Plastics have only been around since the 1950s and yet they’re everywhere: scattered in our lakes, rivers, floating on the top of oceans, and dropping to the bottom. Back in the 1950s and 60s, plastics were hailed as a useful invention, used primarily in packaging materials. In a 2017 study published in Science Advances, scientists estimated that 8300 million metric tons (Mt) of virgin plastic had been produced to date, and as of 2015, approximately 6300 mt of plastic waste had been generated. If current trends continue, approximately 12,000 mt of plastic waste will be in landfills or in the natural environment by 2050.
Now microplastics, which are tiny plastic particles of less than 5 mm in diameter, are everywhere, and they just don’t go away. The buildup in oceans threatens marine life and ultimately ends up in the human body. There’s an estimated 24.4 trillion pieces of microplastics in our oceans, weighing between 82,000 and 578,000 tons.
Microplastics have also hindered light transmission through the ocean waters, negatively affecting the marine ecosystem by hindering light transmission and disrupting the functioning of photosynthetic organisms, such as phytoplankton and algae, which can lead to a cascading effect on the entire food chain. Researchers are assessing the radiative properties (absorption and scattering) of microplastics to determine the extent of disruption they cause to light propagation.
In a recent study published in Optical Engineering, researchers from China and Singapore determined the absorption coefficient and reflectivity of polyamide-12 (PA12), a common marine microplastic pollutant that is generated in the clothing, cosmetic, and packing industries. The researchers measured two parameters: the absorption coefficient, which indicates the amount of light absorbed, and the extinction coefficient, which accounts for the light attenuated (absorbed and scattered) by the particles.
The optical experimental setup they used contained a light source, optical fibers, an adjustable cuvette holder to hold the microplastic solution being inspected, and a spectrometer to assess the amount of light transmitted through the sample. For the microplastic solution, they prepared a suspension containing PA12 particles with a mean diameter of about 97.8 µm. They measured the amount of light transmitted through a glass cuvette containing deionized water without any contaminant to set a benchmark. Then they repeated the experiment with microplastic powder suspension in a quartz glass cuvette. From these measurements, they estimated the extinction coefficient of PA12.
The absorption parameters were measured similarly. Using an integrating sphere, the researchers collected the light scattered by the microplastic suspension in different directions and directed it toward a spectrometer for analysis.
The analysis revealed a scattering albedo (ratio of scattering efficiency to total extinction efficiency) of 0.7 for the PA12 suspension, implying that most of the light passing through it was scattered. “The scattering-dominated characteristic of PA12 changes the distribution of light in seawater, which, in turn, affects the marine ecology,” said Dr. Chunyang Ma, the corresponding author of the study.
They also found that the PA12 particles absorbed the incident radiation at certain specific wavelengths. “Absorption peaks of PA12 were observed at wavelengths of 692, 728, 764, 800, 835, and 940 nm. These correspond to the vibrational absorption of methylene and amide groups,” Ma said.
This is just one of many groups of researchers around the globe that are studying the harmful effects of microplastics. This group suggests that the radiative properties of PA12 can be used to optically monitor the flow of microplastics into oceans, which may help us to understand the importance of decreasing our use of plastics in order to prevent their entry into the aquatic as well as terrestrial food chain. While the trend toward eliminating plastics from daily use is a positive step, we’re a long way from curtailing use of plastics altogether. And even if we were to vastly reduce use, the plastic used over the past 50+ years are still with us. Being armed with the facts on the harmful effects of microplastics will go a long way toward understanding the effects of toxins in the human body.
Paper: “Thermal radiative properties of polyamide-12 from 0.2 to 1.1 μm” by Wen et al., Opt. Eng. 62(3) 034102 (2023). doi: 10.1117/1.OE.62.3.034102.
Photo: Microscopic photo of PA12, a marine microplastic pollutant. Credit Wen et al. doi: 10.1117/1.OE.62.3.034102.