Development and characterization of reference environmentally relevant micro-nano-plastics for risk assessment studies

Micro- and nano-plastics (MNPs) have become ubiquitous environmental pollutants. Extensive toxicological studies of MNPs have been conducted in recent years. However, because of the difficulties involved in extraction and collection of MNPs from environmental media, most of these studies have employed simplistic, pristine, spherical, micro- or nano-sized commercial MNPs, whose properties, including morphology, surface chemistry, and size, do not adequately approximate those of environmentally relevant MNPs. Here, we describe the development and use of methods for the synthesis of well characterized, environmentally relevant MNPs across the life cycle of a plastic material, in a property-controlled manner. Multiple degradation scenarios, including mechanical fragmentation (cryogenic milling), UV weathering, and thermal disintegration (incineration) were applied to virgin plastic materials (polyvinyl chloride, PVC; polyethylene terephthalate, PET; High density polyethylene, HDPE; Acrylonitrile butadiene styrene, ABS; polycarbonate, PC; and polypropylene, PP) to simulate the life cycle pathways that likely occur in the environment over time. The MNPs generated from these degradation processes were size fractionated using both "dry" and "wet" separation methods. Detailed physicochemical characterization of the size fractionated reference MNPs was performed to determine size, morphology, chemical and elemental compositions, and hydrophobicity. Microbiological sterility and endotoxin content of reference MNPs were also assessed. Protocols for storage of reference MNPs in controlled oxygen and moisture conditions for future use in toxicological studies are also described. The methodology developed in this study can be used to synthesize environmentally relevant reference MNPs across the life cycle of plastic materials for use in risk assessment studies.