Degradation of polyethylene terephthalate bottle fragments in the marine environment: An empirical study of structural and chemical changes

Relatively little is known about the degradation pathways and impacts of plastic litter on the seafloor. In this study, polyethylene terephthalate (PET) bottle fragments (n = 70) have been retrieved from kelp deposits washed up after storm activity and analysed by a range of visual and spectroscopic techniques. Fragments of various colours consisted of either side walls, sometimes containing the cap, or petaloid bases. Clear samples often exhibited oxidative yellowing and bases were sometimes locally stained or fouled. Stereomicroscopy revealed heterogeneous abrasion, entrapment of silt, and blistering, delamination and thickening of the base, while electron microscopy revealed the formation of microplastic fragments and flakes. Relative to new PET, Fourier-transform infrared spectrometry showed evidence of ester bond scission and oxidative degradation, while X-ray fluorescence spectrometry revealed a reduction in the concentration of Sb. Heterogeneous PET degradation appears to be determined by both intrinsic properties of the polymer (including bottle geometry) and environmental factors and results in the generation of microplastics and mobilisation of Sb. Independent studies of PET degradation exposed to UV light suggest lifetimes of 20 to 1200 years, but while residing on the seafloor persistence could be considerably longer.