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Polymer weathering under simulated solar radiation and comparison to stormwater and estuarine microplastics
Summary
Researchers weathered polyethylene and polypropylene plastics under simulated sunlight in water for 90 days and compared their spectral changes to those found in environmental microplastics from stormwater and estuaries. They found that polypropylene degraded faster than polyethylene and that spectral databases had difficulty accurately identifying heavily weathered plastics. The study highlights challenges in identifying and age-dating microplastics found in the environment.
Accurate spectral identification of weathered plastics and analyses that provide insight into environmental degradation and age are desirable for source tracking and understanding hazards. The objectives of this study were to (1) evaluate the kinetics of spectral changes for lab-weathered polymers and compare to spectra from environmental microplastics (MPs), and (2) assess the accuracy of spectral databases in identifying weathered polymers. For objective 1, polyethylene (PE) and polypropylene (PP) fragments were exposed to simulated solar radiation in water for 90 days. FTIR spectra were collected periodically and degradation was quantified using carbonyl and hydroxyl bond indices. Significant linear increases in carbonyl indices for PP, but not PE, were observed as a function of exposure time. Spectra (via principal component analysis) and bond indices from lab-weathered polymers were then compared to environmental MPs collected from urban stormwater and the Delaware Bay estuary. Estuarine PP carbonyl and hydroxyl indices varied as a function of spectral collection mode (i.e., ATR vs. transmission) and by sampling site, potentially indicating the bond indices provide insight into sources/fate/transport of PP and are worthy of further study. In contrast, no significant differences were observed for PP in stormwater samples, possibly due to the close proximity of collection locations. PE exhibited non-linear trends in bond indices in the laboratory study and showed no significant association with sampling location in environmental samples, suggesting these indices may be less useful for PE degradation analysis. For objective 2, 14 different polymers, eight of which were polymer blends, were exposed to simulated solar radiation for up to 90 days, in dry and wet conditions. FTIR spectra were collected periodically and analyzed with two spectral identification software. OpenSpecy achieved an 88 % true positive rate compared to siMPle's 57 % at a 70 % hit quality threshold. Expanding reference libraries, to include weathered polymers and polymer blends, could improve spectral identification accuracy, and manual interpretation of FTIR spectra is recommended for low-confidence matches.
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