Researchers conduct batch adsorption testing in different soil types to understand the adsorption and aggregation behavior of nanoplastics in soil

Nanoplastics are an increasing threat to the ecosystem; however, their mobility in the soil is still underexplored. Against this backdrop, researchers from Waseda University and the National Institute of Advanced Industrial Science and Technology investigated the adsorption and aggregation behavior of nanoplastics in different types of soil under different pH conditions. The study offers new perspectives on the migration and environmental interactions of nanoplastics, while broadening our knowledge of pollution dynamics and soil contamination processes.

Plastics are everywhere—from packaging and textiles to electronics and medical devices. As plastic waste breaks down, it releases microscopic particles that can penetrate our ecosystems, hinder plant growth, and potentially transfer harmful pollutants to organisms, including humans. Therefore, these plastic particles are a potential threat to the ecosystem, especially in their nanoparticulate form (1–100 nm diameter), which can penetrate the environment through different routes, including the soil beneath our feet.

With this in mind, a team of researchers from Japan set out to study the migration behavior of nanoplastics in different soil types. The study was led by Kyouhei Tsuchida, a PhD student from the National Institute of Advanced Industrial Science and Technology (AIST) and Waseda University, Japan, with fellow students Yukari Imoto, Takeshi Saito, and Junko Hara also from AIST, and Professor Yoshishige Kawabe from the Department of Resources and Environmental Engineering, Waseda University. This study was published online in the journal Science of The Total Environment on April 4, 2025.

The researchers focused on the adsorption of the nanoplastics on soil and the aggregation characteristics of both the nanoplastics and soil particles under varying pH conditions. “The aggregation properties of nanoplastics and their adsorption onto soil particle surfaces are known to affect their migration in soil,” notes Tsuchida, “We conducted experiments to analyze these traits to get a better understanding of the migration of nanoplastics.” The research team focused on three major aspects. First, the homo or self-aggregation of the nanoplastics. Second, the adsorption properties of the nanoplastics onto soil, and third, how the adsorption of nanoplastics affects the aggregation of soil particles.

To understand the behavior of the nanoplastics under different soil conditions, the researchers used two different types of soil: andosol (volcanic soil) and fine sand. “Both andosol and fine sand have extremely different properties, and we utilized these two to get a broader idea of how the behavior of nanoplastics changes with respect to soil composition and surface characteristics,” explains co-author Hara.
For the self-aggregation studies of the nanoplastics, the team first prepared a suspension of polystyrene nanoparticles under three different pH conditions. Further, they determined its particle size, aggregate particle size, and zeta potential—a measure of the electrical charge on particle surfaces, which helps determine the stability of nanoparticles.

Additionally, the researchers tested the adsorption properties of the polystyrene nanoparticles onto the two soil types under varying pH conditions. To analyze the adsorption behavior, the researchers used batch adsorption testing. “We used batch adsorption testing to gain a deeper insight into how plastic particles accumulate in soil pores. This property hasn’t been well explained in column studies,” explains co-author Kawabe.

The analysis of aggregation and adsorption involved advanced instrumental techniques, including laser diffraction, UV spectroscopy, and zeta potential analysis. According to the results, no aggregation was observed in the polystyrene nanoparticles owing to the high negative charge on the polystyrene nanoparticles. “The highly negative zeta potential of the polystyrene nanoparticles causes repulsion between the particles and remains unaffected by pH changes,” reports Tsuchida. “This was in contrast to that observed for the adsorption properties of the nanoplastics onto soil. Polystyrene nanoparticles adsorbed onto soil, which was influenced by pH, and further, aggregation of the soil particles.”

The results, therefore, suggest that the soil type and pH of the solution can critically alter the movement of nanoplastics in the soil. Understanding these crucial aspects could help to reform policies and strategies for mitigating plastic pollution.

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