Microplastics for Use in Environmental Research

Microplastics < 20 μm are being increasingly reported in treated drinking water as well as in surface waters. As such, ongoing microplastic-related research in various fields is beginning to focus on smaller particle sizes as these appear to be most important from a human health perspective. However, no standardized methods for preparing microplastics of this size have been reported in the literature. This study proposes a cryomilling-based method for preparing microplastics to be used in aqueous stock suspensions. Polymers of 22 different types were obtained from the Centre for Marine Debris Research, Hawai’i Pacific University. Polymers were produced that ranged in size from 2 to 125 μm of which 98% were classified as fragments with 2% as fibers. Size distributions for polyethylene terephthalate (PET) and polypropylene (PP), which are frequently reported in environmental samples, were determined using microscopy. Approximately 80% of the particles resulting from cryomilling were < 20 μm long (major dimension). A stock suspension prepared using PET was employed to illustrate and assess recovery for sampling equipment which incorporated in-line filtration. Recoveries exceeding 80% were observed for individual particle sizes ranging from 2 to 45 μm. Results of these trials suggest that stock suspensions of microplastics are heterogeneous and cannot be treated in a similar manner to chemical solutions. In conclusion, this study represents a forward step towards harmonization of environmental microplastic research methods and improving comparability of future studies.