Identifying Microplastics in the Environment

A group of Danish scientists is using a system from Renishaw to find out if plastic and rubber from tires, that are washed into streams and rivers, contribute to pollution in waste water.

What are Microplastics

Microplastics are particles that are small and hard-to-spot. They make their way from packaging and products into the environment, where they cause significant pollution. The Danish Technology Institute (DTI, Aarhus, Denmark) is characterizing the amount of microplastic pollution entering Danish rainwater and wastewater systems using a Renishaw Raman spectroscopy system.

DTI, a non-profit organization, works with public and private-sector companies to develop and implement new technologies. These include removing plastic particles from the aqueous environment. Although they are difficult to identify and characterize, DTI has already developed a valid research method to determine the presence of microplastics.

Currently, Morten Bormann Nielsen and colleagues in the Life Sciences Division are developing new methods and technology for use in the environmental sector. Dr Nielsen describes his work,

At scales below 100 µm (the width of a hair), it is essentially impossible to determine whether a particle is made of plastic, rubber, stone, glass or organic matter, based on its visual characteristics alone. Therefore, one must use a characterisation technique that yields chemical information about the investigated sample. Doing otherwise will lead to either gross over- or underestimation of the number and types of microplastic present in a sample.

Dr Morten Bormann Nielsen, Life Sciences Division, Danish Technology Institute

Renishaw inVia™ Qontor® Raman Microscope

The research group chose a comprehensive analysis system, a Renishaw inVia™ Qontor® Raman microscope, in order to overcome this problem. The team has already amassed over ten years of experience working with Raman systems from Renishaw.

"We wished to characterise the extent of microplastic pollution entering Danish wastewater and rainwater systems, as well as the degree to which these particles can be retained with different technologies. We also wanted to be able to identify microplastic and microrubber particles from tyres, as the Danish Environmental Protection Agency has estimated that up to 60% of all microplastics that enter the ocean from Denmark originate from this source – primarily via rainwater run-off from roads," continued Dr Nielsen.

It is extremely difficult to guarantee valid microplastic counts from environmental samples. This is due in part to the fact that the particles of interest make up such a minute fraction of each sample (on the order of 150 milligrams per cubic meter of waste).

Care must be taken during isolation of the plastic particles in order to avoid contamination. Furthermore, so that it does not shield the particles, organic matter must be removed. Therefore, a delicate chemical process is required that does not degrade the particles. DTI has the advanced equipment and high level of expertise needed to achieve this.

DTI uses an inVia Qontor Raman microscope because it enables highly automated and efficient experiments to be carried out on samples that contain microplastics. Most notable of the many essential features of the inVia system that facilitate this process are: batch Raman measurements with automatic stage movement to the center of each particle, automatic focusing on uneven particles with LiveTrack™ focus tracking technology, and large area optical microscope montages for particle finding.

Dr Nielsen explains, “The speed of the inVia Qontor system is an essential part of being able to obtain chemical information on the many thousands of particles that are naturally present in samples taken from the environment. Despite sample preparation steps designed to remove sand and organic matter, the vast majority of all microscopic particles in wastewater and rainwater are still not plastic. Because of this, it is essential to measure many thousands of individual particles in the sample to get statistically valid results. Using the features of the inVia Qontor system it is possible to run such measurements overnight, saving us a lot of valuable time.”

“The inVia system is extremely versatile when it comes to chemical characterisation across all life science-related fields. I struggle to think of any other single instrument that is as useful across samples as diverse as pharmaceuticals, plastics, wood-coatings, bacteria, tissue or inks. We sometimes compare it to a synchrotron, in that it really allows you to collect vast amounts of high-quality data in a very short amount of time – only this one fits on a table and not on a football field. Aside from the high-quality spectroscopy that the system delivers, the motorised stage is my favourite part of the system. Throughout my scientific career I have used a lot of different brands of xy-stages, and very few of these come even close to the reliability and speed of the Renishaw stage. Being such an essential part in enabling both batch and mapping measurements, it is worth highlighting.”

Quantifying Microplastics in Wastewater

Already, the DTI has developed and successfully implemented a method for quantifying microplastics in wastewater. This method was used to show a retention rate of at least 99% in two Danish wastewater treatment plants.

The measurement protocol has been further developed so that it can differentiate correctly between fine particles of organic matter, tire and asphalt. Therefore, DIT is very close to being able to fully characterize all of the particles in a sample and classify them as microrubber, microplastics or of natural origin. The advantage of this is that there is no need for intervening sample preparation as the same method is used. As far as DIT is aware, this has not been previously achieved anywhere else.

This combined method is currently being used at different locations in Denmark to quantify the amount of microrubber and microplastics in rainwater. Figure 1 gives an example, highlighting all particles that are larger than 10 mm and classifying them by type. Here, all non-plastic particles are black, whilst the different plastic types are shown in color.

Conclusion

The need for very thorough sampling is highlighted by the fact that, out of more than 4000 particles measured, only 33 matched one of the 11 most commonly used reference plastic types. Insets in Figure 1 show examples of four types of plastic alongside the recorded spectra and the matched reference spectrum: Green: Polyethylene terephthalate (polyester), Orange: PTFE (Teflon™), Cyan: Polypropylene, Red: Polyethylene.

Microscope montage, measuring 5.2 mm by 5.3 mm, of a rainwater sample, after post-processing to identify all particles larger than 10 µm. The vast majority of the particles are not plastic (black). The inserts show microplastic particles identified by spectral matching to different reference plastic types. Orange: PTFE (Teflon™), Green: Polyethylene terephthalate (polyester), Red: Polyethylene, Cyan: Polypropylene.

Figure 1: Microscope montage, measuring 5.2 mm by 5.3 mm, of a rainwater sample, after post-processing to identify all particles larger than 10 µm. The vast majority of the particles are not plastic (black). The inserts show microplastic particles identified by spectral matching to different reference plastic types. Orange: PTFE (Teflon™), Green: Polyethylene terephthalate (polyester), Red: Polyethylene, Cyan: Polypropylene.

This information has been sourced, reviewed and adapted from materials provided by Renishaw plc - Spectroscopy.

For more information on this source, please visit Renishaw plc - Spectroscopy.

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