Assessing the Environmental Impact of Silver Nanoparticles

Interview conducted by Will SoutterApr 30 2013

In this interview, Dr. Ben Colman from Duke University talks to AZoNano about his work assessing the long-term impact of low-level concentration of silver nanoparticles.

Can you give us an overview of your research on silver nanoparticles in the environment?

Due to their antimicrobial properties, engineered silver nanoparticles are used in a wide range of consumer products and have the potential to enter the environment through product use and disposal. We are interested in the impacts of nanosilver on organisms and ecosystems.

We expect the majority of silver nanoparticles will enter the environment through land application of biosolids from wastewater treatment. In this experiment, we simulated this exposure scenario using large planters filled with a local soil, and planted with several locally common plant species.

To these ‘mesocosms’ we then added biosolids with or without the addition of silver nanoparticles at a low concentration that was well within the range measured by the United States Environmental Protection Agency in a recent survey of biosolids.

After fifty days, we measured plant and microbial community composition, abundance, and activity to see how the ecosystem responded.

What did the results show? Did the results differ from your expectations?

Based on our previous lab experiments using similar concentrations of silver nanoparticles, we did not expect nanosilver to have any measureable effect on microbes or plants. However, we found that in our mesocosms nanosilver impacted both microbes and plants.

In the first week of the experiment, microbes showed an increase in the production of nitrous oxide, which is both an important greenhouse gas, and currently the most important stratospheric ozone depleting substance. We also saw a decrease in microbial biomass and activity in surface soil at the end of the experiment, which we expect could slow decomposition given the central role of microbes in this process.

Several plant species were able to take up silver to concentrations at or above those in soil, which is concerning since this silver has the potential to be transferred to animals that feed on or are fed these plants. We also documented a large decrease in biomass in one species of plant in response to nanosilver, suggesting that nanosilver has the potential to alter community composition.

To truly understand how nanomaterials are transported and transformed in the environment, and impacts they have on organisms and ecosystems, there is no substitute for doing experiments at the mesocosm scale.

How were the "mesocosms" for your study designed, and what are the benefits of studies conducted in these environments compared to lab-based studies?

The mesocosms were designed to be as realistic as possible while avoiding the addition of nanosilver to the environment. We used a natural soil with an established microbial community, planted a diverse plant community, and ran the experiment outdoors to allow exposure to sun and rain. This is in marked contrast with the majority of studies which are lab based, conducted with single species, and in growth media which is physically, chemically, and biologically much less variable and complex than soil.

While lab studies can get at complex biochemical mechanisms of toxicity, extrapolating from laboratory experiments to potential environmental impacts can be misleading. To truly understand how nanomaterials are transported and transformed in the environment, and impacts they have on organisms and ecosystems, there is no substitute for doing experiments at the mesocosm scale.

Why was nanosilver chosen as the focus of the study, over other nanomaterials which are being released into the environment?

Silver is known to be toxic to a wide range of organisms, and was used as an antimicrobial agent for thousands of years before we knew microbes existed.

Since it is much less toxic to humans than dissolved silver, nanosilver has been increasingly used as an antimicrobial in a large range of consumer products such as socks and other clothing, supplements, and skin care products.

Since many of these products lose their nanosilver over time, it has a high potential for getting into the environment. All of these factors made nanosilver a logical choice for this study.

What effect will these results have on attempts to effectively regulate harmful nanomaterials?

Our results demonstrate that engineered nanoparticles can have impacts on the environment. More importantly, the magnitude of the impacts observed for AgNPs were as high as or higher in all cases than our positive control treatment, which received biosolids treated with fourfold higher silver concentrations in the form of AgNO₃. This suggests that existing regulations for dissolved silver may need revisiting to best determine if they are adequate for regulating nanosilver.

Thinking beyond the effects of our results on regulation, I hope our work will push other researchers to look beyond individual organisms in the lab and towards organisms in complex environments, and ultimately ecosystems. While biochemical mechanisms of toxicity are less readily observed in such an experimental design, mesocosms are really the only tool we have to observe larger scale direct and indirect impacts of chronic low-level exposures to nanomaterials on organisms and ecosystems. This is especially important given that these low-level exposures are the type that are likely to occur in the environment.

Do you think there is a risk that regulation of nanotechnology will stifle innovation and slow growth in the field?

At present, innovation and growth in nanotechnology are far ahead of our understanding of nanotechnology’s environmental impacts. As with all sectors, as our understanding of the potential impacts of nanomaterials increases, we will need to strike a balance between the need for advancement of technology and the need to protect the environment.

My reading of the literature suggests that there is only a low risk to human health from silver nanoparticles at the concentrations we are likely to encounter them.

Is there any reliable way to estimate the quantity of nanosilver being released into the environment?

There are many ways of estimating both the amount of nanosilver that is being released into the environment as well as the uncertainty around those estimates. Several studies have been published that do this at different scales of space and time.

Your study focused mainly on plants - is there a chance that silver nanoparticles may have a comparable direct effect on human health?

The risk of nanoparticles to humans is outside of my specialty, so my knowledge on this is limited. Given that humans have more microbial cells than we do human cells, and many of these are commensal, I suppose there is the possibility that silver nanoparticles and other antimicrobials could have indirect effects on human health.

In terms of direct effects on humans, my reading of the literature suggests that at the concentrations we are likely to encounter silver nanoparticles, there exists only a low risk. This is in part why nanosilver can be such a useful antimicrobial for select applications like wound dressings; it is toxic to microbes while causing minimal toxicity to our own cells which are trying to heal.

Are there any antimicrobial agents that could be used in place of nanosilver in consumer products?

Triclosan and Microban are antimicrobials used in similar applications to nanosilver, but these have also been shown to have negative impacts when released into the environment.

Alternatively, I think the real questions we should be asking ourselves are: Do products with antimicrobial coatings actually help prevent the spread of disease? Do we need so many of the products that we use and the surfaces we touch to be antimicrobial? While I do not know the answer to these questions, I think that there is the possibility that we are incurring risks to the environment with no guarantee of reward.

What are the next stages for this research?

We are currently working on writing up the results from an experiment simulating the impacts of silver nanoparticles released into aquatic ecosystems as wastewater effluent, and we are setting up another set of terrestrial and aquatic mesocosms. One of our major focuses moving forward is examining the impacts of chronic low-dose exposures in these systems, as well as impacts on food web dynamics.

Where can we find more information about your work?

The paper that details our findings from this study can be downloaded for free from PLOS ONE. Information about the broader work of the Center for the Environmental Implications of NanoTechnology (CEINT), can be found at our website, and more information about my work can be found at my personal website.

About Ben Colman

Benjamin P. Colman received his Ph.D. in 2009 in Ecology, Evolution, and Marine Biology from the University of California, Santa Barbara. Currently. He is a postdoctoral associate with Dr. Emily Bernhardt at Duke University, and a member of the Center for Environmental Implications of NanoTechnology (CEINT). Ben's research interests range from studying the fundamental controls of energy and nutrient cycling in terrestrial and aquatic ecosystems, to understanding how these controls are altered by chemical perturbations, such as engineered nanomaterials.

The approach Ben and his CEINT colleagues take towards understanding the environmental impacts of nanomaterials is to study them through the lenses of ecosystem ecology, microbial ecology, and biogeochemsitry. Simultaneously, they are also working to better understand the impacts of ecosystems on the engineered nanomaterials in terms of their fate and transformations, which ultimately feeds back to determine their impacts.