Could a future ocean acidification scenario influence the photodegradation of microplastics?

Microplastics (MPs) in the marine environment are subject to photodegradation, a process in which they easily get fragmented and leach potentially dangerous compounds. Ocean acidification (OA), owing to the enrichment of carbon dioxide (CO 2 ), is one of the main chemical changes occurring in the marine environment and may be a factor that influences photodegradation. This study aims to investigate the influence of OA on the photodegradation of three types of MPs: polypropylene (PP), expanded polystyrene (EPS), and ethylene-vinyl acetate (EVA). MPs were weathered by exposing them simultaneously for 8 hours to accelerated ultraviolet (UV) radiation and to three pH levels ( i.e. , 8.1, 7.8, and 7.5), which were achieved by injecting CO 2 into a simulated marine medium. The acidification system reproduced the current environmental conditions and those calculated for the future. As expected, the higher the partial pressure of CO2 , total inorganic carbon, bicarbonate ion, and CO2 , the more acidic the pH, and the opposite is true for carbonate ion. Structural changes were assessed by Fourier transform infrared spectroscopy, differential scanning calorimetry, gel permeation chromatography, and scanning electron microscopy. All weathered samples showed a higher degradation rate than the virgin samples. The MPs of PP and EVA presented the highest degradation rates, indicating the development of oxygen-containing functional groups and an increase in crystalline fraction. The oxidation state and crystallinity were higher in samples exposed to the lowest pH. There was no significant difference (p > 0.05) in the degradation rate of EPS samples. The results allow us to infer that an increase in OA predicted for the future could interfere with the photodegradation of some types of MP polymers, accelerating this process.

Balbela C., Soroldoni S., Kessler F., Fernandes A. & Pinho G. L. L., 2022. Could a future ocean acidification scenario influence the photodegradation of microplastics? SSRN. doi: 10.2139/ssrn.4199146. Article.

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