Abstract
Polyethylene (PE) is an intensely utilized polymer, which has consequently led to it becoming a common environmental contaminant. PE and other plastic waste are known to be highly persistent in surface waters; however, chemical and physical changes do take place over time, dependent mostly on highly variable natural conditions, such as oxygen (O2) availability. Gamma radiation was used to generate reactive oxygen species, namely hydroxyl radicals, in initially aerated aqueous solutions to simulate the natural weathering of microplastics in waters where there are fluctuations and often depletions in dissolved O2. The headspace of the irradiated PE-containing solutions was probed for the formation of degradation products using solid-phase microextraction (SPME) fibers in combination with gas chromatography mass spectrometry (GCMS). The major species detected were n-dodecane, with trace levels of tridecane, 2-dodecanone, and hexadecane, which were believed to be predominately adsorbed in the PE microplastics in excess of their aqueous solubility limits. Surface characterization by Raman spectroscopy and light and dark field microscopy indicated no change in the chemical composition of the irradiated PE microplastics under low O2 to anaerobic conditions. However, morphological changes were observed, indicating radical combination reactions.
Highlights
Polyethylene (PE) is the most highly manufactured and utilized polymer in the world, with continuously projected growth as consumer demand in automobile, construction, packaging and other applications continues to rise [1,2]
PE microplastics were purchased from MilliporeSigma (Burlington, MA, USA) with an average molecular weight (MW) of ~4000 g mol−1 and an average Mn~1700, as determined by gel permeation chromatography (GPC), and a density of 0.92 g/mL at 25 ◦C
Beyond ~4 kGy of absorbed gamma dose, dissolved O2 in the water was no longer present for chemical transformations. Since these PE stock microplastics were subjected to gamma doses between 0.5 and 2.0 MGy, O2 was present only during the very early stages of these irradiations
Summary
Polyethylene (PE) is the most highly manufactured and utilized polymer in the world, with continuously projected growth as consumer demand in automobile, construction, packaging and other applications continues to rise [1,2]. A problematic outcome of the over nine billion metric tons of plastic generated since 1950 is a global environment containing a growing amount of persistent waste plastic [11] Much of this waste contaminates surface waters, where it fragments into even more problematic microplastics [12]. Microplastics that encounter low O2 to anaerobic conditions have not been thoroughly studied, yet these low O2 concentration regimes exist in stagnant waters, lower water column areas, and transiently in all waters These conditions may be part of the reason for prolonged lifetimes in the environment and lead to varying morphologies and products of microplastics, some of which may remain adsorbed [39]. We present a radiation chemistry investigation into the radical-induced weathering of PE microplastics that experience fluctuations in dissolved O2 concentrations by utilizing initially aerated water mixtures where dissolved O2 is depleted over time by radical consumption (Equation (2))
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