Deicing salts release micro- and nano-plastics and accelerate their photoaging and algal toxicity in urban waters.

Commercial deicers may represent an overlooked seasonal source of micro- and nanoplastics (MNPs) in urban aquatic environments. This study quantified MNP occurrence and polymer composition in six commercial deicers based on NaCl-, CaCl2-, and CH3COOK and examined their subsequent transformation and biological effects. MNP mass concentrations ranged from 14.5 to 112.6 mg/kg in the >0.22 μm fraction and from 5.0 to 36.3 μg/kg in the <0.22 μm fraction. The tested chloride-based products generally contained higher MNP loads than the organic alternatives, with polypropylene predominating in both fractions. During 10-d photoaging at a fixed anion concentration of 500 mmol/L, saline media enhanced particle oxidation and fragmentation relative to ultrapure water, with CH3COOK generally producing the greatest dimensional reductions. Salt-aged particles exhibited 7-d EC20 values of 19.2-82.0 mg/L for Chlorella pyrenoidosa, compared with 76.6-233.9 mg/L for pristine particles. NaCl-aged particles generally had the lowest EC20 values, indicating that fragmentation alone did not determine toxicity. Direct salt-particle co-exposure at 1.4 mmol/L produced salt- and polymer-dependent responses associated with changes in particle surface charge and particle-cell interactions. One year of government procurement records indicated a potential application-stage MNP input of 7.13 t Together, these findings provide a basis for deicer-product screening and targeted roadside snowmelt management while highlighting the need to consider salinity-mediated particle transformations in ecological risk assessment.