Sludging through the nano lifecycle: Caution ahead

Richard Denison, Ph.D., is a Senior Scientist.

Researchers at Virginia Tech have identified and characterized silver nanoparticles (AgNPs) in the sewage sludge produced by an operating municipal wastewater treatment plant.  The study is notable in several respects:  It is the first time AgNPs have been detected in a field-scale study, one of a real-world operation representative of a real-world exposure scenario to boot.  It shows that silver can exist in wastewater treatment products as nanoparticles.  It indicates such particles may be most likely to partition to sludge under common treatment technologies.  And it suggests that silver may be chemically transformed in the course of wastewater treatment.

The study did not demonstrate that the AgNPs detected in the sludge originated from products containing such nanoparticles, as some news stories have suggested, although the authors indicate such a source "is likely."  But the findings have important implications for nano safety nonetheless. 

The study, by Michael Hochella and colleagues at Virginia Tech, appears in Environmental Science & Technology (web publication dated September 14, 2010).  The authors were motivated no doubt by the fact that nanosilver is already in widespread use in a host of consumer products, which they identify as including food storage containers, coating materials, liquid fabric softeners and detergents, fabrics and clothing, sporting goods, dietary supplements, cleaning and personal-care products, cosmetics, and medical appliances based on data from the Project on Emerging Nanotechnologies product database.

Given that AgNPs are almost certainly being released into municipal wastewater from at least some of these products (as has been shown, for example, from the laundering of nanosilver-impregnated socks), the authors looked for AgNPs in sewage sludge from a publically-owned treatment works (POTW) they indicated is "located in a metropolitan area of the Midwest."  They chose it because of the high concentration of silver in the sludge (856 milligrams per kilogram), and the absence of any known industrial sources of silver that might be releasing silver into the wastewater treated by the plant.

Nanosilver detected

The authors detected AgNPs in the sludge using electron microscopy, which revealed elliptical particles of 5-20 nanometers (nm) in diameter, clustered in "very small, loosely packed aggregates."  They identified the particles to be comprised of silver sulfide (Ag2S) nanocrystals, and speculate that the particles were most likely formed during wastewater treatment under anaerobic conditions, starting either with AgNPs or silver ions, as has been shown in lab studies.

Ag2S has very low water solubility, which may at least partly explain the partitioning of such nanoparticles to the sludge during wastewater treatment.  Of course, its removal from wastewater is by no means the end of the road, as the sludge must subsequently be managed.  That typically means landfilling, incineration or – for about half of all sewage sludge in the U.S. – application to agricultural lands as a fertilizer or soil amendment.

In other words, any nanosilver released from products will eventually re-enter the environment.  We've blogged here before about some of the potential implications of adding more silver (nano or otherwise) into the environment, including the potential to suppress plant growth and soil microbial activity and the potential to create silver-resistant bacteria.

Painted surfaces as sources of nanoparticle release

While this study did not directly demonstrate migration of nanosilver from products into wastewater into sludge and into the environment, it certainly connected some of those dots.  Another study of nano-titanium dioxide (TiO2) used as a pigment in paints connected even more:  It traced such nanoparticles as they moved from both new and weathered painted surfaces into runoff and thence into surface waters.  Based on projected volumes of use and types of applications, some researchers have suggested that migration of such nanoparticles could yield levels in surface waters close to or exceeding the levels at which toxicity to aquatic organisms has been observed.

All of this adds weight to argument that we need to proceed with considerable caution as we consider or pursue nano applications, especially those that are dispersive in nature or can lead to nanoparticle releases from substrates or surfaces that may be exposed to the elements or biota during any stage of the material's lifecycle.

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  1. Rosalind Volpe Silver nanotechnology Working Group
    Posted September 24, 2010 at 5:29 pm | Permalink

    Despite the spin on the actual contents of the research article, this research actually shows what industrial chemists have known for a long time, silver is transformed into inert silver sulfide. The first line of the story is factually incorrect. The Virginia Tech researchers did not find AgNPs (metallic silver nanoparticles) but rather silver sulfide nanoparticles.

    Rosalind Volpe, D.PH
    Director, Silver Nanotechnology Working group

  2. Posted September 26, 2010 at 11:11 am | Permalink

    My post clearly identified the chemical form of the silver in the nanoparticles as silver sulfide. And the shorthand I used to refer to the particles — AgNPs — is that of the authors, not my own creation. I also noted the very low solubility of the AgS.

    Your comment and use of the term "inert" — which suggests you mean to imply there is no cause for concern — misses the larger point of my post and of the paper I cite: that environmental transformations that can alter the chemical form and hence bioavailability of nanoparticles and the elements they contain may occur at any point in the lifecycle and need to be considered.

    The authors' final paragraph makes this clear:

    "With a detailed understanding of the mineralogy of α-Ag2S nanocrystals in sewage sludge materials, it is now necessary to investigate the time-dependent changes in their chemical and physical properties when they move into different environments, for example, when sewage sludge material containing Ag2S NPs is used on agricultural land as soil amendment. Additional processing of sewage sludge materials destined for land application may also have an influence on Ag-containing NP transformation processes. Metal sulfide complexes are, in general, resistant to oxidation and dissociation reactions. In fact, Ag2S is known to be one of the most insoluble minerals, with extremely low water solubility. However, the dissolution rate of galena (PbS) nanocrystals was at least 1 order of magnitude higher than that of galena microcrystals, thus displaying a size-dependent reactivity at the nanoscale. Sulfide minerals and mineral NPs often play central roles in heavy metal release and mobility over great distances. Therefore, future studies of size-dependent reactivity, particularly solubility, of Ag2S NPs will be useful in understanding the environmental fate, influence, and entire life cycle of engineered Ag NPs."