Plastic Color Shapes How Aged Microplastics Soak Up Antibiotics

Microplastics are everywhere in the environment, from mountain soils to the deepest ocean trenches, and scientists have spent years cataloging the many factors that determine how these tiny fragments interact with the pollutants around them. A new study published in the Archives of Environmental Contamination and Toxicology adds a surprisingly vivid variable to that list: color. Researchers led by Ruixin Jin and Maocai Shen at Anhui University of Technology in China found that the pigment embedded in plastic particles strongly influences how the particles age in sunlight and, in turn, how effectively they bind tetracycline, one of the world’s most widely used antibiotics. The finding suggests that environmental risk assessments may be systematically incomplete if they treat microplastics as a chemically uniform class of pollutant rather than a spectrum of differently colored, differently weathered materials.

The research team focused on two of the most common plastics in consumer products and packaging: polypropylene, known as PP, and polymethyl methacrylate, known as PMMA. Both were obtained in four colors: red, yellow, blue, and green. Before any testing, the particles were characterized in their virgin state, and then the team subjected them to controlled aging designed to mimic the ultraviolet radiation and oxidative stress that plastics experience in sunlit surface waters. Aging is not a cosmetic process. As polymer chains break under photochemical attack, surfaces crack, oxygen-containing functional groups accumulate, and particle dimensions shrink, all of which change how a fragment interacts with dissolved molecules in its surroundings.

The physical consequences of aging turned out to be strikingly color-dependent. Under microscopic examination, aged red polypropylene developed a dense network of wrinkles across its surface, while blue polypropylene responded differently, showing irregular flaking in which small pieces of the weathered surface peeled away. These distinct degradation morphologies imply that pigments do not merely sit inertly inside the polymer matrix; they alter how the material absorbs light, generates reactive species, and ultimately disintegrates. Particle size measurements confirmed that aging matters quantitatively as well as visually. Across the polypropylene samples, the average particle size decreased by 6 to 30 percent after aging, with red and yellow particles showing even greater reductions than the overall average, indicating that those pigments either accelerate photodegradation or produce more fragile weathered surfaces.

With the aged and virgin particles in hand, the researchers turned to the central question of the study: how do these differently colored, differently weathered plastics adsorb tetracycline hydrochloride, the hydrochloride salt form of the antibiotic commonly used in medicine and in intensive livestock farming. Tetracycline is a useful probe pollutant because it carries multiple ionizable groups, meaning its charge state shifts with the acidity of the surrounding water, and it can engage plastics through hydrogen bonding, electrostatic attraction, and other surface interactions. Adsorption onto microplastics matters environmentally because particles that soak up antibiotics can transport them far from their point of release, potentially concentrating the drugs in the tissues of organisms that ingest the particles and fostering antibiotic resistance along the way.

The adsorption results revealed a clear hierarchy among the colors that shifted after weathering. For virgin microplastics, the adsorption capacity followed the order red greater than blue greater than yellow approximately equal to green. After aging, the ranking rearranged itself to red greater than yellow approximately equal to green greater than blue, with the yellow and green particles showing significant improvements in adsorption performance. In other words, weathering did not simply boost every color equally; it reshuffled the leaderboard. Red polypropylene remained the strongest binder both before and after aging, but blue particles, which had held second place in their pristine state, dropped to the bottom of the pack once weathered. This reordering demonstrates that the pigment-driven degradation pathways interact with the surface chemistry changes that aging induces, producing net outcomes that cannot be predicted from either factor alone.

The study also examined how two master variables of aquatic chemistry, pH and ionic strength, modulate these interactions. For virgin polypropylene, adsorption of tetracycline hydrochloride reached its maximum at pH 9, whereas aged polypropylene achieved its maximum at pH 7, near neutral conditions. That shift matters because natural freshwaters span a range of pH values, and a weathered particle that binds antibiotics most strongly at neutral pH will behave very differently in a river than a fresh fragment that prefers alkaline conditions. For PMMA, the picture was simpler: maximum adsorption capacity occurred at pH 5 both before and after aging, suggesting that this acrylic polymer’s surface chemistry responds to acidity in a way that is more robust to weathering than polypropylene’s.

Ionic strength produced its own characteristic pattern. Low concentrations of sodium chloride enhanced the adsorption capacity of the colored microplastics, but as the salt concentration increased further, adsorption declined. This non-monotonic response likely reflects competing effects of dissolved ions on the electrical double layers surrounding both the plastic surfaces and the tetracycline molecules, as well as on the antibiotic’s own speciation. Because salinity varies enormously across environments, from soft freshwater streams to brackish estuaries and open seawater, the result implies that the same fragment of colored plastic could act as a strong or weak antibiotic carrier depending on where it drifts. For risk modelers, the message is that color, weathering history, pH, and salinity must be considered jointly rather than as isolated factors.

Why would color exert such power over a polymer’s environmental chemistry? The authors point to the role of pigments in mediating photodegradation. Different pigments absorb different portions of the light spectrum, and some can act as photosensitizers that accelerate the formation of reactive oxygen species within the polymer, while others may shield the matrix or promote specific failure modes such as the flaking seen in blue polypropylene. Prior work by the same group, published in the Journal of Contaminant Hydrology, showed that microplastic color influences the release of dissolved organic matter during photoaging, and earlier studies on polyvinyl chloride found that color affects biofilm development and the chemodynamics of heavy metals on plastic surfaces. The new results extend that theme to antibiotic adsorption on two additional polymers, strengthening the case that color is a first-order variable in microplastic science rather than a cosmetic footnote.

The broader implications reach into public health and environmental policy. Antibiotic pollution drives the evolution of resistance genes, and microplastics are increasingly recognized as vectors that can carry both antibiotics and resistant bacteria through water systems, as documented in studies of plastisphere communities and biofilm-antibiotic interactions. If red and yellow weathered polypropylene bind tetracycline more strongly than other colors, then fragments from red agricultural film, packaging, or consumer goods may pose disproportionate risks in watersheds affected by pharmaceutical runoff. The authors state that their findings provide a theoretical basis for assessing the ecological risks posed by different colored microplastics in complex pollution scenarios, and the work was supported by the Natural Science Foundation of Anhui Province, the Engineering Research Center of Biofilm Water Purification and Utilization Technology of the Ministry of Education, and Anhui University of Technology’s Innovation Training Program.

For now, the study stands as a reminder that the plastic pollution crisis is more chemically intricate than it appears. Two particles of identical polymer type, size, and shape can behave entirely differently in a river if one is red and the other is blue, and both will change again after months of sunlight. As monitoring programs worldwide begin quantifying microplastics in drinking water sources, groundwater, and agricultural soils, incorporating color as a measurable parameter alongside polymer identity and weathering state could sharpen the accuracy of exposure models. The next step, the researchers suggest, is applying this theoretical basis to real-world mixtures, where colored microplastics, antibiotics, salts, and shifting pH coexist, and where the humble pigment inside a fragment of plastic may quietly decide how much of humanity’s pharmaceutical burden hitchhikes through the environment on its surface.

Subject of Research: Color-dependent aging and adsorption of tetracycline by polypropylene and PMMA microplastics

Article Title: Color-Dependent Adsorption Behavior of Tetracycline onto Aged Microplastics

Article References: Jin, R., Li, X., Li, M., & Shen, M. (2026). Color-Dependent Adsorption Behavior of Tetracycline onto Aged Microplastics. Archives of Environmental Contamination and Toxicology, 91(3), Article 19. https://doi.org/10.1007/s00244-026-01221-5

Image Credits: AI Generated

DOI: 10.1007/s00244-026-01221-5

Keywords: microplastics, polypropylene, PMMA, tetracycline, adsorption, photoaging, plastic color, pH, ionic strength, antibiotic pollution, ecological risk, Color-Dependent