Polystyrene nanoplastics modulate the transformation of silver ions in the dark: Key role of environmentally persistent free radicals

• EPFRs generate on PS MPs surface in darkness • PS MPs reduce Ag + to Ag 0 NPs via EPFRs. • EPFRs generate O 2 •− that can further reduce Ag + . • EPFRs also yield 1 O 2 and •OH, oxidizing Ag 0 NPs to Ag 2 O. • PS MPs-mediated Ag + transformation suppresses toxicity of Ag + The co-occurrence of nanoplastics (NPs) and Ag + has drawn public attention, and NPs-driven transformation of Ag⁺ has been widely documented under sunlight. However, the role of NPs in Ag + transformation remains poorly understood under dark conditions. We found that environmentally persistent free radicals (EPFRs) generated on polystyrene (PS) NPs surfaces in dark are fundamentally distinct from those formed under sunlight. The Ag + markedly amplified EPFR formation and accelerated oxidative aging of PS NPs, as reflected by a higher carbonyl index compared with PS NPs alone (CI; 0.313 vs 0.107). Moreover, these EPFRs drive Ag + transformation in the dark. Specifically, the EPFRs reduce Ag + to Ag 0 nanoparticles ( k reduction = 2.7 × 10 ‒3 d ‒1 ), either via direct electron transfer or indirectly by generating O 2 •− as the primary reductant that subsequently drives Ag + reduction. These EPFRs can further oxidize newly formed Ag 0 to Ag 2 O nanoparticles by generating 1 O 2 , and •OH, yielding Ag 0 /Ag 2 O hetero-nanoparticles. Correspondingly, this NPs-mediated change in Ag speciation significantly suppressed Ag accumulation and alleviated oxidative stress in zebrafish larvae in natural waters, thereby significantly increasing 8-h survival (56.7% vs 40.0% for Ag + alone; p < 0.05). Therefore, the presence of PS NPs should be explicitly considered when assessing Ag + risks in natural aquatic ecosystems, including under dark conditions.