Microplastic samples collected from the oceans. Plastic fragments less than 5 mm in length are categorized as microplastics. The scale bar shows a length of 2000 µm or 2 mm.
Newswise — Fukuoka, Japan— A collaborative effort between Kyushu University and Asahi Kasei Corporation has resulted in the development of a novel method for determining the age of microplastics discovered in the upper oceans. This approach combines the analysis of plastic oxidation levels with the consideration of environmental factors like UV exposure and ambient temperature. By integrating these variables, researchers can estimate the age of the microplastics present in the ocean, providing valuable insights into the persistence and degradation of plastic pollution.
The research team utilized their newly developed method to estimate the age of microplastics located in both nearshore and offshore areas of the North Pacific Ocean. Their analysis revealed that microplastics in nearshore regions were estimated to be between 0 and 5 years old, while microplastics in offshore regions ranged from 1 to 3 years old. These findings, which provide valuable insights into the age distribution of microplastics in different oceanic areas, were published in the scientific journal Marine Pollution Bulletin.
Plastics have become the predominant form of pollution in marine environments, spanning from lakes to oceans. Over time, plastic waste undergoes degradation and fragmentation due to exposure to various environmental factors. When plastic waste reaches a size smaller than 5 mm in length, it is referred to as "microplastics." These tiny particles constitute a significant portion of plastic pollution in aquatic ecosystems.
Professor Atsuhiko Isobe, from Kyushu University's Research Institute for Applied Mechanics and the leader of the study, acknowledges that microplastic pollution is a global issue of concern. In a previous study, his team discovered an astonishing estimate of approximately 24 trillion microplastic particles floating on the ocean's surface layer. However, the environmental impacts of microplastics and their effects on living organisms remain largely unknown. Furthermore, understanding the duration of microplastic drift in the ocean is another significant question that researchers are seeking to address.
To find out how old microplastics found in the ocean can be, Isobe and his team began by investigating what metrics could be used to measure microplastic age in the first place.
Rie Okubo, a researcher at Asahi Kasei Corporation and the first author of the study, provides insights into the degradation process of the most prevalent plastic material, polyethylene. Polyethylene undergoes oxidation and degradation as it interacts with the environment. The extent of this degradation can be assessed by measuring changes in the material's molecular weight and a parameter called the carbonyl index. When polyethylene degrades, its carbonyl index increases, while its molecular weight decreases. These indicators serve as measures to quantify the degradation level of polyethylene and its aging process.
To comprehensively understand the impact of temperature and UV radiation on plastic degradation, the research team recognized the need to establish standardized conditions. They conducted a series of exposure experiments using polyethylene material, systematically varying combinations of UV radiation and temperature. By collecting data on how these factors influenced the material's molecular weight and carbonyl index, the team gained insights into the specific effects of different temperature and UV exposure levels on the degradation process of microplastics. This allowed them to establish a framework for assessing the age of microplastics based on environmental factors.
The team found that UVER—ultraviolet erythemal radiation, a measurement of UV radiation at ground level—and seawater temperature were the two biggest contributors of plastic degradation.
With the collected data on the effects of UV radiation and temperature on polyethylene degradation, the research team proceeded to analyze their microplastic samples. The samples were obtained exclusively from the upper ocean, within a depth of one meter from the water surface. They sourced microplastics from various locations, including nearshore regions near Japan, spanning distances of 10 to 80 km off the coast. Additionally, they collected samples from offshore areas situated in the middle of the North Pacific Ocean and the Philippine Sea. By examining these samples, the researchers aimed to apply their findings and estimate the age of the microplastics based on the observed degradation levels.
By analyzing the collected microplastics, the team was able to estimate the age of each induvial sample. They found that nearshore microplastics ranged from 0 to 5 years old, whereas offshore samples ranged from 1 to 3 years old.
Rie Okubo puts forth a hypothesis to explain the observed age differences between nearshore and offshore microplastics. The team speculates that nearshore microplastics have a broader age range of 0 to 5 years because they are more frequently washed ashore and persist for longer periods in nearshore environments. In contrast, offshore microplastics take longer to reach those regions of the ocean, which could explain why the team did not find microplastics older than 3 years in offshore samples. Additionally, the offshore microplastics might be subjected to settling deeper into the waters, thereby being removed from the upper ocean layers where they were originally sampled.
The researchers hope that the new method will give them better insights into how microplastics are generated and spread in the environment. The data will also help in developing more accurate simulations to track microplastics across the ocean.
Professor Atsuhiko Isobe acknowledges that the research on microplastics is still in its early stages, and the data obtained in this study has contributed to enhancing our understanding of the fundamental science behind microplastics. Building upon these findings, the next phase of their research will focus on investigating the effects of mechanical stimuli such as ocean waves and currents on plastic degradation. By studying these factors, the team aims to gather more precise and comprehensive data, further advancing our knowledge in the field of microplastics.
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