Abstract
Microplastic (MP; 1-5000 µm) and nanoplastic (NP; <1 µm) particles observed in the natural environment are typically (i) irregular-shaped, (ii) partially degraded, (iii) a wide range of polymer types, (iv) a continuum of sizes, shapes and densities, and (v) a reservoir for complex mixtures of plastic-associated chemicals. There is a need for environmentally relevant MNP test materials for assessing fate and effects. Small MP (sMP; <50 µm) and NP are thought to be the highest risk size ranges, but cryomilling struggles to produce meaningful yields. Here, we present the results from an assessment and optimisation of three previously reported strategies for producing sMP and NP [1,2]. Strategy #1 used pre-cryomilling, thermal treatment, UVC irradiation and probe sonication of PS, PE and PET materials. Strategy #2 involved the testing of different mechanical degradation processes after pre-cryomilling, including bath sonication, probe sonication and wet grinding (Ultra-Turrax). Strategy #3 investigated the suitability of partial solubilisation with a long chain alkane.
Strategy #1 was viable for producing PS, PE and PET sMPs. A measurable increase in the yield of particles in the range 1-10 µm was observed, with the percentage mass yield increasing from ~0.01% to ~0.7% for PS, from ~0.5% to ~3% for PE and from ~0.1% to ~2.5% for PET. However, overall yields remained very low and there was no measurable increase in the amount of NP. Strategy #2 resulted in no change in the average particle size (z-ave) for cryomilled PE under any of the treatments, with high PDIs indicating a broad particle size distribution. Some reduction in z-ave was observed for cryomilled PET, with probe sonication and wet grinding the most effective treatments, reducing the z-ave from ~6000 nm to ~1800 nm and ~2000 nm, respectively. The PDI was correspondingly reduced, but there was no increase in the proportion of NPs. Strategy #3 resulted in PE particles with z-ave of ~650 nm and a high PDI. The z-ave of PET, PA, PAN and wool ranged from ~200-300 nm, with reduced PDI values suggesting a narrower particle size distribution. Full physicochemical characterisation is currently being completed. The alternative top-down methods evaluated in this study for producing environmentally relevant sMP and NP test materials show potential. Partial solubilisation appears promising for producing, but more characterisation of the resulting materials is need to confirm their environmental relevance.
Strategy #1 was viable for producing PS, PE and PET sMPs. A measurable increase in the yield of particles in the range 1-10 µm was observed, with the percentage mass yield increasing from ~0.01% to ~0.7% for PS, from ~0.5% to ~3% for PE and from ~0.1% to ~2.5% for PET. However, overall yields remained very low and there was no measurable increase in the amount of NP. Strategy #2 resulted in no change in the average particle size (z-ave) for cryomilled PE under any of the treatments, with high PDIs indicating a broad particle size distribution. Some reduction in z-ave was observed for cryomilled PET, with probe sonication and wet grinding the most effective treatments, reducing the z-ave from ~6000 nm to ~1800 nm and ~2000 nm, respectively. The PDI was correspondingly reduced, but there was no increase in the proportion of NPs. Strategy #3 resulted in PE particles with z-ave of ~650 nm and a high PDI. The z-ave of PET, PA, PAN and wool ranged from ~200-300 nm, with reduced PDI values suggesting a narrower particle size distribution. Full physicochemical characterisation is currently being completed. The alternative top-down methods evaluated in this study for producing environmentally relevant sMP and NP test materials show potential. Partial solubilisation appears promising for producing, but more characterisation of the resulting materials is need to confirm their environmental relevance.