Tackling the Challenge of Extracting Microplastics from Soils: A Protocol to Purify Soil Samples for Spectroscopic Analysis

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-Fouriertransform 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.

Municipal biowaste treatment plants contribute to the contamination of the environment with residues of biodegradable plastics with putative higher persistence potential

“…This combination of enzymatic digestion and Fenton oxidation was necessary since for these types of samples Fenton treatment alone was not sufficient to remove enough of the organic material to allow µ-FTIR imaging. A detailed analysis of the challenge of sample preparation for µ-FTIR in case of complex samples has recently been published by some members of our group 33 , where further details can be found.…”

Relevant and Realistic Assessments of Micro‐ and Nanoplastics in the Environment

“…On the first topic, both Moller et al (2022) and Knauss et al (2022) report advances in analytical methodologies that include a soil purification protocol with a high purification efficiency (Moller et al, 2022) and a method to create standardized plastic microfibers. Microfibers are an important environmental contaminant.…”

Comparison of different salt solutions for density separation of conventional and biodegradable microplastic from solid sample matrices

“…An amount of 250 g was used per sample. This is, together with Möller et al (2021), the largest sample mass that has been investigated compared to previously published studies that measure MPs concentrations in soil with density separation.…”

“…If the polymer type were the reason, this would also have been visible in the tests of influence. Thus, materials like separatory funnels and spiral conveyors which are not compressing particles between stirrer and vessel should be reconsidered (Enders et al 2020;Möller et al 2021).…”