Measuring microplastics in soil using spectroscopy

Japanese researchers have devised a novel and simple method to measure nano/microplastic (N/MP) concentrations in different soil types using spectroscopy at two wavelengths, enabling them to accurately quantify N/MPs regardless of their size.

Current techniques for measuring N/MP concentrations in soil require separating the soil organic matter content through chemical and physical processes. Subsequently, the isolated N/MPs are analysed using a microscope, Fourier-transform infrared spectroscopy, Pyrolysis–gas chromatography/mass spectrometry or Raman spectrometry. However, these techniques require advanced skills and have limited resolution for analysing N/MPs smaller than 1 µm. Moreover, often some of the N/MPs in the soil are lost during the separation process, leading to inaccurate measurements.

Looking to detect particles ≤1 µm, the researchers created a method to measure N/MP concentrations in soil using spectroscopy without separating the soil organic matter. Their results have been published in the journal Ecotoxicology and Environmental Safety.

Spectroscopy can determine the concentration of N/MPs in soils based on how much light of a particular wavelength passes through the sample and how much gets absorbed. In this way, spectroscopy can potentially detect N/MPs regardless of size, provided the correct wavelengths are used to distinguish between the N/MPs and soil. Accordingly, the researchers developed a method to use the difference between the absorbance spectra of N/MPs and soil particles to quantify the N/MPs.

Six soil suspensions were created from soil samples with different characteristics, such as particle size distribution and organic content, and were mixed with polystyrene nanoparticles sized 203 nm. This created six different simulated N/MP-contaminated soil suspensions, with the N/MP concentration maintained at 5 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 research leader Kyouhei Tsuchida, from Waseda University and Japan’s National Institute of Advanced Industrial Science and Technology. “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 researchers found that a wavelength combination of 220–260 nm and 280–340 nm had the lowest error level for the six samples and was thus found to be suitable for measuring N/MP concentrations in different soil types. They also created a calibration curve between the concentration of N/MPs in the soil suspensions and 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/MPs on soil particles. This enabled accurate estimation of the concentration of N/MPs in the soil.

These results demonstrate the efficacy of this simple spectroscopy-based method to correctly measure the concentration of N/MPs in soil, without any cumbersome separation process. According to Tsuchida, “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 behaviour of N/MPs in the geosphere environment.”

Image credit: iStock.com/Svetlozar Hristov