Photo aging of polyester microfiber in freshwater and seawater environments: kinetics, mechanisms, and influencing factors

Microfibers, particularly polyethylene terephthalate (PET, commonly known as polyester), are the predominant form of microplastic pollution in aquatic environments. However, the process by which PET microfibers form in these environments remains unclear. To investigate this, we exposed PET microfibers to both freshwater and seawater environments and subjected them to ultraviolet irradiation for 12 days. According to atomic force microscopy, X-ray photoelectron spectroscopy, differential scanning calorimetry, and gel permeation chromatography analyses, PET microfibers exhibited diverse photoaging behavior in freshwater and seawater environments, with the photoaging rate in seawater higher than in freshwater and ultrapure water. Photochemically active ions, including Cl-, Br-, and NO3-, are identified as the dominant factors controlling the aging rate of PET microfibers, particularly NO3-. Mechanistic insights suggest that this effect is due to the higher steady-state concentration of •OH produced in solutions containing these ions (6.04 × 10-15 M for Cl-, 4.93 × 10-15 M for Br-, and 8.00 × 10-15 M for NO3-) compared to pure water (3.72 × 10-15 M), which further accelerates PET photoaging. These findings provide an in-depth understanding of the formation and fate of PET microfibers in freshwater and seawater environments.