Transformation of Silver Nanoparticles in Sewage Sludge

From Environmental Protection Online

Carol Potera

Carol Potera, based in Montana, has written for EHP since 1996. She also writes for Microbe, Genetic Engineering News, and the American Journal of Nursing.

Citation: Potera C 2010. Transformation of Silver Nanoparticles in Sewage Sludge. Environ Health Perspect 118:a526-a527. doi:10.1289/ehp.118-a526a
Online: 01 December 2010

The release and environmental fate of nanoparticles throughout the life cycle of “nanoenabled” goods is an area of growing research interest. In the first known field study of the fate of silver nanoparticles in the wastewater treatment system, researchers now report these nanoparticles transform into silver sulfide in the sludge produced by sewage treatment plants.1 This new information about the life cycle of silver nanoparticles provides a starting point for further exploring their impact on the environment.

Silver has been used as an antimicrobial agent for millennia,2 and the increased surface area offered by the nanoparticle form of the metal offers greater germ-killing capacity.3 Today, manufacturers add silver nanoparticles to hundreds of consumer products, including food storage containers, clothing, computer keyboards, cosmetics, pillows, cell phones, and medical appliances.4

Silver is water soluble, so contact with any type of moisture—such as a bath or a spin in the washing machine—washes some out and sends it into wastewater systems. “We wanted to know what form of silver enters the environment after it goes down the drain and passes through sewage treatment plants,” says Michael Hochella, a geochemist at Virginia Polytechnic Institute and State University and director of natural and incidental nanoparticles for the multi-institute Center for the Environmental Implications of NanoTechnology.5

Sludge from sewage treatment facilities can end up as landfill or soil amendments in agricultural fertilizers, or it can be burned in incinerators. In 2006 and 2007 the U.S. Environmental Protection Agency (EPA) analyzed sewage sludge samples from 74 municipal wastewater treatment facilities nationwide and tested for 28 metals, including silver (which was detected in all the samples).6 Through the EPA, Hochella and postdoctoral fellow Bojeong Kim obtained frozen samples of sludge from a Midwest facility. They suspected it would contain the nanosilver particles now used in consumer products—although the EPA’s goal in sampling was simply to obtain national estimates of the concentrations of selected analytes, not identify nanoparticles.


Numbers of Goods Containing Silver Nanoparticles4
Kim developed analytical methods to determine the size, chemistry, and atomic structure of silver nanoparticles in the samples. The samples tested high in silver, but the silver could not be attributed to an industrial source. Scanning transmission electron microscopy revealed the nanoparticles were 5–20 nm in diameter and formed small, loosely packed aggregates no more than 100 nm in size. Energy-dispersive X-ray spectrometry showed that sulfur (which is produced by microorganisms that digest sewage) combined with the silver in a 2:1 ratio, and the crystal structure confirmed the formation of silver sulfide nanoparticles.1

The results underscore the complexity of environmental fate. “What we start with is not what ends up in the environment,” Hochella says. The researchers don’t know how many silver nanoparticles were introduced to the wastewater treatment plants or how much incoming nanosilver ended up as silver sulfide nanoparticles. However, Kim notes that no pure silver nanoparticles were found in the sludge.

In general, silver sulfide is highly insoluble and settles out of water.7 But no one knows if silver sulfide nanoparticles behave in the same way. Properties of metals can change dramatically as particle size decreases.3 “It’s hard to predict whether the solubility of nanoparticles will increase, decrease, or stay the same,” Kim says. The bioavailability, toxicity, and reactivity of silver sulfide nanoparticles also are unknown.

If silver sulfide nanoparticles do prove toxic, the environmental implications could be unfavorable. Antimicrobial nanoparticles could adversely impact desirable microorganisms that decompose waste in sewage treatment plants, says Murray McBride, director of the Cornell Waste Management Institute. Furthermore, McBride says, nanosized silver sulfide applied to agricultural land could oxidize in soils and release toxic silver ions that kill beneficial soil microorganisms. On the other hand, one study of laboratory-grown Pseudomonas putida biofilms indicated some bacteria bind silver ions, potentially rendering them less toxic.8

References and Notes Top
1. Kim B, et al. Environ Sci Technol 44(19):7509–7514. 2010. doi:10.1021/es101565j Find this article online
2. Alexander JW Surg Infect (Larchmt) 10(3):)289–292. 2009. doi:10.1089/sur.2008.9941 Find this article online
3. Chen X, Schluesener HJ Toxicol Lett 176(1):1–12. 2008. doi:10.1016/j.toxlet.2007.10.004 Find this article online
4. The Project on Emerging Nanotechnologies. Nanotechnology Consumer Product Inventory. Washington, DC:The Woodrow Wilson International Center for Scholars (2010). Available: http://tinyurl.com/5sa88q [accessed 3 Nov 2010].
5. Funded by the National Science Foundation and the U.S. Environmental Protection Agency, the Center for the Environmental Implications of NanoTechnology is a consortium of scientists from Duke University, Carnegie Mellon University, Howard University, Virginia Polytechnic Institute and State University, the University of Kentucky, and Stanford University who study the biological, environmental, and ecological consequences of nanomaterials.
6. EPA. Targeted National Sewage Sludge Survey Statistical Analysis Report. EPA-822-R-08-018. Washington, DC:Office of Water, U.S. Environmental Protection Agency (2009). Available: http://tinyurl.com/33mlma4 [accessed 3 Nov 2010].
7. Lytle PE Environ Toxicol Chem 3(1):21–30. 1984. Find this article online

Nanomaterial risk review

The federal government's plan for researching the health and environmental risks of nanomaterials has "serious weaknesses," says this report from the National Research Council.

Links:
http://books.nap.edu/openbook.php?record_id=12559&page=R1

EPA accepting comments on nanoscale silver

EPA has published in the Federal Register a notice requesting public comment on a petition filed by the International Center for Technology Assessment (ICTA) et al. The petitioners note the rapid increase in the number of products containing manufactured or engineered nanoscale materials.

They further point out that scientists have identified that nanoscale materials can have fundamentally different properties from the non-nanoscale or bulk forms of the same compounds, and that these unique properties may pose new environmental and human health risks.

Additionally, the petitioners review EPA’s procedural history with respect to nanotechnology and conclude that the Agency has not provided adequate regulatory oversight for this emerging technology.

The petition requests that the EPA:

• classify nanoscale silver as a pesticide,
• require formal pesticide registration,
• analyze the potential human health and environmental risks of nanoscale silver, and
• develop appropriate regulatory oversights for nanoscale silver products.

The Agency has determined that the petition raises issues that potentially affect private and public sector stakeholders. Through this notice, EPA is asking for public comment, which may be submitted through January 20, 2009.

Using Living Cells as Nanotechnology Factories

In the tiny realm of nanotechnology, scientists have used a wide variety of materials to build atomic scale structures. But just as in the construction business, nanotechnology researchers can often be limited by the amount of raw materials. Now, Biodesign Institute at Arizona State University researcher Hao Yan has avoided these pitfalls by using cells as factories to make DNA based nanostructures inside a living cell. (More....)

Former EPA Leader Offers Nanotechnology Oversight Roadmap

Nanotechnology will significantly change virtually every facet of the American lifestyle. The next president has the opportunity to shape these changes and to ensure that nanotechnology's benefits will be maximized and its risks identified and controlled.

A new report by former EPA official J. Clarence (Terry) Davies lays out a clear roadmap for the next presidential administration and describes the immediate and longer term steps necessary to deal with the current shortcomings of nanotechnology oversight. (Read more... )

Broken CFL's and use of nanomaterial sorbents

Wondering how to effectively capture mercury vapor for broken compact fluorescent lamps (CFL's)? If you said yes, you are in the same group as Natalie Johnson at the Superfund Basic Research Program (SBRP) laboratory of Robert Hurt, Ph.D. (director of the Institute for Molecular and Nanoscale Innovation at Brown University). Natalie is the lead author on a new study reporting the lab’s latest findings, titled “Mercury Vapor Release from Broken Compact Fluorescent Lamps and In Situ Capture by New Nanomaterial Sorbents” (in press, Environmental Science and Technology).

You can read a write-up of their study in the July 2008 edition of the NIEHS Environmental Factor. Among their findings: a few of the common sorbents, such as powdered sulfur or zinc, require greater than 10 kilograms to treat vapor release for a single CFL, while small quantities of other sorbents (for instance, nano-silver and sulfur-impregnated activated carbon forms) require less than 1 gram of sorbent to capture the vapor.