Spectroscopic Method Measures Soil Contaminants with Ease, Accuracy

A method developed at Waseda University and the National Institute of Advanced Industrial Science and Technology (AIST) provides a simple route to accurately assess the concentration of nano- and microplastics (N/MPs) in different soil types.

Current methods for measuring N/MP concentrations in soil require the separation of soil organic matter content through chemical and physical processes. Afterwards, the isolated plastics are analyzed using a microscope, Fourier-transform infrared spectroscopy, Pyrolysis-gas chromatography/mass spectrometry, or Raman spectrometry.

The problem is that these methods require advanced skills and are limited in their ability to measure N/MPs smaller than a micron. Making matters worse, some of the N/MPs in the soil are lost during the separation process which further compromises accuracy.

A spectroscopy technique developed by researchers at Waseda University and the National Institute of Advanced Industrial Science and Technology (AIST) can measure nano- and microplastic (N/MP) concentrations in the soil with greater accuracy and simplicity compared to conventional methods. Courtesy of Kyouhei Tsuchida/Waseda University.

The spectroscopic method developed at Waseda University and AIST allows accurate measurement of N/MP concentration regardless of their size, and without separating them from organic matter. It does this by measuring how much light of a particular wavelength passes through the sample and how much gets absorbed.

To clarify the optimal conditions and wavelength combinations required to measure N/MP concentrations in soil via spectroscopy, the researchers prepared six types of soil containing N/MPs from soil samples where characteristics like particle size distribution and organic content differed. They mixed the different soil types with 203-nm polystyrene nanoparticles, creating six different, simulated N/MP-contaminated soil suspensions. N/MP concentration was maintained at 5 milligrams per liter (mg/L).

“We measured the absorbance of these soil suspensions at various wavelengths ranging from 200 to 500 nm using a spectrophotometer and based on this, determined the N/MP concentrations in the soil,” said Kyouhei Tsuchida, a researcher at Waseda University. “Then the best combination of two wavelengths was identified for measuring N/MPs, which helped negate the interference from soil particles and leached components in the suspension.”

The team found that a wavelength combination of 220 nm to 260 nm and 280 nm to 340 nm had the lowest error level for the six samples, making this wavelength combination the most suitable for measuring N/MP concentrations in different soil types.

The adsorption potential of N/MPs in the soil varies depending on the soil characteristics, making it important to consider soil adsorption when measuring N/MP concentrations. To take this into account, the researchers created a calibration curve between the concentration of N/MPs in the soil suspensions and the N/MP content added to the dry soil samples. The calibration curve showed a linear relationship between the two variables and allowed for the adsorption of N/MPs on soil particles.

“Our novel measurement approach can quantify different N/MPs, including polyethylene and polyethylene terephthalate, in a variety of soils and can easily be used as an initial assessment tool. Moreover, it can help further our understanding of the distribution and migration behavior of N/MPs in the geosphere environment,” said Tsuchida.

The researchers believe the simplified process of measuring N/MP concentration could encourage studies of N/MP migration. Such studies could provide a greater understanding of the effect of N/MP on human and environmental health.

The research was published in Ecotoxicology and Environmental Safety (www.doi.org/10.1016/j.ecoenv.2024.116366).