Our oceans all too often end up being the dumping ground and collection point for our waste and pollutants. This encompasses several waste streams, from chemical spillages to raw sewage and solid plastics.
Image Credit: David Pereiras/Shutterstock.com
Over recent years, there has been growing concern about the impact that growing amounts of microplastics might have on our oceans and their ecosystems.1 Microplastics are small particles of plastic that arise from the breakdown of larger pieces of plastic through physical abrasion and weathering or through breakdown from UV light and chemical reactions in the environment.
Microplastics are usually less than 5 mm in length. Part of current concerns arises from their sheer abundance in the environment, including remote locations both in the deep ocean and on mountains.1 The other concern is their potential toxicity. There is evidence to suggest that, particularly for young, developing aquatic life, microplastics are toxic.2 They can accumulate in the organs of fish and other sea life. Whether microplastics are also toxic for humans is unclear but their environmental abundance and persistence is a current cause of concern and for the exercise in caution to try and reduce microplastic production.
An important part of detecting and understanding the types of microplastics in our oceans so that measures can be taken to tackle the problem is having tools for the identification and classification of microplastics. Methods such as infrared spectroscopy and Raman have proved effective for particle size classification and identification, though infrared spectroscopy is somewhat less effective for microplastics with very small dimensions.3,4
Not all microplastics are made of entirely polymer-based materials. One other very common source of microplastic pollution in the oceans is paint flakes.5 Paint flakes differ greatly from other types of microplastic pollution not just in their dimensions but also in their physical properties. They tend to be more brittle with low polymer content and may also have many metal ions present. Generally, they have less heterogeneous than purely polymer samples and have the potential to pose even greater environmental threats as they often contain antifouling and anti-corrosion treatments.
X-Ray Spectroscopy
One of the first steps to characterizing paint flakes is to work out their elemental composition. X-ray spectroscopy coupled with scanning electron microscopy can help provide spatially resolved images that provide local information on the elemental composition.
X-ray spectroscopy is an ideal way of performing elemental identification as the high-energy photons used to interact with the electrons in the tightly bound core orbitals in a molecule. These orbitals tend to be very localized on a specific atom, meaning that even in a complex molecular species, the orbitals more closely resemble that of the isolated element rather than being extensively involved in the bonding framework of a molecule.
The effect of this localization is that ionization or interaction with the electrons in core levels tends to happen at very specific energies that are characteristic of the element being probed. While these energies are somewhat sensitive to the local chemical environment of the atom in question, the energies are usually distinct enough that the elemental type can be confidently identified. With sufficiently high incident photon energies, all the elements of the Periodic Table can be probed in this way, from the lighter organic elements such as carbon, to the heavy metals.
X-Ray and Microplastics
Recent work from the University of Plymouth and the Marine Biological Association as part of a multi-year ongoing study of marine pollutants has been using X-ray spectroscopy coupled with scanning electron microscopy to analyze seawater samples collected by the Marine Biological Association’s Continuous Plankton Recorder.
The Continuous Plankton Recorder is fitted with a mesh and towed behind a boat to sample seawater at depths that would be occupied by marine mammals. The Recorder has been all over the world – traversing the North Atlantic region to the Arctic Ocean, as well as Spain, the eastern United States, and Sweden. In that time, it has collected over 3000 samples that the team has subsequently analyzed using X-ray fluorescence techniques.
The main conclusions were that fibers and strands were among the most common types of pollutants – found in nearly 50% of all samples. Flakes were less common, present in 2.8% of the total samples taken. The North Atlantic was the region that seemed to be most heavily affected by paint debris – with many samples tested giving good matches to reference data taken from the paint on ships’ hulls.
X-ray fluorescence has been widely used for elemental identification in a range of sample types, including for the identification of pigments in art and other types of contaminants in soil samples. The results from the current studies about the abundance of metal-containing microplastics are likely to prompt further investigation and studies as to the exact nature of their environmental threat as well as differences in the behavior of more common polymers such as polyethylene and its microplastic formation.
References and Further Reading
1. Hale, R. C., Seeley, M. E., La Guardia, M. J., Mai, L., & Zeng, E. Y. (2020). A Global Perspective on Microplastics. Journal of Geophysical Research: Oceans, 125(1), 1–40. https://doi.org/10.1029/2018JC014719
2. Ma, H., Pu, S., Liu, S., Bai, Y., Mandal, S., & Xing, B. (2020). Microplastics in aquatic environments : Toxicity to trigger ecological consequences. Environmental Pollution, 261, 114089. https://doi.org/10.1016/j.envpol.2020.114089
3. Araujo, C. F., Nolasco, M. M., Ribeiro, A. M. P., & Ribeiro-Claro, P. J. A. (2018). Identification of microplastics using Raman spectroscopy: Latest developments and future prospects. Water Research, 142, 426–440. https://doi.org/10.1016/j.watres.2018.05.060
4. Jung, M. R., Horgen, F. D., Orski, S. V., Rodriguez C., V., Beers, K. L., Balazs, G. H., Jones, T. T., Work, T. M., Brignac, K. C., Royer, S. J., Hyrenbach, K. D., Jensen, B. A., & Lynch, J. M. (2018). Validation of ATR FT-IR to identify polymers of plastic marine debris, including those ingested by marine organisms. Marine Pollution Bulletin, 127(2017), 704–716. https://doi.org/10.1016/j.marpolbul.2017.12.061
5. Turner, A., Ostle, C., & Wootton, M. (2022). Occurrence and chemical characteristics of microplastic paint flakes in the North Atlantic Ocean. Science of the Total Environment, 806, 150375. https://doi.org/10.1016/j.scitotenv.2021.150375
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