Differential scanning calorimetry (DSC): An important tool for polymer identification and characterization of plastic marine debris.

Differential scanning calorimetry (DSC), a routine thermoanalytical method in material science, is gaining utility in plastic pollution research to improve polymer identification. We optimized a DSC method, experimentally testing pan types, temperature ramps, number of melts, and minimum sample masses. Using the optimized method, we created an in-house thermogram library from 201 polymer reference standards. We determined peak melting temperature cutoffs for differentiating variants of PE and nylon. PE cutoffs remained stable after experimentally weathering standards outdoors or for severely weathered HDPE debris found on Hawaii's beaches. Marine debris samples, across a range of weathering severity and previously identified as either low-density or high-density polyethylene (LDPE or HDPE) based on the 1377 cm-1 peak indicating methyl groups by attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR), were analyzed by DSC to confirm or challenge the ATR-FTIR PE differentiation. ATR-FTIR was correct for >80% of the HDPE samples, but <40% of those initially identified as LDPE by ATR-FTIR. Accuracy did not relate to weathering extent. Most samples mis-identified as LDPE were HDPE that had formed methyl groups likely from chain scission during photooxidation. ATR-FTIR alone is unreliable for differentiating weathered PE, DSC is required. We provide a multiple-method workflow for complete and accurate polymer identification, even for microplastics ≥0.03 mg. Applying these methods can better identify the polymer composition of marine debris, essential for sourcing and recycling efforts.

Marine Debris Polymers on Main Hawaiian Island Beaches, Sea Surface, and Seafloor.

Polymeric differences of plastic debris were assessed across four compartments of the Main Hawaiian Islands (sea surface, windward beaches, leeward beaches, and seafloor) to better describe sources and fate. Plastic debris pieces (n = 4671) were collected from 11 beaches, three sea surface tows, and three seafloor dives. Fourier transform infrared spectroscopy identified the polymers of 3551 pieces. Significant differences (p < 0.05) in concentration, types, polymer composition, and weathering were found among four compartments. Windward beaches had 1-2 orders of magnitude more plastic pollution (g/m2) than leeward beaches, despite smaller human populations on windward sides. Sea surface and windward beaches were dominated by severely weathered, less dense floating polymers (polyethylene and polypropylene comprised 92.7 and 93.5% on average, respectively, of the total debris mass), while leeward beaches and the seafloor debris consisted of less weathered and more dense sinking polymers (e.g., 41.0 and 44.7% of total mass consisted of the sum of polystyrene, nylon, cellulose acetate, polyethylene terephthalate, and additive-masked debris). These results are some of the first to provide evidence of polymeric stratification in the marine environment and emphasize that the majority of marine debris in Hawaii is floating in from distant sources rather than from Hawaii's residents or tourists.

Polymer Identification of Plastic Debris Ingested by Pelagic-Phase Sea Turtles in the Central Pacific.

Pelagic Pacific sea turtles eat relatively large quantities of plastic (median 5 g in gut). Using Fourier transform infrared spectroscopy, we identified the polymers ingested by 37 olive ridley, 9 green, and 4 loggerhead turtles caught as bycatch in Hawaii- and American Samoa-based longline fisheries. Unidentifiable samples were analyzed using high-temperature size exclusion chromatography with multiple detectors and/or X-ray photoelectron spectroscopy. Regardless of species differences in dive depths and foraging strategies, ingested plastics were primarily low-density, floating polymers (51% low-density polyethylene (LDPE), 26% polypropylene (PP), 10% unknown polyethylene (PE), and 5% high-density PE collectively). Albeit not statistically significant, deeper diving and deeper captured olive ridley turtles ate proportionally more plastics expected to sink (3.9%) than intermediate-diving green (1.2%) and shallow-diving loggerhead (0.3%) turtles. Spatial, but no sex, size, year, or hook depth differences were observed in polymer composition. LDPE and PP, some of the most produced and least recycled polymers worldwide, account for the largest percentage of plastic eaten by sea turtles in this region. These novel data inform managers about the threat of plastic ingestion to sea turtles and may motivate development of more environmentally friendly practices for plastic production, use, and waste management.

Validation of ATR FT-IR to identify polymers of plastic marine debris, including those ingested by marine organisms.

Polymer identification of plastic marine debris can help identify its sources, degradation, and fate. We optimized and validated a fast, simple, and accessible technique, attenuated total reflectance Fourier transform infrared spectroscopy (ATR FT-IR), to identify polymers contained in plastic ingested by sea turtles. Spectra of consumer good items with known resin identification codes #1-6 and several #7 plastics were compared to standard and raw manufactured polymers. High temperature size exclusion chromatography measurements confirmed ATR FT-IR could differentiate these polymers. High-density (HDPE) and low-density polyethylene (LDPE) discrimination is challenging but a clear step-by-step guide is provided that identified 78% of ingested PE samples. The optimal cleaning methods consisted of wiping ingested pieces with water or cutting. Of 828 ingested plastics pieces from 50 Pacific sea turtles, 96% were identified by ATR FT-IR as HDPE, LDPE, unknown PE, polypropylene (PP), PE and PP mixtures, polystyrene, polyvinyl chloride, and nylon.