Retention of ZnO nanoparticles onto polypropylene and polystyrene microplastics: Aging-associated interactions and the role of aqueous chemistry

Microplastics (MPs) are pervasive and undergo environmental aging processes, which alters potential interaction with the co-contaminants. Hence, to assess their contaminant-carrying capacity, mimicking the weathering characteristics of secondary MPs is crucial. To this end, the present study investigated the interaction of Zinc oxide (nZnO) nanoparticles with non-irradiated (NI) and UV-irradiated (UI) forms of the most abundant MPs, such as polypropylene (PP) and polystyrene (PS), in aqueous environments. SEM images revealed mechanical abrasions on the surfaces of NI-MPs and their subsequent photoaging caused the formation of close-ended and open-ended cracks in UI-PP and UI-PS, respectively. Batch-sorption experiments elucidated nZnO uptake kinetics by PP and PS MPs, suggesting a sorption-desorption pathway due to weaker and stronger sorption sites until equilibrium was achieved. UI-PP showed higher nZnO (∼3000 mg/kg) uptake compared to NI-PP, while UI-PS showed similar or slightly decreased nZnO (∼2000 mg/kg) uptake compared to NI-PS. FTIR spectra and zeta potential measurements revealed electrostatic interaction as the dominant interaction mechanism. Higher nZnO uptake by MPs was noted between pH 6.5 and 8.5, whereas it decreased beyond this range. Despite DOM, MPs always retained ∼874 mg/kg nZnO irrespective of MPs type and extent of aging. The experimental results in river water showed higher nZnO uptake on MPs compared to DI water, attributed to mutual effect of ionic competition, DOM, and MP hydrophobicity. In the case of humic acids, complex synthetic and natural water matrices, NI-MPs retained more nZnO than UI-MPs, suggesting that photoaged MPs sorb less nZnO under environmental conditions than non-photoaged MPs. These findings enhance our understanding on interaction of the MPs with co-contaminants in natural environments.