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Nanotechnology: Applications for Environmental Remediation
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A nanometer is one billionth of a meter — about one ten-thousandth the thickness of a human hair. By this definition, any submicron-sized particle falls under the category of nanoscale materials. The National Nanotechnology Initiative defines nanotechnology as understanding and controlling matter at dimensions between approximately 1 and 100 nanometers, where unique phenomena enable novel applications (NNI 2008). Figure 1 shows the scale of nanoscale materials by comparing a nanowire to a human hair.
Figure 1. Micrograph of a looped nanowire against the backdrop of a human hair (Mazur Group 2008)
Nanoscale materials can be grouped into three categories: natural, incidental, and engineered. Examples of naturally occurring nanoscale materials include clays, organic matter, and iron oxides within soil that play an important role in biogeochemical processes (Klaine et al. 2008). Incidental nanoscale materials enter the environment through atmospheric emissions, solid or liquid waste streams from nanoscale material production facilities, agricultural operations, fuel combustion, and weathering (Klaine et al. 2008; U.S. EPA 2008). Engineered or manufactured nanoscale materials are designed with specific properties and may be released into the environment through industrial or environmental applications (U.S. DHHS 2006; U.S. EPA 2007). Nanoscale materials may be produced via a "top down" approach, such as by milling or grinding macroscale materials or, most commonly, via a "bottom up" approach, such as borohydride reduction, which creates nanoscale materials from component atoms or molecules (Lien et al. 2006; U.S. EPA 2007).
Several terms are used to describe nanoscale materials, including nanoparticles, nanoscale particles, nanomaterials, nanosized particles, nanosized materials, nano-objects, and nanostructured materials. This website uses the term "nanoscale materials." This term also includes materials that may be microscale or macroscale in size with an active component that is nanoscale in range.
Nanoscale materials are being used in a variety of applications within the scientific, environmental, industrial, and medical arenas. The list below includes examples of nanoscale materials, as well as their properties and uses (Gil and Parak 2008; Powell and Kanarek 2006; U.S EPA 2007; U.S. EPA 2008; Klaine et al. 2008; Watlington 2005; Wiesner et al. 2006).
- Nanoscale materials that may exist naturally or may be engineered:
- Fullerenes and carbon nanotubes exist as hollow spheres (buckyballs), ellipsoids, or tubes (nanotubes), which are composed entirely of carbon. They are strong antioxidants, are stable, have limited reactivity, and have excellent thermal and electrical conductivity. Their use includes biomedical, super-capacitor, sensor, and photovoltaic applications (U.S. EPA 2009).
- Nanosized metal oxides include titanium dioxide (TiO2), zinc oxide (ZnO), cerium oxide (CeO2), and iron oxide (Fe3O4), some of which are able to block ultraviolet light. They consist of closely packed semiconductor crystals, which are composed of hundreds or thousands of atoms. Uses of metal oxides include applications in photocatalysts, pigments, drugs (to control release), medical diagnostics, and sunscreen (U.S. EPA 2009).
- Nanoscale materials that are engineered:
- Zero-valent metals, such as nanoscale zero-valent iron (nZVI), have high surface reactivity and are used in the remediation of water, sediments, and soils. See Nanoscale Materials for more information.
- Quantum dots are semiconductors whose excitons (bound electron-hole pairs) are confined in all three spatial dimensions. They range in size from 10 to 50 nanometers. Quantum dot applications include medical imaging, photovoltaics, telecommunications, and sensors.
- Dendrimers are highly branched polymers that can be designed and manufactured to incorporate a variety of functional groups. Common shapes include cones, spheres, and disc-like structures. Dendrimers are used in drug delivery, chemical sensors, modified electrodes, and as DNA transferring agents.
- Composite nanoscale materials are made from two or more different nanoscale materials or one nanoscale material that is combined with bulk type materials. Composite nanoscale materials may be integrated with biological and synthetic molecules, which provide novel electrical, catalytic, magnetic, mechanical, thermal, or imaging capabilities. Potential applications include drug delivery and cancer detection. They are used in auto parts and packaging materials to enhance mechanical and flame-retardant properties.
Applications for Environmental Remediation
An increasing variety of nanoscale materials with environmental applications has been developed over the past several years. For example, nanoscale materials have been used to remediate contaminated soil and groundwater at hazardous waste sites, such as sites contaminated by chlorinated solvents or oil spills. As indicated above, many types of nanoscale materials are being applied across various fields of science and technology; this website focuses on the use of engineered nanoscale materials for environmental site remediation. Nanoscale materials are of interest for environmental applications because the surface areas of the particles are large when compared with their volumes; therefore, their reactivity in chemical or biological surface mediated reactions can be greatly enhanced in comparison to the same material at much larger sizes (U.S. EPA 2007). They can be manipulated for specific applications to create novel properties not present in particles of the same material at the micro- or macroscale. Nanoscale materials can be highly reactive in part because of the large surface area to volume ratio and the presence of a larger number of reactive sites; but may also exhibit altered reaction rates that surface-area alone cannot account for. These properties allow for increased contact with contaminants, thereby resulting in rapid reduction of contaminant concentrations. Furthermore, because of their minute size, nanoscale materials may pervade very small spaces in the subsurface and remain suspended in groundwater if appropriate coatings are used. Appropriate coating may allow the particles to travel farther than macro-sized particles, achieve wider distribution, and therefore improve contaminant reduction. More information on the application of nanotechnology for use in environmental remediation can be found in the Applications section.
Some applications of nanoscale materials for environmental remediation are in the research phase but others are rapidly progressing from pilot-scale to full-scale implementation. For example, certain nanoscale materials hold promise for environmental applications in addressing challenging sites, such as sites contaminated with chlorinated solvents. Ongoing bench- and pilot-scale research is being performed to investigate particles such as TiO2, self-assembled monolayers on mesoporous supports (SAMMSTM), dendrimers, carbon nanotubes, metalloporphyrinogens, and swellable organically modified silica (SOMS). This research is evaluating how to apply the unique chemical and physical properties of these nanoscale materials for use in full-scale environmental remediation (see the Nanotechnology Products with Potential Remediation Applications section). In addition, there are many unanswered questions about nanotechnology. For example, more research is needed to understand the fate and transport of free nanoscale materials in the environment, whether they are persistent, whether they have toxicological effects on various biological systems, and whether the theoretical benefits of nanoscale materials can be realized in broad commercial use (U.S. EPA 2008). Furthermore, nanoscale materials are also being considered for use in sensing and monitoring environmental contaminants; however, research and development of nanosensors are still in progress (U.S. EPA 2007).
Nanotechnology with Potential Remediation Applications
Currently, nanoscale iron is being used in environmental remediation. Detailed information on the use of nanoscale iron for remediation is available within the Application section of this website. Researchers are developing a variety of other nanoscale materials for potential use to adsorb or destroy contaminants as part of either in situ or ex situ processes. The stage of development ranges from bench to pilot scale. These particles include TiO2, SAMMSTM, nanotubes, ferritin, dendrimers, metalloporphyrinogens, and SOMS. More information on these nanoscale materials can be found in the Nanotechnology Products with Potential Remediation Applications section.
Gil, P.R., and W.J Parak. 2008. Composite Nanoparticles Take Aim at Cancer. ACS Nano. 2(11):2200-2205.
Klaine, S.J., P.J.J. Alvarez, G.E. Batley, T.E. Fernandes, R.D. Handy, D.Y. Lyon, S. Mahendra, M.J. McLaughlin, and J.R. Lead. 2008. Nanoparticles in the Environment: Behavior, Fate, Bioavailability, and Effects. Environmental Toxicology and Chemistry. 27(9):1825-1851.
Lien H-l., D.W. Elliott, Y-P. San, and W-X. Zhang. 2006. Recent Progress in Zero-Valent Iron Nanoparticles for Groundwater Remediation. Journal of Environmental Engineering and Management. 16(6):371-380.
Mazur Group, Harvard University. 2008. Available at: http://www.nsf.gov/od/lpa/news/03/pr03147.htm. Accessed May 2009.
National Nanotechnology Initiative (NNI). 2008. What is Nanotechnology? Available at: http://www.nano.gov/nanotech-101/what/definition. Accessed September 25, 2008.
Powell, M.C. and M.S Kanarek. 2006. Nanomaterial Health Effects - Part 2: Uncertainties and Recommendations for the Future. Wisconsin Medical Journal. 105(3):18-23.
U.S. Department of Health and Human Services (U.S. DHHS). Centers for Disease Control and Prevention. 2006. Approaches to Safe Nanotechnology: An Information Exchange with NIOSH. Available at: http://www.cdc.gov/niosh/topics/nanotech/.
U.S. Environmental Protection Agency (U.S. EPA). 2008. Nanotechnology for Site Remediation Fact Sheet. Solid Waste and Emergency Response. EPA 542-F-08-009. October 2008. Available at: http://www.clu-in.org/download/remed/542-f-08-009.pdf.
U.S. EPA. Science Policy Council. 2007. Nanotechnology White Paper. U.S. Environmental Protection Agency. February 2007. Available at: http://www.epa.gov/ncer/nano/publications/whitepaper12022005.pdf.
U.S. EPA. 2012. Emerging Contaminants - Nanomaterials Fact Sheet. Solid Waste and Emergency Response. EPA 505-F-11-009. May 2012. Available at: http://www.epa.gov/fedfac/pdf/emerging_contaminants_nanomaterials.pdf.
Watlington, K. 2005. Emerging Nanotechnologies for Site Remediation and Wastewater Treatment. Available at: http://www.clu-in.org/download/studentpapers/K_Watlington_Nanotech.pdf.
Wiesner, M.R., G.V. Lowry, P. Alvarez, D. Dionysiou, and P. Biswas. 2006. Assessing the Risks of Manufactured Nanoparticles. Environmental Science & Technology. 40(14):4336-4365.