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Conservation - Scientific Papers
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Conservation - Scientific Papers
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 Production of methane and ethylene from plastic in the environment
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Abstract
Mass production of plastics started nearly 70 years ago and the production rate is expected to double over the next two decades. While serving many applications because of their durability, stability and low cost, plastics have deleterious effects on the environment. Plastic is known to release a variety of chemicals during degradation, which has a negative impact on biota. Here, we show that the most commonly used plastics produce two greenhouse gases, methane and ethylene, when exposed to ambient solar radiation. Polyethylene, which is the most produced and discarded synthetic polymer globally, is the most prolific emitter of both gases. We demonstrate that the production of trace gases from virgin low-density polyethylene increase with time, with rates at the end of a 212-day incubation of 5.8 nmol g-1 d-1 of methane, 14.5 nmol g-1 d-1 of ethylene, 3.9 nmol g-1 d-1 of ethane and 9.7 nmol g-1 d-1 of propylene. Environmentally aged plastics incubated in water for at least 152 days also produced hydrocarbon gases. In addition, low-density polyethylene emits these gases when incubated in air at rates ~2 times and ~76 times higher than when incubated in water for methane and ethylene, respectively. Our results show that plastics represent a heretofore unrecognized source of climate-relevant trace gases that are expected to increase as more plastic is produced and accumulated in the environment. Introduction
Over the past 50 years, polymer manufacturing has accelerated, from 2x106 metric tonnes (Mt) per year in 1950 to 381x106 Mt per year in 2015, and is expected to double in the next 20 years [1]. The total global production of plastics to date is estimated at 8300x106 Mt, with polyethylene being the most common polymer [2,3], accounting for approximately 36% of all plastic types [1]. In the environment, plastics are vulnerable to weathering and degradation processes, caused by environmental factors such as light, heat, moisture, chemical oxidation and biological activity that are responsible for physical and chemical changes in the structure of the polymer [4].
Polyethylene, like other plastics, is not inert and is known to release additives and other degradation products into the environment throughout its lifetime. For example, the additive bisphenol-A used in the manufacture of many plastic products [5] is leached as plastics age, and hydrocarbon gases are produced during high-temperature decomposition (>202°C) [6]. These chemicals vary amongst different types of plastic and, once released, some can be toxic and have adverse effects on the environment and human health [7–9]. Degradation processes not only affect the chemical integrity of the plastic but also ultimately results in the fragmentation of the polymer into smaller units increasing the surface area exposed to the elements. Most plastic is synthesized from natural gases [10] and leaching is expected to occur during the aging processes. However, to the best of our knowledge, no previous study has reported hydrocarbon gas emissions from plastics under natural conditions. This study seeks to investigate this phenomenon and its potential environmental consequences. Our research investigated the production of hydrocarbon gases from polyethylene and other plastics at ambient temperature, with an emphasis on methane (CH4), one of the most potent atmospheric greenhouse gases [11–13] and ethylene (C2H4), which reacts with OH in the atmosphere and increases carbon monoxide concentrations [14,15]. Given the substantial rise in plastic production worldwide, understanding the extent of CH4 and C2H4 emissions from plastic is essential. In addition, we report production rates of ethane (C2H6), the second most abundant hydrocarbon in the atmosphere after CH4, known to enhance the level of tropospheric ozone and carbon monoxide [14,16], and propylene (C3H6), also a hydrocarbon pollutant in the atmosphere [17]. Since plastics come in different compositions and morphologies, we conducted a series of experiments to evaluate gas production under a variety of environmental conditions. We show that solar radiation initiates the production of these gases for the polymers tested. |
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