Hunting for engineered nanomaterials in the environment ticle, further complicating detection. So far, nobody has identified a technique “that gives you all the important information you need,” says von der Kammer, who is working with other researchers to compare methods to best distinPAUL WESTERHOFF ET AL., DOI 10.1021/ES901102N
Most environmental research related to nanomaterials has focused on their toxicity in idealized lab settings. But researchers are slowly shifting their lab methods to look for real nanomaterials in the environment, which is key for determining which nanomaterials to study, as well as where and how they might cause harm. Last year, researchers from the Swiss Federal Laboratories for Materials Testing and Research (Empa) demonstrated some early success: they traced titanium dioxide (TiO2) nanoparticles shed from the paint on building exteriors into soils nearby and possibly streams (Environ. Pollut. 2008, DOI 10.1016/j.envpol.2008.08.004). The team used electron microscopy to detect the nanoparticles and bulk chemical analysis to confirm their presence. But finding the nanoparticles in the environment is just one part of the problem. “The task that we have actually is to separate the particles from the surrounding background,” says Frank von der Kammer of the University of Vienna. That’s because some nanoparticles occur naturally or are shed from products that take advantage of a material’s normal sizesor “bulk” form. For example, a large amount of bulk TiO2 has been used for decades as a paint pigment and for other applications. This bulk form can release tagalong nanoparticles. The presence of either type of TiO2 in the environment could throw off measurements of the engineered nanoparticles. Researchers have suggested that crystal sizes and a particle’s accompanying organic matrix could distinguish manufactured forms from natural ones. In some cases, a particle might look like TiO2 stuck to organic matter when it is really an organic-coated TiO2 par-
TiO2 that comes from toothpaste (top) could wind up suspended in effluent (bottom) from a wastewater treatment plant and then could easily be dispersed in the environment by piggybacking on biosolids.
guish nanomaterials. In fact, a single, comprehensive technique may never exist, considering the number of nanomaterials out there with potentially different properties, he adds. But each method can give information about some parameters, and putting those elements together can be like “overlaying ten blurred pictures to get one sharp image in the end.” One approach, presented by researchers from the University of Gothenburg (Sweden) at the Society of Environmental Toxicology and Chemistry’s European meeting in June 2009, is field-flow fractionation coupled with inductively coupled plasma mass spec-
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trometry (ICPMS). The particles settle out according to size as the sample passes through the flow, and then ICPMS can be used to determine the composition of individual particles. Researchers have yet to find 100% of the test materials that they put into their measuring devices, von der Kammer adds. Nanoparticles tend to stick to vessel walls, are difficult to nebulize, or require extreme pretreatment for certain devices. Furthermore, most engineered nanoparticles that will make their way into the environment are not likely to be the naked ones that researchers have studied in the lab. For example, the TiO2 nanoparticles used in a commercially available sunscreen have an aluminum oxide coating and a hydrophobic outer layer, and they lose their outer hydrophobic coating quickly, as reported by Ce´line Botta of the Centre Europe´en de Recherche et d’Enseignement des Ge´osciences de l’Environnement (France) and her colleagues. However, as Botta described at the American Chemical Society meeting in March 2009 and the Goldschmidt Conference in June, the intermediate aluminum coating tends to remain intact. That residual layer could affect how the TiO2 nanoparticles aggregate and travel through water, as well as their possible toxicity to animals such as daphnidsssomething Botta and colleagues are testing now. Bernd Nowack of Empa notes that there are already 200 different forms of TiO2. After nanoTiO2sor any nanomaterial for that mattersis functionalized with coatings, it may have thousands of possible forms. “Do we have to study every single one of them?” he asks. Instead, Nowack suggests that inventorying how these nanomaterials are usedsi.e., full lifecycle assessmentsswill allow
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researchers to refine their search in the real world. The amount of engineered nano-TiO2 on the market will only increase in the next few years, potentially replacing the majority of the bulk form in use by 2025 (Environ. Sci. Technol. 2009, DOI 10.1021/es8032549), according to Christine Ogilvie Robichaud of Duke University and her colleagues. (Current global production levels of bulk TiO2 are about 4 million metric tons per year, with 1.3 million metric tons per year produced in the U.S. alone. Nano-TiO2 production is probably far less than thatsand much less than most high-production-volume materials.) Coauthor Mark Wiesner says his team hopes to “keep knocking more off the list”
of nanomaterials. Inventories lead to estimates of exposure, he says, which are key to investigating toxicity. In a new ES&T article (DOI 10.1021/es803621k), Wiesner and his partners in the Center for Environmental Implications of Nanotechnology at Duke University assess the issues further. This month, several meetings may further focus such research. Scientists will gather in Vienna to share advances related to nanomaterials in general, and the U.S. Environmental Protection Agency plans to host an invitation-only meeting about TiO2 nanoparticles in the environment. TiO2’s photochemical reactions in conjunction with its possible widespread occurrence in the environment make it of particular
interest, says Paul Westerhoff of Arizona State University. Westerhoff is a coauthor of new research in ES&T (DOI 10.1021/es901102n) showing that nano-TiO2 from food and cosmetics, for example, can enter the waste treatment system and travel readily through waste treatment plants into the environment. “Most of [the TiO2] ends up in biosolids,” says Westerhoff, and “wherever biosolids go, so goes TiO2”sfrom agricultural fields, for example, into streams and rivers. From a life-cycle perspective, he says, nano-TiO2 is the only nanomaterial “that looks like there will be much of it out there.” —NAOMI LUBICK
September 1, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 6447
