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.
Highlights
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 20 years [1]
The influence of plastic composition on hydrocarbon gas production Our results show that different types of plastics, that are commonly used and dispersed in the environment worldwide, produce CH4 and C2H4 under environmental conditions
We hypothesize that the relative amounts of low-molecular-weight hydrocarbon gas molecules that are released from plastic substrates depend on the molecular structure of the plastic including the degree of branching, the addition of plasticizers, as well as the manufacturing process
Summary
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 20 years [1]. 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 [7,8,9]. 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 [11,12,13] and ethylene (C2H4), which reacts with OH in the atmosphere and increases carbon monoxide concentrations [14,15]. We show that solar radiation initiates the production of these gases for the polymers tested
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