The oceans are flooded with hundreds of thousands of tons of microplastics. But this figure does not take into account the smallest particles, the abundance of which is confirmed by a study, thanks to a specific observation technique.
There microplastic pollution observed so far would only be the visible part of the iceberg. In the study (1) published on April 18 in the journal Bulletin on marine pollutionthree American researchers from Stony Brook University in New York State confirm that current estimates do not take into account what makes up the vast majority of particles: those smaller than 50 microns. “This fraction is completely neglected in almost all samples of microplastics”confirms Luis Medina Faull, one of the co-authors of the study.
Sampling limit
As part of their work, the three scientists did not favor a conventional method for identifying and characterizing this microscopic contaminant. As a general rule, such sampling involves the use of nets: “manta” type, trolling, for surface studies, or “closure” for deep observation. The nets used are particularly dense and can capture particles up to one hundred microns. The latest reference study is based on this (2) on that topic, published in 2021.
This has already led to a re-evaluation of the scope pollution (between 80,000 and 580,000 tons) by collecting observations from twenty different samples. However, the entire corpus (including samples from the surface or up to a depth of three meters) relied on nets whose mesh best captured particles of 200 microns (the maximum size to be considered for a microplastic being of 5 millimeters, or 5,000 microns). “Conventional net sampling captures plankton-sized microparticles but allows smaller ones to slip throughunderlined by Luis Medina Faull. However, choosing Raman spectroscopy to examine our samples opened up a whole new world of microplastics that are still invisible. »
The abundance is five to six times greater
The technique used by the researchers is generally used for the detection and chemical characterization of particles in a sample of liquid media. It uses a physical phenomenon, called the “Raman effect”, which enables knowledge of the molecular composition, but also the structure (thus size and mass) of materials or particles present in the medium according to their reaction (in frequency or vibration) to light.
By examining three water samples (from Venezuela, the Caribbean and the Pacific coast of the Arctic) with this spectroscopy, the researchers were able to identify plastic microparticles to the nearest micron. In terms of numbers, between 68 and 86% of the identified microplastics were less than 5 microns, with the remainder remaining below 15 microns. “We have not identified any particles larger than 53 microns”, the researchers add. And this, while in terms of concentration, their samples included up to five to six times more particles than in the previous sampling of a comparable order, but limited to particles of 300 microns or more. “This is potentially several hundred or thousands more microparticles for a sample of similar volume. »
Article published on May 24, 2024