Nanoparticles and the Environment Series, Part I (click here for part II)

More than a year ago on this site, I talked to you about how nanoparticles were already all around us, and have been for most of human history. That day, I was hoping to show you that while nanotechnology may sound cool and cutting edge (It is!), nanoparticles themselves aren’t necessarily new, and that nanotechnology is not inherently hazardous or mysterious. Nanotechnology is not the grey goo it has been portrayed to be in science fiction.

Like all new technologies, though, nanotechnology will introduce both new advantages (applications) and new hazards (implications) for human society and the planet. You may already know that the unique properties of nanomaterials give rise to many amazing potential applications: turning light into a weapon against cancer, changing how we generate or store energy, speeding up chemical reactions, or allowing us to make thin sheets of plastic that are stronger than steel. But every time a new technology comes along, we have to consider how we can use the technology responsibly for society’s benefit while minimizing the hazards that the new technology poses. Nanotechnology is no different. In this series of blog posts, we are going to talk about the flipside of nanotechnology’s amazing applications; we are going to look at how human-made (synthetic) nanoparticles may unintentionally enter the environment, and look at some of the risks to human and environmental health these fantastic materials may pose.

Why Nanoparticles in the Environment May Be a Big Deal

As an industrialized society, we already deal everyday with the consequences of human-made chemical contamination in our environment. These contaminants may be too subtle to easily see (BPA leaching into your personal water bottle, the proliferation of steroids and hormones in drinking water) or so obvious as to be impossible to ignore (the Cuyahoga River fire of 1967, the Gulf oil spill), but what is undeniable is the impact synthetic chemical contaminants¹ can have on human health and local environments. As society becomes more interested in reaping the potential benefits of nanotechnology, nanoparticle production will increase, the number of nanoparticle-enabled products will increase, and synthetic nanoparticles will join more traditional synthetic chemicals as environmental contaminants.

After nearly 50 years of detailed study, we know quite a bit about the effect of traditional chemical contaminants (pesticides, plasticizers, pharmaceuticals, etc.) on human or environmental health.² However, the possibility that synthetic nanoparticles could soon join them in the environment raises new concerns, because nanoparticles may impact the health and stability of local ecosystems in ways that are difficult to predict. Today, we are drawing a distinction between synthetic and naturally-occurring nanoparticles; when and where nanoparticles occur naturally, they may actually be an important part of the biological and geochemical processes that are essential for what we consider “normal” ecosystem function.

Just because some nanoparticles occur naturally doesn’t mean that many synthetic nanoparticles don’t pose a serious environmental concern. Ironically, it is nanomaterials’ unique size-dependent properties (which may allow us to unlock new and amazing innovations), that could also pose unforeseen problems if nanoparticles are unintentionally released into the environment. Like traditional chemical contaminants, some synthetic nanoparticles may be directly toxic to microbes, plants, and animals. Silver nanoparticles, for instance (though they are not toxic to humans), dissolve in water and release silver ions (which are antibacterial). If silver nanoparticles are released into the environment, these types of concentrated silver ion releases could devastate local bacterial populations, with drastic consequences for the affected ecosystems.

There are also several types of nanomaterials that may be detrimental to the environment because they facilitate chemical reactions that can harm plankton, bacteria, and small animals. Many metal and metal oxide nanomaterials are excellent catalysts (materials that speed up the rate of different chemical reactions). If these catalytic nanomaterials are released into the environment, they can enable chemical reactions that generate toxic chemicals, such as free radicals or reactive oxygen species (aka ROSs). One of these nanomaterials is titanium dioxide (TiO₂), which is an excellent photocatalyst (ultraviolet light exposure activates its catalytic properties). When illuminated by sunlight, titanium dioxide nanoparticles can catalyze chemical reactions that increase the concentrations of several ROSs (including hydroxide {●OH} or superoxide {●O₂^–} radicals) in natural waters.³ These reactive oxygen species are known to be harmful to many aquatic organisms, including plankton and small fish.

How Nanoparticles Get into the Environment

In my previous post, I briefly touched on some of the different ways in which synthetic nanoparticles can enter the environment, but here, I would like to describe a few of the most important pathways in detail. Like any chemical contaminant, nanoparticles could accidentally be released into the atmosphere or nearby natural water during their production or through a factory’s waste stream. By 2020, the total amount of nanomaterials produced by industry is expected to increase from 1000 to 58000 tons, making the release of nanomaterials during production a significant concern.⁴ However, environmental regulations will ultimately limit the amount of nanoparticle waste that can enter the environment through dumping or accidental release. Therefore, we will focus this discussion on the two most likely pathways of nanomaterial release: release through ordinary human activities and the use of nano-enabled products.

There are a number of human activities that release nanoparticles. These include burning fossil fuels, large-scale mining/demolition, and automobile traffic. Traffic emissions and fossil fuel combustion typically produce ultrafine (read: nano) particles of soot or carbon black with diameters <100 nanometers. Fossil fuel combustion (including natural gas burning) has also been shown to produce more formally recognized types of carbon nanoparticles including carbon nanotubes and fullerenes. Mining and metal refinery operations have recently been shown to generate metal and metal oxide nanoparticles.⁵ One particularly damaging aspect of nanoparticle release from combustion reactions or large-scale demolition is that the nanoparticles are often released directly into the air (although mine tailings may also introduce these nanoparticles into natural waters). Airborne nanoparticles are a source of special environmental health concerns for two reasons: (1) airborne pollutants are difficult to contain and can rapidly spread to other ecosystems (both near and far) and (2) airborne particulate contaminants are being implicated in many pulmonary conditions (including chronic obstructive pulmonary disease, pulmonary inflammation, and pulmonary fibrosis), because their unique size makes it easy for them to pass deep into lung tissue, where they may cause severe respiratory irritation.

The other most likely source of nanomaterial contaminants is nanoparticle-enabled consumer products. More than several thousand consumer products, including many items of clothing, personal care products, next-generation batteries, and sporting goods now contain nanomaterials.⁶ In the next ten years, nanoparticles will likely be incorporated into many more sophisticated products, including pharmaceuticals and next-generation solar cells or batteries. While the inclusion of nanomaterials in these products can enhance their performance, the breakdown of these products at the end of their useful life also provides several key points of entry for synthetic nanoparticles into the environment. When nanoparticles are incorporated into products intended for domestic use (like anti-microbial fabrics or UV-blocking clothing), nanoparticles can end up in landfills or washed down the drain when clothes have been laundered. Nanoparticles that end up in drain water or in landfills can then enter the environment by many different routes.

Once nanoparticles have been disposed of in household garbage or down the drain, their journey through the environment is just beginning. Nanoparticles that escape with water down the drain will eventually enter a wastewater treatment facility. Here, nanoparticle-contaminated water will undergo several purification processes, including mechanical filtering and settling treatments (designed to remove large particles [larger than a grain of sand]), followed by digestion with microbes, and ultimately chemical disinfection. Unfortunately, none of these treatment stages are specifically designed to eliminate nanoparticles from the waste stream. As a result, following waste water treatment, nanoparticles may remain in the purified water that is released back into the environment, or nanoparticles may remain trapped in the microbe-bearing sludge left over from the purification process. Often, the bio-sludge left over from wastewater purification is repurposed as fertilizer for farm land, which can potentially allow man-made nanoparticles to enter soils or small rivers. Alternatively, when nanoparticle-enabled products end up in a landfill, the original product can break down, allowing man-made nanoparticles to leach into soil in and around the landfill area, providing a route for synthetic nanomaterials to enter new environments via soils or even ground water sources.

To wrap up, let’s take a look at an example of how synthetic nanoparticles can be released into the environment when you use a common nanoparticle-enabled consumer product- a UV-blocking running shirt with titanium dioxide (or zinc oxide) nanoparticles incorporated into the fibres.⁷ After an invigorating run (or possibly anything from an easy jog to a jaw-dropping parkour workout, it’s up to you) you peel off your shirt and drop it into the laundry bin. The shirt has served its purpose. A few days later, into the laundry the shirt goes, then the dryer, and back into your drawer. But the shirt left something behind in the water that went down the drain behind the washing machine- a small amount of titanium dioxide nanoparticles. Ultimately, these nanoparticles end up (with a lot of other unmentionables) in a wastewater treatment facility, where they are filtered, chewed up by microbes, and treated with a small amount of chlorine. Likely, some of the TiO₂ nanoparticles end up in the bio-sludge and some remain in the treated water that is released back into the environment. Once the nanoparticles are released into the environment, they may do harm (as we have seen) to local microbe or plankton populations, depending on the amount of UV light exposure they receive, as well as other factors. There is even a chance that nanoparticles released into soil or surface waters may ultimately end up in drinking water, because drinking water treatment facilities also rely on a series of filtering and disinfection procedures that are not specifically designed to remove nanoparticles from water.

We now know a little bit about how nanoparticle contaminants may escape into the environment. Unfortunately, we still know far less about the specific environmental or human health hazards that some synthetic nanoparticles may pose. Understanding the health implications of synthetic nanoparticles is just one of many challenges still remaining if we want to understand the environmental behavior of these nanoparticles in great detail. For instance, we don’t understand how the various components of a given environment (like available ions, minerals, local bacteria, and organic matter) may interact with different nanoparticles to change their size, shape, composition, and bioavailability. Furthermore, we are still unable to easily detect and monitor synthetic nanoparticles that have been released into the environment. As a result, nanoparticles are difficult to track through real ecosystems and many different time-intensive assays are required to even determine whether humans or animals have been exposed to nanoparticles.

Nanotechnology is still a young field. While this means there are many things we still don’t know about both the applications and implications of these new materials, this also gives us a new and unique opportunity to develop a chemical industry that could be truly sustainable from its inception. Hopefully, in the near future, we can overcome many of the barriers described above and help nanotechnology grow into a sustainable industry–one in which we can prevent environmental contamination by nanoparticles, rather than just clean up chemical waste after the fact. Look for the next post in this series soon, where we discuss the specific issues of nanoscale titanium dioxide (a very common industrially produced nanoparticle) as an environmental contaminant.

Footnotes

1. Just a long note on the difference between contaminants and pollutants: Environmental regulatory agencies define a contaminant as a chemical that is present in the environment in higher levels than the chemical would naturally occur. If this chemical has a detrimental effect on the environment, then the chemical is also a pollutant. We will refer to synthetic nanoparticles as contaminants in this post, because for many nanoparticles we don’t yet know the extent of their environmental impact.
2. “Quite a bit” is a relative term. Environmental researchers are still continually surprised by new insights into the persistence and effects of chemical contaminants in the environment.
3. Radicals are chemicals that possess an unpaired electron. As a result, they are very reactive (and therefore, potentially toxic). Under the right circumstances, radicals can even damage an organism’s DNA.
4. Reference: Nanoscience and Nanotechnologies: Opportunities and Uncertainties; Highlights from Royal Society and Royal Academy of Engineering Final Report. Royal Society: London, 2004.
5. Reference:http://www.cdc.gov/niosh/mining/works/coversheet99.html
6. Reference:http://www.nanotechproject.org/news/archive/9242/
7. Some recent research has also shown that if you wear nanoparticle-impregnated clothing, your sweat can cause some of those nanoparticles to leach off onto your skin, but we’ll leave that for another time. http://cen.acs.org/articles/91/web/2013/07/Nanoparticles-Athletic-Apparel-Seep-Sweat.html

 

Further Investigation:
How Did That Get There?: Water Pollution activity
Storm Water Runoff Pollution activity
Chemical Footprint—family activity
Pollution Diffusion activity
Amphibian Skin activity

Note: This post was updated on Sept 10, 2015 to replace the term “man-made” with “human-made” and “synthetic.”