Aging kinetics and mechanisms of polystyrene microplastic in water under sunlight irradiation: Effects of inorganic cations

The effects of chemical components in water on the photochemical transformation of microplastics (MPs) have been well documented, but inorganic cations are often considered inert and ignored. This work studied the photoaging kinetics and mechanisms of polystyrene microplastic (PS-MP) in natural water bodies when common inorganic cations were present at ambient concentrations, including Fe(Ⅲ), NH4+, Ca2+, and Na+. Unlike previous understanding, Fe(Ⅲ) inhibited the photodegradation of MPs by mediating the direct photodegradation of MPs rather than the indirect pathway. Also, NH4+ inhibited PS-MP photoaging, while Ca2+ and Na+ had no effect. Specifically, Fe(Ⅲ) (5 μmol/L) and NH4+ (0.1 mmol/L) reduced the photoaging rate constant (k) to 0.00618 d-1 and 0.00644 d-1, respectively, compared with PS-H2O (k = 0.00705 d-1). With increasing cations’ concentration, the inhibitory effect of Fe(Ⅲ) and NH4+ on PS-MP photoaging was strengthened. According to the results of the reactive oxygen species quenching experiment and ultraviolet-visible spectrum, it was found that light shielding by Fe(Ⅲ) and depletion of hydroxyl radical (•OH) by NH4+ were responsible for the inhibition of photoaging of PS-MP. This work provides new theoretical support for further research on the photoaging process of MPs in the natural water environment.