Photoreactive Bromide Ions as Overlooked Regulators of Nanoplastic Surface Chemistry and Aggregation in Sunlit Seawater

Nanoplastics (NPs) are an emerging concern in marine environments due to their persistence and ecological risks. Bromide ions (Br⁻) are recognized as photoreactive drivers in plastic photoaging, yet their influence on NP aggregation and the underlying mechanisms remains largely unexplored. This study investigates the influence of Br⁻ on the aggregation of polystyrene (PS) and polylactic acid (PLA) NPs under UV irradiation. Br⁻ enhances hydroxyl radical production by PS NPs and forms bromine radicals (Br and Br), which accelerated surface coating degradation, oxidation and bromination in a surface-coating-dependent manner. In NaBr and NaCl/NaBr solution, irradiated PS-NH showed a 3-fold increase in aggregation rate due to the removal of amino group, Br-C bonds, and surface adsorbed Br⁻. In contrast, irradiated PS-Plain and PS-COOH exhibited decreased or minor aggregation due to the enrichment of oxygen-containing and Br-C groups that increased surface hydrophilic and electrostatic repulsion. In seawater, all PS NPs exhibited enhanced aggregation after UV aging, with irradiated PS-Plain and PS-COOH rapidly forming microscale aggregates through calcium bridging facilitated by increased carboxyl groups. PLA NPs remained dispersed regardless of UV irradiation and Br⁻, owing to limited photoaging and strong hydrophilic repulsion. By revealing bromide-mediated regulation of NP photoaging and aggregation, this work reshapes current understanding of NP environmental fate and highlights the need to incorporate overlooked halide chemistry into marine NP risk assessments.