Letter Cite This: Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/journal/estlcu
Identification of Chain Scission Products Released to Water by Plastic Exposed to Ultraviolet Light Berit Gewert, Merle Plassmann, Oskar Sandblom, and Matthew MacLeod* Department of Environmental Science and Analytical Chemistry, Stockholm University, Svante Arrhenius väg 8, 11418 Stockholm, Sweden S Supporting Information *
ABSTRACT: Buoyant plastic in the marine environment is exposed to sunlight, oxidants, and physical stress, which may lead to degradation of the plastic polymer and the release of compounds that are potentially hazardous. We report the development of a laboratory protocol that simulates the exposure of plastic floating in the marine environment to ultraviolet light (UV) and nontarget analysis to identify degradation products of plastic polymers in water. Plastic pellets [polyethylene, polypropylene, polystyrene, and poly(ethylene terephthalate)] suspended in water were exposed to a UV light source for 5 days. Organic chemicals in the water were concentrated by solid phase extraction and then analyzed by ultra-high-performance liquid chromatography coupled to high-resolution mass spectrometry using a nontarget approach with a C18 LC column coupled to a Q Exactive Orbitrap HF mass spectrometer. We designed a data analysis scheme to identify chemicals that are likely chain scission products from degradation of the plastic polymers. For all four polymers, we found homologous series of low-molecular weight polymer fragments with oxidized end groups. In total, we tentatively identified 22 degradation products, which are mainly dicarboxylic acids.
■
INTRODUCTION
To date, attention to the possible environmental hazards from chemicals associated with plastic in the marine environment has mainly been drawn to persistent organic pollutants sorbed to the plastic and additives used in plastic to enhance certain characteristics of plastic products.9,10 Persistent organic pollutants are a well-studied class of environmental contaminants, and many additives are high-production volume chemicals that have undergone hazard and risk assessments. Chemicals derived from the plastic polymer itself during weathering have, however, received much less attention for identification and hazard and risk assessment. Therefore, it is important to develop methods to identify degradation products from polymers to support assessment of potential environmental hazards. To address this research need, we initiated development of a laboratory-based artificial weathering protocol for plastic materials under conditions that could be extrapolated to environmental conditions. We monitored the weathering of plastic materials by analysis of free chemicals that leached into water with liquid chromatography coupled to high-resolution mass spectrometry (LC−HRMS).
Since mass production started in the 1940s, vast amounts of plastics have been produced globally.1,2 Plastics are very persistent, and it is estimated that 60% of all floating debris in the world’s oceans is plastic.3−6 Plastic floating on the ocean surface is exposed to sunlight, oxidants, and physical stress. These factors cause weathering of the plastic, which is evident in changes in their brittleness, density, size, and surface charge, and weathering can lead to degradation of the plastic polymers.7 As plastic is weathered on the ocean surface, additives, residual chemicals from the polymerization process, and free chemicals that are degradation products of the plastic polymer are expected to leach into the surrounding waters. Different polymers follow different degradation pathways; however, photo-oxidation by incident ultraviolet (UV) light is likely the most important mechanism causing chain scission reactions.7 Among polymers with a carbon−carbon backbone, degradation of polyethylene (PE) and polypropylene (PP) by chain scission is expected to produce olefins, aldehydes, and ketones with a range of molecular weights, while end chain scission is expected to be predominant for polystyrene (PS).7,8 Plastic polymers with heteroatoms in the main chain, like poly(ethylene terephthalate) (PET), are expected to degrade to low-molecular weight polymer fragments, including monomers and oligomers with oxidized end groups, especially carboxylic acids.7 © XXXX American Chemical Society
Received: March 7, 2018 Revised: March 28, 2018 Accepted: March 29, 2018
A
DOI: 10.1021/acs.estlett.8b00119 Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
Letter
Environmental Science & Technology Letters
Figure 1. Workflow for degradation product identification.
■
MATERIALS AND METHODS Weathering Experiments. Because we were interested in identification of chemicals that are degradation products of the plastic polymer itself rather than sorbed POPs or additives, we worked with pristine plastic materials that according to the distributer (Goodfellow GmbH) contain as few additives as possible. Low-density polyethylene (PE), polypropylene (PP), polystyrene (PS), and poly(ethylene terephthalate) (PET) were chosen for study (Table S2). Together, these four polymers make up 62.5% of the European demand for plastics.11 A high-intensity UV lamp (HTC 400-241 SUPRATEC HTC/HTT, purchased from OSRAM GmbH) with a spectral radiation distribution from 275 to 450 nm was used to artificially age the plastic. The intensity of the lamp was measured using a UV A/B light meter from Lutron (model UV340A) at a range of distances from 15 to 70 cm; at shorter distances, the intensity was off the scale of the meter. The extrapolated UV light intensity at a distance of 8 cm as in our weathering experiments was 702.8 W/m2. The 5 day exposures in our experiments therefore correspond to approximately 510 days of sunlight exposure to European mean solar irradiance (calculations in the Supporting Information). We used a custom-built rotating mixer to artificially age the plastic in the laboratory. Both the abstract graphic and a schematic drawing in Figure S1 illustrate the rotating mixer. This “weathering wheel” is constructed of aluminum-covered particle board and holds up to six 350 mL quartz glass tubes horizontally. An electric motor rotated the tubes around the UV lamp, which was centrally mounted, and ensures equal UV exposure of both floating (PE and PP) and sinking (PS and PET) plastics in each tube. A strong air flow was used to maintain a constant temperature of the lamp and samples of ∼35 °C. Four samples (i.e., one each of PE, PP, PS, and PET), each containing 5 g of plastic and 250 mL of Milli-Q water, and one blank, with only 250 mL of Milli-Q water, were exposed for 5 days while rotating around the lamp at 1 rpm. Additionally, dark control samples of 5 g of polymer in 250 mL of Milli-Q water were rotated in darkness for 5 days. The leachates from the plastics were concentrated on Waters Oasis HLB Plus Short Cartridge solid phase extraction (SPE) columns (225 mg of sorbent, 60 μm particle size). The HLB SPE cartridges were successively conditioned with 5 mL of an ethyl acetate/ methanol (1/1) mixture, 5 mL of methanol, and 5 mL of MilliQ water. The leachates were then loaded onto the cartridges, which were dried under vacuum for 30 min. The cartridges were eluted with 5 mL of an ethyl acetate/methanol (1/1) mixture followed by 10 mL of methanol. The combined extracts were concentrated to 1 mL under a stream of nitrogen and subsequently analyzed by LC−HRMS. Ultra-High-Performance LC−HRMS (UHPLC−HRMS). Nontarget analysis of compounds liberated from the plastic materials during the artificial weathering experiments was conducted using an UltiMate 3000 Rapid Separation Liquid Chromatography system (Dionex, Germering, Germany) coupled to a Q Exactive HF Hybrid Quadrupole-Orbitrap
mass spectrometer (Thermo Scientific, Bremen, Germany). A volume of 20 μL was injected onto a Hypersil GOLD aQ C18 column (2.1 mm × 100 mm, 1.9 μm inside diameter, Thermo Scientific). The LC flow rate was set to 400 μL/min using H2O (0.1% formic acid) and acetonitrile (0.1% formic acid) as eluents. The gradient started with 100% H2O for 1.0 min and was changed to 100% acetonitrile over the course of 4 min, maintained for 1.5 min, then returned to 100% H2O within 0.2 min, and equilibrated for 1.3 min prior to the next injection. The heated electrospray ionization source had a capillary temperature of 350 °C with a spray voltage of 3.7 kV, a sheath gas flow rate of 30 arbitrary units (au), and an auxiliary gas flow rate of 5 au. The mass spectrometer was run in full scan (100− 1000 Da) at a resolution of 120.000 (at m/z 200) with a datadependent MS/MS scan of the five highest peaks present in the full scan. Each sample was analyzed three times, and an average of the peak areas was calculated. Data Processing. Peak detection and alignment of the LC−MS data were performed using Compound Discoverer 2.0 (Thermo Scientific) to obtain a peak list with peak areas with the following settings: S/N threshold, 5; mass tolerance, 5 ppm; maximum retention time shift, 0.5 min; minimum peak intensity, 1 × 106. With the help of the software, a possible molecular formula fitting the exact mass and isotope patterns was calculated. Furthermore, compounds matching the exact masses were searched for in ChemSpider12,13 and the MS/MS fragments were compared to the mzCloud database.14,15 Data Evaluation and Identification of Polymer Degradation Products. The aim of this study was to identify chemicals produced by degradation of the plastic polymers and leached into water. Therefore, we analyzed the leachates from the experimental setup using a nontarget approach (Figure 1). Because we were specifically interested in degradation products of the polymer itself, our first step (1) was to remove compounds that did not match a chemical formula consisting solely of C, H, and O within a tolerance of the measured and theoretical exact mass of 5 ppm. In the second step (2), compounds with a sample/blank peak area ratio of
