More stories

  • in

    Recycling rare earth elements is hard. Science is trying to make it easier

    Our modern lives depend on rare earth elements, and someday soon we may not have enough to meet growing demand.

    Because of their special properties, these 17 metallic elements are crucial ingredients in computer screens, cell phones and other electronics, compact fluorescent lamps, medical imaging machines, lasers, fiber optics, pigments, polishing powders, industrial catalysts – the list goes on and on (SN Online: 1/16/23). Notably rare earths are an essential part of the high-powered magnets and rechargeable batteries in the electric vehicles and renewable energy technologies needed to get the world to a low- or zero-carbon future.

    Science News headlines, in your inbox

    Headlines and summaries of the latest Science News articles, delivered to your email inbox every Thursday.

    Thank you for signing up!

    There was a problem signing you up.

    In 2021, the world mined 280,000 metric tons of rare earths — roughly 32 times as much as was mined in the mid-1950s. And demand is only going to increase. By 2040, experts estimate, we’ll need up to seven times as much rare earths as we do today.

    Satisfying that appetite won’t be easy. Rare earth elements are not found in concentrated deposits. Miners must excavate huge amounts of ore, subject it to physical and chemical processes to concentrate the rare earths, and then separate them. The transformation is energy intensive and dirty, requiring toxic chemicals and often generating a small amount of radioactive waste that must be safely disposed of. Another concern is access: China has a near monopoly on both mining and processing; the United States has just one active mine (SN Online: 1/1/23).

    For most of the jobs rare earths do, there are no good substitutes. So to help meet future demand and diversify who controls the supply — and perhaps even make rare earth recovery “greener” — researchers are looking for alternatives to conventional mining.   

    Proposals include everything from extracting the metals from coal waste to really out-there ideas like mining the moon. But the approach most likely to make an immediate dent is recycling. “Recycling is going to play a very important and central role,” says Ikenna Nlebedim, a materials scientist at Ames National Laboratory in Iowa and the Department of Energy’s Critical Materials Institute. “That’s not to say we’re going to recycle our way out of the critical materials challenge.”

    Still, in the rare earth magnets market, for instance, by about 10 years from now, recycling could satisfy as much as a quarter of the demand for rare earths, based on some estimates. “That’s huge,” he says.

    But before the rare earths in an old laptop can be recycled as regularly as the aluminum in an empty soda can, there are technological, economic and logistical obstacles to overcome.

    Why are rare earths so challenging to extract?

    Recycling seems like an obvious way to get more rare earths. It’s standard practice in the United States and Europe to recycle from 15 to 70 percent of other metals, such as iron, copper, aluminum, nickel and tin. Yet today, only about 1 percent of rare earth elements in old products are recycled, says Simon Jowitt, an economic geologist at the University of Nevada, Las Vegas.

    “Copper wiring can be recycled into more copper wiring. Steel can just be recycled into more steel,” he says. But a lot of rare earth products are “inherently not very recyclable.”

    Rare earths are often blended with other metals in touch screens and similar products, making removal difficult. In some ways, recycling rare earths from tossed-out items resembles the challenge of extracting them from ore and separating them from each other. Traditional rare earth recycling methods also require hazardous chemicals such as hydrochloric acid and a lot of heat, and thus a lot of energy. On top of the environmental footprint, the cost of recovery may not be worth the effort given the small yield of rare earths. A hard disk drive, for instance, might contain just a few grams; some products offer just milligrams.

    Chemists and materials scientists, though, are trying to develop smarter recycling approaches. Their techniques put microbes to work, ditch the acids of traditional methods or attempt to bypass extraction and separation.

    Microbial partners can help recycle rare earths

    One approach leans on microscopic partners. Gluconobacter bacteria naturally produce organic acids that can pull rare earths, such as lanthanum and cerium, from spent catalysts used in petroleum refining or from fluorescent phosphors used in lighting. The bacterial acids are less environmentally harmful than hydrochloric acid or other traditional metal-leaching acids, says Yoshiko Fujita, a biogeochemist at Idaho National Laboratory in Idaho Falls. Fujita leads research into reuse and recycling at the Critical Materials Institute. “They can also be degraded naturally,” she says.

    In experiments, the bacterial acids can recover only about a quarter to half of the rare earths from spent catalysts and phosphors. Hydrochloric acid can do much better — in some cases extracting as much as 99 percent. But bio-based leaching might still be profitable, Fujita and colleagues reported in 2019 in ACS Sustainable Chemistry & Engineering.

    In a hypothetical plant recycling 19,000 metric tons of used catalyst a year, the team estimated annual revenues to be roughly $1.75 million. But feeding the bacteria that produce the acid on-site is a big expense. In a scenario in which the bacteria are fed refined sugar, total costs for producing the rare earths are roughly $1.6 million a year, leaving around just $150,000 in profits. Switching from sugar to corn stalks, husks and other harvest leftovers, however, would slash costs by about $500,000, raising profits to about $650,000.

    One experimental recycling approach uses organic acids made by bacteria to extract rare earths from waste products. This reactor at the Idaho National Laboratory prepares an organic acid mixture for such recycling.Idaho National Lab

    Other microbes can also help extract rare earths and take them even further. A few years ago, researchers discovered that some bacteria that metabolize rare earths produce a protein that preferentially grabs onto these metals. This protein, lanmodulin, can separate rare earths from each other, such as neodymium from dysprosium — two components of rare earth magnets. A lanmodulin-based system might eliminate the need for the many chemical solvents typically used in such separation. And the waste left behind — the protein — would be biodegradable. But whether the system will pan out on a commercial scale is unknown.

    How to pull rare earths from discarded magnets

    Another approach already being commercialized skips the acids and uses copper salts to pull the rare earths from discarded magnets, a valuable target. Neodymium-iron-boron magnets are about 30 percent rare earth by weight and the single largest application of the metals in the world. One projection suggests that recovering the neodymium in magnets from U.S. hard disk drives alone could meet up about 5 percent of the world’s demand outside of China before the end of the decade.

    Nlebedim led a team that developed a technique that uses copper salts to leach rare earths out of shredded electronic waste that contains magnets. Dunking the e-waste in a copper salt solution at room temperature dissolves the rare earths in the magnets. Other can be scooped out for their own recycling, and the copper can be reused to make more salt solution. Next, the rare earths are solidified and, with the help of additional chemicals and heating, transformed into powdered minerals called rare earth oxides. The process, which has also been used on material left over from magnet manufacturing that typically goes to waste, can recover 90 to 98 percent of the rare earths, and the material is pure enough to make new magnets, Nlebedim’s team has demonstrated.

    In a best-case scenario, using this method to recycle 100 tons of leftover magnet material might produce 32 tons of rare earth oxides and net more than $1 million in profits, an economic analysis of the method suggests.

    That study also evaluated the approach’s environmental impacts. Compared with producing one kilogram of rare earth oxide via one of the main types of mining and processing currently used in China, the copper salt method has less than half the carbon footprint. It produces an average of about 50 kilograms of carbon dioxide equivalent per kilogram of rare earth oxide versus 110, Nlebedim’s team reported in 2021 in ACS Sustainable Chemistry & Engineering.

    But it’s not necessarily greener than all forms of mining. One sticking point is that the process requires toxic ammonium hydroxide and roasting, which consumes a lot of energy, and it still releases some carbon dioxide. Nlebedim’s group is now tweaking the technique. “We want to decarbonize the process and make it safer,” he says.

    Meanwhile, the technology seems promising enough that TdVib, an Iowa company that designs and manufactures magnetic materials and products, has licensed it and built a pilot plant. The initial aim is to produce two tons of rare earth oxides per month, says Daniel Bina, TdVib’s president and CEO. The plant will recycle rare earths from old hard disk drives from data centers.

    Noveon Magnetics, a company in San Marcos, Texas, is already making recycled neodymium-iron-boron magnets. In typical magnet manufacturing, the rare earths are mined, transformed into metal alloys, milled into a fine powder, magnetized and formed into a magnet. Noveon knocks out those first two steps, says company CEO Scott Dunn.

    After demagnetizing and cleaning discarded magnets, Noveon directly mills them into a powder before building them back up as new magnets. Unlike with other recycling methods, there’s no need to extract and separate the rare earths out first. The final product can be more than 99 percent recycled magnet, Dunn says, with a small addition of virgin rare earth elements — the “secret sauce,” as he puts it — that allows the company to fine-tune the magnets’ attributes.

    Compared with traditional magnet mining and manufacturing, Noveon’s method cuts energy use by about 90 percent, Miha Zakotnik, Noveon’s chief technology officer, and other researchers reported in 2016 in Environmental Technology & Innovation. Another 2016 analysis estimated that for every kilogram of magnet produced via Noveon’s method, about 12 kilograms of carbon dioxide equivalent are emitted. That’s about half as much of the greenhouse gas as conventional magnets.

    Dunn declined to share what volume of magnets Noveon currently produces or how much its magnets cost. But the magnets are being used in some industrial applications, for pumps, fans and compressors, as well as some consumer power tools and other electronics.

    To help with recycling, Apple developed the robot Daisy (shown), which can dismantle 23 models of iPhones. Other robots in the works — Taz and Dave — will specialize in recovering rare earth magnets.Apple

    Rare earth recycling has logistical hurdles

    Even as researchers clear technological hurdles, there are still logistical barriers to recycling. “We don’t have the systems for collecting end-of-life products that have rare earths in them,” Fujita says, “and there’s the cost of dismantling those products.” For a lot of e-waste, before rare earth recycling can begin, you have to get to the bits that contain those precious metals.

    Noveon has a semiautomated process for removing magnets from hard disk drives and other electronics.

    Apple is also trying to automate the recycling process. The company’s Daisy robot can dismantle iPhones. And in 2022, Apple announced a pair of robots called Taz and Dave that facilitate the recycling of rare earths. Taz can gather magnet-containing modules that are typically lost during the shredding of electronics. Dave can recover magnets from taptic engines, Apple’s technology for providing users with tactile feedback when, say, tapping an iPhone screen.

    Even with robotic aids, it would still be a lot easier if companies just designed products in a way that made recycling easy, Fujita says.

    No matter how good recycling gets, Jowitt sees no getting around the need to ramp up mining to feed our rare earth–hungry society. But he agrees recycling is necessary. “We’re dealing with intrinsically finite resources,” he says. “Better we try and extract what we can rather than just dumping it in the landfill.” More

  • in

    Heat waves in U.S. rivers are on the rise. Here’s why that’s a problem

    U.S. rivers are getting into hot water. The frequency of river and stream heat waves is on the rise, a new analysis shows.

    Like marine heat waves, riverine heat waves occur when water temperatures creep above their typical range for five or more days (SN: 2/1/22). Using 26 years of United States Geological Survey data, researchers compiled daily temperatures for 70 sites in rivers and streams across the United States, and then calculated how many days each site experienced a heat wave per year. From 1996 to 2021, the annual average number of heat wave days per river climbed from 11 to 25, the team reports October 3 in Limnology and Oceanography Letters.

    Sign Up For the Latest from Science News

    Headlines and summaries of the latest Science News articles, delivered to your inbox

    Thank you for signing up!

    There was a problem signing you up.

    The study is the first assessment of heat waves in rivers across the country, says Spencer Tassone, an ecosystem ecologist at the University of Virginia in Charlottesville. He and his colleagues tallied nearly 4,000 heat wave events — jumping from 82 in 1996 to 198 in 2021 — and amounting to over 35,000 heat wave days. The researchers found that the frequency of extreme heat increased at sites above reservoirs and in free-flowing conditions but not below reservoirs — possibly because dams release cooler water downstream.

    Most heat waves with temperatures the highest above typical ranges occurred outside of summer months between December and April, pointing to warmer wintertime conditions, Tassone says.

    Human-caused global warming plays a role in riverine heat waves, with heat waves partially tracking air temperatures — but other factors are probably also driving the trend. For example, less precipitation and lower water volume in rivers mean waterways warm up easier, the study says.

    “These very short, extreme changes in water temperature can quickly push organisms past their thermal tolerance,” Tassone says. Compared with a gradual increase in temperature, sudden heat waves can have a greater impact on river-dwelling plants and animals, he says. Fish like salmon and trout are particularly sensitive to heat waves because the animals rely on cold water to get enough oxygen, regulate their body temperature and spawn correctly.

    There are chemical consequences to the heat as well, says hydrologist Sujay Kaushal of the University of Maryland in College Park who was not involved with the study. Higher temperatures can speed up chemical reactions that contaminate water, in some cases contributing to toxic algal blooms (SN: 2/7/18). 

    The research can be used as a springboard to help mitigate heat waves in the future, Kaushal says, such as by increasing shade cover from trees or managing stormwater. In some rivers, beaver dams show promise for reducing water temperatures (SN: 8/9/22). “You can actually do something about this.” More

  • in

    Mangrove forests expand and contract with a lunar cycle

    The glossy leaves and branching roots of mangroves are downright eye-catching, and now a study finds that the moon plays a special role in the vigor of these trees.

    Long-term tidal cycles set in motion by the moon drive, in large part, the expansion and contraction of mangrove forests in Australia, researchers report in the Sept. 16 Science Advances. This discovery is key to predicting when stands of mangroves, which are good at sequestering carbon and could help fight climate change, are most likely to proliferate (SN: 11/18/21). Such knowledge could inform efforts to protect and restore the forests.

    Sign Up For the Latest from Science News

    Headlines and summaries of the latest Science News articles, delivered to your inbox

    Thank you for signing up!

    There was a problem signing you up.

    Mangroves are coastal trees that provide habitat for fish and buffer against erosion (SN: 9/14/22). But in some places, the forests face a range of threats, including coastal development, pollution and land clearing for agriculture. To get a bird’s-eye view of these forests, Neil Saintilan, an environmental scientist at Macquarie University in Sydney, and his colleagues turned to satellite imagery. Using NASA and U.S. Geological Survey Landsat data from 1987 to 2020, the researchers calculated how the size and density of mangrove forests across Australia changed over time.

    After accounting for persistent increases in these trees’ growth — probably due to rising carbon dioxide levels, higher sea levels and increasing air temperatures — Saintilan and his colleagues noticed a curious pattern. Mangrove forests tended to expand and contract in both extent and canopy cover in a predictable manner. “I saw this 18-year oscillation,” Saintilan says.

    That regularity got the researchers thinking about the moon. Earth’s nearest celestial neighbor has long been known to help drive the tides, which deliver water and necessary nutrients to mangroves. A rhythm called the lunar nodal cycle could explain the mangroves’ growth pattern, the team hypothesized.

    Over the course of 18.6 years, the plane of the moon’s orbit around Earth slowly tips. When the moon’s orbit is the least tilted relative to our planet’s equator, semidiurnal tides — which consist of two high and two low tides each day — tend to have a larger range. That means that in areas that experience semidiurnal tides, higher high tides and lower low tides are generally more likely. The effect is caused by the angle at which the moon tugs gravitationally on the Earth.  

    Saintilan and his colleagues found that mangrove forests experiencing semidiurnal tides tended to be larger and denser precisely when higher high tides were expected based on the moon’s orbit. The effect even seemed to outweigh other climatic drivers of mangrove growth, such as El Niño conditions. Other regions with mangroves, such as Vietnam and Indonesia, probably experience the same long-term trends, the team suggests.

    Having access to data stretching back decades was key to this discovery, Saintilan says. “We’ve never really picked up before some of these longer-term drivers of vegetation dynamics.”

    It’s important to recognize this effect on mangrove populations, says Octavio Aburto-Oropeza, a marine ecologist at the Scripps Institution of Oceanography in La Jolla, Calif., who was not involved in the research.

    Scientists now know when some mangroves are particularly likely to flourish and should make an extra effort at those times to promote the growth of these carbon-sequestering trees, Aburto-Oropeza says. That might look like added limitations on human activity nearby that could harm the forests, he says. “We should be more proactive.”  More

  • in

    Common, cheap ingredients can break down some ‘forever chemicals’

    There’s a new way to rip apart harmful “forever chemicals,” scientists say.

    Perfluoroalkyl and polyfluoroalkyl substances, also known as PFAS, are found in nonstick pans, water-repellent fabrics and food packaging and they are pervasive throughout the environment. They’re nicknamed forever chemicals for their ability to stick around and not break down. In part, that’s because PFAS have a super strong bond between their carbon and fluorine atoms (SN: 6/4/19). Now, using a bit of heat and two relatively common compounds, researchers have degraded one major type of forever chemical in the lab, the team reports in the Aug. 19 Science. The work could help pave the way for a process for breaking down certain forever chemicals commercially, for instance by treating wastewater.

    Sign Up For the Latest from Science News

    Headlines and summaries of the latest Science News articles, delivered to your inbox

    Thank you for signing up!

    There was a problem signing you up.

    “The fundamental knowledge of how the materials degrade is the single most important thing coming out of this study,” organic chemist William Dichtel said in an August 16 news conference.

    While some scientists have found relatively simple ways of breaking down select PFAS, most degradation methods require harsh, energy-intensive processes using intense pressure — in some cases over 22 megapascals — or extremely high temperatures — sometimes upwards of 1000⁰ Celsius — to break the chemical bonds (SN: 6/3/22).

    Dichtel, of Northwestern University in Evanston, Ill., and his team experimented with two substances found in nearly every chemistry lab cabinet: sodium hydroxide, also known as lye, and a solvent called dimethyl sulfoxide, or DMSO. The team worked specifically with a group of forever chemicals called PFCAs, which contain carboxylic acid and constitute a large percentage of all PFAS. Some of these kinds of forever chemicals are found in water-resistant clothes.

    When the team combined PFCAs with the lye and DMSO at 120⁰ C and with no extra pressure needed, the carboxylic acid fell off the chemical and became carbon dioxide in a process called decarboxylation. What happened next was unexpected, Dichtel said. Loss of the acid led to a process causing “the entire molecule to fall apart in a cascade of complex reactions.” This cascade involved steps that degraded the rest of the chemical into fluoride ions and smaller carbon-containing products, leaving behind virtually no harmful by-products.     .

    “It’s a neat method, it’s different from other ones that have been tried,” says Chris Sales, an environmental engineer at Drexel University in Philadelphia who was not involved in the study. “The biggest question is, how could this be adapted and scaled up?” Northwestern has filed a provisional patent on behalf of the researchers.

    Understanding this mechanism is just one step in undoing forever chemicals, Dichtel’s team said. And more research is needed: There are other classes of PFAS that require their own solutions. This process wouldn’t work to tackle PFAS out in the environment, because it requires a concentrated amount of the chemicals. But it could one day be used in wastewater treatment plants, where the pollutants could be filtered out of the water, concentrated and then broken down. More

  • in

    Electrical bacteria may help clean oil spills and curb methane emissions

    The small motorboat anchors in the middle of the Chesapeake Bay. Shrieks of wintering birds assault the vessel’s five crew members, all clad in bright orange flotation suits. One of the crew slowly pulls a rope out of the water to retrieve a plastic tube, about the length of a person’s arm and filled with mud from the bottom of the bay. As the tube is hauled on board, the stench of rotten eggs fills the air.

    “Chesapeake Bay mud is stinky,” says Sairah Malkin, a biogeochemist at the University of Maryland Center for Environmental Science in Cambridge who is aboard the boat. The smell comes from sulfuric chemicals called sulfides within the mud. They’re quite toxic, Malkin explains.

    Malkin and her team venture out onto the bay every couple of months to sample the foul muck and track the abundance of squiggling mud dwellers called cable bacteria. The microbes are living wires: Their threadlike bodies — thinner than a human hair — can channel electricity.

    Sairah Malkin, of the University of Maryland Center for Environmental Science, cuts holes in a large sediment coring tube to sample mud collected from the bottom of the Chesapeake Bay.Clara Fuchsman

    Cable bacteria use that power to chemically rewire their surroundings. While some microbes in the area produce sulfides, the cable bacteria remove those chemicals and help prevent them from moving up into the water column. By managing sulfides, cable bacteria may protect fish, crustaceans and other aquatic organisms from a “toxic nightmare,” says Filip Meysman, a biogeochemist at the University of Antwerp in Belgium. “They’re kind of like guardian angels in these coastal ecosystems.”

    Now, scientists are studying how these living electrical filaments might do good in other ways. Laboratory experiments show that cable bacteria can support other microbes that consume crude oil, so researchers are investigating how to encourage the bacteria’s growth to help clean up oil spills. What’s more, researchers have shown that cable bacteria could help slash emissions of a potent greenhouse gas — methane — into the atmosphere.

    There’s plenty of evidence that cable bacteria exert a strong influence over their microbial neighbors, Meysman says. The next step, he says, is to figure out how to channel that influence for the greater good.

    Sign Up For the Latest from Science News

    Headlines and summaries of the latest Science News articles, delivered to your inbox

    Thank you for signing up!

    There was a problem signing you up.

    Electric life

    Under the microscope, cable bacteria resemble long sausage links. Their multicellular bodies can grow up to 5 centimeters long. Embedded in the envelope of each cell are parallel “wires” of conductive proteins, which the bacteria use to channel electrons. According to Meysman, the wires are more conductive than the semi­conductors found in electronics.

    About a decade ago, a team of scientists first discovered cable bacteria, in sediment collected from the bottom of Denmark’s Aarhus Bay. Since then, cable bacteria have been found on at least four continents, in streams, lakes, estuaries and coastal environments. “Name me a country, and I’ll show you where the cable bacteria are,” Meysman says.

    Most often, cable bacteria nestle shallow in the sediment, with one end positioned near the surface where there is oxygen and the other end plugged into deeper, sulfide-rich zones. Using their filamentous bodies as electrical conduits, cable bacteria snatch electrons from sulfides on one end and off-load them to oxygen — an eager electron acceptor — at the other, says Nicole Geerlings, a biogeochemist at Utrecht University in the Netherlands. Similar to how batteries charge and release energy by transferring electrons between an anode and cathode, cable bacteria power themselves by channeling electrons, she says. “The electron transport gives [cable bacteria] energy.”

    This unique lifestyle allows cable bacteria to survive in an environment that many organisms could not endure.

    A cable bacterium (right) has a multicellular, segmented body that can grow up to 5 centimeters. Electrically conductive, parallel fibers (visible in the close-up at left) encase the body.From left: N. Geerlings/Utrecht Univ.; Silvia Hidalgo Martinez/Univ. of Antwerp

    Toxic fire wall

    In 2015, Malkin, Meysman and colleagues reported that cable bacteria may help to counteract the onset of euxinia — a fatal buildup of sulfides in oxygen-starved bodies of water. Euxinia can trigger mass die-offs of fish, crustaceans and other aquatic life.

    The lethal phenomenon can occur after fertilizers or sewage are washed into the sea or lakes. That flow of nutrients can trigger algal blooms. When those nutrients are depleted, the blooms die, and large quantities of organic matter sink and accumulate on the sediment. Microbes then decompose the dead material, devouring much of the oxygen in the surrounding water in the process. When oxygen levels become critically low, sulfides may begin to leak from the sediment into the water, giving rise to euxinia.

    Sediments near the bottom of this core sample, taken from the Chesapeake Bay, are probably dark due to the presence of sulfides, while sediments near the top are lighter because cable bacteria have removed the sulfides.S. Malkin

    While studying cable bacteria in a brackish body of water in the Netherlands, Malkin and colleagues discovered a thin layer of rust coating the lake’s bottom. As the cable bacteria pulled electrons from sulfides, converting the noxious chemicals into less-harmful sulfates, the water within the sediment became more acidic, which dissolved some minerals containing iron. The now-mobile iron percolated upward in the sediment, until it interacted with oxygen to form rust.

    This layer of rust could capture sulfides that would otherwise flow into the water, acting as a “fire wall” that could delay euxinia for over a month, or even prevent it altogether, the researchers reported. Even when the cable bacteria’s population dropped, the rust layer persisted, protecting other aquatic creatures from sulfide exposure. The rust may explain why even though instances of nutrient pollution, algal blooms and oxygen depletion are relatively common, reports of euxinia are rare.

    Oil cleanup

    Some researchers are trying to harness the bacteria’s electrical abilities to tackle another devastating threat to coastal ecosystems — oil spills.

    When an oil spill happens in a body of water, booms, skimmers or sorbents are often deployed to limit the spread of hydrocarbons on the surface. But oil may also wash onto beaches, mix with sediments in shallow waters and aggregate onto sinking particles of organic debris, hitching a ride to the seafloor.

    Cleaning up oil at the bottom of the sea is a difficult job, says Ugo Marzocchi, a biogeochemist at Aarhus University in Denmark. “I am not aware of a very effective way to remove hydrocarbons from the seafloor,” he says. “In inland freshwater systems, what is generally done is to dig out the sediments,” he says, an expensive strategy that would be even more costly at sea.

    [embedded content]
    Electrical cable bacteria (white filaments) emerge from the seafloor sediment (at bottom) stretching their bodies to reach a zone of oxygen in the water. The stringy organisms use the oxygen to offload electrons they’ve harvested from harmful sulfides found in the sediment. As sulfide concentrations go down, the water becomes more habitable to microbes that can clean up oil spills.

    Some soil-dwelling microorganisms can use hydrocarbons to fuel their metabolism, and researchers have been studying how some of these oil burners might assist in the cleanup of contaminated sediments. But as they break down hydrocarbons, the microbes generate those concerning sulfides, which are detrimental to the microbes’ own survival, Marzocchi says. In other words, the microbes can help clean up the oil for only so long before they’re overwhelmed by their own toxic waste.

    Cable bacteria might be just the solution, Marzocchi thought. In 2016, researchers reported finding evidence of the electrical microbes in a tar oil-contaminated groundwater aquifer in Germany. Knowing that cable bacteria could occupy sediments contaminated with hydrocarbons, Marzocchi and colleagues reasoned that these bacteria might be able to assist oil-burning microbes and accelerate oil cleanup.

    The researchers filled several containers with oil-contaminated sediment from Aarhus Bay — which contained naturally occurring oil-eating bacteria. The group then injected a few containers with cable bacteria and monitored the degree of hydrocarbon degradation in all of the containers over seven weeks. By the end of the test, the concentration of alkanes — a type of hydrocarbon — in the sediment with cable bacteria had dropped from 0.125 milligrams per gram of sediment to 0.086 milligrams per gram — a 31 percent drop. That’s 23 percentage points more than the 9 percent decrease in the control samples. Cable bacteria helped accelerate the metabolic activity of their oil-eating neighbors by converting the toxic sulfides into sulfates. The sulfates didn’t harm the oil-eating microbes — in fact, they used the chemicals as fuel.

    The researchers are now trying to develop methods to promote cable bacteria growth in the field and see if it’s possible to enhance their effect on oil degradation. One catch is that in oil-contaminated sediment, oxygen is quickly used up by the microbes that break down hydrocarbons. That’s a problem since cable bacteria need access to oxygen. Salts that slowly release oxygen or nitrate — which cable bacteria can use in place of oxygen — might help spur the electrical organisms’ growth at oil spills. But more work is needed to identify the right chemical components and dosage, Marzocchi says.

    Meanwhile, scientists are investigating how cable bacteria might help reduce emission of another hydrocarbon — one that accumulates in the sky.

    Methane at the root

    Colorless, odorless methane is the simplest hydrocarbon (SN: 8/15/20, p. 8). It consists of a single carbon atom attached to a quartet of hydrogen atoms. And it’s a potent greenhouse gas — more than 25 times as effective at trapping heat in the atmosphere as carbon dioxide.

    One major source of methane is rice paddies (SN: 9/25/21, p. 16). During the growing season, rice farmers typically flood their fields to help stave off weeds and pests. Methane-producing microbes — aptly named methanogens — thrive in these waterlogged soils. Paddy-dwelling methanogens are so prolific that rice fields are estimated to generate about 11 percent of all human-induced methane emissions.

    But cable bacteria like paddies too. In 2019, Vincent Scholz, a microbiologist at Aarhus University, and colleagues reported that cable bacteria could flourish among the roots of rice plants and several other aquatic plant species.

    In an experiment, pots of rice plants grown in soils with cable bacteria (right) developed orange layers of rust and emitted less methane than pots without cable bacteria (left).V.V. Scholz/Aarhus Univ.

    That discovery inspired the researchers to investigate how the bacteria interact with methanogens in soils that grow rice. The team grew its own rice plants — some potted in soils with cable bacteria, and some without — and monitored methane emissions.

    To the researchers’ surprise, adding cable bacteria reduced rice soil methane emissions by 93 percent. In the process of removing electrons from sulfides, the bacteria generate sulfates, which other microbes can use as fuel. These sulfate-consuming microbes outcompeted methanogens for nutrients such as hydrogen and acetate in the rice soils, the researchers found. The results were “quite amazing,” Scholz says, though the effectiveness of the electrical microbes in real rice fields has yet to be tested.

    There are signs that cable bacteria are already plugged into real rice paddy soils. After analyzing genetic data collected from rice paddies in the United States, India, Vietnam and China, Scholz and colleagues reported in 2021 the presence of cable bacteria at sites in all four countries. Scholz is in Northern California this summer studying how cable bacteria live in rice fields and whether they’re already impacting methane emissions. He is also exploring ways to introduce cable bacteria to rice fields where they don’t yet exist or enhance the microbes’ numbers in fields where they do.

    There is still much to discover about how the wispy electrical conductors influence our world, Malkin says. Back in the Chesapeake Bay, she and colleagues have found that cable bacteria tend to flourish in the spring, a surge that has also been observed in the Netherlands. The findings add to a growing body of work that suggests cable bacteria are opportunistic organisms that interact with their environments in similar ways all around the world.

    If cable bacteria are already hard at work across the planet, then a bit of coaxing from researchers may be all it takes to turn the mud-dwelling creatures into the most helpful neighbors that a living thing could ask for. More

  • in

    How to make jet fuel from sunlight, air and water vapor

    Jet fuel can now be siphoned from the air.

    Or at least that’s the case in Móstoles, Spain, where researchers demonstrated that an outdoor system could produce kerosene, used as jet fuel, with three simple ingredients: sunlight, carbon dioxide and water vapor. Solar kerosene could replace petroleum-derived jet fuel in aviation and help stabilize greenhouse gas emissions, the researchers report in the July 20 Joule.

    Burning solar-derived kerosene releases carbon dioxide, but only as much as is used to make it, says Aldo Steinfeld, an engineer at ETH Zurich. “That makes the fuel carbon neutral, especially if we use carbon dioxide captured directly from the air.”

    Sign Up For the Latest from Science News

    Headlines and summaries of the latest Science News articles, delivered to your inbox

    Thank you for signing up!

    There was a problem signing you up.

    Kerosene is the fuel of choice for aviation, a sector responsible for around 5 percent of human-caused greenhouse gas emissions. Finding sustainable alternatives has proven difficult, especially for long-distance aviation, because kerosene is packed with so much energy, says chemical physicist Ellen Stechel of Arizona State University in Tempe who was not involved in the study.

    In 2015, Steinfeld and his colleagues synthesized solar kerosene in the laboratory, but no one had produced the fuel entirely in a single system in the field. So Steinfeld and his team positioned 169 sun-tracking mirrors to reflect and focus radiation equivalent to about 2,500 suns into a solar reactor atop a 15-meter-tall tower. The reactor has a window to let the light in, ports that supply carbon dioxide and water vapor as well as a material used to catalyze chemical reactions called porous ceria.

    Within the solar reactor, porous ceria (shown) gets heated by sunlight and reacts with carbon dioxide and water vapor to produce syngas, a mixture of hydrogen gas and carbon monoxide.ETH Zurich

    When heated with solar radiation, the ceria reacts with carbon dioxide and water vapor in the reactor to produce syngas — a mixture of hydrogen gas and carbon monoxide. The syngas is then piped to the tower’s base where a machine converts it into kerosene and other hydrocarbons.

    Over nine days of operation, the researchers found that the tower converted about 4 percent of the used solar energy into roughly 5,191 liters of syngas, which was used to synthesize both kerosene and diesel. This proof-of-principle setup produced about a liter of kerosene a day, Steinfeld says.

    “It’s a major milestone,” Stechel says, though the efficiency needs to be improved for the technology to be useful to industry. For context, a Boeing 747 passenger jet burns around 19,000 liters of fuel during takeoff and the ascent to cruising altitude. Recovering heat unused by the system and improving the ceria’s heat absorption could boost the tower’s efficiency to more than 20 percent, making it economically practical, the researchers say. More

  • in

    Underground heat pollution could be tapped to mitigate climate change

    The secret to efficiently heating some buildings might lurk beneath our feet, in the heat that humans have inadvertently stored underground. 

    Just as cities warm the surrounding air, giving rise to urban heat islands, so too does human infrastructure warm the underlying earth (SN: 3/27/09). Now, an analysis of groundwater well sites across Europe and parts of North America and Australia reveals that roughly a couple thousand of those locations possess excess underground heat that could be recycled to warm buildings for a year, researchers report July 8 in Nature Communications.

    Sign Up For the Latest from Science News

    Headlines and summaries of the latest Science News articles, delivered to your inbox

    Thank you for signing up!

    There was a problem signing you up.

    What’s more, even if humans managed to remove all this accumulated thermal pollution, existing infrastructure at about a quarter of the locations would continue to warm the ground enough that heat could be harvested for many years to come. That could reduce reliance on fossil fuels, and help mitigate climate change.

    This work showcases the impact that underground heat recycling could have if harnessed on a large scale, says hydrogeologist Grant Ferguson of the University of Saskatchewan in Saskatoon, Canada, who was not involved in the study. “There’s a lot of untapped potential out there.”

    Heat leaks into the subsurface from the warm roots of structures such as buildings, parking garages and tunnels, and from artificial surfaces such as asphalt, which absorb solar radiation. In Lyon, France, for example, researchers in 2016 found that human infrastructure warmed groundwater by more than 4 degrees Celsius.

    Scientists don’t fully understand how heat pollution alters underground environments. But warming of the subsurface can cause contaminants, such as arsenic, to move through groundwater more readily.

    Extracting the thermal pollution could be accomplished by piping groundwater to heat pumps at the surface. The water, warmed underground by all that trapped heat, could then warm buildings as it releases heat into their cooler interiors, says Susanne Benz, an environmental scientist at Dalhousie University in Halifax, Canada.

    Harnessing underground heat in this way could provide some communities with a reliable and low-energy means to warm their homes, Benz says. “And if we don’t use it, it will just continue to accumulate,” she says.

    Benz and her colleagues analyzed the population size, heating demand and groundwater temperature at more than 6,000 locations, most of which were in Europe. The researchers found that at about 43 percent of the locations — mostly those near highly populated areas — enough heat had accumulated in the top 20 meters of earth to satisfy a year’s worth of the local heating demand.

    Curious about sustainability, the researchers also identified places where the continuous flow of heat into the underground — and not just the stockpiled thermal pollution — was high. Their calculations show that if all of the accumulated heat was first extracted, the heat that continued leaking from existing infrastructure could be harvested at about 25 percent of the 6,000 locations. At 18 percent of locations, this recycled heat could satisfy at least a quarter of the heating demand of the local population.

    Constructing systems to take advantage of human heat pollution today could one day help residents harvest heat from climate change, the researchers say.

    Using climate projections for the end of the century, the team probed the feasibility of extracting underground heat in a warmer world. In the most optimistic warming scenario considered, which assumes greenhouse gas emissions peak about the year 2040, the researchers found that climate change would warm the ground enough by the end of the century that underground heat recycling at 81 percent of the studied locations could meet more than a quarter of locals’ heating demands. If there are no efforts to curb emissions, that number rises to 99 percent of locations.

    Though the researchers focused mostly on Europe, Benz says that other continents probably also possess abundant underground heat that could be harnessed. In Europe and elsewhere, heat recycling might be most feasible in suburban areas, she says, where there is sufficient accumulated underground heat to help meet local heating demands, and space to install heat recycling systems.

    Looking ahead, Benz plans to investigate whether cooling the subsurface can help reduce aboveground temperatures in urban environments. “This might actually be a little additional tool to control [aboveground] urban heat.” More

  • in

    Flower shape and size impact bees’ chances of catching gut parasites

    Bees that land on short, wide flowers can fly away with an upset stomach.  

    Common eastern bumblebees (Bombus impatiens) are more likely to catch a diarrhea-inducing gut parasite from purple coneflowers, black-eyed Susans and other similarly shaped flora than other flowers, researchers report in the July Ecology. Because parasites and diseases contribute to bee decline, the finding could help researchers create seed mixes that are more bee-friendly and inform gardeners’ and land managers’ decisions about which flower types to plant.

    Sign Up For the Latest from Science News

    Headlines and summaries of the latest Science News articles, delivered to your inbox

    Thank you for signing up!

    There was a problem signing you up.

    The parasite (Crithidia bombi) is transmitted when the insects accidentally ingest contaminated bee feces, which “tends to make the bees dopey and lethargic,” says Rebecca Irwin, a community and evolutionary ecologist at North Carolina State University in Raleigh. “It isn’t the number one bee killer out there,” but bees sickened with it can struggle with foraging.    

    In laboratory experiments involving caged bees and 16 plant species, Irwin and her colleagues studied how different floral attributes affected transmission of the gut parasite. They focused on three factors of transmission: the amount of poop landing on flowers when bees fly and forage, how long the parasite survives on the plants and how easily the parasite is transmitted to new bees. Multiplied together, these three factors show the overall transmission rate.

    Compared with plants with long, narrow flowers like phlox and bluebeards, short, wide flowers had more feces land on them and transmitted the parasite more easily to the pollinators, increasing the overall parasite transmission rate for these flowers. However, parasite survival times were reduced on these blooms. This is probably due to the open floral shapes increased exposure to ultraviolet light, speeding the drying out of parasite-laden “fecal droplets,” Irwin says.

    The findings confirm a new theory suggesting that traits, such as flower shape, are better predictors of disease transmission than individual species of plants, says Scott McArt, an entomologist focusing on pollinator health at Cornell University who wasn’t involved with the study. Therefore, “you don’t need to know everything about every plant species when designing your pollinator-friendly garden or habitat restoration project.”

    Instead, to limit disease transmission among bees, it’s best to choose plants that have narrower, longer flowers, he says. “Wider and shorter flowers are analogous to the small, poorly ventilated rooms where COVID is efficiently transmitted among humans.”  

    If ripping out coneflowers or black-eyed Susans isn’t palatable, don’t fret. Irwin recommends continuing to plant a diversity of flower types. This helps if one type of flower is “a high transmitting species,” she notes. In the future, she plans to conduct field experiments examining other factors that could influence parasite transmission, such as whether bees are driven to visit certain types of flowers more often in nature.   More