Session: Presidential Symposium: Plastics and the Oceans: Chemistry for the Journey Ahead:

It is widely recognized that sunlight exposure is a primary control of the long-term fate of plastics in the environment. Sunlight converts plastic into oxygenated products that span the polymer, dissolved, and gas phases. However, our understanding of how plastic formulation influences the formation rate and composition of these photo-products is incomplete. The present work characterized the composition of four single-use consumer polyethylene (PE) bags from major retailers (Walmart, CVS, and Target) and one pure PE film, as well as the dissolved photo-products from each. All consumer PE bags contained inorganic additives, ranging from 15-36% of the total mass. Calcium carbonate was the most abundant inorganic additive (13-34%), followed by titanium dioxide (TiO₂; 1-2%). For all PE types, sunlight exposure produced 3- to 70-fold more DOC than was leached in the dark, and all consumer PE bags produced more DOC during sunlight exposure than the pure PE. DOC leached in the dark and produced by sunlight was analyzed using high resolution mass spectrometry, revealing that sunlight exposure substantially increased the number of formulas assigned (1.1- to 50-fold). The average molecular weight of the DOC consistently increased with light exposure, as expected from the breakdown of high molecular weight polymer chains. The photo-produced DOC from the consumer PE bags was compositionally distinct from the pure PE. The greatest similarities were observed between the three TiO₂-containing bags, for which at least 77% of formulas were in common. In contrast, only 28% of the photo-produced formulas from the pure PE were detected in the consumer bags. Overall, these findings suggest that plastic formulation, including TiO₂, plays a determining role the amount and composition of DOC generated by sunlight. Consequently, studies on pure polymers are not representative of the plastic entering the ocean or the photochemical fate of plastics in the ocean.

Anna Walsh

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