Photodegradation-driven microparticle release from commercial plastic water bottles

Plastic pollution is widespread in the environment and has been detected in natural and drinking water sources. In this study, seventy polyethylene terephthalate (PET) bottles with five different wall thicknesses were exposed to sunlight for ten weeks to evaluate the role of photochemical degradation as a potential source of microplastics in bottled drinking water. Solar flux measurements were used to calculate the cumulative photon dose received by the bottles, which enabled the extraction of apparent first-order reaction rates for microparticle formation as detected by flow immersion microscopy. Buildup of microparticles was observed up to 14-20 μg L^-1 during the first 30 days of the experiment, corresponding to a cumulative UVA + UVB photon dose of ∼30 × 10²⁴ photons cm^-2, followed by a plateau at higher photon doses. The observed plateau is likely due to a combination of factors, including the limited extent of the water-plastic interface susceptible to decomposition induced by photolysis and continued photodegradation and breakdown of previously released microparticles into smaller, sub-detectable fragments. Fourier-transform infrared spectroscopy analysis of plastic bottles before and after sunlight exposure revealed carbonyl _<i>v</i>[CO] stretching band reductions, indicating chemical degradation through bond scission and decarboxylation. Comparative analysis of the spectroscopic and kinetic data suggests that PET with higher amorphous content is more susceptible to photochemical degradation, leading to accelerated breakdown and increased microplastic release. High-resolution mass spectrometry analysis of filtered microparticle samples revealed repeating units of terephthalic acid and ethylene glycol (<i>m/z</i> 192.035), consistent with the primary monomer unit of PET polymer. Furthermore, the mass spectra of the filtered microparticle material closely resembled that of PET, sharing many of the most abundant spectral features. These findings confirm that the microparticle residues detected in the bottled water originated from the plastic material.