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    How to get rid of toxic ‘forever chemical’ pollution

    This February, 14 lorries set out from Wilmington, North Carolina, with a toxic cargo: more than 150 tonnes of grit-like carbon that had soaked up harmful chemicals from the city’s drinking water.The lorries took the carbon to one of the closest available ‘reactivation’ kilns, 1,200 kilometres north in Buffalo, New York. There, temperatures of nearly 1,000 °C burnt off the chemicals, breaking them into simple gas molecules, which were later turned into minerals. This month, the refreshed carbon will ride the lorries back down south.The manager of Wilmington’s drinking-water plant, Benjamin Kearns, says he checks the weather forecast for Buffalo all winter — fearing disruptions to his water-purifying operations, which depend on a carefully timed supply of fresh carbon every month. “If there’s a snowstorm, I am concerned,” he says.The carbon, technically called granular activated carbon (GAC), is at the heart of a US$43-million system that began operating in 2022 to rid Wilmington’s drinking water of PFASs, or per- and poly-fluoroalkyl substances. These synthetic chemicals now pervade the world — they’re used to make computer chips, lithium-ion batteries, medical devices, stain-resistant textiles and smudge-proof coatings, among many other products — and some of them endanger human health. Also known as forever chemicals, they resist natural destruction because of their strong carbon–fluorine bonds.Kearns’s plant is in the vanguard of an almighty remediation effort. In April 2024, the US Environmental Protection Agency (EPA) put strict nationwide limits on the concentrations of six PFASs in drinking water. The agency estimates that the rule will reduce PFAS exposure for around 100 million US residents.How best to get rid of PFASs is now a multibillion-dollar question. The EPA estimated that US utilities might have to spend up to $1.5 billion annually for treatment systems; an industry group that is suing the agency argues that costs could be up to $48 billion over the next 5 years. Utilities must have systems in place by 2029.European nations have rules restricting PFAS levels in drinking water, too. European Union rules take effect from 2026, but allow higher concentrations than the EPA does; however, countries such as Denmark and Germany have set stricter limits.The concern and the expectation of a booming market for cleaning up PFAS pollution has sparked a rush for better ways to capture and destroy forever chemicals. Although GAC does work — it is a porous material (or sorbent) that can trap and house pollutants — it doesn’t capture all PFASs equally well. Trucking carbon around so that the collected PFASs can be destroyed in reactivation kilns also exacerbates climate change, adds Frank Leibfarth, a polymer chemist at the University of North Carolina at Chapel Hill.Drinking water is filtered through nearly 4 metres of granular activated carbon in huge tanks at the Sweeney Water Treatment Plant.Credit: Cape Fear Public Utility AuthorityAnd although the EPA has focused on drinking water, scientists want to stop PFASs from ever reaching the water by removing them from other environmental sources. The industrial facilities that produce and use PFASs, ranging from fluorochemical manufacturers to paper and textile mills, often send their waste to municipal wastewater (sewage treatment) plants. But these aren’t usually equipped to remove PFASs, so their outflow adds forever chemicals into rivers. From there, the PFASs can reach drinking water directly or do so indirectly by infiltrating soils.The sludge that is left over from sewage treatment also accumulates PFASs. In some parts of the world, this nutrient-rich sludge, known as biosolids, has been spread onto farmland as fertilizer. In states such as Maine, farms that yield PFAS-tainted food have shut down. And a type of fire-fighting foam that is formulated with PFASs has contaminated soils and seeped into groundwater around military bases and airports worldwide, because of its frequent past use in fire-training exercises there.With looming deadlines, academic researchers and companies are developing methods to gather and destroy PFASs from these sources. “Loads of evolving techniques are out there,” says PFAS specialist Ian Ross at CDM Smith, an engineering firm in Boston, Massachusetts, that works on PFAS remediation.Capturing contaminantsOn the second floor of Kearns’s facility — the Sweeney Water Treatment Plant — drinking water held in up to eight concrete tanks sinks silently through nearly four metres of GAC. “It’s amazing how much carbon you need to treat for PFAS,” says Orlando Coronell, an engineer at the University of North Carolina at Chapel Hill who is collaborating with Leibfarth to test a new sorbent at the facility.Operated by the Cape Fear Public Utility Authority (CFPUA), this plant serves 200,000 people in the coastal city of Wilmington. It draws water from the Cape Fear River, which in 2017 was shown to have high levels of one of the six EPA-regulated PFASs, called GenX (see ‘Cape Fear and PFAS pollution’). The molecules had come from 160 kilometres upstream, where fluorochemical manufacturer Chemours makes PFASs for electronics and battery manufacturing, among other uses, and had discharged them into the river.Source: RTI International (Adapted from https://go.nature.com/429E3DL)After the CFPUA installed the sorbent system, levels of GenX and several other PFASs fell (and are below the new EPA limits). While the system was being designed and constructed, North Carolina’s state environmental agency sued Chemours, and a local non-profit group added pressure by suing both organizations together. The parties agreed to settle: Chemours denied wrongdoing, but installed better controls on its PFAS emissions, including a 1.6-kilometre-long underground barrier wall paired with a GAC filtering system that collects surface and groundwater near the plant and removes PFASs. The firm says it has invested more than $400 million at its plant to remediate PFAS emissions and limit future ones. The CFPUA is currently suing Chemours to pay for the Sweeney filtration system.GAC is generally effective, but it is a ‘broad-spectrum’ sorbent that traps everything it attracts into its hydrophobic (water-repellent) pores, not just PFASs, says Coronell. The Sweeney plant receives water with much higher levels of dissolved organic matter than of PFASs, which compete for space in GAC’s pores. The six molecules on the EPA’s list stick well enough, but any PFAS with a shorter, hydrophobic fluorine-bearing tail does not. As GAC’s pores fill up, short-chain PFASs can break through the pores and re-enter drinking water.In particular, ultrashort-chain PFASs (those with a three-carbon fluorinated tail or shorter) are worrying researchers(see ‘PFAS pollutants’), because the molecules are being found in waters downstream of Chemours and near semiconductor manufacturing facilities1. After the CFPUA detected two ultrashort PFASs in its drinking water after treatment, it began switching out the GAC about every 200 days, instead of the roughly 300 days it had used when capturing GenX. That has almost doubled its carbon-regeneration costs.Other established ways to capture PFASs have pros and cons. A type of sorbent called ion-exchange resin traps contaminants broadly through electrostatic interactions: the six EPA-regulated PFASs all carry a negative charge, and they stick by trading places with a negatively charged component on the resin.Less resin is needed to treat the same amount of water than with GAC, but it costs five to six times more, says Detlef Knappe, an environmental scientist at North Carolina State University in Raleigh. Nitrate salt ions in the water can clog the resin and reduce its cost-effectiveness, and the resins are used just once in drinking-water facilities, because cleansing them often involves washing in methanol, an unacceptably toxic solvent.Another method uses membranes to separate contaminants from water. In reverse osmosis, mechanical pressure forces water through a membrane with tiny pores: water that is almost pure passes through, while everything else stays on the other side in a gradually saltier mix. Membrane systems are more expensive to build — reverse osmosis was three times the cost of a sorbent system when the CFPUA evaluated the options. (But if sorbents had to be changed more frequently, then membrane systems would become cost-effective, says Knappe.) Reverse osmosis also generates massive volumes of a watery, PFAS-laced brine that is difficult to manage.Targeted PFAS trapsMany researchers are inventing sorbents that can trap PFASs more selectively, often involving multiple chemical interactions at once. On the first floor at the Sweeney plant, beneath the GAC tanks, Coronell and Leibfarth are testing a proprietary sorbent. So far, it has lasted three times as long as the CFPUA’s GAC and 40% longer than a top-performing ion-exchange resin before the short-chain molecules have broken through. One possibility, says Kearns, is to add a layer of a new sorbent to capture escapees from GAC, thereby lengthening the time between trips to the reactivation kiln.Some researchers are testing their sorbents on dirtier, more complex PFAS sources, such as wastewater. The dirtiest of all is the liquid that pools at the bottom of a landfill (landfill leachate), which must be pumped out and treated, often by being transported to the nearest wastewater treatment plant by lorry. “It’s pretty gross,” says William Dichtel, a chemist at Northwestern University in Evanston, Illinois, who plans to test a sorbent on the leachate.In general, sorbents capture long-chained PFASs better than short-chained ones. Costly membrane systems might prove necessary for waters enriched in short-chained PFASs: one study2 found that nanofiltration, which uses membranes with slightly larger pores and produces less waste than reverse osmosis, captured more than 90% of ultrashort-chain PFASs from semiconductor wastewater.Another idea is to reconfigure GAC itself. The material’s pores are irregularly shaped, but carbon chemist Pan Ni at the University of Missouri in Columbia and his colleagues have reported preliminary work at a conference suggesting that the pores could be aligned into nano-sized channels instead. With the right channel diameters, GAC might begin targeting just the short-chained molecules.Destroying captured PFASsEvery sorbent eventually becomes full. How best to destroy the accumulated PFASs is now a key question and a billion-dollar market.Utilities that choose to clean their water using GAC could follow the Sweeney example and drive it to a reactivation kiln. An alternative is incineration, which is also a common way to dispose of spent single-use resins. Incineration simply destroys materials by burning them in the presence of oxygen — “a runaway reaction”, says Knappe — whereas GAC reactivation is controlled and works without oxygen.Ideally, both kinds of treatment would break every carbon–fluorine bond and release fluorine as hydrogen fluoride gas. The gas could then pass through ‘scrubbers’ containing alkaline reagents similar to baking soda to convert it into harmless minerals such as sodium fluoride.But it is not clear that the PFASs are completely mineralized, because some laboratory studies can’t match up the mass of the fluorine going in with that recovered from the products3. This suggests that some PFASs might have been broken up only into smaller gaseous PFAS molecules, which are harder to capture and might be spread into the air.Such gases can, however, be captured with the types of filter already installed at incinerator facilities that routinely deal with hazardous waste, says AnnieLu DeWitt, an analytical chemist at Clean Harbors, a waste-management firm headquartered in Norwell, Massachusetts, that specializes in hazardous-waste incineration and landfill.Tests in which PFASs were incinerated with added calcium minerals suggest that fluorine gets locked effectively into calcium fluoride. In Australia, some PFAS-containing wastes have been fed to cement kilns, which run at high temperatures and contain lots of calcium. However, total mineralization remains unproven.Because of questions over the effectiveness of incineration, the US Department of Defense has temporarily forbidden its facilities from incinerating fire-fighting foam that has high concentrations of PFASs. The department is working with the EPA and Clean Harbors to check whether incineration produces some PFAS gases. If not, the expectation is that incineration will become the go-to technique, says Ross. In the meantime, a bevy of start-up firms, often spun out of academic labs, have developed alternative ways to destroy PFASs. Many of these use high-energy conditions to rip the molecules apart. The firms say that the techniques can treat fire-fighting foam and PFAS-laced brines or biosolids that aren’t suitable for incineration.Vials of samples containing GenX, a PFAS chemical, in an EPA analysis lab in Cincinnati.Credit: Joshua A. Bickel/AP/AlamyA Pennsylvania start-up firm called OnVector uses plasmas (ionized gases) to break the molecules apart, while the company 374Water in Morrisville, North Carolina, uses supercritical water (water that behaves like a gas and a liquid, owing to high pressure and temperature). A technique being commercialized by Aquagga, a firm in Washington state, uses water at a lower temperature and pressure than other firms do, but adds an alkali chemical to kick-start the PFAS destruction.Milder destruction methods

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    How I connect Colombia’s remote communities to safer water

    “I’m an electrical engineer, but I think the most important connections are those between humans. This is what drives my work managing Monitoreo De Agua En Colombia (Water Monitoring in Colombia) a project at the University of the Andes in Bogotá. In Colombia, many rivers are contaminated, for example with mercury used in illegal gold mining. Through this project, the university’s engineering students work with groups in remote areas to co-design water probes, such as the one I’m using in the picture.I lead this project alongside my research using nanotechnology to create innovative materials for energy applications. Utilizing my engineering skills and resources to make humanitarian technologies for people here in Colombia is a hugely satisfying and important part of my job.In this picture, taken last November, I’m demonstrating how to use a custom-designed probe to record the pH, conductivity, dissolved-oxygen level and temperature of the water. We upload the results to our website to form a publicly accessible data set that shows the safety of water across the country. As of February, we have worked with 8 communities and have made around 50 probes. I am proud that this project is open science, so that any community can build a probe for themselves.

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    Meet the ice-hunting robots headed for the Moon right now

    A lander and an orbiter are on their way to the Moon to look for water at the lunar south pole (pictured).Credit: Alan Dyer/VW PICS/Universal Images Group via GettyTwo US spacecraft launched to the Moon today from Florida’s Cape Canaveral, on their way to hunt for water that scientists think exists at the lunar south pole. What the craft finds could have big ramifications for NASA’s plans to send astronauts to this part of the Moon in the coming years.Moon mission failure: why is it so hard to pull off a lunar landing?One of the missions is a commercial lander; it aims to touch down closer to the Moon’s south pole than any previous mission, carrying NASA instruments including an ice-hunting robot drill. The other spacecraft, NASA’s Lunar Trailblazer, is an orbiter with the goal of producing the highest-resolution maps of water on the Moon.Lunar water could provide a resource for expanded lunar exploration, such as by supplying the raw ingredients for rocket fuel at Moon bases. Scientists have known since 2009 that such water exists, but they want to know much more about where it is and how much there is. The two new spacecraft “are going after really important pieces of that puzzle,” says Parvathy Prem, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, who is not affiliated with either mission.The lander is expected to touch down on 6 March. It is the second attempt by Intuitive Machines, a company based in Houston, Texas, whose first lunar spacecraft tipped over on landing last year.Lunar Trailblazer will take a leisurely trajectory and reach the Moon in several months. If all goes well, it will enter its final science-mapping orbit around August.Searching for waterMany space agencies and scientists are keen to learn more about water at the lunar poles, which hold a geological record of the Solar System’s early history. The Indian mission Chandrayaan-2 is currently orbiting the Moon and building up its own data on where water might exist, as is a Korean probe that carries a NASA instrument to peer into shadowed, potentially ice-rich craters.Intuitive Machines’ new lander, named Athena, is headed for the Mons Mouton region of the Moon. Researchers think there is water in the lunar soil there, perhaps bound up in minerals or in pores in the soil.These six countries are about to go to the Moon — here’s whyAthena will search for water in several ways, including the use of NASA’s ice-mining drill, TRIDENT. If Athena lands successfully, operators will command TRIDENT to penetrate the lunar soil, drilling up to one metre deep to pull up the soil and leave it in a crumbly pile on the surface. A mass spectrometer on board will analyse the pile for signs of water or other volatile substances that might be escaping as gases. That ability to drill and analyse simultaneously provides “critical data on how lunar soils behave”, says Jackie Quinn, the drill’s project manager at NASA’s Kennedy Space Center on Merritt Island, Florida.

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    International action is needed now to save the Pantanal

    The Pantanal, the world’s largest freshwater wetland, straddles Brazil, Paraguay and Bolivia. National and international action is urgently required to counter an unprecedented conservation crisis there.
    Competing Interests
    The authors declare no competing interests. More

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    ‘Unacceptable’: a staggering 4.4 billion people lack safe drinking water, study finds

    People gather around a roadside pipeline to collect drinking water in Bangladesh.Credit: Mamunur Rashid/NurPhoto/Getty

    Approximately 4.4 billion people drink unsafe water — double the previous estimate — according to a study published today in Science1. The finding, which suggests that more than half of the world’s population is without clean and accessible water, puts a spotlight on gaps in basic health data and raises questions about which estimate better reflects reality.That this many people don’t have access is “unacceptable”, says Esther Greenwood, a water researcher at the Swiss Federal Institute of Aquatic Science and Technology in Dübendorf and an author on the Science paper. “There’s an urgent need for the situation to change.”The United Nations has been tracking access to safely managed drinking water, recognized as a human right, since 2015. Before this, the UN reported only whether global drinking-water sources were ‘improved’, meaning they were probably protected from outside contamination with infrastructure such as backyard wells, connected pipes and rainwater-collection systems. According to this benchmark, it seemed that 90% of the global population had its drinking water in order. But there was little information on whether the water itself was clean, and, almost a decade later, statisticians are still relying on incomplete data.“We really lack data on drinking-water quality,” Greenwood says. Today, water-quality data exist for only about half of the global population. That makes calculating the exact scale of the problem difficult, Greenwood adds.Crunching numbersIn 2015, the UN created its Sustainable Development Goals to improve human welfare. One of them is to “achieve universal and equitable access to safe and affordable drinking water for all” by 2030. The organization updated its criteria for safely managed drinking-water sources: they must be improved, consistently available, accessible where a person lives and free from contamination.
    The world faces a water crisis — 4 powerful charts show how
    Using this framework, the Joint Monitoring Programme for Water Supply, Sanitation and Hygiene (JMP), a research collaboration between the World Health Organization (WHO) and the UN children’s agency UNICEF, estimated in 2020 that there are 2.2 billion people without access to safe drinking water. To arrive at this figure, the programme aggregated data from national censuses, reports from regulatory agencies and service providers and household surveys.But it assessed drinking-water availability differently from the method used by Greenwood and her colleagues. The JMP examined at least three of the four criteria in a given location, and then used the lowest value to represent that area’s overall drinking-water quality. For instance, if a city had no data on whether its water-source was consistently available, but 40% of the population had uncontaminated water, 50% had improved water sources and 20% had water access at home, then the JMP estimated that 20% of that city’s population had access to safely managed drinking water. The programme then scaled this figure across a nation’s population using a simple mathematical extrapolation.By contrast, the Science paper used survey responses about the four criteria from 64,723 households across 27 low- and middle-income countries between 2016 and 2020. If a household failed to meet any of the four criteria, it was categorized as not having safe drinking water. From this, the team trained a machine-learning algorithm and included global geospatial data — including factors such as regional average temperature, hydrology, topography and population density — to estimate that 4.4 billion people lack access to safe drinking water, of which half are accessing sources tainted with the pathogenic bacteria Escherichia coli.The model also suggested that almost half of the 4.4 billion live in south Asia and sub-Saharan Africa (see ‘Water woes’).

    Source: Ref 1.

    ‘A long way to go’It’s “difficult” to say which estimate — the JMP’s or the new figure — is more accurate, says Robert Bain, a statistician at UNICEF’s Middle East and North Africa Regional Office, based in Amman, Jordan, who contributed to the calculation of both numbers. The JMP brings together many data sources but has limitations in its aggregation approach, whereas the new estimation takes a small data set and scales it up with a sophisticated model, he says.The study by Greenwood and colleagues really highlights “the need to pay closer attention to water quality”, says Chengcheng Zhai, a data scientist at the University of Notre Dame in Indiana. Although the machine-learning technique used by the team is “very innovative and clever”, she says, water access is dynamic, so the estimation might still not be quite right. Wells can be clean of E. coli one day and become contaminated the next, and the household surveys don’t capture that, Zhai suggests.“Whichever number you run with — two billion or four billion — the world has a long way to go” towards ensuring that people’s basic rights are fulfilled, Bain says. More

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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