Microplastics: How Do They Spread?

Listicle 1 Microplastics: How Do They Spread? Alexander Beadle From remote and pristine regions of the Pyrenees1 to the deep-sea floor,2 small fragments of plastic can be found the world over. With such a broad geographic spread, these microplastics — the umbrella term given to tiny plastic particles or fibers typically measuring less than 5 mm — have become ubiquitous in the modern world. Broadly, microplastics can be divided into two categories: primary and secondary microplastics.3 Primary microplastics are plastic particulates designed for commercial use, such as the microbeads sometimes included in personal care products or the synthetic fibers shed from clothes during washing. In contrast, secondary microplastics are formed when larger plastic products, such as discarded bottles or plastic bags, begin to decompose and break down in the environment. Once these particles enter the environment, there is a risk that the microplastics could be ingested by fish, local animals or humans who unknowingly eat contaminated foods. While the health effects of microplastics are still not fully understood, current research indicates that microplastic exposure can cause tissue damage, oxidative stress and changes in immune-related gene expression for marine life.4 In humans, accidental consumption is also thought to be linked to problems with cytotoxicity, neurotoxicity, immune system disruption and the accumulation of microplastics in different body tissues.4 But how does this exposure happen? Research suggests that our soil, waterways and even the air around us can be a source of microplastic contamination. Fortunately, researchers have a number of tools at their disposal that can help to identify and quantify cases of serious contamination,5 with several commercial-scale cleanup technologies also helping to stop the continued spread of such microplastics. Routes of microplastic contamination There are three main avenues through which humans, animals and the wider environmental ecosystem might be exposed to microplastics. These are: Soil There are multiple routes that microplastics might take to enter the soil.6 The breakdown of discarded plastic items that have been sent to landfill is a major contributor. Other sources of contamination might include tire wear and tear, soil amendments or the spreading of contaminated sewage sludges or wastewater on farmland. MICROPLASTICS: HOW DO THEY SPREAD? 2 Listicle Once they are in the soil, microplastics can have a significant impact on their local environment. Plants may struggle with proper nutrient metabolism and absorption if microplastics are present at high enough levels, as the microplastics can disrupt soil microorganisms and thus affect the normal microbiome.7 There is also the risk that microplastics could enter the broader food chain once they have been introduced to fertile land, increasing the risk of accidental consumption by animals and humans. Current research indicates that microplastics may also act as a vector for other harmful pollutants, carrying residual chemicals into new environments where they could present additional risks to life.8 Water Microplastics may be present in seawater, freshwater, wastewater or groundwater, although the exact source of the plastic varies significantly. Microplastic fibers may be relatively common in seawater, for example, as fishing nets begin to degrade and release material into the oceans. The microplastics found in wastewater are more likely to be primary microplastics from personal care products being used and washed down the drain.9 Rainwater is another interesting case, as this water can act as a powerful vector that transports microplastics out into soil or other aquatic environments, often picking up other contaminants along the way. One study conducted in the Nakdong River, South Korea, found that 70–80% of the annual microplastic load is moved during the country’s rainy season.10 In a marine environment, microplastics will gradually fragment into greater numbers of smaller microplastic particles through physical abrasion, ultraviolet (UV) irradiation and biodegradation. At such small particle sizes, these microplastics may pose even more substantial risks to aquatic life by penetrating cell membranes and causing cellular damage.9 This presents a significant danger to the rest of the food chain too, as predators run the risk of consuming prey containing large amounts of plastic accumulated in their tissues, thus furthering the spread of these microplastics. Air Due to their incredibly small size and low density, it is also possible for microplastics to be picked up in the wind and blown from place to place.11 This means that humans and animals are actively breathing in atmospheric microplastics as they go through daily life. Indeed, current research simulations suggest that humans may breathe in nearly 300 microplastic particles during a day of light activity.12 Microplastic particles have already been found in human lung tissue samples; however, there is still a general lack of information regarding the effects of these particles on the respiratory system and the mechanisms through which they might contribute to lung disease.13 How is microplastic contamination detected? Visual inspection using a microscope or stereoscope can be sufficient to detect the presence of microplastics in simple sample matrices. Despite being a relatively time-consuming technique, even when assisted by the use of special dyes, a recent review estimates that visual inspection is still used in just under one-third of microplastic analyses of water and sediment samples.14 According to the same review, the most frequently used methods of analysis for water and sediment samples are Fourier-transform infrared (FTIR) spectroscopy-based methods. This spectroscopy technique uses the vibrational modes of different molecules in order to identify certain characteristic structures within a sample that will indicate its chemical makeup. Raman spectroscopy, another vibrational spectroscopy method, is also commonly used for microplastic analysis. MICROPLASTICS: HOW DO THEY SPREAD? 3 Listicle Pyrolysis-gas chromatography-mass spectrometry (pyro-GC-MS) is another frequently used method of analysis wherein microplastic samples are pyrolyzed in an inert atmosphere and the resulting gases are analyzed.14 By its nature, this method is destructive and does not allow an analyst to glean any information regarding the number of microplastic particles in a given sample. However, for the bulk analysis of a large sample, this technique can provide very accurate characterizations of the different plastics present within very complex matrices. Microplastic remediation An effective microplastic remediation technique must take care not to create more microplastics inadvertently, either through incomplete degradation or additional fragmentation. For this reason, coagulation is one of the most frequently used techniques for removing microplastics at commercial wastewater treatment facilities.15 The technique works by using different chemical coagulants to destabilize the microplastic particles suspended in the wastewater, promoting sedimentation and easy removal when passed through a filter. However, this is not a perfect solution to microplastic pollution. At the industrial level, the coagulation process will produce significant amounts of sludge that may pose its own risk to the environment if not adequately treated. There are other, more experimental techniques for microplastic remediation that are currently being trialed.15 One of these is photocatalytic degradation. This method makes use of a semiconductor material that absorbs visible or UV light and, in the process, generates free radicals, which degrade the microplastics. However, while this technique does effectively destroy microplastic particles, it still generates some sludge waste that must be monitored and disposed of carefully to prevent damage to the environment. Another novel technique that is attracting significant attention is electrocoagulation. A sustainable and costeffective alternative, electrocoagulation operates on the same principle as traditional coagulation but without the use of chemical coagulating agents. In electrocoagulation, metallic electrodes are used to produce cations that can promote flocculation between the microplastic particles. This produces significantly less sludge waste than the traditional approach, which is a boon for both the environment and the wastewater treatment plant, that no longer has to deal with the expensive post-processing and monitoring steps needed for proper sludge disposal. As a material, plastic has revolutionized our modern world. Its impressive durability and strength-to-weight ratio has enabled significant advancement in a wide variety of disciplines, including building and construction, electronics, textiles, consumer products and transportation. But now more than ever, scientists are aware of the dangers presented when these products are disposed of improperly and are allowed to break down to form harmful microplastics. For today’s researchers, it is crucial that we improve our understanding of how these plastic particles are being spread, so that actions can be taken to remediate cases of contamination and stem the flow of these plastics into our environment. Sponsored by MICROPLASTICS: HOW DO THEY SPREAD? 4 Listicle References 1. Allen S, Allen D, Phoenix VR, et al. Atmospheric transport and deposition of microplastics in a remote mountain catchment. Nat Geosci. 2019;12(5):339-344. doi:10.1038/s41561-019-0335-5 2. Barrett J, Chase Z, Zhang J, et al. Microplastic pollution in deep-sea sediments from the Great Australian Bight. Front Mar Sci. 2020;7. doi:10.3389/fmars.2020.576170 3. Microplastics: sources, effects and solutions. European Parliament. Published November 22, 2018. Accessed June 1, 2023. https://www.europarl.europa.eu/news/en/headlines/society/20181116STO19217/microplastics-sources-effects-and-solutions 4. Bhuyan MdS. Effects of microplastics on fish and in human health. Front Environ Sci. 2022;10. doi:10.3389/fenvs.2022.827289 5. Bose P. Microplastic detection. Technology Networks. Published August 31 2021. Accessed June 1, 2023. http://www.technologynetworks.com/applied-sciences/articles/microplastic-detection-353032 6. Guo JJ, Huang XP, Xiang L, et al. Source, migration and toxicology of microplastics in soil. Environ Int. 2020;137:105263. doi:10.1016/j.envint.2019.105263 7. Sun Y, Yang C, Liang H, et al. Health risk analysis of microplastics in soil in the 21st century: A scientometrics review. Front Environ Sci. 2022;10. doi: https://doi.org/10.3389/fenvs.2022.976237 8. Sajjad M, Huang Q, Khan S, et al. Microplastics in the soil environment: A critical review. Environ Technol Innov. 2022;27:102408. doi:10.1016/j.eti.2022.102408 9. Kye H, Kim J, Ju S, Lee J, Lim C, Yoon Y. Microplastics in water systems: A review of their impacts on the environment and their potential hazards. Heliyon. 2023;9(3):e14359. doi:10.1016/j.heliyon.2023.e14359 10. Eo S, Hong SH, Song YK, Han GM, Shim WJ. Spatiotemporal distribution and annual load of microplastics in the Nakdong River, South Korea. Water Res. 2019;160:228-237. doi:10.1016/j.watres.2019.05.053 11. Abad López AP, Trilleras J, Arana VA, Garcia-Alzate LS, Grande-Tovar CD. Atmospheric microplastics: exposure, toxicity, and detrimental health effects. RSC Adv. 13(11):7468-7489. doi:10.1039/d2ra07098g 12. Vianello A, Jensen RL, Liu L, Vollertsen J. Simulating human exposure to indoor airborne microplastics using a Breathing Thermal Manikin. Sci Rep. 2019;9:8670. doi:10.1038/s41598-019-45054-w 13. Lu K, Zhan D, Fang Y, et al. Microplastics, a potential threat to patients with lung diseases. Front Toxicol. 2022;4:958414. doi:10.3389/ftox.2022.958414 14. Prata JC, da Costa JP, Duarte AC, Rocha-Santos T. Methods for sampling and detection of microplastics in water and sediment: A critical review. TrAC Trends Anal Chem. 2019;110:150-159. doi:10.1016/j.trac.2018.10.029 15. Osman AI, Hosny M, Eltaweil AS, et al. Microplastic sources, formation, toxicity and remediation: a review. Environ Chem Lett. 2023. doi:10.1007/s10311-023-01593-3