By Ching Nam Wong (Jimmy)
Plastics are one of the most abundant materials being used daily. Plastics are based on polymers and are processed by different chemical additives which give the plastic different properties suitable for use in cosmetics, clothing, and industrial processes. ‘Microplastics’, or plastics that are less than 5mm in diameter, often pollute the environment as they take a long time (>1000 years) to be naturally degraded. This property makes microplastics a harmful pollutant that can persist and accumulate in the food chain and cause harm for many years to come (INTERNATIONAL MARITIME ORGANIZATION, 2015). Microplastics are divided into Primary Microplastics and Secondary Microplastics. Primary Microplastics are manufactured directly as small plastic beads for uses such as in toothpaste, cosmetics, industrial purposes. Conversely, Secondary Microplastics are the product of bigger plastic debris being broken down into smaller pieces through biological/physical/chemical/photo-degradation (Wagner & Lambert, 2018).
Microplastics have been present in our ecosystem since as early as the 1970s, where Carpenter et al. reported the abundance of plastics debris in the ocean (E J Carpenter et al., 1972). However, microplastic pollution did not get the attention it warrants as an environmental concern until the early 2000s.
Most of our understanding of how plastics degrade are based on laboratory experiments investigating that focus on a single mechanism, such as photo-degradation, biodegradation, or thermal degradation (Scott Lambert et al., 2013). In these processes, big plastic debris is degraded into microplastics, nano-plastics, and eventually non-polymer organic compounds. Kinetic models have been developed to calculate the rate of plastic fragmentation and predict plastic end-product fragment based on chain scission of the polymer backbone, which is crucial in plastic degradation. Chain scission generally happens through the combination following four ways: random chain scission by oxidation; chain scission of polymer mid-point by mechanical degradation; chain-end succession by photo and thermal degradation; and inhomogeneity, which is the probability each bond will be broken and dispersed (McCoy & Madras, 1997). All these factors make prediction of degradation patterns difficult. Furthermore, the type and amount of chemical additives in plastics gives the prediction another layer of complexity. For example, antioxidants and antimicrobial chemicals tend to slow down plastic degradation, whereas biological compounds such as starch in biodegradable plastics speed up degradation.
Overall, how plastics degrade depends on the environmental condition, amount/types of chemical additives, and the physical properties of the plastic polymer itself. This understanding of plastic degradation is important for understanding more about the distribution, formation, and accumulation of microplastics in the environment. With the widespread and persistent microplastic pollution around the globe, microplastics are attracting more research attention for further investigation.
Once primary/secondary microplastic is been released into the environment, they are often taken up by organisms through either ingestion/drinking or dermal uptake (i.e. gills). A study by Rosenkranz show that zooplanktons Bosmina coregoni and Daphnia cucullate cannot distinguish between microplastic beads and algae, so they ingest microplastics (Philipp Rosenkranz et al., 2009). How microplastics cause physical stress to organisms depends on microplastic size and shape. As a bigger microplastic diameter could affect the retention rate, and sharper/triangular microplastics could lead to irritation the digestive tract. Studies of Daphnia magna show exposure to high levels (105,000 particles L-1) of secondary microplastics with mean diameter 2.6 μm will lead to higher mortality, longer inter-brood period, and lower reproduction rate. Surprisingly, an identical experiment was carried out on primary microplastics, but no harmful effect was (Shima Ziajahromi et al., 2017).
Apart from physical stress, chemicals from plastics could leach out creating chemical stress to the environment. These chemicals can be impurities, solvents, catalysts, etc. In aqueous environments, some of these chemicals that are soluble in water can leach out, forming a leachate of chemical mixture. Lithner’s team studied such mixture’s effect on D. magna. He obtained leachate mixture from polyvinyl chloride (PVC), polyurethane (PUR), and polycarbonate (PC), for liquid to solid ratio (L/S) = 10 and 1 day leaching time. The results showed these leachates of chemicals are toxic to D. magna, with EC50 values of 5 to 69 g plastic L-1. This toxicity can be increased with longer leaching time (Delilah Lithner, Ildikó Nordensvan & Göran Dave, 2011).
It is certain that microplastics are harmful to the environment. But will it affect humans and other organisms not in the aqueous environment? A recent study shows the possibility of microplastic transfer between different trophic levels, where microplastics transfer from Mytilus edulis to Carcinus maenas through ingestion (Paul Farrell & Kathryn Nelson, 2013). It is clear that physical and chemical stress imposed by microplastics could potentially move all the way up the food chain, even affecting humans. However, it is still unknown whether bioaccumulation and biomagnification will occur for microplastic and its toxins. If they do, microplastic pollution will become an even bigger environmental concern.
With rising concerns over microplastic pollution, governments and international organizations around the globe are implementing policies to reduce plastic wastes. The UN’s Agenda for Sustainable Development aims to reduce the release of chemicals and wastes including plastics air, soil, and water by internationally agreed frameworks (United Nations, 2015). The G7 also acknowledged that marine litter – especially plastic litter poses a global challenge, affecting marine life, ecosystem, and potentially human health. The G7 action plan listed that they would reduce the use of single-use disposable packaging by giving incentives. They also agreed to encourage businesses to develop more sustainable packages to reduce plastic waste (G7, 2015). The Green Dot Initiative in Europe give companies the responsibility of reducing their packaging waste and take partake in re-use take-back schemes (European Parliament on packaging and packaging waste. 1994). The Clean Oceans Initiative launched in 2018 aim to provide money lending and technical assistant for any project that removes the waste in waterways, including microplastics.
Technology-wise, The Ocean Cleanup foundation has plastic collecting devices which reduce microplastics near coastal areas. They also launched Wilson Ocean clean-up system at the Great Pacific Garbage Patch to clean up plastic wastes (Martini Kim, Goldstein & Miriam, 2014).
In conclusion, microplastic pollution is indeed a global challenge, as they persist in the environment for a long time and have the ability to move up the food chain, while threatening the environment and potentially human health both physically and chemically. Therefore, we must raise public awareness and give more research attention to understand microplastic pollution.
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