Photic zone plastic: isolation of microplastics in environmental samples and improved understanding of their fate in sunlit water

Plastic pollution in natural environments is a marker of the current human-driven Anthropocene. The abundance and chemistry of plastic debris in aquatic ecosystems provide insight into the interaction of plastic and plastic-derived carbon as novel components of complex biogeochemical cycles. This dissertation is comprised of three chapters; Chapters 2 and 3 present analytical techniques advancing isolation and analysis of plastics in aquatic ecosystems, and Chapter 4 examines the role of photofragmentation and photodissolution in the degradation of photic zone plastics in aquatic ecosystems.Chapter 2 describes an acidic/oxidative chemical digestion for isolation of microplastics co-collected with environmental material (i.e., natural particulate matter and biological material). The chemical digestion consists of an overnight, one-pot, method, specifically optimized for digestion of cellulose acetate to enable filtration of water onto cellulose acetate filters with subsequent isolation of plastics by digestion of the filter material. >99% by mass of carbon-carbon backbone chain polymers polyethylene (PE), polypropylene (PP), and polystyrene (PS) was retained during digestion, and >96% of polyethylene terephthalate (PET), containing susceptible carbon-oxygen bonds. This method quantitatively removed the cellulose acetate matrix (below analytical limits of detection), expanding the accessible size limits for microplastic collection to 0.2 μm, the pore size limit of easily accessible commercial filters. This method also performed well for digestion of common materials in aquatic ecosystems, with near quantitative removal of algae and Albacore tuna tissue (>99%). Lower digestion efficiency was observed for particulate matter in the urban Muddy River (Boston, MA, 86%), likely indicating recalcitrant terrestrial and/or anthropogenic inputs. This digestion method meets and exceeds other chemical digestions for microplastic isolation, and significantly improves plastic stability compared to previously reported acidic digestions, particularly for PET. Chapter 3 builds upon the digestion method as developed for the isolation and cleanup of microplastics greater than 1 mm in size in Chapter 2 to develop a method for the collection, purification, identification and quantification of sub-mm and sub-micron plastics. This method tests the efficacy of the digestion method for the isolation and preservation of sub-mm plastics when collected on 0.45 μm cellulose acetate filters and relies upon the development of a pyrolysis gas chromatography mass spectrometry (Pyr-GC/MS) framework for simultaneous characterization and quantification of plastics that are too small to analyze via Raman or Fourier transform infrared (FT-IR) microscopy (i.e., are below about 10 μm). Pyr-GC/MS quantification of polymer mixtures is challenging due to thermochemical reactions that occur when polymers are co-pyrolyzed but are absent during single polymer calibration, which can result in previously reported uncertainties of up to 130% for polymer pairs. In this study mixtures of three to four polymers (i.e., combinations of PE, PP, PS, and PET) were characterized and quantified. Under these challenging conditions our quantification framework achieved accuracies of 108±13% for PE, 105±11% for PP, 104±20% for PET, and 125±17% PS at μg masses. The Pyr-GC/MS framework was then combined with the digestion method in Chapter 2 to demonstrate the capabilities of this techniques for characterization and mass quantification of plastics in aquatic environments at sizes down to 0.45 μm, with potential to be extended to 0.2 μm. In Chapter 4 the fragmentation and dissolution effects of photodegradation as experienced by plastic debris floating in the photic zone of the ocean are determined in a year-long irradiation experiment. There are trillions of plastic pieces afloat in the ocean, yet models of buoyant ocean plastics are not effective at predicting the size distributions of sub-mm plastic debris. To understand the photochemistry of sub-mm buoyant plastic this study tracked the fragmentation and carbon- transformations (i.e., from solid to dissolved) of ~600 μm PP particles over one year of irradiation in a solar simulator. While plastic-mass and plastic-carbon loss were linear over time, with a loss of 69% of plastic-carbon after one year of irradiation, dissolved organic carbon (DOC) accumulation followed sigmoidal kinetics. Previous studies with shorter irradiation times have reported exponential DOC kinetics, which was also observed over the first 181 days of our experiment. However, the extended 1-year irradiation time clearly demonstrated that the current understanding of DOC accumulation kinetics is incomplete and that linear kinetics should be used to model the photochemical losses of plastics from the ocean's surface. Photofragmentation was not observed, although extensive cracking at the particle surface was visible with scanning electron microscopy. This study finds that photodissolution rather than photofragmentation is the main photodegradation mechanism of plastics in sub-mm sizes when no mechanical force is applied. Understanding removal mechanisms of plastics in aquatic environments is a key step to constraining models of plastic abundance in the ocean. The results presented in this dissertation contribute to a growing body of knowledge regarding isolation, analysis, fate and chemistry of plastic pollution in the environment. As the amount of plastic-carbon present in the Earth system grows to rival biogeochemically relevant pools of natural, biogenic carbon, improved understanding of how plastics travel and interact with aquatic ecosystems is critically important. This work presents powerful methods for isolation, chemical characterization, and quantification of plastics in aquatic environments. This dissertation also advances understanding of plastic photochemistry, finding photodissolution is the primary mechanism of polypropylene photodegradation, and describing previously unknown kinetics for DOC accumulation and plastic loss during photoirradiation.--Author's abstract