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Occurrence of Microplastics and Cytotoxic Effects of Organic Extracts from Isolated Mesoplastics in Compost
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Compost made from food waste and manure contains tiny plastic bits (mostly from packaging), which can end up in garden and farm soil. The good news: the composting process itself actually seemed to reduce both the amount of plastic fragments and their toxicity compared to the raw waste going in, suggesting composting may help break down or neutralize some of these contaminants rather than concentrate them. Still, since plastics are entering the food-growing system through compost at all, this points to a need for better plastic separation before waste becomes fertilizer.
High Resolution Image Download MS PowerPoint Slide Compost application is a widely recommended practice to maintain and improve soil fertility. However, such a practice could be a main entry path for plastic into soil. Accordingly, in the present work, two different compost samples, obtained with and without biochar, were analyzed to investigate how composting can affect the presence of microplastics (MPs). The substrate of both samples (consisting of a mixture of household food waste and animal manure) was also analyzed for comparative purposes. Samples were processed by oxidation, flotation, and filtration. MPs on the filters were observed, counted, and size-calibrated using both a stereomicroscope and an inverted microscope. MPs larger than 1 mm were further characterized by attenuated total reflectance Fourier-transformed infrared spectroscopy (ATR-FTIR). In parallel, mesoplastics (0.5–2 cm) were recovered from substrate and compost and extracted in methanol for testing in vitro cytotoxicity. The estimated concentration of MPs ranged from 820 to 1340 fragments/kg of dry sample, depending upon the sample. Three polymers represented the totality of identified plastic items: polyethylene (PE, including both low and high density), polyethylene terephthalate (PET), and polypropylene (PP) in order of abundance. Nevertheless, cytotoxicity was only observed in mesoplastic extracts from the substrate and could not be attributed to the identified plastic items themselves, suggesting that cytotoxic effects could have been caused by contaminants adsorbed to plastics or by the leaching of plastic additives during the extraction process. In summary, the composting process reduced the cytotoxicity of plastic extracts and the presence of MPs in compost, which could be attributed to the fragmentation of plastics.
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