Connecting consumer plastic formulations to marine fates and impacts
Solutions to plastic pollution have been impeded by knowledge gaps surrounding plastic’s environmental persistence and implications. To fill some of these gaps, this thesis aims to connect consumer plastic formulations (the specific mixture of polymers and additives) to marine fates and impacts. First, I explored relationships between consumer polyethylene (PE) bag formulations, degradation by sunlight, and dissolved organic carbon (DOC) release to seawater. I found that the bags contained 15-36% inorganic additives, mainly calcium carbonate and titanium dioxide (TiO2). Bags and pure PE produced 3- to 80-fold more DOC during sunlight exposure than darkleaching, with more DOC generated by the bags than the pure PE. High resolution mass spectrometry revealed that photo-produced DOC comprised tens of thousands of unstudied chemicals. Additives strongly influenced degradation rates and DOC compositions. Second, I examined the interplay between sunlight and marine microbes on degradation of pure and TiO2-containing cellulose diacetate (CDA) fabrics. I found that sunlight reduced CDA’s average molecular weight (MW) and, ultimately, converted it to CO2. TiO2 accelerated MW reduction 2-fold and conversion to CO2 24-fold. Prior degradation by sunlight expedited microbial degradation in both fabrics. Finally, I assessed inorganic additive compositions in consumer plastics, their potential for liberation by sunlight, and potential impacts on local and global biogeochemistry. Consumer plastics contained ~8% inorganic additives comprising nearly 50 elements. Additive zinc (Zn) isotopic signatures appeared unique relative to other marine sources, which may be evident in the marine Zn isotopic balance. Light exposure accelerated release of elements into water relative to dark-leaching. Based on the most-cited estimate of plastic leakage to the ocean, plastic-derived antimony and Zn may be 3% and 1%, respectively, of natural riverine fluxes and quadruple by 2060. Proportions in heavily polluted rivers appear even greater. However, plastic leakage estimates span orders of magnitude, translating to high uncertainty in element fluxes. Collectively, this thesis demonstrates that additives and sunlight are overlooked drivers of marine plastic fates and impacts; integrating them into studies and models may transform our understanding of plastic pollution. Furthermore, leveraging the connection between formulation and fate may enable us to reduce environmental impacts using existing materials.
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Massachusetts Institute of Technology and Woods Hole Oceanographic Institution