Alkaline extraction yields a higher number of microplastics in forest canopy leaves: implication for microplastic storage

Airborne microplastics are a type of suspended particulate matter less than 100 µm in size. They have drawn attention recently due to their potential impact on human health and the environment. However, knowledge on airborne microplastics in forest and their interaction with plant leaves is limited. Here, we analyzed microplastics on konara oak leaves collected at a small forest in Tokyo. Leaves were water-washed to yield a first extract, sonicated in water to yield a second extract and then extracted with 10%w potassium hydroxide to yield a third extract. We employed micro-Fourier transform infrared spectroscopy with attenuated total reflection imaging to analyze microplastics, identifying polymer materials and quantifying their concentration. Results show that the average number of microplastics in leaf were 0.01 piece/cm² in the water extract (7.6%), 0.05 piece/cm² by sonication (38.4%), and 0.07 piece/cm² in the potassium hydroxide extract (53.8%). Microscopic data reveal that potassium hydroxide extraction allows to remove epicuticular waxes including adhering substances. These findings highlight the need to use a strong basic reagent, potassium hydroxide, to extract most airborne microplastics in leaf. The findings also suggest that canopy leaves could be a long-term sink for airborne microplastics, rather than merely temporary accumulators.