New Study Shows Soil Type and pH Impact Nanoplastic Movement in Soil

Plastic contamination is still an ongoing environmental challenge, especially due to the prevalence of nanoplastics—minuscule plastic fragments less than 100 nanometers in size. They will pass through the environment via water, air, but most particularly soil, eliciting serious apprehension regarding their capacity to lead to severe devastation for agriculture, vegetation, and even the earth. In a bid to know more about how nanoplastics act once they are present in the soil environment, researchers from Japan investigated the effect of pH and soil type on plastic nanoparticles' mobility in a comprehensive study.

The study was led by a team from Waseda University and the National Institute of Advanced Industrial Science and Technology (AIST), and it involved Kyouhei Tsuchida. The study aimed to investigate nanoplastic behavior in various soils as well as movement caused by environmental factors. The April 4, 2025, publication by Science of the Total Environment brings fresh understanding of nanoplastic aggregation, adsorption onto the soil, and influencing soil particle behavior.

Three behaviors were examined in the research: nanoplastic self-aggregation, adsorption onto soil particles, and the effect of this adsorption on the soil aggregation. The experiments involved polystyrene nanoparticles, one of the most widely occurring forms of nanoplastic, and two soils—andosol, a volcanic and organic matter-rich soil, and fine sand, which is characterized by its relatively inertness.

The nanoplastics were suspended into solutions of three varying pH to monitor changes in behavior for the particles. The particle size, aggregate size, and zeta potential—a measure of surface charge that is characteristic of particle stability—were characterized. Zeta potential measurements confirmed that the polystyrene nanoparticles were significantly negatively charged and thus did not aggregate at any of the tested pH values. This suggests that pH change had no influence on the stability of the nanoplastic particles themselves.

Soil interactions, however, yielded varied results. Though the nanoplastics did not self-aggregate, they adsorbed to soil surfaces, and the procedure was significantly dependent on the surrounding solution's pH. Adsorption testing was also performed, and it showed that acidic or alkaline conditions modified the extent to which nanoplastics could bond to the soil particles. These adsorption behaviors also affected the manner in which the soil particles were coming together, which is potentially affecting water movement and soil structure.

The scientists employed batch adsorption testing to examine how nanoplastics get stuck in soil pores. Using this technique, they were able to test how particles interact when combined with soil, as opposed to simply passing through it. In contrast to earlier column-based experiments, this technique gave a more accurate picture of how plastics may settle or get stuck in soil conditions.

Andosol and fine sand responded very differently to the experiments. Andosol, being more organic and with a higher surface area, showed stronger interactions with nanoplastics compared to fine sand. These results suggest that the soil composition plays a significant role in determining how nanoplastics flow and settle in different environments.

Instrumental techniques such as laser diffraction, UV spectrometry, and determination of zeta potential were employed to validate the results. That nanoplastics do not auto-aggregate and adsorb on soil pH-dependently suggests that nanoplastics will be more prone to aggregate more under some soils compared to others. Such results can guide future studies on nanoplastic behavior and the development of more effective environmental monitoring and control measures.

To identify how soil type and pH level affect the migration of nanoplastics is central to evaluating the long-term hazard to agriculture and natural ecosystems. Because of soil variability globally from composition to pH, the study emphasized the importance of regional area-specific pollution management techniques in response to soil types.

Conclusion
The present work provides valuable information regarding the relationship between nanoplastics and varying soil types in varying pH. Although the particles themselves are pH-stable, adsorption to soil and resultant changes in soil aggregation depend on soil type and pH. These findings have the potential to shape policy to reduce the extent and severity of nanoplastic presence in the environment. As plastic use continues to grow, studies like this become increasingly important to direct international strategy regarding micro and nanoplastic pollution.

Source/Credits:
Source: Waseda University
Published Research: Science of the Total Environment (DOI: 10.1016/j.scitotenv.2025.178712)