Multiphase photochemical reactions as sinks of nanoplastic photodissolution products in aqueous environments: a model study for benzene

Carcinogenic benzene is the most concerning product of the irradiation of polystyrene nanoplastics in aqueous suspension. Interestingly, benzene formed in water from polystyrene can volatilise to the gas phase or react with aqueous-phase hydroxyl radicals (•OH(w)) to produce toxic phenol. The persistence of benzene in water would range from some weeks to some months, and the branching ratio between the •OH(w) reaction and volatilisation mainly depends on water depth and the DOC (dissolved organic carbon) concentration. Actually, benzene volatilisation is particularly important in shallow waters (1-2 m depth), or even in relatively deep waters (> 5 m) if the DOC value is high enough (> 5 mgC L-1). Aqueous phenol formed from benzene + •OH(w) reacts in turn with •OH(w), the carbonate radical (CO3•-(w)), and the triplet states of chromophoric dissolved organic matter (3CDOM*(w)) in different proportions, depending on water chemistry. In the gas phase, benzene reacts with •OH(g) to produce phenol, which in turn reacts with •OH(g) and especially with the nitrate radical (•NO3 (g)). The overall degradation is fast enough for phenol to reach an extremely low steady-state concentration in the atmosphere. However, up to 50% of the initial water-dissolved benzene would produce gas-phase phenol as intermediate compound and, eventually, yield phytotoxic nitrophenols. Among the latter, 4-nitrophenol has strong potential to partition into atmospheric waters and reach back aqueous environments (or soil) via wet depositions. To a lesser extent, similar phenomena would involve the highly phytotoxic 2,4-dinitrophenol.