The research team, led by Zuzanna Prus-Frączek and including senior authors Katarzyna Styszko and Małgorzata Wilk, focused on polyethylene terephthalate, or PET, the polymer of water bottles and food packaging, and polystyrene, or PS, the aromatic plastic found in insulation, packaging foam, and countless disposable items. Both are among the most common polymers recovered from treated sludge, which can harbor hundreds to thousands of microplastic particles per kilogram of dry matter. Because European Union policy increasingly promotes the reuse of sludge as fertilizer and soil amendment on farmland, understanding whether hydrothermal treatment truly breaks these particles down is not an academic detail. It is central to whether plastic pollution quietly rides the circular economy back into the food chain.

Earlier studies had reported that hydrothermal carbonization can slash the number of microplastic particles detectable in sludge by up to 90 percent, with one study showing removal of particles larger than 50 micrometers jumping from about 11 percent to 85 percent as temperatures rose from 170 to 220 degrees Celsius. But a reduction in detectable particles is not the same as destruction. Fragments can shatter below the size limits of analytical instruments, and polymer-derived compounds can dissolve into the surrounding water without ever being mineralized. The Polish team set out to determine which of these fates actually befalls PET and PS, treating the two polymers separately in pure water at 220 degrees Celsius for one, two, and three hours, conditions carefully chosen to mirror real sewage sludge processing while eliminating interference from sludge-derived organic matter.

The results revealed a striking asymmetry between the two plastics. PET, whose backbone contains ester bonds that water can chemically attack, proved far more vulnerable. Thermogravimetric analysis showed that its degradation onset temperature, which stands at roughly 402 degrees Celsius in the raw polymer, plummeted to between 246 and 284 degrees Celsius after treatment. The pyrolytic residue left at 800 degrees Celsius collapsed from 12.7 percent to as little as 1.3 percent, indicating a severely diminished tendency to form stable carbon structures. Infrared and Raman spectroscopy painted a consistent picture: the sharp ester carbonyl band at 1714 inverse centimeters gave way to new dominant components near 1671 to 1677 inverse centimeters, signaling a heterogeneous mix of partially hydrolyzed ester bonds and newly formed oxygen-containing structures. Scanning electron microscopy showed surfaces transformed from smooth and compact into rough, cracked, and cavity-riddled landscapes.

Polystyrene, by contrast, emerged from the reactor remarkably intact. Its hydrophobic aromatic skeleton, held together by carbon-carbon bonds that water cannot easily cleave, resisted the conditions that visibly weakened PET. Treated PS samples retained a single-step thermal degradation profile with maximum decomposition temperatures of around 416 to 417 degrees Celsius, essentially unchanged from the raw material. The principal FTIR and Raman bands, including the characteristic aromatic ring breathing mode near 1001 inverse centimeters, remained within measurement uncertainty of their original positions. Only slight band broadening after three hours hinted at limited local disorder at the surface. The finding aligns with earlier work showing that complete PS decomposition demands 360 degrees Celsius and four hours, conditions far more severe than any sewage sludge reactor would employ.

Perhaps the most consequential discovery came from analyzing the process water itself. When the researchers measured chemical oxygen demand and total organic carbon in the liquid phase, PET proved to be a prodigious polluter. Normalized to the initial polymer mass, PET released approximately 737 to 876 milligrams of COD and 320 to 343 milligrams of TOC per gram of plastic into the water, with absolute concentrations reaching 6145 to 7300 milligrams per liter of COD and around 2700 to 2855 milligrams per liter of TOC. Polystyrene, in comparison, contributed only 35 to 83 milligrams of COD and 99 to 140 milligrams of TOC per gram of polymer. The pH of the PET process water remained stubbornly acidic, between 3.8 and 3.9, consistent with the accumulation of acidic products from ester bond cleavage, while phenolic compounds slowly increased with longer treatment, suggesting the formation of refractory aromatic derivatives that resist further degradation.

These numbers carry a direct message for engineers designing full-scale sludge hydrothermal carbonization plants. Process water from sludge HTC is already heavily loaded with dissolved organics and can contain inhibitory or toxic compounds, and adding a substantial polymer-derived burden, particularly from PET-rich sludge, would amplify the challenge of treating or safely recirculating that water. Previous pilot-scale studies have shown that biological treatment of HTC process water can achieve COD removals of roughly 71 to 75 percent in membrane bioreactors and sequencing batch reactors, but the researchers caution that the added plastic-derived load could strain these systems further. Hydrochar yields added another layer of nuance: the most intensive apparent conversion for both polymers occurred at two hours, with yields for PS dropping to 75.6 percent and PET to 53.6 percent, and the non-monotonic trends hint at competing processes of depolymerization, dissolution, and secondary condensation of soluble intermediates back onto the solid phase.

The environmental implications extend beyond the treatment plant. Hydrochar from PET-containing sludge, with its weakened structure and earlier degradation onset, would likely be more susceptible to further fragmentation, oxidation, and weathering if applied to soils, potentially generating new generations of smaller plastic particles. The dramatic reduction in pyrolytic residue also suggests a diminished capacity for stable carbon sequestration in PET-derived char, undermining one of the climate arguments for hydrochar use in agriculture. Polystyrene-derived material, meanwhile, may remain embedded within or incorporated into the carbonized matrix despite surface alteration, retaining its aromatic identity. In neither case does the plastic vanish. As the authors put it, HTC should be understood primarily as a polymer-transformation and redistribution process, shifting material between the solid, liquid, and gas phases rather than eliminating it from the system.

The study does have limits that the team acknowledges candidly. The experiments used pristine, manually fragmented waste plastics in distilled water at a single temperature, rather than environmentally aged particles in a real sludge matrix, and the analyses could not identify specific molecular transformation products, distinguish hydrolysis from oxidation, or assess nanoplastic formation because samples were ground before imaging. Future work, the researchers argue, should incorporate real sewage sludge, replicate experiments across broader operating windows, and deploy gas and liquid chromatography coupled with mass spectrometry to identify individual compounds, alongside leaching and ecotoxicity tests to evaluate the fate of released organics in soils. For now, the message for the water sector is clear and somewhat uncomfortable: hydrothermal carbonization remains a valuable technology for valorizing wet sludge, but claims of microplastic removal must be read as claims of microplastic transformation, and any credible plant design must treat the process water as a first-class waste stream, not an afterthought.

Subject of Research: Hydrothermal carbonization of PET and polystyrene microplastics under sewage sludge treatment conditions

Article Title: Hydrothermal carbonization of PET and PS microplastics under conditions relevant to sewage sludge treatment

Article References: Prus-Frączek, Z., Kalemba-Rec, I., Magdziarz, A., Dróżdż, A., Chwiej, J., Styszko, K., & Wilk, M. (2026). Hydrothermal carbonization of PET and PS microplastics under conditions relevant to sewage sludge treatment. Energy Reports, 16, Article 109697. https://doi.org/10.1016/j.egyr.2026.109697

Image Credits: AI Generated

DOI: 10.1016/j.egyr.2026.109697

Keywords: microplastics, hydrothermal carbonization, sewage sludge, PET, polystyrene, hydrochar, process water, wastewater treatment, COD, TOC, polymer degradation, circular economy

The research, led by Amandine Passin and corresponding author Fabienne Lagarde of the Institut des Molécules et des Matériaux du Mans at Le Mans Université, together with colleagues from the French industrial plastics research centre CT-IPC, set out to answer a deceptively simple question: does the initial formulation of a plastic determine its environmental fate? Although additives such as stabilisers, antioxidants and processing aids are incorporated into polymers specifically to improve durability, their influence on the generation of microplastics and nanoplastics, and on the leaching of chemical compounds into water, has remained surprisingly poorly characterised. Understanding this link is critical because microplastics and their dissolved or gaseous degradation products follow very different pathways through ecosystems and pose distinct challenges for environmental monitoring.

To probe this question, the team compared two polypropylene formulations under controlled laboratory conditions. The first was a reference polypropylene containing no added additives, representing the bare polymer matrix. The second, designated PP + 6, contained six industrially representative additives chosen to reflect the kinds of compounds commonly used in real-world plastic products. Plastic pellets from both formulations were subjected to accelerated ultraviolet weathering designed to mimic prolonged sunlight exposure, and the weathered pellets were subsequently placed in water and agitated mechanically to reproduce the physical wear that plastics experience in rivers, oceans and soils. This combination of photochemical ageing followed by mechanical stress allowed the researchers to simulate, in compressed laboratory timescales, the sequential insults that plastic debris suffers outdoors.

The analytical toolkit behind the study was deliberately multi-technique. Gravimetric measurements tracked how much mass each formulation lost over time, while morphometric analysis and scanning electron microscopy revealed how particle sizes and surface textures evolved. Total organic carbon analysis quantified the soluble degradation products dissolved into the water phase, allowing the team to distinguish between material that broke off as intact microplastic and nanoplastic particles and material that left the polymer as dissolved organic species. By following these endpoints in parallel, the researchers could build a mass-balance picture of degradation rather than focusing narrowly on particle counts alone.

The results revealed a dramatic and unexpected divergence between the two formulations. The additive-free reference polypropylene degraded through a clearly defined three-phase process, ultimately losing a cumulative 82 plus-or-minus 8 percent of its mass after 50 days of ultraviolet exposure. In the first phase, degradation was dominated by the release of volatile compounds and soluble species, as ultraviolet photons cleaved polymer chains and oxidation reactions produced small molecules that either evaporated or dissolved. The second phase was marked by surface ablation, in which thin layers of the embrittled polymer flaked away as microplastic particles. The third and most dramatic phase saw a sharp acceleration in the release of microplastics, nanoplastics and soluble products as entire granules fragmented, a process that alone accounted for 62 plus-or-minus 7 percent of the total mass loss measured at the 50-day mark.

The additive-containing formulation told a strikingly different story. PP + 6 degraded far more slowly, reaching only a cumulative mass loss of 24 plus-or-minus 3 percent after the same 50 days of ultraviolet weathering, and its degradation followed two distinct phases rather than three. The six additives, by scavenging radicals and shielding the polymer from photo-oxidation, effectively bought the material time, delaying chain scission and postponing the catastrophic fragmentation seen in the additive-free sample. From a durability standpoint, the additives did precisely what they were designed to do. But the study makes clear that slowing degradation is not the same as eliminating its environmental consequences.

Indeed, even as PP + 6 resisted fragmentation, the researchers observed a continuous increase in two other forms of pollution. Emissions of volatile compounds rose steadily throughout the weathering period, and the additives themselves leached progressively into the aqueous phase. This means that a plastic product which appears, to the naked eye, to be weathering gracefully may nonetheless be quietly releasing its chemical constituents into surrounding water over months and years. Because many plastic additives are known or suspected to be ecotoxic, endocrine-active or persistent, this slow-release pathway represents an environmental exposure route that particle-focused monitoring programmes can easily miss entirely.

The broader significance of the work lies in its demonstration that plastic is not a monolithic pollutant. Two samples of the same base polymer, differing only in their additive packages, can follow fundamentally different degradation trajectories, generate different proportions of microplastics, nanoplastics, dissolved organics and volatile emissions, and therefore pose different environmental risks. Most laboratory studies of plastic weathering to date have used simplified, additive-free or minimally formulated materials for the sake of experimental control, yet the study shows that such model systems can dramatically overstate or mischaracterise the behaviour of the formulated plastics that actually populate the environment. Environmental fate models, exposure assessments and risk frameworks that ignore formulation chemistry may consequently be built on shaky ground.

The findings also complicate the emerging policy conversation around plastics. As negotiators and regulators consider rules governing plastic composition, additives and recyclability, this research suggests that decisions about what goes into a plastic product have consequences far beyond the use phase of the item. An additive package that extends service life may simultaneously delay microplastic generation while extending the period over which chemicals leach out, shifting the timing and nature of the environmental burden rather than simply reducing it. Conversely, formulations that fragment quickly may flood ecosystems with particles but exhaust their leachable additives sooner. Neither pathway is inherently benign, and the authors argue that both microplastic and nanoplastic generation and soluble and volatile emissions must be considered together when assessing the true environmental impact of plastic materials.

For the researchers, the next frontier is extending this mass-balance approach to other polymer types, other additive combinations and real environmental matrices, where sunlight, temperature, salinity, microbes and mechanical forces interact in far more complex ways than any accelerated weathering chamber can reproduce. But the central message of the study already stands: when it comes to how plastic ages in the environment, formulation is destiny. The invisible ingredients blended into a plastic pellet at the factory gate shape not only how long the product lasts, but what it becomes, particle by particle and molecule by molecule, as the sun slowly takes it apart.

Polypropylene is particularly vulnerable to photo-oxidation because its backbone contains tertiary carbon atoms, where hydrogen abstraction by UV-generated radicals initiates a self-propagating chain reaction. Once oxygen is incorporated, the polymer forms carbonyl groups that absorb light and accelerate further degradation, a process known as auto-accelerating photo-oxidation. This inherent chemical susceptibility explains why polypropylene items left outdoors become brittle, chalky and prone to crumbling within months, and why stabiliser packages are considered indispensable in nearly every commercial application of the resin.

The distinction between microplastics and nanoplastics matters scientifically because the two fractions behave differently once released. Particles in the micrometre range tend to settle, aggregate with natural organic matter and be ingested by filter feeders, whereas nanoplastic particles have far higher surface-area-to-mass ratios, can cross biological barriers more readily and are notoriously difficult to detect with conventional sampling methods. By tracking fragmentation phases gravimetrically rather than relying solely on particle counting, the study sidesteps some of the analytical blind spots that have hampered earlier weathering experiments, in which a large share of degrading material simply vanished from the measured mass budget.

Total organic carbon measurements offer a complementary advantage: they capture the dissolved fraction of degradation, including short-chain oxidation products and leached additives that carry no particle signature at all. Environmental monitoring programmes, which overwhelmingly target intact particles collected by nets and filters, are structurally blind to this soluble pool. The finding that volatile emissions also rise continuously during weathering adds a third, even less visible compartment, since evaporated fragments enter the atmosphere and may undergo further photochemical transformation far from the site of release.

The accelerated weathering approach used in the study compresses years of outdoor exposure into weeks, a standard technique in polymer science, though translating laboratory doses into real environmental lifetimes remains an acknowledged challenge. Even so, the relative comparison between the two formulations is robust, because both materials experienced identical conditions. The threefold difference in cumulative mass loss between the additive-free and additive-containing pellets therefore reflects genuine formulation effects rather than experimental artefact.

Because the work was published as an open-access article in Microplastics and Nanoplastics, with supplementary material available for readers seeking the full morphometric and carbon datasets, other laboratories can replicate the mass-balance framework directly. Extending it to polyethylene, polystyrene and formulated bioplastics, and to seawater matrices where salt and biofilms alter leaching behaviour, would allow the field to build the comparative database of formulation-specific degradation pathways that current risk assessments conspicuously lack.

Subject of Research: How plastic additives influence UV-induced degradation of polypropylene and the release of microplastics, nanoplastics and chemical compounds

Article Title: Influence of additives on UV-induced degradation of polypropylene: micro and nanoplastic formation and additives release

Article References: Passin, A., Glais, M., Arib, C., Montembault, V., Falher, T., & Lagarde, F. (2026). Influence of additives on UV-induced degradation of polypropylene: micro and nanoplastic formation and additives release. Microplastics and Nanoplastics. https://doi.org/10.1186/s43591-026-00227-z

Image Credits: AI Generated

DOI: 10.1186/s43591-026-00227-z

Keywords: polypropylene, plastic additives, UV degradation, microplastics, nanoplastics, additives leaching, volatile emissions, plastic weathering, polymer degradation, environmental pollution, plastic formulation, mass loss