Interactions between soil physical fractions and microplastics – An attenuated total reflectance-mid infrared and chemometrics study

Abstract

As the volume of plastics produced, used, and discarded around the world continues to increase, the environmental pollution presented by these polymers and their by-products continues to be impacted. Different methods and techniques have been proposed to evaluate plastics in environmental samples, including soils. However, in most cases, the effect of variables such as plastic particle size, soil physical fractions, and interactions between them, was not evaluated or not well understood. The aim of this study was to evaluate the effect of different polystyrene (PS) fractions added to laboratory standard soil particle sizes (clay, sand, and silt) on the attenuated total reflectance (ATR) mid-infrared (MIR) spectra of the samples. Additive-free PS was sieved in four particle sizes, namely fraction 1 (∼500–251 µm), fraction 2 (∼250–181 µm), fraction 3 (∼180–107 µm) and fraction 4 (∼106–46 µm). The PS fractions were used to spike four physical soil fractions such as clay, loam, sand and sandy and analysed using ATR-MIR spectroscopy (4000–400 cm−1). The results of this study indicated that the MIR absorbances at specific frequencies of the binary soil-PS mixtures are different and depend on the combination of the soil physical fraction and PS particle size. Overall, no linearities were observed depending on the fraction analysed. This is of importance as the data indicates a different behaviour on the MIR spectral properties of the sample. Although further validation is needed, these results might have a direct effect on the development of robust protocols to monitor MPs in complex matrices such as soil using MIR spectroscopy.