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ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Perspective either landfilled or enter the natural environment, where plastics accumulate and persist for a long period of time.1 In the United States, where the landfill rate for discarded plastics exceeds 75%, such polymers are now responsible for a significant fraction (19%) of all municipal solid waste.8 At current growth rates, the accumulation of plastics waste in landfills and/or in the natural environment is projected to reach nearly 12,000 Mt globally by 2050.1,9 The amount of plastic waste entering the oceans has emerged as a major concern. Large-scale concentrated accumulations of plastics have been found in the South Pacific subtropical gyre and the Eastern Pacific Ocean gyre.10−12 Even in a medium waste-to-debris conversion rate scenario, the total amount of plastics waste in the ocean is expected to grow from 50 Mt in 2015 to 150 Mt by 2025.13 The sources of this material are highly correlated with the absence of effective waste management infrastructures. It has been estimated that nearly 90% of the plastics entering the ocean comes from just 10 rivers, all located in Asia or Africa.14 Ocean plastic debris is associated with persistent organic pollutants (POPs), including polychlorinated biphenyls (PCB), pesticides, and polycyclic aromatic hydrocarbons (PAHs), due to the higher affinity of these hydrophobic molecules for plastics compared to their affinities for sediments or water.15,16 Finally, nearly 700 marine species have been observed to interact directly with plastics marine debris through ingestion, entanglement, and/or smothering.17 There are often vast differences between plastics degradation rates reported in the peer-reviewed literature and those reported by the popular press. A few media reports acknowledge the deficit of knowledge about the degradation rates of plastics,18,19 but more often, they present degradation times as known, despite the paucity of scientific evidence. Media estimates of degradation times for plastic bags tend to fall into one of two ranges: 10−20 years20 or 500−1000 years,21 while that for “plastic” bottles is reported as over 70 up to 450 years.21 Some media have reported that “plastics” do not degrade at all.22 In these claims, however, the type of plastics is often unclear, and the environmental conditions are not specified. Also, the extrapolation method is unknown. Each of these factors has a large impact on degradation times. Furthermore, scientific studies of plastics degradation times are evolving, and estimated lifetimes can change dramatically based new evidence. For example, a recent study found that polystyrene exposed to sunlight degrades on much shorter time scales than the thousands of years in previous estimates.23 This study aims to present an overview of plastics degradation pathways in the environment and to summarize current knowledge about degradation rates for different types of commodity plastics under various environmental conditions. The results should help researchers and policymakers to more accurately describe the times needed for various plastics to degrade in the environment. ■ ABIOTIC DEGRADATION PATHWAYS The environmental degradation mechanisms for plastics can be classified as either (i) physical, referring to changes in the bulk structure, such as cracking, embrittlement, and flaking, or (ii) chemical, referring to changes at the molecular level such as bond cleavage or oxidation of long polymer chains to create new molecules, usually with significantly shorter chain lengths. The potential environmental hazards associated with the soluble chemical byproducts of plastics degradation must be considered,24 as well as with the leaching of small molecules added during product formulation. Typically, chemical degradation at near-ambient temperatures in the environment involves either hydrolysis (requiring H2O) or oxidation (requiring O2), both of which can be accelerated by microbial action, heat, light, or combinations thereof.25,26 In the sections below, we focus on natural abiotic processes that lead to the chemical degradation of polyethylene (PE), polyethylene terephthalate (PET), and polylactic acid (PLA). Although biotic degradation pathways are also undoubtedly important,27 degradation is typically initiated abiotically (light, heat, acids, etc.).28,29 Abiotic and biotic processes often work in tandem, with abiotic degradation leading to smaller molecules that are subsequently mineralized by microbes.30 In this section, we focus on degradation pathways for PE, PET, and PLA due to the high relative number of studies of these plastics described later in this Perspective. Degradation mechanisms for other commodity plastics (including polyvinyl chloride (PVC),31,32 polypropylen (PP),33−35 and polystyrene (PS)36,37) have been reviewed elsewhere.38 Polyethylene (PE). Although PE is the most inert of the polyolefins, it does degrade slowly in the natural environment. The backbone chains of PE are constructed exclusively from C−C single bonds which do not readily undergo hydrolysis and which resist photo-oxidative degradation due to the lack of UV−visible chromophores. Adventitious impurities or struc- tural defects that form in PE during its manufacturing, or during subsequent weathering,38 can act as chromophores.39 PE may also contain a small number of unsaturated (CC) bonds in the main chain or at the chain ends (typically, vinyl groups in HDPE and vinylidenes in LDPE). These sites are readily oxidized by O3, NOx, or other tropospheric radicals, often to highly unstable hydroperoxides, which are then converted to more stable UV-absorbing carbonyl groups.40 An increased rate of photo-oxidation was reported for LDPE, relative to HDPE, due to the higher frequency of reactive branch points in the low density polymer.41 In the absence of sunlight, thermal oxidative degradation of PE does not occur at appreciable rates at temperatures below 100 °C.42 Since the role of light in photo-oxidative degradation is only to initiate chain reactions,42 similar product distributions are generated in both photochemical and thermal processes (Figure 1). In environments lacking both sunlight and oxygen (e.g., landfills), anaerobic thermal degradation is unlikely to proceed naturally due to the high temperatures required (≥350 °C).43,44 3495 Figure 1. Common products in the thermal- and photo-oxidative degradation pathways for polyethylene (R, R′, and R′′ are polymer chains of variable length). https://dx.doi.org/10.1021/acssuschemeng.9b06635 ACS Sustainable Chem. Eng. 2020, 8, 3494−3511PDF Image | Degradation Rates of Plastics in the Environment
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