Walsh
Anna N.
Walsh
Anna N.
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ArticlePlastic formulation is an emerging control of its photochemical fate in the ocean(American Chemical Society, 2021-09-08) Walsh, Anna N. ; Reddy, Christopher M. ; Niles, Sydney F. ; McKenna, Amy M. ; Hansel, Colleen M. ; Ward, Collin P.Sunlight exposure is a control of long-term plastic fate in the environment that converts plastic into oxygenated products spanning the polymer, dissolved, and gas phases. However, our understanding of how plastic formulation influences the amount and composition of these photoproducts remains incomplete. Here, we characterized the initial formulations and resulting dissolved photoproducts of four single-use consumer polyethylene (PE) bags from major retailers and one pure PE film. Consumer PE bags contained 15–36% inorganic additives, primarily calcium carbonate (13–34%) and titanium dioxide (TiO2; 1–2%). Sunlight exposure consistently increased production of dissolved organic carbon (DOC) relative to leaching in the dark (3- to 80-fold). All consumer PE bags produced more DOC during sunlight exposure than the pure PE (1.2- to 2.0-fold). The DOC leached after sunlight exposure increasingly reflected the 13C and 14C isotopic composition of the plastic. Ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry revealed that sunlight exposure substantially increased the number of DOC formulas detected (1.1- to 50-fold). TiO2-containing bags photochemically degraded into the most compositionally similar DOC, with 68–94% of photoproduced formulas in common with at least one other TiO2-containing bag. Conversely, only 28% of photoproduced formulas from the pure PE were detected in photoproduced DOC from the consumer PE. Overall, these findings suggest that plastic formulation, especially TiO2, plays a determining role in the amount and composition of DOC generated by sunlight. Consequently, studies on pure, unweathered polymers may not accurately represent the fates and impacts of the plastics entering the ocean.
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ArticleSynergy between sunlight, titanium dioxide, and microbes enhances cellulose diacetate degradation in the ocean(American Chemical Society, 2022-09-16) Walsh, Anna N. ; Mazzotta, Michael G. ; Nelson, Taylor F. ; Reddy, Christopher M. ; Ward, Collin P.Sunlight chemically transforms marine plastics into a suite of products, with formulationthe specific mixture of polymers and additivesdriving rates and products. However, the effect of light-driven transformations on subsequent microbial lability is poorly understood. Here, we examined the interplay between photochemical and biological degradation of fabrics made from cellulose diacetate (CDA), a biobased polymer used commonly in consumer products. We also examined the influence of ∼1% titanium dioxide (TiO2), a common pigment and photocatalyst. We sequentially exposed CDA to simulated sunlight and native marine microbes to understand how photodegradation influences metabolic rates and pathways. Nuclear magnetic resonance spectroscopy revealed that sunlight initiated chain scission reactions, reducing CDA’s average molecular weight. Natural abundance carbon isotope measurements demonstrated that chain scission ultimately yields CO2, a newly identified abiotic loss term of CDA in the environment. Measurements of fabric mass loss and enzymatic activities in seawater implied that photodegradation enhanced biodegradation by performing steps typically facilitated by cellulase. TiO2 accelerated CDA photodegradation, expediting biodegradation. Collectively, these findings (i) underline the importance of formulation in plastic’s environmental fate and (ii) suggest that overlooking synergy between photochemical and biological degradation may lead to overestimates of marine plastic persistence.
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ThesisConnecting consumer plastic formulations to marine fates and impacts(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2023-09) Walsh, Anna N. ; Ward, Collin P. ; Reddy, Christopher M.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.