Enhanced Photoagingof Functionalized Nanoplasticsby Cadmium Ions and Corresponding Diverse Transport Behaviors of Productsin Porous Media: Mechanisms and Modeling

The photoaging of nanoplastics (NPs) mediated by heavy metals and the transport mechanisms of the products have been widely overlooked. This study demonstrated that cadmium ion (Cd­(II)) mediation accelerated the photoaging of polystyrene NPs (carboxyl-modified CNPs, amino-modified ANPs, sulfonate, and amino comodified SANPs) by generating more •OH and 1O2, thereby altering their physicochemical properties and consequent transport behavior. Kinetic attachment models and Derjaguin–Landau–Verwey–Overbeek theory proved that the Cd­(II)-mediated photoaging process had diverse effects on the transport and retention of surface-functionalized NPs in water-saturated sand media. In particular, Cd­(II) mediation at 50 mg/L increased the hydroxyl and carboxyl groups on aged CNPs, facilitating their transport by 20.3% with reduced k2 (0.036 vs 0.021) and increased φmax (74.804 KBT vs 85.127 KBT) values. Conversely, enhanced aged ANP agglomerates driven by amino-carboxyl electrostatic attraction had ripening adsorption on additional heterogeneous surfaces of sand. The presence of reinforced fragments of aged ANPs deposited on sand led to remarkable detachment. Aged SANPs retained their original mobility after Cd­(II) mediation due to negligible changes in surface groups, whereas their retention associated with a higher k1d (0.347 vs 0.183) became more reversible to be flushed out by water. This study provided insights for evaluating cocontamination risks in watersheds and surrounding soils.