Abstract
Most microplastic (MP) and nanoplastic (NP) environmental fate and hazard assessments utilise spherical, monodisperse polymer particles that do not represent the continuum of partially degraded, irregular-shaped MPs in the natural environment. A such, there is a strong need for environmentally relevant test and reference materials (TRMs). Cryomilling approaches can readily generate bulk amounts of MP TRMs >100 µm, but only negligible amounts of TRMs below this size and into the nanoscale (generally <<0.1% by mass). New methods need to be developed for the production of small MP (sMP; 1-100 µm) and NP (<1 µm). In the current study, pristine polypropylene (PP), polyethylene (PE, HDPE and LDPE), polyethylene terephthalate (PET), polystyrene (PS) and Polytetrafluoroethylene (PTFE) pellets were cryomilled and sieved to produce core stocks of TRMs <100 µm in size. Characterisation by a Morphology G3 particle size and shape image analyser showed that all TRMs had a mean particle size of ~100 µm by volume and 0.5-2 µm by particle number, indicating high numbers of sMP and NP, but representing a very low mass. Cryomilled PET, PE and PS were selected for use in the development of a secondary TRM production step that combined UVC ozonation to partially UV degrade the surface of the particles, followed by probe sonication to promote further fragmentation. At the same time, a hyphenated field flow fractionation (FFF) and pyrolysis GC-MS workflow was developed for the mass-based quantification of sMP and NPs of defined size fractions. This involved the use of solvent extraction of the particles from aqueous dispersions prior to analysis and quantification. The approach was able to extract ~100% of sMP and ~80% of NP from aqueous dispersion and quantification limits were identified as ~0.1 ng, ~1 ng and ~1 µg for PET, PE and PS respectively. Morphology G3 was used to quantify the number of sMP, while nanotracking analysis was used to quantify the size distribution and number of NPs. Results suggested that the UVC ozonation and probe sonication strongly increased the quantity of sMP and NP, with mass fraction increases of 30x (PS), 15x (PET) and 6x (PE), respectively. However, this still represented µg quantities being produced from mg quantities of stock material. The results suggest that environmentally relevant sMP and NP TRMs can be produced, but the method requires optimisation to generate quantities suitable for use in environmental fate and effects studies.