Issue 1, 2022

Characterizing photochemical ageing processes of microplastic materials using multivariate analysis of infrared spectra

Abstract

Microplastics in the environment are an emerging concern due to impacts on human and environmental health. In addition to direct effects on biota, microplastics influence the fate and distribution of trace organic contaminants through sorption and transport. Environmental weathering may influence the rate and extent of chemical sorption. Changes in the surface characteristics of four common plastics including low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP), and polystyrene (PS) were followed under the influence of both artificial light (UV-B) and natural sunlight for up to six months. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra were collected at regular intervals. Principal component analysis (PCA) of the full dataset of UV-B weathered samples (n >500 spectra) simultaneously discriminated plastic type and extent of photochemical weathering. The magnitude of PCA scores correlated with exposure time and the loadings were consistent with surface chemistry changes including photooxidation. Projecting sunlight and UV-C exposed samples onto this PCA model demonstrated that similar chemical changes occurred, albeit at different rates. The results were compared to the carbonyl index (CI) with similar weathering trends indicating PP weathered at a faster initial rate than LDPE and HDPE. We propose that a multivariate approach is more widely applicable than CI as illustrated by PS, which lacked a stable reference peak. Kinetic analysis of the time series indicated that outdoor weathering occurred 5–12 times slower than the artificial exposure used here, depending on the plastic and the light source employed. The results provide unique insights into weathering processes and the photochemical age of naturally weathered plastics.

Graphical abstract: Characterizing photochemical ageing processes of microplastic materials using multivariate analysis of infrared spectra

Supplementary files

Article information

Article type
Paper
Submitted
17 sep 2021
Accepted
08 dec 2021
First published
08 dec 2021

Environ. Sci.: Processes Impacts, 2022,24, 52-61

Characterizing photochemical ageing processes of microplastic materials using multivariate analysis of infrared spectra

M. Zvekic, L. C. Richards, C. C. Tong and E. T. Krogh, Environ. Sci.: Processes Impacts, 2022, 24, 52 DOI: 10.1039/D1EM00392E

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