Microplastics are tiny plastic particles less than 5 millimeters in diameter that have emerged as a novel marine environment pollutant. Micro­plastics usually result from a breakdown of larger plastic debris but can also be generated from plastic microbeads used in personal care products. Over the years, there has been a significant buildup of microplastic pollutants in our oceans, with a recent estimate that the world’s oceans contain over 24.4 trillion pieces of microplastics weighing between 82,000 and 578,000 tons. It is highly likely that micro­plastics are being consumed by marine organisms and have pervaded the aquatic food chain.

Addi­tionally, micro­plastics have negatively affected the marine ecosystem by hindering light transmission through the ocean waters. This has disrupted the functioning of photosynthetic organisms, such as phytoplankton and algae, which can lead to a cascading effect on the entire food chain. It is, therefore, imperative to assess the radiative properties – absorption and scattering – of microplastics to determine the extent of disruption they cause to light propa­gation. Fortunately, in a new study researchers from China and Singapore rose to the occasion. In their study, they 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 specifically focused on measuring two important 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. Accordingly, they constructed an optical experimental setup containing 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 micrometers. To assess the degree of light scattering, they measured the amount of light trans­mitted through a glass cuvette containing deionized water without any contaminant to set a benchmark. They then 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. The researchers used an inte­grating sphere to collect 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 Chunyang Ma.

Addi­tionally, the PA12 particles were found to absorb the incident radiation at certain specific wavelengths. “Absorption peaks of PA12 were observed at wavelengths of 692, 728, 764, 800, 835, and 940 nanometers. These correspond to the vibra­tional absorption of methylene and amide groups,” Ma said. With these insights, the researchers suggest that the radiative properties of PA12 can be used to optically monitor the flow of micro­plastics into oceans. This, in turn, could help us prevent their entry into the aquatic as well as terrestrial food chain, mitigating its threat to all forms of life, including humans. (Source: SPIE)

Reference: H. Wen et al.: Thermal radiative properties of polyamide-12 from 0.2 to 1.1 μm, Opt. Eng. 62, 034102 (2023); DOI: 10.1117/1.OE.62.3.034102

Link: School of Mechanical and Electrical Engineering, Nanchang University, Nanchang, China