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ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Perspective Since experimental assessment of polymer degradation rate laws would require kinetic studies that last multiple decades or even longer, we assume a simple first-order dependence on SA (eq 3) and further assume that SA is constant over time (i.e., pseudo-zeroth-order behavior). However, we must point out that these assumptions have substantial consequences and can lead to extrapolation errors that represent decades or even centuries of additional lifetime. For example, a 100 μm thick film with a SSDR of 1 μm year−1 would require 100 years to degrade completely following a pseudo-zeroth-order rate law (constant surface area), while the same film would be 99% degraded in nearly 500 years if the reaction is first-order degradation. The error is much less important at lower extents of the reaction: using the same SSDR, pseudo-zeroth-order degradation would require 50 years to reach 50% completion, compared to 68 years for first-order degradation. In addition, we note that the polymer is likely to have undergone extensive chemical and morphological changes as the reaction approaches high conversion. For example, disintegration into smaller pieces (e.g., microplastics) may alter the rate of reaction dramatically. Although little is known about the details of such processes, recent reports suggest that mechanical forces can cause the flaking of weathered, or oxidized, surfaces, leading to ablation, in addition to macro- fragmentation.166 Studies of several polymer types in a weathering chamber revealed exponential growth in the numbers of nanometer- and micron-sized particles released over several weeks.125,167 However, since nearly two-thirds of microplastics in the ocean are estimated to originate from the washing of synthetic fabrics and the abrasion of rubber tires,168 fragmentation of bottles, bags, pipes, and other large rigid pieces may not be major sources of microplastics and nanoplastics. No extrapolation model is likely to describe the rates of complex phenomena involved in polymer degradation accurately. Consequently, we choose to compare degradation rates by calculating the first half-life, or the time in which the material loses 50% of its original mass. The values are still associated with large uncertainties but are probably much more accurate than attempting to extrapolate the time for “complete” degradation. Subsequent half-lives may be very different from the first half-life, depending on the rate law. In addition, the representativeness of literature values for degradation rates is unknown, and averaging them does not allow us to assess uncertainties in a statistically meaningful way. Nevertheless, in view of the public interest in estimated plastics lifetimes, we averaged the available SSDRs for each plastics type under each type of degradation condition in Figure 8 in order to estimate a first half-life for several common household plastic items, shown in Table 1. Ranges for these half-lives, obtained by multiplying SSDRs based on reported minimum and maximum values for degradation rates by the typical thickness of each plastics type for the specified application, highlight the large uncertainty in the extrapolation. The degradation was assumed to be unidirectional, proceeding from the exterior of the material toward the center. According to Table 1, common single-use plastics items like LDPE plastic bags and HDPE milk bottles and laundry detergent containers have estimated half-lives of 5 and 250 years, respectively, in landfill/compost/soil conditions. In the marine environment, the estimated half-lives are shorter, at 3.4 and 58 years, respectively. All values are subject to additional uncertainty because some data suggest much longer lifetimes, based on degradation rates that were immeasurably slow in both landfill/ compost/soil and marine environments. For heavier industrial items like HDPE pipes, complete degradation may require thousands of years, regardless of the environment (we note that the durability of such items is often desirable for their intended purpose). In order to obtain estimated times for complete degradation, we can assume a constant rate of degradation, a constant surface area:volume ratio, a constant reaction order, uniform crystallinity, and a mechanism for microplastic degradation identical to that of the parent material. These assumptions make such estimates highly uncertain. Nevertheless, the complete degradation of an HDPE bottle is estimated to require 500 and 116 years in the land and marine environments, respectively. Environmental Effects on Degradation Rates. Pro- longed exposure to environmental factors such as moisture, heat, light, or microbial action causes polymers to be abraded into smaller pieces (eventually to microplastics) as well as cleaved into small molecules.11 The effect of a particular environmental factor on the degradation rate depends strongly on the type of plastic. For example, some studies of petrochemical-based polymers show that degradation rates are lower in the marine environment compared to land- fills.169,170 The differences are typically attributed to lower ambient temperatures and low dissolved oxygen concen- trations in the marine environment.25 However, our literature analysis shows that average degradation rates for HDPE and LDPE are actually slightly higher in marine environments compared to degradation on land (Table 1), although the differences could be statistically insignificant due to the large uncertainties of the averages. The effects of lower temperatures and oxygen concentrations in the ocean may be outweighed by the more intense UV radiation, relative to a landfill. In other situations, plastics exposed to sunlight on land can experience “heat buildup”, reaching temperatures higher than the surrounding air and experiencing accelerated degrada- tion.171 Temperatures in some landfills and industrial com- posters have been reported to reach 80−100 °C,152 accelerating degradation rates provided sufficient oxygen and/or moisture are present for the thermal-oxidative degradation and hydrolysis pathways, respectively. For example, PLA undergoes ester hydrolysis under industrial composting conditions (≥60 °C), although it is very slow to degrade at lower temperatures.56 Consequently, PLA appears to be just as recalcitrant as its petrochemical counterparts in marine environments, where temperatures are well below 60 °C (Figure 8). Landfill/soil/compost conditions typically apply to buried materials that experience little solar UV radiation, hindering photodegradation. Biofouling can hinder the rate of photo- degradation by decreasing sunlight penetration.25,139,171 In the ocean, biofouling can also increase the overall density of plastic pieces, causing them to sink172,173 (although some plastics, including PET, PVC, PLA, do not float anyway).174−177 The process may be time dependent, as plastic debris has been observed to undergo repeated cycles of sinking and floating. After the fouled plastic debris sinks in the water column, it can undergo defouling due to the absence of sunlight needed to maintain the film, causing the density to decrease and the debris to resurface.174 Such studies have been largely conducted on petrochemical-based plastics, but similar changes in buoyancy are expected to affect the degradation kinetics of https://dx.doi.org/10.1021/acssuschemeng.9b06635 3503 ACS Sustainable Chem. Eng. 2020, 8, 3494−3511PDF Image | Degradation Rates of Plastics in the Environment
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