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ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Perspective biodegradable plastics as well. Interestingly, average photo- degradation rates for “biodegradable” plastics do not differ much from those for petrochemical-based plastics. Effects of Accelerating Conditions. UV Irradiation. Solar UV radiation is necessary to initiate photo-oxidation of most polymers,37 which proceeds via a radical chain mechanism that results in bond cleavage and a decrease in molecular weight (see above). Shorter chains, often with oxygen-containing functional groups at the chain ends, are more prone to attack by microorganisms and mineralization due to their increased hydrophilicity (which enhances microbial adhesion).178 Several experimental studies reported synergy between photo-oxidation and biodegradation of polyethylene.179−181 In another study, the abundance of carbonyl groups in polyethylene increased upon exposure to UV radiation for 60 h, then decreased when the photo-oxidized polyethylene was subsequently incubated with microorgan- isms, suggesting microbial degradation.182 Transient Thermal Treatment. Moderate heating in air enhances rates of polyolefin oxidative degradation significantly. The increase in polymer hydrophilicity arising from incorpo- ration of oxygen-containing functional groups facilitates surface attachment of microorganisms.27,178 In a study conducted in a soil culture over 12 months, films of LDPE, HDPE, or PP that were thermally pretreated at 80 °C for 10 days showed enhanced biodegradation rates of 12, 4.5, and 2.7 μm year−1, respectively, compared to 6.3, 1.8, and 0.1 μm year−1, respectively, without thermal pretreatment.130 In the marine environment, LDPE and HDPE films thermally pretreated at 80 °C in an oven for 10 days showed 4- to 7-fold increases in marine bacterial colonization compared to the untreated materials over six months. Higher mass losses were also observed for thermally pretreated LDPE and HDPE (17% and 5.5%, respectively), compared to the untreated materials (10% and 1%, respectively) over the same time period.135 Humidity. Elevated humidity levels accelerate polyester degradation by promoting hydrolysis. For example, chain scission of a PET in a plastic bottle was 5 times greater at 60 °C and 100% relative humidity compared to 45% relative humidity. However, at temperatures of 80 °C or higher, there was no significant increase in the rate of hydrolysis with increased humidity, since the rate of thermal-oxidative degradation outpaces that of hydrolysis at these elevated temperatures.153 Humidity has also been shown to accelerate the photodegradation of PLA,183 and polyolefins such as PE,184 PP,185 and PVC,186 by promoting an increase in the concentration of hydroxyl radicals. ■ CONCLUSIONS Each year, 400 Mt of plastic waste is generated, of which 175 Mt enters landfills and the natural environment.1 The amount of plastics that enters landfills and the environment is sufficient for rebuilding the currently standing Great Wall of China with a span of 6000 km187 every 12 months. Despite this massive scale, the literature on plastics degradation under environ- mental conditions is disproportionately thin. Relatively little has been reported in the literature regarding the following: How quickly do plastics degrade in the environment? What are the degradation pathways? What are the factors affecting the degradation processes? What are the degradation byproducts? When juxtaposed, existing literature on plastics degradation rates shows a wide range of numbers. Furthermore, the various degradation byproducts may include larger molecules, as well as microscale and nanoscale plastics with increased bioavailability and associated potential adverse impacts to ecosystems throughout the food web. Additional research is needed to better understand the mechanisms of polymer degradation under various environ- mental conditions. Given the long life-span of plastics, methods to use short-term experimental results to predict long-term degradation pathways and methods to simulate degradation, for example, using modern computational chemistry techniques, must also be further refined and vetted. Our review highlights the need to standardize the metrics and the experimental conditions used in plastics degradation research. Studies on plastics degradation often omit key information, such as the temperature, microbial loading, and the size and shape of the sample, which are essential to interpret the results properly. These deficiencies, combined with the sparseness of the literature, limit the ability to conduct meaningful meta-analyses. The SSDR metric proposed here is a step in that direction; however, it only measures how much material, or mass, is lost from the sample. Therefore, neither structural changes nor the extent of mineralization of plastics can be addressed using SSDR alone. Furthermore, extrap- olations are fraught with uncertainty. We anticipate the need for development and implementation of multiple well-defined standard metrics to quantify the rates of polymer degradation in the environment. 3504 ■ Corresponding Authors Susannah L. Scott − Department of Chemistry & Biochemistry and Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, California 93106, United States; orcid.org/0000-0003-1161-0499; Email: sscott@engineering.ucsb.edu Sangwon Suh − Bren School of Environmental Science and Management, University of California, Santa Barbara, Santa Barbara, California 93106, United States; orcid.org/0000- 0001-8290-6276; Email: suh@bren.ucsb.edu Authors Ali Chamas − Department of Chemistry & Biochemistry, University of California, Santa Barbara, Santa Barbara, California 93106, United States; orcid.org/0000-0001- 8739-8880 Hyunjin Moon − Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, California 93106, United States Jiajia Zheng − Bren School of Environmental Science and Management, University of California, Santa Barbara, Santa Barbara, California 93106, United States Yang Qiu − Bren School of Environmental Science and Management, University of California, Santa Barbara, Santa Barbara, California 93106, United States Tarnuma Tabassum − Department of Chemistry & Biochemistry, University of California, Santa Barbara, Santa Barbara, California 93106, United States Jun Hee Jang − Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, California 93106, United States; orcid.org/0000-0002-1879-0544 Mahdi Abu-Omar − Department of Chemistry & Biochemistry and Department of Chemical Engineering, University of https://dx.doi.org/10.1021/acssuschemeng.9b06635 AUTHOR INFORMATION ACS Sustainable Chem. Eng. 2020, 8, 3494−3511PDF Image | Degradation Rates of Plastics in the Environment
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