Cal Baier-Anderson, Ph.D., is a Health Scientist.
The proliferation of nanoscale materials in consumer products is impressive: nano titanium dioxide and zinc oxide in sunscreen, buckyballs in face creams, and nanosilver in socks are but a few examples of what is currently available for purchase. But they make me wonder: what happens when the nanomaterials in or released from these products are washed down the drain?
A recent study published in Environmental Science & Technology (ES & T) demonstrates that nanoscale silver, added to socks to reduce the growth of odor-causing bacteria, readily washes out into wash water. Testing of the wash water demonstrates that some of the silver is present in nanoparticle form, although dissolved silver ions are also present. And of course, at the end of the day, nanoparticles present in personal care products, such as nanoscale zinc oxide and titanium dioxide in sunscreen and carbon fullerenes in face creams, will also be washed down the drain in the sink, shower, or tub.
For most communities, the wastewater treatment plant is the next downstream recipient of these down-the-drain nanoparticles. The impact that wastewater treatment has on nanomaterials, or conversely, the impact that nanomaterials have on wastewater treatment, is largely unknown.
According to Cynthia Finley, Ph.D., Director for Regulatory Affairs at the National Association of Clean Water Agencies, “Our first concern is that nanoparticles could interfere with the wastewater treatment process and undermine our ability to meet our obligations under the Clean Water Act.”
“Our second concern is that the nanomaterials will make it through the wastewater treatment process and be discharged into receiving waters. Although currently there is no requirement for us to treat nanomaterials, we take our responsibilities seriously, and are mindful of the potential impacts of unregulated chemicals. Right now we don’t know how to treat nanomaterials in wastewater, so research will be very important in determining the appropriate response of wastewater treatment agencies.”
Depending on their characteristics, nanoparticles could wind up in the sludge (the solid materials removed from wastewater in the course of its treatment) or could be discharged with the treated wastewater. The disposition of nanoparticles following wastewater treatment will determine their subsequent fate and transport pathways; if nanoparticles primarily find their way into sewage sludge, then impact studies will need to focus on potential releases following land application and incineration, the main means by which we manage such sludge. If, however, nanoparticles primarily flow out in the discharge water, impact studies must consider their potential effects on aquatic organisms and ecosystems.
Another new paper published in ES & T describes a computer model that seeks to predict the movement of three types of nanoparticles – nano silver, nano titanium dioxide, and carbon nanotubes – throughout the materials’ life cycles. If validated for nanomaterials, models such as these can be useful, because they help us predict where nanoparticles may go, and hence where we might want to search for possible adverse impacts.
Of course, given how little data are actually available, modeling studies like these are constructed on a mountain of assumptions, or even best guesses. In this study, the authors assume that 60% of nano titanium dioxide produced is used in cosmetics and sunscreens, and that 95% of this material winds up in wastewater treatment plants. In contrast, the authors assume that carbon nanotubes are more likely to be used as additives to solid materials, such as plastics or other composites, and therefore they are more likely to be disposed of with solid waste. Because the uses of nano silver are currently much more diverse, the authors assume some would end up in wastewaters, some would be released to air and some would be disposed of as solid waste.
As more real data are acquired, models need to be adjusted and or updated. For example, the model I just described assumes that dissolved silver, rather than silver nanoparticles, will be the dominant form of silver released into wastewaters. Based on the results of the sock study, however, we now have good reason to expect that silver still in the form of nanoparticles will be released into wastewaters – something any good model would need to account for.
All this is to say that we are a long way from understanding the environmental fate and transport of down-the-drain nanomaterials. The development of models, as imperfect as they may be, can still help to guide research by allowing us to make predictions that we can then empirically test. Through such bootstrapping, we can hopefully move toward developing better guidance and standards that wastewater treatment system operators will need to protect the environment and public health.
But better models and more research don’t ensure safety. Our concerns regarding down-the-drain nanomaterial releases are yet another reason we urge that nanomaterials not be used in dispersive applications until their potential effects to human health and the environment are fully understood and mitigated.
Update: Jen Jackson of the East Bay Municipal Utilities District wasn't able to get back to me with her thoughts until after I published this post. Here's what she says:
“There is growing concern among wastewater agencies about the proliferation of products containing nanomaterials, such as nanosilver and nanocopper. Nanomaterials may behave differently than larger molecules when they reach wastewater treatment plants. We don’t know whether some nanomaterials may impair the wastewater treatment process, or sail through treatment plants into local waterways. Unfortunately, research on the impact of nanoparticles to the wastewater treatment process has not kept pace with the number of uses and applications of nanomaterials."