Sunlight and Additives Change Which Compounds Leach from Plastic Bags

Plastics are usually very durable, but over time, plastic waste can split apart into microscopic pieces and produce new compounds that can end up in the environment, e.g., in aquatic ecosystems. Some plastic items, such as polyethylene (PE) shopping bags, can float in water, which exposes them to direct sunlight. Pure polymers can produce water-soluble molecules and gases when placed under ultraviolet light. However, plastics in consumer goods often contain a variety of additives. How the additives in these materials influence the decomposition process under sunlight is not well-understood.

Collin P. Ward, Woods Hole Oceanographic Institution, MA, USA, and colleagues have investigated how the composition of single-use shopping bags influences the dissolved compounds generated under sunlight over short periods. Using X-ray diffraction, the researchers examined four polyethylene bags from U.S. retailers and a pure polyethylene film. No inorganic additives were identified in the pure PE, but calcium carbonate (13–34 %) and titanium dioxide (0.1–2.2 %) were found in the bags.

The researchers stored samples of the plastic bags and the pure PE in separate containers containing water, either in the dark or under simulated daylight, for up to a week. Small amounts of water-soluble compounds were released in the dark, while under simulated sunlight, much larger concentrations were reached. The team analyzed the leached compounds using Fourier-transform ion-cyclotron-resonance mass spectrometry (FT-ICR-MS). The number of assigned formulas ranged from over 5,300 to over 15,300 under sunlight, representing 1.1-fold to 50-fold increases over the analogous numbers in the dark. Of the approximately 9,000 assigned formulas generated by the pure PE when exposed to sunlight, only about 28 % overlapped with those from the bags.

According to the researchers, these findings suggest that plastic formulations influence the amount and composition of water-soluble compounds generated under sunlight. They caution that studies on pure polymers may not accurately represent the impact of plastic waste on the environment.