Hazard potential of polystyrene and polybutylene succinate microplastics: Cellular toxicity following long-term exposure and Mercury(II) carrier effects

Microplastics (MPs), such as polystyrene (PS) and polybutylene succinate (PBS), are widespread environmental contaminants. While PBS is deemed biodegradable, its environmental impact remains uncertain. This study assesses the cytotoxicity and mercury (HgCl₂) adsorption capacity of PS-MPs and PBS-MPs (~16 μm) using the HT-29 human intestinal epithelial cell line. During long-term exposure, both microplastic types accumulated within and were trapped by the intestinal mucus layer, yet their toxicological profiles diverged significantly. Pristine PS-MPs significantly reduced cell viability to approximately 67% by day 28, whereas PBS-MPs had minimal cytotoxic effects. Short-term exposure (7 days) showed minimal cytotoxicity; however, PS-MPs increased reactive oxygen species (ROS) production and induced apoptosis. In contrast, PBS-MPs exhibited a higher mercury adsorption capacity, absorbing nearly 3-7 times more mercury than PS-MPs and releasing it more rapidly. Mercury-laden PBS (PBS-Hg) caused a significant increase in intracellular mercury levels, chromatin condensation in about 22% of cells, and an approximately 20% reduction in cell viability by day 14. Mercury-laden PS (PS-Hg), conversely, induced minimal genotoxicity or loss of viability. These findings suggest that the chemical properties facilitating PBS's biodegradability may also enhance its ability to adsorb and transport heavy metals. This "Trojan horse" mechanism indicates that labeling a plastic as "biodegradable" does not inherently reduce environmental hazards, underscoring the need to assess both polymer toxicity and contaminant vector potential in developing safer materials.