Spotting invisible microplastics with a bit more light

Microplastics – plastic fragments less than a few millimetres in length – are becoming a serious environmental concern.

But it’s hard to judge the exact effects of microplastics, or even how many there are at the moment, because they’re difficult to detect.

Best practice involves identifying every single microplastic fragment by hand – which, given they exist in the trillions, is so time-consuming it can only give us tiny slivers of information.

But researchers are working on better methods to spot microplastics: particularly by using light.

A team of Chinese and Singaporean researchers has published some tell-tale signatures of polyamide-12, a common microplastic substance, in the journal Optical Engineering.

These signatures could be used to estimate amounts of polyamide in much bigger sections of lakes, rivers and oceans.

Every type of molecule interacts slightly differently with light: absorbing some shades of light, and reflecting others.

This is also true of different types of plastic. Researchers have already collected data on the way polystyrene, another common form of plastic, interacts with light.

This research adds polyamide-12 to the mix.

Also called nylon-12, polyamide-12 is a common type of plastic, used in everything from 3D printing ink to face cream thickeners.

In a lab, the researchers shone different wavelengths of light through a tube containing polyamide-12 particles suspended in water.

They measured the absorption coefficient and extinction coefficient of polyamide-12: the amount of light the substance can absorb, and the light that gets scattered and absorbed by the particles respectively.

They also found that polyamide-12 scattered a lot of light: in fact, most of the light passing through water with polyamide-12 in it gets scattered by the particles.

If they’re changing sunlight this much, it may affect photosynthesising organisms in the ocean, which could then send ripples up the food chain.

“The scattering-dominated characteristic of polyamide-12 changes the distribution of light in seawater, which, in turn, affects the marine ecology,” says corresponding author Dr Chunyang Ma, a researcher at Nanchang University in China and the National University of Singapore.

But with the data the researchers have collected, it should be easier to monitor for polyamide-12 microplastics.

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