Microplastics in wastewater may do more than simply carry pollutants. A new study shows that aged microplastics can actively alter the chemical reactions used to remove pharmaceutical contaminants during advanced wastewater treatment, accelerating pollutant degradation while also changing the abundance of potentially harmful transformation products.
Researchers investigated how aged polyamide microplastics affect the breakdown of carbamazepine, a widely used pharmaceutical frequently detected in surface water, wastewater, and even drinking water. The team examined three ultraviolet-based advanced oxidation processes, known as UV/PMS, UV/H₂O₂, and UV/Cl, which use ultraviolet light and oxidants to generate highly reactive chemical species that destroy organic contaminants.
"Our results show that aged microplastics should not always be regarded as passive particles during wastewater treatment," said corresponding author Xiaohui Wang of Beijing University of Chemical Technology. "Their aged surfaces can participate in chemical reactions, increase the production of reactive species, and ultimately influence both pollutant removal and the products formed during treatment."
The researchers found that aging caused environmentally persistent free radicals, or EPFRs, to develop on the surface of polyamide microplastics. These surface radicals promoted electron transfer and enhanced the formation of reactive species, particularly hydroxyl radicals and singlet oxygen.
As a result, aged microplastics increased the observed degradation rate constant of carbamazepine by 1.2 to 1.8 times across the three treatment systems. The largest relative enhancement occurred in the UV/Cl system, where the degradation rate constant increased by about 1.8 times.
However, faster disappearance of the original pharmaceutical does not necessarily mean lower environmental risk.
Using mass spectrometry, the researchers identified numerous transformation products and found that aged microplastics increased the abundance of several intermediates. The fundamental degradation pathways remained broadly similar, involving reactions such as hydroxylation, oxidation, ring condensation, and ring cleavage, but the distribution of the resulting products changed.
Toxicity modeling further indicated that some transformation products were more toxic to aquatic organisms than the original carbamazepine molecule. In particular, TP 194 showed substantially higher acute and chronic toxicity, while several other intermediates were classified as harmful or toxic. Structural analysis suggested that removal of the amide group, hydroxyl substitution, and formation of unsaturated rings could contribute to increased toxicity.
The study also showed that wastewater chemistry matters. Nitrate promoted carbamazepine degradation in all three systems, while bicarbonate and humic acid generally inhibited it. The influence of chloride depended on its concentration and the oxidation system being used.
The findings highlight the need to consider microplastics as chemically active components of wastewater treatment environments, rather than inert contaminants. They also suggest that evaluating treatment efficiency solely by measuring the disappearance of a parent pollutant may overlook changes in transformation-product toxicity.
The authors note that the experiments were conducted under controlled laboratory conditions and used only polyamide microplastics. Future studies will need to test a broader range of plastics and verify these effects in complex, environmentally realistic waters.
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Journal reference: Wang Y, Zhou X, Xu Z, Liu H, Wang X, et al. 2026. Oxidant-dependent carbamazepine transformation: how aged microplastics modulate degradation pathways in UV-AOPs. New Contaminants 2: e024 doi: 10.48130/newcontam-0026-0021
https://www.maxapress.com/article/doi/10.48130/newcontam-0026-0021
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About the Journal:
New Contaminants (e-ISSN 3069-7603) is an open-access journal focusing on research related to emerging pollutants and their remediation.
