Water is life, but ever scarcer. The most promising approaches to a mounting global problem may be molecular.
Zheng-Tian Lu, SM’91, first heard about krypton 81 dating at a meeting of physicists in Germany in 1996. The use of radioactive isotopes like krypton 81 and the more commonly known carbon 14 to date organic objects goes back to 1907, when Bertram Boltwood first measured the age of rocks containing radioactive uranium. Carbon 14 dating reaches its limit at about 30,000 years, but krypton 81, with a half-life of 229,000 years, has the potential to date much older things—among them subterranean aquifers.
In an age of shrinking water supplies, a better understanding of groundwater is one critical front. Almost half the world’s drinking water comes from underground (rather than from surface sources like lakes and streams). And in many regions where people rely on groundwater, these resources are under threat. But questions about aquifers have been largely matters of guesswork and conjecture, much of it wrong: How long has the water been in the ground? How fast and in which direction is it flowing? And, most important, at what rate is it being replenished from above? With better answers, those resources could be managed more effectively. Krypton 81 dating, Lu and others recognized, might help, but first a number of daunting challenges had to be solved.
The Water Research Initiative, a joint project of UChicago, Argonne, and Ben-Gurion University of the Negev in Israel, is taking on challenges like this. The initiative applies the insights of basic research in physics, chemistry, biology, and other disciplines, especially at the molecular level, to developing new technologies for addressing scarcity, pollution, and other real-world water problems. A project of the Institute for Molecular Engineering (IME), it springs from the conviction that water is among the most high-stakes challenges of our time—and likely to grow still more so as the global population climbs and climate change worsens.
The initiative encourages researchers from the three institutions to work together on common projects, bringing to bear a wide range of expertise. In 2013 the initiative awarded its first grants, totaling more than $1 million, to five research groups; it added a sixth project this year. The teams are designing sophisticated new membranes to filter out viruses, the smallest pathogens, from water; developing catalysts that can destroy dangerous organic pollutants; and figuring out ways to prevent bacteria from fouling the membranes used to desalinate seawater, a technology that water-starved regions around the world are turning to more and more. And Lu’s team, continuing to refine work that began almost 20 years ago, has already unlocked some of the secrets of ancient aquifers by capturing and measuring the elusive krypton 81.
Discovered in 1950 by Argonne’s John Reynolds, SM’48, PhD’50, krypton 81 is formed when cosmic rays strike ordinary krypton atoms high in the atmosphere. Water on the surface absorbs air, including krypton 81, but once underground it is cut off from the atmosphere and no longer absorbs new gases. Over time—a very long time—the krypton 81 decays, turning into bromine. Fill a jar with rainwater, close the lid, and in 229,000 years you will have only half the krypton 81 you started with. In a million years you will be down to one-sixteenth. By extracting the gas from a sample of groundwater and measuring the amount of krypton 81 in it, scientists can determine with considerable accuracy how long the water has been in the ground.
But krypton 81 is exceedingly rare. It is difficult to detect and almost impossible to measure using conventional methods of radioisotope dating. In the air around us, only 1.14 particles in a million are krypton. Of these, only one in a trillion is krypton 81. A liter of air contains about 20,000 atoms of the isotope—and 10 sextillion (10) of everything else. If all the sand on earth were air, four grains would be krypton 81. It’s the needle in the atmospheric haystack.
Zheng-Tian Lu (second from right in the back row) and colleagues with an ATTA device. (Photography by George Joch, courtesy Argonne National Laboratory)
Lu found it. In 1997 the physicist and part-time professor at UChicago joined Argonne National Laboratory, where he and his colleagues worked for two years to build an atom trap that could detect and measure the isotope. Made of stainless steel, coiled copper, ceramic, and other materials, it stretched the length of a kitchen table. He called it ATTA, for Atom Trap Trace Analysis. It worked by releasing a small amount of krypton gas into a vacuum, sending it down a long tube, and then trapping individual atoms by striking them with lasers on six sides. Set to the right frequency, the lasers could single out krypton 81 atoms and make them glow like fireflies at dusk. And Lu could count them.
Since 1999, when Lu first published the ATTA method, krypton 81 dating has begun to transform scientists’ understanding of aquifers around the world.“Especially now that water is getting more and more precious, with climate change … it becomes more and more important to manage wisely whatever water resources there are,” says Neil Sturchio, a geochemist at the University of Delaware who has worked with Lu. “In many parts of the world, groundwater is the only source of water that’s available, especially in arid regions.”
For this reason, Sturchio calls krypton 81 dating “the most important new tool in hydrology in 50 years.” It has shed light on the age and movement of underground water on every continent; it has been used to date ice from Antarctic glaciers and water in Yellowstone’s geyser basins. In many cases the ATTA method has revealed that deep aquifers are much older than scientists thought, suggesting that the water takes much longer to be replenished and can be more easily depleted than was supposed.
Last summer Lu and a team of researchers from Argonne and Ben-Gurion spent two weeks traveling across Israel, sampling deep wells. They wanted to determine the age and movements of water in a critical sandstone aquifer 1,000 meters below the Negev desert. Using equipment designed by Reika Yokochi, a UChicago geoscientist, that could fit in the back of a van, they extracted the gases from several hundred liters of groundwater in just an hour. In two weeks they sampled more than 30 wells and sent tanks of compressed gas, each about the size of a scuba tank, to Chicago.
Using another device she designed, a tangle of tubes and aluminum foil, Yokochi extracted the krypton from each sample. Each tank of gas yielded a vial of krypton about the size of a pencil stub, which she sent to the group at Argonne. By measuring the krypton 81 in each vial, they discovered that in some places the water was quite young—in the ground 30,000 years or less—while in others it was as old as 600,000 years. “People don’t expect that,” Lu says. Further analysis showed that water beneath Israel was flowing very slowly, about a meter a year, from the Sinai Desert in the west to the Dead Sea region in the east. “They knew the water level everywhere,” says Lu. “What they didn’t know is how this water moves underneath.”
The method keeps yielding surprises, and critical data. Lu first used it a decade ago to date water from the Nubian Aquifer in western Egypt. Sturchio, who collected the samples, said conventional wisdom had held that the aquifer was 30,000 to 40,000 years old. But Lu discovered that the water had been underground for a million years. “A lot of the old literature that had become gospel about groundwater is pretty much all wrong,” Sturchio says. Hydrologists can use information about the age and movement of underground water to make more realistic models than previously possible. Given the dependence of agriculture on groundwater, this knowledge can have far-reaching implications. Sturchio points to the example of drought-stricken California, where aquifers are being depleted faster than they are being replenished with rain and snowmelt.
If a better hydrologic model had been available, it could have been used to place regulatory limits on the annual amount of groundwater withdrawal so that even in times of drought the groundwater resource would not be depleted, he says. Such models can be used in conjunction with climate models to predict maximum sustainable food production—and thus maximum sustainable population density in a region. This is especially important in arid regions, Sturchio says, but relevant in more temperate regions that have high population density too.
Since Lu built the first ATTA device, he has built two more atom traps to measure krypton 81 and similar trace gases. Several other traps have been built around the world, including in China, in Germany, and at Columbia University. The third version, ATTA-3, is more sensitive than its predecessors. It can measure krypton 81 using much smaller samples of gas, which require much smaller samples of water. His first machine needed a million liters of water to measure the krypton in it, while ATTA-3 requires a minimum sample of about 100 liters. Lu hopes to make it possible for scientists to one day map all the world’s ice and groundwater resources. To do this, he wants to refine his method so that it requires as little as 10 liters of water, making it conceivable that hydrologists might be able to ship samples directly from field to lab instead of degassing large quantities of water in the field. “We think we can do better,” he says.
The Water Research Initiative dates to the summer of 2012, when two old friends met in Chicago to discuss how their universities might collaborate. Moshe Gottlieb, a chemical engineer from Ben-Gurion, and Matthew Tirrell, the Pritzker Director of UChicago’s just-formed Institute for Molecular Engineering, had known each other since the 1970s, when they were both starting out at the University of Minnesota. Now in their 60s, they spent a warm July weekend discussing research topics that might link UChicago’s strength in basic science with Ben-Gurion’s expertise in engineering.
The IME was only a year old. Tirrell, who had come to UChicago from the University of California, Berkeley, presided over a faculty of one—himself. For most of its existence UChicago had left engineering to others, focusing on basic science. But the rise of molecular engineering had blurred the old distinction between basic and applied science. Molecular engineers use insights into the properties of atoms and molecules to create nanotechnologies to advance computing, medicine, energy, and other fields. Tirrell was starting to hire a faculty and define a research agenda.
Over two days, he and Gottlieb considered different possibilities for collaboration. They talked about renewable energy, biomedicine, information technology, and other potential topics, some of which the IME is pursuing today. All were important, all were feasible, and yet none seemed right for this collaboration.
“We didn’t want to do something that people already do and were far in advance,” Gottlieb recalls. “There are lots of people working on green energy. We would be just another one.” On the second day they recognized that the emerging crisis in the supply of fresh water, from California to South Asia to sub-Saharan Africa, was ideally suited to the institutions’ strengths, and in need of new ideas and approaches. “The quality of water is threatened in many parts of the world,” says Tirrell. “And water interacts with other large-scale systems we all depend on, including energy.”
Water Research Initiative director Steven Sibener. (Photography by Dan Dry)
It was a problem of global scope that could be approached through many disciplines, including molecular science, and it was suited to the collaboration. Israel is a world leader in the conservation, desalination, and purification of water. In the past decade it has built four big desalination plants (with a fifth under construction), which produce 40 percent of the country’s domestic water consumption. Israel recycles 86 percent of its sewage, yielding half the water used in agriculture. The waste from Tel Aviv’s two million residents waters crops in the Negev. Researchers at Ben-Gurion have been part of this effort, developing, among other technologies, new ways to monitor groundwater pollution, manage water used in agriculture, and improve the efficiency of desalination technology.
After Gottlieb returned home, a group of researchers from UChicago and Argonne traveled to Israel to talk with scientists at Ben-Gurion about what areas of water research they might cooperate on. A group from Ben-Gurion made a return visit, and in 2013 UChicago president Robert J. Zimmer and Rivka Carmi, Ben-Gurion’s president, signed an agreement that made the initiative official. The signing took place at the residence of Israeli president Shimon Peres, with Chicago mayor Rahm Emanuel looking on. Steven Sibener, the Carl William Eisendrath Distinguished Service Professor in Chemistry and the James Franck Institute, became the initiative’s director. His research has revealed previously unknown ways that carbon dioxide and other gases become embedded in ice.
Water is central to our lives not only for drinking and household use but for agriculture, manufacturing, and energy production. And the demand for it is growing. The United Nations estimates that population growth boosts water use by 64 billion cubic meters a year. Over the next 35 years, it says, the world demand for water will rise 55 percent.
This portends increasing scarcity for an already thirsty planet. Water covers almost 71 percent of the earth’s surface, yet only 2.5 percent is fresh. Of that 2.5 percent, 70 percent is bound up in ice and snow. Much of the rest lies underground. Only 0.3 percent of all freshwater is on the surface. By 2030, the UN estimates, 47 percent of the world’s population will live in areas of high water stress.
At the same time, in many regions the quality of the world’s water supply is as big a problem as the quantity. According to the World Health Organization and UNICEF, about one in 10 persons on the planet—748 million—lacks access to clean drinking water. In developing countries, 70 percent of industrial waste is dumped untreated into the water. Even in the United States, more than half the nation’s rivers and streams are in poor condition, according a 2008–09 Environmental Protection Agency survey. The US Geological Survey reported in January that tests on 6,600 wells over two decades showed that one in five contained a man-made or natural contaminant that posed a health risk.
The close links between water, food, and energy make the problem of water even more challenging. Most of the fresh water we consume is used in agriculture. Humans need two to four liters of drinking water a day to survive, but it takes 2,000 to 5,000 liters to produce one day’s food. The rising standard of living in many developing countries is increasing the agricultural use of water. According to the UN, it takes 3,500 liters of water to grow a kilogram of rice but 15,000 liters to raise the same amount of beef. Meanwhile, the spread of irrigation has tripled water use in agriculture over the past 50 years. The UN estimates that 20 percent of aquifers are overexploited.
Producing energy also requires immense quantities of water. Water is needed to cool electric turbines. It’s used in hydraulic fracturing, to break the rock underground. Some forms of renewable energy, like biofuels, guzzle large quantities of water. According to the UN, 15 percent of all the water we use goes toward making energy. Ninety percent of energy production relies on intensive, unsustainable water consumption. And the demand for energy is growing.
In the 1960s two scientists developed the Loeb-Sourirajan method of reverse osmosis that for the first time made it commercially feasible to remove salt from seawater by forcing the water through a semipermeable plastic membrane. Reverse osmosis is not the only way to take salt out of seawater, but it is the least expensive and most efficient. Advances in reverse osmosis in recent years have created a boom in desalination plants, with more than 16,000 now operating worldwide.
New technology has brought reverse osmosis close to its theoretical limit of efficiency. But the membranes used in desalination are efficient only as long as they are clean, and they are prone to fouling. Electrostatic energy and van der Waals forces—the weak attraction between molecules—combine to attract organic molecules like sugars and proteins. These molecules, in turn, attract bacteria, which form colonies that clog membranes. This problem isn’t unique to desalination plants; aquarium walls accumulating slime or dishes left too long in the sink suffer from the same effect. In desalination, the result is a drop in efficiency. The plant must pump harder to keep water flowing through the desalination membranes, or the membranes must be cleaned with chemicals that can damage them. “It’s a universal problem for all water-related applications,” says Jing Yu, a postdoctoral appointee at Argonne and a visiting scientist at UChicago.
In high-tech labs at Argonne, Yu and his colleagues are designing polymers—the long molecules that make up plastics, gelatins, and DNA—that can help membranes resist fouling. These special polymers repel both organic molecules and the bacteria that feed on them. Called zwitterionic polymer brushes, they stand up like the nap on carpet. They also exhibit polarity, making them attractive to water, which washes away organic molecules that might otherwise collect and attract bacteria.
Making polymers to these specifications is not easy. It requires high heat and a vacuum, and takes from several days to a week of intensive work in the lab. The researchers at Argonne cook up small amounts and then send them in glass jars or plastic tubes to Ben-Gurion. There, researchers with expertise in testing high-tech materials paint the polymers on membranes to see how well they function. Some of the polymers, says Yu, have proven “quite effective.”
But effectiveness is only a start. The goal of Yu’s research—and of the Water Research Initiative—is not merely to create materials and processes that work in the lab but to develop them for commercial use. Researchers in the antifouling project, including Tirrell and Gottlieb, are testing different versions of the polymers to see how they react to different levels of salt and pH. They’re also trying to find simpler and more efficient ways of making them, such as in ambient conditions rather than in a vacuum. They want to be able to use inexpensive raw materials and environmentally friendly, nontoxic methods, perhaps using sunlight to start the polymerization. “Our goal is to make it cheap and easy to make,” says Yu.
Concern over water pollution focused for many years on chemicals used in steelmaking and other industrial processes. Today concern is mounting over a new generation of contaminants, including hormones, pharmaceuticals, and other organic compounds that can react in the body, disrupting endocrine systems and causing cancer. One way to attack these organic pollutants is to use metal catalysts to break them down into smaller molecules that are safe for humans. But most catalysts don’t work in water; the oxygen in it destroys them. They need organic solvents instead. As part of the water initiative, Sibener and Dmitri Talapin, professor of chemistry, with Ben-Gurion professors Miron Landau and Moti Herskowitz, are using molecular engineering techniques to design and build catalysts that can work in water. These catalysts will be incorporated into membranes; as water flows through, the harmful chemicals will be destroyed. “It’s a fairly big breakthrough,” says Gottlieb.
Cleaning water can also mean removing harmful microorganisms, like viruses. Current technology does this imperfectly. A virus may be as small as 20 nanometers wide, or 20 billionths of a meter. The pores in commercially available filters cannot be made consistently that size—some are small enough, but others are too large and allow some viruses to pass. “With something like a virus, removing 80 or 90 percent of it is not good enough,” says Seth Darling, PhD’02, a nanoscientist at Argonne and a fellow at the IME. “Even a little can make you sick. You want to remove it all.”
Darling has experimented for about eight years with block copolymers. These materials link together two or more different polymer chains, often with different properties. They interest scientists because they can be engineered to create nanostructures with specific combinations of characteristics, directed to meet specific technological needs. For example, they are used to make ABS, a plastic found in protective headgear and canoes that is lightweight but tough. Darling and his Argonne colleagues have been trying to use them to create low-cost photovoltaic surfaces for solar power. They thought block copolymer technology, combined with a new technique called sequential infiltration synthesis (SIS), might help construct more effective membranes to clean water.
Block copolymers have another quality that is useful to molecular engineers: they self-assemble. The trick is to create versions of them that will order up in useful ways. Darling and Jeffrey Elam, PhD’95, use a polymer that self-assembles into an array of tiny cylinders that stand up like a bundle of pencils. Then they expose this polymer layer to two gases that are precursors for titanium dioxide, enhancing it. In just a few minutes, the metal oxide embeds itself in the interstices between the cylinders. Next, using heat, they decompose the copolymer. “You just cook it away,” Darling says. “It’s relatively easy to remove polymer.”
What’s left is a tough titanium dioxide film full of tiny holes—all the same size—where the copolymer cylinders once stood. By altering the copolymers, they can change the diameter of the cylinders and hence the size of the holes. With this method they produce what up until now has been unavailable: a membrane with tiny pores of consistent size.
The titanium dioxide gives the membrane one other useful characteristic. When you shine a light on titanium dioxide, the energy of the light makes it act as a catalyst. In this way a membrane containing titanium dioxide can destroy harmful organic pollutants and membrane foulants at the same time it filters out dangerous microorganisms.
Several challenges remain before the technology is commercially viable. One is to figure out how to attach the film to a commercial membrane without leaving gaps. “You can’t have gaps,” Darling says. “It sort of defeats the purpose.” They are also tinkering with the film’s photochemical capability, trying to shift it away from ultraviolet light toward the range of visible light.
Energy, says Darling, is “the biggest challenge we face in this century. If we want to combat climate change, we need to develop lower-cost, highly scalable renewable energy technology. The same can be argued about water.”
The Water Research Initiative is nearing the end of its second year. At winter’s end, the University launched a national search to hire the next water expert to succeed Sibener as the initiative’s director and build on the foundation that he and Tirrell have put in place. Meanwhile, the inaugural research projects entered their second year making encouraging progress. “It’s clear that our concept of assembling multidisciplinary research teams was the correct approach,” says Sibener. Those teams “are working in areas that hold realistic promise for making innovative and significant discoveries in fundamental water science, but also with very direct conduits to applied science and technology.” Progress shown with the first grants could attract new and deeper sources of funding, from government agencies, private foundations, and businesses that see commercial potential in the research.
There will also likely be new opportunities for a broader palette of researchers to tackle further scientific problems—more efficient water use in agriculture, for instance, or too much water in places like Chicago, where more frequent heavy rains brought on by climate change have increased flooding risk. The vision for the Water Research Initiative is to broaden its focus beyond engineering and nanoscience to questions of law, economics, and public policy, drawing on other parts of the University. Water scarcity is “a challenging scientific and engineering problem, which we’re very interested in” says Sibener, “but it also has societal and political ramifications that are becoming more obvious every day.”
In all, water “is the grand issue for a generation—a topic whose time has come.” Students, he adds, are especially drawn to the topic. After a few talks he’s given about it to undergraduate groups, he says, the students’ “eyes were gleaming.” More than 10 graduate students, undergraduate students, and postdocs are directly involved in the six projects; their sense of these issues’ importance for their own futures is palpable. “We are going to have to figure out how to maintain and shepherd our precious water supplies, and to do that we are going to have to learn how to make fresh water from seawater very cost effectively,” Sibener says. “Research at the intersection of basic and applied science will be crucial to this endeavor.”
Richard Mertens is a writer in Chicago. He last wrote for the Magazine on the University of Chicago Harris School of Public Policy’s Gary Project.
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