Effect of aging on the release of additives and the toxicity of microplastics

Microplastics (MPs) that contaminate water and food persist for a certain time in the environment and undergo an aging process that could affect their toxicity. The present study evaluates the effect of aging on the chemical composition of MPs and their toxicity. Two commercial plastics (PET and PVC) were aged under artificial and natural conditions. Their surface composition, additive contents and bioaccessibility were analyzed before and after the treatment. Toxicity was evaluated by exposing an intestinal cell model (Caco-2/HT29-MTX/THP-1) to aged and non-aged PVC MPs (50-100 µg/mL, median size 4-6 µm, 6 days), and to their bioaccessible fractions, with the analysis of different parameters (mitochondrial activity, generation of reactive species (ROS/RNS), secretion of IL-1β, mucus and intercellular junction morphology, and paracellular permeability). Although aging had no significant effects on PET plastics, it led to an increase in oxygen-containing functional groups on the surface of PVC MPs. In these plastics, the additive content decreased with aging, particularly over extended periods (21-69%), and the formation of MEHP [mono(2-ethylhexyl) phthalate], a degradation product of DEHP [di(2-ethylhexyl) phthalate] present in the plastic, was observed. The aged and non-aged PVC MPs increased paracellular permeability (24-89%), generating a pro-oxidant response (ROS/RNS increase: 21-53%) and morphological changes (increased mucus secretion and decreased intercellular junction tightness), with the effect being more marked in aged MPs. The bioaccessible fractions also showed toxicity, though to a lesser degree, indicating that the additives or compounds released during transit of the MPs through the digestive tract can also exert an adverse effect.