Testing For Microplastics: Challenges And Solutions
Microplastics have been present in the water cycle for a long time but only came to international attention as a problem in the marine environment in the 1970s. Lacking a way to test for microplastics, for decades scientists, governments, and academics merely observed this problem. But as regulation and solutions become a priority on a global scale, so too has the development of standards for testing.
Developing testing standards for microplastics has been a challenge in and of itself. Water Online spoke with Lindsay Lozeau, senior R&D scientist at MilliporeSigma, about microplastics, the methods and technologies currently used for testing, their limitations, and how to use them to get the most accurate measurements possible.
How has microplastic contamination become such a significant challenge?
Lozeau: The sheer quantity of plastics we consume worldwide is incredible: We produced nearly 370 million tons of plastic in 2019, and there is evidence that the COVID-19 pandemic has only accelerated this with the increased use of single-use plastic personal protective equipment on a global scale. The textiles industry, responsible for another 95 million tons of plastic fiber, accounts for 35% of microplastics released into the environment (2019).
There are two sources of microplastics in water sources today. Primary sources are manufactured and come largely from the cosmetics industry in the form of microbeads. These tiny pieces of plastic are deliberately included in products for scrubbing and texture, etc., or from resin pellets in general plastic production. Secondary sources come from the degradation of larger plastic products (i.e., macrolitter, or macroplastics), such as bags, bottles, straws, and so forth.
In addition, the term “plastic” refers to a huge range of chemicals and materials, all of which vary in size, density, opacity, color, and shape. We loosely define microplastics as being less than 5 millimeters in size down to 1 micrometer, and they come in various shapes. This variation makes it hard for automated methods to analyze the pieces. Another challenge for analysts is to differentiate between plastic and non-plastic particles in a sample. For example, it can be difficult to determine if a fiber is made of cellulose (such as from toilet paper) or plastic (such as from clothing).
Given how difficult it is to test for microplastics, what are governments doing to regulate them?
Lozeau: The focus of the first major regulations has been on banning primary sources of microplastics, meaning the intentional manufacturing of microbeads for products like cosmetics. This has already happened or is being proposed in many regions such as the U.S., Canada, EU, New Zealand, China, and others. In the U.S., different states have taken varied approaches to learning about and regulating microplastics. But secondary sources are another matter. You can’t ask people to just stop making or using plastic, and there’s already so much of it in water sources. The key to understanding potential solutions to remediation is testing to determine what you have and how much.
Normally, when governments regulate emerging contaminants, they develop a standard analytical method for laboratories to follow. However, the rate at which we have been discovering new contaminants, such as per- and polyfluorinated substances (PFAS) and more, has been faster than regulating bodies can keep up with. A lot of the actual test methods are coming from academia and private industry, and this is true for microplastics as well. While government efforts are primarily focused on understanding toxicity, the industry focuses on developing analytical methods.
What are the primary analytical methods being used to test for microplastics in water? And how effective are they?
Lozeau: A lot of labs are using optical identification, on its own or in combination with other methods. In this process, they will take a water sample and then clean it up using physical separation methods such as sedimentation and density separation, or chemical methods such as the Fenton reaction to remove any cellulose materials in the sample. After that, the sample is run through a filter to extract the plastics, which can then be identified under a microscope. The accuracy of this method is heavily dependent on the skills of the analyst to recognize plastic from other contaminants.
As a result of this inconsistency, there has been a push for the development of more robust measurement techniques, such as spectroscopy, including Raman and infrared (IR), as well as pyrolysis gas chromatography-mass spectrometry (Py-GC-MS). In September 2022, California’s State Water Resources Control Board approved a policy handbook that details a phased approach for testing and reporting of microplastics in drinking water and supports harmonization with standards organizations, including ASTM International. This multi-lab validation study involves spectroscopic-based workflows, either infrared or Raman spectroscopy. The method describes extraction and collection of microplastics on a filter, followed by examination of the microplastics under a microscope for high-level observation, before spectroscopic characterization. ASTM has currently published two water sampling methods for microplastics, D8332 and D8333, and is working on a spectroscopy-based method expected to be published for comment phase at the end of 2022. There are several other work items for best practices in development.
How important is filtration to these testing methods?
Lozeau: Very! Filters are used in most processes I’ve seen, both to create microplastics analysis grade (MAG) water as well as to separate the microplastics from the sampled water. The type of filter used in each process can make a difference in the results of the test. For creating MAG water, California Water Boards specifically recommends “high purity water filtered through a filter with pore-size 1 µm or smaller (of any appropriate material; glass fiber filters are suitable).” Table 1 shows different types of Millipore® membranes having pore size of 1 µm or smaller and their efficiency in retaining polystyrene beads of various diameters (to mimic microplastics). All of these membranes can be used to prepare MAG water.
When analyzing microplastics, different membranes will work better for different analytical workflows. For visualization and optical-based methods, we suggest a 1 µm glass fiber membrane. We’ve tested this material with ALS Global, which developed a fluorescent microscopy-based analytical process for microplastics in bottled water using Nile Red dye with no background fluorescence, based on a study from State University of New York (SUNY) Fredonia. Figure 1 shows the results from this study.
Glass fiber membranes are also used in Py-GC-MS analysis, largely being developed in academia, though also starting to appear as part of testing lab protocols and is gaining regulatory attention.
Raman and IR spectroscopy methods tend to use filters as the substrate for the microplastics when loaded into the instruments, because they allow direct analysis after collection of the microplastics. Because of this, filters that are IR-transparent or those that have low spectral interference with Raman signals may work better. In this sense, some studies suggesting using metallic-coated polycarbonate membranes which appear invisible, thus allowing the instrument to only detect the microplastics. The California Water Board’s study suggests the use of plain polycarbonate membranes or glass slides for Raman and IR methods. Of course, workflows can vary, and if combined with digestion methods, it is always important to make sure that filter material is chemically compatible with the filtrate. Other membranes, including aluminum oxide, glass fiber, mixed cellulose ester (MCE), and silicon have appeared in literature, so the most appropriate membrane should be determined on a case-by-case basis.
Where can labs go to get more information about testing for microplastics?
These resources are a great place to get started:
NOAA Method For Analyzing Microplastics https://marinedebris.noaa.gov/sites/default/files/publications-files/noaa_microplastics_methods_manual.pdf
California Water Board’s Handbook For Testing And Reporting Microplastics https://www.waterboards.ca.gov/drinking_water/certlic/drinkingwater/documents/microplastics/rs2022-0032.pdf
U.S. EPA Microplastic Beach Protocol
https://www.epa.gov/system/files/documents/2021-09/microplastic-beach-protocol_sept-2021.pdf
Canada’s Plastic Pollution Information Sheet
European Chemicals Agency’s (ECHA) Microplastics Information Website
https://echa.europa.eu/hot-topics/microplastics
Nile Red Method For Determining Microplastics
Mason, S.A., et al. Synthetic polymer contamination in bottled water. Frontiers in Chemistry, 2018, 407.