Introduction
Most of the time, when we think of microplastic pollution, we think of little fragments of plastic collected in soil. However, plastic contains chemical additives, and those chemicals can leach into the water around the fragment. This leachate is the other, invisible side of plastic pollution, and we actually know very little about it. Hence, this created various questions for scientists, like when different plastics leach into water, what chemicals do they release, and could they have an effect on a plant before it even gets a chance to sprout? To investigate, the authors turned to lettuce, one of the most commonly consumed leafy vegetables.
Methods
The researchers produced leachate from two generic plastics, polyamide and polyethylene (the same plastic used in bags and films). They then soaked microplastic fibers for each at 25C and 50 C, to simulate gentle and high-temperature environments. They then characterized the leachates by the amount released, carbon and nitrogen, aromaticity, hydrophobic content, and molecular weight. The leachates were then applied to lettuce seed and tested for germination.
Results
Through this experiment, these two plastics were found to be vastly different. Dissolved carbon and nitrogen levels were way higher in the nylon leachate than the polyethylene, with values climbing along with temperature. Aromaticity and hydrophobicity in each leachate were largely governed by the plastic type itself, rather than temperature. The nature of the leachate is more dependent on the kind of plastic than the environment it is in. To test whether these anomalies held for plants, the scientists tracked the germination scores of lettuce seeds in each leachate.
Conclusion:
This work changes our perspective on plastic pollution. Microplastics aren't one problem, but several. Each variety is introducing its chemical profile into the ground. A nylon fiber and a polyethylene sheet may look similarly innocuous, but they impart profoundly different chemical doses into the soil-and it's not just their chemical dose, but the dose of their heat-plastic that warms the soil may do so even more. For the plants drawing nutrition from that soil, it might just be the plastic that they are physically in contact with that is least dangerous-and finding out which plastics are the biggest leakers is the key to prioritizing which chemicals to study.
