四日市大学論集
吉崎海岸の漂着マイクロプラスチックの動態と劣化特性の研究
2025 年 37 巻 2 号 p. 335-357
詳細
2025 年 37 巻 2 号 p. 335-357
We conducted a long-term survey of microplastics (MP) drifting ashore on the Yoshizaki Beach using a spatially dense arrangement of observation points. The average value of MP number density for all observation points and all observation periods was 9,356 pieces/m2, and polymer coatings of control release fertilizer (CRF) accounted for approximately 75% of the total. The number density of MP increased rapidly from June to August, suggesting a causal relationship with paddy plowing works. The gravity center of the number of MP along the beach showed a temporal variation and the MP types with smaller apparent specific gravity, such as pieces of foamed plastics, travel a longer distance than other plastic pieces. Most of the MP moved toward the southern part of the beach during autumn to winter and drifted away to the ocean until spring. The seasonal wind might be the main cause the drifting motion. A 150-day or 120-day sunlight exposure test was conducted on five types of polymer coatings of CRF. All samples developed discoloration and fragmentation during the test period, but the changes varied considerably with types. The weight loss rate during the test period ranged from 2.5% to 33.5%. The carbonyl index of the polyethylene (PE) coatings increased linearly as the number of days during the test. The carbonyl index of PE coatings collected during the survey of Yoshizaki Beach showed the annual fluctuations and it reached its maximum and minimum in February and from August to October, respectively. The recruitment of new plastic pieces of CRF during spring to summer and the photodegradation on the beach during summer to autumn is assumed to be the cause of the fluctuations. Comparison with the results of the sunlight exposure test showed that the number of exposure days of the PE coatings of CRF collected at Yoshizaki Beach ranges from 28.9 to 55.5 days. This number of days may be much smaller than the manufacturers of the fertilizer expected
