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Plastics are important materials for a number of applications, products and industries. While many of them can be recycled, there are a number of growing concerns that plastics are having a significantly detrimental impact on the environment, be it through not decomposing in landfills to micro and nanoplastics, to being found in most of the fish within our oceans.
It’s likely that their use will continue for many years to come but more efforts are being sought to see whether a plastic will be mineralized or remain in the environment for many years. One of these methods is to use carbon-labelled (13C) polymers.
Bulk polymers are a concern because they take many, many years to decompose and break down. However, to the wildlife, and in particular the marine life, on our planet, nanoplastics present a much more challenging problem. Nanoplastics are nanosized plastic particles that are typically smaller than a few micrometers and are formed as larger plastics break down over time into smaller fragments.
Nanoplastics can present many issues in the environment. One of the most documented is their presence in the gastrointestinal tract (GIT) of various fish and marine animals. However, implications could be more severe if research finds that these nanosized polymers can migrate into tissues (which is possible given their small size).
Evidence of these migrations has not been confirmed, but regardless of the findings of that research, the presence of nanoplastics in our oceans is challenging on many levels.
The Need for Assessing Plastics in the Environment
One reason for assessing plastics is to determine their distribution in the environment. These are the issues mentioned above and are known as transfer processes. Plastics also need to be assessed from a chemical perspective to see how likely they are to biodegrade over time, especially if they are a polymer which is marked as biodegradable. These are known as transformation processes and can alter the physiochemical properties of a polymer, leading to them disintegrate and degrade into nanoplastics
While transfer processes need to be analyzed to determine the macro scale of polymer pollution, on an individual level, analyzing the transformation processes is important for determining if a polymer is indeed biodegradable.
However, this is much easier said than done as there are number of factors that need to be considered — including material properties such as size and morphology as well as system properties such as temperature and water chemistry. Many studies use an extrinsic label, that is, a label that is extrinsic to the carbon backbone of the polymer, but these are only useful for studying transfer processes.
To study the transformation processes of plastics, an intrinsic label is needed because the transformation process directly affects the carbon backbone of the polymer chains. These can include processes such as depolymerization, crosslinking and oxidation reactions that destabilize the polymer backbone, causing them to break down into nanosized pieces of plastic, i.e. nanoplastic.
From here, the nanoplastics can either form other derivative molecules that will be taken up by microorganisms or mineralized into the Earth, or they can stay as harmful nanoplastic materials.
What drives these processes, and what is the likely end point in the polymer’s life cycle, are two key questions currently being investigated for a range of polymers, with the aim of seeing what the likely exposure rate is for various organisms and ecosystems that are exposed to these materials.
What are C-Labelled Polymers and Why Use Them?
Carbon-labelled (C-labelled) polymers are a type of intrinsic label which are labelled with carbon-13 (13C) along parts of the polymer backbone. Carbon-13 is a stable isotope of carbon with an extra neutron in its nucleus and can be used to selectively track the plastic as it goes through the different transformation processes from a bulk material to a nanoplastic.
Carbon-13 gives off different signals to analysis instruments than carbon-12 does, so the fragments with carbon-13 atoms can be easily identified with techniques such as gas chromatography (GC), isotope ratio mass spectrometry (IRMS), and 13C-isotope sensitive cavity ringdown spectroscopy.
Many analyses in other areas use radioactive isotopes such as carbon-14 (14C), but the extra handling and safety requirements needed for this kind of analysis makes radioactive isotopes an unfeasible choice. Conversely, carbon-13 does not leave any radioactive waste behind and their incorporation into polymers is much simpler than with radioactive isotopes.
Using C-Labelled Polymers as Intrinsic Labels for Better Environmental Analyses
Because carbon-13 isotopes can be analyzed easily and tracked, they enable researchers to see how a plastic breaks down into a nanoplastic and what happens to the nanoplastic in different environments.
The use of C-labelled polymers is also a useful technique for the polymers which are specifically marked as biodegradable, as their use is often directly related to their release in some applications, e.g. in agriculture for reducing the accumulation of plastics in soil compared to non-biodegradable plastics.
In these scenarios, one of the requirements is to show that the plastic (and subsequent nanoplastic) will break down into biomass and carbon dioxide gas, and this is a specific process that can be monitored with C-labelling methods.
There is an ongoing debate with agricultural polymer materials as to whether some “biodegradable” polyethylene films are fully biodegradable or whether they are partially degradable and therefore generate nanoplastics into the nearby soil and any surrounding environments (through being carried by moving water, soil movement etc.), but C-labelling is an approach that could be used to solve this debate.
C-labelling methods may also prove useful for assessing the transformation and environmental impacts of using polymers that are made up of more than one different monomer unit (or plastics that contain a blend of different polymers), as the analyses will show if all the organic constituents break down fully.
C-labelling can also be extended to polymer additives, such as plasticizers and photostabilizers to see if they have an adverse effect on the environment when disposed.
References and Further Reading
“Nanoplastic should be better understood”- Nature Nanotechnology, (2019), DOI: 10.1038/s41565-019-0437-7
“Things we know and don’t know about nanoplastic in the environment”- Wagner S. and Reemtsma T., Nature Nanotechnology¸ (2019), DOI: 10.1038/s41565-019-0424-z
“Assessing the environmental transformation of nanoplastic through 13C-labelled polymers”- Sander M. et al, Nature Nanotechnology, (2019), DOI: 10.1038/s41565-019-0420-3