Nanoplastic in the environment

Plastic is ubiquitous as packaging material or as part of many products of our daily life. However, due to the steadily increasing global plastic production, plastic particles can now be found everywhere in the environment. It is estimated that between four and twelve million tonnes of plastic enter the seas and oceans every year ^[1].
Plastic particles, which are intentionally manufactured, are called primary plastic particles. They are added to daily life products like cosmetics or are used in research and diagnostics. Fragmentation of larger plastic items into smaller pieces yields so called secondary plastics and the degradation is caused by exposure to sun, wind or water.
In order to assess the potential risk of plastic particles in various sizes, details on the amount of particles released into the environment, their origin, the underlying transformation and fragmentation processes as well as the environmental effects are needed ^[2].

What is nanoplastic?

Nanoplastics are polymer-based particles (e.g. Polyethylene terephthalate (PET) or polystyrene) in the nanometre size range. They are either intentionally manufactured (primary nanoplastic particles) for different products (e.g. medical devices, drugs or electronics) with a defined size and composition or formed by the degradation of larger plastic items (e.g. bottles; Secondary nanoplastics).

However, currently there is no official definition of the term “nanoplastics” because it does not consist of a uniform material or composition. The scientific community is using the following size categories for classification for the different plastic particle groups: nanoplastics (1nm – 1µm), microplastics (1µm – 1mm), mesoplastics (1mm – 1cm) and macroplastics (1cm – 100cm) ^[3].
The continuous process of unintentional fragmentation of mismanaged plastic waste by exposure to sun, wind or water leads to the formation of particles in the size range of 1µm- 5mm (meso- and microplastics), and subsequently to nanoplastic particles smaller than 1µm ^[2,4,5].

Detection of nanoplastics

Despite several laboratory studies, comprehensive field data on exposure with nanoplastic particles are lacking ^[6].
Measuring environmental concentrations of nanoplastic particles is quite challenging for current analytical methods. First, the analytical detection methods have to distinguish between natural particles and plastic compartments. Secondly, real-life concentrations of nanoplastics can be very low down to nanograms per unit with particle sizes between 1-1000 nm. However, with the existing detection methods for nanoplastics both detection and quantification are hard to achieve (see cross-cutting article Detecting nanomaterials in the environment) ^[5,7].

Therefore, computational models based on microplastic data are used to estimate the number and environmental concentration of nanoplastic particles. Using microplastic data, the total mass contribution of nanoplastics to all plastic particles was predicted to be small. However, with 1 microplastic particle per 100.000.000.000.000.000 (10¹⁷) nanoplastics particles, nanoplastic particles significantly outnumber the microplastic particles ^[4,8].

Environmental behaviour of nanoplastic and its effects on the ecosystem

At present, there is little information on the emergence and environmental behaviour of nanoplastic particles, meaning all transport and transformation processes. This is due to the diverse sources of nanoplastic particles, varying physical properties, various types and timeframes for degradation and different ways of transportation ^{[7, 9, 10]}. What is certain is that the number of nano- and microplastic particles is going to increase due to the huge amount of large plastic compartments located in the environment. Nanoplastic particles also undergo environmental transformation processes such as agglomeration with other particles thus accumulating in the different environmental compartments. Likewise, they are able to bind and later on release unwanted chemicals such as flame retardants or plasticiser into the environment. However, the contribution of nano- and microplastic-mediated chemicals’ transfer to the overall exposure levels of environmental organisms is small and does not increase the risk for these organisms ^[11].

Nanoplastic particles can also interact with various environmental organisms. Most research projects have been conducted employing primary polystyrene nanoparticles in laboratory short-term studies. Nanoplastic particles were found to attach to organisms surfaces and being taken up into the gut, which may lead to an impairment of their normal functioning. They do not exert very severe acute effects, but provoke sublethal effects after longer exposure periods. In some organisms, the effects of nanoplastic particles differ from those of microscale plastic particles ^[6].
Current estimates assume that environmental concentrations of nanoplastics are too low to provoke effects under environmental conditions. However, as the emission of nanoplastic particles into the environment will significantly increase in the next decades, long-term studies and chronic exposure levels are needed for a comprehensive risk assessment ^[8].

Therefore, reducing the emission of plastic in general and thereby indirectly also that of nanoplastic particles and accordingly the environmental burden is of great importance. Necessary steps towards this include the reduction of mismanaged plastic waste by the establishment of proper waste management systems all over the world. In addition, replacement or ban of one-way plastic products and microplastics in consumer products will help in reducing the plastic load in the environment and accordingly the emergence of nanoplastics. Also strategies to remove plastic particles from the environment are currently under development, but those will not be applicable to nanoplastic particles ^{[1, 8 12-16]}.

Current Research Activities related to nanoplastics

Many funding programmes have been set up both at German (BMBF-Initiative Plastic in the Environment) and European (JPI-Ocean) level to investigate the occurrence of plastics in the environment and the associated effects in more detail. For example, from 2016-2018, the WEATHER-MIC project studied the ecological effects of microplastics and its fragmentation into nanoplastic by weathering processes. The researchers used artificial weathering to fragment plastic debris to investigate distribution processes as well as the toxicity of the resulting plastic particles. As a result the researchers could show, that chemical leachates from different types of plastic induces oxidative stress response ^[17].

In 2019, the German research vessel “SONNE” went on a 5-week expedition to collect and analyse plastic particles of various size ranges across vertical and horizontal transects of the Pacific Ocean in order to understand the transport and transformation of plastic particles. Another ambitious project aiming at the removal of plastics from the oceans is “The Ocean Clean Up”. By using the waves to collect and subsequently recycle plastic debris they “aim to clean up 90% of ocean plastic pollution”. However, nanoplastic particles are not accessible for the developed technology, but the removal of macroplastics from the environment will reduce the future formation of nanoplastic ^[18].

The durability of plastic leads to a long-term accumulation of plastic particles in various shapes and sizes up to the nanoscale in the environment. However, reliable data of how nanoplastic is generated and distributed in the environment is rare. Current analytical methods are not yet able to distinguish plastic nanoparticles non-plastic nanoparticles. At present, nanoplastic in its estimated concentration range has no severe effects towards plants and animals. In the future, long-term effects need to be studied and analytical methods for detection need to be improved.

Literature

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