Microplastics have been quantified across a wide range of matrixes, most commonly in water samples from the aquatic environment due to the ease of laboratory extraction and isolation of suspected microplastic particles. Subsequently, few published methods exist for the extraction of microplastics from terrestrial soils and biowastes, and few studies have quantified and characterised the microplastics present in these matrixes, especially composts.1 Isolating microplastics from those matrixes presents a number of challenges, particularly the separation of plastic particles from organic rich materials.
Methods were developed for the extraction of microplastics from four different solid biowastes (biosolids, bulk compost, bagged compost, and vermicompost) and soil irrigated with effluent. A combination of digestion with wet peroxide oxidation, and isolation of the supernatant by two density separations (ultra-pure water and saturated sodium iodide) using a separating unit were used. Suspected microplastics were identified under stereomicroscope and the polymer type was confirmed by micro Fourier-transform infrared spectroscopy (µFTIR). Plastic particles were extracted from the five different matrix types, with five sites for each matrix sampled across Aotearoa | New Zealand. Microplastics were categorised by morphotype, polymer type, and size. The highest concentrations of microplastics were isolated from biosolids and vermicompost, with an average of 2724 and 2687 particles/kg, respectively, followed by bulk compost, bagged compost, and soil irrigated with effluent (1944, 1102, and 213 particles/kg respectively). The most common polymer types were polypropylene, polyethylene, and acrylic (35.3%, 26.2%, and 16.3%, respectively). Fragments were the most common morphotype isolated at 57.1%, followed by films and fibres (29.9%, and 12.5%, respectively). Glitter and kitchen sponge fragments made up 7.1% and 5.6% of all particles, respectively, and were found across all sample types. The prominence of microplastics in these matrixes calls for the regulation of plastic products globally to reduce plastic contamination in these beneficial biowastes.