system to function off grid.Cost is a crucial consideration, especially given that passive AWH will — at least initially — be targeted at resource-limited populations. Fortunately, many of the sorbents now under development should be affordable. Yaghi says that MOF manufacture is already being done at an industrial scale, and that the cost is largely determined by the metal involved. For MOF-303, that means aluminium, which he says costs just $1–2 per kilogram. Some hydrogel polymers can be expensive to produce, but others can be made more cheaply. Yu’s team is even exploring whether hydrogel ingredients can be directly extracted from biomass. The opportunity for low-cost production from easily accessible materials is a key asset of Li’s cellulose fabrics. Her group is working on deploying its system in coastal communities in Senegal where fresh water is scarce. “The burden of getting fresh drinkable water there falls onto the teenage girls,” she says. “We’re trying to educate the girls, and developed a curriculum so that they can build a set-up themselves with locally available cellulose sources.”Li’s system simply requires a textile drape that can be wrung out by hand. Other sorbent-based systems depend on more sophisticated apparatus for the harvesting process — but even those do not need to be expensive. For example, Wang recalls a prototype hydrogel-sorbent-based device that he developed about five years ago7. Apart from the sorbent itself, Wang says, all the materials for the system were purchased from a local supermarket. For just $3.20, Wang and his colleagues estimated that they could construct a device that would supply roughly 3 litres — the minimum amount of water needed daily by a typical adult.Of course, there is also the issue of ensuring that the water pulled from the air is free of dangerous substances. Yaghi says that his experiences in field testing in US deserts have been reassuring. “We tested the water for metal and organics, and it was like the purest water you could find,” he says. But this is not a certainty in every environment, particularly near sources of industrial pollution. Careful assessments will be needed to ensure that collected water is separated efficiently from contaminants.Pollution has been a particular concern when harvesting fog, Schutzius says. In August, his group described a fog-harvesting net enhanced with a titanium dioxide coating, which efficiently breaks down organic pollutants such as diesel after being activated by ultraviolet light from the Sun8. He thinks that researchers should take similar considerations into account for other domains of AWH. “The whole point of adsorption is you can concentrate a lot of stuff that’s otherwise dilute,” he says.Opening the tapSome passive AWH systems are already moving into commercial development. Yaghi’s lab, for example, has spun off a start-up firm in Irvine, California, called Atoco, which aims to roll out first-generation MOF-based harvesters in the next year or so. Different water-harvesting technologies will find different applications. The robust performance of MOFs in extremely arid conditions will make them a versatile choice, whereas systems based on cellulose or hydrogels might be restricted to more humid environments.

    A fog-collector park (left) in the mountains of Morocco traps water vapour on nets (right).Credit:

    These technologies are unlikely to fully replace existing systems such as seawater desalination, which has a proven track record of high-volume water production. But AWH could greatly reduce dependency on centralized water processing, making it accessible at the village or even single-household scale. Yaghi sees a future in which any house with electricity could reliably address its drinking-water needs with an appliance roughly the size of a microwave oven.And there are abundant opportunities beyond simply producing drinking water. For example, Wang’s group has described a harvesting system that piggybacks on existing photovoltaic solar panels, using the waste heat and energy from these panels to power water production9; the resulting water helps to cool the panels and therefore improves their efficiency. Similar approaches have been described for managing — and exploiting — waste heat in industrial settings. AWH also has agricultural applications; Yu’s group, for example, is working on using hydrogel-based materials to produce self-watering soils that directly draw moisture from the air10.It is indisputable that, as the ongoing climate catastrophe worsens, society will need to leverage every solution at its disposal to meet the planet’s water needs. “I worked in Saudi Arabia, and people there say water security is national security — that’s 100% true,” says Wang. “It’s getting more serious, and we need to do things more effectively.” More