Nano- and microplastics are a well-known threat found almost everywhere in nature, including soil, oceans, drinking water, air, and even the human body. Studies have shown that soils in particular account for a significant portion of N/MP. The problem with these N/MPs is that, due to their microscopic size, they can easily migrate through the soil into groundwater or freshwater bodies due to rainwater leaching. From there they enter the human body. Therefore, understanding the distribution and movement of N/MP in soil is essential to measure and mitigate threats.
Current techniques for measuring N/MP concentrations in soil require separation of soil organic matter content through chemical and physical processes. The isolated N/MPs are then analyzed using microscopy, Fourier transform infrared spectroscopy, pyrolysis gas chromatography/mass spectrometry, or Raman spectroscopy. However, these techniques require advanced techniques and have limited resolution for analyzing N/MPs smaller than 1 μm. Additionally, during the separation process, some of the N/MP in the soil is often lost, making measurements inaccurate. Therefore, it is necessary to develop a simple yet accurate method to detect and measure N/MPs below 1 μm in soil.
To this end, a team of researchers led by Tsuchida Kyouhei, a researcher at the National Institute of Advanced Industrial Science and Technology at Waseda University, and Dr. Yukari Imoto, Dr. Takeshi Saito, and Dr. Junko Hara from the National Institute for Advanced Study formed a team. Dr. Yoshishige Kawabe of the Department of Industrial Science and Technology and Waseda University has devised a new and simple method to measure N/MP concentrations in soil using spectroscopy without isolating soil organic matter. Spectroscopy can determine N/MP concentrations in soil based on how much light of a specific wavelength passes through the sample and how much is absorbed. In this way, spectroscopy can potentially detect N/MPs regardless of their size if the correct wavelength is used to distinguish N/MPs from soil. Therefore, the researchers developed a method that uses the difference in the absorption spectra of N/MP and soil particles to quantify N/MP. Their research results Ecotoxicology and environmental safetyDiary dated May 28, 2024.
Six soil suspensions were generated from soil samples with different characteristics, including particle size distribution and organic matter content, and mixed with 203 nm sized polystyrene nanoparticles. This resulted in six different simulated N/MP contaminated soil suspensions where the N/MP concentration was maintained at 5 mg/L. “We used a spectrophotometer to measure the absorbance of these soil suspensions at different wavelengths ranging from 200 to 500 nm and determined the N/MP concentration in the soil based on this. We then determined the best of the two wavelengths for N/MP measurements. The combination was confirmed, which helped nullify the interference of soil particles and leached components in the suspension,” explains Tsuchida.
The researchers found that a combination of wavelengths of 220-260 nm and 280-340 nm had the lowest error level for six samples and was therefore suitable for measuring N/MP concentrations in various soil types. They also created a calibration curve between the N/MP concentration in the soil suspension and the N/MP content added to the dry soil samples. The calibration curve showed a linear relationship between the two variables and took into account the adsorption of N/MP onto soil particles. This allowed us to accurately estimate the N/MP concentration in the soil.
These results demonstrate the efficacy of a simple spectroscopy-based method to accurately measure N/MP concentrations in soil without cumbersome separation procedures. “Our new measurement approach can quantify a variety of N/MPs, including polyethylene and polyethylene terephthalate, in a variety of soils and can be easily used as an initial assessment tool. It also helps to better understand the distribution and transport behavior of N. This could be an MP in the geospheric environment,” Tsuchida concluded.