Abstract
Microplastic pollution in soils is an emerging topic in the scientific community, with researchers striving to determine the occurrence and the impact of microplastics on soil health, ecology, and functionality. However, information on the microplastic contamination of soils is limited because of a lack of suitable analytical methods. Because micro-Fourier-transform infrared spectroscopy (µ-FTIR), next to Raman spectroscopy, is one of the few methods that allows the determination of the number, polymer type, shape, and size of microplastic particles, the present study addresses the challenge of purifying soil samples sufficiently to allow a subsequent µ-FTIR analysis. A combination of freeze-drying, sieving, density separation, and a sequential enzymatic-oxidative digestion protocol enables removal of the mineral mass (>99.9% dry wt) and an average reduction of 77% dry weight of the remaining organic fraction. In addition to visual integrity, attenuated total reflectance FTIR, gel permeation chromatography, and differential scanning calorimetry showed that polyamide, polyethylene, polyethylene terephthalate, and polyvinyl chloride in the size range of 100 to 400 µm were not affected by the approach. However, biodegradable polylactic acid showed visible signs of degradation and reduced molecular weight distribution after protease treatment. Nevertheless, the presented purification protocol is a reliable and robust method to purify relatively large soil samples of approximately 250 g dry weight for spectroscopic analysis in microplastic research and has been shown to recover various microplastic fibers and fragments down to a size of 10 µm from natural soil samples. Environ Toxicol Chem 2022;41:844-857. © 2021 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.
Highlights
Introduction of macro‐ as well as microplastics into soils is estimated to exceed the numbers emitted into water bodies by far (Kawecki and Nowack 2019)
We investigated visual integrity and changes in the functional groups on the polymer surfaces via attenuated total reflectance (ATR)–FTIR spectroscopy, mean molar mass distribution via gel permeation chromatography (GPC), as well as changes in the thermal transition characteristics via differential scanning calorimetry (DSC)
The vast majority of the soil mass is already removed in the density separation step: of a 250‐g soil sample, 52, 38, and 160 mg solid particulate matter (>10 μm) remained in the supernatant phase of the ZnCl2 solution for samples 1, 2, and 3, respectively
Summary
Introduction of macro‐ as well as microplastics into soils is estimated to exceed the numbers emitted into water bodies by far (Kawecki and Nowack 2019). A substantial proportion of plastic pollution is expected to enter and remain permanently in the soil column (Rillig et al 2017; Hurley and Nizzetto 2018; Rochman 2018). De Souza Machado et al (2019) conducted a study in which soils and spring onions were exposed to 6 microplastic types, including beads, fibers, and fragments. The exposure experiments resulted in altered physical soil parameters as well as changes in plant performance. Treatments with polyamide (PA) beads and polyester fibers elicited the largest differences from the control treatments, wileyonlinelibrary.com/ETC
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