Identification of Chain Scission Products Released to Water by Plastic Exposed to Ultraviolet Light

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.

Effects of Leachates from UV-Weathered Microplastic in Cell-Based Bioassays

“…A detailed description on the weathering setup is given by Gewert et al (2018). 24 In short, triplicates of 50 g of each test material, suspended in 200 mL (i.e., liquid− solid ratio of four) of ASW (Instant Ocean Sea salt, Blacksburg, Virginia U.S.A.) in quartz glass vessels were weathered by intense UV A+B light irradiation (OSRAM Supratec HTC400−241 R7s UVA/UVB lamp), combined with horizontal rotation of the vessels around the lamp. Six vessels were weathered at a time using a custom-made wheel, which rotates the quartz glass vessels around the UV lamp to ensure equal UV exposure and to provide gentle mixing of the particles (SI Figure S2).…”

“…The 96-h UV treatment in the rotating vessels simulated about 410 days of Middle European sun exposure. 24 Procedural blanks were generated by completing the weathering protocol with ASW without microplastic. A detailed description of the lamp properties and solar simulation equivalence calculation can be found in the SI (Section S1.…”

“…The dried aliquots of the concentrated leachates were taken up in 100 μL of methanol/Milli-Q water (1/1) and analysis of dicarboxylic acids was performed using an UltiMate 3000 Rapid Separation Liquid Chromatography system (Dionex, Germering, Ger-many) coupled to a Q Exactive HF Hybrid quadrupole-Orbitrap mass spectrometer (Thermo Scientific, Bremen, Germany). The method was adopted from Gewert et al (2018) 24 as specified in the SI (Section S3. Instrumental Analysis).…”

“…The most important abiotic degradation process for plastic in the environment is UV radiation-initiated autocatalytic radical oxidation. 2,23 Recently, Gewert et al (2018) 24 identified a set of low molecular weight polymer chain scission products liberated from commercially important polymers exposed to UV light. They were mainly dicarboxylic acids, but also included other oxidized end-groups.…”

“…31,32 Liquid chromatography coupled to high-resolution mass spectrometry (LC-HRMS) was used to identify potential degradation products, i.e., carboxylic acids, in the leachates. To better account for PPARγ results, we tested the hypothesis that dicarboxylic acids, previously identified as degradation products of UV-weathered PE, 24 could explain the observed explicit induction of PPARγ, by dosing reference mono-and dicarboxylic acids with a range of carbon chain lengths (C5− C18) into the PPARγ assay. In a last step, we applied a concentration addition model to compare the observed bioanalytical effects to the chemical analytical data.…”