Aggregation behavior of photoaging nanoplastics in artificial sweat solutions

The aging process and aggregation behavior of nanoplastics govern their fate and ecological risk in aquatic environments. Unfortunately, the aggregation behavior of nanoplastics in sweat and the effect of aging on this process remains unknown. This study investigated the aggregation kinetics of polystyrene nanoplastics (PS-NPs) in three types of artificial sweat before and after photoaging. The aggregation rates (k) of PS-NPs before and after photoaging followed the order ofAmerican-Association-of-Textile-Chemists-and-Colorists-pH-4.3 (k_aged =0.6381 nm/s, k_original =0.4337 nm/s) > British-Standard-pH-6.5 (k_aged =0.3589 nm/s, k_original =0.1297 nm/s) >International-Standard-Organization-pH-8.0 (k_aged =0 nm/s, k_original =0 nm/s). Photoaging decreased the C-O content on the surface of PS-NPs from 4.47 % to 1.97 %, thus to promote the aggregation of PS-NPs. Moreover, decrease of the pH value of three types of artificial sweat (from 8.0 to 4.3) all increased the aggregation rate of the PS-NPs. Inorganic constituents (NaCl and Na₂HPO₄) promoted the aggregation of PS-NPs by increasing the positive charges on the surface of PS-NPs, while organic constituents (L-histidine, lactic acid, and urea) stabilized PS-NPs by adsorbing on the surface of PS-NPs. These findings demonstrated that the solution conditions of sweat and photoaging process together determined the transport and distribution of nanoplastics in sweat, offering new insights for assessing and predicting the skin exposure risk of nanoplastics.