Sorption processes of wastewater contaminants on virgin and aged polystyrene microplastics: physicochemical changes and cellular toxicity assessment

Microplastics (MPs) are pervasive pollutants, subject to environmental ageing processes. This enhances their capacity to adsorb various contaminants from environmental matrices, posing growing threats to both ecosystems and human health. This study investigates the adsorption behaviour and cytotoxicity of 1 μm polystyrene microplastics (mPS), both virgin (v-mPS) and home-aged by thermo-oxidation (ox-mPS), following exposure to wastewater (ww). Hydrodynamic diameter measurements and FTIR analysis revealed significant adsorption of organic and inorganic substances onto v- and ox-mPS, proving the formation of an environmental corona. Approximately, 25% of the entire oxidisable load of the ww, assessed by COD, was adsorbed to the mPS. ww-v-mPS exhibited a higher increase in particle size and a thicker coating due to their hydrophobic nature and affinity for apolar or weakly polar compounds. Conversely, ww-ox-mPS, enriched with polar functional groups, displayed a more moderate increase in diameter. Zeta potential measurements indicated significant adsorption of negatively charged species on v-mPS. In human colorectal cells, both ww-mPS caused cell mortality increases (%Δ 243.3 and 83.2 in v- and ox-mPS in comparison to untreated mPS), highlighting potential risks from ingestion of contaminated food.