Understanding microplastics: a simple guide to a complex problem

Microplastics (MPs) have emerged as a pressing environmental concern, drawing increasing attention for their widespread presence and complex behavior in the environment. These tiny plastic particles, some are often invisible to the naked eye, have been detected in oceans, soils, drinking water, and even the food we consume raising urgent questions about their long-term ecological and human health impacts.  In a previous paper (Microplastics in water: Understanding The Risks | GHD Insights), we discussed the basics of MPs.  This paper builds on that, offering a more in-depth examination of MPs through the materials science lens, with a focus on the complex interplay among polymer composition, morphology, and surface chemistry, weathering, and environmental fate and transport.  In doing so, it also explores the significant challenges MPs pose to current water and wastewater treatment systems, as well as the limitations of existing testing methods. Understanding MPs from both a structural and systems perspective is essential for managing MPs and other co-contaminants.

MPs are tiny plastic fragments smaller than 5 millimeters—about the size of a sesame seed—while those smaller than one micrometer are classified as nanoplastics (NPs). MPs are ubiquitous in the environment, found from oceans and freshwater sources to the air we breathe and the food we eat. Some MPs are intentionally manufactured, such as microbeads used in face wash. However, most MPs originate from the gradual breakdown of larger plastic items like bags, bottles, and tires. Environmental factors including weathering, sunlight exposure, and physical abrasion slowly fragment these plastics into smaller MPs particles.

MPs are a growing concern because they can enter the human body through ingestion and inhalation. The smallest particles, especially NPs have the potential to cross cell membranes in the gut and accumulate in critical organs. Emerging studies have even detected MPs in human brain tissue, raising concerns about their possible links to inflammation and adverse health effects [Nihart, A. J., et al. “Bioaccumulation of microplastics in decedent human brains”. Nature Medicine, 2025, 31(4), 1114–1119. https://doi.org/10.1038/s41591-024-03453-1].

Natural environmental forces play a major role in transforming everyday plastic items into MPs. Sunlight, temperature fluctuations, and mechanical forces such as abrasion and wave action contribute to the gradual fragmentation of plastic—a process known as weathering. Weathering not only breaks plastics into smaller particles but also enhances their chemical reactivity and toxicity.

As plastics degrade, additives and residual chemicals used in their manufacturing processes, e.g., pigments, plasticizers, stabilizers, and catalysts, can leach out more readily. For instance, PET often contains antimony as a catalyst, which can leach into the environment under prolonged exposure to heat and ultraviolet (UV) radiation. One study found measurable antimony release from PET bottles exposed to sunlight and elevated temperatures [Westerhoff, P., et al.  “Antimony leaching from polyethylene terephthalate (PET) plastic used for bottled drinking water” Water Research, 2008, 42(3), 551–556. https://doi.org/10.1016/j.watres.2007.07.048].

Furthermore, as plastic fragments become smaller, their surface area to volume ratio also increases, enhancing their ability to adsorb other environmental pollutants and interact with biological systems. This increase in surface area also raises the likelihood of pollutant desorption, potentially making weathered microplastics both contaminant carriers and sources of toxic substances in aquatic and terrestrial environments.

MPs can adsorb and transport environmental pollutants, including per- and polyfluoroalkyl substances (PFAS), which are persistent and widely distributed in the environment. The strength and nature of the interactions between PFAS and MPs depend on several key factors:

• Chemical structure of both the MP polymer and the specific PFAS compound (e.g., chain length, functional groups)
• Surface characteristics of MPs, such as texture, porosity, and the presence of functional groups or oxidative weathering products
• Environmental conditions including pH, temperature, ionic strength, and the presence of dissolved organic matter

It is generally observed that longer-chain PFAS (e.g., PFOA, PFOS) exhibit stronger adsorption onto certain MP types, especially hydrophobic polymers, than shorter-chain PFAS. However, natural organic matter and microbial biofilms can form coatings on MPs, altering their surface chemistry and either enhancing or inhibiting PFAS binding [Salawu, O. A., et al. “Adsorption of PFAS onto secondary microplastics: A mechanistic study”. Journal of Hazardous Materials, 2024, 470, 134185. https://doi.org/10.1016/j.jhazmat.2024.134185;  Hatinoglu, M. D., et al. “Modified linear solvation energy relationships for adsorption of perfluorocarboxylic acids by polystyrene microplastics” Science of The Total Environment, 2023, 860, 160524. https://doi.org/10.1016/j.scitotenv.2022.160524].

Once adsorbed, PFAS-laden MPs can act as mobile vectors, facilitating the long-range transport of PFAS across air, water, and soil compartments—potentially introducing them into previously uncontaminated ecosystems.

Identifying and measuring MPs in the environment is challenging due to their small size, diverse shapes, and wide range of polymer types. Most current analytical methods can reliably detect only particles larger than 10–20 micrometers, potentially overlooking smaller and possibly more harmful NPs.

Sampling methods commonly used include:

• Grab Samples: Simple collection using bottles, followed by filtration in the lab. While straightforward, this method is highly susceptible to contamination and may not represent spatial or temporal variability.
• Sieve Filtration: Involves stacked metal mesh screens to separate MPs by size. It allows for processing larger water volumes but is ineffective for capturing fine particles below the mesh size.
• In-line Filtration: Enclosed systems installed directly in the flow stream, enabling real-time filtration with reduced contamination risk and improved capture of smaller MPs.

Analytical techniques include:

• Pyrolysis-Gas Chromatography/Mass Spectrometry (Py-GC/MS): Thermally decomposes the plastic to identify polymer types and quantify mass. While highly accurate for composition, it does not provide size or shape information.
• Spectroscopic Methods (e.g., Fourier Transform Infrared Spectroscopy-FTIR, Raman Spectroscopy, and Laser Direct Infrared Imaging-LDIR): Use infrared or laser-based techniques to identify polymers and determine particle size and morphology. These methods offer high-resolution data but are time-intensive, costly, and require skilled operators.

Each method has strengths and limitations, and no single technique provides a complete picture. As a result, a combination of sampling and analytical approaches is often used to improve accuracy, resolution, and representativeness in microplastic monitoring.

Microplastics (MPs) present significant challenges for water treatment and environmental remediation. Due to their small size, diverse morphologies, and surface properties, MPs are difficult to remove and can transport other pollutants such as PFAS, heavy metals, and hydrophobic organic compounds.

In wastewater treatment plants (WWTPs), an estimated 80–95% of larger MPs are removed during treatment processes [Adegoke, K. et al “Microplastics toxicity, detection, and removal from water/wastewater” Marine Pollution Bulletin, 2023, 187, 114546]. However, smaller MPs often bypass conventional filtration and remain in the effluent. The captured fraction frequently accumulates in biosolids, which, when applied to land, can reintroduce MPs into terrestrial and aquatic environments. Floating or buoyant plastics may evade removal systems, and weathered or aged plastics exhibit increased surface reactivity, enhancing their ability to bind with co-contaminants.

In environmental remediation, MPs complicate efforts by transporting pollutants through soil, groundwater, and sediments. They can interfere with sorbents and other treatment technologies by acting as both contaminant carriers and physical obstructions. Their persistence and mobility underscore the need to account for MPs in both the design and evaluation of cleanup strategies.

Plastic was engineered for durability, which partly explains the persistence of MPs in the environment. As plastics degrade into MPs and further into NPs, they become more difficult to detect but easier for humans and other organisms to ingest, raising significant health concerns. In addition to carrying other pollutants, MPs can also leach toxic additives, such as plasticizers, flame retardants, stabilizers, and fillers originally incorporated during plastics manufacturing. Understanding MPs requires examining their chemical composition, morphology, synthesis, formulation, and environmental behavior. The latter is especially critical, as MPs not only leach hazardous compounds but also act as carriers for other environmental contaminants such as PFAS and toxic heavy metals. Effective management of MPs and their co-pollutants requires a comprehensive understanding of the complex interactions among these substances in environmental systems.