TY - JOUR
AU1 - Wilson, Niki
AB - The growing need for interdisciplinary research Wild winds and rough seas make cleaning up oceanic oil spills a tough business. No single technique is completely effective in removing toxic compounds from a large-scale slick. However, scientists are increasingly enlisting tiny helpers for the job. Oil-eating microbes are already part of the cleanup crew, but there is something even smaller on the horizon. Imagine a compound that, when sprayed across the contaminated sea, would trap the oil in a brown jelly that could be scooped from the surface. It sounds like science fiction, but Huaqiang Zeng, of the Agency for Science, Technology, and Research in Singapore, and his team have developed this technology (doi:10.1021/acs.chemmater.6b01367) and are working to make it available for commercial use. At the heart of their solution: nanoparticles. Nanoparticles are almost too small to imagine, measuring 1 to 100 nanometers—about 0.00002 the width of a human hair. These tiny, usually inorganic particles can be made of anything from metals to clay and are fundamentals of the nanomaterials industry. They provide hope for solving some of our biggest environmental problems. Technologies that incorporate them are rapidly showing promise to help reduce waste, increase energy efficiency, and clean up industrial contaminants and spills. The unique properties that come with their small size have unlocked a potential treasure trove of opportunity that fuels a multibillion-dollar industry. View largeDownload slide Nanoparticles of titanium dioxide (TiO2) are used widely in commercial industry, including for sunblock. Shown here are TiO2 nanoparticles dispersed in an organic solvent. Images: Advanced Water Research Lab. View largeDownload slide Nanoparticles of titanium dioxide (TiO2) are used widely in commercial industry, including for sunblock. Shown here are TiO2 nanoparticles dispersed in an organic solvent. Images: Advanced Water Research Lab. Nanoparticles are being used not only as part of the green tech revolution but also for a myriad of purposes, from cancer-drug delivery to scratch-proof eyeglasses (http://ec.europa.eu/health/scientific_committees/opinions_layman/en/nanotechnologies/l-3/5-nanoparticles-­consumer-products.htm). Titanium dioxide (TiO2) nanoparticles are used as a sunblock in cosmetics. Carbon nanotubes can be found in tennis racquets. Silver nanoparticles are added to socks, underwear, and sports clothes to reduce odor, and they are added to bandages to prevent bacterial infections. The same advantageous properties that come with their size, though, can potentially have harmful effects on soil and water when nanoparticles make their way into the environment. Risk assessment and research to understand their potential deleterious effects have not kept pace with their rapid proliferation, and environmental toxicologists are racing to address this knowledge gap as the seemingly rocket-propelled nanomaterials industry forges ahead. Solving big problems with tiny particles The technological promise of nanoparticles comes as a result of the unique behavior that accompanies their small size. As ecotoxicologist Ryan Otter, of Middle Tennessee State University, explains, “When you take an element like silver or copper and make it very small, what you do is dramatically increase the surface-to-volume ratio.” At the nanoscale, this means that more of the particle is in contact with the outside world than is not. That leads to some important changes in the material's properties. A nanoparticle may conduct electricity better, become stronger, or become capable of catalyzing reactions, says Otter. These traits constitute an amazing toolbox when it comes to environmental problem solving. Nanoparticles are being used to clean up soil and groundwater contamination, a significant problem in the United States. Many of these contaminated areas are Superfund sites, designated by the US Environmental Protection Agency as requiring a long-term response to the cleanup of hazardous material contaminants. In his paper “Nanoscale Iron Particles for Environmental Remediation: An Overview”, ­Wei-xian Zhang, of Lehigh University, writes, “A few blocks from this author's office is one of the largest brownfields in the US, the former Bethlehem Steel site occupying more than 2500 acres. There are more than 50 Superfund sites within 2 [hours] of driving time from Lehigh University. Nationwide, more than 1500 sites have been put on the Superfund list. Less than one-third of them have been cleaned up in the past decade.” But, he writes, “Nanoscale particles may hold the potential to cost-­effectively address some of the challenges of site remediation.” These particles can be used to transform, detoxify, and immobilize contaminants such as polychlorinated biphenyls (PCBs). Zhang writes that the unique properties of nanoparticles mean they can be put in a solution that can easily be injected underground, where they can remain in suspension to establish an on-site treatment zone. Nanoparticles are also being used to strengthen existing materials. For example, wind turbine blades can be coated in an epoxy containing ­carbon nanotubes to reduce the weight of the blades and increase their durability. This increases energy production. Eagle Tuulivoima Oy, a Finnish Wind Turbine Company, claims that these blades can produce 30 percent more wind power than conventional ones. View largeDownload slide Nanocomposite membranes filter for different particles based on porosity. Microfiltration (MF) is used for the separation of large particulates that are 0.1–1 microns (such as oil), ultrafiltration (UF) for particles 0.01–0.1 microns (such as virus and bacteria), nanofiltration (NF) for particles 0.001–0.01 microns, and reverse osmosis (RO) for particles less than a nanometer. Images: Advanced Water Research Lab. View largeDownload slide Nanocomposite membranes filter for different particles based on porosity. Microfiltration (MF) is used for the separation of large particulates that are 0.1–1 microns (such as oil), ultrafiltration (UF) for particles 0.01–0.1 microns (such as virus and bacteria), nanofiltration (NF) for particles 0.001–0.01 microns, and reverse osmosis (RO) for particles less than a nanometer. Images: Advanced Water Research Lab. Mohtada Sadrzadeh, assistant professor of mechanical engineering and principal investigator at the Advanced Water Research Lab at the University of Alberta, is using nanoparticles to solve what he considers to be “a threat to freshwater” in Alberta's oil sands region. There, oil (in the form of bitumen) is removed from the ground in two ways: through open pit mines or, more often, through wells in situ. In situ operations commonly use an extraction method that pumps steam underground in one well to liquefy the bitumen, which is collected in a second well and pumped to the ­surface. This process contaminates the freshwater used to create steam with salts, silica, calcium, magnesium, and organic matter. Currently, attempts to decontaminate and recycle the water are limited to ceramic filters, which can be cost prohibitive and clog easily, and polymeric membranes that are not as strong and do not filter out many of the contaminants. Only calcium, magnesium, and silica are removed, leaving organic matter and salt. At best, 90 percent of low-quality water is recovered and recycled. Sadrzadeh and his colleagues hope to change this through the development of advanced membranes that are affordable and can capture the majority of contaminants. These membranes are nanocomposites—polymeric membranes embedded with nano­materials (including nanoparticles) such as TiO2, graphene oxide, and indium tin oxide. Although using a polymeric membrane keeps costs down, the nanomaterials embedded within it increase strength and functionality. “One hundred percent of the silica and more than 90 percent of the organic material are removed,” says Sadrzadeh. As an added bonus, better filtration means less wear and tear on boilers and other machinery and improves the life cycle of the parts, which cuts back on waste, he explains. It also reduces operational costs. Nanocomposite membranes are not yet used widely by water treatment companies, says Sadrzadeh. “More than 80 percent use polymeric ­membranes, and the share of nanocomposites is less than 1 percent.” But because nanocomposite membranes are affordable and their functional properties—such as conductivity or antimicrobial behaviors—can be ­tailored on the basis of what nanomaterial is used, they represent the future of membrane ­technology, he says. The downside of nanoparticles Scientists now know that nanoparticles are being released into the environment. Because they behave differently from their bulk parent material, toxicologists are racing to develop new ways to test for potentially toxic effects and to understand how nanoparticles disperse into and affect aquatic and terrestrial ecosystems. Chris Metcalfe, a professor in the School of the Environment at Trent University, in Ontario, Canada, studies the potential for materials and chemicals to be toxic in the environment. He has focused on those that “might be flushed down the drain, find their way to surface waters, and potentially impact organisms or drinking water,” he says. These include pharmaceuticals, personal care products, cleaners, and now nanoparticles. Metcalfe says that studying nanoparticle toxicity has involved a steep learning curve. “When we started this work 10 years ago, we didn’t know how to suspend the stuff in water so we could test it on aquatic organisms,” he says, adding that he and his colleagues also had to develop the analytical capability to figure out how many particles were present in the water. “It's taken a while to develop those kinds of methods.” In one study, he and his students added 15 kilograms of silver nanoparticles into Lake 222 in Ontario's Experimental Lakes Area (an amount he says would yield concentrations in water about 10 times those likely to be expelled from a drinking-water treatment plant). Within a few weeks of the start of additions to the lake, they found that silver nanoparticles were widely distributed. They remained in their nano form (as opposed to clumping together) and could be found throughout the water column. The particles were ingested by aquatic organisms and appeared to bioaccumulate in animals further up the food chain. Although Metcalfe and his ­colleagues have yet to see any toxic effects on plankton and other creatures at the bottom of the food chain (the data analysis is in progress), there were “indications of stress in northern pike at the top of the food chain, so something was happening,” he says. Environmental toxicologist Aaron Roberts investigates the impacts of potentially toxic substances in aquatic systems. He is trying to understand the impacts of oil spills and mercury accumulation in fish. Roberts is a professor in the Department of Biological Sciences at the University of North Texas, and his work on nanoparticles began a decade ago, when a colleague began using single-walled carbon nanotubes (CNTs) coated in phospholipids for biomedical purposes and asked Roberts to help identify potential toxicity. View largeDownload slide Shown here is a solar-powered system used to pump nano silver into Experimental Lake 222 in Ontario, Canada. Researchers found that the silver nanoparticles are ingested by aquatic organisms. Photograph: Chris Metcalfe. View largeDownload slide Shown here is a solar-powered system used to pump nano silver into Experimental Lake 222 in Ontario, Canada. Researchers found that the silver nanoparticles are ingested by aquatic organisms. Photograph: Chris Metcalfe. View largeDownload slide Single-walled carbon nanotubes, shown here, are coated in phospholipids and used in biomedicine. Image: Taner Yildirim, the National Institute of Standards and Technology. View largeDownload slide Single-walled carbon nanotubes, shown here, are coated in phospholipids and used in biomedicine. Image: Taner Yildirim, the National Institute of Standards and Technology. Roberts designed a lab experiment in which he exposed the zooplankton Daphnia magna, a microscopic crustacean, to the CNTs suspended in a water solution. Daphnia ingested the CNTs and used the phospholipid coating as a food source. They then excreted the now-lipidless CNTs into the water in a form prone to clumping together. These aggregated chunks settled into the sediment as opposed to staying in the water column. “The takeaway was that organisms in the environment could change the properties of these materials,” says Roberts. Roberts has been working on nanoparticles ever since, and like Metcalfe, he says it has been a game of catch-up—being asked to test for potential toxic effects while simultaneously learning how to do it. Early in his career of working on nanoparticles, he and his colleagues would joke, “Two years ago, we couldn’t spell nanotoxicologist, and now we are one.” One challenge has been determining how to even identify nanoparticles in the environment, especially carbon. “We were literally looking for carbon in a sea of other carbon.” Along with improved testing has come the awareness of potentially unknown complex environmental interactions by nanoparticles and the need to understand how these particles behave outside of laboratory experiments. Roberts gives the example of nano TiO2, one of the most widely used forms of nanoparticles in commercial industry. Production in the United States is estimated to be 2.4 million metric tons by 2025 (doi:10.1021/es8032549), and 3–30 percent of that may end up in water bodies (doi:10.1021/ez400106t). It comes in two forms: The “rutile” form is used for blocking ultraviolet (UV) light in sunscreen and other products, whereas the “anatase” form is activated by UV and is used in photocatalytic processes such as treating water or creating self-sterilizing surfaces. When researchers test the toxicity of anatase TiO2 to organisms in an environmental chamber in a lab with minimal exposure to UV radiation, “You might see that material as nontoxic,” says Roberts. But when UV is added, as would be the case in clear water, “it increases the toxicity by over an order of magnitude.” Juliane Filser, a professor of ecology at the University of Bremen, Germany, also advocates for studies in “realistic environments” where many species are sampled, or where long-term experiments are conducted under field conditions. Along with colleague Moira McKee, in 2016, she undertook an extensive review of the known impacts of silver and copper nanoparticles on soil communities. These communities are composed of the plants, fungi, bacteria, insects, and earthworms that maintain the structural and chemical composition of the soil. Through her review, she observed not only the direct impacts of nanoparticles on soil organisms but also cascading effects that have the potential to change entire communities. View largeDownload slide Transmission electron microscopy image of silver nanoparticles formed from silver ions in solution with humic acid. The acid tends to coat the nanoparticles (visible here as a pale cloud), keeping them in a colloidal suspension instead of clumping together. (Color added by the photographer for clarity.) Image: State University of New York, Buffalo. View largeDownload slide Transmission electron microscopy image of silver nanoparticles formed from silver ions in solution with humic acid. The acid tends to coat the nanoparticles (visible here as a pale cloud), keeping them in a colloidal suspension instead of clumping together. (Color added by the photographer for clarity.) Image: State University of New York, Buffalo. Further reading. Blake D, Nar M, D’Souza NA, et al. 2014. Treatment with coated layer double hydroxide clays decreases the toxicity of copper-contaminated water. Archives of Environmental Contamination and Toxicology 43: 4227–4233. Mansfield CM, Alloy MM, Hamilton J, Verbeck GF, Newton K, Klaine SJ, Roberts AP. 2015. Photo-induced toxicity of titanium dioxide nanoparticles to Daphnia magna under natural sunlight. Chemosphere 120: 206–210. McKee MS, Filser J. 2016. Impacts of metal-based engineered nanomaterials on soil communities. Environmental Science Nano 3: 506–533. doi:10.1039/c6en00007j Zhang W. 2003. Nanoscale iron particles for environmental remediation: An overview. Journal of Nanoparticle Research 5: 323–332. doi:10.1023/A:1025520116015 Filser says that the same properties that make silver nanoparticles important antibacterial agents also make them deadly to microorganisms in the soil. Many of these microorganisms are an important food source for ­earthworms—species that play the critical roles of aerating the soil and moving organic plant detritus to deeper layers. Silver nanoparticles also kill more bacteria than they do fungi, shifting the nature of the soil community. Some of the fungi that proliferate as a result are themselves highly toxic to other organisms in the soil. Others are highly resistant to the toxic effect of metals, having evolved the ability to ingest them without harm, and secrete them back into the environment. These fungi keep “producing the same toxicant again,” says Filser. One of the most important findings of their review was the amazingly strong effects of several types of nanoparticles at low concentrations. Traditionally, when scientists looked at the potential toxic effects of a substance, they assumed a dose–response curve, she explains. “A small dose should have a small effect, and a big dose should have a large effect. But this is not the case with nanoparticles,” says Filser. Instead, many studies have found the opposite. For example, with silver nanoparticles, no effect was seen at hundreds of milligrams per kilogram, but a negative effect was seen at 3 to 5 milligrams per ­kilogram. The reason is not known, but there are a few theories. One suggests that at high concentrations, most nanoparticles stick together, or “aggregate,” and behave differently. They do not stay suspended in solutions in the soil, which reduces toxicity. Another theory suggests they form “heteroaggregates”—a complex interaction that includes bonds with the soil that nullify some of the negative effects. Still another theory ­suggests that some organisms detect and avoid toxicity at high concentrations of metal nanoparticles, but when concentrations are low enough, they are less likely to show avoidance behavior and more likely to be harmed by persistent low-level exposure. The point, says Filser, is that “if regulations are being based on traditional dose–response curves, then they may not protect us at all,” adding that it is important to look carefully at this issue and others identified in her review of nanotoxicity in soils in order to establish adequate environmental assessment and regulation. View largeDownload slide This is a cross-section showing nanoparticles of copper zinc tin sulfide laid down to create a solar cell. Photograph: Oregon State University. View largeDownload slide This is a cross-section showing nanoparticles of copper zinc tin sulfide laid down to create a solar cell. Photograph: Oregon State University. Risk and reward To prevent toxicity from nanoparticles, “we need to do effective risk assessment before [products] appear on the market,” says Metcalfe. However, environmental risk assessment is only as good as the data going into it. “With nano, we have a lot of incomplete information about where they go and how they act,” says Roberts. He explains that there are still cases in which scientists do not know whether nanoparticles are even dispersing into the environment in large amounts or whether they remain as nanoparticles. Interactions with the environment may alter the particles and how they behave, making it difficult to predict what they will do. Still, there are substantially more data—and better ways of obtaining them—than a decade ago. For example, scientists studying nanoscale processes and products are now much better at characterizing particles. This means identifying their exact size, shape, parent material, and coating. This is critical when sharing experimental evidence and predicting their behavior. For example, Filser and McKee write that parent material, particle size, and shape could be related to bacterial toxicity. There is also international recognition that a sustainable nanotech industry requires research and oversight. Since 2007, the Organization for Economic Co-operation and Development (OECD) has formed the Working Party on Nanotechnology, as well as the Programme on the Safety of Manufactured Nanomaterials. The goal is to help the 35 signatory countries—the United States included—to develop national policies guaranteeing the responsible development of nanotechnologies. Among many outcomes, the OECD has developed guidelines for manufacturing and testing. Still, regulatory agencies struggle with how to employ risk assessments for nanomaterials, explains Metcalfe, because previous testing processes were not designed for nanomaterials. Filser agrees. “I appreciate the standard procedures proposed by the OECD. They help us a lot with comparability of results,” she says. Nevertheless, “there is still the need to bridge the gap from the lab to the environment.” Including environmental ecologists is important in this endeavor to identify what cannot be assessed with standardized tests. Filser also says that the public needs to be better educated to think critically about nanotechnology and to participate in the global discussion about how it should be used and handled. Her lab is part of the Nano Competence Graduate School at the University of Bremen, which is aimed at facilitating research and communication in order to increase public awareness of the potential impacts of nanomaterials in the environment. The school fosters collaboration and communication across the disciplines of natural, social, and applied scientists. This crosstalk is important, because many of the scientists developing nanotechnology are chemists and engineers, whereas those evaluating the potential environmental impacts tend to be biologists, ecologists, and toxicologists. Such collaboration may be critical to a sustainable nano industry going forward. Roberts recently worked with a colleague at the University of Texas who wanted to use a type of clay nanoparticle to remove metals from contaminated water. His team came aboard to help identify any toxicity issues associated with the technology. “We were trying to design something that was really efficient at removing metals but also was not going to pose a risk to aquatic life. It was a pretty cool collaboration,” he says. As a result, the team was able to show that coated clay nanoparticles decreased the toxicity of copper-containing solutions to D. magna and that the particles hold promise as a ­remediation tool. With a broad potential for societal, environmental, and economic benefit from nanomaterials, it is unlikely that the nanotech industry will slow down while the biological sciences play catch-up. In the words of the OECD, “There is a need for a responsible and coordinated approach to ensure that potential safety issues are being addressed at the same time as the technology is developing.” Closing the gap between the rapid development of this technology and scientists’ ability to understand its impacts will be ­critical to the sustainability of the industry and the avoidance of unintended consequences. © The Author(s) 2018. Published by Oxford University Press on behalf of the American Institute of Biological Sciences. All rights reserved. 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TI - Nanoparticles: Environmental Problems or Problem Solvers?
JF - BioScience
DO - 10.1093/biosci/biy015
DA - 2018-04-01
UR - https://www.deepdyve.com/lp/oxford-university-press/nanoparticles-environmental-problems-or-problem-solvers-0TH4PMRuDv
SP - 241
EP - 246
VL - 68
IS - 4
DP - DeepDyve
ER -