Microplastics, defined as solid polymeric particles smaller than 5 millimeters, have become an emerging concern worldwide. They arise either from deliberately manufactured microscopic materials, so-called primary microplastics, or from the progressive fragmentation of larger plastic objects exposed to physical, chemical, and biological weathering, known as secondary microplastics. Wastewater treatment plants sit squarely on the front lines of this pollution pathway. They receive enormous loads of plastic fibers and fragments from household laundry, cosmetic products, and urban runoff, yet they also act as concentrators, retaining a significant fraction of the incoming plastic in the sludge line. When that sludge is reused on land or improperly disposed of, it can become a secondary source of contamination for soils and waterways. Understanding exactly how many particles flow through these facilities—and what they are made of—depends on analytical methods that can reliably pry plastic particles out of a sea of organic matter without destroying the evidence in the process. That is precisely the challenge the new research set out to solve.

Led by Ivanilson da Silva de Aquino of the University of Brasília, together with colleagues at the UnB Planaltina Faculty and the Federal University of Jequitinhonha and Mucuri Valleys, the team built its methodological framework from the ground up. The first step was to manufacture reference microplastics from the six commodity polymers that dominate urban plastic waste streams: polyethylene terephthalate, high-density polyethylene, low-density polyethylene, polyvinyl chloride, polypropylene, and polystyrene. These laboratory-made particles, produced as irregular fragments ranging from 0.1 to 3.0 millimeters, served as benchmarks against which every subsequent processing step could be tested. Before any chemistry was attempted, the researchers confirmed by attenuated total reflectance Fourier-transform infrared spectroscopy—ATR-FTIR for short—that each batch of reference particles displayed the characteristic absorption patterns of its parent polymer, ensuring that the starting materials were spectroscopically unambiguous.

The heart of the new protocol is a digestion step based on Fenton’s reagent, a mixture of hydrogen peroxide and iron catalyst that generates highly reactive hydroxyl radicals capable of shredding organic matter. Wastewater and sludge samples are so loaded with organic material, microbial biomass, and inorganic debris that microscopic plastic particles would otherwise be invisible. The catch is that the digestion must be aggressive enough to clear the matrix but gentle enough to leave the plastics themselves chemically intact. If the polymers are oxidized too severely, their infrared fingerprints can shift, complicating identification, and in some cases particles can fragment or lose mass entirely. The team therefore subjected its six reference polymers to five different Fenton protocols spanning temperatures from 30 to 80 degrees Celsius and various reaction durations, then scrutinized the consequences using FTIR and a metric known as the carbonyl index.

The carbonyl index tracks the relative abundance of carbon-oxygen double bonds on polymer surfaces, a hallmark of oxidative degradation. Because polyolefins such as polyethylene and polypropylene naturally lack carbonyl groups, even small increases in the index reveal genuine surface oxidation. The results were revealing. At 30 degrees Celsius the reaction was too sluggish to digest organic matter effectively. Above 80 degrees Celsius oxidation became severe, with polyethylene terephthalate and polypropylene showing the largest changes in carbonyl absorption. But in the moderate window between 40 and 60 degrees Celsius, with a reaction time of about two hours, the Fenton treatment delivered the best of both worlds: organic matter was substantially degraded, while the carbonyl index of all tested polymers showed no significant changes. FTIR spectra recorded before and after digestion showed no meaningful spectral deviations, meaning the polymeric fingerprints needed for reliable identification survived the chemical assault intact.

With digestion optimized, the researchers turned to the second major hurdle: separating buoyant plastic particles from the remaining debris. Density separation exploits the simple physical fact that most common polymers are less dense than suitable salt solutions, so they float while heavier mineral and organic particles sink. The team built a custom separation device and systematically compared separation media. Plain water, with a density of 1.0 grams per cubic centimeter, performed poorly because more than half of the target polymers are denser than water and simply sank. Saturated sodium chloride solution, the cheap and inert classic, reached about 1.2 grams per cubic centimeter and successfully floated roughly two-thirds of the studied plastics—but it excluded the high-density polymers polyvinyl chloride, at 1.10 to 1.45 grams per cubic centimeter, and polyethylene terephthalate, at 1.37 to 1.45 grams per cubic centimeter. Relying on sodium chloride alone, the authors caution, could systematically underestimate microplastic abundance.

Zinc chloride proved to be the decisive upgrade. At concentrations reaching 1.6 and 1.8 grams per cubic centimeter, zinc chloride solutions kept every polymer tested in suspension, allowing effective recovery of even the heaviest particles. In the validation experiments, the densest solution ensured that all target polymers remained in the floating fraction, minimizing losses during sample processing. The researchers note, however, that the analytical superiority of zinc chloride comes with trade-offs: its toxicity, higher operational cost, and disposal requirements must be weighed when designing routine monitoring programs, and reuse of the solution can help mitigate cost and waste.

The true test came when the optimized protocol was unleashed on real samples from a full-scale facility, the Brasília North wastewater treatment plant. The researchers deliberately chose two matrices representing opposite ends of the processing complexity spectrum: raw wastewater, with a chemical oxygen demand averaging 882.7 milligrams of oxygen per liter, and dewatered sewage sludge, whose chemical oxygen demand averaged a staggering 30,600 milligrams of oxygen per liter—roughly thirty-five times higher. Applying the optimized Fenton protocol cut chemical oxygen demand by 81.3 percent in the raw wastewater and 82.3 percent in the sludge, leaving values of 165.4 and 5,420 milligrams of oxygen per liter respectively. Visually, the transformation was dramatic: dark, turbid samples became noticeably clearer and more translucent after treatment, although sludge samples, given their enormous organic load, sometimes required a second digestion cycle to remove residual suspended material.

Following digestion, density separation with zinc chloride at 1.8 grams per cubic centimeter produced striking stratification in the custom unit, with a floating fraction enriched in microplastics accumulating at the top and residual organic matter and denser inorganic particles settling at the bottom. Spiked reference particles of every polymer type were successfully recovered from both matrices, including the notoriously difficult polyethylene terephthalate and polyvinyl chloride. The researchers then applied the validated workflow to hunt for native microplastics—particles that arrived in the samples from the real world rather than from the laboratory. Stereomicroscopy revealed a rich menagerie of fragments, films, and fibers varying widely in color, size, and surface texture, exactly the heterogeneity expected from the diverse plastic sources entering urban sewer networks.

ATR-FTIR analysis of the recovered native particles confirmed the method’s analytical power. Polyethylene emerged as the dominant polymer in both wastewater and sludge, identified through its characteristic absorption bands near 2915, 2845, 1462, and 717 wavenumbers, corresponding to carbon-hydrogen stretching, methylene bending, and rocking vibrations. A subtle band near 1377 wavenumbers even allowed the team to distinguish low-density from high-density polyethylene. Crucially, the spectra of environmental particles retained all the features needed for identification, confirming that the upstream pretreatment had not compromised the spectroscopic evidence. The predominance of polyethylene aligns with the ubiquity of packaging films and containers in urban life and echoes findings from previous studies around the world.

The significance of this work extends well beyond a single treatment plant in the Brazilian capital. By demonstrating that Fenton digestion, zinc chloride density separation, stereomicroscopy, and ATR-FTIR can operate as a coherent, validated pipeline across both liquid and solid waste streams, the study provides exactly the kind of methodological scaffolding that the field has lacked. The authors emphasize that their framework can support future monitoring programs aimed at evaluating the occurrence, transport, and fate of microplastics throughout wastewater treatment systems, and that it contributes to the ongoing effort to establish standardized protocols for microplastic analysis in complex environmental matrices. As regulators and researchers worldwide push to quantify how much plastic flows from drains to rivers to oceans—and how much lingers in the sludge spread across farmland—reliable, reproducible measurements will be the foundation of every policy decision. This new protocol offers a tested blueprint for generating them.

The research highlights the critical role that sewage treatment plants (STPs) play in managing waste, yet they also inadvertently become reservoirs for harmful substances. Most notably, PFAS, recognized for their water-repellent properties, have been widely used in various industrial applications and consumer goods. Unfortunately, their resilience means they do not break down easily in the environment, leading to accumulation in water sources and sediment. The implications of this presence are profound, sparking considerable concern among scientists, environmental advocates, and public health officials alike.

Alves and colleagues meticulously devised a study aiming to identify and quantify the concentrations of PFAS in wastewater treatment facilities. To achieve this, they employed LTPE extraction, a method noted for its efficiency in isolating trace levels of contaminants from complex matrices such as sewage and sludge samples. This technique allows for a more straightforward extraction process while minimizing the risk of sample degradation, ultimately improving the reliability of the analytical results.

Following extraction, the researchers utilized LC–MS/MS, a method celebrated for its sensitivity and specificity when detecting various substances. This analytical technique allows for the precise measurement of the concentration of PFAS, enabling the researchers to map their presence in STP outputs. The study identifies a diverse array of PFAS compounds, reinforcing concerns regarding these substances’ ubiquity in urban water systems, where they can subsequently migrate into drinking water supplies.

The findings of Alves et al. are striking. They reveal that many sewage treatment plants are pathways for PFAS into the environment. By analyzing both sewage inflow samples and sludge produced during the treatment process, the team found alarming levels of certain PFAS compounds. This research not only illuminates the severe contamination issues related to wastewater processing but also casts a spotlight on the need for comprehensive wastewater treatment solutions to address harmful legacy pollutants.

Moreover, the researchers engaged in comparative analysis with existing literature to place their findings within the broader context of PFAS research. The data reflect regional variations, responding to previous studies highlighting that different geographical areas may harbor distinct PFAS concentrations. Such analyses are crucial, as they inform the development of localized strategies to manage and mitigate the impacts of these persistent pollutants.

The implications of this study extend beyond environmental science; they touch upon public health policies, regulatory frameworks, and community awareness. Given the documented links between PFAS exposure and adverse health outcomes, including reproductive, developmental, and carcinogenic effects, there is an urgent need for effective policy measures. This study underscores the importance of robust research to enable informed decision-making by policymakers and stakeholders.

Public engagement and awareness are also critical to addressing contamination issues. This study reiterates the necessity for communities to be educated about the sources of PFAS and their potential hazards, which ultimately benefits public health. Environmental groups can leverage these findings to advocate for cleaner alternatives and stricter regulations governing the release and disposal of PFAS-contaminated waste.

Looking forward, the research conducted by Alves et al. presents opportunities for further investigation into the pathways by which these substances enter the environment. Understanding the dynamics of PFAS dispersion can assist scientists and environmental engineers in devising targeted strategies for remediation. Additionally, through collaboration with industry partners, potential alternatives to PFAS in manufacturing processes could be explored to mitigate new inputs.

Moreover, this study pushes the envelope on analytical chemistry applied to environmental science, showcasing the continuous need for technological advancement in monitoring pollutants. Enhanced analytical methods will yield more profound insights into the fate and transport of contaminants, ultimately leading to improved strategies for pollution control.

In conclusion, the work of Alves, Cunha, and Sanson represents a significant contribution to the understanding of PFAS in the context of sewage treatment plants. Their findings herald the importance of maintaining diligence in environmental monitoring and necessitate a unified response from researchers, policymakers, and the public to mitigate the impact of these persistent pollutants on both ecosystems and public health. This timely study serves as a call to action, prompting stakeholders across disciplines to engage in meaningful dialogue and collaborative efforts aimed at safeguarding the environment.

As the scientific community continues to unravel the complexities associated with PFAS, it becomes increasingly clear that our responsibility extends beyond research. There is an ethical imperative to translate scientific insights into effective policies and practices that will protect our natural resources and human health for generations to come.

Subject of Research: Analysis of perfluorinated substances in sewage and sludge from sewage treatment plants.

Article Title: LTPE extraction and LC–MS/MS analysis of perfluorinated substances in sewage and sludge from sewage treatment plants.

Article References:

Alves, M.C.P., Cunha, L.R., Sanson, A.L. et al. LTPE extraction and LC–MS/MS analysis of perfluorinated substances in sewage and sludge from sewage treatment plants.
Environ Sci Pollut Res (2025). https://doi.org/10.1007/s11356-025-37261-y

Image Credits: AI Generated

DOI: https://doi.org/10.1007/s11356-025-37261-y

Keywords: PFAS, sewage treatment plants, environmental science, contamination, public health, analytical chemistry, Liquid-Liquid Extraction (LTPE), Liquid Chromatography-Tandem Mass Spectrometry (LC–MS/MS).